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Sample records for metal-poor halo stars

  1. Age determination of metal-poor halo stars using nucleochronometry

    NASA Astrophysics Data System (ADS)

    Christlieb, N.

    2016-09-01

    I describe the method of nucleochronometry for determining individual ages of stars, and report on results of the application of this method to old, metal-poor stars belonging to the Galactic halo population. I discuss uncertainties and caveats of this age determination method.

  2. La and Eu Abundances in Metal-poor Halo Stars

    NASA Astrophysics Data System (ADS)

    Cardillo, Harrison; Burris, Debra L.

    2016-01-01

    Elements with atomic number greater than Z=26 (the Iron Peak) cannot be formed through fusion in a star's core; the majority of these elements are produced through one of two neutron-capture processes. Early in the history of the Galaxy, the rapid neutron-capture process (r-process) is believed to be responsible for the production of elements Z=56 and beyond. These elements require at least one generation of stars to have completed their life cycle in order to be synthesized. Therefore, if we observe the heavy metal abundances in what are called Population II stars (metal-poor stars), then we can begin to make inferences about the chemistry of the earliest stars in the Galaxy. To contribute to this picture of the early universe, the Lanthanum and Europium abundances of low-metallicity stars will be measured and trends in these abundances based on comparisons to existing related literature will be sought.

  3. Armchair cartography - A map of the Galactic halo based on observations of local, metal-poor stars

    NASA Astrophysics Data System (ADS)

    Sommer-Larsen, Jesper; Zhen, Chen

    1990-01-01

    The velocity distribution of metal-poor halo stars in the solar neighborhood is studied to extract data on the global spatial and kinematic properties of the Galactic stellar halo. A global model of the solar neighborhood stars is constructed from observed positions and three-dimensional velocity of local, metal-poor halo stars in terms of a discrete sum of orbits. The characteristics of the reconstructed halo are examined and used to study the evolution of the halo subsystems.

  4. Very Metal-poor Outer-halo Stars with Round Orbits

    NASA Astrophysics Data System (ADS)

    Hattori, Kohei; Yoshii, Yuzuru; Beers, Timothy C.; Carollo, Daniela; Lee, Young Sun

    2013-01-01

    The orbital motions of halo stars in the Milky Way reflect the orbital motions of the progenitor systems in which they formed, making it possible to trace the mass-assembly history of the Galaxy. Direct measurement of three-dimensional velocities, based on accurate proper motions and line-of-sight velocities, has revealed that the majority of halo stars in the inner-halo region move in eccentric orbits. However, our understanding of the motions of distant, in situ halo-star samples is still limited, due to the lack of accurate proper motions for these stars. Here we explore a model-independent analysis of the line-of-sight velocities and spatial distribution of a recent sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars within 30 kpc of the Galactic center. We find that the mean rotational velocity of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that the relatively more metal-rich BHB stars are dominated by stars with eccentric orbits, as previously observed for other stellar samples in the inner-halo region. By contrast, the very metal-poor BHB stars are dominated by stars on rounder, lower-eccentricity orbits. Our results indicate that the motion of the progenitor systems of the Milky Way that contributed to the stellar populations found within 30 kpc correlates directly with their metal abundance, which may be related to their physical properties such as gas fractions. These results are consistent with the existence of an inner/outer halo structure for the halo system, as advocated by Carollo et al.

  5. VERY METAL-POOR OUTER-HALO STARS WITH ROUND ORBITS

    SciTech Connect

    Hattori, Kohei; Yoshii, Yuzuru; Beers, Timothy C.; Carollo, Daniela; Lee, Young Sun

    2013-01-20

    The orbital motions of halo stars in the Milky Way reflect the orbital motions of the progenitor systems in which they formed, making it possible to trace the mass-assembly history of the Galaxy. Direct measurement of three-dimensional velocities, based on accurate proper motions and line-of-sight velocities, has revealed that the majority of halo stars in the inner-halo region move in eccentric orbits. However, our understanding of the motions of distant, in situ halo-star samples is still limited, due to the lack of accurate proper motions for these stars. Here we explore a model-independent analysis of the line-of-sight velocities and spatial distribution of a recent sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars within 30 kpc of the Galactic center. We find that the mean rotational velocity of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that the relatively more metal-rich BHB stars are dominated by stars with eccentric orbits, as previously observed for other stellar samples in the inner-halo region. By contrast, the very metal-poor BHB stars are dominated by stars on rounder, lower-eccentricity orbits. Our results indicate that the motion of the progenitor systems of the Milky Way that contributed to the stellar populations found within 30 kpc correlates directly with their metal abundance, which may be related to their physical properties such as gas fractions. These results are consistent with the existence of an inner/outer halo structure for the halo system, as advocated by Carollo et al.

  6. HIERARCHICAL FORMATION OF THE GALACTIC HALO AND THE ORIGIN OF HYPER METAL-POOR STARS

    SciTech Connect

    Komiya, Yutaka; Habe, Asao; Suda, Takuma; Fujimoto, Masayuki Y.

    2009-05-01

    Extremely metal-poor (EMP) stars in the Galactic halo are unique probes into the early universe and the first stars. We construct a new program to calculate the formation history of EMP stars in the early universe with the chemical evolution, based on the merging history of the Galaxy. We show that the hierarchical structure formation model reproduces the observed metallicity distribution function and also the total number of observed EMP stars, when we take into account the high-mass initial mass function and the contribution of binaries, as proposed by Komiya et al. The low-mass survivors divide into two groups of those born before and after the mini-halos are polluted by their own first supernovae. The former has observational counterparts in the hyper metal-poor (HMP) stars below [Fe/H] < -4, while the latter represents the majority of EMP stars with {approx}<[Fe/H]> - 4. In this Letter, we focus on the origin of the extremely small iron abundances of HMP stars. We compute the change in the surface abundances of individual stars through the accretion of the metal-enriched interstellar gas along with the dynamical and chemical evolution of the Galaxy, to demonstrate that after-birth pollution of Population III stars is sufficiently effective to explain the observed abundances of HMP stars. Metal pre-enrichment by possible pair instability supernovae is also discussed, to derive constraints on their roles and on the formation of the first low-mass stars.

  7. r-Process abundances in metal-poor Galactic halo stars

    NASA Astrophysics Data System (ADS)

    Siqueira-Mello, C.; Barbuy, B.; Spite, M.; Spite, F.; Caffau, E.; Hill, V.; Wanajo, S.; François, P.; Bonifacio, P.; Cayrel, R.

    The site of the r-process is not completely defined, and several models try to explain the origin of the trans-Fe elements. Observed abundances are the best clues to bring some light to this multiplicity of possible mechanisms, and the extremely metal-poor (EMP) Galactic halo stars have a special role in this problem. In this contribution we present the solution of a long-standing problem about the origin of the heavy elements in the metal-poor halo subgiant star HD 140283, and its correlation with the Truran's theory. Next, we describe the results obtained with the EMP r-II star CS 31082-001 in the frame of the ESO Large Program ``First Stars''. Using STIS/HST observations we provide abundances for elements never presented before in this stars, making CS 31082-001 the most complete r-II object studied, with a total of 37 detections of neutron-capture elements. Finally, we present the results obtained from a sample of seven r-I stars, showing how those objects can help us solving the heavy elements problem. Conclusions are also described.

  8. FORMATION HISTORY OF METAL-POOR HALO STARS WITH THE HIERARCHICAL MODEL AND THE EFFECT OF INTERSTELLAR MATTER ACCRETION ON THE MOST METAL-POOR STARS

    SciTech Connect

    Komiya, Yutaka; Habe, Asao; Suda, Takuma; Fujimoto, Masayuki Y.

    2010-07-01

    We investigate star formation and chemical evolution in the early universe by considering the merging history of the Galaxy in the {Lambda} cold dark matter scenario according to the extended Press-Schechter theory. We give some possible constraints from comparisons with observation of extremely metal-poor (EMP) stars, made available by the recent large-scale surveys and by the follow-up high-resolution spectroscopy. We demonstrate that (1) the hierarchical structure formation can explain the characteristics of the observed metallicity distribution function including a break around [Fe/H] = -4; (2) a high-mass initial mass function (IMF) of peak mass {approx}10 M{sub sun} with the contribution of binaries, derived from the statistics of carbon-enhanced EMP stars, predicts the frequency of low-mass survivors consistent with the number of EMP stars observed for -4 {approx_lt} [Fe/H] {approx_lt} -2.5; (3) the stars formed from primordial gas before the first supernova (SN) explosions in their host mini-halos are assigned to the hyper metal-poor (HMP) stars with [Fe/H] {approx} -5; and (4) there is no indication of significant changes in the IMF and the binary contribution at metallicities -4 {approx_gt} [Fe/H] {approx_gt} -2.5, or even larger, as far as the field stars of the Galactic halo are concerned. We further study the effects of surface pollution through the accretion of interstellar matter (ISM) along the chemical and dynamical evolution of the Galaxy for low-mass Population III and EMP survivors. Because of the shallower potential of smaller halos, the accretion of ISM in the mini-halos in which these stars were born dominates the surface metal pollution. This can account for the surface iron abundances as observed for the HMP stars if the cooling and concentration of gas in their birth mini-halos are taken into account. We also study the feedback effect from the very massive Population III stars. The metal pre-pollution by pair-instability SNe is shown to be

  9. CARBON-ENHANCED METAL-POOR STARS IN THE INNER AND OUTER HALO COMPONENTS OF THE MILKY WAY

    SciTech Connect

    Carollo, Daniela; Norris, John E.; Freeman, Ken C.; Beers, Timothy C.; Lee, Young Sun; Kennedy, Catherine R.; Bovy, Jo; Sivarani, Thirupathi; Aoki, Wako E-mail: kcf@mso.anu.edu.au E-mail: beers@pa.msu.edu E-mail: kenne257@msu.edu E-mail: sivarani@iiap.res.in

    2012-01-10

    Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30,000 calibration stars from the Sloan Digital Sky Survey. Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ({sup c}arbonicity{sup )} in excess of [C/Fe] =+0.7 are considered CEMP stars, the global frequency of CEMP stars in the halo system for [Fe/H] <-1.5 is 8%, for [Fe/H] <-2.0 it is 12%, and for [Fe/H] <-2.5 it is 20%. We also confirm a significant increase in the level of carbon enrichment with declining metallicity, growing from ([C/Fe]) {approx}+1.0 at [Fe/H] =-1.5 to ([C/Fe]) {approx}+1.7 at [Fe/H] =-2.7. The nature of the carbonicity distribution function (CarDF) changes dramatically with increasing distance above the Galactic plane, |Z|. For |Z| <5 kpc, relatively few CEMP stars are identified. For distances |Z| >5 kpc, the CarDF exhibits a strong tail toward high values, up to [C/Fe] > +3.0. We also find a clear increase in the CEMP frequency with |Z|. For stars with -2.0 < [Fe/H] <-1.5, the frequency grows from 5% at |Z| {approx}2 kpc to 10% at |Z| {approx}10 kpc. For stars with [Fe/H] <-2.0, the frequency grows from 8% at |Z| {approx}2 kpc to 25% at |Z| {approx}10 kpc. For stars with -2.0 < [Fe/H] <-1.5, the mean carbonicity is ([C/Fe]) {approx}+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for [Fe/H] <-2.0, ([C/Fe]) {approx}+1.5, again roughly independent of |Z|. Based on a statistical separation of the halo components in velocity space, we find evidence for a significant contrast in the frequency of CEMP stars between the inner- and outer-halo components-the outer halo possesses roughly twice the fraction of CEMP stars as the inner halo. The carbonicity distribution also differs between the inner-halo and outer-halo components-the inner halo has a greater portion of stars with modest carbon

  10. Carbon-enhanced metal-poor stars: CEMP-s and CEMP-no subclasses in the halo system of the Milky Way

    SciTech Connect

    Carollo, Daniela; Freeman, Ken; Beers, Timothy C.; Placco, Vinicius M.; Tumlinson, Jason; Martell, Sarah L. E-mail: kcf@mso.anu.edu.au E-mail: vplacco@gemini.edu E-mail: smartell@aao.gov.au

    2014-06-20

    We explore the kinematics and orbital properties of a sample of 323 very metal-poor stars in the halo system of the Milky Way, selected from the high-resolution spectroscopic follow-up studies of Aoki et al. and Yong et al. The combined sample contains a significant fraction of carbon-enhanced metal-poor (CEMP) stars (22% or 29%, depending on whether a strict or relaxed criterion is applied for this definition). Barium abundances (or upper limits) are available for the great majority of the CEMP stars, allowing for their separation into the CEMP-s and CEMP-no subclasses. A new method to assign membership to the inner- and outer-halo populations of the Milky Way is developed, making use of the integrals of motion, and applied to determine the relative fractions of CEMP stars in these two subclasses for each halo component. Although limited by small-number statistics, the data suggest that the inner halo of the Milky Way exhibits a somewhat higher relative number of CEMP-s stars than CEMP-no stars (57% versus 43%), while the outer halo possesses a clearly higher fraction of CEMP-no stars than CEMP-s stars (70% versus 30%). Although larger samples of CEMP stars with known Ba abundances are required, this result suggests that the dominant progenitors of CEMP stars in the two halo components were different; massive stars for the outer halo, and intermediate-mass stars in the case of the inner halo.

  11. The role of binaries in the enrichment of the early Galactic halo. III. Carbon-enhanced metal-poor stars - CEMP-s stars

    NASA Astrophysics Data System (ADS)

    Hansen, T. T.; Andersen, J.; Nordström, B.; Beers, T. C.; Placco, V. M.; Yoon, J.; Buchhave, L. A.

    2016-04-01

    Context. Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high- and low-C bands, mostly populated by binary and single stars, respectively. Aims: Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Methods: We have systematically monitored the radial velocities of a sample of 22 CEMP-s stars for several years with ~monthly, high-resolution, low S/N échelle spectra obtained at the Nordic Optical Telescope (NOT) at La Palma, Spain. From these spectra, radial velocities with an accuracy of ≈100 m s-1 were determined by cross-correlation with optimised templates. Results: Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82 ± 10%, while four stars appear to be single (18 ± 10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for eleven of the binaries and provisional orbits for six long-period systems (P > 3000 days), and orbital circularisation timescales are discussed. Conclusions: The conventional scenario of local mass transfer from a former asymptotic giant branch (AGB) binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their

  12. Deep SDSS optical spectroscopy of distant halo stars. III. Chemical analysis of extremely metal-poor stars

    NASA Astrophysics Data System (ADS)

    Fernández-Alvar, E.; Allende Prieto, C.; Beers, T. C.; Lee, Y. S.; Masseron, T.; Schneider, D. P.

    2016-09-01

    Aims: We present the results of an analysis of 107 extremely metal-poor (EMP) stars with metallicities lower than [Fe/H] =- 3.0, identified in medium-resolution spectra in the Sloan Digital Sky Survey (SDSS). Our analysis provides estimates of the stellar effective temperatures and surface gravities, as well as iron, calcium, and magnesium abundances. Methods: We followed the same method as in previous papers of this series. The method is based on comparisons of the observed spectra with synthetic spectra. The abundances of Fe, Ca, and Mg were determined by fitting spectral regions that are dominated by lines of each element. In addition, we present a technique to determine upper limits for elements whose features are not detected in a given spectrum. We also analyzed our sample with the SEGUE stellar parameter pipeline to obtain additional determinations of the atmospheric parameters and iron and alpha-element abundances, which we thend compare with ours. In addition, we used these parameters to infer [C/Fe] ratios. Results: Ca is typically the only element in these spectra with a moderate to low signal-to-noise ratio and medium resolution in this metallicity regime with lines that are sufficiently strong to reliably measure its abundance. Fe and Mg exhibit weaker features that in most cases only provide upper limits. We measured [Ca/Fe] and [Mg/Fe] for EMP stars in the SDSS spectra and conclude that most of the stars exhibit the typical enhancement level for α-elements, ~+0.4, although some stars for which only [Fe/H] upper limits could be estimated indicate higher [α/Fe] ratios. We also find that 26% of the stars in our sample can be classified as carbon-enhanced metal-poor (CEMP) stars and that the frequency of CEMP stars also increases with decreasing metallicity, as has been reported for previous samples. We identify a rare, bright (g = 11.90) EMP star, SDSS J134144.61+474128.6, with [Fe/H] =- 3.27, [C/Fe] = + 0.95, and elevated magnesium ([Mg/Fe] =+ 0

  13. Constraints on First-Stars Models From Observations of Local Low-Mass Dwarf Galaxies and Galactic Metal-Poor Halo Stars

    NASA Astrophysics Data System (ADS)

    Yung, Long Yan; Venkatesan, A.

    2014-01-01

    The first metal-free stars in the universe had hard ionizing photon spectra and unique element yields from their supernovae, leaving signatures in the reionization of the intergalactic medium and in the metal enrichment of gas in the early universe. Here, we examine the metal abundances in a variety of systems in the local universe, from very metal-poor Galactic halo stars to ultra-faint dwarf spheroidal galaxies, and compare them with the latest theoretical models of massive stars with and without rotation. We confirm the similar abundance patterns found in the ultra-faint dwarfs and metal-poor halo stars by recent studies, and find new trends of interest in a variety of individual elements spanning metallicity values of [Fe/H] from about -2 to -5. We also compare our results with the abundances found in the very metal-deficient nearby dwarf irregular galaxy Leo P, which was recently discovered in the Arecibo ALFALFA survey. We comment on the similarities and differences between abundance trends in gas-rich dwarf galaxy systems like Leo P versus gas-poor ones like the ultra-faint dwarf spheroidals, and on the possibility of such systems hosting populations of the first stars. This work has been supported by NSF grant AST-1211005 and by Research Corporation through the Cottrell College Science Award.

  14. Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

    NASA Astrophysics Data System (ADS)

    Chiba, Masashi; Beers, Timothy C.

    2000-06-01

    We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H]<=-0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H]<=-2.2, which likely represent a ``pure'' halo component, are characterized by a radially elongated velocity ellipsoid (σU,σV,σW)=(141+/-11, 106+/-9, 94+/-8) km s-1 and small prograde rotation =30 to 50 km s-1, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (Δ/Δ|Z|=-52+/-6 km s-1 kpc-1), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high-e stars at [Fe/H]~-1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H]<=-2.2. The kinematics of the intermediate

  15. The role of binaries in the enrichment of the early Galactic halo. II. Carbon-enhanced metal-poor stars: CEMP-no stars

    NASA Astrophysics Data System (ADS)

    Hansen, T. T.; Andersen, J.; Nordström, B.; Beers, T. C.; Placco, V. M.; Yoon, J.; Buchhave, L. A.

    2016-02-01

    Context. The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20-70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon; these are the so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic chemical evolution models is whether this non-standard composition reflects that of the stellar natal clouds or is due to local, post-birth mass transfer of chemically processed material from a binary companion; CEMP stars should then all be members of binary systems. Aims: Our aim is to determine the frequency and orbital parameters of binaries among CEMP stars with and without over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars, respectively - as a test of this local mass-transfer scenario. This paper discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider a similar sample of CEMP-s stars. Methods: High-resolution, low S/N spectra of the stars were obtained at roughly monthly intervals over a time span of up to eight years with the FIES spectrograph at the Nordic Optical Telescope. Radial velocities of ~100 m s-1 precision were determined by cross-correlation after each observing night, allowing immediate, systematic follow-up of any variable object. Results: Most programme stars exhibit no statistically significant radial-velocity variation over this period and appear to be single, while four are found to be binaries with orbital periods of 300-2000 days and normal eccentricity; the binary frequency for the sample is 17 ± 9%. The single stars mostly belong to the recently identified low-C band, while the binaries have higher absolute carbon abundances. Conclusions: We conclude that the nucleosynthetic process responsible for the strong carbon excess in these ancient stars is unrelated to their binary status; the carbon was imprinted on their natal molecular clouds in the early Galactic interstellar

  16. Explaining the Ba, Y, Sr, and Eu abundance scatter in metal-poor halo stars: constraints to the r-process

    NASA Astrophysics Data System (ADS)

    Cescutti, G.; Chiappini, C.

    2014-05-01

    Context. Thanks to the heroic observational campaigns carried out in recent years we now have large samples of metal-poor stars for which measurements of detailed abundances exist. In particular, large samples of stars with metallicities -5 < [Fe/H] <-1 and measured abundances of Sr, Ba, Y, and Eu are now available. These data hold important clues on the nature of the contribution of the first stellar generations to the enrichment of our Galaxy. Aims: We aim to explain the scatter in Sr, Ba, Y, and Eu abundance ratio diagrams unveiled by the metal-poor halo stars. Methods: We computed inhomogeneous chemical evolution models for the Galactic halo assuming different scenarios for the r-process site: the electron-capture (EC) supernovae and the magnetorotationally driven (MRD) supernovae scenarios. We also considered models with and without the contribution of fast-rotating massive stars (spinstars) to an early enrichment by the s-process. A detailed comparison with the now large sample of stars with measured abundances of Sr, Ba, Y, Eu, and Fe is provided (both in terms of scatter plots and number distributions for several abundance ratios). Results: The scatter observed in these abundance ratios of the very metal-poor stars (with [Fe/H] <-2.5) can be explained by combining the s-process production in spinstars, and the r-process contribution coming from massive stars. For the r-process we have developed models for both the EC and the MRD scenarios that match the observations. Conclusions: With the present observational and theoretical constraints we cannot distinguish between the EC and the MRD scenarios in the Galactic halo. Independently of the r-process scenarios adopted, the production of elements by an s-process in spinstars is needed to reproduce the spread in abundances of the light neutron capture elements (Sr and Y) over heavy neutron capture elements (Ba and Eu). We provide a way to test our suggestions by means of the distribution of the Ba isotopic

  17. The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars

    NASA Astrophysics Data System (ADS)

    Hansen, T. T.; Andersen, J.; Nordström, B.; Beers, T. C.; Yoon, J.; Buchhave, L. A.

    2015-11-01

    Context. The detailed chemical composition of most metal-poor halo stars has been found to be highly uniform, but a minority of stars exhibit dramatic enhancements in their abundances of heavy neutron-capture elements and/or of carbon. The key question for Galactic chemical evolution models is whether these peculiarities reflect the composition of the natal clouds, or if they are due to later (post-birth) mass transfer of chemically processed material from a binary companion. If the former case applies, the observed excess of certain elements was implanted within selected clouds in the early ISM from a production site at interstellar distances. Aims: Our aim is to determine the frequency and orbital properties of binaries among these chemically peculiar stars. This information provides the basis for deciding whether local mass transfer from a binary companion is necessary and sufficient to explain their unusual compositions. This paper discusses our study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element enhanced VMP and EMP stars. Methods: High-resolution, low signal-to-noise spectra of the stars were obtained at roughly monthly intervals over eight years with the FIES spectrograph at the Nordic Optical Telescope. From these spectra, radial velocities with an accuracy of ~100 m s-1 were determined by cross-correlation against an optimized template. Results: Fourteen of the programme stars exhibit no significant radial-velocity variation over this temporal window, while three are binaries with orbits of typical eccentricity for their periods, resulting in a normal binary frequency of ~18 ± 6% for the sample. Conclusions: Our results confirm our preliminary conclusion from 2011, based on partial data, that the chemical peculiarity of the r-I and r-II stars is not caused by any putative binary companions. Instead, it was imprinted on the natal molecular clouds of these stars by an external, distant source. Models of the ISM in early galaxies

  18. The Best and Brightest Metal-Poor Stars

    NASA Astrophysics Data System (ADS)

    Schlaufman, Kevin

    2015-01-01

    The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. High-resolution follow up has revealed that 3.8% of our candidates have [Fe/H] < -3.0 and 32.5% have -3.0 < [Fe/H] < -2.0. We are using the Automated Planet Finder, Gemini, and Magellan to follow up all of our metal-poor candidates with V < 12 in both hemispheres with the goal of collecting the most information-rich sample of metal-poor stars ever assembled.

  19. The Best and Brightest Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Schlaufman, Kevin C.; Casey, Andrew R.

    2014-12-01

    The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. The acquisition of high signal-to-noise spectra for faint EMP stars requires a major telescope time commitment, making the construction of large samples of EMP star abundances prohibitively expensive. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. We have used our selection to identify 11,916 metal-poor star candidates with V < 14, increasing the number of publicly available candidates by more than a factor of five in this magnitude range. Their bright apparent magnitudes have greatly eased high-resolution follow-up observations that have identified seven previously unknown stars with [Fe/H] <~ -3.0. Our follow-up campaign has revealed that 3.8+1.3-1.1% of our candidates have [Fe/H] <~ -3.0 and 32.5+3.0-2.9% have -3.0 <~ [Fe/H] <~ -2.0. The bulge is the most likely location of any existing Galactic Population III stars, and an infrared-only variant of our selection is well suited to the identification of metal-poor stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H] <~ -2.7 are within about 2 kpc of the Galactic center. They are among the most metal-poor stars known in the bulge. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  20. The best and brightest metal-poor stars

    SciTech Connect

    Schlaufman, Kevin C.; Casey, Andrew R. E-mail: arc@ast.cam.ac.uk

    2014-12-10

    The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. The acquisition of high signal-to-noise spectra for faint EMP stars requires a major telescope time commitment, making the construction of large samples of EMP star abundances prohibitively expensive. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. We have used our selection to identify 11,916 metal-poor star candidates with V < 14, increasing the number of publicly available candidates by more than a factor of five in this magnitude range. Their bright apparent magnitudes have greatly eased high-resolution follow-up observations that have identified seven previously unknown stars with [Fe/H] ≲ –3.0. Our follow-up campaign has revealed that 3.8{sub −1.1}{sup +1.3}% of our candidates have [Fe/H] ≲ –3.0 and 32.5{sub −2.9}{sup +3.0}% have –3.0 ≲ [Fe/H] ≲ –2.0. The bulge is the most likely location of any existing Galactic Population III stars, and an infrared-only variant of our selection is well suited to the identification of metal-poor stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H] ≲ –2.7 are within about 2 kpc of the Galactic center. They are among the most metal-poor stars known in the bulge.

  1. Dependence of the Sr-to-Ba and Sr-to-Eu Ratio on the Nuclear Equation of State in Metal-poor Halo Stars

    NASA Astrophysics Data System (ADS)

    Famiano, M. A.; Kajino, T.; Aoki, W.; Suda, T.

    2016-10-01

    A model is proposed in which the dependence on the equation of state (EOS) of the scatter of [Sr/Ba] in metal-poor stars is studied. Light r-process element enrichment in these stars has been explained via a truncated r-process, or “tr-process.” The truncation of the r-process from a generic core-collapse event followed by a collapse into an accretion-induced black hole is examined in the framework of a galactic chemical evolution model. The constraints on this model imposed by observations of extremely metal-poor stars are explained, and the upper limits in the [Sr/Ba] distributions are found to be related to the nuclear EOS in a collapse scenario. The scatter in [Sr/Ba] and [Sr/Eu] as a function of metallicity has been found to be consistent with turbulent ejection in core-collapse supernovae. Adaptations of this model are evaluated to account for the scatter in isotopic observables. This is done by assuming mixing in ejecta in a supernova event. Stiff EOS are eliminated by this model.

  2. Carbon-enhanced metal-poor stars in different environments

    NASA Astrophysics Data System (ADS)

    Salvadori, S.; Skúladóttir, Á.; de Bennassuti, M.

    2016-09-01

    The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connections with the chemical elements produced by the first stellar generations is still highly debated. We briefly review observations of CEMP stars in different environments (Galactic stellar halo, ultra-faint, and classical dwarf galaxies) and interpret their properties using cosmological chemical-evolution models for the formation of the Local Group. We discuss the implications of current observations for the properties of the first stars, clarify why the fraction of carbon-enhanced to carbon-normal stars varies in dwarf galaxies with different luminosity, and discuss the origin of the first CEMP(-no) star found in the Sculptor dwarf galaxy.

  3. Toward ab initio extremely metal poor stars

    NASA Astrophysics Data System (ADS)

    Ritter, Jeremy S.; Safranek-Shrader, Chalence; Milosavljević, Miloš; Bromm, Volker

    2016-09-01

    Extremely metal poor stars have been the focus of much recent attention owing to the expectation that their chemical abundances can shed light on the metal and dust yields of the earliest supernovae. We present our most realistic simulation to date of the astrophysical pathway to the first metal enriched stars. We simulate the radiative and supernova hydrodynamic feedback of a 60 M⊙ Population III star starting from cosmological initial conditions realizing Gaussian density fluctuations. We follow the gravitational hydrodynamics of the supernova remnant at high spatial resolution through its freely-expanding, adiabatic, and radiative phases, until gas, now metal-enriched, has resumed runaway gravitational collapse. Our findings are surprising: while the Population III progenitor exploded with a low energy of 1051 erg and injected an ample metal mass of 6 M⊙, the first cloud to collapse after the supernova explosion is a dense surviving primordial cloud on which the supernova blast wave deposited metals only superficially, in a thin, unresolved layer. The first metal-enriched stars can form at a very low metallicity, of only 2 - 5 × 10-4 Z⊙, and can inherit the parent cloud's highly elliptical, radially extended orbit in the dark matter gravitational potential.

  4. DETAILED ABUNDANCES OF TWO VERY METAL-POOR STARS IN DWARF GALAXIES

    SciTech Connect

    Kirby, Evan N.; Cohen, Judith G.

    2012-12-01

    The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[{alpha}/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at -3 < [Fe/H] < -2 sample SNe with [Fe/H] << -3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] {approx} -3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo.

  5. THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] {<=} -3.5 , ,

    SciTech Connect

    Yong, David; Norris, John E.; Bessell, M. S.; Asplund, M.; Christlieb, N.; Beers, Timothy C.; Barklem, P. S.; Frebel, Anna; Ryan, S. G. E-mail: jen@mso.anu.edu.au E-mail: martin@mso.anu.edu.au E-mail: beers@pa.msu.edu E-mail: afrebel@mit.edu

    2013-01-01

    We present a homogeneous chemical abundance analysis of 16 elements in 190 metal-poor Galactic halo stars (38 program and 152 literature objects). The sample includes 171 stars with [Fe/H] {<=} -2.5, of which 86 are extremely metal poor, [Fe/H] {<=} -3.0. Our program stars include 10 new objects with [Fe/H] {<=} -3.5. We identify a sample of 'normal' metal-poor stars and measure the trends between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for this sample. Using this mean trend, we identify objects that are chemically peculiar relative to 'normal' stars at the same metallicity. These chemically unusual stars include CEMP-no objects, one star with high [Si/Fe], another with high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic processes at lowest metallicity that are distinct from the main r-process. Finally, for many elements, we find a significant trend between [X/Fe] versus T {sub eff}, which likely reflects non-LTE and/or three-dimensional effects. Such trends demonstrate that care must be exercised when using abundance measurements in metal-poor stars to constrain chemical evolution and/or nucleosynthesis predictions.

  6. Exploring the early Universe with extremely metal-poor stars

    NASA Astrophysics Data System (ADS)

    Hansen, Terese T.; Christlieb, Norbert; Hansen, Camilla J.; Beers, Timothy C.

    2016-08-01

    The earliest phases of Galactical chemical evolution and nucleosynthesis can be investigated by studying the old metal-poor stars. It has been recognized that a large fraction of metal-poor stars possess significant over-abundances of carbon relative to iron. Here we present the results of a 23-star homogeneously analyzed sample of metal-poor candidates from the Hamburg/ESO survey. We have derived abundances for a large number of elements ranging from Li to Pb. The sample includes four ultra metal-poor stars ([Fe/H] < -4.0), six CEMP-no stars, five CEMP-s stars, two CEMP-r stars and two CEMP-r/s stars. This broad variety of the sample stars gives us an unique opportunity to explore different abundance patterns at low metallicity.

  7. The EMBLA survey - metal-poor stars in the Galactic bulge

    NASA Astrophysics Data System (ADS)

    Howes, Louise M.; Asplund, Martin; Keller, Stefan C.; Casey, Andrew R.; Yong, David; Lind, Karin; Frebel, Anna; Hays, Austin; Alves-Brito, Alan; Bessell, Michael S.; Casagrande, Luca; Marino, Anna F.; Nataf, David M.; Owen, Christopher I.; Da Costa, Gary S.; Schmidt, Brian P.; Tisserand, Patrick

    2016-07-01

    Cosmological models predict the oldest stars in the Galaxy should be found closest to the centre of the potential well, in the bulge. The Extremely Metal-poor BuLge stars with AAOmega survey (EMBLA) successfully searched for these old, metal-poor stars by making use of the distinctive SkyMapper photometric filters to discover candidate metal-poor stars in the bulge. Their metal-poor nature was then confirmed using the AAOmega spectrograph on the Anglo-Australian Telescope. Here we present an abundance analysis of 10 bulge stars with -2.8 < [Fe/H] < -1.7 from MIKE/Magellan observations, in total determining the abundances of 22 elements. Combining these results with our previous high-resolution data taken as part of the Gaia-ESO Survey, we have started to put together a picture of the chemical and kinematic nature of the most metal-poor stars in the bulge. The currently available kinematic data are consistent with the stars belonging to the bulge, although more accurate measurements are needed to constrain the stars' orbits. The chemistry of these bulge stars deviates from that found in halo stars of the same metallicity. Two notable differences are the absence of carbon-enhanced metal-poor bulge stars, and the α element abundances exhibit a large intrinsic scatter and include stars which are underabundant in these typically enhanced elements.

  8. The Chemical Abundances of Stars in the Halo (CASH) Project. III. A New Classification Scheme for Carbon-enhanced Metal-poor Stars with s-process Element Enhancement

    NASA Astrophysics Data System (ADS)

    Hollek, Julie K.; Frebel, Anna; Placco, Vinicius M.; Karakas, Amanda I.; Shetrone, Matthew; Sneden, Christopher; Christlieb, Norbert

    2015-12-01

    We present a detailed abundance analysis of 23 elements for a newly discovered carbon-enhanced metal-poor (CEMP) star, HE 0414-0343, from the Chemical Abundances of Stars in the Halo Project. Its spectroscopic stellar parameters are Teff = 4863 K, {log}g=1.25,\\ξ = 2.20 km s-1, and [Fe/H] = -2.24. Radial velocity measurements covering seven years indicate HE 0414-0343 to be a binary. HE 0414-0343 has {{[C/Fe]}}=1.44 and is strongly enhanced in neutron-capture elements but its abundances cannot be reproduced by a solar-type s-process pattern alone. Traditionally, it could be classified as a “CEMP-r/s” star. Based on abundance comparisons with asymptotic giant branch (AGB) star nucleosynthesis models, we suggest a new physically motivated origin and classification scheme for CEMP-s stars and the still poorly understood CEMP-r/s. The new scheme describes a continuous transition between these two so-far distinctly treated subgroups: CEMP-sA, CEMP-sB, and CEMP-sC. Possible causes for a continuous transition include the number of thermal pulses the AGB companion underwent, the effect of different AGB star masses on their nucleosynthetic yields, and physics that is not well approximated in 1D stellar models such as proton ingestion episodes and rotation. Based on a set of detailed AGB models, we suggest the abundance signature of HE 0414-0343 to have arisen from a >1.3 M⊙ mass AGB star and a late-time mass transfer that transformed HE 0414-0343 into a CEMP-sC star. We also find that the [Y/Ba] ratio well parametrizes the classification and can thus be used to easily classify any future such stars. Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

  9. The lithium isotopic ratio in very metal-poor stars

    NASA Astrophysics Data System (ADS)

    Lind, K.; Melendez, J.; Asplund, M.; Collet, R.; Magic, Z.

    2013-06-01

    Context. Un-evolved, very metal-poor stars are the most important tracers of the cosmic abundance of lithium in the early universe. Combining the standard Big Bang nucleosynthesis model with Galactic production through cosmic ray spallation, these stars at [Fe / H] < - 2 are expected to show an undetectably small 6Li / 7Li isotopic signature. Evidence to the contrary may necessitate an additional pre-galactic production source or a revision of the standard model of Big Bang nucleosynthesis. It would also cast doubts on Li depletion from stellar atmospheres as an explanation for the factor 3-5 discrepancy between the predicted primordial 7Li from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. Aims: We revisit the isotopic analysis of four halo stars, two with claimed 6Li-detections in the literature, to investigate the influence of improved model atmospheres and line formation treatment. Methods: For the first time, a combined 3D, non-local thermodynamic equilibrium (NLTE) modelling technique for Li, Na, and Ca lines is utilised to constrain the intrinsic line-broadening and to determine the Li isotopic ratio. We discuss the influence of 3D NLTE effects on line profile shapes and assess the realism of our modelling using the Ca excitation and ionisation balance. Results: By accounting for NLTE line formation in realistic 3D hydrodynamical model atmospheres, we can model the Li resonance line and other neutral lines with a consistency that is superior to LTE, with no need for additional line asymmetry caused by the presence of 6Li. Contrary to the results from 1D and 3D LTE modelling, no star in our sample has a significant (2σ) detection of the lighter isotope in NLTE. Over a large parameter space, NLTE modelling systematically reduces the best-fit Li isotopic ratios by up to five percentage points. As a bi-product, we also present the first ever 3D NLTE Ca and Na abundances of halo stars, which reveal significant departures from LTE

  10. What Are These Blue Metal-Poor Stars?

    NASA Astrophysics Data System (ADS)

    Preston, George W.; Sneden, Christopher

    2000-08-01

    intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose from improper use of the globular cluster BS specific frequency, SBS=n(BS)/n(HB)~1, to estimate the halo BS space density. We use a simple model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction fB. Our model calculations return values of SBS~5 for fB=0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.

  11. Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way.

    PubMed

    Howes, L M; Casey, A R; Asplund, M; Keller, S C; Yong, D; Nataf, D M; Poleski, R; Lind, K; Kobayashi, C; Owen, C I; Ness, M; Bessell, M S; Da Costa, G S; Schmidt, B P; Tisserand, P; Udalski, A; Szymański, M K; Soszyński, I; Pietrzyński, G; Ulaczyk, K; Wyrzykowski, Ł; Pietrukowicz, P; Skowron, J; Kozłowski, S; Mróz, P

    2015-11-26

    The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.

  12. Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way.

    PubMed

    Howes, L M; Casey, A R; Asplund, M; Keller, S C; Yong, D; Nataf, D M; Poleski, R; Lind, K; Kobayashi, C; Owen, C I; Ness, M; Bessell, M S; Da Costa, G S; Schmidt, B P; Tisserand, P; Udalski, A; Szymański, M K; Soszyński, I; Pietrzyński, G; Ulaczyk, K; Wyrzykowski, Ł; Pietrukowicz, P; Skowron, J; Kozłowski, S; Mróz, P

    2015-11-26

    The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon. PMID:26560034

  13. LITHIUM ABUNDANCES IN CARBON-ENHANCED METAL-POOR STARS

    SciTech Connect

    Masseron, Thomas; Johnson, Jennifer A.; Lucatello, Sara; Karakas, Amanda; Plez, Bertrand; Beers, Timothy C.; Christlieb, Norbert E-mail: jaj@astronomy.ohio-state.edu

    2012-05-20

    Carbon-enhanced metal-poor (CEMP) stars are believed to show the chemical imprints of more massive stars (M {approx}> 0.8 M{sub Sun }) that are now extinct. In particular, it is expected that the observed abundance of Li should deviate in these stars from the standard Spite lithium plateau. We study here a sample of 11 metal-poor stars and a double-lined spectroscopic binary with -1.8 < [Fe/H] < -3.3 observed with the Very Large Telescope/UVES spectrograph. Among these 12 metal-poor stars, there are 8 CEMP stars for which we measure or constrain the Li abundance. In contrast to previous arguments, we demonstrate that an appropriate regime of dilution permits the existence of 'Li-Spite plateau and C-rich' stars, whereas some of the 'Li-depleted and C-rich' stars call for an unidentified additional depletion mechanism that cannot be explained by dilution alone. We find evidence that rotation is related to the Li depletion in some CEMP stars. Additionally, we report on a newly recognized double-lined spectroscopic binary star in our sample. For this star, we develop a new technique from which estimates of stellar parameters and luminosity ratios can be derived based on a high-resolution spectrum alone, without the need for input from evolutionary models.

  14. THE RAVE SURVEY: RICH IN VERY METAL-POOR STARS

    SciTech Connect

    Fulbright, Jon P.; Wyse, Rosemary F. G.; Ruchti, Gregory R.; Gilmore, G. F.; Grebel, Eva; Bienayme, O.; Siebert, A.; Binney, J.; Bland-Hawthorn, J.; Campbell, R.; Freeman, K. C.; Gibson, B. K.; Helmi, A.; Munari, U.; Navarro, J. F.; Siviero, A.; Parker, Q. A.; Reid, W.; Seabroke, G. M.; Steinmetz, M.

    2010-11-20

    Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H] <-2 dex, and down to below -4 dex, can be efficiently identified within the Radial Velocity Experiment (RAVE) survey of bright stars, without requiring additional confirmatory observations. We determine a calibration of the equivalent width of the calcium triplet lines measured from the RAVE spectra onto true [Fe/H], using high spectral resolution data for a subset of the stars. These RAVE iron abundances are accurate enough to obviate the need for confirmatory higher-resolution spectroscopy. Our initial study has identified 631 stars with [Fe/H] {<=}-2, from a RAVE database containing approximately 200,000 stars. This RAVE-based sample is complete for stars with [Fe/H] {approx_lt}-2.5, allowing statistical sample analysis. We identify three stars with [Fe/H] {approx_lt}-4. Of these, one was already known to be 'ultra metal-poor', one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H] = -4.0, rather than the published [Fe/H] = -3.3, and derive [C/Fe] = +0.9, and [N/Fe] = +3.2, and the third is at the limit of our signal-to-noise ratio. RAVE observations are ongoing and should prove to be a rich source of bright, easily studied, very metal-poor stars.

  15. Very Metal-poor Stars Observed by the RAVE Survey

    NASA Astrophysics Data System (ADS)

    Matijevič, Gal

    2016-08-01

    Radial Velocity Experiment (RAVE) observed ~500,000 southern sky stars between 2003 and 2013 in the infra-red calcium triplet (CaII) spectral region. In this study we extended the analysis of RAVE very metal-poor stars ([Fe/H] < -2) presented by Fulbright et al. (2010). We employed a novel method for identifying the metal-poor stars and developed a tool for modeling CaII lines where we also modeled the background noise to avoid systematical biases in the equivalent width (EW) measurements. Final metallicity values were derived with a flexible calibration approach using only 2MASS photometric data and EW measurements obtained from the RAVE spectra.

  16. Chemical Abundances of Metal-poor stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Jablonka, Pascale; Hill, Vanessa; Starkenburg, Else; Lemasle, Bertrand; Shetrone, Matthew; Irwin, Mike; Norris, John; Yong, David; Gilmore, Gerry; Salvadori, Stephania; Skuladottir, Asa; Tolstoy, Eline

    2016-08-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. The slower star formation efficiency make dwarf galaxies ideal systems for testing nucleosynthetic yields. Not only are alpha-poor stars found at lower metallicities, and a higher fraction of carbon-enhanced stars, but we are also finding stars in dwarf galaxies that appear to be iron-rich. These are compared with yields from a variety of supernova predictions.

  17. CHEMICAL ABUNDANCES OF METAL-POOR RR LYRAE STARS IN THE MAGELLANIC CLOUDS

    SciTech Connect

    Haschke, Raoul; Grebel, Eva K.; Duffau, Sonia; Frebel, Anna; Hansen, Camilla J.; Koch, Andreas

    2012-09-01

    We present for the first time a detailed spectroscopic study of chemical element abundances of metal-poor RR Lyrae stars in the Large and Small Magellanic Cloud (LMC and SMC). Using the MagE echelle spectrograph at the 6.5 m Magellan telescopes, we obtain medium resolution (R {approx} 2000-6000) spectra of six RR Lyrae stars in the LMC and three RR Lyrae stars in the SMC. These stars were chosen because their previously determined photometric metallicities were among the lowest metallicities found for stars belonging to the old populations in the Magellanic Clouds. We find the spectroscopic metallicities of these stars to be as low as [Fe/H]{sub spec} = -2.7 dex, the lowest metallicity yet measured for any star in the Magellanic Clouds. We confirm that for metal-poor stars, the photometric metallicities from the Fourier decomposition of the light curves are systematically too high compared to their spectroscopic counterparts. However, for even more metal-poor stars below [Fe/H]{sub phot} < -2.8 dex this trend is reversed and the spectroscopic metallicities are systematically higher than the photometric estimates. We are able to determine abundance ratios for 10 chemical elements (Fe, Na, Mg, Al, Ca, Sc, Ti, Cr, Sr, and Ba), which extend the abundance measurements of chemical elements for RR Lyrae stars in the Clouds beyond [Fe/H] for the first time. For the overall [{alpha}/Fe] ratio, we obtain an overabundance of 0.36 dex, which is in very good agreement with results from metal-poor stars in the Milky Way halo as well as from the metal-poor tail in dwarf spheroidal galaxies. Comparing the abundances with those of the stars in the Milky Way halo we find that the abundance ratios of stars of both populations are consistent with another. Therefore, we conclude that from a chemical point of view early contributions from Magellanic-type galaxies to the formation of the Galactic halo as claimed in cosmological models are plausible.

  18. LITHIUM ABUNDANCES OF EXTREMELY METAL-POOR TURNOFF STARS

    SciTech Connect

    Aoki, Wako; Inoue, Susumu; Barklem, Paul S.; Beers, Timothy C.; Christlieb, Norbert; Perez, Ana E. GarcIa; Norris, John E.; Carollo, Daniela E-mail: Paul.Barklem@physics.uu.se E-mail: N.Christlieb@lsw.uni-heidelberg.de E-mail: jen@mso.anu.edu.au E-mail: inoue@tap.scphys.kyoto-u.ac.jp

    2009-06-20

    We have determined Li abundances for eleven metal-poor turnoff stars, among which eight have [Fe/H] <-3, based on LTE analyses of high-resolution spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope. The Li abundances for four of these eight stars are determined for the first time by this study. Effective temperatures are determined by a profile analysis of H{alpha} and H{beta}. While seven stars have Li abundances as high as the Spite Plateau value, the remaining four objects with [Fe/H] <-3 have A(Li) =log (Li/H)+ 12 {approx}< 2.0, confirming the existence of extremely metal-poor (EMP) turnoff stars having low Li abundances, as reported by previous work. The average of the Li abundances for stars with [Fe/H]<-3 is lower by 0.2 dex than that of the stars with higher metallicity. No clear constraint on the metallicity dependence or scatter of the Li abundances is derived from our measurements for the stars with [Fe/H]<-3. Correlations of the Li abundance with effective temperatures, with abundances of Na, Mg, and Sr, and with the kinematical properties are investigated, but no clear correlation is seen in the EMP star sample.

  19. Binarity in carbon-enhanced metal-poor stars

    NASA Astrophysics Data System (ADS)

    Starkenburg, Else; Shetrone, Matthew D.; McConnachie, Alan W.; Venn, Kim A.

    2014-06-01

    A substantial fraction of the lowest metallicity stars show very high enhancements in carbon. It is debated whether these enhancements reflect the stars' birth composition, or if their atmospheres were subsequently polluted, most likely by accretion from an asymptotic giant branch binary companion. Here we investigate and compare the binary properties of three carbon-enhanced subclasses: The metal-poor CEMP-s stars that are additionally enhanced in barium; the higher metallicity (sg)CH- and Ba II stars also enhanced in barium; and the metal-poor CEMP-no stars, not enhanced in barium. Through comparison with simulations, we demonstrate that all barium-enhanced populations are best represented by a ˜100 per cent binary fraction with a shorter period distribution of at maximum ˜20 000 d. This result greatly strengthens the hypothesis that a similar binary mass transfer origin is responsible for their chemical patterns. For the CEMP-no group we present new radial velocity data from the Hobby-Eberly Telescope for 15 stars to supplement the scarce literature data. Two of these stars show indisputable signatures of binarity. The complete CEMP-no data set is clearly inconsistent with the binary properties of the CEMP-s class, thereby strongly indicating a different physical origin of their carbon enhancements. The CEMP-no binary fraction is still poorly constrained, but the population resembles more the binary properties in the solar neighbourhood.

  20. AN EXTREMELY CARBON-RICH, EXTREMELY METAL-POOR STAR IN THE SEGUE 1 SYSTEM

    SciTech Connect

    Norris, John E.; Yong, David; Gilmore, Gerard; Wyse, Rosemary F. G.; Frebel, Anna

    2010-10-10

    We report the analysis of high-resolution, high signal-to-noise ratio, spectra of an extremely metal-poor, extremely C-rich red giant, Seg 1-7, in Segue 1-described in the literature alternatively as an unusually extended globular cluster or an ultra-faint dwarf galaxy. The radial velocity of Seg 1-7 coincides precisely with the systemic velocity of Segue 1, and its chemical abundance signature of [Fe/H] = -3.52, [C/Fe] = +2.3, [N/Fe] = +0.8, [Na/Fe] = +0.53, [Mg/Fe] = +0.94, [Al/Fe] = +0.23, and [Ba/Fe] < -1.0 is similar to that of the rare and enigmatic class of Galactic halo objects designated CEMP-no (carbon-rich, extremely metal-poor with no enhancement (over solar ratios) of heavy neutron-capture elements). This is the first star in a Milky Way 'satellite' that unambiguously lies on the metal-poor, C-rich branch of the Aoki et al. bimodal distribution of field halo stars in the ([C/Fe], [Fe/H])-plane. Available data permit us only to identify Seg 1-7 as a member of an ultra-faint dwarf galaxy or as debris from the Sgr dwarf spheroidal galaxy. In either case, this demonstrates that at extremely low abundance, [Fe/H ] <-3.0, star formation and associated chemical evolution proceeded similarly in the progenitors of both the field halo and satellite systems. By extension, this is consistent with other recent suggestions that the most metal-poor dwarf spheroidal and ultra-faint dwarf satellites were the building blocks of the Galaxy's outer halo.

  1. Heavy-elements in metal-poor stars: an UV perspective

    NASA Astrophysics Data System (ADS)

    Siqueira-Mello, C.; Barbuy, B.

    2014-11-01

    The site(s) of the r-process(es) is(are) not completely defined, and several models have been proposed. Observed abundances are the best clues to bring some light to this field, especially the study of the extremely metal-poor (EMP) Galactic halo stars. Many elements can be measured using ground-based facilities already available, but the ultraviolet window also presents a rich opportunity in terms of chemical abundances of heavy elements. In fact, for some elements only the UV transitions are strong enough to be useful. Focusing on the project of the Cassegrain U-Band Brazilian Spectrograph (CUBES), we discuss the science case for heavy elements in metal-poor stars, describing the useful lines of trans-Fe elements present in the UV region. Lines in the far UV are also discussed.

  2. Behavior of sulfur in extremely metal-poor stars

    NASA Astrophysics Data System (ADS)

    Takada-Hidai, Masahide; Sargent, Wallace L. W.

    The LTE abundances of sulfur (S) were explored in the sample of 15 metal-poor stars with the metallicity range of -4<[Fe/H]<-1.5, based on the equivalent widths of the S I(1) 9212 and 9237 Å lines measured on high-resolution spectra, which were observed by the Keck I HIRES. Combining our results and those of Takada-Hidai et al. (2005), we found that the behavior of [S/Fe] against [Fe/H] shows a nearly flat trend in the range of metallicity down to [Fe/H]˜-4.

  3. VERY METAL-POOR STARS IN THE OUTER GALACTIC BULGE FOUND BY THE APOGEE SURVEY

    SciTech Connect

    Garcia Perez, Ana E.; Majewski, Steven R.; Hearty, Fred R.; Cunha, Katia; Shetrone, Matthew; Johnson, Jennifer A.; Zasowski, Gail; Smith, Verne V.; Beers, Timothy C.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Allende Prieto, Carlos; Bizyaev, Dmitry; Ebelke, Garrett; Malanushenko, Elena; Malanushenko, Viktor; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Leo; and others

    2013-04-10

    Despite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] {<=} -1.7), including two that are very metal-poor [Fe/H] {approx} -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the {alpha}-elements O, Mg, and Si without significant {alpha}-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.

  4. Searching for dust around hyper metal poor stars

    SciTech Connect

    Venn, Kim A.; Divell, Mike; Starkenburg, Else; Puzia, Thomas H.; Côté, Stephanie; Lambert, David L.

    2014-08-20

    We examine the mid-infrared fluxes and spectral energy distributions for stars with iron abundances [Fe/H] <–5, and other metal-poor stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excesses. These non-detections rule out many types of circumstellar disks, e.g., a warm debris disk (T ≤ 290 K), or debris disks with inner radii ≤1 AU, such as those associated with the chemically peculiar post-asymptotic giant branch spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g., a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 μm is detected at the 2σ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.

  5. A Photometric Method for Discovering Extremely Metal Poor Stars

    NASA Astrophysics Data System (ADS)

    Miller, Adam

    2015-01-01

    I present a new non-parametric machine-learning method for predicting stellar metallicity ([Fe/H]) based on photometric colors from the Sloan Digital Sky Survey (SDSS). The method is trained using a large sample of ~150k stars with SDSS spectra and atmospheric parameter estimates (Teff, log g, and [Fe/H]) from the SEGUE Stellar Parameters Pipeline (SSPP). For bright stars (g < 18 mag) with 4500 K < Teff < 7000 K and log g > 2, corresponding to the stars for which the SSPP estimates are most reliable, the method is capable of predicting [Fe/H] with a typical scatter of ~0.16 dex. This scatter is smaller than the typical uncertainty associated with [Fe/H] measurements from a low-resolution spectrum. The method is suitable for the discovery of extremely metal poor (EMP) stars ([Fe/H] < -3), as high purity (P > 50%), but low efficiency (E ~ 10%), samples of EMP star candidates can be generated from the sources with the lowest predicted [Fe/H]. To improve the efficiency of EMP star discovery, an alternative machine-learning model is constructed where the number of non-EMP stars is down-sampled in the training set, and a new regression model is fit. This alternate model improves the efficiency of EMP candidate selection by a factor of ~2. To test the efficacy of the model, I have obtained low-resolution spectra of 56 candidate EMP stars. I measure [Fe/H] for these stars using the well calibrated Ca II K line method, and compare our spectroscopic measurements to those from the machine learning model. Once applied to wide-field surveys, such as SDSS, Pan-STARRS, and LSST, the model will identify thousands of previously unknown EMP stars.

  6. Inefficient star formation in extremely metal poor galaxies.

    PubMed

    Shi, Yong; Armus, Lee; Helou, George; Stierwalt, Sabrina; Gao, Yu; Wang, Junzhi; Zhang, Zhi-Yu; Gu, Qiusheng

    2014-10-16

    The first galaxies contain stars born out of gas with few or no 'metals' (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities, and while dust has been used to trace gas in low-metallicity galaxies, low spatial resolution in the far-infrared has typically led to large uncertainties. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.

  7. Inefficient star formation in extremely metal poor galaxies.

    PubMed

    Shi, Yong; Armus, Lee; Helou, George; Stierwalt, Sabrina; Gao, Yu; Wang, Junzhi; Zhang, Zhi-Yu; Gu, Qiusheng

    2014-10-16

    The first galaxies contain stars born out of gas with few or no 'metals' (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities, and while dust has been used to trace gas in low-metallicity galaxies, low spatial resolution in the far-infrared has typically led to large uncertainties. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe. PMID:25318522

  8. TESTING THE ASTEROSEISMIC MASS SCALE USING METAL-POOR STARS CHARACTERIZED WITH APOGEE AND KEPLER

    SciTech Connect

    Epstein, Courtney R.; Johnson, Jennifer A.; Tayar, Jamie; Pinsonneault, Marc; Elsworth, Yvonne P.; Chaplin, William J.; Shetrone, Matthew; Mosser, Benoît; Hekker, Saskia; Harding, Paul; Silva Aguirre, Víctor; Basu, Sarbani; Beers, Timothy C.; Bizyaev, Dmitry; Bedding, Timothy R.; Frinchaboy, Peter M.; García, Rafael A.; and others

    2014-04-20

    Fundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation frequency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher (<ΔM > =0.17 ± 0.05 M {sub ☉}) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 M {sub ☉} level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of ∼100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.

  9. EXTREMELY METAL-POOR STARS IN THE MILKY WAY: A SECOND GENERATION FORMED AFTER REIONIZATION

    SciTech Connect

    Trenti, Michele; Shull, J. Michael E-mail: michael.shull@colorado.ed

    2010-03-20

    Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M {approx}> 100 M{sub sun}) formed in minihalos at redshift z {approx}> 20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely metal-poor (EMP) stars in the Milky Way (MW) halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose a new solution to the problem in which the IMF of second-generation stars formed at z {approx}> 10, before reionization, is deficient in sub-solar mass stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z {approx}< 10 in pockets of gas enriched to metallicity Z {approx}> 10{sup -3.5} Z{sub sun} by winds from Population II stars. Our cosmological simulations of dark matter halos like the MW show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z {approx}< 13 in halos with virial temperature T{sub vir} {approx} 10{sup 4} K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z {approx} 10{sup -3.5} Z{sub sun}.

  10. Identifying Bright Carbon-Enhanced Metal-Poor Stars in the RAVE Catalog

    NASA Astrophysics Data System (ADS)

    Placco, Vinicius; Beers, Timothy C.

    2016-01-01

    Bright metal-poor stars are of great importance for high-resolution spectroscopic follow-up, since their brightness allows for detailed studies of the chemical compositions of their atmospheres, obtainable with short integration times on 4m-8m class telescopes. We have carried out a medium-resolution spectroscopic follow-up survey of very metal-poor ([Fe/H] < -2.0) stars selected from the RAVE catalog.Over the course of four semesters we observed over 1,200 stars with the Gemini North, Gemini South, SOAR, KPNO/Mayall, and ESO/NTT telescopes. These spectra are used to confirm the estimated atmospheric parameters from RAVE, as well as to determine [C/Fe], using our spectroscopic analysis pipeline. This information has already enabled the identification of many new carbon-enhanced metal-poor (CEMP) stars, including representatives of the inner- and outer-halo populations of the Milky Way, for which high-resolution spectroscopy is in progress from the ground with the Magellan/Clay Telescope and with the South African Large Telescope (SALT). The most interesting stars from the high-resolution follow-up will be observed from space with HST/STIS or COS. In this talk I will present the results of the medium-resolution follow-up, and preliminary results from the high-resolution effort.We acknowledge partial support from the grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation.

  11. R-Process Abundances and Chronometers in Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Cowan, John J.; Pfeiffer, B.; Kratz, K.-L.; Thielemann, F.-K.; Sneden, Christopher; Burles, Scott; Tytler, David; Beers, Timothy C.

    1999-08-01

    Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (quasi-particle random-phase approximation [QRPA] β-decay properties and the recent extended Thomas-Fermi with Strutinsky integral and quenching (ETFSI-Q) nuclear mass model), have been performed. These theoretical computations assume the classical waiting-point approximation of (n,γ)⇄(γ,n) equilibrium. The calculations reproduce the solar isotopic r-abundances in detail, including the heaviest stable Pb and Bi isotopes. These calculations are then compared with ground-based and Hubble Space Telescope observations of neutron-capture elements in the metal-poor halo stars CS 22892-052, HD 115444, HD 122563, and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z>=56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This indicates also that it is unlikely that the solar r-process abundances resulted from a random superposition of varying abundance patterns from different r-process nucleosynthesis sites. This further suggests that there is one r-process site in the Galaxy, at least for elements Z>=56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, we present predictions for the zero decay-age abundances of the radioactive elements Th and U. We compare these predictions (obtained with the mass model ETFSI-Q, which reproduces solar r-abundances best) with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892-052, and HD 122563. Within the observational errors the ratio of [Th/Eu] is the same in both CS 22892-052 and HD 115444. Comparing with the theoretical ratio suggests an average age

  12. Diversity of abundance patterns of neutron-capture elements in very metal-poor stars

    SciTech Connect

    Aoki, Misa; Ishimaru, Yuhri; Aoki, Wako; Wanajo, Shinya

    2014-05-02

    Observations of Very Metal-Poor stars indicate that there are at least two sites to r-process; “weak r-process” and “main r-process”. A question is whether these two are well separated or there exists a variation in the r-process. We present the results of abundance analysis of neutron-capture elements in the two Very Metal-Poor stars HD107752 and HD110184 in the Milky Way halo observed with the Subaru Telescope HDS. The abundance patterns show overabundace at light n-capture elements (e.g. Sr, Y), inferring the element yielding of weak r-process, while heavy neutron-capture elements (e.g. Ba, Eu) are deficient; however, the overabundance of light ones is not as significant as that previously found in stars representing the weak r-process (e.g. HD122563; Honda et al. 2006). Our study show diversity in the abundance patterns from light to heavy neutron-capture elements in VMP stars, suggesting a variation in r-process, which may depend on electron fraction of environment.

  13. High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey

    NASA Astrophysics Data System (ADS)

    Jacobson, Heather R.; Keller, Stefan; Frebel, Anna; Casey, Andrew R.; Asplund, Martin; Bessell, Michael S.; Da Costa, Gary S.; Lind, Karin; Marino, Anna F.; Norris, John E.; Peña, José M.; Schmidt, Brian P.; Tisserand, Patrick; Walsh, Jennifer M.; Yong, David; Yu, Qinsi

    2015-07-01

    The SkyMapper Southern Sky Survey is carrying out a search for the most metal-poor stars in the Galaxy. It identifies candidates by way of its unique filter set which allows for estimation of stellar atmospheric parameters. The set includes a narrow filter centered on the Ca ii K 3933 Å line, enabling a robust estimate of stellar metallicity. Promising candidates are then confirmed with spectroscopy. We present the analysis of Magellan Inamori Kyocera Echelle high-resolution spectroscopy of 122 metal-poor stars found by SkyMapper in the first two years of commissioning observations. Forty-one stars have [{Fe}/{{H}}]≤slant -3.0. Nine have [{Fe}/{{H}}]≤slant -3.5, with three at [{Fe}/{{H}}]∼ -4. A 1D LTE abundance analysis of the elements Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, Ba, and Eu shows these stars have [X/Fe] ratios typical of other halo stars. One star with low [X/Fe] values appears to be “Fe-enhanced,” while another star has an extremely large [Sr/Ba] ratio: \\gt 2. Only one other star is known to have a comparable value. Seven stars are “CEMP-no” stars ([{{C}}/{Fe}]\\gt 0.7, [{Ba}/{Fe}]\\lt 0). 21 stars exhibit mild r-process element enhancements (0.3≤slant [{Eu}/{Fe}]\\lt 1.0), while four stars have [{Eu}/{Fe}]≥slant 1.0. These results demonstrate the ability to identify extremely metal-poor stars from SkyMapper photometry, pointing to increased sample sizes and a better characterization of the metal-poor tail of the halo metallicity distribution function in the future. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  14. A Wide-Field Photometric Survey for Extratidal Tails Around Five Metal-Poor Globular Clusters in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Chun, Sang-Hyun; Kim, Jae-Woo; Sohn, Sangmo T.; Park, Jang-Hyun; Han, Wonyong; Kim, Ho-Il; Lee, Young-Wook; Lee, Myung Gyoon; Lee, Sang-Gak; Sohn, Young-Jong

    2010-02-01

    Wide-field deep g'r'i' images obtained with the Megacam of the Canada-France-Hawaii Telescope are used to investigate the spatial configuration of stars around five metal-poor globular clusters M15, M30, M53, NGC 5053, and NGC 5466, in a field-of-view ~3°. Applying a mask filtering algorithm to the color-magnitude diagrams of the observed stars, we sorted cluster's member star candidates that are used to examine the characteristics of the spatial stellar distribution surrounding the target clusters. The smoothed surface density maps and the overlaid isodensity contours indicate that all of the five metal-poor globular clusters exhibit strong evidence of extratidal overdensity features over their tidal radii, in the form of extended tidal tails around the clusters. The orientations of the observed extratidal features show signatures of tidal tails tracing the clusters' orbits, inferred from their proper motions, and effects of dynamical interactions with the Galaxy. Our findings include detections of a tidal bridge-like feature and an envelope structure around the pair of globular clusters M53 and NGC 5053. The observed radial surface density profiles of target clusters have a deviation from theoretical King models, for which the profiles show a break at 0.5-0.7rt , extending the overdensity features out to 1.5-2rt . Both radial surface density profiles for different angular sections and azimuthal number density profiles confirm the overdensity features of tidal tails around the five metal-poor globular clusters. Our results add further observational evidence that the observed metal-poor halo globular clusters originate from an accreted satellite system, indicative of the merging scenario of the formation of the Galactic halo. Based on observations carried out at the Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii. This is part of the

  15. Follow-up observations of extremely metal-poor stars identified from SDSS

    NASA Astrophysics Data System (ADS)

    Aguado, D. S.; Allende Prieto, C.; González Hernández, J. I.; Carrera, R.; Rebolo, R.; Shetrone, M.; Lambert, D. L.; Fernández-Alvar, E.

    2016-08-01

    Context. The most metal-poor stars in the Milky Way witnessed the early phases of formation of the Galaxy, and have chemical compositions that are close to the pristine mixture from Big Bang nucleosynthesis, polluted by one or few supernovae. Aims: Only two dozen stars with ([Fe/H] < -4) are known, and they show a wide range of abundance patterns. It is therefore important to enlarge this sample. We present the first results of an effort to identify new extremely metal-poor stars in the Milky Way halo. Methods: Our targets have been selected from low-resolution spectra obtained as part of the Sloan Digital Sky Survey, and followed-up with medium resolution spectroscopy on the 4.2 m William Herschel Telescope and, in a few cases, at high resolution on the 9.2 m Hobby-Eberly Telescope. Stellar parameters and the abundances of magnesium, calcium, iron, and strontium have been inferred from the spectra using classical model atmospheres. We have also derived carbon abundances from the G band. Results: We find consistency between the metallicities estimated from SDSS and those from new data at the level of 0.3 dex. The analysis of medium resolution data obtained with ISIS on the WHT allows us to refine the metallicities and in some cases measure other elemental abundances. Our sample contains 11 new metal-poor stars with [Fe/H] < -3.0, one of them with an estimated metallicity of [Fe/H] ~ -4.0. We also discuss metallicity discrepancies of some stars in common with previous works in the literature. Only one of these stars is found to be C-enhanced at about [C/Fe] ~ + 1, whereas the other metal-poor stars show C abundances at the level of [C/Fe] ~ + 0.45. Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.The reduced spectra as FITS files are only available at

  16. NEW HUBBLE SPACE TELESCOPE OBSERVATIONS OF HEAVY ELEMENTS IN FOUR METAL-POOR STARS

    SciTech Connect

    Roederer, Ian U.; Thompson, Ian B.; Lawler, James E.; Sobeck, Jennifer S.; Beers, Timothy C.; Cowan, John J.; Frebel, Anna; Ivans, Inese I.; Schatz, Hendrik; Sneden, Christopher

    2012-12-15

    Elements heavier than the iron group are found in nearly all halo stars. A substantial number of these elements, key to understanding neutron-capture nucleosynthesis mechanisms, can only be detected in the near-ultraviolet. We report the results of an observing campaign using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to study the detailed heavy-element abundance patterns in four metal-poor stars. We derive abundances or upper limits from 27 absorption lines of 15 elements produced by neutron-capture reactions, including seven elements (germanium, cadmium, tellurium, lutetium, osmium, platinum, and gold) that can only be detected in the near-ultraviolet. We also examine 202 heavy-element absorption lines in ground-based optical spectra obtained with the Magellan Inamori Kyocera Echelle Spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the High Resolution Echelle Spectrometer on the Keck I Telescope on Mauna Kea. We have detected up to 34 elements heavier than zinc. The bulk of the heavy elements in these four stars are produced by r-process nucleosynthesis. These observations affirm earlier results suggesting that the tellurium found in metal-poor halo stars with moderate amounts of r-process material scales with the rare earth and third r-process peak elements. Cadmium often follows the abundances of the neighboring elements palladium and silver. We identify several sources of systematic uncertainty that must be considered when comparing these abundances with theoretical predictions. We also present new isotope shift and hyperfine structure component patterns for Lu II and Pb I lines of astrophysical interest.

  17. Beryllium and boron in metal-poor stars

    NASA Astrophysics Data System (ADS)

    Primas, Francesca

    2010-04-01

    Knowledge of lithium, beryllium, and boron abundances in stars of the Galactic halo and disk plays a major role in our understanding of Big Bang nucleosynthesis, cosmic-ray physics, and stellar interiors. 9Be and 10B are believed to originate entirely from spallation reactions in the interstellar medium (ISM) between α-particles and protons and heavy nuclei like carbon, nitrogen, and oxygen (CNO), whereas 11B may have an extra production channel via neutrino-spallation. Beryllium and boron are both observationally challenging, with their main resonant doublets falling respectively at 313 nm and at 250 nm. The advent of 8-10m class telescopes equipped with highly sensitive (in the near-UV/blue) spectrographs has opened up a new era of Be abundance studies. Here, I will review and discuss the most interesting results of recent observational campaigns in terms of formation and evolution of these two light elements.

  18. Carbon-enhanced metal-poor stars: relics from the dark ages

    SciTech Connect

    Cooke, Ryan J.; Madau, Piero

    2014-08-20

    We use detailed nucleosynthesis calculations and a realistic prescription for the environment of the first stars to explore the first episodes of chemical enrichment that occurred during the dark ages. Based on these calculations, we propose a novel explanation for the increased prevalence of carbon-enhanced metal-poor (CEMP) stars with decreasing Fe abundance: the observed chemistry for the most metal-poor Galactic halo stars is the result of an intimate link between the explosions of the first stars and their host minihalo's ability to retain its gas. Specifically, high-energy supernovae produce a near solar ratio of C/Fe, but are effective in evacuating the gas from their host minihalo, thereby suppressing the formation of a second generation of stars. On the other hand, minihalos that host low-energy supernovae are able to retain their gas and form a second stellar generation, but, as a result, the second stars are born with a supersolar ratio of C/Fe. Our models are able to accurately reproduce the observed distributions of [C/Fe] and [Fe/H], as well as the fraction of CEMP stars relative to non-CEMP stars as a function of [Fe/H] without any free parameters. We propose that the present lack of chemical evidence for very massive stars (≳ 140 M {sub ☉}) that ended their lives as a highly energetic pair-instability supernova does not imply that such stars were rare or did not exist; the chemical products of these very massive first stars may have been evacuated from their host minihalos and were never incorporated into subsequent generations of stars. Finally, our models suggest that the most Fe-poor stars currently known may have seen the enrichment from a small multiple of metal-free stars, and need not have been exclusively enriched by a solitary first star. These calculations also add further support to the possibility that some of the surviving dwarf satellite galaxies of the Milky Way are the relics of the first galaxies.

  19. Looking for imprints of the first stellar generations in metal-poor bulge field stars

    NASA Astrophysics Data System (ADS)

    Siqueira-Mello, C.; Chiappini, C.; Barbuy, B.; Freeman, K.; Ness, M.; Depagne, E.; Cantelli, E.; Pignatari, M.; Hirschi, R.; Frischknecht, U.; Meynet, G.; Maeder, A.

    2016-09-01

    Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] ≈-1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims: The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ~ 45 000) and high-signal-to-noise (S/N> 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods: High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code. Results: We confirm that the analysed stars are moderately metal-poor (-1.04 ≤ [Fe/H] ≤-0.43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤ + 0.2, and α-enhanced. We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na - O, Al - O, Al - Mg anti-correlations) were

  20. Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

    NASA Astrophysics Data System (ADS)

    Abate, C.; Pols, O. R.; Stancliffe, R. J.; Izzard, R. G.; Karakas, A. I.; Beers, T. C.; Lee, Y. S.

    2015-09-01

    The stellar population in the Galactic halo is characterised by a large fraction of carbon-enhanced metal-poor (CEMP) stars. Most CEMP stars have enhanced abundances of s-process elements (CEMP-s stars), and some of these are also enriched in r-process elements (CEMP-s/r stars). In one formation scenario proposed for CEMP stars, the observed carbon excess is explained by invoking wind mass transfer in the past from a more massive thermally-pulsing asymptotic giant branch (AGB) primary star in a binary system.In this work we generate synthetic populations of binary stars at metallicity Z = 0.0001 ([Fe/H] ≈ - 2.3), with the aim of reproducing the observed fraction of CEMP stars in the halo. In addition, we aim to constrain our model of the wind mass-transfer process, in particular the wind-accretion efficiency and angular-momentum loss, and investigate under which conditions our model populations reproduce observed distributions of element abundances.We compare the CEMP fractions determined from our synthetic populations and the abundance distributions of many elements with observations. Several physical parameters of the binary stellar population of the halo are uncertain, in particular the initial mass function, the mass-ratio distribution, the orbital-period distribution, and the binary fraction. We vary the assumptions in our model about these parameters, as well as the wind mass-transfer process, and study the consequent variations of our synthetic CEMP population.The CEMP fractions calculated in our synthetic populations vary between 7% and 17%, a range consistent with the CEMP fractions among very metal-poor stars recently derived from the SDSS/SEGUE data sample. The resulting fractions are more than a factor of three higher than those determined with default assumptions in previous population-synthesis studies, which typically underestimated the observed CEMP fraction. We find that most CEMP stars in our simulations are formed in binary systems with periods

  1. DETECTION OF A DISTINCT METAL-POOR STELLAR HALO IN THE EARLY-TYPE GALAXY NGC 3115

    SciTech Connect

    Peacock, Mark B.; Strader, Jay; Romanowsky, Aaron J.; Brodie, Jean P.

    2015-02-10

    We present the resolved stellar populations in the inner and outer halo of the nearby lenticular galaxy NGC 3115. Using deep Hubble Space Telescope observations, we analyze stars 2 mag fainter than the tip of the red giant branch (TRGB). We study three fields along the minor axis of this galaxy, 19, 37, and 54 kpc from its center—corresponding to 7, 14, and 21 effective radii (r{sub e} ). Even at these large galactocentric distances, all of the fields are dominated by a relatively enriched population, with the main peak in the metallicity distribution decreasing with radius from [Z/H] ∼ –0.5 to –0.65. The fraction of metal-poor stars ([Z/H] < –0.95) increases from 17% at 16-37 kpc to 28% at ∼54 kpc. We observe a distinct low-metallicity population (peaked at [Z/H] ∼ –1.3 and with total mass 2 × 10{sup 10} M {sub ☉} ∼ 14% of the galaxy's stellar mass) and argue that this represents the detection of an underlying low-metallicity stellar halo. Such halos are generally predicted by galaxy formation theories and have been observed in several late-type galaxies, including the Milky Way and M31. The metallicity and spatial distribution of the stellar halo of NGC 3115 are consistent with the galaxy's globular cluster system, which has a similar low-metallicity population that becomes dominant at these large radii. This finding supports the use of globular clusters as bright chemodynamical tracers of galaxy halos. These data also allow us to make a precise measurement of the magnitude of the TRGB, from which we derive a distance modulus of NGC 3115 of 30.05 ± 0.05 ± 0.10{sub sys} (10.2 ± 0.2 ± 0.5{sub sys} Mpc)

  2. AN ELEMENTAL ASSAY OF VERY, EXTREMELY, AND ULTRA-METAL-POOR STARS

    SciTech Connect

    Hansen, T.; Christlieb, N.; Hansen, C. J.; Beers, T. C.; Placco, V. M.; Yong, D.; Bessell, M. S.; Norris, J. E.; Asplund, M.; Frebel, A.; Pérez, A. E. García

    2015-07-10

    We present a high-resolution elemental-abundance analysis for a sample of 23 very metal-poor ([Fe/H] < −2.0) stars, 12 of which are extremely metal-poor ([Fe/H] < −3.0), and 4 of which are ultra-metal-poor ([Fe/H] < −4.0). These stars were targeted to explore differences in the abundance ratios for elements that constrain the possible astrophysical sites of element production, including Li, C, N, O, the α-elements, the iron-peak elements, and a number of neutron-capture elements. This sample substantially increases the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars—our program stars include eight that are considered “normal” metal-poor stars, six CEMP-no stars, five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three of the six CEMP-no stars, all of which are Li depleted with respect to the Spite plateau. The majority of the CEMP stars have [C/N] > 0. The stars with [C/N] < 0 suggest a larger degree of mixing; the few CEMP-no stars that exhibit this signature are only found at [Fe/H] < −3.4, a metallicity below which we also find the CEMP-no stars with large enhancements in Na, Mg, and Al. We confirm the existence of two plateaus in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. We also present evidence for a “floor” in the absolute Ba abundances of CEMP-no stars at A(Ba) ∼ −2.0.

  3. An Elemental Assay of Very, Extremely, and Ultra-metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Hansen, T.; Hansen, C. J.; Christlieb, N.; Beers, T. C.; Yong, D.; Bessell, M. S.; Frebel, A.; García Pérez, A. E.; Placco, V. M.; Norris, J. E.; Asplund, M.

    2015-07-01

    We present a high-resolution elemental-abundance analysis for a sample of 23 very metal-poor ([Fe/H] < ‑2.0) stars, 12 of which are extremely metal-poor ([Fe/H] < ‑3.0), and 4 of which are ultra-metal-poor ([Fe/H] < ‑4.0). These stars were targeted to explore differences in the abundance ratios for elements that constrain the possible astrophysical sites of element production, including Li, C, N, O, the α-elements, the iron-peak elements, and a number of neutron-capture elements. This sample substantially increases the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars—our program stars include eight that are considered “normal” metal-poor stars, six CEMP-no stars, five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three of the six CEMP-no stars, all of which are Li depleted with respect to the Spite plateau. The majority of the CEMP stars have [C/N] > 0. The stars with [C/N] < 0 suggest a larger degree of mixing; the few CEMP-no stars that exhibit this signature are only found at [Fe/H] < ‑3.4, a metallicity below which we also find the CEMP-no stars with large enhancements in Na, Mg, and Al. We confirm the existence of two plateaus in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. We also present evidence for a “floor” in the absolute Ba abundances of CEMP-no stars at A(Ba) ∼ ‑2.0. Based on observations made with the European Southern Observatory telescopes.

  4. Extremely metal-poor stars and chemical signature of the first stars

    NASA Astrophysics Data System (ADS)

    Komiya, Yutaka

    2014-05-01

    Extremely metal-poor (EMP) stars in the Local Universe are expected as useful probes to the first stars. A portion of EMP stars should be second generation stars which are formed with nucleosynthetic yields of the first supernovae. In addition, if low-mass first stars were formed, they survive to date in the Local Universe. We constructed a new chemical evolution model taking into account the hierarchical galaxy formation process, and investigate the formation history of the first stars and the EMP stars. In this paper, we derive number and distribution of low-mass Population III (Pop. III) stars in the Local Universe and discuss the possibility to observe Pop. III survivors. We also show the expected chemical properties of the second generation stars and possible nucleosynthetic signatures of the first supernovae.

  5. Possible evidence for metal accretion onto the surfaces of metal-poor main-sequence stars

    SciTech Connect

    Hattori, Kohei; Yoshii, Yuzuru; Beers, Timothy C.; Carollo, Daniela; Lee, Young Sun

    2014-04-01

    The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parameterized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time.

  6. New Hubble Space Telescope Observations of Heavy Elements in Four Metal-Poor Stars

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Lawler, James E.; Sobeck, Jennifer S.; Beers, Timothy C.; Cowan, John J.; Frebel, Anna; Ivans, Inese I.; Schatz, Hendrik; Sneden, Christopher; Thompson, Ian B.

    2012-12-01

    Elements heavier than the iron group are found in nearly all halo stars. A substantial number of these elements, key to understanding neutron-capture nucleosynthesis mechanisms, can only be detected in the near-ultraviolet. We report the results of an observing campaign using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to study the detailed heavy-element abundance patterns in four metal-poor stars. We derive abundances or upper limits from 27 absorption lines of 15 elements produced by neutron-capture reactions, including seven elements (germanium, cadmium, tellurium, lutetium, osmium, platinum, and gold) that can only be detected in the near-ultraviolet. We also examine 202 heavy-element absorption lines in ground-based optical spectra obtained with the Magellan Inamori Kyocera Echelle Spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the High Resolution Echelle Spectrometer on the Keck I Telescope on Mauna Kea. We have detected up to 34 elements heavier than zinc. The bulk of the heavy elements in these four stars are produced by r-process nucleosynthesis. These observations affirm earlier results suggesting that the tellurium found in metal-poor halo stars with moderate amounts of r-process material scales with the rare earth and third r-process peak elements. Cadmium often follows the abundances of the neighboring elements palladium and silver. We identify several sources of systematic uncertainty that must be considered when comparing these abundances with theoretical predictions. We also present new isotope shift and hyperfine structure component patterns for Lu II and Pb I lines of astrophysical interest. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 8111 and

  7. THE MOST METAL-POOR STARS. III. THE METALLICITY DISTRIBUTION FUNCTION AND CARBON-ENHANCED METAL-POOR FRACTION , ,

    SciTech Connect

    Yong, David; Norris, John E.; Bessell, M. S.; Asplund, M.; Christlieb, N.; Beers, Timothy C.; Barklem, P. S.; Frebel, Anna; Ryan, S. G. E-mail: jen@mso.anu.edu.au E-mail: martin@mso.anu.edu.au E-mail: beers@pa.msu.edu E-mail: afrebel@mit.edu

    2013-01-01

    We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with [Fe/H] {<=} -3.0, based on high-resolution, high signal-to-noise spectroscopy, of which some 32 objects lie below [Fe/H] = -3.5. After accounting for the completeness function, the 'corrected' MDF does not exhibit the sudden drop at [Fe/H] = -3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to [Fe/H] = -4.1. Similar results are obtained from the 'raw' MDF. We find that the fraction of CEMP objects below [Fe/H] = -3.0 is 23% {+-} 6% and 32% {+-} 8% when adopting the Beers and Christlieb and Aoki et al. CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers and Christlieb criterion.

  8. The Frequency of Warm Carbon-Enhanced Metal-Poor Stars in SDSS-I DR-5

    NASA Astrophysics Data System (ADS)

    Marsteller, Brian E.; Beers, T. C.; Sivarani, T.; Rossi, S.; Knapp, J.; Plez, B.; Johnson, J.; Masseron, T.

    2006-12-01

    There exists current a debate concerning the frequency of stars with large enhancements of carbon ([C/Fe] > +1.0) among very metal-poor ([Fe/H] <-2.0) stars in the Galactic halo. Some authors, e.g., Marsteller et al. (2005) and Lucatello et al. (2006), have concluded that a rather high frequency, on the order of 20%-25% exists, while other authors (e.g., Cohen et al. 2005) have claimed lower frequencies. One of the difficulties in making a precise estimate is that many previous samples of stars are dominated by giants, which are subject to alteration of the surface carbon abundance due to evolutionary effects. Fortunately, there is now an attractive alternative. The publicly available stellar database from SDSS-I (DR-5) contains large numbers (more than 24,000) warm (Teff >= 5700 K), very metal-poor stars (many of which were selected as calibration objects during the course of SDSS-I) which are not expected to have evolved to the point where carbon can be diluted on their surfaces. An estimate of the frequency of carbon-enhanced stars from this sample should provide one of the best available estimates of the true value of this quantity. In order to obtain estimates of [Fe/H] and [C/Fe] for this large sample, I have developed an automated spectral synthesis technique, making use of Sneden's MOOG program. With reasonable first estimates of the atmospheric parameters for our sample (obtained by the SDSS/SEGUE spectroscopic pipeline discussed elsewhere in this meeting), this approach quickly converges to the best available combination of [Fe/H] and [C/Fe] required to fit the spectral regions around the CaII K and CH G-bands. I will discuss the resulting frequency of carbon-enhanced metal-poor stars among the very metal-poor stars in this sample.

  9. Study of Neutron-Capture Element Abundances in Metal-Poor Stars

    NASA Astrophysics Data System (ADS)

    Li, Hongjie; Shen, Xiaojing; Liang, Shuai; Cui, Wenyuan; Zhang, Bo

    2013-02-01

    This work describes a study of elemental abundances for 30 metal-poor stars whose chemical abundances provide excellent information for setting constraints on models of neutron-capture processes. Based on the abundances of main r- and weak r-process stars, the abundance patterns of main r-process and weak r-process are obtained. The two r-process component coefficients are defined to determine the relative contributions from individual neutron-capture process to abundances of metal-poor stars. Based on the component coefficients, we find that metal-poor stars BD+42621 and HD 4306 are also weak r-process stars, which means that the abundance pattern produced by weak r-process is stable. All metal-poor star abundances contain the contributions of both main r-process and weak r-process. The elements produced by weak r-process have increased along with Fe over the polluted history. Most of the metal-poor star abundances do not follow the pattern observed in the solar system, but there is a small fraction that do. For the low-[Sr/Fe] star BD-185550 ([Sr/Fe] lsim -1), neutron-capture element abundances can be explained by the mixture of two r-process components. Since lighter elements in this star cannot be fitted by the two components, the abundance pattern of P-component is estimated from those abundances.

  10. SPECTROSCOPIC STUDIES OF EXTREMELY METAL-POOR STARS WITH THE SUBARU HIGH DISPERSION SPECTROGRAPH. V. THE Zn-ENHANCED METAL-POOR STAR BS 16920-017

    SciTech Connect

    Honda, Satoshi; Aoki, Wako; Beers, Timothy C.; Takada-Hidai, Masahide E-mail: aoki.wako@nao.ac.jp E-mail: hidai@apus.rh.u-tokai.ac.jp

    2011-04-01

    We report Zn abundances for 18 very metal-poor stars studied in our previous work, covering the metallicity range -3.2< [Fe/H] <-2.5. The [Zn/Fe] values of most stars show an increasing trend with decreasing [Fe/H] in this metallicity range, confirming the results found by previous studies. However, the extremely metal-poor star BS 16920-017([Fe/H] =-3.2) exhibits a significantly high [Zn/Fe] ratio ([Zn/Fe] = +1.0). Comparison of the chemical abundances of this object with HD 4306, which has similar atmospheric parameters to BS 16920-017, clearly demonstrates a deficiency of {alpha} elements and neutron-capture elements in this star, along with enhancements of Mn and Ni, as well as Zn. The association with a hypernova explosion that has been proposed to explain the high Zn abundance ratios found in extremely metal-poor stars is a possible explanation, although further studies are required to fully interpret the abundance pattern of this object.

  11. In the thick of it: metal-poor disc stars in RAVE

    NASA Astrophysics Data System (ADS)

    Kordopatis, G.; Gilmore, G.; Wyse, R. F. G.; Steinmetz, M.; Siebert, A.; Bienaymé, O.; McMillan, P. J.; Minchev, I.; Zwitter, T.; Gibson, B. K.; Seabroke, G.; Grebel, E. K.; Bland-Hawthorn, J.; Boeche, C.; Freeman, K. C.; Munari, U.; Navarro, J. F.; Parker, Q.; Reid, W. A.; Siviero, A.

    2013-12-01

    By selecting in the Radial Velocity Experiment-fourth data release (RAVE-DR4) survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest three-component model (thin disc, thick disc and halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to [M/H] = ‒2 dex, contributing roughly 3 per cent of the entire thick disc population and having a shorter scalelength compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity ([M/H]) and the orbital velocity (Vφ), which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find ∂Vφ/∂[M/H]≈ 50 km s-1 dex-1, which is in very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc and suggest that given the above-mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely.

  12. EXTREMELY METAL-POOR STARS AND A HIERARCHICAL CHEMICAL EVOLUTION MODEL

    SciTech Connect

    Komiya, Yutaka

    2011-07-20

    Early phases of the chemical evolution of the Galaxy and formation history of extremely metal-poor (EMP) stars are investigated using hierarchical galaxy formation models. We build a merger tree of the Galaxy according to the extended Press-Schechter theory. We follow the chemical evolution along the tree and compare the model results to the metallicity distribution function and abundance ratio distribution of the Milky Way halo. We adopt three different initial mass functions (IMFs). In a previous study, we argued that the typical mass, M{sub md}, of EMP stars should be high, M{sub md} {approx} 10 M{sub sun}, based on studies of binary origin carbon-rich EMP stars. In this study, we show that only the high-mass IMF can explain an observed small number of EMP stars. For relative element abundances, the high-mass IMF and the Salpeter IMF predict similar distributions. We also investigate dependence on nucleosynthetic yields of supernovae (SNe). The theoretical SN yields by Kobayashi et al. and Chieffi and Limongi show reasonable agreement with observations for {alpha}-elements. Our model predicts a significant scatter of element abundances at [Fe/H] < -3. We adopted the stellar yields derived in the work of Francois et al., which produce the best agreement between the observational data and the one-zone chemical evolution model. Their yields well reproduce a trend of the averaged abundances of EMP stars but predict much larger scatter than do the observations. The model with hypernovae predicts Zn abundance, in agreement with the observations, but other models predict lower [Zn/Fe]. Ejecta from the hypernovae with large explosion energy is mixed in large mass and decreases the scatter of the element abundances.

  13. High-resolution spectroscopic studies of ultra metal-poor stars found in the LAMOST survey

    NASA Astrophysics Data System (ADS)

    Li, Haining; Aoki, Wako; Zhao, Gang; Honda, Satoshi; Christlieb, Norbert; Suda, Takuma

    2015-10-01

    We report on the observations of two ultra metal-poor (UMP) stars with [Fe/H] ˜ -4.0, including one new discovery. The two stars are studied in the on-going and quite efficient project to search for extremely metal-poor (EMP) stars with LAMOST and Subaru. Detailed abundances or upper limits of abundances have been derived for 15 elements from Li to Eu based on high-resolution spectra obtained with the High Dispersion Spectrograph (HDS) mounted in the Subaru Telescope. The abundance patterns of both UMP stars are consistent with the "normal population" among the low-metallicity stars. Both of the two program stars show carbon-enhancement without any excess of heavy neutron-capture elements, indicating that they belong to the subclass of (carbon-enhanced metal-poor) CEMP-no stars, as is the case of most UMP stars previously studied. The [Sr/Ba] ratios of both CEMP-no UMP stars are above [Sr/Ba] ˜ -0.4, suggesting the origin of the carbon-excess is not compatible with the mass transfer from an asymptotic giant branch companion where the s-process has operated. Lithium abundance is measured in the newly discovered UMP star LAMOST J125346.09+075343.1, making it the second UMP turnoff star with Li detection. The Li abundance of LAMOST J125346.09+075343.1 is slightly lower than the values obtained for less metal-poor stars with similar temperatures, and provides a unique data point at [Fe/H] ˜ -4.2 to support the "meltdown" of the Li Spite plateau at extremely low metallicity. Comparison with the other two UMP and HMP (hyper metal-poor, with [Fe/H] < -5.0) turnoff stars suggests that the difference in lighter elements such as CNO and Na might cause notable difference in lithium abundances among CEMP-no stars.

  14. Metal-poor Stars Observed with the Magellan Telescope. III. New Extremely and Ultra Metal-poor Stars from SDSS/SEGUE and Insights on the Formation of Ultra Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Placco, Vinicius M.; Frebel, Anna; Lee, Young Sun; Jacobson, Heather R.; Beers, Timothy C.; Pena, Jose M.; Chan, Conrad; Heger, Alexander

    2015-08-01

    We report the discovery of one extremely metal-poor (EMP; [{Fe}/{{H}}] \\lt -3) and one ultra metal-poor (UMP; [{Fe}/{{H}}] \\lt -4) star selected from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration survey. These stars were identified as EMP candidates based on their medium-resolution (R ˜ 2000) spectra, and were followed up with high-resolution (R ˜ 35,000) spectroscopy with the Magellan/Clay Telescope. Their derived chemical abundances exhibit good agreement with those of stars with similar metallicities. We also provide new insights on the formation of the UMP stars, based on comparisons with a new set of theoretical models of supernovae (SNe) nucleosynthesis. The models were matched with 20 UMP stars found in the literature, together with one of the program stars (SDSS J1204+1201), with [{Fe}/{{H}}]=-4.34. From fitting their abundances, we find that the SNe progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 {M}⊙ to 28 {M}⊙ and explosion energies from 0.3 to 0.9× {10}51 {erg}. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for a future high-resolution follow-up of UMP candidates. In addition, we are able to reproduce the different CNO abundance patterns found in UMP stars with a single progenitor type by varying its mass and explosion energy. Magellan Telescopes located at Las Campanas Observatory, Chile.

  15. HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: LEO IV

    SciTech Connect

    Simon, Joshua D.; McWilliam, Andrew; Thompson, Ian B.; Frebel, Anna; Kirby, Evan N. E-mail: andy@ociw.ed E-mail: afrebel@cfa.harvard.ed

    2010-06-10

    We present high-resolution Magellan/MIKE spectroscopy of the brightest star in the ultra-faint dwarf galaxy Leo IV. We measure an iron abundance of [Fe/H] = -3.2, adding to the rapidly growing sample of extremely metal-poor (EMP) stars being identified in Milky Way satellite galaxies. The star is enhanced in the {alpha} elements Mg, Ca, and Ti by {approx}0.3 dex, very similar to the typical Milky Way halo abundance pattern. All of the light and iron-peak elements follow the trends established by EMP halo stars, but the neutron-capture elements Ba and Sr are significantly underabundant. These results are quite similar to those found for stars in the ultra-faint dwarfs Ursa Major II, Coma Berenices, Booetes I, and Hercules, suggesting that the chemical evolution of the lowest-luminosity galaxies may be universal. The abundance pattern we observe is consistent with predictions for nucleosynthesis from a Population III supernova explosion. The extremely low metallicity of this star also supports the idea that a significant fraction ({approx}>10%) of the stars in the faintest dwarfs have metallicities below [Fe/H] = -3.0.

  16. Spectroscopic Analysis of Metal-poor Stars from LAMOST: Early Results

    NASA Astrophysics Data System (ADS)

    Li, Hai-Ning; Zhao, Gang; Christlieb, Norbert; Wang, Liang; Wang, Wei; Zhang, Yong; Hou, Yonghui; Yuan, Hailong

    2015-01-01

    We report on early results from a pilot program searching for metal-poor stars with LAMOST and follow-up high-resolution observation acquired with the MIKE spectrograph attached to the Magellan II telescope. We performed detailed abundance analysis for eight objects with iron abundances [Fe/H] < -2.0, including five extremely metal-poor (EMP; [Fe/H] < -3.0) stars with two having [Fe/H] < -3.5. Among these objects, three are newly discovered EMP stars, one of which is confirmed for the first time with high-resolution spectral observations. Three program stars are regarded as carbon-enhanced metal-poor (CEMP) stars, including two stars with no enhancement in their neutron-capture elements, which thus possibly belong to the class of CEMP-no stars; one of these objects also exhibits significant enhancement in nitrogen, and is thus a potential carbon and nitrogen-enhanced metal-poor star. The [X/Fe] ratios of the sample stars generally agree with those reported in the literature for other metal-poor stars in the same [Fe/H] range. We also compared the abundance patterns of individual program stars with the average abundance pattern of metal-poor stars and find only one chemically peculiar object with abundances of at least two elements (other than C and N) showing deviations larger than 0.5 dex. The distribution of [Sr/Ba] versus [Ba/H] agrees that an additional nucleosynthesis mechanism is needed aside from a single r-process. Two program stars with extremely low abundances of Sr and Ba support the prospect that both main and weak r-processes may have operated during the early phase of Galactic chemical evolution. The distribution of [C/N] shows that there are two groups of carbon-normal giants with different degrees of mixing. However, it is difficult to explain the observed behavior of the [C/N] of the nitrogen-enhanced unevolved stars based on current data.

  17. SPECTROSCOPIC ANALYSIS OF METAL-POOR STARS FROM LAMOST: EARLY RESULTS

    SciTech Connect

    Li, Hai-Ning; Zhao, Gang; Wang, Liang; Wang, Wei; Yuan, Hailong; Christlieb, Norbert; Zhang, Yong; Hou, Yonghui E-mail: gzhao@nao.cas.cn

    2015-01-10

    We report on early results from a pilot program searching for metal-poor stars with LAMOST and follow-up high-resolution observation acquired with the MIKE spectrograph attached to the Magellan II telescope. We performed detailed abundance analysis for eight objects with iron abundances [Fe/H] < -2.0, including five extremely metal-poor (EMP; [Fe/H] < -3.0) stars with two having [Fe/H] < -3.5. Among these objects, three are newly discovered EMP stars, one of which is confirmed for the first time with high-resolution spectral observations. Three program stars are regarded as carbon-enhanced metal-poor (CEMP) stars, including two stars with no enhancement in their neutron-capture elements, which thus possibly belong to the class of CEMP-no stars; one of these objects also exhibits significant enhancement in nitrogen, and is thus a potential carbon and nitrogen-enhanced metal-poor star. The [X/Fe] ratios of the sample stars generally agree with those reported in the literature for other metal-poor stars in the same [Fe/H] range. We also compared the abundance patterns of individual program stars with the average abundance pattern of metal-poor stars and find only one chemically peculiar object with abundances of at least two elements (other than C and N) showing deviations larger than 0.5 dex. The distribution of [Sr/Ba] versus [Ba/H] agrees that an additional nucleosynthesis mechanism is needed aside from a single r-process. Two program stars with extremely low abundances of Sr and Ba support the prospect that both main and weak r-processes may have operated during the early phase of Galactic chemical evolution. The distribution of [C/N] shows that there are two groups of carbon-normal giants with different degrees of mixing. However, it is difficult to explain the observed behavior of the [C/N] of the nitrogen-enhanced unevolved stars based on current data.

  18. BOO-1137-AN EXTREMELY METAL-POOR STAR IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY BOOeTES I

    SciTech Connect

    Norris, John E.; Yong, David; Gilmore, Gerard; Wyse, Rosemary F. G.

    2010-03-01

    We present high-resolution (R {approx} 40,000), high-signal-to-noise ratio (20-90) spectra of an extremely metal-poor giant star Boo-1137 in the 'ultra-faint' dwarf spheroidal galaxy (dSph) Booetes I, absolute magnitude M{sub V} {approx} -6.3. We derive an iron abundance of [Fe/H] = -3.7, making this the most metal-poor star as yet identified in an ultra-faint dSph. Our derived effective temperature and gravity are consistent with its identification as a red giant in Booetes I. Abundances for a further 15 elements have also been determined. Comparison of the relative abundances, [X/Fe], with those of the extremely metal-poor red giants of the Galactic halo shows that Boo-1137 is 'normal' with respect to C and N, the odd-Z elements Na and Al, the iron-peak elements, and the neutron-capture elements Sr and Ba, in comparison with the bulk of the Milky Way halo population having [Fe/H] {approx}<-3.0. The alpha-elements Mg, Si, Ca, and Ti are all higher by DELTA[X/Fe] {approx} 0.2 than the average halo values. Monte Carlo analysis indicates that DELTA[alpha/Fe] values this large are expected with a probability {approx}0.02. The elemental abundance pattern in Boo-1137 suggests inhomogeneous chemical evolution, consistent with the wide internal spread in iron abundances we previously reported. The similarity of most of the Boo-1137 relative abundances with respect to halo values, and the fact that the alpha-elements are all offset by a similar small amount from the halo averages, points to the same underlying galaxy-scale stellar initial mass function, but that Boo-1137 likely originated in a star-forming region where the abundances reflect either poor mixing of supernova (SN) ejecta, or poor sampling of the SN progenitor mass range, or both.

  19. Mass-loss predictions for evolved very metal-poor massive stars

    NASA Astrophysics Data System (ADS)

    Muijres, L.; Vink, J. S.; de Koter, A.; Hirschi, R.; Langer, N.; Yoon, S.-C.

    2012-10-01

    Context. The first couple of stellar generations may have been massive, of order 100 M⊙, and to have played a dominant role in galaxy formation and the chemical enrichment of the early Universe. Some fraction of these objects may have died as pair-instability supernovae or gamma-ray bursts. The winds of these stars may have played an important role in determining these outcomes. As the winds are driven by radiation pressure on spectral lines, their strengths are expected to vary with metallicity. Until now, most mass-loss predictions for metal-poor O-type stars have assumed a scaled-down solar-abundance pattern. However, Population III evolutionary tracks show significant surface enrichment through rotational mixing of CNO-processed material, because even metal-poor stars switch to CNO-burning early on. Aims: We address the question of whether the CNO surface enhanced self-enrichment in the first few generations of stars could impact their mass-loss properties. Methods: We employ Monte Carlo simulations to establish the local line-force and solve for the momentum equation of the stellar outflow, testing whether an outflow can actually be established by assessing the net acceleration at the sonic point of the flow. Stellar evolution models of rotating metal-poor stars are used to specify the surface chemical composition, focussing on the phases of early enrichment. Results: We find that the mass-loss rates of CNO enhanced metal-poor stars are higher than those of non-enriched stars, but they are much lower than those rates where the CNO abundance is included in the total abundance Z. Metal-poor stars hotter than ~50 000 K, in the metallicity range investigated here (with an initial metallicity Z ≲ 10-4) are found to have no wind, as the high-ionization species of the CNO elements have too few strong lines to drive an outflow. We present a heuristic formula that provides mass-loss estimates for CNO-dominated winds in relation to scaled-down solar abundances

  20. Hot, Massive Stars in the Extremely Metal-Poor Galaxy, I Zw 18

    NASA Technical Reports Server (NTRS)

    Heap, Sara R.; Malumuth, Eliot M.

    2010-01-01

    The extremely metal-poor galaxy I Zw 18, is the Rosetta Stone for understanding z=7-8 galaxies now being discovered by Hubb|e's Wide Field Camera 3 (HST/WFC3). Using HST/STIS images and recently obtained HST/COS ultraviolet spectra, we derive information about the hot, massive stars in this galaxy including stellar abundances, constraints on the stellar IMF and mass distribution of young clusters containing hot, massive stars.

  1. THE EXTREME OVERABUNDANCE OF MOLYBDENUM IN TWO METAL-POOR STARS

    SciTech Connect

    Peterson, Ruth C.

    2011-11-20

    We report determinations of the molybdenum abundances in five mildly to extremely metal-poor turnoff stars using five Mo II lines near 2000 A. In two of the stars, the abundance of molybdenum is found to be extremely enhanced, as high or higher than the neighboring even-Z elements ruthenium and zirconium. Of the several nucleosynthesis scenarios envisioned for the production of nuclei in this mass range in the oldest stars, a high-entropy wind acting in a core-collapse supernova seems uniquely capable of the twin aspects of a high molybdenum overproduction confined to a narrow mass range. Whatever the details of the nucleosynthesis mechanism, however, this unusual excess suggests that very few individual nucleosynthesis events were responsible for the synthesis of the light trans-Fe heavy elements in these cases, an unexpected result given that both are only moderately metal-poor.

  2. MOLYBDENUM, RUTHENIUM, AND THE HEAVY r-PROCESS ELEMENTS IN MODERATELY METAL-POOR MAIN-SEQUENCE TURNOFF STARS

    SciTech Connect

    Peterson, Ruth C.

    2013-05-01

    The ratios of elemental abundances observed in metal-poor stars of the Galactic halo provide a unique present-day record of the nucleosynthesis products of its earliest stars. While the heaviest elements were synthesized by the r- and s-processes, dominant production mechanisms of light trans-ironic elements were obscure until recently. This work investigates further our 2011 conclusion that the low-entropy regime of a high-entropy wind (HEW) produced molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed extreme overabundances of those elements with respect to iron. Only a few, rare nucleosynthesis events may have been involved. Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28 similar stars by matching spectral calculations to well-exposed near-UV Keck HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff stars with Mo or Ru line detections and no evidence for s-process production (therefore old), we find Mo and Ru to be three to six times overabundant. In contrast, the maximum overabundance is reduced to factors of three and two for the neighboring elements zirconium and palladium. Since the overproduction peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin. The overabundance level of the heavy r-process elements varies significantly, from none to a factor of four, but is uncorrelated with Mo and Ru overabundances. Despite their moderate metallicity, stars in this group trace the products of different nucleosynthetic events: possibly very few events, possibly events whose output depended on environment, metallicity, or time.

  3. Molybdenum, Ruthenium, and the Heavy r-process Elements in Moderately Metal-poor Main-sequence Turnoff Stars

    NASA Astrophysics Data System (ADS)

    Peterson, Ruth C.

    2013-05-01

    The ratios of elemental abundances observed in metal-poor stars of the Galactic halo provide a unique present-day record of the nucleosynthesis products of its earliest stars. While the heaviest elements were synthesized by the r- and s-processes, dominant production mechanisms of light trans-ironic elements were obscure until recently. This work investigates further our 2011 conclusion that the low-entropy regime of a high-entropy wind (HEW) produced molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed extreme overabundances of those elements with respect to iron. Only a few, rare nucleosynthesis events may have been involved. Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28 similar stars by matching spectral calculations to well-exposed near-UV Keck HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff stars with Mo or Ru line detections and no evidence for s-process production (therefore old), we find Mo and Ru to be three to six times overabundant. In contrast, the maximum overabundance is reduced to factors of three and two for the neighboring elements zirconium and palladium. Since the overproduction peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin. The overabundance level of the heavy r-process elements varies significantly, from none to a factor of four, but is uncorrelated with Mo and Ru overabundances. Despite their moderate metallicity, stars in this group trace the products of different nucleosynthetic events: possibly very few events, possibly events whose output depended on environment, metallicity, or time.

  4. Origins of the thick disk of the Milky Way Galaxy as traced by the elemental abundances of metal-poor stars

    NASA Astrophysics Data System (ADS)

    Ruchti, Gregory Randal

    2010-12-01

    Understanding the formation and evolution of disks in galaxies in the early universe is very important for understanding the forms of galaxies today. Recent studies of the Milky Way Galaxy, an ideal galaxy for analyzing individual stars within its disk, indicate that the formation of the Galactic disk is very complex. Most of these studies, however, contain very few stars at low metallicities. Metal-poor stars are important, because they are potential survivors of the earliest star formation in the disk of the Milky Way Galaxy. I therefore measured elemental abundances of a statistically significant sample of metal-poor ([Fe/H] ≲ - 1.0) stars in the disk of the Galaxy, chosen from the RAVE survey in order to study the early formation history of the Galactic disk. I report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 dwarf/sub-giant metal-poor thick-disk candidate stars. I found that the [alpha/Fe] ratios are enhanced implying that enrichment proceeded by purely core-collapse supernovae. This requires that star formation in each star forming region had a short duration. The relative lack of scatter in the [alpha/Fe] ratios implies good mixing in the interstellar medium prior to star formation. In addition, the ratios resemble that of the halo, indicating that the halo and thick disk share a similar massive star initial mass function. I further looked for radial or vertical gradients in metallicity or alpha-enhancement for the metal-poor thick disk, never before done for such a sample. I found no radial gradient and a moderate vertical gradient in my derived iron abundance, and only minimal-amplitude gradients in [alpha/Fe]. In addition, I show that the distribution of orbital eccentricities for my metal-poor thick-disk stars requires that the thick disk was formed primarily in situ, with direct accretion being extremely minimal. I conclude that the alpha-enhancement of the metal-poor thick disk, and the lack of obvious radial or

  5. THE ORIGIN OF LOW [α/Fe] RATIOS IN EXTREMELY METAL-POOR STARS

    SciTech Connect

    Kobayashi, Chiaki; Ishigaki, Miho N.; Tominaga, Nozomu; Nomoto, Ken'ichi

    2014-04-10

    We show that the low ratios of α elements (Mg, Si, and Ca) to Fe recently found for a small fraction of extremely metal-poor stars can be naturally explained with the nucleosynthesis yields of core-collapse supernovae, i.e., 13-25 M {sub ☉} supernovae, or hypernovae. For the case without carbon enhancement, the ejected iron mass is normal, consistent with observed light curves and spectra of nearby supernovae. On the other hand, the carbon enhancement requires much smaller iron production, and the low [α/Fe] of carbon-enhanced metal-poor stars can also be reproduced with 13-25 M {sub ☉} faint supernovae or faint hypernovae. Iron-peak element abundances, in particular Zn abundances, are important to put further constraints on the enrichment sources from galactic archaeology surveys.

  6. EXPLAINING THE Sr AND Ba SCATTER IN EXTREMELY METAL-POOR STARS

    SciTech Connect

    Aoki, W.; Suda, T.; Boyd, R. N.; Kajino, T.; Famiano, M. A. E-mail: takuma.suda@nao.ac.jp E-mail: kajino@nao.ac.jp

    2013-03-20

    Compilations of abundances of strontium and barium in extremely metal-poor stars show that an apparent cutoff is observed for [Sr/Ba] at [Fe/H] < -3.6 and large fluctuations for [Fe/H] > -3.6 with a clear upper bound depending on metallicity. We study the factors that place upper limits on the logarithmic ratio [Sr/Ba]. A model is developed in which the collapses of type II supernovae are found to reproduce many of the features seen in the data. This model is consistent with galactic chemical evolution constraints of light-element enrichment in metal-poor stars. Effects of turbulence in an explosive site have also been simulated, and are found to be important in explaining the large scatter observed in the [Sr/Ba] data.

  7. DETECTION OF ELEMENTS AT ALL THREE r-PROCESS PEAKS IN THE METAL-POOR STAR HD 160617

    SciTech Connect

    Roederer, Ian U.; Lawler, James E. E-mail: jelawler@wisc.edu

    2012-05-01

    We report the first detection of elements at all three r-process peaks in the metal-poor halo star HD 160617. These elements include arsenic and selenium, which have not been detected previously in halo stars, and the elements tellurium, osmium, iridium, and platinum, which have been detected previously. Absorption lines of these elements are found in archive observations made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We present up-to-date absolute atomic transition probabilities and complete line component patterns for these elements. Additional archival spectra of this star from several ground-based instruments allow us to derive abundances or upper limits of 45 elements in HD 160617, including 27 elements produced by neutron-capture reactions. The average abundances of the elements at the three r-process peaks are similar to the predicted solar system r-process residuals when scaled to the abundances in the rare earth element domain. This result for arsenic and selenium may be surprising in light of predictions that the production of the lightest r-process elements generally should be decoupled from the heavier r-process elements.

  8. Discovery of a strongly r-process enhanced extremely metal-poor star LAMOST J110901.22+075441.8

    NASA Astrophysics Data System (ADS)

    Li, Hai-Ning; Aoki, Wako; Honda, Satoshi; Zhao, Gang; Christlieb, Norbert; Suda, Takuma

    2015-08-01

    We report the discovery of an extremely metal-poor (EMP) giant, LAMOST J110901.22+075441.8, which exhibits a large excess of r-process elements with [Eu/Fe] ˜ +1.16. The star is one of the newly discovered EMP stars identified from the LAMOST low-resolution spectroscopic survey and a high-resolution follow-up observation with the Subaru Telescope. Stellar parameters and elemental abundances have been determined from the Subaru spectrum. Accurate abundances for a total of 23 elements including 11 neutron-capture elements from Sr through Dy have been derived for LAMOST J110901.22+075441.8. The abundance pattern of LAMOST J110901.22+075441.8 in the range of C through Zn is in line with the “normal” population of EMP halo stars, except that it shows a notable underabundance in carbon. The heavy element abundance pattern of LAMOST J110901.22+075441.8 is in agreement with other well studied cool r-II metal-poor giants such as CS 22892-052 and CS 31082-001. The abundances of elements in the range from Ba through Dy match the scaled solar r-process pattern well. LAMOST J110901.22+075441.8 provides the first detailed measurements of neutron-capture elements among r-II stars at such low metallicity with [Fe/H] ≲ -3.4, and exhibits similar behavior as other r-II stars in the abundance ratio of Zr/Eu as well as Sr/Eu and Ba/Eu.

  9. First high-precision differential abundance analysis of extremely metal-poor stars

    NASA Astrophysics Data System (ADS)

    Reggiani, Henrique; Meléndez, Jorge; Yong, David; Ramírez, Ivan; Asplund, Martin

    2016-02-01

    Context. Studies of extremely metal-poor stars indicate that chemical abundance ratios [X/Fe] have a root mean square scatter as low as 0.05 dex (12%). It remains unclear whether this reflects observational uncertainties or intrinsic astrophysical scatter arising from physical conditions in the interstellar medium at early times. Aims: We measure differential chemical abundance ratios in extremely metal-poor stars to investigate the limits of precision and to understand whether cosmic scatter or observational errors are dominant. Methods: We used high-resolution (R ~ 95 000) and high signal-to-noise (S/N = 700 at 5000 Å) HIRES/Keck spectra to determine high-precision differential abundances between two extremely metal-poor stars through a line-by-line differential approach. We determined stellar parameters for the star G64-37 with respect to the standard star G64-12. We performed EW measurements for the two stars for the lines recognized in both stars and performed spectral synthesis to study the carbon abundances. Results: The differential approach allowed us to obtain errors of σ(Teff) = 27 K, σ(log g) = 0.06 dex, σ( [Fe/H] ) = 0.02 dex and σ(vt) = 0.06 km s-1. We estimated relative chemical abundances with a precision as low as σ([X/Fe]) ≈ 0.01 dex. The small uncertainties demonstrate that there are genuine abundance differences larger than the measurement errors. The observed Li difference cannot be explained by the difference in mass because the less massive star has more Li. Conclusions: It is possible to achieve an abundance precision around ≈ 0.01-0.05 dex for extremely metal-poor stars, which opens new windows on the study of the early chemical evolution of the Galaxy. Table A.1 is also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A67

  10. Metal-poor stars observed with the Magellan telescope. II. Discovery of four stars with [Fe/H] ≤ –3.5

    SciTech Connect

    Placco, Vinicius M.; Beers, Timothy C.; Frebel, Anna; Christlieb, Norbert; Lee, Young Sun; Kennedy, Catherine R.; Rossi, Silvia; Santucci, Rafael M.

    2014-01-20

    We report on the discovery of seven low-metallicity stars selected from the Hamburg/ESO Survey, six of which are extremely metal-poor (EMP, [Fe/H] ≤ –3.0), with four having [Fe/H] ≤ –3.5. Chemical abundances or upper limits are derived for these stars based on high-resolution (R ∼ 35,000) Magellan/MIKE spectroscopy, and are in general agreement with those of other very and extremely metal-poor stars reported in the literature. Accurate metallicities and abundance patterns for stars in this metallicity range are of particular importance for studies of the shape of the metallicity distribution function of the Milky Way's halo system, in particular for probing the nature of its low-metallicity tail. In addition, taking into account suggested evolutionary mixing effects, we find that six of the program stars (with [Fe/H] ≤ –3.35) possess atmospheres that were likely originally enriched in carbon, relative to iron, during their main-sequence phases. These stars do not exhibit overabundances of their s-process elements, and hence may be, within the error bars, additional examples of the so-called CEMP-no class of objects.

  11. Hot, Massive Stars in the Extremely Metal-Poor Galaxy, I Zw 18

    NASA Technical Reports Server (NTRS)

    Heap, Sara R.; Malumuth, Eliot M.

    2010-01-01

    The carbon-enhanced metal-poor galaxy, I Zw 18, is the Rosetta Stone for understanding galaxies in the early universe by providing constraints on the IMF of massive stars, the role of galaxies in reionization of the universe, mixing of newly synthesized material in the ISM, and gamma-ray bursts at low metallicity, and on the earliest generations of stars producing the observed abundance pattern. We describe these constraints as derived from analyses of HST/COS spectra of I Zw 18 including stellar atmosphere analysis and photo-ionization modeling of both the emission and absorption spectra of the nebular material and interstellar medium.

  12. A giant planet around a metal-poor star of extragalactic origin.

    PubMed

    Setiawan, Johny; Klement, Rainer J; Henning, Thomas; Rix, Hans-Walter; Rochau, Boyke; Rodmann, Jens; Schulze-Hartung, Tim

    2010-12-17

    Stars in their late stage of evolution, such as horizontal branch stars, are still largely unexplored for planets. We detected a planetary companion around HIP 13044, a very metal-poor star on the red horizontal branch, on the basis of radial velocity observations with a high-resolution spectrograph at the 2.2-meter Max-Planck Gesellschaft-European Southern Observatory telescope. The star's periodic radial velocity variation of P = 16.2 days caused by the planet can be distinguished from the periods of the stellar activity indicators. The minimum mass of the planet is 1.25 times the mass of Jupiter and its orbital semimajor axis is 0.116 astronomical units. Because HIP 13044 belongs to a group of stars that have been accreted from a disrupted satellite galaxy of the Milky Way, the planet most likely has an extragalactic origin. PMID:21097905

  13. TOPoS: chemical study of extremely metal-poor stars.

    NASA Astrophysics Data System (ADS)

    Caffau, E.; Sbordone, L.; Bonifacio, P.; Cayrel, R.; Christlieb, N.; Clark, P.; François, P.; Glover, S.; Klessen, R.; Koch, A.; Ludwig, H.-G.; Monaco, L.; Plez, B.; Spite, F.; Spite, M.; Steffen, M.; Zaggia, S.

    The extremely metal-poor (EMP) stars hold in their atmospheres the fossil record of the chemical composition of the early phases of the Galactic evolution. The chemical analysis of such objects provides important constraints on these early phases. EMP stars are very rare objects; to dig them out, large amounts of data have to be processed. With an automatic procedure, we analysed objects with colours of Turn-Off stars from the Sloan Digital Sky Survey to select a sample of good candidate EMP stars. In the latest years, we observed a sample of these candidates with X-Shooter and UVES, and we have an ongoing ESO large programme to use these spectrographs to observe EMP stars. I will report here the results on metallicity and Strontium abundance. Based on observations obtained at ESO Paranal Observatory, programme 189.D-0165(A)

  14. A giant planet around a metal-poor star of extragalactic origin.

    PubMed

    Setiawan, Johny; Klement, Rainer J; Henning, Thomas; Rix, Hans-Walter; Rochau, Boyke; Rodmann, Jens; Schulze-Hartung, Tim

    2010-12-17

    Stars in their late stage of evolution, such as horizontal branch stars, are still largely unexplored for planets. We detected a planetary companion around HIP 13044, a very metal-poor star on the red horizontal branch, on the basis of radial velocity observations with a high-resolution spectrograph at the 2.2-meter Max-Planck Gesellschaft-European Southern Observatory telescope. The star's periodic radial velocity variation of P = 16.2 days caused by the planet can be distinguished from the periods of the stellar activity indicators. The minimum mass of the planet is 1.25 times the mass of Jupiter and its orbital semimajor axis is 0.116 astronomical units. Because HIP 13044 belongs to a group of stars that have been accreted from a disrupted satellite galaxy of the Milky Way, the planet most likely has an extragalactic origin.

  15. The Synthetic-Oversampling Method: Using Photometric Colors to Discover Extremely Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Miller, A. A.

    2015-09-01

    Extremely metal-poor (EMP) stars ([Fe/H] ≤ -3.0 dex) provide a unique window into understanding the first generation of stars and early chemical enrichment of the universe. EMP stars are exceptionally rare, however, and the relatively small number of confirmed discoveries limits our ability to exploit these near-field probes of the first ˜500 Myr after the Big Bang. Here, a new method to photometrically estimate [Fe/H] from only broadband photometric colors is presented. I show that the method, which utilizes machine-learning algorithms and a training set of ˜170,000 stars with spectroscopically measured [Fe/H], produces a typical scatter of ˜0.29 dex. This performance is similar to what is achievable via low-resolution spectroscopy, and outperforms other photometric techniques, while also being more general. I further show that a slight alteration to the model, wherein synthetic EMP stars are added to the training set, yields the robust identification of EMP candidates. In particular, this synthetic-oversampling method recovers ˜20% of the EMP stars in the training set, at a precision of ˜0.05. Furthermore, ˜65% of the false positives from the model are very metal-poor stars ([Fe/H] ≤ -2.0 dex). The synthetic-oversampling method is biased toward the discovery of warm (˜F-type) stars, a consequence of the targeting bias from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding survey. This EMP selection method represents a significant improvement over alternative broadband optical selection techniques. The models are applied to >12 million stars, with an expected yield of ˜600 new EMP stars, which promises to open new avenues for exploring the early universe.

  16. THE SYNTHETIC-OVERSAMPLING METHOD: USING PHOTOMETRIC COLORS TO DISCOVER EXTREMELY METAL-POOR STARS

    SciTech Connect

    Miller, A. A.

    2015-09-20

    Extremely metal-poor (EMP) stars ([Fe/H] ≤ −3.0 dex) provide a unique window into understanding the first generation of stars and early chemical enrichment of the universe. EMP stars are exceptionally rare, however, and the relatively small number of confirmed discoveries limits our ability to exploit these near-field probes of the first ∼500 Myr after the Big Bang. Here, a new method to photometrically estimate [Fe/H] from only broadband photometric colors is presented. I show that the method, which utilizes machine-learning algorithms and a training set of ∼170,000 stars with spectroscopically measured [Fe/H], produces a typical scatter of ∼0.29 dex. This performance is similar to what is achievable via low-resolution spectroscopy, and outperforms other photometric techniques, while also being more general. I further show that a slight alteration to the model, wherein synthetic EMP stars are added to the training set, yields the robust identification of EMP candidates. In particular, this synthetic-oversampling method recovers ∼20% of the EMP stars in the training set, at a precision of ∼0.05. Furthermore, ∼65% of the false positives from the model are very metal-poor stars ([Fe/H] ≤ −2.0 dex). The synthetic-oversampling method is biased toward the discovery of warm (∼F-type) stars, a consequence of the targeting bias from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding survey. This EMP selection method represents a significant improvement over alternative broadband optical selection techniques. The models are applied to >12 million stars, with an expected yield of ∼600 new EMP stars, which promises to open new avenues for exploring the early universe.

  17. The s-PROCESS Nucleosynthesis in Massive Metal-Poor Stars

    NASA Astrophysics Data System (ADS)

    Iwamoto, Nobuyuki

    2005-12-01

    We present the s-process nucleosynthesis in massive stars with a wide range of metallicity, using the recent sets of reaction rates and stellar input physics. The decreasing metallicity makes poisoning effects of primary 16O larger at the late phase of core He burning, at which the s-process occurs actively in solar metallicity stars, and prevents the synthesis of heavy elements from being efficient. However, we find that the s-process proceeds very efficiently via neutron source reaction of 13C(α,n)16O at the end of core H burning phase when the metallicity decreases below Z ~ 10-8. These massive, extremely low metallicity stars may have an important contribution of light s-elements to observed extremely metal-poor stars.

  18. Abundance Profiling of Extremely Metal-poor Stars and Supernova Properties in the Early Universe

    NASA Astrophysics Data System (ADS)

    Tominaga, Nozomu; Iwamoto, Nobuyuki; Nomoto, Ken'ichi

    2014-04-01

    After the big bang nucleosynthesis, the first heavy element enrichment in the universe was made by a supernova (SN) explosion of a population (Pop) III star (Pop III SN). The abundance ratios of elements produced from Pop III SNe are recorded in abundance patterns of extremely metal-poor (EMP) stars. The observations of the increasing number of EMP stars have made it possible to statistically constrain the explosion properties of Pop III SNe. We present Pop III SN models whose nucleosynthesis yields well reproduce, individually, the abundance patterns of 48 such metal-poor stars as [Fe/H] <~ - 3.5. We then derive relations between the abundance ratios of EMP stars and certain explosion properties of Pop III SNe: the higher [(C + N)/Fe] and [(C + N)/Mg] ratios correspond to the smaller ejected Fe mass and the larger compact remnant mass, respectively. Using these relations, the distributions of the abundance ratios of EMP stars are converted to those of the explosion properties of Pop III SNe. Such distributions are compared with those of the explosion properties of present day SNe: the distribution of the ejected Fe mass of Pop III SNe has the same peak as that of the present day SNe but shows an extended tail down to ~10-2-10-5 M ⊙, and the distribution of the mass of the compact remnant of Pop III SNe is as wide as that of the present-day, stellar-mass black holes. Our results demonstrate the importance of large samples of EMP stars obtained by ongoing and future EMP star surveys and subsequent high-dispersion spectroscopic observations in clarifying the nature of Pop III SNe in the early universe.

  19. Abundance profiling of extremely metal-poor stars and supernova properties in the early universe

    SciTech Connect

    Tominaga, Nozomu; Iwamoto, Nobuyuki; Nomoto, Ken'ichi E-mail: iwamoto.nobuyuki@jaea.go.jp

    2014-04-20

    After the big bang nucleosynthesis, the first heavy element enrichment in the universe was made by a supernova (SN) explosion of a population (Pop) III star (Pop III SN). The abundance ratios of elements produced from Pop III SNe are recorded in abundance patterns of extremely metal-poor (EMP) stars. The observations of the increasing number of EMP stars have made it possible to statistically constrain the explosion properties of Pop III SNe. We present Pop III SN models whose nucleosynthesis yields well reproduce, individually, the abundance patterns of 48 such metal-poor stars as [Fe/H] ≲ – 3.5. We then derive relations between the abundance ratios of EMP stars and certain explosion properties of Pop III SNe: the higher [(C + N)/Fe] and [(C + N)/Mg] ratios correspond to the smaller ejected Fe mass and the larger compact remnant mass, respectively. Using these relations, the distributions of the abundance ratios of EMP stars are converted to those of the explosion properties of Pop III SNe. Such distributions are compared with those of the explosion properties of present day SNe: the distribution of the ejected Fe mass of Pop III SNe has the same peak as that of the present day SNe but shows an extended tail down to ∼10{sup –2}-10{sup –5} M {sub ☉}, and the distribution of the mass of the compact remnant of Pop III SNe is as wide as that of the present-day, stellar-mass black holes. Our results demonstrate the importance of large samples of EMP stars obtained by ongoing and future EMP star surveys and subsequent high-dispersion spectroscopic observations in clarifying the nature of Pop III SNe in the early universe.

  20. G64-12 and G64-37 Are Carbon-enhanced Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Placco, Vinicius M.; Beers, Timothy C.; Reggiani, Henrique; Meléndez, Jorge

    2016-10-01

    We present new high-resolution chemical-abundance analyses for the well-known high proper-motion subdwarfs G64-12 and G64-37, based on very high signal-to-noise ratio spectra ({{S}}/{{N}}˜ 700/1) with resolving power R ˜ 95,000. These high-quality data enable the first reliable determination of the carbon abundances for these two stars; we classify them as carbon-enhanced metal-poor (CEMP) stars based on their carboni cities, which both exceed [C/Fe] = +1.0. They are sub-classified as CEMP-no Group-II stars, based on their location in the Yoon-Beers diagram of absolute carbon abundance, A(C) versus [Fe/H], as well as on the conventional diagnostic [Ba/Fe]. The relatively low absolute carbon abundances of CEMP-no stars, in combination with the high effective temperatures of these two stars ({T}{eff}˜ 6500 {{K}}), weakens their CH molecular features to the point that accurate carbon abundances can only be estimated from spectra with very high S/N. A comparison of the observed abundance patterns with the predicted yields from massive, metal-free supernova models reduces the inferred progenitor masses by factors of ˜2-3, and explosion energies by factors of ˜10-15, compared to those derived using previously claimed carbon-abundance estimates. There are certainly many more warm CEMP-no stars near the halo main-sequence turnoff that have been overlooked in past studies, directly impacting the derived frequencies of CEMP-no stars as a function of metallicity, a probe that provides important constraints on Galactic chemical evolution models, the initial mass function in the early universe, and first-star nucleosynthesis.

  1. Seven new carbon-enhanced metal-poor RR Lyrae stars

    SciTech Connect

    Kennedy, Catherine R.; Stancliffe, Richard J.; Kuehn, Charles; Beers, Timothy C.; Kinman, T. D.; Placco, Vinicius M.; Reggiani, Henrique; Rossi, Silvia; Lee, Young Sun

    2014-05-20

    We report estimated carbon-abundance ratios, [C/Fe], for seven newly discovered carbon-enhanced metal-poor (CEMP) RR Lyrae stars. These are well-studied RRab stars that had previously been selected as CEMP candidates based on low-resolution spectra. For this pilot study, we observed eight of these CEMP RR Lyrae candidates with the Wide Field Spectrograph on the ANU 2.3 m telescope. Prior to this study, only two CEMP RR Lyrae stars had been discovered: TY Gru and SDSS J1707+58. We compare our abundances to new theoretical models of the evolution of low-mass stars in binary systems. These simulations evolve the secondary stars, post accretion from an asymptotic giant-branch (AGB) donor, all the way to the RR Lyrae stage. The abundances of CEMP RR Lyrae stars can be used as direct probes of the nature of the donor star, such as its mass, and the amount of material accreted onto the secondary. We find that the majority of the sample of CEMP RR Lyrae stars is consistent with AGB donor masses of around 1.5-2.0 M {sub ☉} and accretion masses of a few hundredths of a solar mass. Future high-resolution studies of these newly discovered CEMP RR Lyrae stars will help disentangle the effects of the proposed mixing processes that occur in such objects.

  2. The chemical imprint of silicate dust on the most metal-poor stars

    SciTech Connect

    Ji, Alexander P.; Frebel, Anna; Bromm, Volker E-mail: afrebel@mit.edu

    2014-02-20

    We investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (<1 M {sub ☉}) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming that the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H]{sub crit}). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H]{sub crit} = –4.5 ± 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H]{sub crit} = –5.3 ± 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine-structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].

  3. Abundance anomalies in metal-poor stars from Population III supernova ejecta hydrodynamics

    NASA Astrophysics Data System (ADS)

    Sluder, Alan; Ritter, Jeremy S.; Safranek-Shrader, Chalence; Milosavljević, Miloš; Bromm, Volker

    2016-02-01

    We present a simulation of the long-term evolution of a Population III supernova remnant in a cosmological minihalo. Employing passive Lagrangian tracer particles, we investigate how chemical stratification and anisotropy in the explosion can affect the abundances of the first low-mass, metal-enriched stars. We find that reverse shock heating can leave the inner mass shells at entropies too high to cool, leading to carbon enhancement in the recollapsing gas. This hydrodynamic selection effect could explain the observed incidence of carbon-enhanced metal-poor stars at low metallicity. We further explore how anisotropic ejecta distributions, recently seen in direct numerical simulations of core-collapse explosions, may translate to abundances in metal-poor stars. We find that some of the observed scatter in the Population II abundance ratios can be explained by an incomplete mixing of supernova ejecta, even in the case of only one contributing enrichment event. We demonstrate that the customary hypothesis of fully mixed ejecta clearly fails if post-explosion hydrodynamics prefers the recycling of some nucleosynthetic products over others. Furthermore, to fully exploit the stellar-archaeological programme of constraining the Pop III initial mass function from the observed Pop II abundances, considering these hydrodynamical transport effects is crucial. We discuss applications to the rich chemical structure of ultrafaint dwarf satellite galaxies, to be probed in unprecedented detail with upcoming spectroscopic surveys.

  4. THE MOST METAL-POOR STARS. I. DISCOVERY, DATA, AND ATMOSPHERIC PARAMETERS

    SciTech Connect

    Norris, John E.; Bessell, M. S.; Yong, David; Asplund, M.; Murphy, Simon J.; Christlieb, N.; Barklem, P. S.; Beers, Timothy C.; Frebel, Anna; Ryan, S. G. E-mail: bessell@mso.anu.edu.au E-mail: martin@mso.anu.edu.au E-mail: paul.barklem@physics.uu.se E-mail: afrebel@mit.edu

    2013-01-01

    We report the discovery of 34 stars in the Hamburg/ESO Survey for metal-poor stars and the Sloan Digital Sky Survey that have [Fe/H] {approx}< -3.0. Their median and minimum abundances are [Fe/H] = -3.1 and -4.1, respectively, while 10 stars have [Fe/H] < -3.5. High-resolution, high signal-to-noise spectroscopic data-equivalent widths and radial velocities-are presented for these stars, together with an additional four objects previously reported or currently being investigated elsewhere. We have determined the atmospheric parameters, effective temperature (T {sub eff}), and surface gravity (log g), which are critical in the determination of the chemical abundances and the evolutionary status of these stars. Three techniques were used to derive these parameters. Spectrophotometric fits to model atmosphere fluxes were used to derive T {sub eff}, log g, and an estimate of E(B - V); H{alpha}, H{beta}, and H{gamma} profile fitting to model atmosphere results provided the second determination of T {sub eff} and log g; and finally, we used an empirical T {sub eff}-calibrated H{delta} index, for the third, independent T {sub eff} determination. The three values of T {sub eff} are in good agreement, although the profile fitting may yield systematically cooler T {sub eff} values, by {approx}100 K. This collective data set will be analyzed in future papers in the present series to utilize the most metal-poor stars as probes of conditions in the early universe.

  5. Halo Star Lithium Depletion

    SciTech Connect

    Pinsonneault, M. H.; Walker, T. P.; Steigman, G.; Narayanan, Vijay K.

    1999-12-10

    The depletion of lithium during the pre-main-sequence and main-sequence phases of stellar evolution plays a crucial role in the comparison of the predictions of big bang nucleosynthesis with the abundances observed in halo stars. Previous work has indicated a wide range of possible depletion factors, ranging from minimal in standard (nonrotating) stellar models to as much as an order of magnitude in models that include rotational mixing. Recent progress in the study of the angular momentum evolution of low-mass stars permits the construction of theoretical models capable of reproducing the angular momentum evolution of low-mass open cluster stars. The distribution of initial angular momenta can be inferred from stellar rotation data in young open clusters. In this paper we report on the application of these models to the study of lithium depletion in main-sequence halo stars. A range of initial angular momenta produces a range of lithium depletion factors on the main sequence. Using the distribution of initial conditions inferred from young open clusters leads to a well-defined halo lithium plateau with modest scatter and a small population of outliers. The mass-dependent angular momentum loss law inferred from open cluster studies produces a nearly flat plateau, unlike previous models that exhibited a downward curvature for hotter temperatures in the 7Li-Teff plane. The overall depletion factor for the plateau stars is sensitive primarily to the solar initial angular momentum used in the calibration for the mixing diffusion coefficients. Uncertainties remain in the treatment of the internal angular momentum transport in the models, and the potential impact of these uncertainties on our results is discussed. The 6Li/7Li depletion ratio is also examined. We find that the dispersion in the plateau and the 6Li/7Li depletion ratio scale with the absolute 7Li depletion in the plateau, and we use observational data to set bounds on the 7Li depletion in main-sequence halo

  6. Radiative levitation in carbon-enhanced metal-poor stars with s-process enrichment

    NASA Astrophysics Data System (ADS)

    Matrozis, E.; Stancliffe, R. J.

    2016-07-01

    A significant fraction of all metal-poor stars are carbon-rich. Most of these carbon-enhanced metal-poor (CEMP) stars also show enhancement in elements produced mainly by the s-process (CEMP-s stars), and evidence suggests that the origin of these non-standard abundances can be traced to mass transfer from a binary asymptotic giant branch (AGB) companion. Thus, observations of CEMP-s stars are commonly used to infer the nucleosynthesis output of low-metallicity AGB stars. A crucial step in this exercise is understanding what happens to the accreted material after mass transfer ceases. Here we present models of the post-mass-transfer evolution of CEMP-s stars considering the physics of thermohaline mixing and atomic diffusion, including radiative levitation. We find that stars with typical CEMP-s star masses, M ≈ 0.85 M⊙, have very shallow convective envelopes (Menv ≲ 10-7 M⊙). Hence, the surface abundance variations arising from the competition between gravitational settling and radiative levitation should be orders of magnitude larger than observed (e.g. [C/Fe] < -1 or [C/Fe] > +4). Lower-mass stars (M ≈ 0.80 M⊙) retain thicker convective envelopes and thus show variations more in line with observations, but are generally too unevolved (log g > 4) when they reach the age of the Universe. We are therefore unable to reproduce the spread in the observed abundances with these models and conclude that some other physical process must largely suppress atomic diffusion in the outer layers of CEMP-s stars. We demonstrate that this could be achieved by some additional (turbulent) mixing process operating at the base of the convective envelope, as found by other authors. Alternatively, mass-loss rates around 10-13 M⊙yr-1 could also negate most of the abundance variations by eroding the surface layers and forcing the base of the convective envelope to move inwards in mass. Since atomic diffusion cannot have a substantial effect on the surface abundances of CEMP

  7. Fluorine in carbon-enhanced metal-poor stars: a binary scenario

    NASA Astrophysics Data System (ADS)

    Lugaro, M.; de Mink, S. E.; Izzard, R. G.; Campbell, S. W.; Karakas, A. I.; Cristallo, S.; Pols, O. R.; Lattanzio, J. C.; Straniero, O.; Gallino, R.; Beers, T. C.

    2008-06-01

    Aims: A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. Methods: We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars. Results: The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 {M}⊙ of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z ≃ 0.0001 and mass ≃2 {M}⊙. Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process.

  8. Stellar Populations and Star Formation History of the Metal-poor Dwarf Galaxy DDO 68

    NASA Astrophysics Data System (ADS)

    Sacchi, E.; Annibali, F.; Cignoni, M.; Aloisi, A.; Sohn, T.; Tosi, M.; van der Marel, R. P.; Grocholski, A. J.; James, B.

    2016-10-01

    We present the star formation history (SFH) of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only 12+{log}({{O}}/{{H}})=7.15 and a very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only ∼0.15 Gyr ago. Our data provide a deep and uncontaminated optical color–magnitude diagram (CMD) that allows us to disprove this hypothesis since we find a population of at least ∼1 Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is ≃0.01 M ⊙ yr‑1, corresponding to a total astrated mass of ≃1.3 × 108 M ⊙. Our photometry allows us to infer the distance from the tip of the red giant branch, D = 12.08 ± 0.67 Mpc; however, to let our synthetic CMD reproduce the observed ones, we need a slightly higher distance, D = 12.65 Mpc, or (m ‑ M)0 = 30.51, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long tail, probably due to tidal interactions. The SFH of the tail differs from that of the main body mainly for enhanced activity at recent epochs likely triggered by the interaction. Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA Contract NAS5-26555.

  9. Three New Planetary Systems Orbiting Metal-Poor Thick Disk Stars

    NASA Astrophysics Data System (ADS)

    Cochran, William D.; Endl, M.; Brugamyer, E. J.; MacQueen, P. J.

    2013-10-01

    We report the detection of Jovian mass planets orbiting three nearby metal-poor thick disk stars. These discoveries were all made using precise radial velocity measurements from the High Resolution Spectrograph of the Hobby-Eberly Telescope. All of the planets are of Jovian mass or larger, with orbital periods ranging from about a year to over six years. HIP 14342 shows two planetary companions with orbital periods near a 2:1 resonance. The other planets detected orbit HIP 13366 and HIP 109384. All three of these stars are kinematic members of the galactic "thick disk", which is a population of stars with a larger vertical scale height and a larger velocity dispersion that the thin disk to which the Sun belongs. The thick disk stars are of lower total metallicity than the Sun, and are also chemically different than thin disk stars, having the abundances of their alpha-capture elements (e.g. O, Ne, Mg, Si, S, Ca, Ti) enhanced by 0.2 to 0.4 dex over those of thin disk stars of the same [Fe/H]. The majority of planets found among stars with [Fe/H] < ~-0.2 orbit thick disk stars, even though thin disk stars significantly outnumber thick disk stars in this metallicity range. Thus, the enhanced abundance of the alpha-capture elements, which are also key elements in the chemistry of planet-forming materials, may be responsible for the large fraction of low-metallicity thick-disk stars with planetary companions.

  10. Carbon Abundance Plateaus among Carbon-Enhanced Metal-Poor Stars

    NASA Astrophysics Data System (ADS)

    Yoon, Jinmi; He, Siyu; Placco, Vinicius; Carollo, Daniela; Beers, Timothy C.

    2016-01-01

    A substantial fraction of low-metallicity stars in the Milky Way, the Carbon-Enhanced Metal-Poor (CEMP) stars, exhibit enhancements of their carbon-to-iron relative to the solar value ([C/Fe] > +0.7). They can be divided into several sub-classes, depending on the nature and degree of the observed enhancements of their neutron-capture elements, providing information on their likely progenitors. CEMP-s stars (which exhibit enhanced s-process elements) are thought to be enhanced by mass transfer from an evolved AGB companion, while CEMP-no stars (which exhibit no over-abundances of neutron-capture elements) appear to be associated with explosions of the very first generations of stars. High-resolution spectroscopic analyses are generally required in order to make these sub-classifications.Several recent studies have suggested the existence of bimodality in the distribution of absolute carbon abundances among CEMP stars -- most CEMP-no stars belong to a low-C band ((A(C) ˜ 6.5), while most CEMP-s stars reside on a high-C band (A(C) ˜ 8.25). The number of CEMP stars considered by individual studies is, however, quite small, so we have compiled all available high-resolution spectroscopic data for CEMP stars, in order to further investigate the existence of the claimed carbon bi-modality, and to consider what can be learned about the progenitors of CEMP-s and CEMP-no stars based on the observed distribution of A(C) on the individual plateaus.We acknowledge partial support from the grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation.

  11. The r-Process in Metal Poor Stars and Black Hole Formation

    SciTech Connect

    Boyd, R N; Famiano, M A; Meyer, B S; Motizuki, Y; Kajino, T; Roederer, I U

    2011-11-30

    Nucleosynthesis of heavy nuclei in metal-poor stars is generally ascribed to the r-process, as the abundance pattern in many such stars agrees with the inferred Solar r-process abundances. Nonetheless, a significant number of these stars do not share this r-process template. they suggest that many such stars have begun an r-process, but it was prevented from running to completion in more massive stars by collapse to black holes, creating a 'truncated r-process,' or 'tr-process'. The observed fraction of tr-process stars is found to be consistent with expectations from the initial mass function (IMF), and they suggest that an apparent sharp truncation observed at around mass 160 could result from a combination of collapses to black holes and the difficulty of observing the higher mass rare earths. They test the tr-process hypothesis with calculations that are terminated before all r-process trajectories have been ejected. These produce qualitative agreement with observation when both black hole collapse and observational realities are taken into account.

  12. Abundances of carbon-enhanced metal-poor stars as constraints on their formation

    NASA Astrophysics Data System (ADS)

    Hansen, C. J.; Nordström, B.; Hansen, T. T.; Kennedy, C. R.; Placco, V. M.; Beers, T. C.; Andersen, J.; Cescutti, G.; Chiappini, C.

    2016-04-01

    Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] =-2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims: Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods: Atmospheric parameters for our programme stars were estimated from a combination of V-K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12C/13C ratios. Results: Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (~10-40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions: The derived stellar abundances trace the formation

  13. CHEMICAL ANALYSIS OF THE NINTH MAGNITUDE CARBON-ENHANCED METAL-POOR STAR BD+44 Degree-Sign 493

    SciTech Connect

    Ito, Hiroko; Aoki, Wako; Beers, Timothy C.; Tominaga, Nozomu; Honda, Satoshi; Carollo, Daniela E-mail: beers@noao.edu E-mail: honda@kwasan.kyoto-u.ac.jp

    2013-08-10

    We present detailed chemical abundances for the bright carbon-enhanced metal-poor (CEMP) star BD+44 Degree-Sign 493, previously reported on by Ito et al. Our measurements confirm that BD+44 Degree-Sign 493 is an extremely metal-poor ([Fe/H] =-3.8) subgiant star with excesses of carbon and oxygen. No significant excesses are found for nitrogen and neutron-capture elements (the latter of which place it in the CEMP-no class of stars). Other elements that we measure exhibit abundance patterns that are typical for non-CEMP extremely metal-poor stars. No evidence for variations of radial velocity has been found for this star. These results strongly suggest that the carbon enhancement in BD+44 Degree-Sign 493 is unlikely to have been produced by a companion asymptotic giant-branch star and transferred to the presently observed star, nor by pollution of its natal molecular cloud by rapidly-rotating, massive, mega metal-poor ([Fe/H] < - 6.0) stars. A more likely possibility is that this star formed from gas polluted by the elements produced in a ''faint'' supernova, which underwent mixing and fallback, and only ejected small amounts of elements of metals beyond the lighter elements. The Li abundance of BD+44 Degree-Sign 493 (A(Li) = log (Li/H)+12 =1.0) is lower than the Spite plateau value, as found in other metal-poor subgiants. The upper limit on Be abundance (A(Be) = log (Be/H)+12 < - 1.8) is as low as those found for stars with similarly extremely-low metallicity, indicating that the progenitors of carbon- (and oxygen-) enhanced stars are not significant sources of Be, or that Be is depleted in metal-poor subgiants with effective temperatures of {approx}5400 K.

  14. The low Sr/Ba ratio on some extremely metal-poor stars

    NASA Astrophysics Data System (ADS)

    Spite, M.; Spite, F.; Bonifacio, P.; Caffau, E.; François, P.; Sbordone, L.

    2014-11-01

    Context. It has been noted that, in classical extremely metal-poor (EMP) stars, the abundance ratio of two well-observed neutron-capture elements, Sr and Ba, is always higher than [Sr/Ba] = -0.5, which is the value of the solar r-only process; however, a handful of EMP stars have recently been found with a very low Sr/Ba ratio. Aims: We try to understand the origin of this anomaly by comparing the abundance pattern of the elements in these stars and in the classical EMP stars. Methods: For a rigorous comparison with previous data, four stars with very low Sr/Ba ratios were observed and analyzed in the same way as in the First Stars program: analysis within LTE approximation through 1D (hydrostatic) model atmosphere, providing homogeneous abundances of nine neutron-capture elements. Results: In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in fact, found to be higher than the r-only solar ratio, so the star is discarded. The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved giants. They do not present a clear carbon enrichment, but in evolved giants C is partly burned into N, and owing to their high N abundance, they could still have initially been carbon-rich EMP stars (CEMP). The abundances of Na to Mg present similar anomalies to those in CEMP stars. The abundance patterns of the neutron-capture elements in the three stars are strikingly similar to a theoretical s-process pattern. This pattern could at first be attributed to pollution by a nearby AGB, but none of the stars presents a clear variation in the radial velocity indicating the presence of a companion. The stellar parameters seem to exclude any internal pollution in a TP-AGB phase for at least two of these stars. The possibility that the stars are early-AGB stars polluted during the core He flash does not seem compatible with the theory. Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (ID 077.D-0299(A) PI

  15. Binary properties of CH and carbon-enhanced metal-poor stars

    NASA Astrophysics Data System (ADS)

    Jorissen, A.; Van Eck, S.; Van Winckel, H.; Merle, T.; Boffin, H. M. J.; Andersen, J.; Nordström, B.; Udry, S.; Masseron, T.; Lenaerts, L.; Waelkens, C.

    2016-02-01

    The HERMES spectrograph installed on the 1.2-m Mercator telescope has been used to monitor the radial velocity of 13 low-metallicity carbon stars, among which seven carbon-enhanced metal-poor (CEMP) stars and six CH stars (including HIP 53522, a new member of the family, as revealed by a detailed abundance study). All stars but one show clear evidence for binarity. New orbits are obtained for eight systems. The sample covers an extended range in orbital periods, extending from 3.4 d (for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26, the longest known among barium, CH, and extrinsic S stars). Three systems exhibit low-amplitude velocity variations with periods close to 1 yr superimposed on a long-term trend. In the absence of an accurate photometric monitoring of these systems, it is not clear yet whether these variations are the signature of a very low-mass companion or of regular envelope pulsations. The period - eccentricity (P - e) diagram for the 40 low-metallicity carbon stars with orbits now available shows no difference between CH and CEMP-s stars (the latter corresponding to those CEMP stars enriched in s-process elements, as are CH stars). We suggest that they must be considered as one and the same family and that their different names only stem from historical reasons. Indeed, these two families have as well very similar mass-function distributions, corresponding to companions with masses in the range 0.5-0.7 M⊙, indicative of white-dwarf companions, adopting 0.8-0.9 M⊙ for the primary component. This result confirms that CH and CEMP-s stars obey the same mass-transfer scenario as their higher-metallicity analogues, barium stars. The P - e diagrams of barium, CH, and CEMP-s stars are indeed very similar. They reveal two different groups of systems: one with short orbital periods (P< 1000 d) and mostly circular or almost circular orbits, and another with longer period and eccentric (e> 0.1) orbits. These two groups either

  16. Metal-poor Stars Observed with the Magellan Telescope. II. Discovery of Four Stars with [Fe/H] <= -3.5

    NASA Astrophysics Data System (ADS)

    Placco, Vinicius M.; Frebel, Anna; Beers, Timothy C.; Christlieb, Norbert; Lee, Young Sun; Kennedy, Catherine R.; Rossi, Silvia; Santucci, Rafael M.

    2014-01-01

    We report on the discovery of seven low-metallicity stars selected from the Hamburg/ESO Survey, six of which are extremely metal-poor (EMP, [Fe/H] <= -3.0), with four having [Fe/H] <= -3.5. Chemical abundances or upper limits are derived for these stars based on high-resolution (R ~ 35,000) Magellan/MIKE spectroscopy, and are in general agreement with those of other very and extremely metal-poor stars reported in the literature. Accurate metallicities and abundance patterns for stars in this metallicity range are of particular importance for studies of the shape of the metallicity distribution function of the Milky Way's halo system, in particular for probing the nature of its low-metallicity tail. In addition, taking into account suggested evolutionary mixing effects, we find that six of the program stars (with [Fe/H] <= -3.35) possess atmospheres that were likely originally enriched in carbon, relative to iron, during their main-sequence phases. These stars do not exhibit overabundances of their s-process elements, and hence may be, within the error bars, additional examples of the so-called CEMP-no class of objects. Based on observations gathered with: The 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile; the Southern Astrophysical Research (SOAR) telescope (SO2011B-002), which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU); and the New Technology Telescope (NTT) of the European Southern Observatory (088.D-0344A), La Silla, Chile.

  17. A Survey of Proper-Motion Stars. XIV. Spectroscopic Binaries among Metal-poor Field Blue Stragglers

    NASA Astrophysics Data System (ADS)

    Carney, Bruce W.; Latham, David W.; Laird, John B.; Grant, Catherine E.; Morse, Jon A.

    2001-12-01

    We summarize the results from a program of monitoring the radial velocities of 10 metal-poor, high-velocity field stars whose colors are 0.01 to 0.13 mag bluer than main-sequence turnoffs of comparable-metallicity globular clusters. Two of the candidate halo blue stragglers (BD +72 94 and BD +40 1166) show no signs of velocity variability, one (HD 84937) shows only weak signs of variability, one (BD +25 1981) appears to be a very long-period binary, and six (BD -12 2669, HD 97916, HD 106516, BD +51 1817, G66-30, and G202-65) are single-lined spectroscopic binaries, with periods ranging from 167 to 844 days. Velocity coverage for the four candidates without orbital solutions ranges from 15.9 to 19.0 years. The orbital eccentricities are all low, e<0.30 and =0.11. Five of the six binary orbits have very low eccentricities, with =0.07. We have reanalyzed the velocity data from Preston & Sneden and have derived orbital solutions similar to theirs for 10 of the spectroscopic binaries among their ``blue metal-poor'' stars with [Fe/H]<=-0.6. We confirm their conclusion that the binary frequency is high; we find 47+/-10% if we include only the definite binaries with [Fe/H]<=-0.6. Our orbital solutions for the seven binaries with periods longer than 20 days all have low eccentricities, with e<=0.26 and =0.11. These orbital characteristics are very similar to the Ba II, CH, subgiant CH, and dwarf carbon stars, suggesting that mass transfer has been involved in their formation. Of the five binary stars in our program with published abundances of lithium, all have been found to be deficient (and one in beryllium as well). In contrast, two of the three apparently single stars have published lithium abundances and show no deficiency. The mass functions for the six binaries in our program and seven similar systems studied by Preston & Sneden are consistent with their unseen companions all being white dwarfs with M~0.55 Msolar and random orbital inclinations. Taking all

  18. High-resolution abundance analysis of very metal-poor r-I stars

    NASA Astrophysics Data System (ADS)

    Siqueira Mello, C.; Hill, V.; Barbuy, B.; Spite, M.; Spite, F.; Beers, T. C.; Caffau, E.; Bonifacio, P.; Cayrel, R.; François, P.; Schatz, H.; Wanajo, S.

    2014-05-01

    Context. Moderately r-process-enriched stars (r-I; +0.3 ≤ [Eu/Fe] ≤ +1.0) are at least four times as common as those that are greatly enriched in r-process elements (r-II; [Eu/Fe] > +1.0), and the abundances in their atmospheres are important tools for obtaining a better understanding of the nucleosynthesis processes responsible for the origin of the elements beyond the iron peak. Aims: The main aim of this work is to derive abundances for a sample of seven metal-poor stars with -3.4 ≤ [Fe/H] ≤ -2.4 classified as r-I stars, to understand the role of these stars for constraining the astrophysical nucleosynthesis event(s) that is (are) responsible for the production of the r-process, and to investigate whether they differ, in any significant way, from the r-II stars. Methods: We carried out a detailed abundance analysis based on high-resolution spectra obtained with the VLT/UVES spectrograph, using spectra in the wavelength ranges 3400-4500 Å, 6800-8200 Å, and 8700-10 000 Å, with resolving power R ~ 40 000 (blue arm) and R ~ 55 000 (red arm). The OSMARCS LTE 1D model atmosphere grid was employed, along with the spectrum synthesis code Turbospectrum. Results: We have derived abundances of the light elements Li, C, and N, the α-elements Mg, Si, S, Ca, and Ti, the odd-Z elements Al, K, and Sc, the iron-peak elements V, Cr, Mn, Fe, Co, and Ni, and the trans-iron elements from the first peak (Sr, Y, Zr, Mo, Ru, and Pd), the second peak (Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb), the third peak (Os and Ir, as upper limits), and the actinides (Th) regions. The results are compared with values for these elements for r-II and "normal" very and extremely metal-poor stars reported in the literature, ages based on radioactive chronometry are explored using different models, and a number of conclusions about the r-process and the r-I stars are presented. Hydrodynamical models were used for some elements, and general behaviors for the 3D corrections

  19. S-process in extremely metal-poor, low-mass stars

    NASA Astrophysics Data System (ADS)

    Cruz, M. A.; Serenelli, A.; Weiss, A.

    2013-11-01

    Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12C(p,γ)13N(β)13C(α,n)16O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims: We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods: We present evolutionary and post-processing s-process calculations of a 1 M⊙ stellar model with metallicities of Z = 0, 10-8, and 10-7. We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results: Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12C/^{13C} abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions: Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

  20. Simulating Metal-Poor and Metal-Free Star Formation in the Earliest Galaxies

    NASA Astrophysics Data System (ADS)

    Safranek-Shrader, Chalence

    2014-01-01

    The end of the cosmic dark ages was brought about by the formation of the first stars and galaxies. Since this epoch is currently outside of observational reach, numerical studies are key in understanding this uncharted cosmic epoch. In this dissertation talk, I will describe my work using high-resolution, zoom-in simulations to understand the formation of these earliest stellar associations in a cosmological setting. The overarching focus will be on the fragmentation of collapsing gas and how this process is moderated by the gas chemistry, radiation fields, and realistic cosmological initial conditions. A key aspect of this work has been the development of sophisticated physics modules for the hydrodynamics code FLASH, including non-equilibrium chemistry, radiative transfer schemes, and sink particles. I will begin by describing how more moderate mass Population III stars ended their lives with a relatively quick heavy-element enrichment of their host dark matter halos, resulting in prompt Population II star formation. The introduction of metals from the first supernovae is believed to induce a star formation mode transition from high to low characteristic mass. I will show how the fragmentation of such metal enriched gas depends strongly on the metallicity, with fragmentation setting in when gas hits the CMB temperature floor. If present, an H2 photo-dissociating Lyman-Werner radiation background can delay the formation of the first stars and potentially result in clustered metal-free star formation in more massive, self-shielding halos at lower redshift. I will present results from recent simulations that follow the collapse and fragmentation of the first dust enriched gas to high densities (n ~ 10^14 cm^-3), analyzing the interplay of dust cooling with a CMB temperature floor and gauging the effect that dust heating from protostellar feedback has on the outcome of star formation. Finally, I will discuss this work’s implications for next

  1. New detections of arsenic, selenium, and other heavy elements in two metal-poor stars

    SciTech Connect

    Roederer, Ian U.; Schatz, Hendrik; Beers, Timothy C.; Lawler, James E.; Cowan, John J.; Frebel, Anna; Ivans, Inese I.; Sneden, Christopher; Sobeck, Jennifer S.

    2014-08-10

    We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to obtain new high-quality spectra covering the 1900 ≤λ ≤ 2360 Å wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been detected previously in either star. Abundances derived for Ge I, Te I, Os II, and Pt I confirm those derived from lines at longer wavelengths. We also derive upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean [As/Fe] ratio derived from these two stars and five others in the literature is unchanged over the metallicity range –2.8 < [Fe/H] <–0.6, ([As/Fe]) = +0.28 ± 0.14 (σ = 0.36 dex). The mean [Se/Fe] ratio derived from these two stars and six others in the literature is also constant, ([Se/Fe]) = +0.16 ± 0.09 (σ = 0.26 dex). The As and Se abundances are enhanced relative to a simple extrapolation of the iron-peak abundances to higher masses, suggesting that this mass region (75 ≤A ≤ 82) may be the point at which a different nucleosynthetic mechanism begins to dominate the quasi-equilibrium α-rich freezeout of the iron peak. ([Cu II/Cu I]) = +0.56 ± 0.23 in HD 108317 and HD 128279, and we infer that lines of Cu I may not be formed in local thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and [Os/Fe] ratios are also derived from neutral and ionized species, and each ratio pair agrees within the mutual uncertainties, which range from 0.15 to 0.52 dex.

  2. Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP-s stars

    NASA Astrophysics Data System (ADS)

    Abate, C.; Pols, O. R.; Izzard, R. G.; Karakas, A. I.

    2015-09-01

    Many of the carbon-enhanced metal-poor (CEMP) stars that we observe in the Galactic halo are found in binary systems and show enhanced abundances of elements produced by the slow neutron-capture process (s-elements). The origin of the peculiar chemical abundances of these CEMP-s stars is believed to be accretion in the past of enriched material from a primary star in the asymptotic giant branch (AGB) phase of its evolution.We investigate the mechanism of mass transfer and the process of nucleosynthesis in low-metallicity AGB stars by modelling the binary systems in which the observed CEMP-s stars were formed.For this purpose we compare a sample of 67 CEMP-s stars with a grid of binary stars generated by our binary evolution and nucleosynthesis model. We classify our sample CEMP-s stars in three groups based on the observed abundance of europium. In CEMP-s/r stars the europium-to-iron ratio is more than ten times higher than in the Sun, whereas it is lower than this threshold in CEMP-s/nr stars. No measurement of europium is currently available for CEMP-s/ur stars.On average our models reproduce the abundances observed in CEMP-s/nr stars well, whereas in CEMP-s/r stars and CEMP-s/ur stars the abundances of the light-s elements (strontium, yttrium, zirconium) are systematically overpredicted by our models, and in CEMP-s/r stars the abundances of the heavy-s elements (barium, lanthanum) are underestimated. In all stars our modelled abundances of sodium overestimate the observations. This discrepancy is reduced only in models that underestimate the abundances of most of the s-elements. Furthermore, the abundance of lead is underpredicted in most of our model stars, independent of the metallicity. These results point to the limitations of our AGB nucleosynthesis model, particularly in the predictions of the element-to-element ratios. In our models CEMP-s stars are typically formed in wide systems with periods above 10 000 days, while most of the observed CEMP-s stars

  3. High-Resolution Spectroscopy of Extremely Metal-Poor Stars from SDSS/SEGUE. II. Binary Fraction

    NASA Astrophysics Data System (ADS)

    Aoki, Wako; Suda, Takuma; Beers, Timothy C.; Honda, Satoshi

    2015-02-01

    The fraction of binary systems in various stellar populations of the Galaxy and the distribution of their orbital parameters are important but not well-determined factors in studies of star formation, stellar evolution, and Galactic chemical evolution. While observational studies have been carried out for a large sample of nearby stars, including some metal-poor Population II stars, almost no constraints on the binary nature for extremely metal-poor (EMP; [Fe/H] \\lt -3.0) stars have yet been obtained. Here we investigate the fraction of double-lined spectroscopic binaries and carbon-enhanced metal-poor (CEMP) stars, many of which could have formed as pairs of low-mass and intermediate-mass stars, to estimate the lower limit of the fraction of binary systems having short periods. The estimate is based on a sample of very metal-poor stars selected from the Sloan Digital Sky Survey and observed at high spectral resolution in a previous study by Aoki et al. That survey reported 3 double-lined spectroscopic binaries and 11 CEMP stars, which we consider along with a sample of EMP stars from the literature compiled in the SAGA database. We have conducted measurements of the velocity components for stacked absorption features of different spectral lines for each double-lined spectroscopic binary. Our estimate indicates that the fraction of binary stars having orbital periods shorter than 1000 days is at least 10%, and possibly as high as 20% if the majority of CEMP stars are formed in such short-period binaries. This result suggests that the period distribution of EMP binary systems is biased toward short periods, unless the binary fraction of low-mass EMP stars is significantly higher than that of other nearby stars.

  4. High-resolution spectroscopy of extremely metal-poor stars from SDSS/Segue. II. Binary fraction

    SciTech Connect

    Aoki, Wako; Suda, Takuma; Beers, Timothy C.; Honda, Satoshi E-mail: takuma.suda@nao.ac.jp E-mail: honda@nhao.jp

    2015-02-01

    The fraction of binary systems in various stellar populations of the Galaxy and the distribution of their orbital parameters are important but not well-determined factors in studies of star formation, stellar evolution, and Galactic chemical evolution. While observational studies have been carried out for a large sample of nearby stars, including some metal-poor Population II stars, almost no constraints on the binary nature for extremely metal-poor (EMP; [Fe/H] <−3.0) stars have yet been obtained. Here we investigate the fraction of double-lined spectroscopic binaries and carbon-enhanced metal-poor (CEMP) stars, many of which could have formed as pairs of low-mass and intermediate-mass stars, to estimate the lower limit of the fraction of binary systems having short periods. The estimate is based on a sample of very metal-poor stars selected from the Sloan Digital Sky Survey and observed at high spectral resolution in a previous study by Aoki et al. That survey reported 3 double-lined spectroscopic binaries and 11 CEMP stars, which we consider along with a sample of EMP stars from the literature compiled in the SAGA database. We have conducted measurements of the velocity components for stacked absorption features of different spectral lines for each double-lined spectroscopic binary. Our estimate indicates that the fraction of binary stars having orbital periods shorter than 1000 days is at least 10%, and possibly as high as 20% if the majority of CEMP stars are formed in such short-period binaries. This result suggests that the period distribution of EMP binary systems is biased toward short periods, unless the binary fraction of low-mass EMP stars is significantly higher than that of other nearby stars.

  5. Abundance analysis of SDSS J134338.67+484426.6; an extremely metal-poor star from the MARVELS pre-survey

    NASA Astrophysics Data System (ADS)

    Susmitha Rani, A.; Sivarani, T.; Beers, T. C.; Fleming, S.; Mahadevan, S.; Ge, J.

    2016-05-01

    We present an elemental-abundance analysis of an extremely metal-poor (EMP; [Fe/H] <-3.0) star, SDSS J134338.67+484426.6, identified during the course of the Multi-object Apache Point Observatory Radial Velocity Exoplanet Large-area Survey spectroscopic pre-survey of some 20 000 stars to identify suitable candidates for exoplanet searches. This star, with an apparent magnitude V = 12.14, is the lowest metallicity star found in the pre-survey, and is one of only ˜20 known EMP stars that are this bright or brighter. Our high-resolution spectroscopic analysis shows that this star is a subgiant with [Fe/H] = -3.42, having `normal' carbon and no enhancement of neutron-capture abundances. Strontium is underabundant, [Sr/Fe] = -0.47, but the derived lower limit on [Sr/Ba] indicates that Sr is likely enhanced relative to Ba. This star belongs to the sparsely populated class of α-poor EMP stars that exhibit low ratios of [Mg/Fe], [Si/Fe], and [Ca/Fe] compared to typical halo stars at similar metallicity. The observed variations in radial velocity from several epochs of (low- and high-resolution) spectroscopic follow-up indicate that SDSS J134338.67+484426.6 is a possible long-period binary. We also discuss the abundance trends in EMP stars for r-process elements, and compare with other magnesium-poor stars.

  6. Exploring the origin of lithium, carbon, strontium, and barium with four new ultra metal-poor stars

    SciTech Connect

    Hansen, T.; Hansen, C. J.; Christlieb, N. E-mail: cjhansen@lsw.uni-heidelberg.de; and others

    2014-06-01

    We present an elemental abundance analysis for four newly discovered ultra metal-poor stars from the Hamburg/ESO survey, with [Fe/H] ≤ –4. Based on high-resolution, high signal-to-noise spectra, we derive abundances for 17 elements in the range from Li to Ba. Three of the four stars exhibit moderate to large overabundances of carbon, but have no enhancements in their neutron-capture elements. The most metal-poor star in the sample, HE 0233–0343 ([Fe/H] = –4.68), is a subgiant with a carbon enhancement of [C/Fe] = +3.5, slightly above the carbon-enhancement plateau suggested by Spite et al. No carbon is detected in the spectrum of the fourth star, but the quality of its spectrum only allows for the determination of an upper limit on the carbon abundance ratio of [C/Fe] < +1.7. We detect lithium in the spectra of two of the carbon-enhanced stars, including HE 0233–0343. Both stars with Li detections are Li-depleted, with respect to the Li plateau for metal-poor dwarfs found by Spite and Spite. This suggests that whatever site(s) produced C either do not completely destroy lithium, or that Li has been astrated by early-generation stars and mixed with primordial Li in the gas that formed the stars observed at present. The derived abundances for the α elements and iron-peak elements of the four stars are similar to those found in previous large samples of extremely and ultra metal-poor stars. Finally, a large spread is found in the abundances of Sr and Ba for these stars, possibly influenced by enrichment from fast rotating stars in the early universe.

  7. A search for stars of very low metal abundance. VI. Detailed abundances of 313 metal-poor stars

    SciTech Connect

    Roederer, Ian U.; Preston, George W.; Thompson, Ian B.; Shectman, Stephen A.; Burley, Gregory S.; Kelson, Daniel D.; Sneden, Christopher

    2014-06-01

    We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10 yr observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coudé Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by ≈0.25 dex for red giants and ≈0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] ≤–3.5, 84 stars with [Fe/H] ≤–3.0, and 210 stars with [Fe/H] ≤–2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] ≤–2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.

  8. Footprints of the weak s-process in the carbon-enhanced metal-poor star ET0097

    NASA Astrophysics Data System (ADS)

    Yang, Guochao; Li, Hongjie; Liu, Nian; Cui, Wenyuan; Liang, Yanchun; Zhang, Bo

    2016-09-01

    Historically, the weak s-process contribution to metal-poor stars is thought to be extremely small, due to the effect of the secondary-like nature of the neutron source 22Ne(α , n)25Mg in massive stars, which means that metal-poor "weak s-process stars" could not be found. ET0097 is the first observed carbon-enhanced metal-poor (CEMP) star in the Sculptor dwarf spheroidal galaxy. Because C is enriched and the elements heavier than Ba are not overabundant, ET0097 can be classified as a CEMP-no star. However, this star shows overabundances of lighter n-capture elements (i.e., Sr, Y and Zr). In this work, having adopted the abundance decomposition approach, we investigate the astrophysical origins of the elements in ET0097. We find that the light elements and iron-peak elements (from O to Zn) of the star mainly originate from the primary process of massive stars and the heavier n-capture elements (heavier than Ba) mainly come from the main r-process. However, the lighter n-capture elements such as Sr, Y and Zr should mainly come from the primary weak s-process. The contributed fractions of the primary weak s-process to the Sr, Y and Zr abundances of ET0097 are about 82 %, 84 % and 58 % respectively, suggesting that the CEMP star ET0097 should have the footprints of the weak s-process. The derived result should be a significant evidence that the weak s-process elements can be produced in metal-poor massive stars.

  9. Deriving Stellar Effective Temperatures of Metal-poor Stars with the Excitation Potential Method

    NASA Astrophysics Data System (ADS)

    Frebel, Anna; Casey, Andrew R.; Jacobson, Heather R.; Yu, Qinsi

    2013-05-01

    It is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures ("excitation temperatures") of metal-poor stars based on a literature sample with -3.3 < [Fe/H] < -2.5. Excitation temperatures were determined from Fe I line abundances in high-resolution optical spectra in the wavelength range of ~3700-~7000 Å, although shorter wavelength ranges, up to 4750-6800 Å, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it brings the excitation temperatures in good agreement with photometrically derived values. The modified temperature also influences other stellar parameters, as the Fe I-Fe II ionization balance is simultaneously used to determine the surface gravity, while also forcing no abundance trend on the absorption line strengths to obtain the microturbulent velocity. As a result of increasing the temperature, the often too low gravities and too high microturbulent velocities in red giants become higher and lower, respectively. Our adjustment scheme thus continues to build on the advantage of deriving temperatures from spectroscopy alone, independent of reddening, while at the same time producing stellar chemical abundances that are more straightforwardly comparable to studies based on photometrically derived temperatures. Hence, our method may prove beneficial for comparing different studies in the literature as well as the many high-resolution stellar spectroscopic surveys that are or will be carried out in the next few years. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  10. DERIVING STELLAR EFFECTIVE TEMPERATURES OF METAL-POOR STARS WITH THE EXCITATION POTENTIAL METHOD

    SciTech Connect

    Frebel, Anna; Casey, Andrew R.; Jacobson, Heather R.; Yu Qinsi

    2013-05-20

    It is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures (''excitation temperatures'') of metal-poor stars based on a literature sample with -3.3 < [Fe/H] < -2.5. Excitation temperatures were determined from Fe I line abundances in high-resolution optical spectra in the wavelength range of {approx}3700-{approx}7000 A, although shorter wavelength ranges, up to 4750-6800 A, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it brings the excitation temperatures in good agreement with photometrically derived values. The modified temperature also influences other stellar parameters, as the Fe I-Fe II ionization balance is simultaneously used to determine the surface gravity, while also forcing no abundance trend on the absorption line strengths to obtain the microturbulent velocity. As a result of increasing the temperature, the often too low gravities and too high microturbulent velocities in red giants become higher and lower, respectively. Our adjustment scheme thus continues to build on the advantage of deriving temperatures from spectroscopy alone, independent of reddening, while at the same time producing stellar chemical abundances that are more straightforwardly comparable to studies based on photometrically derived temperatures. Hence, our method may prove beneficial for comparing different studies in the literature as well as the many high-resolution stellar spectroscopic surveys that are or will be carried out in the next few years.

  11. THE OXYGEN ABUNDANCE OF THE ULTRA-METAL-POOR STAR HE 0557-4840

    SciTech Connect

    Norris, John E.; Bessell, M. S.; Asplund, M.; Christlieb, N.; Eriksson, K.; Korn, A. J.

    2012-07-10

    We present a high-resolution ultraviolet (UV) spectrum of the ultra-metal-poor (UMP) carbon-enhanced red giant HE 0557-4840 (T{sub eff}/log g/[Fe/H] = 4900/2.2/-4.8). Combining these data with earlier observations, the radial velocity is 212.0 {+-} 0.4 km s{sup -1}, with no evidence of variability during 2006 February to 2007 December. One-dimensional (1D) LTE model-atmosphere analysis of UV Fe and CH lines confirms the iron and carbon abundances obtained previously ([Fe/H] = -4.8 and [C/Fe]{sub 1D} = +1.7), and places a more stringent limit on nitrogen abundance of [N/Fe]{sub 1D} < +1.0. Analysis of the UV OH lines yields [O/Fe]{sub 1D} = +2.3 {+-} 0.4. When corrections are made for three-dimensional (3D) effects we obtain [C/Fe]{sub 3D} = +1.1, [N/Fe]{sub 3D} < +0.1, and [O/Fe]{sub 3D} +1.4. Comparison of the abundances of HE 0557-4840 with those of supernova models of Nomoto et al. and Joggerst et al. suggests that none is able to explain fully the observed abundance pattern. For HE 0557-4840, the Frebel et al. transition discriminant D{sub trans}(log(10{sup [C/H]} + 0.3 Multiplication-Sign 10{sup [O/H]}) = -3.4 {+-} 0.2, consistent with fine-structure transitions of C II and O I being a major cooling mechanism of star-forming regions at the earliest times. Of the four stars known to have [Fe/H] {approx}< -4.3, three are strongly carbon and oxygen enhanced. If the suggestion by Caffau et al. that SDSS J102915+172927 ([Fe/H] = -4.7) does not belong to the class of C-rich, O-rich, UMP stars is supported by future similar discoveries, one will need to consider multiple channels for the production of stars having [Fe/H] {approx}< -4.3.

  12. BERYLLIUM AND ALPHA-ELEMENT ABUNDANCES IN A LARGE SAMPLE OF METAL-POOR STARS

    SciTech Connect

    Boesgaard, Ann Merchant; Rich, Jeffrey A.; Levesque, Emily M.; Bowler, Brendan P. E-mail: jrich@ifa.hawaii.edu E-mail: bpbowler@ifa.hawaii.edu

    2011-12-20

    The light elements, Li, Be, and B, provide tracers for many aspects of astronomy including stellar structure, Galactic evolution, and cosmology. We have made observations of Be in 117 metal-poor stars ranging in metallicity from [Fe/H] = -0.5 to -3.5 with Keck I/HIRES. Our spectra are high resolution ({approx}42,000) and high signal to noise (the median is 106 per pixel). We have determined the stellar parameters spectroscopically from lines of Fe I, Fe II, Ti I, and Ti II. The abundances of Be and O were derived by spectrum synthesis techniques, while abundances of Fe, Ti, and Mg were found from many spectral line measurements. There is a linear relationship between [Fe/H] and A(Be) with a slope of +0.88 {+-} 0.03 over three orders of magnitude in [Fe/H]. We find that Be is enhanced relative to Fe; [Be/Fe] is +0.40 near [Fe/H] {approx}-3.3 and drops to 0.0 near [Fe/H] {approx}-1.7. For the relationship between A(Be) and [O/H], we find a gradual change in slope from 0.69 {+-} 0.13 for the Be-poor/O-poor stars to 1.13 {+-} 0.10 for the Be-rich/O-rich stars. Inasmuch as the relationship between [Fe/H] and [O/H] seems robustly linear (slope = +0.75 {+-} 0.03), we conclude that the slope change in Be versus O is due to the Be abundance. Much of the Be would have been formed in the vicinity of Type II supernova (SN II) in the early history of the Galaxy and by Galactic cosmic-ray (GCR) spallation in the later eras. Although Be is a by-product of CNO, we have used Ti and Mg abundances as alpha-element surrogates for O in part because O abundances are rather sensitive to both stellar temperature and surface gravity. We find that A(Be) tracks [Ti/H] very well with a slope of 1.00 {+-} 0.04. It also tracks [Mg/H] very well with a slope of 0.88 {+-} 0.03. We have kinematic information on 114 stars in our sample and they divide equally into dissipative and accretive stars. Almost the full range of [Fe/H] and [O/H] is covered in each group. There are distinct differences in

  13. Fe-Group Elements in the Metal-Poor Star HD 84937: Abundances and their Implications

    NASA Astrophysics Data System (ADS)

    Sneden, Chris; Cowan, John J.; Kobayashi, Chiaki; Pignatari, Marco; Lawler, James E.; Den Hartog, Elizabeth; Wood, Michael P.

    2016-01-01

    We have derived accurate relative abundances of the Fe-group elements Sc through Zn in the very metal-poor main-sequence turnoff star HD 84937. For this study we analyzed high resolution, high signal-to-noise HST/STIS and VLT/UVES spectra over a total wavelength range 2300-7000 Å. We employed only recent or newly-applied reliable laboratory transition data for all species. Abundances from more than 600 lines of non-Fe species were combined with about 550 Fe lines in HD 84937 to yield abundance ratios of high precision. From parallel analyses of solar photospheric spectra we also derived new solar abundances of these elements. This in turn yielded internally-consistent relative HD 84937 abundances with respect to the Sun. For seven of the ten Fe-group elements the HD 84937 abundances were from both neutral and ionized transitions. In all of these cases the neutral and ionized species yield the same abundances within the measurement uncertainties. Therefore standard Saha ionization balance appears to hold in the HD 84937 atmosphere. We derived metallicity [Fe/H] = -2.32 with sample standard deviation of 0.06. Solid evidence is seen for departures from the solar abundance mix in HD 84937, for example [Co/Fe] = +0.14, [Cu/Fe] = -0.83, and <[Sc,Ti,V/Fe]> = +0.31. Combining our Sc, Ti, and V abundances for this star with those from large-sample spectroscopic surveys suggests that these elements are positively correlated in stars with [Fe/H] < -2. HD 84937 is unusually enriched in Sc, Ti, and V. Our analysis strongly suggests that different types of supernovae with a large scatter of explosion energies and asymmetries contributed to the creation of the Fe-group elements early in the Galaxy's history.This work has been supported in part by NASA grant NNX10AN93G (J.E.L.), by NSF grants AST-1211055 (J.E.L.), AST-1211585 (C.S.), PHY-1430152 (through JINA, J.J.C. and M.P.), EU MIRGCT-2006-046520 (M.P.), and by the ``Lendlet-2014'' Programme of the Hungarian Academy of

  14. A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-poor Stars

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Preston, George W.; Thompson, Ian B.; Shectman, Stephen A.; Sneden, Christopher; Burley, Gregory S.; Kelson, Daniel D.

    2014-06-01

    We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10 yr observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coudé Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by ≈0.25 dex for red giants and ≈0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] <=-3.5, 84 stars with [Fe/H] <=-3.0, and 210 stars with [Fe/H] <=-2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] <=-2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and The McDonald Observatory of The University of Texas at Austin. The Hobby-Eberly Telescope is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University

  15. HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS FROM SDSS/SEGUE. I. ATMOSPHERIC PARAMETERS AND CHEMICAL COMPOSITIONS

    SciTech Connect

    Aoki, Wako; Suda, Takuma; Beers, Timothy C.; Lee, Young Sun; Honda, Satoshi; Ito, Hiroko; Takada-Hidai, Masahide; Frebel, Anna; Fujimoto, Masayuki Y.; Carollo, Daniela; Sivarani, Thirupathi E-mail: takuma.suda@nao.ac.jp E-mail: lee@pa.msu.edu E-mail: hidai@apus.rh.u-tokai.ac.jp E-mail: fujimoto@astro1.sci.hokudai.ac.jp E-mail: sivarani@iiap.res.in

    2013-01-01

    Chemical compositions are determined based on high-resolution spectroscopy for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). High-resolution spectra with moderate signal-to-noise (S/N) ratios were obtained with the High Dispersion Spectrograph of the Subaru Telescope. Most of the sample (approximately 80%) are main-sequence turnoff stars, including dwarfs and subgiants. Four cool main-sequence stars, the most metal-deficient such stars known, are included in the remaining sample. Good agreement is found between effective temperatures estimated by the SEGUE stellar parameter pipeline, based on the SDSS/SEGUE medium-resolution spectra, and those estimated from the broadband (V - K){sub 0} and (g - r){sub 0} colors. Our abundance measurements reveal that 70 stars in our sample have [Fe/H] < -3, adding a significant number of EMP stars to the currently known sample. Our analyses determine the abundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe] > +0.7) among the 25 giants in our sample is as high as 36%, while only a lower limit on the fraction (9%) is estimated for turnoff stars. This paper is the first of a series of papers based on these observational results. The following papers in this series will discuss the higher-resolution and higher-S/N observations of a subset of this sample, the metallicity distribution function, binarity, and correlations between the chemical composition and kinematics of extremely metal-poor stars.

  16. Behavior of [S/Fe] in Very Metal-Poor Stars from the S I 1.046 µm Lines Revisited

    NASA Astrophysics Data System (ADS)

    Takeda, Yoichi; Takada-Hidai, Masahide

    2012-04-01

    With an aim to establish how the [S/Fe] ratios behave in the very low metallicity regime down to [Fe/H] ˜ -3, we conducted a non-LTE analysis of near-IR S I triplet lines (multiplet 3) at 10455-10459 Å for a dozen very metal-poor stars (-3.2 ≲ [Fe/H] ≲ -1.9) based on new observational data obtained with IRCS+AO188 of the Subaru Telescope. It turned out that the resulting [S/ Fe] values are only moderately supersolar at [S/Fe] ˜ +0.2-0.5, irrespective of the metallicity. While this ``flat'' tendency is consistent with a trend recently corroborated by Spite et al. (2011, A&A, 528, A9) based on the S I 9212/9228/9237 lines (multiplet 1), it disaffirms the possibility of a conspicuously large [S/Fe] (up to ˜ +0.8) at [Fe/H] ˜ -3 that we once suggested in our first report on the S abundances of disk/halo stars using S I 10455-10459 lines (Takeda & Takada-Hidai 2011, PASJ, 63, S537). Given these new observational facts, we withdraw our previous argument, since we consider that [S/Fe]'s of some most metal-poor objects were overestimated in that paper; the likely cause for this failure is also discussed.

  17. SEARCHES FOR METAL-POOR STARS FROM THE HAMBURG/ESO SURVEY USING THE CH G BAND

    SciTech Connect

    Placco, Vinicius M.; Rossi, Silvia; Kennedy, Catherine R.; Beers, Timothy C.; Lee, Young Sun; Christlieb, Norbert; Sivarani, Thirupathi; Reimers, Dieter; Wisotzki, Lutz

    2011-12-15

    We describe a new method to search for metal-poor candidates from the Hamburg/ESO objective-prism survey (HES) based on identifying stars with apparently strong CH G-band strengths for their colors. The hypothesis we exploit is that large overabundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. The selection was made by considering two line indices in the 4300 A region, applied directly to the low-resolution prism spectra. This work also extends a previously published method by adding bright sources to the sample. The spectra of these stars suffer from saturation effects, compromising the index calculations and leading to an undersampling of the brighter candidates. A simple numerical procedure, based on available photometry, was developed to correct the line indices and overcome this limitation. Visual inspection and classification of the spectra from the HES plates yielded a list of 5288 new metal-poor (and by selection, carbon-rich) candidates, which are presently being used as targets for medium-resolution spectroscopic follow-up. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for 117 of the first candidates, based on follow-up medium-resolution spectra obtained with the SOAR 4.1 m and Gemini 8 m telescopes. We demonstrate that our new method improves the metal-poor star fractions found by our pilot study by up to a factor of three in the same magnitude range, as compared with our pilot study based on only one CH G-band index. Our selection scheme obtained roughly a 40% success rate for identification of stars with [Fe/H] <-1.0; the primary contaminant is late-type stars with near-solar abundances and, often, emission line cores that filled in the Ca II K line on the prism spectrum. Because the selection is based on carbon, we greatly increase the numbers of known carbon-enhanced metal-poor stars from the HES with intermediate metallicities -2

  18. Metallicity inhomogeneities in local star-forming galaxies as a sign of recent metal-poor gas accretion

    SciTech Connect

    Sánchez Almeida, J.; Morales-Luis, A. B.; Muñoz-Tuñón, C.; Méndez-Abreu, J.; Elmegreen, D. M.; Elmegreen, B. G. E-mail: abml@iac.es E-mail: elmegreen@vassar.edu E-mail: jma20@st-andrews.ac.uk

    2014-03-01

    We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show ≅0.5 dex metallicity decrements in inner regions with enhanced star formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sánchez Almeida et al., and was interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications. (1) It proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one-tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star formation activity in the local universe. We argue that the star formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, is naturally interpreted as side effects of pristine gas infall. Alternatives to the metal-poor gas accretion are examined as well.

  19. Stellar Dust Production in Chemically Primitive Environments: Infrared Lightcurves and Mass Loss in Extremely Metal-poor AGB Stars

    NASA Astrophysics Data System (ADS)

    Sonneborn, George

    In their final stage of evolution, asymptotic giant branch (AGB) stars inject a substantial amount of dust into the surrounding interstellar medium, potentially dominating the total stellar dust budgets of their host galaxies. However, stellar models conflict over whether metal-poor AGB stars can condense enough dust to drive a strong stellar wind, so it is unclear what role AGB stars play in the early Universe compared to other dust sources, e.g., in high-redshift quasars that show evidence for massive dust reservoirs. Empirically, AGB stars that are massive enough to contribute in the early Universe are only well studied in the Milky Way and the nearby Magellanic Clouds; all three environments are relatively metal-rich and thus unlikely to be representative of high-redshift AGB stars. This lack of observations of metal-poor AGB stars motivated the survey of DUST in Nearby Galaxies with Spitzer (DUSTiNGS), which imaged 50 nearby dwarf galaxies in the infrared and identified 526 dusty "extreme" AGB stars. The DUSTiNGS stars confirm that dust can form at metallicities as low as 0.008 solar, more than an order of magnitude lower than had been previously observed. However, very little is known about the DUSTiNGS stars; among the unknowns are the photospheric chemistries, stellar masses, temperatures, luminosities, pulsation periods and amplitudes, dust-production rates, and even their statuses as bona fide AGB stars. To eliminate these unknowns, we were awarded 56 hours of Priority 1 observing time in Spitzer's cycle 11 to obtain 6 new epochs of imaging for a subset of the DUSTiNGS variables over an 18 month baseline. These will be the first infrared light curves of metal-poor, dust-producing AGB stars, allowing us to study the influence of metallicity on pulsation and dust production. Combined with additional archival data, our cycle-11 Spitzer program will allow estimates of all of the parameters listed above, enabling the first direct comparisons to models of AGB

  20. FORMATION OF CARBON-ENHANCED METAL-POOR STARS IN THE PRESENCE OF FAR-ULTRAVIOLET RADIATION

    SciTech Connect

    Bovino, S.; Schleicher, D. R. G.; Latif, M. A.; Grassi, T.

    2014-08-01

    Recent discoveries of carbon-enhanced metal-poor stars like SMSS J031300.36–670839.3 provide increasing observational insights into the formation conditions of the first second-generation stars in the universe, reflecting the chemical conditions after the first supernova explosion. Here, we present the first cosmological simulations with a detailed chemical network including primordial species as well as C, C{sup +}, O, O{sup +}, Si, Si{sup +}, and Si{sup 2+} following the formation of carbon-enhanced metal-poor stars. The presence of background UV flux delays the collapse from z = 21 to z = 15 and cool the gas down to the cosmic microwave background temperature for a metallicity of Z/Z {sub ☉} = 10{sup –3}. This can potentially lead to the formation of lower-mass stars. Overall, we find that the metals have a stronger effect on the collapse than the radiation, yielding a comparable thermal structure for large variations in the radiative background. We further find that radiative backgrounds are not able to delay the collapse for Z/Z {sub ☉} = 10{sup –2} or a carbon abundance as in SMSS J031300.36–670839.3.

  1. Is HE 0107-5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars

    NASA Astrophysics Data System (ADS)

    Suda, Takuma; Aikawa, Masayuki; Machida, Masahiro N.; Fujimoto, Masayuki Y.; Iben, Icko, Jr.

    2004-08-01

    We discuss the origin of HE 0107-5240, which, with a metallicity of [Fe/H]=-5.3, is the most iron-poor star yet observed. Its discovery has an important bearing on the question of the observability of first-generation stars in our universe. In common with other stars of very small metallicity (-4<~[Fe/H]<~-2.5), HE 0107-5240 shows a peculiar abundance pattern, including large enhancements of C, N, and O, and a more modest enhancement of Na. The observed abundance pattern can be explained by nucleosynthesis and mass transfer in a first-generation binary star, which, after birth, accretes matter from a primordial cloud mixed with the ejectum of a supernova. We elaborate the binary scenario on the basis of our current understanding of the evolution and nucleosynthesis of extremely metal-poor, low-mass model stars and discuss the possibility of discriminating this scenario from others. In our picture, iron-peak elements arise in surface layers of the component stars by accretion of gas from the polluted primordial cloud, pollution occurring after the birth of the binary. To explain the observed C, N, O, and Na enhancements, as well as the 12C/ 13C ratio, we suppose that the currently observed star, once the secondary in a binary, accreted matter from a chemically evolved companion, which is now a white dwarf. To estimate the abundances in the matter transferred in the binary, we rely on the results of computations of model stars constructed with up-to-date input physics. Nucleosynthesis in a helium-flash-driven convective zone into which hydrogen has been injected is followed, allowing us to explain the origin in the primary of the observed O and Na enrichments and to discuss the abundances of s-process elements. From the observed abundances, we conclude that HE 0107-5240 has evolved from a wide binary (of initial separation ~20 AU) with a primary of initial mass in the range 1.2-3 Msolar. On the assumption that the system now consists of a white dwarf and a red giant

  2. Standard Stars for the High-velocity and Metal-poor project at San Pedro Mártir

    NASA Astrophysics Data System (ADS)

    Schuster, W. J.; Parrao, L.; Contreras, M. E.

    2016-04-01

    The main documentation for the primary and secondary standard stars used in the high-velocity and metal-poor stars project is presented. Observations were taken using the Strömgren-Crawford, uvby-Hβ, 6-channel, spectrophotometric equipment with the H.L. Johnson 1.5-m telescope at the Observatorio Astronómico Nacional, San Pedro Mártir, between 1987 and 2007. Standard photometric values from the literature are reported for our standard stars, as well as transformed standard values, errors in the instrumental system, the transformation coefficients obtained for the standard system, the transformation errors, and the methods used to obtain such photometric observations and their standard transformations.

  3. A New View of the Dwarf Spheroidal Satellites of the Milky Way From VLT/FLAMES: Where are the Very Metal Poor Stars?

    SciTech Connect

    Helmi, Amina; Irwin, M.J.; Tolstoy, E.; Battaglia, G.; Hill, V.; Jablonka, P.; Venn, K.; Shetrone, M.; Letarte, B.; Arimoto, N.; Abel, T.; Francois, P.; Kaufer, A.; Primas, F.; Sadakane, K.; Szeifert, T.; /Kapteyn Astron. Inst., Groningen /Cambridge U., Inst. of Astron. /Meudon Observ. /LASTRO Observ. /Victoria U. /Texas U., McDonald Observ. /Tokyo, Astron. Observ. /KIPAC, Menlo Park /European Southern Obs., Chile /European Southern Observ. /Osaka Kyoiku U.

    2006-11-20

    As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph): Sculptor, Sextans, Fornax and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early Universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] {approx} -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

  4. High-Resolution Spectroscopy of the Metal-Poor Star HD 187216

    NASA Astrophysics Data System (ADS)

    Barzdis, A.; Začs, L.; Galazutdinov, G.

    Abundance analysis of the metal-poor, carbon-rich giant HD 187216 using high-resolution (R ≈ 45 000) spectrum was performed. An LTE abundance analysis was done for carefully selected clean atomic lines, using the Uppsala atmospheric model with Teff = 4000 K, log g = 0.75, ξt = 2.8 km s-1 and [Z] = --2.0. The mean metallicity [Fe/H] = --1.7 derived by using singly ionized iron lines is much higher than previously believed. It seems likely that Fe I lines, like many other neutral atomic lines, suffer from non-LTE effects that are significant at low metallicity and gravity. The abundances of the neutron capture elements are found to be enhanced by about 1.3 dex relative to the iron group elements. Possible causes of chemical peculiarities of HD 187216 are discussed.

  5. Preliminary determination of the Non-LTE Calcium abundance in a sample of extremely metal-poor stars*

    NASA Astrophysics Data System (ADS)

    Spite, M.; Spite, F.; Bonifacio, P.; Caffau, E.; Andrievsky, S.; Korotin, S.; Cayrel, R.; François, P.

    2011-12-01

    The abundance ratios of the elements found in the extremely metal-poor stars (EMP) are a test of the yields predicted by the models of supernovae. For precise comparisons, it is of course preferable to avoid the approximation of LTE. The difference of LTE and NLTE profiles is displayed for three strong lines. The NLTE abundances of Ca are derived from the profiles of about 15 Ca I lines in the EMP giants and about 10 lines in the turnoff stars. The improved abundance trends are consistent with a [Ca/Fe] ratio constant vs. [Fe/H], and with a [Ca/Mg] ratio slightly declining when [Mg/H] increases. Also [Ca/Mg] presents a scatter larger than [Ca/Fe]. As far as the comparison with sulfur (another alpha elment) is concerned we find that [S/Ca] presents a scatter smaller than [S/Mg].

  6. NUCLEOSYNTHESIS IN HIGH-ENTROPY HOT BUBBLES OF SUPERNOVAE AND ABUNDANCE PATTERNS OF EXTREMELY METAL-POOR STARS

    SciTech Connect

    Izutani, Natsuko; Umeda, Hideyuki E-mail: umeda@astron.s.u-tokyo.ac.j

    2010-09-01

    There have been suggestions that the abundance of extremely metal-poor (EMP) stars can be reproduced by hypernovae (HNe), not by normal supernovae (SNe). However, recently it was also suggested that if the innermost neutron-rich or proton-rich matter is ejected, the abundance patterns of ejected matter are changed, and normal SNe may also reproduce the observations of EMP stars. In this Letter, we calculate explosive nucleosynthesis with various Y {sub e} and entropy, and investigate whether normal SNe with this innermost matter, which we call the 'hot-bubble' component, can reproduce the abundance of EMP stars. We find that neutron-rich (Y {sub e} = 0.45-0.49) and proton-rich (Y {sub e} = 0.51-0.55) matter can increase Zn/Fe and Co/Fe ratios as observed, but tend to overproduce other Fe-peak elements. In addition, we find that if slightly proton-rich matter with 0.50 {<=} Y {sub e} < 0.501 with s/k {sub b} {approx} 15-40 is ejected as much as {approx}0.06 M {sub sun}, even normal SNe can reproduce the abundance of EMP stars, though it requires fine-tuning of Y {sub e}. On the other hand, HNe can more easily reproduce the observations of EMP stars without fine-tuning. Our results imply that HNe are the most likely origin of the abundance pattern of EMP stars.

  7. Detection of Phosphorus, Sulphur, and Zinc in the Carbon-enhanced Metal-poor Star BD+44 493

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Placco, Vinicius M.; Beers, Timothy C.

    2016-06-01

    The carbon-enhanced metal-poor star BD+44°493 ([Fe/H] = -3.9) has been proposed as a candidate second-generation star enriched by metals from a single Pop III star. We report the first detections of P and S and the second detection of Zn in any extremely metal-poor carbon-enhanced star, using new spectra of BD+44°493 collected by the Cosmic Origins Spectrograph on the Hubble Space Telescope. We derive [P/Fe] = -0.34 ± 0.21, [S/Fe] = +0.07 ± 0.41, and [Zn/Fe] = -0.10 ± 0.24. We increase by 10-fold the number of Si i lines detected in BD+44°493, yielding [Si/Fe] = +0.15 ± 0.22. The [S/Fe] and [Zn/Fe] ratios exclude the hypothesis that the abundance pattern in BD+44°493 results from depletion of refractory elements onto dust grains. Comparison with zero-metallicity supernova (SN) models suggests that the stellar progenitor that enriched BD+44°493 was massive and ejected much less than 0.07 M ⊙ of 56Ni, characteristic of a faint SN. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS 5-26555. These observations are associated with program GO-14231.

  8. Detection of Phosphorus, Sulphur, and Zinc in the Carbon-enhanced Metal-poor Star BD+44 493

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Placco, Vinicius M.; Beers, Timothy C.

    2016-06-01

    The carbon-enhanced metal-poor star BD+44°493 ([Fe/H] = ‑3.9) has been proposed as a candidate second-generation star enriched by metals from a single Pop III star. We report the first detections of P and S and the second detection of Zn in any extremely metal-poor carbon-enhanced star, using new spectra of BD+44°493 collected by the Cosmic Origins Spectrograph on the Hubble Space Telescope. We derive [P/Fe] = ‑0.34 ± 0.21, [S/Fe] = +0.07 ± 0.41, and [Zn/Fe] = ‑0.10 ± 0.24. We increase by 10-fold the number of Si i lines detected in BD+44°493, yielding [Si/Fe] = +0.15 ± 0.22. The [S/Fe] and [Zn/Fe] ratios exclude the hypothesis that the abundance pattern in BD+44°493 results from depletion of refractory elements onto dust grains. Comparison with zero-metallicity supernova (SN) models suggests that the stellar progenitor that enriched BD+44°493 was massive and ejected much less than 0.07 M ⊙ of 56Ni, characteristic of a faint SN. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS 5-26555. These observations are associated with program GO-14231.

  9. Iron-group Abundances in the Metal-poor Main-Sequence Turnoff Star HD~84937

    NASA Astrophysics Data System (ADS)

    Sneden, Christopher; Cowan, John J.; Kobayashi, Chiaki; Pignatari, Marco; Lawler, James E.; Den Hartog, Elizabeth A.; Wood, Michael P.

    2016-01-01

    We have derived new, very accurate abundances of the Fe-group elements Sc through Zn (Z = 21-30) in the bright main-sequence turnoff star HD 84937 based on high-resolution spectra covering the visible and ultraviolet spectral regions. New or recent laboratory transition data for 14 species of seven elements have been used. Abundances from more than 600 lines of non-Fe species have been combined with about 550 Fe lines in HD 84937 to yield abundance ratios of high precision. The abundances have been determined from both neutral and ionized transitions, which generally are in agreement with each other. We find no substantial departures from the standard LTE Saha ionization balance in this [Fe/H] = -2.32 star. Noteworthy among the abundances are [Co/Fe] = +0.14 and [Cu/Fe] = -0.83, in agreement with past studies of abundance trends in this and other low-metallicity stars, and < [{{Sc,Ti,V/Fe}}]> = +0.31, which has not been noted previously. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys reveals that they are positively correlated in stars with [Fe/H] < -2 HD 84937 lies at the high end of this correlation. These trends constrain the synthesis mechanisms of Fe-group elements. We also examine the Galactic chemical evolution abundance trends of the Fe-group elements, including a new nucleosynthesis model with jet-like explosion effects.

  10. VizieR Online Data Catalog: Metal-poor stars towards the Galactic bulge (Koch+, 2016)

    NASA Astrophysics Data System (ADS)

    Koch, A.; McWilliam, A.; Preston, G. W.; Thompson, I. B.

    2015-11-01

    The stars studied here were identified in a search for EMP stars in the Galactic bulge (Preston et al. unpublished), near b=-10°, employing the 2.5-m du Pont and 1-m Swope telescopes at Las Campanas Observatory. Observations of seven EMP candidates presented here were taken spread over six nights in July 2007 with a median seeing of 0.95", while individual exposures reached as high as 2" and notably better conditions (~0.6") during several nights. Our chosen set-up included a 0.5" slit, 2x1 binning in spectral and spatial dimensions and resulted in a resolving power of R~45000. An observing log is given in Table 1. (3 data files).

  11. DISCOVERY OF MIRA VARIABLE STARS IN THE METAL-POOR SEXTANS DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Sakamoto, Tsuyoshi; Matsunaga, Noriyuki; Nakada, Yoshikazu; Hasegawa, Takashi

    2012-12-10

    We report the discovery of two Mira variable stars (Miras) toward the Sextans dwarf spheroidal galaxy (dSph). We performed optical long-term monitoring observations for two red stars in the Sextans dSph. The light curves of both stars in the I{sub c} band show large-amplitude (3.7 and 0.9 mag) and long-period (326 {+-} 15 and 122 {+-} 5 days) variations, suggesting that they are Miras. We combine our own infrared data with previously published data to estimate the mean infrared magnitudes. The distances obtained from the period-luminosity relation of the Miras (75.3{sup +12.8}{sub -10.9} and 79.8{sup +11.5}{sub -9.9} kpc, respectively), together with the radial velocities available, support memberships of the Sextans dSph (90.0 {+-} 10.0 kpc). These are the first Miras found in a stellar system with a metallicity as low as [Fe/H] {approx} -1.9 than any other known system with Miras.

  12. Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars. II. s-process nucleosynthesis during the core He flash

    NASA Astrophysics Data System (ADS)

    Campbell, S. W.; Lugaro, M.; Karakas, A. I.

    2010-11-01

    Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims: We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods: We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M_⊙ and a metallicity of [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously. Results: We find that the mixing and burning of protons into the hot convective core leads to the production of 13C, which then burns via the 13C(α, n)16O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Conclusions: Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ˜ ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early

  13. THE NUCLEOSYNTHETIC IMPRINT OF 15-40 M{sub sun} PRIMORDIAL SUPERNOVAE ON METAL-POOR STARS

    SciTech Connect

    Joggerst, C. C.; Woosley, S. E.; Almgren, A.; Bell, J.; Heger, Alexander; Whalen, Daniel

    2010-01-20

    The inclusion of rotationally induced mixing in stellar evolution can alter the structure and composition of pre-supernova stars. We survey the effects of progenitor rotation on nucleosynthetic yields in Population III and II supernovae (SNe) using the new adaptive mesh refinement code CASTRO. We examine piston-driven spherical explosions in 15, 25, and 40 M{sub sun} stars at Z = 0 and 10{sup -4} Z{sub sun} with three explosion energies and two rotation rates. Rotation in the Z = 0 models resulted in primary nitrogen production and a stronger hydrogen burning shell which led all models to die as red supergiants (in contrast to the blue supergiant progenitors made without rotation). On the other hand, the Z = 10{sup -4} Z{sub sun} models that included rotation ended their lives as compact blue stars. Because of their extended structure, the hydrodynamics favors more mixing and less fallback in the metal-free stars than the Z = 10{sup -4} models. As expected, higher energy explosions produce more enrichment and less fallback than do lower energy explosions, and at constant explosion energy, less massive stars produce more enrichment and leave behind smaller remnants than do more massive stars. We compare our nucleosynthetic yields to the chemical abundances in the three most iron-poor stars yet found and reproduce the abundance pattern of one, HE 0557-4840, with a zero metallicity, 15 M{sub sun}, 2.4 x 10{sup 51} erg SN. A Salpeter IMF-averaged integration of our yields for Z = 0 models with explosion energies of 2.4 x 10{sup 51} erg or less is in good agreement with the abundances observed in larger samples of extremely metal-poor (EMP) stars, provided 15 M{sub sun} stars are included. Since the abundance patterns of EMP stars likely arise from a representative sample of progenitors, our yields suggest that 15-40 M{sub sun} core-collapse SNe with moderate explosion energies contributed the bulk of the metals to the early universe.

  14. THE ORIGINS OF LIGHT AND HEAVY R-PROCESS ELEMENTS IDENTIFIED BY CHEMICAL TAGGING OF METAL-POOR STARS

    SciTech Connect

    Tsujimoto, Takuji; Shigeyama, Toshikazu

    2014-11-01

    Growing interests in neutron star (NS) mergers as the origin of r-process elements have sprouted since the discovery of evidence for the ejection of these elements from a short-duration γ-ray burst. The hypothesis of a NS merger origin is reinforced by a theoretical update of nucleosynthesis in NS mergers successful in yielding r-process nuclides with A > 130. On the other hand, whether the origin of light r-process elements are associated with nucleosynthesis in NS merger events remains unclear. We find a signature of nucleosynthesis in NS mergers from peculiar chemical abundances of stars belonging to the Galactic globular cluster M15. This finding combined with the recent nucleosynthesis results implies a potential diversity of nucleosynthesis in NS mergers. Based on these considerations, we are successful in the interpretation of an observed correlation between [light r-process/Eu] and [Eu/Fe] among Galactic halo stars and accordingly narrow down the role of supernova nucleosynthesis in the r-process production site. We conclude that the tight correlation by a large fraction of halo stars is attributable to the fact that core-collapse supernovae produce light r-process elements while heavy r-process elements such as Eu and Ba are produced by NS mergers. On the other hand, stars in the outlier, composed of r-enhanced stars ([Eu/Fe] ≳ +1) such as CS22892-052, were exclusively enriched by matter ejected by a subclass of NS mergers that is inclined to be massive and consist of both light and heavy r-process nuclides.

  15. An extremely primitive star in the Galactic halo.

    PubMed

    Caffau, Elisabetta; Bonifacio, Piercarlo; François, Patrick; Sbordone, Luca; Monaco, Lorenzo; Spite, Monique; Spite, François; Ludwig, Hans-G; Cayrel, Roger; Zaggia, Simone; Hammer, François; Randich, Sofia; Molaro, Paolo; Hill, Vanessa

    2011-09-01

    The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as 'metallicity'. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10(-5), it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10(-8) to 1.5 × 10(-6) (ref. 8), although competing theories claiming the contrary do exist. (We use 'low-mass' here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10(-7), which is 4.5 × 10(-5) times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars--that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium. PMID:21886158

  16. The Chemical Composition of Halo Stars on Extreme Orbits

    NASA Astrophysics Data System (ADS)

    Stephens, Alex

    1999-04-01

    Presented within is a fine spectroscopic analysis of 11 metal-poor (-2.15<[Fe/H]<-1.00) dwarf stars on orbits that penetrate the outermost regions of the Galactic halo. Abundances for a select group of light metals (Na, Mg, Si, Ca, and Ti), Fe-peak nuclides (Cr, Fe, and Ni), and neutron-capture elements (Y and Ba) were calculated using line strengths measured from high-resolution (R~48,000), high signal-to-noise ratio (S/N~110pixel^-1) echelle spectra acquired with the Keck I 10 m telescope and HIRES spectrograph. Ten of the stars have apogalactica, a proxy for stellar birthplace, which stretch between 25 and 90 kpc; however, these ``outer halo'' stars exhibit strikingly uniform abundances. The average, Fe-normalized abundances-<[Mg/Fe]>=+0.23+/-0.09, <[Si/Fe]>=+0.24+/-0.10, <[Ca/Fe]>=+0.22+/-0.07, <[Ti/Fe]>=+0.20+/-0.08, <[Cr/Fe]>=0.02+/-0.07, <[Ni/Fe]>=-0.09+/-0.07, and <[Ba/Fe]>=+0.01+/-0.12-exhibit little intrinsic scatter; moreover, the evolution of individual ratios (as a function of [Fe/H]) is generally consistent with the predictions of galactic chemical evolution models dominated by the ejecta of core-collapse supernovae. Only <[Y/Fe]>=-0.13+/-0.21 exhibits a dispersion larger than observational uncertainties, which suggests a different nucleosynthesis site for this element. It has been conjectured that stars on high-energy orbits-either those that penetrate the remote halo or ones with extreme retrograde velocities-were once associated with a cannibalized satellite galaxy. Such stars, as shown here, are indistinguishable from metal-poor dwarfs of the inner Galactic halo. The uniformity of the abundances, regardless of kinematic properties, suggests that physically, spatially, and temporally distinct star-forming regions within (or near) the growing Milky Way experienced grossly similar chemical evolution histories. Implications for galaxy formation scenarios are discussed.

  17. Atomic diffusion in metal poor stars. The influence on the Main Sequence fitting distance scale, subdwarfs ages and the value of Delta Y/ Delta Z

    NASA Astrophysics Data System (ADS)

    Salaris, M.; Groenewegen, M. A. T.; Weiss, A.

    2000-03-01

    The effect of atomic diffusion on the Main Sequence (MS) of metal-poor low mass stars is investigated. Since diffusion alters the stellar surface chemical abundances with respect to their initial values, one must ensure - by calibrating the initial chemical composition of the theoretical models - that the surface abundances of the models match the observed ones of the stellar population under scrutiny. When properly calibrated, our models with diffusion reproduce well within the errors the Hertzsprung-Russell diagram of Hipparcos subdwarfs with empirically determined T_eff values and high resolution spectroscopical [Fe/H] determinations. Since the observed surface abundances of subdwarfs are different from the initial ones due to the effect of diffusion, while the globular clusters stellar abundances are measured in Red Giants, which have practically recovered their initial abundances after the dredge-up, the isochrones to be employed for studying globular clusters and Halo subdwarfs with the same observational value of [Fe/H] are different and do not coincide. This is at odds with the basic assumption of the MS-fitting technique for distance determinations. However, the use of the rather large sample of Hipparcos lower MS subdwarfs with accurate parallaxes keeps at minimum the effect of these differences, for two reasons. First, it is possible to use subdwarfs with observed [Fe/H] values close to the cluster one; this minimizes the colour corrections (which are derived from the isochrones) needed to reduce all the subdwarfs to a mono-metallicity sequence having the same [Fe/H] than the cluster. Second, one can employ objects sufficiently faint so that the differences between the subdwarfs and cluster MS with the same observed value of [Fe/H] are small (they increase for increasing luminosity). We find therefore that the distances based on standard isochrones are basically unaltered when diffusion is taken properly into account. On the other hand, the absolute ages

  18. THE MOST METAL-POOR STARS. IV. THE TWO POPULATIONS WITH [Fe/H] {approx}< -3.0

    SciTech Connect

    Norris, John E.; Yong, David; Bessell, M. S.; Asplund, M. E-mail: bessell@mso.anu.edu.au; and others

    2013-01-01

    We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] {approx}< -3.0, utilizing chemical abundances from high-resolution, high signal-to-noise model-atmosphere analyses. The C-rich population represents {approx}28% of stars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no stars, and two others with [Fe/H] {approx} -5.5 and uncertain classification. The population is O-rich ([O/Fe] {approx}> +1.5); the light elements Na, Mg, and Al are enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little evidence for enhancements relative to solar values. These results are best explained in terms of the admixing and processing of material from H-burning and He-burning regions as achieved by nucleosynthesis in zero-heavy-element models in the literature of 'mixing and fallback' supernovae (SNe); of rotating, massive, and intermediate-mass stars; and of Type II SNe with relativistic jets. The available (limited) radial velocities offer little support for the C-rich stars with [Fe/H] < -3.1 being binary. More data are required before one could conclude that binarity is key to an understanding of this population. We suggest that the C-rich and C-normal populations result from two different gas cooling channels in the very early universe of material that formed the progenitors of the two populations. The first was cooling by fine-structure line transitions of C II and O I (to form the C-rich population); the second, while not well defined (perhaps dust-induced cooling?), led to the C-normal group. In this scenario, the C-rich population contains the oldest stars currently observed.

  19. METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. I. CONSTRAINTS ON PROGENITOR MASS AND METALLICITY OF AGB STARS UNDERGOING s-PROCESS NUCLEOSYNTHESIS

    SciTech Connect

    Placco, Vinicius M.; Rossi, Silvia; Frebel, Anna; Beers, Timothy C.; Karakas, Amanda I.; Kennedy, Catherine R.; Christlieb, Norbert; Stancliffe, Richard J.

    2013-06-20

    We present a comprehensive abundance analysis of two newly discovered carbon-enhanced metal-poor (CEMP) stars. HE 2138-3336 is a s-process-rich star with [Fe/H] = -2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if non-local thermodynamic equilibrium corrections are included ([Pb/Fe] = +3.84). HE 2258-6358, with [Fe/H] = -2.67, exhibits enrichments in both s- and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R {approx} 2000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R {approx} 30, 000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical asymptotic giant branch (AGB) nucleosynthesis models of 1.3 M{sub Sun} with [Fe/H] = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 M{sub Sun} at [Fe/H] = -2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138-3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258-6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0 {<=} M(M{sub Sun }) {<=} 1.3) and metallicities (-2.8 {<=} [Fe/H] {<=}-2.5) yield the best agreement with our observed elemental abundance patterns.

  20. Metal-Poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship Between Stellar Mass, Temperature-Based Metallicity, and Star Formation Rate

    NASA Technical Reports Server (NTRS)

    Ly, Chun; Rigby, Jane R.; Cooper, Michael; Yan, Renbin

    2015-01-01

    We report on the discovery of 28 redshift (z) approximately 0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O (sub III)] lambda 4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) extends to low stellar mass and high SFR. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFRs, with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 plus or minus 0.61 decimal exponent (dex) above the redshift (z) approximately equal to 1 stellar mass-SFR relation, and 0.23 plus or minus 0.23 decimal exponent (dex) below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 decimal exponent (dex), but significant dispersion remains (0.29 decimal exponent (dex) with 0.16 decimal exponent (dex) due to measurement uncertainties). This dispersion suggests that gas accretion, star formation and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of approximately 100 (sup plus 310) (sub minus 75) million years that suggests stochastic star formation. Combining our sample with other redshift (z) of approximately 1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 97.3 percent confidence. We interpret this positive correlation as recent star formation that has enriched the gas, but has not had time to drive the metal-enriched gas out with feedback mechanisms.

  1. Metal-Poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship Between Stellar Mass, Temperature-Based Metallicity, and Star Formation Rate

    NASA Technical Reports Server (NTRS)

    Ly, Chun; Rigby, Jane R.; Cooper, Michael; Yan, Renbin

    2015-01-01

    We report on the discovery of 28 redshift (z) approximately equal to 0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O (sub III)] lambda 4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFR with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 plus or minus 0.61 dex above the redshift (z) approximately 1 stellar mass-SFR relation and 0.23 plus or minus 0.23 dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation, and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of approximately equal to 100 (sup plus 310) (sub minus 75) million years which suggests stochastic star formation. Combining our sample with other redshift (z) of approximately 1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4 percent confidence. We interpret this positive correlation as recent star formation that has enriched the gas but has not had time to drive the metal-enriched gas out with feedback mechanisms.

  2. Stellar yields of rotating first stars. I. Yields of weak supernovae and abundances of carbon-enhanced hyper-metal-poor stars

    SciTech Connect

    Takahashi, Koh; Umeda, Hideyuki; Yoshida, Takashi

    2014-10-10

    We perform a stellar evolution simulation of first stars and calculate stellar yields from the first supernovae. The initial masses are taken from 12 to 140 M {sub ☉} to cover the whole range of core-collapse supernova progenitors, and stellar rotation is included, which results in efficient internal mixing. A weak explosion is assumed in supernova yield calculations, thus only outer distributed matter, which is not affected by the explosive nucleosynthesis, is ejected in the models. We show that the initial mass and the rotation affect the explosion yield. All the weak explosion models have abundances of [C/O] larger than unity. Stellar yields from massive progenitors of >40-60 M {sub ☉} show enhancement of Mg and Si. Rotating models yield abundant Na and Al, and Ca is synthesized in nonrotating heavy massive models of >80 M {sub ☉}. We fit the stellar yields to the three most iron-deficient stars and constrain the initial parameters of the mother progenitor stars. The abundance pattern in SMSS 0313–6708 is well explained by 50-80 M {sub ☉} nonrotating models, rotating 30-40 M {sub ☉} models well fit the abundance of HE 0107-5240, and both nonrotating and rotating 15-40 M {sub ☉} models explain HE 1327-2326. The presented analysis will be applicable to other carbon-enhanced hyper-metal-poor stars observed in the future. The abundance analyses will give valuable information about the characteristics of the first stars.

  3. CHEMICAL COMPOSITIONS OF KINEMATICALLY SELECTED OUTER HALO STARS

    SciTech Connect

    Zhang Lan; Zhao Gang; Ishigaki, Miho; Chiba, Masashi; Aoki, Wako E-mail: zhanglan@bao.ac.c E-mail: chiba@astr.tohoku.ac.j

    2009-12-01

    Chemical abundances of 26 metal-poor dwarfs and giants are determined from high-resolution and high signal-to-noise ratio spectra obtained with the Subaru/High Dispersion Spectrograph. The sample is selected so that most of the objects have outer-halo kinematics. Self-consistent atmospheric parameters were determined by an iterative procedure based on spectroscopic analysis. Abundances of 13 elements, including alpha-elements (Mg, Si, Ca, Ti), odd-Z light elements (Na, Sc), iron-peak elements (Cr, Mn, Fe, Ni, Zn), and neutron-capture elements (Y, Ba), are determined by two independent data reduction and local thermodynamic equillibrium analysis procedures, confirming the consistency of the stellar parameters and abundances results. We find a decreasing trend of [alpha/Fe] with increasing [Fe/H] for the range of -3.5< [Fe/H] <-1, as found by Stephens and Boesgaard. [Zn/Fe] values of most objects in our sample are slightly lower than the bulk of halo stars previously studied. These results are discussed as possible chemical properties of the outer halo in the Galaxy.

  4. Models of Metal-poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars

    NASA Astrophysics Data System (ADS)

    VandenBerg, Don A.; Richard, O.; Michaud, G.; Richer, J.

    2002-05-01

    Isochrones for ages between 12 and 18 Gyr have been derived from the evolutionary tracks presented in Paper I (Richard et al.) for masses from 0.5 to 1.0 Msolar and initial chemical abundances corresponding to (1) Y=0.2352, Z=1.69×10-4 ([Fe/H]=-2.31,[α/Fe]=0.3) and (2) Y=0.2370, Z=1.69×10-3 ([Fe/H=-1.31,[α/Fe]=0.3). These are the first models for Population II stars in which both gravitational settling and radiative accelerations have been taken into account. Allowance for these diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. However, in order for the diffusive models to satisfy the constraints from Li and Fe abundance data (see Paper I) and to reproduce the observed morphologies of globular cluster (GC) color-magnitude diagrams (CMDs) in a straightforward way, extra mixing just below the boundary of the convective envelope seems to be necessary. Indeed, when additional turbulent mixing is invoked, the resultant models are able to satisfy all of these constraints, as well as those provided by the CMDs of local subdwarfs, rather well. Moreover, they imply an age near 13.5 Gyr for M92, which is one of the most metal-deficient (and presumably one of the oldest) of the Galaxy's GCs, if the field subgiant HD 140283 is used to derive the cluster distance. Comparisons of field subdwarfs and subgiants with a recently published fiducial for M5 suggests that the cluster has [Fe/H]<~-1.4, in conflict with some estimates based on high-resolution spectroscopy, if the metallicities of the field stars are to be trusted. In addition, an age of ~11.5 Gyr is found for M5, irrespective of whether diffusive or nondiffusive isochrones are employed in the analysis. The implications of our results for the extragalactic distance scale and for the Hubble constant are briefly discussed in the context of the presently favored ΩM~0.35, ΩΛ~0.65 cosmological model.

  5. OBSERVATIONS OF BINARY STARS WITH THE DIFFERENTIAL SPECKLE SURVEY INSTRUMENT. V. TOWARD AN EMPIRICAL METAL-POOR MASS–LUMINOSITY RELATION

    SciTech Connect

    Horch, Elliott P.; Van Altena, William F.; Demarque, Pierre; Howell, Steve B.; Everett, Mark E.; Ciardi, David R.; Teske, Johanna K.; Henry, Todd J.; Winters, Jennifer G. E-mail: william.vanaltena@yale.edu E-mail: steve.b.howell@nasa.gov E-mail: ciardi@ipac.caltech.edu E-mail: thenry@astro.gsu.edu

    2015-05-15

    In an effort to better understand the details of the stellar structure and evolution of metal-poor stars, the Gemini North telescope was used on two occasions to take speckle imaging data of a sample of known spectroscopic binary stars and other nearby stars in order to search for and resolve close companions. The observations were obtained using the Differential Speckle Survey Instrument, which takes data in two filters simultaneously. The results presented here are of 90 observations of 23 systems in which one or more companions was detected, and six stars where no companion was detected to the limit of the camera capabilities at Gemini. In the case of the binary and multiple stars, these results are then further analyzed to make first orbit determinations in five cases, and orbit refinements in four other cases. The mass information is derived, and since the systems span a range in metallicity, a study is presented that compares our results with the expected trend in total mass as derived from the most recent Yale isochrones as a function of metal abundance. These data suggest that metal-poor main-sequence stars are less massive at a given color than their solar-metallicity analogues in a manner consistent with that predicted from the theory.

  6. Observations of Binary Stars with the Differential Speckle Survey Instrument. V. Toward an Empirical Metal-Poor Mass-Luminosity Relation

    NASA Astrophysics Data System (ADS)

    Horch, Elliott P.; van Altena, William F.; Demarque, Pierre; Howell, Steve B.; Everett, Mark E.; Ciardi, David R.; Teske, Johanna K.; Henry, Todd J.; Winters, Jennifer G.

    2015-05-01

    In an effort to better understand the details of the stellar structure and evolution of metal-poor stars, the Gemini North telescope was used on two occasions to take speckle imaging data of a sample of known spectroscopic binary stars and other nearby stars in order to search for and resolve close companions. The observations were obtained using the Differential Speckle Survey Instrument, which takes data in two filters simultaneously. The results presented here are of 90 observations of 23 systems in which one or more companions was detected, and six stars where no companion was detected to the limit of the camera capabilities at Gemini. In the case of the binary and multiple stars, these results are then further analyzed to make first orbit determinations in five cases, and orbit refinements in four other cases. The mass information is derived, and since the systems span a range in metallicity, a study is presented that compares our results with the expected trend in total mass as derived from the most recent Yale isochrones as a function of metal abundance. These data suggest that metal-poor main-sequence stars are less massive at a given color than their solar-metallicity analogues in a manner consistent with that predicted from the theory.

  7. Carbon-enhanced metal-poor stars in SDSS/Segue. II. Comparison of CEMP-star frequencies with binary population-synthesis models

    SciTech Connect

    Lee, Young Sun; Suda, Takuma; Beers, Timothy C.; Stancliffe, Richard J.

    2014-06-20

    We present a comparison of the frequencies of carbon-enhanced metal-poor (CEMP) giant and main-sequence turnoff (MSTO) stars with predictions from binary population-synthesis models involving asymptotic giant-branch (AGB) mass transfer. The giant and MSTO stars are selected from the Sloan Digital Sky Survey and the Sloan Extension for Galactic Understanding and Exploration. We consider two initial mass functions (IMFs)—a Salpeter IMF, and a mass function with a characteristic mass of 10 M {sub ☉}. For giant stars, the comparison indicates a good agreement between the observed CEMP frequencies and the AGB binary model using a Salpeter IMF for [Fe/H] > – 1.5, and a characteristic mass of 10 M {sub ☉} for [Fe/H] < – 2.5. This result suggests that the IMF shifted from high- to low-mass dominated in the early history of the Milky Way, which appears to have occurred at a 'chemical time' between [Fe/H] =–2.5 and [Fe/H] =–1.5. The CEMP frequency for the turnoff stars with [Fe/H] < – 3.0 is much higher than the AGB model prediction from the high-mass IMF, supporting the previous assertion that one or more additional mechanisms, not associated with AGB stars, are required for the production of carbon-rich material below [Fe/H] =–3.0. We also discuss possible effects of first dredge-up and extra mixing in red giants and internal mixing in turnoff stars on the derived CEMP frequencies.

  8. From First Stars to the Spite Plateau: A Possible Reconciliation of Halo Stars Observations with Predictions from Big Bang Nucleosynthesis

    NASA Astrophysics Data System (ADS)

    Piau, L.; Beers, T. C.; Balsara, D. S.; Sivarani, T.; Truran, J. W.; Ferguson, J. W.

    2006-12-01

    Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor ([Fe/H]<-1.3) halo stars near the turnoff has been attributed to a cosmological origin. Closer analysis, however, revealed that the observed abundance lies at Δ7Li~0.4 dex below the predictions of big bang nucleosynthesis (BBN). The measurements of deuterium abundances along the lines of sight toward quasars, and the recent data from the Wilkinson Microwave Anisotropy Probe (WMAP), have independently confirmed this gap. We suggest here that part of the discrepancy (from 0.2 to 0.3 dex) is explained by a first generation of stars that efficiently depleted lithium. Assuming that the models for lithium evolution in halo turnoff stars, as well as the Δ7Li, estimates are correct, we infer that between one-third and one-half of the baryonic matter of the early halo (i.e., ~109 Msolar) was processed through Population III stars. This new paradigm proposes a very economical solution to the lingering difficulty of understanding the properties of the Spite plateau and its lack of star-to-star scatter down to [Fe/H]=-2.5. It is moreover in agreement both with the absence of lithium in the most iron-poor turnoff star currently known (HE 1327-2326) and also with new trends of the plateau suggesting its low-metallicity edge may be reached around [Fe/H]=-2.5. We discuss the role of turbulent mixing associated with enhanced supernovae explosions in the early interstellar medium in this picture. We suggest how it may explain the small scatter and also other recent observational features of the lithium plateau. Finally, we show that other chemical properties of the extremely metal-poor stars (such as carbon enrichment) are also in agreement with significant Population III processing in the halo, provided these models include mass loss and rotationally induced mixing.

  9. OXYGEN ABUNDANCES IN LOW- AND HIGH-{alpha} FIELD HALO STARS AND THE DISCOVERY OF TWO FIELD STARS BORN IN GLOBULAR CLUSTERS

    SciTech Connect

    Ramirez, I.; Melendez, J.

    2012-10-01

    Oxygen abundances of 67 dwarf stars in the metallicity range -1.6 < [Fe/H] < -0.4 are derived from a non-LTE analysis of the 777 nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and {alpha}-element (Mg, Si, Ca, Ti) abundances: thick disk, high-{alpha} halo, and low-{alpha} halo. We find the oxygen abundance trends of thick-disk and high-{alpha} halo stars very similar. The low-{alpha} stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the {alpha} elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (-2.3 < [Fe/H] < +0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3% {+-} 2% of the local field metal-poor star population was born in GCs.

  10. AN INFRARED CENSUS OF DUST IN NEARBY GALAXIES WITH SPITZER (DUSTiNGS). II. DISCOVERY OF METAL-POOR DUSTY AGB STARS

    SciTech Connect

    Boyer, Martha L.; Sonneborn, George; McQuinn, Kristen B. W.; Gehrz, Robert D.; Skillman, Evan; Barmby, Pauline; Bonanos, Alceste Z.; Gordon, Karl D.; Meixner, Margaret; Groenewegen, M. A. T.; Lagadec, Eric; Lennon, Daniel; Marengo, Massimo; McDonald, Iain; Zijlstra, Albert; Sloan, G. C.; Van Loon, Jacco Th.

    2015-02-10

    The DUSTiNGS survey (DUST in Nearby Galaxies with Spitzer) is a 3.6 and 4.5 μm imaging survey of 50 nearby dwarf galaxies designed to identify dust-producing asymptotic giant branch (AGB) stars and massive stars. Using two epochs, spaced approximately six months apart, we identify a total of 526 dusty variable AGB stars (sometimes called ''extreme'' or x-AGB stars; [3.6]-[4.5] > 0.1 mag). Of these, 111 are in galaxies with [Fe/H] < –1.5 and 12 are in galaxies with [Fe/H] < –2.0, making them the most metal-poor dust-producing AGB stars known. We compare these identifications to those in the literature and find that most are newly discovered large-amplitude variables, with the exception of ≈30 stars in NGC 185 and NGC 147, 1 star in IC 1613, and 1 star in Phoenix. The chemical abundances of the x-AGB variables are unknown, but the low metallicities suggest that they are more likely to be carbon-rich than oxygen-rich and comparisons with existing optical and near-IR photometry confirm that 70 of the x-AGB variables are confirmed or likely carbon stars. We see an increase in the pulsation amplitude with increased dust production, supporting previous studies suggesting that dust production and pulsation are linked. We find no strong evidence linking dust production with metallicity, indicating that dust can form in very metal-poor environments.

  11. Cool Carbon Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Gigoyan, K. S.

    2016-06-01

    In this paper we report current status of search and study for Faint High Latitude Carbon Stars (FHLCs). Data for more than 1800 spectroscopically confirmed FHLCs are known, which are found thanks to objective prism surveys and photometric selections. More than half of the detected objects belongs to group of dwarf Carbon (dC) stars. Many-sided investigations based on modern astrophysical databases are necessary to study the space distribution of different groups of the FHLC stars and their possible origin in the Halo of our Galaxy. We report about the selection of FHLCs by the spectroscopic surveys: First Byurakan Survey (FBS), Hamburg/ESO Survey (HES), LAMOST Pilot Survey and SDSS, as well as by photometric selection: APM Survey for Cool Carbon Stars in the Galactic Halo, SDSS and 2MASS JHK colours.

  12. Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars. I. Stellar yield tables and the CEMPs

    NASA Astrophysics Data System (ADS)

    Campbell, S. W.; Lattanzio, J. C.

    2008-11-01

    Context: The growing body of spectral observations of the extremely metal-poor (EMP) stars in the Galactic Halo provides constraints on theoretical studies of the chemical and stellar evolution of the early Universe. Aims: To calculate yields for EMP stars for use in chemical evolution calculations and to test whether such models can account for some of the recent abundance observations of EMP stars, in particular the highly C-rich EMP (CEMP) halo stars. Methods: We modify an existing 1D stellar structure code to include time-dependent mixing in a diffusion approximation. Using this code and a post-processing nucleosynthesis code we calculate the structural evolution and nucleosynthesis of a grid of models covering the metallicity range: -6.5 ≤ [Fe/H] ≤ -3.0 (plus Z = 0), and mass range: 0.85 ≤ M ≤ 3.0 M_⊙, amounting to 20 stars in total. Results: Many of the models experience violent nuclear burning episodes not seen at higher metallicities. We refer to these events as “Dual Flashes” since they are characterised by nearly simultaneous peaks in both hydrogen and helium burning. These events have been reported by previous studies. Some of the material processed by the Dual Flashes is dredged up causing significant surface pollution with a distinct chemical composition. We have calculated the entire evolution of the Z=0 and EMP models, from the ZAMS to the end of the TPAGB, including extensive nucleosynthesis. In this paper, the first of a series describing and analysing this large data set, we present the resulting stellar yields. Although subject to many uncertainties these are, as far as we are aware, the only yields currently available in this mass and metallicity range. We also analyse the yields in terms of C and N, comparing them to the observed CEMP abundances. At the lowest metallicities ([Fe/H] ≲ -4.0) we find the yields to contain ~ 1 to 2 dex too much carbon, in agreement with all previous studies. At higher metallicities ([Fe/H] ~ -3

  13. Carbon Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Totten, E. J.; Irwin, M. J.

    1996-04-01

    A byproduct of the APM high redshift quasar survey (Irwin et al. 1991) was the discovery of ~ 20 distant (20-100kpc) cool AGB carbon stars (all N-type) at high Galactic latitude. In August we used the INT+IDS to survey the rest of the high latitude SGC sky visible from La Palma and found 10 more similar carbon stars. Before this work there were only a handful of published faint high latitude cool carbon stars known (eg. Margon et al., 1984, Mould et al., 1985) and there has been speculation as to their origin (eg. Sanduleak, 1980, van den Bergh & Lafontaine, 1984). Intermediate age carbon stars (3 -- 7 Gyrs) seem unlikely to have formed in the halo in isolation from other star forming regions so how did they get there ? One possiblity that we are investigating, is that they arise from either the disruption of tidally captured dSph galaxies or are a manifestion of the long sought after optical component of the Magellanic Stream. Lack of proper motion rules out the possibility of them being dwarf carbon stars (eg. Warren et al., 1992); indeed no N-type carbon stars have been found to be dwarf carbon stars. Our optical spectroscopy confirms their carbon star type (they are indistinguishable from cool AGB carbon stars in nearby dwarf galaxies) and hence probable large distances. We are extending our survey to the NGC region, obtaining radial velocities and good S:N fluxed spectra for all the carbon stars. This will enable us to investigate their kinematics, true spatial distribution and hence their origin. Even, in the event that these objects are somehow an integral part of the Galactic halo, then their velocities and large distances will enable direct studies of the velocity ellipsoid and rotation of the outer halo (eg. Green et al., 1994).

  14. Metal-poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-forming Galaxy: Direct Evidence for Cold Accretion?

    NASA Astrophysics Data System (ADS)

    Crighton, Neil H. M.; Hennawi, Joseph F.; Prochaska, J. Xavier

    2013-10-01

    In our current galaxy formation paradigm, high-redshift galaxies are predominantly fueled by accretion of cool, metal-poor gas from the intergalactic medium. Hydrodynamical simulations predict that this material should be observable in absorption against background sightlines within a galaxy's virial radius, as optically thick Lyman limit systems (LLSs) with low metallicities. Here we report the discovery of exactly such a strong metal-poor absorber at an impact parameter R = 58 kpc from a star-forming galaxy at z = 2.44. Besides strong neutral hydrogen (N_{{H}^0}=10^{19.50+/- 0.16}\\, cm^{-2}) we detect neutral deuterium and oxygen, allowing a precise measurement of the metallicity: log10(Z/Z ⊙) = -2.0 ± 0.17, or (7-15) × 10-3 solar. Furthermore, the narrow deuterium linewidth requires a cool temperature <20,000 K. Given the striking similarities between this system and the predictions of simulations, we argue that it represents the direct detection of a high-redshift cold-accretion stream. The low-metallicity gas cloud is a single component of an absorption system exhibiting a complex velocity, ionization, and enrichment structure. Two other components have metallicities >0.1 solar, 10 times larger than the metal-poor component. We conclude that the photoionized circumgalactic medium (CGM) of this galaxy is highly inhomogeneous: the majority of the gas is in a cool, metal-poor and predominantly neutral phase, but the majority of the metals are in a highly ionized phase exhibiting weak neutral hydrogen absorption but strong metal absorption. If such inhomogeneity is common, then high-resolution spectra and detailed ionization modeling are critical to accurately appraise the distribution of metals in the high-redshift CGM. .

  15. METAL-POOR, COOL GAS IN THE CIRCUMGALACTIC MEDIUM OF A z = 2.4 STAR-FORMING GALAXY: DIRECT EVIDENCE FOR COLD ACCRETION?

    SciTech Connect

    Crighton, Neil H. M.; Hennawi, Joseph F.; Prochaska, J. Xavier

    2013-10-20

    In our current galaxy formation paradigm, high-redshift galaxies are predominantly fueled by accretion of cool, metal-poor gas from the intergalactic medium. Hydrodynamical simulations predict that this material should be observable in absorption against background sightlines within a galaxy's virial radius, as optically thick Lyman limit systems (LLSs) with low metallicities. Here we report the discovery of exactly such a strong metal-poor absorber at an impact parameter R = 58 kpc from a star-forming galaxy at z = 2.44. Besides strong neutral hydrogen (N{sub H{sup 0}}=10{sup 19.50±0.16} cm{sup -2}) we detect neutral deuterium and oxygen, allowing a precise measurement of the metallicity: log{sub 10}(Z/Z {sub ☉}) = –2.0 ± 0.17, or (7-15) × 10{sup –3} solar. Furthermore, the narrow deuterium linewidth requires a cool temperature <20,000 K. Given the striking similarities between this system and the predictions of simulations, we argue that it represents the direct detection of a high-redshift cold-accretion stream. The low-metallicity gas cloud is a single component of an absorption system exhibiting a complex velocity, ionization, and enrichment structure. Two other components have metallicities >0.1 solar, 10 times larger than the metal-poor component. We conclude that the photoionized circumgalactic medium (CGM) of this galaxy is highly inhomogeneous: the majority of the gas is in a cool, metal-poor and predominantly neutral phase, but the majority of the metals are in a highly ionized phase exhibiting weak neutral hydrogen absorption but strong metal absorption. If such inhomogeneity is common, then high-resolution spectra and detailed ionization modeling are critical to accurately appraise the distribution of metals in the high-redshift CGM.

  16. Improved Co I log(gf) & hfs data and Abundance Determinations in the Photospheres of the Sun & Metal-poor Star HD 84937

    NASA Astrophysics Data System (ADS)

    Lawler, James E.; Sneden, Chris; Cowan, John J.

    2016-01-01

    New emission branching fraction measurements for 898 lines of the first spectrum of cobalt (Co I) from hollow cathode lamp spectra recorded with a 1m Fourier transform spectrometer (FTS) and a high resolution echelle spectrometer are reported. Radiative lifetimes from laser induced fluorescence measurements are combined with the branching fractions to determine accurate log(gf)s for the 898 lines. Selected published hyperfine structure (hfs) constants for levels of neutral Co are used to generate complete hfs component patterns for 195 transitions of Co I. These new laboratory data are applied to determine the Co abundance in the Sun and metal-poor star HD 84937, yielding log eps(Co) = 4.955 ± 0.007 (sigma = 0.059) based on 82 Co I lines and log eps(Co) = 2.785 ± 0.008 (sigma = 0.065) based on 66 Co I lines respectively. A Saha balance test on the photosphere of HD 84937 is performed using 16 UV lines of Co II, and good agreement is found with the Co I result in this metal-poor ([Fe I /H] = -2.32, [Fe II /H] = -2.32) dwarf star. The resulting value of [Co/Fe] = +0.14 supports a rise of Co/Fe at low metallicity that has been suggested in other studies. These new Co I data are part of a continuing effort to explore the limits of 1D/LTE photospheric models in metal-poor stars and to determine the relative abundance of Fe-group elements at low metallicity. This work is supported in part by NASA grant NNX10AN93G (J.E.L.), by NSF grant AST-1211055 (J.E.L.), and by NSF grant AST-1211585 (C.S.).

  17. The GAPS programme with HARPS-N at TNG. II. No giant planets around the metal-poor star HIP 11952

    NASA Astrophysics Data System (ADS)

    Desidera, S.; Sozzetti, A.; Bonomo, A. S.; Gratton, R.; Poretti, E.; Claudi, R.; Latham, D. W.; Affer, L.; Cosentino, R.; Damasso, M.; Esposito, M.; Giacobbe, P.; Malavolta, L.; Nascimbeni, V.; Piotto, G.; Rainer, M.; Scardia, M.; Schmid, V. S.; Lanza, A. F.; Micela, G.; Pagano, I.; Bedin, L. R.; Biazzo, K.; Borsa, F.; Carolo, E.; Covino, E.; Faedi, F.; Hébrard, G.; Lovis, C.; Maggio, A.; Mancini, L.; Marzari, F.; Messina, S.; Molinari, E.; Munari, U.; Pepe, F.; Santos, N.; Scandariato, G.; Shkolnik, E.; Southworth, J.

    2013-06-01

    In the context of the programme Global Architecture of Planetary Systems (GAPS), we have performed radial velocity monitoring of the metal-poor star HIP 11952 on 35 nights during about 150 days using the newly installed high-resolution spectrograph HARPS-N at the TNG and HARPS at the ESO 3.6 m telescope. The radial velocities show a scatter of 7 m s-1, compatible with the measurement errors for such a moderately warm metal-poor star (Teff = 6040 ± 120 K; [Fe/H] = -1.9 ± 0.1). We exclude the presence of the two giant planets with periods of 6.95 ± 0.01 d and 290.0 ± 16.2 d and radial velocity semi-amplitudes of 100.3 ± 19.4 m s-1 and 105.2 ± 14.7 m s-1, respectively, which have recently been announced. This result is important because HIP 11952 was thought to be the most metal-poor star hosting a planetary systemwith giant planets, which challenged some models of planet formation. Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion Galileo Galilei of the INAF at the Spanish Observatorio del Roque de los Muchachos of the IAC in the frame of the programme Global Architecture of Planetary Systems (GAPS). Based on observations collected at the La Silla Observatory, ESO (Chile): Program 185.D-0056.Table 1 is available in electronic form at http://www.aanda.org

  18. HST/STIS abundances in the uranium rich metal poor star CS 31082-001: Constraints on the r-Process

    NASA Astrophysics Data System (ADS)

    Siqueira-Mello, C.; Spite, M.; Barbuy, B.; Spite, F.; Caffau, E.; Hill, V.; Wanajo, S.; Primas, F.; Plez, B.; Cayrel, R.; Andersen, J.; Nordström, B.; Sneden, C.; Beers, T. C.; Bonifacio, P.; François, P.; Molaro, P.

    2016-01-01

    As a brief revision, the origin of heavy elements and the role of abundances in extremely metal-poor (EMP) stars are presented. Heavy element abundances in the EMP uranium-rich star CS 31082-001 based mainly on near-UV spectra from STIS/HST are presented. These results should be useful for a better characterisation of the neutron exposure(s) that produced the r-process elements in this star, as well as a guide for improving nuclear data and astrophysical site modelling, given that the new element abundances not available in previous works (Ge, Mo, Lu, Ta, W, Re, Pt, Au, and Bi) make CS 31082-001 the most completely well studied r-II object, with a total of 37 detections of n-capture elements.

  19. Carbon-enhanced metal-poor star frequencies in the galaxy: corrections for the effect of evolutionary status on carbon abundances

    SciTech Connect

    Placco, Vinicius M.; Frebel, Anna; Beers, Timothy C.; Stancliffe, Richard J.

    2014-12-10

    We revisit the observed frequencies of carbon-enhanced metal-poor (CEMP) stars as a function of the metallicity in the Galaxy, using data from the literature with available high-resolution spectroscopy. Our analysis excludes stars exhibiting clear overabundances of neutron-capture elements and takes into account the expected depletion of surface carbon abundance that occurs due to CN processing on the upper red giant branch. This allows for the recovery of the initial carbon abundance of these stars, and thus for an accurate assessment of the frequencies of carbon-enhanced stars. The correction procedure we develop is based on stellar-evolution models and depends on the surface gravity, log g, of a given star. Our analysis indicates that for stars with [Fe/H] ≤–2.0, 20% exhibit [C/Fe] ≥+0.7. This fraction increases to 43% for [Fe/H] ≤–3.0 and 81% for [Fe/H] ≤–4.0, which is higher than have been previously inferred without taking the carbon abundance correction into account. These CEMP star frequencies provide important inputs for Galactic and stellar chemical evolution models, as they constrain the evolution of carbon at early times and the possible formation channels for the CEMP-no stars. We also have developed a public online tool with which carbon corrections using our procedure can be easily obtained.

  20. Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars

    NASA Astrophysics Data System (ADS)

    Amarsi, A. M.; Lind, K.; Asplund, M.; Barklem, P. S.; Collet, R.

    2016-08-01

    As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic STAGGER model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe I/Fe II excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is overestimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves.We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally- and temporally-averaged 3D STAGGER model atmospheres.

  1. r-Process Elements in EMP stars: Indicators of Inhomogeneous Early Halo Enrichment

    NASA Astrophysics Data System (ADS)

    Andersen, Johannes; Nordström, Birgitta; Thidemann Hansen, Terese

    2015-08-01

    Extremely metal-poor (EMP) halo stars with [Fe/H] below ~ -3 are considered to be fossil records of conditions in the early halo. In the simplest picture where iron is a proxy for overall metallicity and indirectly for time, EMP stars formed before the oldest and most metal-poor Galactic globular clusters. High-resolution spectroscopy with 8m-class telescopes has shown the detailed abundance pattern of these stars to be surprisingly uniform (e.g. Bonifacio+ 2012) and essentially Solar, apart from the α-enhancement typical of SN II nucleosynthesis. A small fraction (~3%) of EMP stars, however, is strongly enhanced in the heaviest (r-process) neutron-capture elements, highlighting that the periodic system of elements was fully populated already this early.These striking departures from the general chemical homogeneity could be produced by local or distant sources. The former case is simple - mass transfer from a binary companion that evolved to produce a highly neutron-rich environment (one or more NS). Alternatively, the r-process elements were formed in a site at interstellar distance and preferentially seeded into the natal clouds of the present-day EMP-r stars. Our long-term, precise monitoring of the radial velocities of a sample of such stars (Hansen+ 2011) disproved the binary hypothesis, which would in fact also fail to explain the existence of r-process poor stars, such as HD 122653. We thus conclude that the chemical enrichment of the early halo was far more complex, patchy and likely anisotropic than assumed in current models of Galactic chemical evolution: The EMP-r stars are not just peculiarities to be ignored, but indicate that a new level of complexity must be invoked. That r-process elements have not (yet) been observed in high-redshift DLA systems is readily explained by their low abundance relative to the lighter species and the rarity of strong enrichment events.

  2. First stars. XVI. HST/STIS abundances of heavy elements in the uranium-rich metal-poor star CS 31082-001

    NASA Astrophysics Data System (ADS)

    Siqueira Mello, C.; Spite, M.; Barbuy, B.; Spite, F.; Caffau, E.; Hill, V.; Wanajo, S.; Primas, F.; Plez, B.; Cayrel, R.; Andersen, J.; Nordström, B.; Sneden, C.; Beers, T. C.; Bonifacio, P.; François, P.; Molaro, P.

    2013-02-01

    Context. The origin and site(s) of the r-process nucleosynthesis is(are) still not known with certainty, but complete, detailed r-element abundances offer our best clues. The few extremely metal-poor (EMP) stars with large r-element excesses allow us to study the r-process signatures in great detail, with minimal interference from later stages of Galactic evolution. CS 31082-001 is an outstanding example of the information that can be gathered from these exceptional stars. Aims: Here we aim to complement our previous abundance determinations for third-peak r-process elements with new and improved results for elements of the first and second r-process peaks from near-UV HST/STIS and optical UVES spectra. These results should provide new insight into the nucleosynthesis of the elements beyond iron. Methods: The spectra were analyzed by a consistent approach based on an OSMARCS LTE model atmosphere and the Turbospectrum spectrum synthesis code to derive abundances of heavy elements in CS 31082-001, and using updated oscillator strengths from the recent literature. Synthetic spectra were computed for all lines of the elements of interest to check for proper line intensities and possible blends in these crowded spectra. Our new abundances were combined with the best previous results to provide reliable mean abundances for the first and second-peak r-process elements. Results: We present new abundances for 23 neutron-capture elements, 6 of which - Ge, Mo, Lu, Ta, W, and Re - have not been reported before. This makes CS 31082-001 the most completely studied r-II star, with abundances for a total of 37 neutron-capture elements. We also present the first NLTE+3D abundance of lead in this star, further constraining the nature of the r-process. Based on observations made with the NASA/ESA Hubble Space Telescope (HST) through the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555; and

  3. IMPROVED Co i log(gf) VALUES AND ABUNDANCE DETERMINATIONS IN THE PHOTOSPHERES OF THE SUN AND METAL-POOR STAR HD 84937

    SciTech Connect

    Lawler, J. E.; Sneden, C.; Cowan, J. J. E-mail: chris@verdi.as.utexas.edu

    2015-09-15

    New emission branching fraction measurements for 898 lines of the first spectrum of cobalt (Co i) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer on Kitt Peak, AZ and a high-resolution echelle spectrometer. Published radiative lifetimes from laser induced fluorescence measurements are combined with the branching fractions to determine accurate absolute atomic transition probabilities for the 898 lines. Hyperfine structure (hfs) constants for levels of neutral Co in the literature are surveyed and selected values are used to generate complete hfs component patterns for 195 transitions of Co i. These new laboratory data are applied to determine the Co abundance in the Sun and metal-poor star HD 84937, yielding log ϵ(Co) = 4.955 ± 0.007 (σ = 0.059) based on 82 Co i lines and log ϵ(Co) = 2.785 ± 0.008 (σ = 0.065) based on 66 Co i lines, respectively. A Saha or ionization balance test on the photosphere of HD 84937 is performed using 16 UV lines of Co ii, and good agreement is found with the Co i result in this metal-poor ([Fe i/H] = −2.32, [Fe ii/H] = −2.32) dwarf star. The resulting value of [Co/Fe] = +0.14 supports a rise of Co/Fe at low metallicity that has been suggested in other studies.

  4. Improved Co I log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

    NASA Astrophysics Data System (ADS)

    Lawler, J. E.; Sneden, C.; Cowan, J. J.

    2015-09-01

    New emission branching fraction measurements for 898 lines of the first spectrum of cobalt (Co i) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer on Kitt Peak, AZ and a high-resolution echelle spectrometer. Published radiative lifetimes from laser induced fluorescence measurements are combined with the branching fractions to determine accurate absolute atomic transition probabilities for the 898 lines. Hyperfine structure (hfs) constants for levels of neutral Co in the literature are surveyed and selected values are used to generate complete hfs component patterns for 195 transitions of Co i. These new laboratory data are applied to determine the Co abundance in the Sun and metal-poor star HD 84937, yielding log ɛ(Co) = 4.955 ± 0.007 (σ = 0.059) based on 82 Co i lines and log ɛ(Co) = 2.785 ± 0.008 (σ = 0.065) based on 66 Co i lines, respectively. A Saha or ionization balance test on the photosphere of HD 84937 is performed using 16 UV lines of Co ii, and good agreement is found with the Co i result in this metal-poor ([Fe i/H] = -2.32, [Fe ii/H] = -2.32) dwarf star. The resulting value of [Co/Fe] = +0.14 supports a rise of Co/Fe at low metallicity that has been suggested in other studies.

  5. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    DOE PAGES

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    In this paper, we study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ~ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ~ 108–1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108–109 M⊙),more » and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (<<1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (~2%–5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (~40%–80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (~20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. In conclusion, we suggest that the MW could be a "transient fossil"; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

  6. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    NASA Astrophysics Data System (ADS)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ∼ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ∼ 108–1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108–109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < ‑2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  7. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    NASA Astrophysics Data System (ADS)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (˜2%-5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (˜40%-80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  8. Uncovering multiple Wolf-Rayet star clusters and the ionized ISM in Mrk 178: the closest metal-poor Wolf-Rayet H II galaxy

    NASA Astrophysics Data System (ADS)

    Kehrig, C.; Pérez-Montero, E.; Vílchez, J. M.; Brinchmann, J.; Kunth, D.; García-Benito, R.; Crowther, P. A.; Hernández-Fernández, J.; Durret, F.; Contini, T.; Fernández-Martín, A.; James, B. L.

    2013-07-01

    New integral field spectroscopy (IFS) has been obtained for the nearby metal-poor Wolf-Rayet (WR) galaxy Mrk 178 to examine the spatial correlation between its WR stars and the neighbouring ionized interstellar medium (ISM). The strength of the broad WR features and its low metallicity make Mrk 178 an intriguing object. We have detected the blue and red WR bumps in different locations across the field of view (˜300 pc × 230 pc) in Mrk 178. The study of the WR content has been extended, for the first time, beyond its brightest star-forming knot uncovering new WR star clusters. Using Large/Small Magellanic Cloud-template WR stars, we empirically estimate a minimum of ˜20 WR stars within the region sampled. Maps of the spatial distribution of the emission lines and of the physical-chemical properties of the ionized ISM have been created and analysed. Here, we refine the statistical methodology by Pérez-Montero et al. (2011) to probe the presence of variations in the ISM properties. An error-weighted mean of 12+log(O/H) = 7.72 ± 0.01 is taken as the representative oxygen abundance for Mrk 178. A localized N and He enrichment, spatially correlated with WR stars, is suggested by this analysis. Nebular He II λ4686 emission is shown to be spatially extended reaching well beyond the location of the WR stars. This spatial offset between WRs and He II emission can be explained based on the mechanical energy input into the ISM by the WR star winds, and does not rule out WR stars as the He II ionization source. We study systematic aperture effects on the detection and measurement of the WR features, using Sloan Digital Sky Survey spectra combined with the power of IFS. In this regard, the importance of targeting low metallicity nearby systems is discussed.

  9. The Diverse Origins of Neutron-capture Elements in the Metal-poor Star HD 94028: Possible Detection of Products of i-Process Nucleosynthesis

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Karakas, Amanda I.; Pignatari, Marco; Herwig, Falk

    2016-04-01

    We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = ‑1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = ‑0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy. These data are associated with Program 072.B-0585(A), PI. Silva. Some data presented in this paper were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute

  10. The Diverse Origins of Neutron-capture Elements in the Metal-poor Star HD 94028: Possible Detection of Products of i-Process Nucleosynthesis

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Karakas, Amanda I.; Pignatari, Marco; Herwig, Falk

    2016-04-01

    We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = -1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = -0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy. These data are associated with Program 072.B-0585(A), PI. Silva. Some data presented in this paper were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute is

  11. VizieR Online Data Catalog: Extremely metal-poor stars CaII triplet (Carrera+, 2013)

    NASA Astrophysics Data System (ADS)

    Carrera, R.; Pancino, E.; Gallart, C.; Del Pino, A.

    2014-09-01

    The CaT calibration obtained in Paper I (Carrera et al., 2007AJ....134.1298C, Cat. J/AJ/134/1298) was obtained from observations of almost 500 RGB stars in 29 Galactic open and globular clusters, covering a total metallicity range of -2.33<=[Fe/H]<=+0.47. We used the same sample in this paper, so we refer the reader to Paper I for a detailed discussion about the observations and data reduction of these stars. (4 data files).

  12. Population studies. XIII. A new analysis of the Bidelman-Macconnell 'weak-metal' stars - confirmation of metal-poor stars in the thick disk of the galaxy

    SciTech Connect

    Beers, Timothy C.; Norris, John E.; Placco, Vinicius M.; Lee, Young Sun; Rossi, Silvia; Carollo, Daniela; Masseron, Thomas E-mail: jen@mso.anu.edu.au E-mail: youngsun@cnu.ac.kr E-mail: daniela.carollo@mq.edu.au

    2014-10-10

    A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R ∼ 3000) optical spectra have been obtained for 302 of the candidate 'weak-metal' stars selected by Bidelman and MacConnell. We use these data to calibrate the recently developed generalization of the Sloan Extension for Galactic Exploration and Understanding and Exploration (SEGUE) Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (T {sub eff}, log g, and [Fe/H]) for these non-Sloan Digital Sky Survey/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities –1.8 < [Fe/H] ≤–0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy versus angular momentum diagram. A comparison is made with recent results for a similar-size sample of Radial Velocity Experiment stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.

  13. IMPROVED Ni I log(gf) VALUES AND ABUNDANCE DETERMINATIONS IN THE PHOTOSPHERES OF THE SUN AND METAL-POOR STAR HD 84937

    SciTech Connect

    Wood, M. P.; Lawler, J. E.; Sneden, C.; Cowan, J. J. E-mail: jelawler@wisc.edu E-mail: cowan@nhn.ou.edu

    2014-04-01

    Atomic transition probability measurements for 371 Ni I lines in the UV through near-IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer and a new echelle spectrograph are combined with published radiative lifetimes to determine these transition probabilities. Generally good agreement is found in comparisons to previously reported Ni I transition probability measurements. Use of the new echelle spectrograph, independent radiometric calibration methods, and independent data analysis routines enable a reduction of systematic errors and overall improvement in transition probability uncertainty over previous measurements. The new Ni I data are applied to high-resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ni abundances. Lines covering a wide range of wavelength and excitation potential are used to search for non-LTE effects.

  14. The Evolution of Pristine Gas: Implications for Milky Way Halo Stars

    NASA Astrophysics Data System (ADS)

    Sarmento, Richard J.; Scannapieco, Evan; Pan, Liubin

    2016-06-01

    We implement a new subgrid model for turbulent mixing to accurately follow the cosmological evolution of the first stars, the mixing of their supernova ejecta and the impact on the chemical composition of the Galactic Halo. Using the cosmological adaptive mesh refinement code RAMSES, we implement a model for the pollution of pristine gas as described in Pan et al. (2013). This allows us to account for the fraction of Z < Zcrit stars formed throughout the simulation volume, even in regions in which the average metallicity is well above Zcrit. Further, as a result of modeling the pristine fraction of gas, we also improve our modeling of the metallicity of the polluted fraction, fpol, of both the gas and stars.Additionally, we track the evolution of the “primordial metals” generated by Pop III supernovae. These metals are taken up by second-generation stars and are likely to lead to unique abundance signatures characteristic of carbon enhanced, metal poor (CEMP) stars. As an illustrative example, we associate primordial metals with abundance ratios used by Keller at al (2014) to explain the source of metals in the star SMSS J031300.36- 670839.3, finding good agreement with the observed [Fe/H], [C/H], [O/H] and [Mg/Ca] ratios in CEMP Milky Way (MW) halo stars.

  15. WASP-37b: A 1.8 M{sub J} EXOPLANET TRANSITING A METAL-POOR STAR

    SciTech Connect

    Simpson, E. K.; Faedi, F.; Barros, S. C. C.; Pollacco, D.; Todd, I.; McCormac, J.; Brown, D. J. A.; Cameron, A. Collier; Miller, G. R. M.; Hebb, L.; Smalley, B.; Anderson, D. R.; Butters, O. W.; Hebrard, G.; Boisse, I.; Santerne, A.; Street, R. A.; Skillen, I.; Triaud, A. H. M. J.; Bento, J.

    2011-01-15

    We report on the discovery of WASP-37b, a transiting hot Jupiter orbiting an m{sub v} = 12.7 G2-type dwarf, with a period of 3.577469 {+-} 0.000011 d, transit epoch T{sub 0} = 2455338.6188 {+-} 0.0006 (HJD; dates throughout the paper are given in Coordinated Universal Time (UTC)), and a transit duration 0.1304{sup +0.0018}{sub -0.0017} d. The planetary companion has a mass M{sub p} = 1.80 {+-} 0.17 M{sub J} and radius R{sub p} = 1.16{sup +0.07}{sub -0.06} R{sub J}, yielding a mean density of 1.15{sup +0.12}{sub -0.15} {rho}{sub J}. From a spectral analysis, we find that the host star has M{sub *} = 0.925 {+-} 0.120 M{sub sun}, R{sub *} = 1.003 {+-} 0.053 R{sub sun}, T{sub eff} = 5800 {+-} 150 K, and [Fe/H] = -0.40 {+-} 0.12. WASP-37 is therefore one of the lowest metallicity stars to host a transiting planet.

  16. KELT-6b: A P ∼ 7.9 day hot Saturn transiting a metal-poor star with a long-period companion

    SciTech Connect

    Collins, Karen A.; Kielkopf, John F.; Eastman, Jason D.; Beatty, Thomas G.; Gaudi, B. Scott; Siverd, Robert J.; Pepper, Joshua; Stassun, Keivan G.; Johnson, John Asher; Howard, Andrew W.; Fulton, Benjamin J.; Fischer, Debra A.; Manner, Mark; Bieryla, Allyson; Latham, David W.; Gregorio, Joao; Buchhave, Lars A.; Jensen, Eric L. N.; Penev, Kaloyan; Crepp, Justin R.; and others

    2014-02-01

    We report the discovery of KELT-6b, a mildly inflated Saturn-mass planet transiting a metal-poor host. The initial transit signal was identified in KELT-North survey data, and the planetary nature of the occulter was established using a combination of follow-up photometry, high-resolution imaging, high-resolution spectroscopy, and precise radial velocity measurements. The fiducial model from a global analysis including constraints from isochrones indicates that the V = 10.38 host star (BD+31 2447) is a mildly evolved, late-F star with T {sub eff} = 6102 ± 43 K, log g{sub ⋆}=4.07{sub −0.07}{sup +0.04}, and [Fe/H] = –0.28 ± 0.04, with an inferred mass M {sub *} = 1.09 ± 0.04 M {sub ☉} and radius R{sub ⋆}=1.58{sub −0.09}{sup +0.16} R{sub ⊙}. The planetary companion has mass M{sub P} = 0.43 ± 0.05 M {sub Jup}, radius R{sub P}=1.19{sub −0.08}{sup +0.13} R{sub Jup}, surface gravity log g{sub P}=2.86{sub −0.08}{sup +0.06}, and density ρ{sub P}=0.31{sub −0.08}{sup +0.07} g cm{sup −3}. The planet is on an orbit with semimajor axis a = 0.079 ± 0.001 AU and eccentricity e=0.22{sub −0.10}{sup +0.12}, which is roughly consistent with circular, and has ephemeris of T {sub c}(BJD{sub TDB}) = 2456347.79679 ± 0.00036 and P = 7.845631 ± 0.000046 days. Equally plausible fits that employ empirical constraints on the host-star parameters rather than isochrones yield a larger planet mass and radius by ∼4)-7). KELT-6b has surface gravity and incident flux similar to HD 209458b, but orbits a host that is more metal poor than HD 209458 by ∼0.3 dex. Thus, the KELT-6 system offers an opportunity to perform a comparative measurement of two similar planets in similar environments around stars of very different metallicities. The precise radial velocity data also reveal an acceleration indicative of a longer-period third body in the system, although the companion is not detected in Keck adaptive optics images.

  17. The first Population II stars formed in externally enriched mini-haloes

    NASA Astrophysics Data System (ADS)

    Smith, Britton D.; Wise, John H.; O'Shea, Brian W.; Norman, Michael L.; Khochfar, Sadegh

    2015-09-01

    We present a simulation of the formation of the earliest Population II stars, starting from cosmological initial conditions and ending when metals created in the first supernovae are incorporated into a collapsing gas cloud. This occurs after a supernova blast-wave collides with a nearby mini-halo, inducing further turbulence that efficiently mixes metals into the dense gas in the centre of the halo. The gas that first collapses has been enriched to a metallicity of Z ˜ 2 × 10-5 Z⊙. Due to the extremely low metallicity, collapse proceeds similarly to metal-free gas until dust cooling becomes efficient at high densities, causing the cloud to fragment into a large number of low-mass objects. This external enrichment mechanism provides a plausible origin for the most metal-poor stars observed, such as SMSS J031300.36-670839.3, that appear to have formed out of gas enriched by a single supernova. This mechanism operates on shorter time-scales than the time for low-mass mini-haloes (M ≤ 5 × 105 M⊙) to recover their gas after experiencing a supernova. As such, metal-enriched stars will likely form first via this channel if the conditions are right for it to occur. We identify a number of other externally enriched haloes that may form stars in this manner. These haloes have metallicities as high as 0.01 Z⊙, suggesting that some members of the first generation of metal-enriched stars may be hiding in plain sight in current stellar surveys.

  18. Very Low-Mass Stars with Extremely Low Metallicity in the Milky Way's Halo

    NASA Astrophysics Data System (ADS)

    Aoki, Wako; Beers, Timothy C.; Suda, Takuma; Honda, Satoshi; Lee, Young Sun

    2016-08-01

    Large surveys and follow-up spectroscopic studies in the past few decades have been providing chemical abundance data for a growing number of very metal-poor ([Fe/H] <-2) stars. Most of them are red giants or main-sequence turn-off stars having masses near 0.8 solar masses. Lower mass stars with extremely low metallicity ([Fe/H] <-3) are yet to be explored. Our high-resolution spectroscopic study for very metal-poor stars found with SDSS has identified four cool main-sequence stars with [Fe/H] <-2.5 among 137 objects (Aoki et al. 2013). The effective temperatures of these stars are 4500-5000 K, corresponding to a mass of around 0.5 solar masses. Our standard analysis of the high-resolution spectra based on 1D-LTE model atmospheres has obtained self-consistent chemical abundances for these objects, assuming small values of micro-turbulent velocities compared with giants and turn-off stars. The low temperature of the atmospheres of these objects enables us to measure their detailed chemical abundances. Interestingly, two of the four stars have extreme chemical-abundance patterns: one has the largest excesses of heavy neutron-capture elements associated with the r-process abundance pattern known to date (Aoki et al. 2010), and the other exhibits low abundances of the α-elements and odd-Z elements, suggested to be signatures of the yields of very massive stars (> 100 solar masses; Aoki et al. 2014). Although the sample size is still small, these results indicate the potential of very low-mass stars as probes to study the early stages of the Milky Way's halo formation.

  19. Evolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production

    NASA Astrophysics Data System (ADS)

    Gil-Pons, P.; Doherty, C. L.; Lau, H.; Campbell, S. W.; Suda, T.; Guilani, S.; Gutiérrez, J.; Lattanzio, J. C.

    2013-09-01

    Aims: Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars. Methods: Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M⊙, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Results: Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars are able to lose most of their envelopes before their cores reach the Chandrasekhar mass (mCh), so our standard models do not predict the occurrence of SNI1/2 for Z = 10-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M⊙ for our 4 M⊙ model and 1.5 M⊙ for our 9 M⊙ model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Conclusions: Our results have implications for the early chemical evolution of the Universe and might provide another piece for the puzzle of the carbon

  20. On the necessity of composition-dependent low-temperature opacity in models of metal-poor asymptotic giant branch stars

    SciTech Connect

    Constantino, Thomas; Campbell, Simon; Lattanzio, John; Gil-Pons, Pilar

    2014-03-20

    The vital importance of composition-dependent low-temperature opacity in low-mass (M ≤ 3 M {sub ☉}) asymptotic giant branch (AGB) stellar models of metallicity Z ≥ 0.001 has recently been demonstrated. Its significance to more metal-poor, intermediate-mass (M ≥ 2.5 M {sub ☉}) models has yet to be investigated. We show that its inclusion in lower-metallicity models ([Fe/H] ≤–2) is essential and that there exists no threshold metallicity below which composition-dependent molecular opacity may be neglected. We find it to be crucial in all intermediate-mass models investigated ([Fe/H] ≤–2 and 2.5 ≤ M/M {sub ☉} ≤ 5), because of the evolution of the surface chemistry, including the orders of magnitude increase in the abundance of molecule-forming species. Its effect on these models mirrors that previously reported for higher-metallicity models—increase in radius, decrease in T {sub eff}, faster mass loss, shorter thermally pulsing AGB lifetime, reduced enrichment in third dredge-up products (by a factor of 3-10), and an increase in the mass limit for hot bottom burning. We show that the evolution of low-metallicity models with composition-dependent low-temperature opacity is relatively independent of initial metal abundance because its contribution to the opacity is far outweighed by changes resulting from dredge-up. Our results imply a significant reduction in the expected number of nitrogen-enhanced metal-poor stars, which may help explain their observed paucity. We note that these findings are partially a product of the macrophysics adopted in our models, in particular, the Vassiliadis and Wood mass loss rate which is strongly dependent on radius.

  1. Post Asymptotic Giant Branch and Central Stars of Planetary Nebulae in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Weston, Simon

    2012-01-01

    database. Combined with previously identified halo post-AGB stars, including the results of a sub-sample from the Palomar-Green (PG) survey, the number of observed and predicted populations are compared. The number of observed post-AGB candidates shows a remarkable deficit to expectations. A survey within a subset of the photometric database of SDSS supports the findings of the PG and SDSS spectroscopic surveys. These findings provide strong evidence for a lack of post-AGB stars in the Galactic halo and thick disc. A plausible explanation is that a large fraction of stars in these old, metal-poor populations are evolving via alternative channels. The implications of such a result are far reaching with knock on effects for stellar evolutionary theory, galactic evolution and extragalactic redshift estimates.

  2. IMPROVED Ti II log(gf) VALUES AND ABUNDANCE DETERMINATIONS IN THE PHOTOSPHERES OF THE SUN AND METAL-POOR STAR HD 84937

    SciTech Connect

    Wood, M. P.; Lawler, J. E.; Sneden, C.; Cowan, J. J. E-mail: jelawler@wisc.edu E-mail: cowan@nhn.ou.edu

    2013-10-01

    Atomic transition probability measurements for 364 lines of Ti II in the UV through near-IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer (FTS) and a new echelle spectrometer are combined with published radiative lifetimes to determine these transition probabilities. The new results are in generally good agreement with previously reported FTS measurements. Use of the new echelle spectrometer, independent radiometric calibration methods, and independent data analysis routines enables a reduction of systematic errors and overall improvement in transition probability accuracy over previous measurements. The new Ti II data are applied to high-resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ti abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. The Ti abundances derived using Ti II for these two stars match those derived using Ti I and support the relative Ti/Fe abundance ratio versus metallicity seen in previous studies.

  3. IMPROVED V I log(gf) VALUES AND ABUNDANCE DETERMINATIONS IN THE PHOTOSPHERES OF THE SUN AND METAL-POOR STAR HD 84937

    SciTech Connect

    Lawler, J. E.; Wood, M. P.; Den Hartog, E. A.; Feigenson, T.; Sneden, C.; Cowan, J. J. E-mail: mpwood@wisc.edu E-mail: tfeigenson@wisc.edu E-mail: cowan@nhn.ou.edu

    2015-01-01

    New emission branching fraction measurements for 836 lines of the first spectrum of vanadium (V I) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer (FTS) and a high-resolution echelle spectrometer. The branching fractions are combined with recently published radiative lifetimes from laser-induced fluorescence measurements to determine accurate absolute atomic transition probabilities for the 836 lines. The FTS data are also used to extract new hyperfine structure A coefficients for 26 levels of neutral vanadium. These new laboratory data are applied to determine the V abundance in the Sun and metal-poor star HD 84937, yielding log ε(V) = 3.956 ± 0.004 (σ = 0.037) based on 93 V I lines and log ε(V) = 1.89 ± 0.03 (σ = 0.07) based on nine V I lines, respectively, using the Holweger-Müller 1D model. These new V I abundance values for the Sun and HD 84937 agree well with our earlier determinations based upon V II.

  4. IMPROVED V II log(gf) VALUES, HYPERFINE STRUCTURE CONSTANTS, AND ABUNDANCE DETERMINATIONS IN THE PHOTOSPHERES OF THE SUN AND METAL-POOR STAR HD 84937

    SciTech Connect

    Wood, M. P.; Lawler, J. E.; Den Hartog, E. A.; Sneden, C.; Cowan, J. J. E-mail: jelawler@wisc.edu E-mail: chris@verdi.as.utexas.edu

    2014-10-01

    New experimental absolute atomic transition probabilities are reported for 203 lines of V II. Branching fractions are measured from spectra recorded using a Fourier transform spectrometer and an echelle spectrometer. The branching fractions are normalized with radiative lifetime measurements to determine the new transition probabilities. Generally good agreement is found between this work and previously reported V II transition probabilities. Two spectrometers, independent radiometric calibration methods, and independent data analysis routines enable a reduction in systematic uncertainties, in particular those due to optical depth errors. In addition, new hyperfine structure constants are measured for selected levels by least squares fitting line profiles in the FTS spectra. The new V II data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to determine new, more accurate V abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. Very good agreement is found between our new solar photospheric V abundance, log ε(V) = 3.95 from 15 V II lines, and the solar-system meteoritic value. In HD 84937, we derive [V/H] = –2.08 from 68 lines, leading to a value of [V/Fe] = 0.24.

  5. Improved log(gf) Values for Lines of V I and V II, New Vanadium Abundances in the Sun and the Metal-Poor Star HD 84937

    NASA Astrophysics Data System (ADS)

    Lawler, James E.; Wood, Michael P.; Den Hartog, Elizabeth; Feigenson, Thomas; Sneden, Chris; Cowan, John J.

    2015-01-01

    New emission branching fraction measurements for 836 lines of the first spectrum of vanadium (V I) and 203 lines of V II are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1m Fourier transform spectrometer (FTS) and a high resolution echelle spectrometer. The branching fractions are combined with new radiative lifetimes from laser induced fluorescence measurements to determine accurate absolute atomic transition probabilities for 1039 lines of V I and V II. The FTS data are also used to extract new hyperfine structure A coefficients for both spectra. These new laboratory data are applied to determine the V abundance in the Sun and metal-poor star HD 84937, yielding log ɛ(V) = 3.96 (σ = 0.04) based on 93 V I lines and log ɛ(V) = 1.89 (σ = 0.07) based on nine V I lines respectively, and yielding log ɛ(V) = 3.95 (σ = 0.05) based on 15 V II lines and log ɛ(V) = 1.87 (σ = 0.07) based on 68 V II lines respectively1-3.1. Wood et al., ApJS 214:18 (2014), 2. Den Hartog et al. ApJS in press (2014), 3. Lawler et al. ApJS submitted (2014). This work is supported by NASA grant NNX10AN93G (JEL), NSF AST-1211055 (EDH & JEL), and NSF AST-1211585 (CS).

  6. Improved V I Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

    NASA Astrophysics Data System (ADS)

    Lawler, J. E.; Wood, M. P.; Den Hartog, E. A.; Feigenson, T.; Sneden, C.; Cowan, J. J.

    2014-12-01

    New emission branching fraction measurements for 836 lines of the first spectrum of vanadium (V I) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer (FTS) and a high-resolution echelle spectrometer. The branching fractions are combined with recently published radiative lifetimes from laser-induced fluorescence measurements to determine accurate absolute atomic transition probabilities for the 836 lines. The FTS data are also used to extract new hyperfine structure A coefficients for 26 levels of neutral vanadium. These new laboratory data are applied to determine the V abundance in the Sun and metal-poor star HD 84937, yielding log ɛ(V) = 3.956 ± 0.004 (σ = 0.037) based on 93 V I lines and log ɛ(V) = 1.89 ± 0.03 (σ = 0.07) based on nine V I lines, respectively, using the Holweger-Müller 1D model. These new V I abundance values for the Sun and HD 84937 agree well with our earlier determinations based upon V II.

  7. Lithium in halo stars from standard stellar evolution

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Demarque, Pierre; Kawaler, Steven D.

    1990-01-01

    A grid has been constructed of theoretical evolution sequences of models for low-metallicity stars from the premain-sequence to the giant branch phases. The grid is used to study the history of surface Li abundance during standard stellar evolution. The Li-7 observations of halo stars by Spite and Spite (1982) and subsequent observations are synthesized to separate the halo stars by age. The theory of surface Li abundance is illustrated by following the evolution of a reference halo star model from the contracting fully convective premain sequence to the giant branch phase. The theoretical models are compared with observed Li abundances. The results show that the halo star lithium abundances can be explained in the context of standard stellar evolution theory using completely standard assumptions and physics.

  8. Deep Imaging of Extremely Metal-Poor Galaxies

    NASA Astrophysics Data System (ADS)

    Corbin, Michael

    2006-07-01

    Conflicting evidence exists regarding whether the most metal-poor and actively star-forming galaxies in the local universe such as I Zw 18 contain evolved stars. We propose to help settle this issue by obtaining deep ACS/HRC U, narrow-V, I, and H-alpha images of nine nearby {z < 0.01} extremely metal-poor {12 + O/H < 7.65} galaxies selected from the Sloan Digital Sky Survey. These objects are only marginally resolved from the ground and appear uniformly blue, strongly motivating HST imaging. The continuum images will establish: 1.} If underlying populations of evolved stars are present, by revealing the objects' colors on scales 10 pc, and 2.} The presence of any faint tidal features, dust lanes, and globular or super star clusters, all of which constrain the objects' evolutionary states. The H-alpha images, in combination with ground-based echelle spectroscopy, will reveal 1.} Whether the objects are producing "superwinds" that are depleting them of their metals; ground-based images of some of them indeed show large halos of ionized gas, and 2.} The correspondence of their nebular and stellar emission on scales of a few parsecs, which is important for understanding the "feedback" process by which supernovae and stellar winds regulate star formation. One of the sample objects, CGCG 269-049, lies only 2 Mpc away, allowing the detection of individual red giant stars in it if any are present. We have recently obtained Spitzer images and spectra of this galaxy to determine its dust content and star formation history, which will complement the proposed HST observations. [NOTE: THIS PROPOSAL WAS REDUCED TO FIVE ORBITS, AND ONLY ONE OF THE ORIGINAL TARGETS, CGCG 269-049, AFTER THE PHASE I REVIEW

  9. Discovery of solar system-size halos around young stars

    NASA Technical Reports Server (NTRS)

    Beckwith, S.; Skrutskie, M. F.; Zuckerman, B.; Dyck, H. M.

    1984-01-01

    Near-infrared speckle interferometric observations of five pre-main-sequence stars reveal a core-halo structure around two of these stars: HL Tau and R Mon. The halo light distribution is shown to arise from scattered light from small circumstellar particles. Halo sizes of 320 x 200 AU (alpha x delta FWHM) and 1300 x 1300 AU are deduced for HL Tau and R Mon, respectively, and the halo light is substantially bluer than the stellar light. The minimum mass of small particles in the scattering regions is comparable to the earth's mass in HL Tau and ten times greater in R Mon. Mass loss from the stars is almost certainly insufficient to produce the halo matter. The halos probably consist of relatively slowly moving matter bound gravitationally to the stars. From the size and mass of the circumstellar matter, it appears likely that these halos are in the early stage in the formation of planet-forming disks around the young stars.

  10. An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres. I. Formation of the G-band in metal-poor dwarf stars

    NASA Astrophysics Data System (ADS)

    Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H.-G.; Steffen, M.; Spite, M.

    2016-09-01

    Context. Recent developments in the three-dimensional (3D) spectral synthesis code Linfor3D have meant that for the first time, large spectral wavelength regions, such as molecular bands, can be synthesised with it in a short amount of time. Aims: A detailed spectral analysis of the synthetic G-band for several dwarf turn-off-type 3D atmospheres (5850 ≲ Teff [ K ] ≲ 6550, 4.0 ≤ log g ≤ 4.5, - 3.0 ≤ [Fe/H] ≤-1.0) was conducted, under the assumption of local thermodynamic equilibrium. We also examine carbon and oxygen molecule formation at various metallicity regimes and discuss the impact it has on the G-band. Methods: Using a qualitative approach, we describe the different behaviours between the 3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the different physics involved inevitably leads to abundance corrections, which differ over varying metallicities. Spectra computed in 1D were fit to every 3D spectrum to determine the 3D abundance correction. Results: Early analysis revealed that the CH molecules that make up the G-band exhibited an oxygen abundance dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen abundances showed zero impact to CH formation. The 3D corrections are also stronger at lower metallicity. Analysis of the 3D corrections to the G-band allows us to assign estimations of the 3D abundance correction to most dwarf stars presented in the literature. Conclusions: The 3D corrections suggest that A(C) in carbon-enhanced metal-poor (CEMP) stars with high A(C) would remain unchanged, but would decrease in CEMP stars with lower A(C). It was found that the C/O ratio is an important parameter to the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally important parameter for OH transitions under 3D. This presents a clear interrelation between the carbon and oxygen abundances in 3D atmospheres through their molecular species, which is not seen in 1D.

  11. Connecting Galaxies, Halos, and Star Formation Rates Across Cosmic Time

    SciTech Connect

    Conroy, Charlie; Wechsler, Risa H.

    2008-06-02

    A simple, observationally-motivated model is presented for understanding how halo masses, galaxy stellar masses, and star formation rates are related, and how these relations evolve with time. The relation between halo mass and galaxy stellar mass is determined by matching the observed spatial abundance of galaxies to the expected spatial abundance of halos at multiple epochs--i.e. more massive galaxies are assigned to more massive halos at each epoch. This 'abundance matching' technique has been shown previously to reproduce the observed luminosity- and scale-dependence of galaxy clustering over a range of epochs. Halos at different epochs are connected by halo mass accretion histories estimated from N-body simulations. The halo-galaxy connection at fixed epochs in conjunction with the connection between halos across time provides a connection between observed galaxies across time. With approximations for the impact of merging and accretion on the growth of galaxies, one can then directly infer the star formation histories of galaxies as a function of stellar and halo mass. This model is tuned to match both the observed evolution of the stellar mass function and the normalization of the observed star formation rate--stellar mass relation to z {approx} 1. The data demands, for example, that the star formation rate density is dominated by galaxies with M{sub star} {approx} 10{sup 10.0-10.5} M{sub {circle_dot}} from 0 < z < 1, and that such galaxies over these epochs reside in halos with M{sub vir} {approx} 10{sup 11.5-12.5} M{sub {circle_dot}}. The star formation rate--halo mass relation is approximately Gaussian over the range 0 < z < 1 with a mildly evolving mean and normalization. This model is then used to shed light on a number of issues, including (1) a clarification of 'downsizing', (2) the lack of a sharp characteristic halo mass at which star formation is truncated, and (3) the dominance of star formation over merging to the stellar build-up of galaxies

  12. Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. IV. Oxygen diagnostics in extremely metal-poor red giants with infrared OH lines

    NASA Astrophysics Data System (ADS)

    Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Caffau, E.; Ludwig, H.-G.; Steffen, M.; Spite, M.

    2015-04-01

    Context. Although oxygen is an important tracer of Galactic chemical evolution, measurements of its abundance in the atmospheres of the oldest Galactic stars are still scarce and rather imprecise. This is mainly because only a few spectral lines are available for the abundance diagnostics. At the lowest end of the metallicity scale, oxygen can only be measured in giant stars and in most of cases such measurements rely on a single forbidden [O i] 630 nm line that is very weak and frequently blended with telluric lines. Although molecular OH lines located in the ultraviolet and infrared could also be used for the diagnostics, oxygen abundances obtained from the OH lines and the [O i] 630 nm line are usually discrepant to a level of ~ 0.3-0.4 dex. Aims: We study the influence of convection on the formation of the infrared (IR) OH lines and the forbidden [O i] 630 nm line in the atmospheres of extremely metal-poor (EMP) red giant stars. Our ultimate goal is to clarify whether a realistic treatment of convection with state-of-the-art 3D hydrodynamical model atmospheres may help to bring the oxygen abundances obtained using the two indicators into closer agreement. Methods: We used high-resolution (R = 50 000) and high signal-to-noise ratio (S/N ≈ 200-600) spectra of four EMP red giant stars obtained with the VLT CRIRES spectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lines located in the spectral range of 1514-1548 and 1595-1632 nm were used to determine oxygen abundances by employing standard 1D local thermodynamic equilibrium (LTE) abundance analysis methodology. We then corrected the 1D LTE abundances obtained from each individual OH line for the 3D hydrodynamical effects, which was done by applying 3D-1D LTE abundance corrections that were determined using 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres. Results: We find that the influence of convection on the formation of [O i] 630 nm line in the atmospheres of EMP giants

  13. DUST-SCATTERED ULTRAVIOLET HALOS AROUND BRIGHT STARS

    SciTech Connect

    Murthy, Jayant; Henry, Richard Conn

    2011-06-10

    We have discovered ultraviolet (UV) halos extending as far as 5 deg. around four (of six) bright UV stars using data from the Galaxy Evolution Explorer satellite. These halos are due to scattering of the starlight from nearby thin, foreground dust clouds. We have placed limits of 0.58 {+-} 0.12 and 0.72 {+-} 0.06 on the phase function asymmetry factor (g) in the FUV (1521 A) and NUV (2320 A) bands, respectively. We suggest that these halos are a common feature around bright stars and may be used to explore the scattering function of interstellar grains at small angles.

  14. The boron-to-beryllium ratio in halo stars - A signature of cosmic-ray nucleosynthesis in the early Galaxy

    NASA Technical Reports Server (NTRS)

    Walker, T. P.; Steigman, G.; Schramm, D. N.; Olive, K. A.; Fields, B.

    1993-01-01

    We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be, and B in the early Galaxy with particular attention to the uncertainties in the predictions of this model. The observed correlation between the Be abundance and the metallicity in metal-poor Population II stars requires that Be was synthesized in the early Galaxy. We show that the observations and such Population II GCR synthesis of Be are quantitatively consistent with the big bang nucleosynthesis production of Li-7. We find that there is a nearly model independent lower bound to B/Be of about 7 for GCR synthesis. Recent measurements of B/Be about 10 in HD 140283 are in excellent agreement with the predictions of Population II GCR nucleosynthesis. Measurements of the boron abundance in additional metal-poor halo stars is a key diagnostic of the GCR spallation mechanism. We also show that Population II GCR synthesis can produce amounts of Li-6 which may be observed in the hottest halo stars.

  15. Lithium abundance in a turnoff halo star on an extreme orbit

    NASA Astrophysics Data System (ADS)

    Spite, M.; Spite, F.; Caffau, E.; Bonifacio, P.

    2015-10-01

    Context. The lithium abundance in turnoff stars of the old population of our Galaxy is remarkably constant in the metallicity interval -2.8 < [Fe/H] < -2.0, defining a plateau. The Li abundance of these turnoff stars is clearly lower than the abundance predicted by the primordial nucleosynthesis in the frame of the standard Big Bang nucleosynthesis. Different scenarios have been proposed for explaining this discrepancy, along with the very low scatter of the lithium abundance around the plateau. Aims: The recently identified very high velocity star, WISE J0725-2351 appears to belong to the old Galactic population, and appears to be an extreme halo star on a bound, retrograde Galactic orbit. In this paper, we study the abundance ratios and, in particular the lithium abundance, in this star. Methods: The available spectra (ESO-Very Large Telescope) are analyzed and the abundances of Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba are determined. Results: The abundance ratios in WISE J0725-2351 are those typical of old turnoff stars. The lithium abundance in this star is in close agreement with the lithium abundance found in the metal-poor turnoff stars located at moderate distance from the Sun. This high velocity star confirms, in an extreme case, that the very small scatter of the lithium plateau persists independent of the dynamic and kinematic properties of the stars. Based on observations obtained at the ESO Paranal Observatory, Chile Programmes 093.D-0127, PI: S. Geier and 189.B-0925, PI: S. Trager.Table 2 (line by line abundances of the elements) is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/582/A74

  16. IDENTIFYING STAR STREAMS IN THE MILKY WAY HALO

    SciTech Connect

    King, Charles III; Brown, Warren R.; Geller, Margaret J.; Kenyon, Scott J. E-mail: wbrown@cfa.harvard.edu E-mail: skenyon@cfa.harvard.edu

    2012-05-01

    We develop statistical methods for identifying star streams in the halo of the Milky Way that exploit observed spatial and radial velocity distributions. Within a great circle, departures of the observed spatial distribution from random provide a measure of the likelihood of a potential star stream. Comparisons between the radial velocity distribution within a great circle and the radial velocity distribution of the entire sample also measure the statistical significance of potential streams. The radial velocities enable construction of a more powerful joint statistical test for identifying star streams in the Milky Way halo. Applying our method to halo stars in the Hypervelocity Star (HVS) survey, we detect the Sagittarius stream at high significance. Great circle counts and comparisons with theoretical models suggest that the Sagittarius stream comprises 10%-17% of the halo stars in the HVS sample. The population of blue stragglers and blue horizontal branch stars varies along the stream and is a potential probe of the distribution of stellar populations in the Sagittarius dwarf galaxy prior to disruption.

  17. Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters

    NASA Astrophysics Data System (ADS)

    Davidge, T. J.

    2001-06-01

    JHK images obtained with the Canada-France-Hawaii Telescope adaptive optics bonnette are used to investigate the near-infrared photometric properties of red giant branch (RGB) and horizontal-branch (HB) stars in eight metal-poor globular clusters with RGC<=2 kpc. The slope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature of these clusters, four of which (NGC 6287, 6293, 6333, and 6355) are found to have metallicities that are comparable to M92. The luminosity functions of RGB stars in inner spheroid and outer halo clusters have similar slopes, although there is a tendency for core-collapsed clusters to have slightly flatter luminosity functions than noncollapsed clusters. The distribution of red HB stars on the (K, J-K) CMDs of inner spheroid clusters with [Fe/H]~-1.5 is very different from that of clusters with [Fe/H]~-2.2, suggesting that metallicity is the main parameter defining HB content among these objects. The RGB bump is detected in four of the inner spheroid clusters, and this feature is used to compute distances to these objects. Finally, the specific frequency of globular clusters in the inner Galaxy is discussed in the context of the early evolution of the bulge. Based on the ratio of metal-poor to metal-rich clusters in the inner Galaxy it is suggested that the metal-poor clusters formed during an early intense burst of star formation. It is also demonstrated that if the globular cluster formation efficiency for the inner Galaxy is similar to that measured in other spheroidal systems, then the main body of the bulge could have formed from gas that was chemically enriched in situ; hence, material from a separate pre-enriched reservoir, such as the disk or outer halo, may not be required to form the bulge.

  18. VVV SURVEY NEAR-INFRARED PHOTOMETRY OF KNOWN BULGE RR LYRAE STARS: THE DISTANCE TO THE GALACTIC CENTER AND ABSENCE OF A BARRED DISTRIBUTION OF THE METAL-POOR POPULATION

    SciTech Connect

    Dékány, I.; Minniti, D.; Catelan, M.; Zoccali, M.; Hempel, M.; Saito, R. K.

    2013-10-20

    We have combined optical and near-infrared data of known RR Lyrae (RRL) stars in the bulge in order to study the spatial distribution of its metal-poor component by measuring precise reddening values and distances of 7663 fundamental-mode RRL stars with high-quality photometry. We obtain a distance to the Galactic center of R {sub 0} = 8.33 ± 0.05 ± 0.14 kpc. We find that the spatial distribution of the RRL stars differs from the structures traced by the predominantly metal-rich red clump (RC) stars. Unlike the RC stars, the RRL stars do not trace a strong bar, but have a more spheroidal, centrally concentrated distribution, showing only a slight elongation in its very center. We find a hint of bimodality in the density distribution at high southern latitudes (b < –5°), which needs to be confirmed by extending the areal coverage of the current census. The different spatial distributions of the metal-rich and metal-poor stellar populations suggest that the Milky Way has a composite bulge.

  19. METAL-POOR LITHIUM-RICH GIANTS IN THE RADIAL VELOCITY EXPERIMENT SURVEY

    SciTech Connect

    Ruchti, Gregory R.; Fulbright, Jon P.; Wyse, Rosemary F. G.; Gilmore, Gerard F.; Grebel, Eva K.; Bienayme, Olivier; Siebert, Arnaud; Bland-Hawthorn, Joss; Freeman, Ken C.; Gibson, Brad K.; Munari, Ulisse; Navarro, Julio F.; Parker, Quentin A.; Watson, Fred G.; Reid, Warren; Seabroke, George M.; Siviero, Alessandro; Steinmetz, Matthias; Williams, Mary; Zwitter, Tomaz

    2011-12-20

    We report the discovery of eight lithium-rich field giants found in a high-resolution spectroscopic sample of over 700 metal-poor stars ([Fe/H] < -0.5) selected from the Radial Velocity Experiment survey. The majority of the Li-rich giants in our sample are very metal-poor ([Fe/H] {approx}< -1.9), and have a Li abundance (in the form of {sup 7}Li), A(Li) = log (n(Li)/n(H)) + 12, between 2.30 and 3.63, well above the typical upper red giant branch (RGB) limit, A(Li) < 0.5, while two stars, with A(Li) {approx} 1.7-1.8, show similar lithium abundances to normal giants at the same gravity. We further included two metal-poor, Li-rich globular cluster giants in our sample, namely the previously discovered M3-IV101 and newly discovered (in this work) M68-A96. This comprises the largest sample of metal-poor Li-rich giants to date. We performed a detailed abundance analysis of all stars, finding that the majority of our sample stars have elemental abundances similar to that of Li-normal halo giants. Although the evolutionary phase of each Li-rich giant cannot be definitively determined, the Li-rich phase is likely connected to extra mixing at the RGB bump or early asymptotic giant branch that triggers cool bottom processing in which the bottom of the outer convective envelope is connected to the H-burning shell in the star. The surface of a star becomes Li-enhanced as {sup 7}Be (which burns to {sup 7}Li) is transported to the stellar surface via the Cameron-Fowler mechanism. We discuss and discriminate among several models for the extra mixing that can cause Li production, given the detailed abundances of the Li-rich giants in our sample.

  20. Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars

    NASA Astrophysics Data System (ADS)

    Beers, Timothy C.; Placco, Vinicius M.; Carollo, Daniela; Santucci, Rafael; Rossi, Siliva; Lee, Young Sun; Denissenkov, Pavel; Tumlinson, Jason; Tissera, Patricia; Lentner, Geoffrey

    2016-01-01

    In a pioneering effort, Preston et al. (1991, AJ 375, 121) reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy.We demonstrate that the mean de-reddened g-r color increases outward in the Galaxy from -0.22 to -0.08 (over a color window spanning [-0.3:0.0]) from regions close to the Galactic center to ~40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15 kpc of the Galaxy. Within this centralregion, which we refer to as the Ancient Chronographic Sphere (ACS), the age difference spans a mean color range of about 0.05 mag (~0.8 Gyrs). Interestingly, the ACS extends far enough to include the Solar Neighborhood, suggesting that ancient metal-poor stars should be readily detectable in the vicinity of the Sun. Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population.We acknowledge partial support from the grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation.

  1. Visibility of stars, halos, and rainbows during solar eclipses.

    PubMed

    Können, Gunther P; Hinz, Claudia

    2008-12-01

    The visibility of stars, planets, diffraction coronas, halos, and rainbows during the partial and total phases of a solar eclipse is studied. The limiting magnitude during various stages of the partial phase is presented. The sky radiance during totality with respect to noneclipse conditions is revisited and found to be typically 1/4000. The corresponding limiting magnitude is +3.5. At totality, the signal-to-background ratio of diffraction coronas, halos, and rainbows has dropped by a factor of 250. It is found that diffraction coronas around the totally eclipsed Sun may nevertheless occur. Analyses of lunar halo observations during twilight indicate that bright halo displays may also persist during totality. Rainbows during totality seem impossible. PMID:19037334

  2. Visibility of stars, halos, and rainbows during solar eclipses.

    PubMed

    Können, Gunther P; Hinz, Claudia

    2008-12-01

    The visibility of stars, planets, diffraction coronas, halos, and rainbows during the partial and total phases of a solar eclipse is studied. The limiting magnitude during various stages of the partial phase is presented. The sky radiance during totality with respect to noneclipse conditions is revisited and found to be typically 1/4000. The corresponding limiting magnitude is +3.5. At totality, the signal-to-background ratio of diffraction coronas, halos, and rainbows has dropped by a factor of 250. It is found that diffraction coronas around the totally eclipsed Sun may nevertheless occur. Analyses of lunar halo observations during twilight indicate that bright halo displays may also persist during totality. Rainbows during totality seem impossible.

  3. Neutron stars and white dwarfs in galactic halos

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1989-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  4. Neutron stars and white dwarfs in galactic halos?

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1990-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  5. Carbon Stars in the Satellites and Halo of M31

    NASA Astrophysics Data System (ADS)

    Hamren, Katherine; Beaton, Rachael L.; Guhathakurta, Puragra; Gilbert, Karoline M.; Tollerud, Erik J.; Boyer, Martha L.; Rockosi, Constance M.; Smith, Graeme H.; Majewski, Steven R.; Howley, Kirsten

    2016-09-01

    We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color-color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample’s eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.

  6. Carbon Stars in the Satellites and Halo of M31

    NASA Astrophysics Data System (ADS)

    Hamren, Katherine; Beaton, Rachael L.; Guhathakurta, Puragra; Gilbert, Karoline M.; Tollerud, Erik J.; Boyer, Martha L.; Rockosi, Constance M.; Smith, Graeme H.; Majewski, Steven R.; Howley, Kirsten

    2016-09-01

    We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color–color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample’s eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.

  7. The chemical composition of TS 01, the most oxygen-deficient planetary nebula. AGB nucleosynthesis in a metal-poor binary star

    NASA Astrophysics Data System (ADS)

    Stasińska, G.; Morisset, C.; Tovmassian, G.; Rauch, T.; Richer, M. G.; Peña, M.; Szczerba, R.; Decressin, T.; Charbonnel, C.; Yungelson, L.; Napiwotzki, R.; Simón-Díaz, S.; Jamet, L.

    2010-02-01

    The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A) with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h) is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling which takes into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Incidentally we find from the new observational constraints that both stellar components contribute to the ionization: the “cool” one provides the bulk of hydrogen ionization, while the “hot” one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be 1/3.5, 1/4.2, 1/70, and 1/11 of the solar value respectively, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of solar. The abundance of He relative to H is 0.089 ± 0.009. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1 M⊙ requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with an initial mass of 1 M⊙, appropriate metallicity, and initial rotation of 100 km s-1, and find that rotation greatly improves the agreement between the predicted and observed abundances. Based on observations obtained at the Canada

  8. Resolved Stellar Halos of M87 and NGC 5128

    NASA Astrophysics Data System (ADS)

    Bird, Sarah A.; Harris, William; Flynn, Chris; Blakeslee, John P.; Valtonen, Mauri

    2015-08-01

    We search halo fields of two giant elliptical galaxies: M87, using HST images at 10 kpc from the center, and NGC 5128 (Cen A), using VIMOS VLT images at 65 kpc from the center and archival HST data from 8 to 38 kpc from the center. We resolve thousands of red-giant-branch stars in these stellar halo fields using V and I filters, and, in addition, measure the metallicity using stellar isochrones. In Cen A, we find that the density of metal-rich and metal-poor halo stars falls off with the same slope in the de Vaucouleurs' law profile, from the inner halo of 8 kpc out to 70 kpc, with no sign of a transition to dominance by metal-poor stars. We also find that the metallicity distribution of the inner stellar halo of M87 is most similar to that of NGC 5128's inner stellar halo.

  9. The abundance of boron in three halo stars

    NASA Technical Reports Server (NTRS)

    Duncan, Douglas K.; Lambert, David L.; Lemke, Michael

    1992-01-01

    B abundances for three halo stars: HD 140283, HD 19445, and HD 201891 are presented. Using recent determinations of the Be abundance in HD 140283, B/Be of 10 +5/-4 is found for this star, and similar ratios are inferred for HD 19445 and HD 201891. This ratio is equal to the minimum value of 10 expected from a synthesis of B and Be by high-energy cosmic-ray spallation reactions in the interstellar medium. It is shown that the accompanying synthesis of Li by alpha on alpha fusion reactions is probably a minor contributor to the observed 'primordial' Li of halo stars. The observed constant ratios of B/O and Be/O are expected if the principal channel of synthesis involves cosmic-ray CNO nuclei from the supernovae colliding with interstellar protons.

  10. Deep SDSS optical spectroscopy of distant halo stars. II. Iron, calcium, and magnesium abundances

    NASA Astrophysics Data System (ADS)

    Fernández-Alvar, E.; Allende Prieto, C.; Schlesinger, K. J.; Beers, T. C.; Robin, A. C.; Schneider, D. P.; Lee, Y. S.; Bizyaev, D.; Ebelke, G.; Malanushenko, E.; Malanushenko, V.; Oravetz, D.; Pan, K.; Simmons, A.

    2015-05-01

    Aims: We analyze a sample of 3944 low-resolution (R ~ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ Teff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods: We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by χ2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results: The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range -1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way. Table 1 and beginning of Tables 2 and 3 are available in electronic form at http://www.aanda.orgFull Tables 2 and 3 are only available at the CDS via anonymous ftp to http

  11. Li-7 abundances in halo stars: Testing stellar evolution models and the primordial Li-7 abundance

    NASA Technical Reports Server (NTRS)

    Chaboyer, Brian; Demarque, P.

    1994-01-01

    A large number of stellar evolution models with (Fe/H) = -2.3 and -3.3 have been calculated in order to determine the primordial Li-7 abundance and to test current stellar evolution models by a comparison to the extensive database of accurate Li abundances in extremely metal-poor halo stars observed by Thorburn (1994). Standard models with gray atmospheres do a very good job of fitting the observed Li abundances in stars hotter than approximately 5600 K. They predict a primordial. Li-7 abundance of log N(Li) = 2.24 +/- 0.03. Models which include microscopic diffusion predict a downward curvature in the Li-7 destruction isochrones at hot temperatures which is not present in the observations. Thus, the observations clearly rule out models which include uninhibited microscopic diffusion of Li-7 from the surface of the star. Rotational mixing inhibits the microscopic diffusion and the (Fe/H) = -2.28 stellar models which include both diffusion and rotational mixing provide an excellent match to the mean trend in T(sub eff) which is present in the observations. Both the plateau stars and the heavily depleted cool stars are well fit by these models. The rotational mixing leads to considerable Li-7 depletion in these models and the primordial Li-7 abundance inferred from these models is log N(Li) = 3.08 +/- 0.1. However, the (Fe/H) = -3.28 isochrones reveal problems with the combined models. These isochrones predict a trend of decreasing log N(Li) with increasing T(sub eff) which is not present in the observations. Possible causes for this discrepancy are discussed.

  12. CHEMICAL ABUNDANCES OF THE MILKY WAY THICK DISK AND STELLAR HALO. I. IMPLICATIONS OF [{alpha}/Fe] FOR STAR FORMATION HISTORIES IN THEIR PROGENITORS

    SciTech Connect

    Ishigaki, Miho N.; Aoki, Wako; Chiba, Masashi E-mail: aoki.wako@nao.ac.jp

    2012-07-01

    We present the abundance analysis of 97 nearby metal-poor (-3.3 < [Fe/H] <-0.5) stars having kinematic characteristics of the Milky Way (MW) thick disk and inner and outer stellar halos. The high-resolution, high-signal-to-noise optical spectra for the sample stars have been obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of Fe, Mg, Si, Ca, and Ti have been derived using a one-dimensional LTE abundance analysis code with Kurucz NEWODF model atmospheres. By assigning membership of the sample stars to the thick disk, inner halo, or outer halo components based on their orbital parameters, we examine abundance ratios as a function of [Fe/H] and kinematics for the three subsamples in wide metallicity and orbital parameter ranges. We show that, in the metallicity range of -1.5 < [Fe/H] {<=}-0.5, the thick disk stars show constantly high mean [Mg/Fe] and [Si/Fe] ratios with small scatter. In contrast, the inner and the outer halo stars show lower mean values of these abundance ratios with larger scatter. The [Mg/Fe], [Si/Fe], and [Ca/Fe] for the inner and the outer halo stars also show weak decreasing trends with [Fe/H] in the range [Fe/H] >-2. These results favor the scenarios that the MW thick disk formed through rapid chemical enrichment primarily through Type II supernovae of massive stars, while the stellar halo has formed at least in part via accretion of progenitor stellar systems having been chemically enriched with different timescales.

  13. The Gaia-ESO Survey: Detailed abundances in the metal-poor globular cluster NGC 4372

    NASA Astrophysics Data System (ADS)

    San Roman, I.; Muñoz, C.; Geisler, D.; Villanova, S.; Kacharov, N.; Koch, A.; Carraro, G.; Tautvaišiene, G.; Vallenari, A.; Alfaro, E. J.; Bensby, T.; Flaccomio, E.; Francois, P.; Korn, A. J.; Pancino, E.; Recio-Blanco, A.; Smiljanic, R.; Bergemann, M.; Costado, M. T.; Damiani, F.; Heiter, U.; Hourihane, A.; Jofré, P.; Lardo, C.; de Laverny, P.; Masseron, T.; Morbidelli, L.; Sbordone, L.; Sousa, S. G.; Worley, C. C.; Zaggia, S.

    2015-07-01

    We present the abundance analysis for a sample of 7 red giant branch stars in the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part of the Gaia-ESO Survey. This is the first extensive study of this cluster from high-resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19 ± 0.03 and find no evidence of any metallicity spread. This metallicity makes NGC 4372 one of the most metal-poor Galactic globular clusters. We also find an α-enhancement typical of halo globular clusters at this metallicity. Significant spreads are observed in the abundances of light elements. In particular, we find a Na-O anticorrelation. Abundances of O are relatively high compared with other globular clusters. This could indicate that NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al spread is also present that spans a range of more than 0.5 dex in Al abundances. Na is correlated with Al and Mgabundances at a lower significance level. This pattern suggests that the Mg-Al burning cycle is active. This behavior can also be seen in giant stars of other massive, metal-poor clusters. A relation between light and heavy s-process elements has been identified.

  14. The lithium abundance in halo stars

    NASA Technical Reports Server (NTRS)

    Hobbs, L. M.; Duncan, Douglas K.

    1987-01-01

    Spectroscopic observations are reported for 23 population II subdwarfs (selected for age homogeneity) and 31 other stars (mainly population I F and G dwarfs). The spectra were obtained in the 670.7-nm line of Li I using coude spectrographs on the 2.7-m reflector at McDonald Observatory, the 3.0-m reflector at Lick Observatory, and the 4-m Mayall reflector at KPNO during 1983-1985. The results are presented (along with selected published data) in extensive tables and graphs and analyzed. For 12 stars with space velocities v(LSR) = 160 km/s or greater and Fe/H = -1.4 or less, the atmospheric Li/H abundance is shown to depend on T(e), the mean value for T(e) = 5600 K or more being Li/H = (1.2 + or - 0.3) x 10 to the -10th. This result, also found for many of the population I stars, is interpreted as a significant constraint on the cosmic baryon/photon ratio.

  15. RADIAL VELOCITIES OF GALACTIC HALO STARS IN VIRGO

    SciTech Connect

    Brink, Thomas G.; Mateo, Mario; Martinez-Delgado, David E-mail: mmateo@umich.ed

    2010-11-15

    We present multi-slit radial velocity measurements for 111 stars in the direction of the Virgo Stellar Stream (VSS). The stars were photometrically selected to be probable main-sequence stars in the Galactic halo. When compared with the radial velocity distribution expected for the halo of the Milky Way, as well as the distribution seen in a control field, we observe a significant excess of negative velocity stars in the field, which can likely be attributed to the presence of a stellar stream. This kinematic excess peaks at a Galactic standard of rest radial velocity of -75 km s{sup -1}. A rough distance estimate suggests that this feature extends from {approx}15 kpc out to, and possibly beyond, the {approx}30 kpc limit of the study. The mean velocity of these stars is incompatible with those of the VSS itself (V{sub gsr} {approx} 130 km s{sup -1}), which we weakly detect, but it is consistent with radial velocity measurements of nearby 2MASS M-giants and SDSS+SEGUE K/M-giants and blue horizontal branch stars that constitute the leading tidal tail of the Sagittarius dwarf spheroidal galaxy. Some oblate models for the shape of the Milky Way's dark matter halo predict that the leading arm of the Sagittarius Stream should pass through this volume, and have highly negative (V{sub gsr} {approx}< -200 km s{sup -1}) radial velocities, as it descends down from the northern Galactic hemisphere toward the Galactic plane. The kinematic feature observed in this study, if it is in fact Sagittarius debris, is not consistent with these predictions, and instead, like other leading stream radial velocity measurements, is consistent with a recently published triaxial halo model, or, if axisymmetry is imposed, favors a prolate shape for the Galactic halo potential. However, a rough distance estimate to the observed kinematic feature places it somewhat closer (D {approx} 15-30 kpc) than the Sagittarius models predict (D {approx} 35-45 kpc).

  16. Infrared Halo Frames a Newborn Star

    NASA Astrophysics Data System (ADS)

    2003-08-01

    Summary: Observations with the VLT of a star-forming cloud have revealed, for the first time, a ring of infrared light around a nascent star. The images also show the presence of jets that emanate from the young object and collide with the surrounding cloud. ESO PR Photo 26a/03 ESO PR Photo 26a/03 [Preview - JPEG: 974 x 400 pix - 404k [Normal - JPEG: 1947 x 800 pix - 1M] The DC303.8-14.2 globule A small and dark interstellar cloud with the rather cryptic name of DC303.8-14.2 is located in the inner part of the Milky Way galaxy. It is seen in the southern constellation Chamaeleon and consists of dust and gas. Astronomers classify it as a typical example of a "globule". As many other globules, this cloud is also giving birth to a star. Some years ago, observations in the infrared spectral region with the ESA IRAS satellite observatory detected the signature of a nascent star at its centre. Subsequent observations with the Swedish ESO Submillimetre Telescope (SEST) at La Silla (Chile) were carried out by Finnish astronomer Kimmo Lehtinen . He revealed that DC303.8-14.2 is collapsing under its own gravity, a process which will ultimately result in the birth of a new star from the gas and dust in this cloud. Additional SEST observations of the millimetre emission of carbon monoxide (CO) molecules demonstrated a strong outflow from the nascent star. A small part of the gas that falls inward onto the central object is re-injected into the surrounding via this outward-bound "bipolar stream" . The structure of DC303.8-14.2 The left panel in PR Photo 26a/03 shows the DC303.8-14.2 globule as it looks in red light. This image was obtained at wavelength 700 nm and has been reproduced from the Digitized Sky Survey (DSS) [1]. It covers a sky region of 20 x 20 arcmin 2 , or about 50% of the area of the full moon. The dust particles in the cloud reflect the light from stars, causing the cloud to appear brighter than the adjacent sky. The brightness distribution over the cloud

  17. Using accurate phase space coordinates of ~100,00 halo field stars to constrain the Milky Way halo

    NASA Astrophysics Data System (ADS)

    Valluri, Monica

    2015-08-01

    The current cosmological paradigm predicts that dark matter halos are triaxial overall, but oblate in regions where baryons dominate. However recent measurements of the shape of the Milky Way dark matter halo find it to be very triaxial with a shape and orientation that are significantly at odds with theoretical predictions. The ESA’s Gaia satellite will soon map the entire Milky Way giving us six phase-space coordinates, ages and abundances for hundreds of thousands of halo stars. I will report progress on a new code based on the Schwarzschild orbit superposition method and orbital frequency mapping, to determine the global shape of the Milky Way's dark matter halo using field stars from Gaia. This technique will simultaneously yield the self-consistent phase-space distribution function of the stellar halo in the inner 20-30kpc region. Detailed analysis of correlations between the chemical abundances, ages and orbits of halo stars in this distribution function will enable us to extract clues to the formation history of the Milky Way that are encoded in orbital properties of halo stars.

  18. Evolutionary models of halo stars with rotation. II - Effects of metallicity on lithium depletion, and possible implications for the primordial lithium abundance

    NASA Technical Reports Server (NTRS)

    Pinsonneault, M. H.; Deliyannis, Constantine P.; Demarque, P.

    1992-01-01

    Models of metal-poor stars with rotation were computed and their lithium depletion was compared with observations of halo stars. The models that have turn-off ages compatible with the observations have a nearly flat Li-T(eff) relationship in the region of the Spite lithium 'plateau'. Depending on the initial angular momentum, the models have a depletion factor ranging between a factor of 5 and a factor of 10 at fixed T(eff), implying a maximum initial lithium abundance of 3.1. Both the dispersion and the overall depletion factor are much smaller for metal-poor models than for solar metallicity ones. The factors that determine lithium depletion in rotational models are discussed and the different depletion patterns in solar metallicity and metal-poor models are traced to differences in their structure and evolution. The dependence of the lithium depletion on age, mass, initial angular momentum, and metallicity is also discussed. The dispersion predicted from these models is not inconsistent with the observations.

  19. HUBBLE'S SEARCH FOR FAINT FIELD STARS IN GALACTIC HALO

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Left A NASA Hubble Space Telescope image of a randomly selected area of sky taken to search for faint red stars that might constitute dark matter in our Milky Way Galaxy. (Dark matter is material of an unknown type that makes up most of the mass of our galaxy). If the dark matter in our Galaxy was made of faint red stars -- as many scientists have previously conjectured -- then about 38 such stars should have been visible in this HST image. The simulated stars (diamond-shaped symbols), based on theoretical calculations, illustrate what scientists would have seen if the dark matter were locked-up in faint red stars. These surprising results rule out dim stars as an explanation for dark matter in our Galaxy. Right The unmodified HST image shows the region is actually so devoid of stars that far more distant background galaxies can easily be seen. The field is in the constellation Eridanus, far outside the plane of our Milky Way Galaxy. This region was chosen to highlight stars in the galactic halo, where dark matter exists, and to avoid the contribution of faint stars in the plane of the Galaxy. Technical Information: The image was constructed from seven exposures totaling almost three hours of searching by HST. The field shown is about 1.5 arc-minutes across. The image was taken in near-infrared light (814 nm) with the Wide Field Planetary Camera 2, on Feb 8, 1994. This observation is part of the HST parallel observing program. Credit: J Bahcall, Institute for Advance Study, Princeton and NASA

  20. Mapping the Galactic Halo. I. The ``Spaghetti'' Survey

    NASA Astrophysics Data System (ADS)

    Morrison, Heather L.; Mateo, Mario; Olszewski, Edward W.; Harding, Paul; Dohm-Palmer, R. C.; Freeman, Kenneth C.; Norris, John E.; Morita, Miwa

    2000-05-01

    We describe a major survey of the Milky Way halo designed to test for kinematic substructure caused by destruction of accreted satellites. We use the Washington photometric system to identify halo stars efficiently for spectroscopic follow-up. Tracers include halo giants (detectable out to more than 100 kpc), blue horizontal-branch (BHB) stars, halo stars near the main-sequence turnoff, and the ``blue metal-poor stars'' of Preston, Beers, & Shectman. We demonstrate the success of our survey by showing spectra of stars we have identified in all these categories, including giants as distant as 75 kpc. We discuss the problem of identifying the most distant halo giants. In particular, extremely metal-poor halo K dwarfs are present in approximately equal numbers to the distant giants for V>18, and we show that our method will distinguish reliably between these two groups of metal-poor stars. We plan to survey 100 deg2 at high Galactic latitude and expect to increase the numbers of known halo giants, BHB stars, and turnoff stars by more than an order of magnitude. In addition to the strong test that this large sample will provide for the question, Was the Milky Way halo accreted from satellite galaxies? we will improve the accuracy of mass measurements of the Milky Way beyond 50 kpc via the kinematics of the many distant giants and BHB stars we find. We show that one of our first data sets constrains the halo density law over Galactocentric radii of 5-20 kpc and z-heights of 2-15 kpc. The data support a flattened power-law halo with b/a of 0.6 and exponent -3.0. More complex models with a varying axial ratio may be needed with a larger data set.

  1. Fragmentation in Dusty Low-metallicity Star-forming Halos

    NASA Astrophysics Data System (ADS)

    Meece, Gregory R.; Smith, Britton D.; O'Shea, Brian W.

    2014-03-01

    The first stars in the universe, termed Population III, are thought to have been very massive compared to the stars that form in the present epoch. As feedback from the first generation of stars altered the contents of the interstellar medium, the universe switched to a low-mass mode of star formation, which continues in the high-metallicity stars formed in the present era. Several studies have investigated the transition between metal-free and metal-enriched star formation, with tentative evidence being found for a metallicity threshold near 10-3.5 Z ⊙ due to atomic and molecular transitions and another threshold near 10-5.5 Z ⊙ due to dust. In this work, we simulate the fragmentation of cooling gas in idealized, low-metallicity halos using the adaptive mesh refinement code Enzo. We conduct several simulations of 106 M ⊙ and 107 M ⊙ halos at z = 20 in which the metal content, initial rotation, and degree of turbulence are varied in order to study the effect of these properties on gas fragmentation over a range of densities. We find tentative support for the idea of a critical metallicity, but the effect of varying metallicity on the gas we observe is not as dramatic as what has been reported in earlier studies. It is theorized that at lower redshifts with a lower cosmic microwave background temperature, variations in metallicity might have a larger effect on cooling and fragmentation. We find no clear relation between the initial spin or the initial level of turbulence in the halo and the final properties of the gas contained therein. Additionally, we find that the degree to which the Jeans length is refined, the initial density profile of the gas, and the inclusion of deuterium chemistry each have a significant effect on the evolution and fragmentation of the gas in the halo—in particular, we find that at least 64 grid cells are needed to cover the Jeans length in order to properly resolve the fragmentation.

  2. Fragmentation in dusty low-metallicity star-forming halos

    SciTech Connect

    Meece, Gregory R.; Smith, Britton D.; O'Shea, Brian W.

    2014-03-10

    The first stars in the universe, termed Population III, are thought to have been very massive compared to the stars that form in the present epoch. As feedback from the first generation of stars altered the contents of the interstellar medium, the universe switched to a low-mass mode of star formation, which continues in the high-metallicity stars formed in the present era. Several studies have investigated the transition between metal-free and metal-enriched star formation, with tentative evidence being found for a metallicity threshold near 10{sup –3.5} Z {sub ☉} due to atomic and molecular transitions and another threshold near 10{sup –5.5} Z {sub ☉} due to dust. In this work, we simulate the fragmentation of cooling gas in idealized, low-metallicity halos using the adaptive mesh refinement code Enzo. We conduct several simulations of 10{sup 6} M {sub ☉} and 10{sup 7} M {sub ☉} halos at z = 20 in which the metal content, initial rotation, and degree of turbulence are varied in order to study the effect of these properties on gas fragmentation over a range of densities. We find tentative support for the idea of a critical metallicity, but the effect of varying metallicity on the gas we observe is not as dramatic as what has been reported in earlier studies. It is theorized that at lower redshifts with a lower cosmic microwave background temperature, variations in metallicity might have a larger effect on cooling and fragmentation. We find no clear relation between the initial spin or the initial level of turbulence in the halo and the final properties of the gas contained therein. Additionally, we find that the degree to which the Jeans length is refined, the initial density profile of the gas, and the inclusion of deuterium chemistry each have a significant effect on the evolution and fragmentation of the gas in the halo—in particular, we find that at least 64 grid cells are needed to cover the Jeans length in order to properly resolve the

  3. The temperatures of very metal-poor subdwarfs

    NASA Technical Reports Server (NTRS)

    Bell, R. A.

    1986-01-01

    The determination of ages for metal poor globular cluster stars using the relationship between stellar effective temperature and color, in order to transform isochrones from the L, Teff plane to the M(V), color plane is described. Estimates of the effective temperatures of metal-poor field subdwarfs deduced from the ratios of the integrated fluxes to the 12000 A fluxes of the 4 subdwarfs are in good agreement with published results. The stars are fainter than the models in the infrared, but the UV fluxes are in good agreement with model predictions.

  4. HAT-P-24b: AN INFLATED HOT JUPITER ON A 3.36 DAY PERIOD TRANSITING A HOT, METAL-POOR STAR

    SciTech Connect

    Kipping, D. M.; Bakos, G. A.; Hartman, J.; Torres, G.; Latham, D. W.; Noyes, R. W.; Beky, B.; Perumpilly, G.; Esquerdo, G. A.; Sasselov, D. D.; Stefanik, R. P.; Shporer, A.; Kovacs, Geza; Howard, A. W.; Marcy, G. W.; Fischer, D. A.; Johnson, J. A.; Lazar, J.; Papp, I.; Sari, P.

    2010-12-20

    We report the discovery of HAT-P-24b, a transiting extrasolar planet orbiting the moderately bright V = 11.818 F8 dwarf star GSC 0774-01441, with a period P = 3.3552464 {+-} 0.0000071 days, transit epoch T{sub c} = 2455216.97669 {+-} 0.00024 (BJD)11, and transit duration 3.653 {+-} 0.025 hr. The host star has a mass of 1.191 {+-} 0.042 M{sub sun}, radius of 1.317 {+-} 0.068 R{sub sun}, effective temperature 6373 {+-} 80 K, and a low metallicity of [Fe/H] = -0.16 {+-} 0.08. The planetary companion has a mass of 0.681 {+-} 0.031 M{sub J} and radius of 1.243 {+-} 0.072 R{sub J} yielding a mean density of 0.439 {+-} 0.069 g cm{sup -3}. By repeating our global fits with different parameter sets, we have performed a critical investigation of the fitting techniques used for previous Hungarian-made Automated Telescope planetary discoveries. We find that the system properties are robust against the choice of priors. The effects of fixed versus fitted limb darkening are also examined. HAT-P-24b probably maintains a small eccentricity of e = 0.052{sup +0.022}{sub -0.017}, which is accepted over the circular orbit model with false alarm probability 5.8%. In the absence of eccentricity pumping, this result suggests that HAT-P-24b experiences less tidal dissipation than Jupiter. Due to relatively rapid stellar rotation, we estimate that HAT-P-24b should exhibit one of the largest known Rossiter-McLaughlin effect amplitudes for an exoplanet ({Delta}V{sub RM} {approx_equal} 95 m s{sup -1}) and thus a precise measurement of the sky-projected spin-orbit alignment should be possible.

  5. POPULATION III STAR FORMATION IN LARGE COSMOLOGICAL VOLUMES. I. HALO TEMPORAL AND PHYSICAL ENVIRONMENT

    SciTech Connect

    Crosby, Brian D.; O'Shea, Brian W.; Smith, Britton D.; Turk, Matthew J.; Hahn, Oliver

    2013-08-20

    We present a semi-analytic, computationally inexpensive model to identify halos capable of forming a Population III star in cosmological simulations across a wide range of times and environments. This allows for a much more complete and representative set of Population III star forming halos to be constructed, which will lead to Population III star formation simulations that more accurately reflect the diversity of Population III stars, both in time and halo mass. This model shows that Population III and chemically enriched stars coexist beyond the formation of the first generation of stars in a cosmological simulation until at least z {approx} 10, and likely beyond, though Population III stars form at rates that are 4-6 orders of magnitude lower than chemically enriched stars by z = 10. A catalog of more than 40,000 candidate Population III forming halos were identified, with formation times temporally ranging from z = 30 to z = 10, and ranging in mass from 2.3 Multiplication-Sign 10{sup 5} M{sub Sun} to 1.2 Multiplication-Sign 10{sup 10} M{sub Sun }. At early times, the environment that Population III stars form in is very similar to that of halos hosting chemically enriched star formation. At later times Population III stars are found to form in low-density regions that are not yet chemically polluted due to a lack of previous star formation in the area. Population III star forming halos become increasingly spatially isolated from one another at later times, and are generally closer to halos hosting chemically enriched star formation than to another halo hosting Population III star formation by z {approx} 10.

  6. An age difference of two billion years between a metal-rich and a metal-poor globular cluster.

    PubMed

    Hansen, B M S; Kalirai, J S; Anderson, J; Dotter, A; Richer, H B; Rich, R M; Shara, M M; Fahlman, G G; Hurley, J R; King, I R; Reitzel, D; Stetson, P B

    2013-08-01

    Globular clusters trace the formation history of the spheroidal components of our Galaxy and other galaxies, which represent the bulk of star formation over the history of the Universe. The clusters exhibit a range of metallicities (abundances of elements heavier than helium), with metal-poor clusters dominating the stellar halo of the Galaxy, and higher-metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and were later engulfed along with their original host galaxies, and which were formed within it. Here we report an absolute age of 9.9 ± 0.7 billion years (at 95 per cent confidence) for the metal-rich globular cluster 47 Tucanae, determined by modelling the properties of the cluster's white-dwarf cooling sequence. This is about two billion years younger than has been inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than metal-poor halo clusters like NGC 6397.

  7. An age difference of two billion years between a metal-rich and a metal-poor globular cluster.

    PubMed

    Hansen, B M S; Kalirai, J S; Anderson, J; Dotter, A; Richer, H B; Rich, R M; Shara, M M; Fahlman, G G; Hurley, J R; King, I R; Reitzel, D; Stetson, P B

    2013-08-01

    Globular clusters trace the formation history of the spheroidal components of our Galaxy and other galaxies, which represent the bulk of star formation over the history of the Universe. The clusters exhibit a range of metallicities (abundances of elements heavier than helium), with metal-poor clusters dominating the stellar halo of the Galaxy, and higher-metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and were later engulfed along with their original host galaxies, and which were formed within it. Here we report an absolute age of 9.9 ± 0.7 billion years (at 95 per cent confidence) for the metal-rich globular cluster 47 Tucanae, determined by modelling the properties of the cluster's white-dwarf cooling sequence. This is about two billion years younger than has been inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than metal-poor halo clusters like NGC 6397. PMID:23903747

  8. Contributions to the Galactic halo from in-situ, kicked-out, and accreted stars

    NASA Astrophysics Data System (ADS)

    Sheffield, Allyson A.; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne V.; Majewski, Steven R.

    2016-08-01

    We report chemical abundances for a sample of 66 M giants with high S/N high-resolution spectroscopy in the inner halo of the Milky Way. The program giant stars have radial velocities that vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, based on kinematics, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants - in-situ or accreted - can be assessed. Of particular interest are a class of stars that form in-situ, deep in the Milky Way's gravitational potential well, but are ``kicked out'' of the disk into the halo due to a perturbation event. We find: (1) our sample is dominated by accreted stars and (2) tentative evidence of a small kicked-out population in our Milky Way halo sample.

  9. Fractional Yields Inferred from Halo and Thick Disk Stars

    NASA Astrophysics Data System (ADS)

    Caimmi, R.

    2013-12-01

    Linear [Q/H]-[O/H] relations, Q = Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, are inferred from a sample (N=67) of recently studied FGK-type dwarf stars in the solar neighbourhood including different populations (Nissen and Schuster 2010, Ramirez et al. 2012), namely LH (N=24, low-α halo), HH (N=25, high-α halo), KD (N=16, thick disk), and OL (N=2, globular cluster outliers). Regression line slope and intercept estimators and related variance estimators are determined. With regard to the straight line, [Q/H]=a_{Q}[O/H]+b_{Q}, sample stars are displayed along a "main sequence", [Q,O] = [a_{Q},b_{Q},Δ b_{Q}], leaving aside the two OL stars, which, in most cases (e.g. Na), lie outside. The unit slope, a_{Q}=1, implies Q is a primary element synthesised via SNII progenitors in the presence of a universal stellar initial mass function (defined as simple primary element). In this respect, Mg, Si, Ti, show hat a_{Q}=1 within ∓2hatσ_ {hat a_{Q}}; Cr, Fe, Ni, within ∓3hatσ_{hat a_{Q}}; Na, Ca, within ∓ rhatσ_{hat a_{Q}}, r>3. The empirical, differential element abundance distributions are inferred from LH, HH, KD, HA = HH + KD subsamples, where related regression lines represent their theoretical counterparts within the framework of simple MCBR (multistage closed box + reservoir) chemical evolution models. Hence, the fractional yields, hat{p}_{Q}/hat{p}_{O}, are determined and (as an example) a comparison is shown with their theoretical counterparts inferred from SNII progenitor nucleosynthesis under the assumption of a power-law stellar initial mass function. The generalized fractional yields, C_{Q}=Z_{Q}/Z_{O}^{a_{Q}}, are determined regardless of the chemical evolution model. The ratio of outflow to star formation rate is compared for different populations in the framework of simple MCBR models. The opposite situation of element abundance variation entirely due to cosmic scatter is also considered under reasonable assumptions. The related differential element abundance

  10. The lithium content of the Galactic Halo stars

    NASA Astrophysics Data System (ADS)

    Charbonnel, C.; Primas, F.

    2005-11-01

    Thanks to the accurate determination of the baryon density of the universe by the recent cosmic microwave background experiments, updated predictions of the standard model of Big Bang nucleosynthesis now yield the initial abundance of the primordial light elements with unprecedented precision. In the case of ^7Li, the CMB+SBBN value is significantly higher than the generally reported abundances for Pop II stars along the so-called Spite plateau. In view of the crucial importance of this disagreement, which has cosmological, galactic and stellar implications, we decided to tackle the most critical issues of the problem by revisiting a large sample of literature Li data in halo stars that we assembled following some strict selection criteria on the quality of the original analyses. In the first part of the paper we focus on the systematic uncertainties affecting the determination of the Li abundances, one of our main goal being to look for the "highest observational accuracy achievable" for one of the largest sets of Li abundances ever assembled. We explore in great detail the temperature scale issue with a special emphasis on reddening. We derive four sets of effective temperatures by applying the same colour {T}_eff calibration but making four different assumptions about reddening and determine the LTE lithium values for each of them. We compute the NLTE corrections and apply them to the LTE lithium abundances. We then focus on our "best" (i.e. most consistent) set of temperatures in order to discuss the inferred mean Li value and dispersion in several {T}_eff and metallicity intervals. The resulting mean Li values along the plateau for [Fe/H] ≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075 when the lowest effective temperature considered is taken equal to 5700 K and 6000 K respectively. This is a factor of 2.48 to 2.81 (depending on the adopted SBBN model and on the effective temperature range chosen to delimit the plateau) lower than the CMB

  11. THE MOST METAL-POOR DAMPED Lyα SYSTEMS: AN INSIGHT INTO DWARF GALAXIES AT HIGH-REDSHIFT

    SciTech Connect

    Cooke, Ryan J.; Pettini, Max; Jorgenson, Regina A.

    2015-02-10

    In this paper we analyze the kinematics, chemistry, and physical properties of a sample of the most metal-poor damped Lyα systems (DLAs), to uncover their links to modern-day galaxies. We present evidence that the DLA population as a whole exhibits a ''knee'' in the relative abundances of the α-capture and Fe-peak elements when the metallicity is [Fe/H] ≅ –2.0, assuming that Zn traces the buildup of Fe-peak elements. In this respect, the chemical evolution of DLAs is clearly different from that experienced by Milky Way halo stars, but resembles that of dwarf spheroidal galaxies in the Local Group. We also find a close correspondence between the kinematics of Local Group dwarf galaxies and of high-redshift metal-poor DLAs, which further strengthens this connection. On the basis of such similarities, we propose that the most metal-poor DLAs provide us with a unique opportunity to directly study the dwarf galaxy population more than ten billion years in the past, at a time when many dwarf galaxies were forming the bulk of their stars. To this end, we have measured some of the key physical properties of the DLA gas, including their neutral gas mass, size, kinetic temperature, density, and turbulence. We find that metal-poor DLAs contain a warm neutral medium with T {sub gas} ≅ 9600 K predominantly held up by thermal pressure. Furthermore, all of the DLAs in our sample exhibit a subsonic turbulent Mach number, implying that the gas distribution is largely smooth. These results are among the first empirical descriptions of the environments where the first few generations of stars may have formed in the universe.

  12. The Abundances of Neutron-capture Species in the Very Metal-poor Globular Cluster M15: A Uniform Analysis of Red Giant Branch and Red Horizontal Branch Stars

    NASA Astrophysics Data System (ADS)

    Sobeck, Jennifer S.; Kraft, Robert P.; Sneden, Christopher; Preston, George W.; Cowan, John J.; Smith, Graeme H.; Thompson, Ian B.; Shectman, Stephen A.; Burley, Gregory S.

    2011-06-01

    The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Δ(RHB - RGB) ≈ 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.

  13. Inhomogeneous chemical enrichment in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Kobayashi, Chiaki

    2016-08-01

    In a galaxy, chemical enrichment takes place in an inhomogeneous fashion, and the Galactic Halo is one of the places where the inhomogeneous effects are imprinted and can be constrained from observations. I show this using my chemodynamical simulations of Milky Way type galaxies. The scatter in the elemental abundances originate from radial migration, merging/accretion of satellite galaxies, local variation of star formation and chemical enrichment, and intrinsic variation of nucleosynthesis yields. In the simulated galaxies, there is no strong age-metallicity relation. This means that the most metal-poor stars are not always the oldest stars, and can be formed in chemically unevolved clouds at later times. The long-lifetime sources of chemical enrichment such as asymptotic giant branch stars or neutron star mergers can contribute at low metallicities. The intrinsic variation of yields are important in the early Universe or metal-poor systems such as in the Galactic halo. The carbon enhancement of extremely metal-poor (EMP) stars can be best explained by faint supernovae, the low [α/Fe] ratios in some EMP stars naturally arise from low-mass (~ 13 - 15M ⊙) supernovae, and finally, the [α/Fe] knee in dwarf spheroidal galaxies can be produced by subclasses of Type Ia supernovae such as SN 2002cx-like objects and sub-Chandrasekhar mass explosions.

  14. FORMATION OF METAL-POOR GLOBULAR CLUSTERS IN Ly{alpha} EMITTING GALAXIES IN THE EARLY UNIVERSE

    SciTech Connect

    Elmegreen, Bruce G.; Malhotra, Sangeeta; Rhoads, James

    2012-09-20

    The size, mass, luminosity, and space density of Ly{alpha} emitting (LAE) galaxies observed at intermediate to high redshift agree with expectations for the properties of galaxies that formed metal-poor halo globular clusters (GCs). The low metallicity of these clusters is the result of their formation in low-mass galaxies. Metal-poor GCs could enter spiral galaxies along with their dwarf galaxy hosts, unlike metal-rich GCs, which form in the spirals themselves. Considering an initial GC mass larger than the current mass to account for multiple stellar populations, and considering the additional clusters that are likely to form with massive clusters, we estimate that each GC with a mass today greater than 2 Multiplication-Sign 10{sup 5} M{sub Sun} was likely to have formed among a total stellar mass {approx}> 3 Multiplication-Sign 10{sup 7} M{sub Sun }, a molecular mass {approx}> 10{sup 9} M{sub Sun }, and 10{sup 7} to 10{sup 9} M{sub Sun} of older stars, depending on the relative gas fraction. The star formation rate would have been several M{sub Sun} yr{sup -1} lasting for {approx}10{sup 7} yr, and the Ly{alpha} luminosity would have been {approx}> 10{sup 42} erg s{sup -1}. Integrating the LAE galaxy luminosity function above this minimum, considering the average escape probability for Ly{alpha} photons (25%), and then dividing by the probability that a dwarf galaxy is observed in the LAE phase (0.4%), we find agreement between the comoving space density of LAEs and the average space density of metal-poor GCs today. The local galaxy WLM, with its early starburst and old GC, could be an LAE remnant that did not get into a galaxy halo because of its remote location.

  15. The Average Star Formation Histories of Galaxies in Dark Matter Halos from z = 0-8

    NASA Astrophysics Data System (ADS)

    Behroozi, Peter S.; Wechsler, Risa H.; Conroy, Charlie

    2013-06-01

    We present a robust method to constrain average galaxy star formation rates (SFRs), star formation histories (SFHs), and the intracluster light (ICL) as a function of halo mass. Our results are consistent with observed galaxy stellar mass functions, specific star formation rates (SSFRs), and cosmic star formation rates (CSFRs) from z = 0 to z = 8. We consider the effects of a wide range of uncertainties on our results, including those affecting stellar masses, SFRs, and the halo mass function at the heart of our analysis. As they are relevant to our method, we also present new calibrations of the dark matter halo mass function, halo mass accretion histories, and halo-subhalo merger rates out to z = 8. We also provide new compilations of CSFRs and SSFRs; more recent measurements are now consistent with the buildup of the cosmic stellar mass density at all redshifts. Implications of our work include: halos near 1012 M ⊙ are the most efficient at forming stars at all redshifts, the baryon conversion efficiency of massive halos drops markedly after z ~ 2.5 (consistent with theories of cold-mode accretion), the ICL for massive galaxies is expected to be significant out to at least z ~ 1-1.5, and dwarf galaxies at low redshifts have higher stellar mass to halo mass ratios than previous expectations and form later than in most theoretical models. Finally, we provide new fitting formulae for SFHs that are more accurate than the standard declining tau model. Our approach places a wide variety of observations relating to the SFH of galaxies into a self-consistent framework based on the modern understanding of structure formation in ΛCDM. Constraints on the stellar mass-halo mass relationship and SFRs are available for download online.

  16. Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-metal" Stars—Confirmation of Metal-poor Stars in the Thick Disk of the Galaxy

    NASA Astrophysics Data System (ADS)

    Beers, Timothy C.; Norris, John E.; Placco, Vinicius M.; Lee, Young Sun; Rossi, Silvia; Carollo, Daniela; Masseron, Thomas

    2014-10-01

    A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R ~ 3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the Sloan Extension for Galactic Exploration and Understanding and Exploration (SEGUE) Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (T eff, log g, and [Fe/H]) for these non-Sloan Digital Sky Survey/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities -1.8 < [Fe/H] <=-0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy versus angular momentum diagram. A comparison is made with recent results for a similar-size sample of Radial Velocity Experiment stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.

  17. Observational probes of the connection between Star Formation Efficiency and Dark Matter halo mass of galaxies

    NASA Astrophysics Data System (ADS)

    Kalinova, Veselina; Colombo, Dario; Rosolowsky, Erik

    2015-08-01

    Modern simulations predict that the stellar mass and the star formation efficiency of a galaxy are tightly linked to the dark matter (DM) halo mass of that galaxy. This prediction relies on a specific model of galaxy evolution and so testing this prediction directly tests our best models of galaxy formation and evolution. Recent DM numerical studies propose relationships between star formation efficiency and the DM halo mass with two domains based on SF feedback (low-mass) vs. AGN feedback (high-mass), see Moster et al. (2013). The observational probe of such parameters in the relationship imply globally important physics that are fundamental as, e.g., the star formation law (e.g., Kennicutt et al., 1998), the universal depletion time (Leroy et al. 2008), and the origin of the cold gas phase with respect to the stellar disc (Davis et al.2011). Thus, we can directly measure whether this parameterization is correct by estimating the stellar mass, star formation efficiency and dynamical (DM) mass for a set of galaxies at strategically selected points to test if they fall on the predicted relationship.We use CO data from the Extragalactic Database for Galaxy Evolution survey (EDGE) in conjunction with archival 21-cm data and spectroscopic data from Calar Alto Legacy Integral Field spectroscopy Area survey (CALIFA) to measure the stellar vs. halo mass and star-formation-efficiency vs. halo mass relations of the galaxies. We also analyze archival 21-cm spectra to estimate rotation speeds, atomic gas masses and halo masses for a set of EDGE galaxies. Data from CALIFA are used for high quality star formation efficiency and stellar mass measurements. By linking these three parameters - stellar mass, star formation efficiency (SFE) and DM halo mass - we can test the simulation models of how the gas is cooling in the potential wells of the dark matter halos and then forms stars.

  18. New cluster members and halo stars of the Galactic globular cluster NGC 1851

    NASA Astrophysics Data System (ADS)

    Navin, Colin A.; Martell, Sarah L.; Zucker, Daniel B.

    2015-10-01

    NGC 1851 is an intriguing Galactic globular cluster, with multiple stellar evolutionary sequences, light and heavy element abundance variations and indications of a surrounding stellar halo. We present the first results of a spectroscopic study of red giant stars within and outside of the tidal radius of this cluster. Our results identify nine probable new cluster members (inside the tidal radius) with heliocentric radial velocities consistent with that of NGC 1851. We also identify, based on their radial velocities, four probable extratidal cluster halo stars at distances up to ˜3.1 times the tidal radius, which are supportive of previous findings that NGC 1851 is surrounded by an extended stellar halo. Proper motions were available for 12 of these 13 stars and all are consistent with that of NGC 1851. Apart from the cluster members and cluster halo stars, our observed radial velocity distribution agrees with the expected distribution from a Besançon disc/N-body stellar halo Milky Way model generated by the GALAXIA code, suggesting that no other structures at different radial velocities are present in our field. The metallicities of these stars are estimated using equivalent width measurements of the near-infrared calcium triplet absorption lines and are found, within the limitations of this method, to be consistent with that of NGC 1851. In addition we recover 110 red giant cluster members from previous studies based on their radial velocities and identify three stars with unusually high radial velocities.

  19. [α/Fe] ABUNDANCES OF FOUR OUTER M31 HALO STARS

    SciTech Connect

    Vargas, Luis C.; Geha, Marla; Tollerud, Erik J.; Gilbert, Karoline M.; Kirby, Evan N.; Guhathakurta, Puragra

    2014-12-10

    We present alpha element to iron abundance ratios, [α/Fe], for four stars in the outer stellar halo of the Andromeda Galaxy (M31). The stars were identified as high-likelihood field halo stars by Gilbert et al. and lie at projected distances between 70 and 140 kpc from M31's center. These are the first alpha abundances measured for a halo star in a galaxy beyond the Milky Way. The stars range in metallicity between [Fe/H] = –2.2 and [Fe/H] = –1.4. The sample's average [α/Fe] ratio is +0.20 ± 0.20. The best-fit average value is elevated above solar, which is consistent with rapid chemical enrichment from Type II supernovae. The mean [α/Fe] ratio of our M31 outer halo sample agrees (within the uncertainties) with that of Milky Way inner/outer halo stars that have a comparable range of [Fe/H].

  20. Formation Rates of Population III Stars and Chemical Enrichment of Halos during the Reionization Era

    NASA Astrophysics Data System (ADS)

    Trenti, Michele; Stiavelli, Massimo

    2009-04-01

    The first stars in the universe formed out of pristine primordial gas clouds that were radiatively cooled to a few hundreds of degrees kelvin either via molecular or atomic (Lyman-α) hydrogen lines. This primordial mode of star formation was eventually quenched once radiative and/or chemical (metal enrichment) feedbacks marked the transition to Population II stars. In this paper, we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model, we derive the star formation history of Population III stars, their contribution to the reionization of the universe and the time of the transition from Population III star formation in minihalos (M ≈ 106 M sun, cooled via molecular hydrogen) to that in more massive halos (M gsim 2 × 107 M sun, where atomic hydrogen cooling is also possible). We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal-free stars contribute only to a minor fraction of the total number of photons required to reionize the universe. In addition, metal-free star formation is primarily located in minihalos, and chemically enriched halos become the dominant locus of star formation very early in the life of the universe—at redshift z ≈ 25—even assuming a modest fraction (0.5%) of enriched gas converted in stars. If instead multiple metal-free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the reionizing radiation budget. In addition

  1. Two New Ultra-Faint Star Clusters in the Milky Way Halo

    NASA Astrophysics Data System (ADS)

    Kim, Dongwon

    2016-08-01

    Kim 1 & 2 are two new star clusters discovered in the Stromlo Missing Satellite Survey. Kim 1, located at a heliocentric distance of 19.8 +/- 0.9 kpc, features an extremely low total luminosity (M V = 0.3 +/- 0.5 mag) and low star concentration. Together with the large ellipticity (ɛ = 0.42 +/- 0.10) and irregular isophotes, these properties suggest that Kim 1 is an intermediate mass star cluster being stripped by the Galactic tidal field. Kim 2 is a rare ultra-faint outer halo globular cluster located at a heliocentric distance of 104.7 +/- 4.1 kpc. The cluster exhibits evidence of significant mass loss such as extra-tidal stars and mass-segregation. Kim 2 is likely to follow an orbit confined to the peripheral region of the Galactic halo, and/or to have formed in a dwarf galaxy that was later accreted into the Galactic halo.

  2. STAR FORMATION HISTORY OF THE MILKY WAY HALO TRACED BY THE OOSTERHOFF DICHOTOMY AMONG GLOBULAR CLUSTERS

    SciTech Connect

    Jang, Sohee; Lee, Young-Wook

    2015-06-22

    In our recent investigation of the Oosterhoff dichotomy in the multiple population paradigm, we have suggested that the RR Lyrae variables in the globular clusters (GCs) of Oosterhoff groups I, II, and III are produced mostly by first, second, and third generation stars (G1, G2, and G3), respectively. Here we show, for the first time, that the observed dichotomies in the inner and outer halo GCs can be naturally reproduced when these models are extended to all metallicity regimes, while maintaining reasonable agreements in the horizontal-branch type versus [Fe/H] correlations. In order to achieve this, however, specific star formation histories are required for the inner and outer halos. In the inner halo GCs, the star formation commenced and ceased earlier with a relatively short formation timescale between the subpopulations (∼0.5 Gyr), while in the outer halo, the formation of G1 was delayed by ∼0.8 Gyr with a more extended timescale between G1 and G2 (∼1.4 Gyr). This is consistent with the dual origin of the Milky Way halo. Despite the difference in detail, our models show that the Oosterhoff period groups observed in both outer and inner halo GCs are all manifestations of the “population-shift” effect within the instability strip, for which the origin can be traced back to the two or three discrete episodes of star formation in GCs.

  3. Constraints on baryonic dark matter in the Galactic halo and Local Group

    NASA Astrophysics Data System (ADS)

    Richstone, Douglas; Gould, Andrew; Guhathakurta, Puragra; Flynn, Chris

    1992-04-01

    A four-color method and deep CCD data are used to search for very faint metal-poor stars in the direction of the south Galactic pole. The results make it possible to limit the contribution of ordinary old, metal-poor stars to the dynamical halo of the Galaxy or to the Local Group. The ratio of the mass of the halo to its ordinary starlight must be more than about 2000, unless the halo is very small. For the Local Group, this ratio is greater than about 400. If this local dark matter is baryonic, the process of compact-object formation must produce very few 'impurities' in the form of stars similar to those found in globular clusters. The expected number of unbound stars with MV not greater than 6 within 100 pc of the sun is less than 1 based on the present 90-percent upper limit to the Local Group starlight.

  4. Constraints on baryonic dark matter in the Galactic halo and Local Group

    NASA Technical Reports Server (NTRS)

    Richstone, Douglas; Gould, Andrew; Guhathakurta, Puragra; Flynn, Chris

    1992-01-01

    A four-color method and deep CCD data are used to search for very faint metal-poor stars in the direction of the south Galactic pole. The results make it possible to limit the contribution of ordinary old, metal-poor stars to the dynamical halo of the Galaxy or to the Local Group. The ratio of the mass of the halo to its ordinary starlight must be more than about 2000, unless the halo is very small. For the Local Group, this ratio is greater than about 400. If this local dark matter is baryonic, the process of compact-object formation must produce very few 'impurities' in the form of stars similar to those found in globular clusters. The expected number of unbound stars with MV not greater than 6 within 100 pc of the sun is less than 1 based on the present 90-percent upper limit to the Local Group starlight.

  5. A Search for Coronal Activity Among Two Metal-poor Subdwarfs and One Subgiant

    NASA Astrophysics Data System (ADS)

    Smith, Graeme H.; Dupree, Andrea K.; Günther, Hans Moritz

    2016-08-01

    A search has been made using the XMM-Newton satellite for coronal soft X-ray emission from HD 19445, HD 25329, and HD 140283, three Population II stars in the Galactic halo having metallicities of {{[Fe/H]}}˜ -2. The program stars, consisting of two subdwarfs and one metal-poor subgiant, were pre-selected from ground-based observations to have He i λ10830 absorption lines with an equivalent width (EW) of 30 mÅ or more. If such stars follow a relation between He i EW and soft X-ray flux applicable to Population I dwarf stars, then they would be expected to have X-ray luminosities ˜ 5× {10}-7 times their bolometric luminosity, and as such would yield detectable sources in 20 ks exposures with the XMM-Newton EPIC-PN and MOS cameras. No detections were found in such exposures made with XMM-Newton. Upper limits to soft X-ray emission from the two program stars that have effective temperatures most similar to that of the Sun, namely HD 19445 and HD 140283, are comparable to the level of the quiet Sun. The star HD 25329, a cooler subdwarf, exhibits an upper limit similar to the Sun at maximum activity. These measurements suggest that coronal activity appears to decrease with age among the oldest G dwarfs, but K-M subdwarfs possibly have maintained a solar-like level of activity. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

  6. VizieR Online Data Catalog: Model SDSS colors for halo stars (Allende Prieto+, 2014)

    NASA Astrophysics Data System (ADS)

    Allende Prieto, C.; Fernandez-Alvar, E.; Schlesinger, K. J.; Lee, Y. S.; Morrison, H. L.; Schneider, D. P.; Beers, T. C.; Bizyaev, D.; Ebelke, G.; Malanushenko, E.; Oravetz, D.; Pan, K.; Simmons, A.; Simmerer, J.; Sobeck, J.; Robin, A. C.

    2014-06-01

    We analyze a sample of tens of thousands of spectra of halo turnoff stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population "in situ" out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as flux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS). Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that best reproduces each observed spectrum. We use an optimization algorithm and evaluate model fluxes by means of interpolation in a pre-computed grid. In our analysis, we account for the spectrograph's varying resolution as a function of fiber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars at logg (cgs units) lower than 2.5. An analysis of stars in the globular cluster M13 reveals a dependence of the inferred metallicity on surface gravity for stars with logg<2.5, confirming the systematics identified in the comparison with the SSPP. We find that our metallicity estimates are significantly more precise than the SSPP results. We also find excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turnoff stars as flux calibrators efficiently excludes stars with high metallicities, but does not significantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with that expected from a dual halo system in the Milky Way. (1 data file).

  7. Accretion in the galactic halo

    NASA Astrophysics Data System (ADS)

    Stephens, Alex Courtney

    2000-10-01

    The Milky Way disk is enveloped in a diffuse, dynamically-hot collection of stars and star clusters collectively known as the ``stellar halo''. Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet, little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way's stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing ``extreme'' kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or ``accreted'') dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line-strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10-m and KPNO 4-m telescopes. The abundances extracted from the spectra reveal: (1)The vast majority of outer halo stars possess supersolar [α/Fe] > 0.0) ratios. (2)The [α/Fe] ratio appears to decrease with increasing metallicity. (3)The outer halo stars have lower ratios of [α/Fe] than inner halo stars at a given metallicity. (4)At the largest metallicities, there is a large spread in the observed [α/Fe] ratios. (5)[α/Fe] anti-correlates with RAPO. (6)Only one star (BD+80° 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the

  8. Halo Stream candidates in the LAMOST DR2

    NASA Astrophysics Data System (ADS)

    Zhao, Jingkun

    2015-08-01

    We have detected eight stellar halo stream candidates in the solar neighborhood using a sample including 64,819 FGK metal-poor ([Fe/H] < -0.7) dwarfs extracted from the cross-match among the LAMOST DR2, WISE, 2MASS and PPMXL catalogues. With the strategy of halo stream detection in Klement et al, several significant ‘phase-space overdensi- ties” of stars on very similar orbits are identified. Three structures are known previously. Five new halo stream candidates are also found. The kinematics and metallicity of these stream candidates are then analyzed. Detailed element abundance are needed to better know the ori-gin of these streams.

  9. Centaurus A: Stellar Metallicity Transition in the Halo

    NASA Astrophysics Data System (ADS)

    Bird, Sarah; Flynn, C.; Harris, W. E.; Valtonen, M.

    2013-01-01

    The very earliest stars in giant galaxies - the most metal-poor halo stars and globular clusters - may have formed before the onset of hierarchical merging, within small pregalactic dwarfs that populated the large-scale dark-matter potential well. Today, these relic stars should be found in a sparse and extremely extended “outermost-halo” component. Finding clear traces of this component in other giant galaxies, and deconvolving it from the more obvious and metal-rich spheroid component generated later by mergers, has been extraordinarily difficult. Now, striking new evidence discovered in M 31 and NGC 3379 suggests that the metal-poor outermost halo can be isolated at very large radii, R > 12Reff . We now have a new deep imaging study with ESO VLT of the nearest giant elliptical and merger remnant, Centaurus A, to search for this extended remnant of the galaxy’s earliest history.

  10. Is main-sequence galaxy star formation controlled by halo mass accretion?

    NASA Astrophysics Data System (ADS)

    Rodríguez-Puebla, Aldo; Primack, Joel R.; Behroozi, Peter; Faber, S. M.

    2016-01-01

    The galaxy stellar-to-halo mass relation (SHMR) is nearly time-independent for z < 4. We therefore construct a time-independent SHMR model for central galaxies, wherein the in situ star formation rate (SFR) is determined by the halo mass accretion rate (MAR), which we call stellar halo accretion rate coevolution (SHARC). We show that the ˜0.3 dex dispersion of the halo MAR matches the observed dispersion of the SFR on the star formation main sequence (MS). In the context of `bathtub'-type models of galaxy formation, SHARC leads to mass-dependent constraints on the relation between SFR and MAR. Despite its simplicity and the simplified treatment of mass growth from mergers, the SHARC model is likely to be a good approximation for central galaxies with M* = 109-1010.5 M⊙ that are on the MS, representing most of the star formation in the Universe. SHARC predictions agree with observed SFRs for galaxies on the MS at low redshifts, agree fairly well at z ˜ 4, but exceed observations at z ≳ 4. Assuming that the interstellar gas mass is constant for each galaxy (the `equilibrium condition' in bathtub models), the SHARC model allows calculation of net mass loading factors for inflowing and outflowing gas. With assumptions about preventive feedback based on simulations, SHARC allows calculation of galaxy metallicity evolution. If galaxy SFRs indeed track halo MARs, especially at low redshifts, that may help explain the success of models linking galaxy properties to haloes (including age-matching) and the similarities between two-halo galaxy conformity and halo mass accretion conformity.

  11. A DIFFERENTIAL CHEMICAL ELEMENT ANALYSIS OF THE METAL-POOR GLOBULAR CLUSTER NGC 6397

    SciTech Connect

    Koch, Andreas; McWilliam, Andrew E-mail: andy@obs.carnegiescience.edu

    2011-08-15

    We present chemical abundances in three red giants and two turnoff (TO) stars in the metal-poor Galactic globular cluster (GC) NGC 6397 based on spectroscopy obtained with the Magellan Inamori Kyocera Echelle high-resolution spectrograph on the Magellan 6.5 m Clay telescope. Our results are based on a line-by-line differential abundance analysis relative to the well-studied red giant Arcturus and the Galactic halo field star Hip 66815. At a mean of -2.10 {+-} 0.02 (stat.) {+-}0.07 (sys.), the differential iron abundance is in good agreement with other studies in the literature based on gf-values. As in previous differential works we find a distinct departure from ionization equilibrium in that the abundances of Fe I and Fe II differ by {approx}0.1 dex, with opposite signs for the red giant branch (RGB) and TO stars. The {alpha}-element ratios are enhanced to 0.4 (RGB) and 0.3 dex (TO), respectively, and we also confirm strong variations in the O, Na, and Al/Fe abundance ratios. Accordingly, the light-element abundance patterns in one of the red giants can be attributed to pollution by an early generation of massive Type II supernovae. TO and RGB abundances are not significantly different, with the possible exception of Mg and Ti, which are, however, amplified by the patterns in one TO star additionally belonging to this early generation of GC stars. We discuss interrelations of these light elements as a function of the GC metallicity.

  12. Attribution of halo merger mass ratio and star formation rate density

    NASA Astrophysics Data System (ADS)

    Kim, Sungeun; Jo, Jeong-woon; Hwang, Jihe; Youn, Soyoung; Park, Boha

    2016-06-01

    We have used codes for implementing the merger tree algorithm by Cole et al. (2007) and Parkinson et al. (2008) and derived the halo merger mass ratio of protocluster of galaxies across the cosmic time. The authors compare the observed and simulated star formation rates reported by the various groups and derive the star formation rate densities at different red-shifts. This study implies that an investigation of different mass variables should be incorporated into the analysis in order to accurately estimate cumulative star formation rates of galaxies and star formation rate densities as a function of red-shifts.

  13. A giant stream of metal-rich stars in the halo of the galaxy M31.

    PubMed

    Ibata, R; Irwin, M; Lewis, G; Ferguson, A M; Tanvir, N

    2001-07-01

    Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.

  14. The Hamburg/ESO R-process Enhanced Star survey (HERES). X. HE 2252-4225, one more r-process enhanced and actinide-boost halo star

    NASA Astrophysics Data System (ADS)

    Mashonkina, L.; Christlieb, N.; Eriksson, K.

    2014-09-01

    dating results in a stellar age of τ = 1.5 ± 1.5 Gyr that is not expected for a very metal-poor halo star. Based on observations collected at the European Southern Observatory, Paranal, Chile (Proposal numbers 170.D-0010, and 280.D-5011).Table 3 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/569/A43

  15. Characterizing stellar halo populations II: The age gradient in blue horizontal-branch stars

    NASA Astrophysics Data System (ADS)

    Das, Payel; Williams, Angus; Binney, James

    2016-08-01

    The distribution of Milky Way halo blue horizontal-branch (BHB) stars is examined using action-based extended distribution functions (EDFs) that describe the locations of stars in phase space, metallicity, and age. The parameters of the EDFs are fitted using stars observed in the Sloan Extension for Galactic Understanding and Exploration-II (SEGUE-II) survey that trace the phase-space kinematics and chemistry out to ˜70 kpc. A maximum a posteriori probability (MAP) estimate method and a Markov Chain Monte Carlo method are applied, taking into account the selection function in positions, distance, and metallicity for the survey. The best-fit EDF declines with actions less steeply at actions characteristic of the inner halo than at the larger actions characteristic of the outer halo, and older ages are found at smaller actions than at larger actions. In real space, the radial density profile steepens smoothly from -2 at ˜2 kpc to -4 in the outer halo, with an axis ratio ˜0.7 throughout. There is no indication for rotation in the BHBs, although this is highly uncertain. A moderate level of radial anisotropy is detected, with βs varying from isotropic to between ˜0.1 and ˜0.3 in the outer halo depending on latitude. The BHB data are consistent with an age gradient of -0.03 Gyr kpc-1, with some uncertainty in the distribution of the larger ages. These results are consistent with a scenario in which older, larger systems contribute to the inner halo, whilst the outer halo is primarily comprised of younger, smaller systems.

  16. THE CLUSTERING AND HALO MASSES OF STAR-FORMING GALAXIES AT z < 1

    SciTech Connect

    Dolley, Tim; Brown, Michael J. I.; Pimbblet, Kevin A.; Palamara, David P.; Beare, Richard; Weiner, Benjamin J.; Jannuzi, Buell T.; Brodwin, Mark; Kochanek, C. S.; Dey, Arjun; Atlee, David W.

    2014-12-20

    We present clustering measurements and halo masses of star-forming galaxies at 0.2 < z < 1.0. After excluding active galactic nuclei (AGNs), we construct a sample of 22,553 24 μm sources selected from 8.42 deg{sup 2} of the Spitzer MIPS AGN and Galaxy Evolution Survey of Boötes. Mid-infrared imaging allows us to observe galaxies with the highest star formation rates (SFRs), less biased by dust obscuration afflicting the optical bands. We find that the galaxies with the highest SFRs have optical colors that are redder than typical blue cloud galaxies, with many residing within the green valley. At z > 0.4 our sample is dominated by luminous infrared galaxies (LIRGs, L {sub TIR} > 10{sup 11} L {sub ☉}) and is composed entirely of LIRGs and ultraluminous infrared galaxies (ULIRGs, L {sub TIR} > 10{sup 12} L {sub ☉}) at z > 0.6. We observe weak clustering of r {sub 0} ≈ 3-6 h {sup –1} Mpc for almost all of our star-forming samples. We find that the clustering and halo mass depend on L {sub TIR} at all redshifts, where galaxies with higher L {sub TIR} (hence higher SFRs) have stronger clustering. Galaxies with the highest SFRs at each redshift typically reside within dark matter halos of M {sub halo} ≈ 10{sup 12.9} h {sup –1} M {sub ☉}. This is consistent with a transitional halo mass, above which star formation is largely truncated, although we cannot exclude that ULIRGs reside within higher mass halos. By modeling the clustering evolution of halos, we connect our star-forming galaxy samples to their local descendants. Most star-forming galaxies at z < 1.0 are the progenitors of L ≲ 2.5 L {sub *} blue galaxies in the local universe, but star-forming galaxies with the highest SFRs (L {sub TIR} ≳ 10{sup 11.7} L {sub ☉}) at 0.6 < z < 1.0 are the progenitors of early-type galaxies in denser group environments.

  17. Contributions to the Galactic Halo from In-Situ, Kicked-Out, and Accreted Stars

    NASA Astrophysics Data System (ADS)

    Sheffield, Allyson; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne; Majewski, Steven

    2015-08-01

    The chemical and kinematical properties of stars in the Galactic halo provide a means to study the formation history of the Milky Way. Stars formed within a satellite galaxy will bear the imprint of their host dark matter subhalo: star formation is less efficient in less massive protogalactic clumps, so we should observe a specific pattern in [Fe/H] as a function of α-elements and slow/rapid neutron capture elements that reflects this efficiency. Due to their formation in Type II supernovae, α-elements probe the relative timescale of formation for populations of stars. The addition of s- and r-process elements gives a more complete evolutionary picture of the Galaxy. The yields of s- and r-process elements, which are synthesized in Type II supernovae and thermally pulsating AGB stars, respectively, are coupled to the Fe seed nuclei present in the formation site; thus, neutron capture element yields vary with metallicity and provide further constraints on the subhalo’s star formation history.We will report chemical abundances for a sample of 109 M giants in the nearby halo of the Milky Way. The stars were selected for high-resolution spectroscopy based upon their radial velocities: the radial velocities vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants can be assessed. Of particular interest are a class of stars that form in situ, deep in the Milky Way’s gravitational potential well, but are “kicked out” of the disk into the halo due to a perturbation event. A kicked-out population has recently been identified in Andromeda. N-body simulations predict a range in the percentage of stars belonging to the kicked-out disk population in galaxies. We will present our results within the context of

  18. Evidence for a dispersion in the lithium abundances of extreme halo stars

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Pinsonneault, M. H.; Duncan, Douglas K.

    1993-01-01

    Evidence is presented to the effect that there exists a small dispersion in the lithium abundances of extreme halo dwarfs. This dispersion cannot be accounted for by standard stellar models alone, particularly toward the turnoff, and would thus require early differential Galactic Li enrichment, perhaps independent of metallicity. The magnitude of the dispersion is also consistent with the predictions of evolutionary models of halo stars with rotation, which do not require, but do not rule out either, early Galactic enrichment. These rotational models also predict a significant depletion in the lithium abundance during the stars' lifetime. The rotational models predict that stars which formed with very low initial angular momentum will have lithium abundances measurably above the plateau.

  19. Dual Stellar Halos in Early-type Galaxies and Formation of Massive Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Myung Gyoon; Jang, In Sung

    2016-08-01

    M105 in the Leo I Group is a textbook example of a standard elliptical galaxy. It is only one of the few elliptical galaxies for which we can study their stellar halos using the resolved stars. It is an ideal target to study the structure and composition of stellar halos in elliptical galaxies. We present photometry and metallicity of the resolved stars in the inner and outer regions of M105. These provide strong evidence that there are two distinct stellar halos in this galaxy, a metal-poor (blue) halo and a metal-rich (red) halo. Then we compare them with those in other early-type galaxies and use the dual halo mode formation scenario to describe how massive galaxies formed.

  20. Highly-Ionized Gas in the Galactic Halo: A FUSE Survey of O 6 Absorption toward 22 Halo Stars

    NASA Astrophysics Data System (ADS)

    Zsargo, J.; Sembach, K. R.; Howk, J. C.; Savage, B. D.

    2002-12-01

    Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of O 6 in the Galactic halo between ~0.3 and ~10 kpc from the Galactic mid-plane. Strong O 6 λ 1031.93 absorption was detected toward 21 stars, and a reliable 3 σ upper limit was obtained toward HD 97991. The weaker member of the O 6 doublet at 1037.62 Å could be studied toward only six stars. The observed columns are reasonably consistent with a patchy exponential O 6 distribution with a mid-plane density of 1.7x10-8 cm-3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O 6 absorption along the Galactic sight lines, which indicates the general absence of high velocity O 6 within 2-5 kpc of the Galactic mid-plane. The correlation between the H 1 and O 6 intermediate velocity absorption is also poor. The O 6 velocity dispersions are much larger than the value of ~18 km/s expected from thermal broadening for gas at T ~ 3x105 K, the temperature at which O 6 is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O 6 profiles. Kinematical comparisons of O 6 with Ar 1 reveal that 9 of 21 sight lines are closely aligned in LSR velocity (|Δ VLSR| <=5 km/s ), while 8 of 21 exhibit significant velocity differences (|Δ VLSR| >= 15 km/s ). This dual behavior may indicate the presence of two different types of O 6-bearing environments toward the Galactic sight lines. Comparison of O 6 with other highly-ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow, and that turbulent mixing also has a significant contribution. The role of turbulent mixing is most important toward sight lines that sample supernova remnants like Loop I and IV. We are also able to show that the O 6 enhancement toward the Galactic center region that was observed in the FUSE

  1. Runaway Stars in the Galactic Halo: Their Origin and Kinematics

    NASA Astrophysics Data System (ADS)

    Duarte de Vasconcelos Silva, Manuel

    2012-03-01

    Star formation in the Milky Way is confined to star-forming regions (OB associ- ation, HII regions, and open clusters) in the Galactic plane. It is usually assumed that these regions are found preferably along spiral arms, as is observed in other spiral galaxies. However, young early-type stars are often found at high Galactic latitudes, far away from their birthplaces in the Galactic disc. These stars are called runaway stars, and it is believed that they were ejected from their birth- places early in their lifetimes by one of two mechanisms: ejection from a binary system following the destruction of the massive companion in a supernova type II event (the binary ejection mechanism), or ejection from a dense cluster following a close gravitational encounter between two close binaries (the dynamical ejection mechanism). The aims of our study were: to improve the current understanding of the nature of high Galactic latitude runaway stars, in particular by investigating whether the theoretical ejection mechanisms could explain the more extreme cases; to show the feasibility of using high Galactic latitude stars as tracers of the spiral arms. The main technique used in this investigation was the tracing of stellar orbits back in time, given their present positions and velocities in 3D space. This technique allowed the determination of the ejection velocities, flight times and birthplaces of a sample of runaway stars. In order to obtain reasonable velocity estimates several recent catalogues of proper motion data were used. We found that the evolutionary ages of the vast majority of runaway stars is consistent with the disc ejection scenario. However, we identified three outliers which would need flight times much larger then their estimated ages in order to reach their present positions in the sky. Moreover, the ejection velocity distribution appears to be bimodal, showing evidence for two populations of runaway stars: a "low" velocity population (89 per cent of the

  2. MAPPING THE GALACTIC HALO WITH BLUE HORIZONTAL BRANCH STARS FROM THE TWO-DEGREE FIELD QUASAR REDSHIFT SURVEY

    SciTech Connect

    De Propris, Roberto; Harrison, Craig D.; Mares, Peter J.

    2010-08-20

    We use 666 blue horizontal branch stars from the 2Qz Redshift Survey to map the Galactic halo in four dimensions (position, distance, and velocity). We find that the halo extends to at least 100 kpc in Galactocentric distance, and obeys a single power-law density profile of index {approx}-2.5 in two different directions separated by about 150{sup 0} on the sky. This suggests that the halo is spherical. Our map shows no large kinematically coherent structures (streams, clouds, or plumes) and appears homogeneous. However, we find that at least 20% of the stars in the halo reside in substructures and that these substructures are dynamically young. The velocity dispersion profile of the halo appears to increase toward large radii while the stellar velocity distribution is non-Gaussian beyond 60 kpc. We argue that the outer halo consists of a multitude of low luminosity overlapping tidal streams from recently accreted objects.

  3. MACHO RR lyrae in the inner halo and bulge

    SciTech Connect

    Drake, A.; Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T.S.; Becker, A.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W.; Tomaney, A.; Vandehei, T.; Welch, D.

    1998-10-01

    The RR Lyrse in the bulge have been proposed to be the oldest populations in the Milky Way, tracers of how the galaxy formed. We study here the distribution of ?{approximately}1600 bulge RR Lyrae stars found by the MACHO Project. The RR Lyrae with 0.4 ? R ? 3 kpc show a density law that is well fit by the extension of the metal-poor stellar halo present in the outer regions of the Milky Way.

  4. New Halo Stars of the Galactic Globular Clusters M3 and M13 in the LAMOST DR1 Catalog

    NASA Astrophysics Data System (ADS)

    Navin, Colin A.; Martell, Sarah L.; Zucker, Daniel B.

    2016-10-01

    M3 and M13 are Galactic globular clusters with previous reports of surrounding stellar halos. We present the results of a search for members and extratidal cluster halo stars within and outside of the tidal radius of these clusters in the LAMOST Data Release 1. We find seven candidate cluster members (inside the tidal radius) of both M3 and M13, respectively. In M3 we also identify eight candidate extratidal cluster halo stars at distances up to ∼9.8 times the tidal radius, and in M13 we identify 12 candidate extratidal cluster halo stars at distances up to ∼13.8 times the tidal radius. These results support previous indications that both M3 and M13 are surrounded by extended stellar halos, and we find that the GC destruction rates corresponding to the observed mass loss are generally significantly higher than theoretical studies predict.

  5. The Origin of the Metal-Poor Common Proper Motion Pair HD 134439/134440: Insights from New Elemental Abundances

    NASA Astrophysics Data System (ADS)

    Chen, Yu; King, Jeremy R.; Boesgaard, Ann M.

    2014-11-01

    The low [α/Fe] ratio in the metal-poor ([Fe/H] ~ -1.50) common proper motion pair HD 134439 and HD 134440 has been variously attributed to chemical evolution in an extragalactic environment with an irregular star formation history, planetesimal accretion, and formation in an environment with an unusually high dust-to-gas ratio. We explore these various putative origins using CNO, Be, Ag, and Eu abundances derived from high-resolution near-UV Keck/HIRES spectroscopy. While we confirm a previously suggested correlation between elemental abundance ratios and condensation temperature at the 95% confidence level, these ratios lie within the continuum of values manifested by extant dSph data. We argue that the most plausible origin of our stars' distinctive abundance distribution relative to the Galactic halo field is formation in an environment chemically dominated by products of Type II SN of low progenitor mass; such a progenitor mass bias has been previously suggested as an explanation of low α-element ratios of dSph stars. The proper motion pair's heavy-to-light n-capture element ratio, which is >=0.3-0.5 dex lower than in the Galactic halo field and dSph stars, is discussed in the context of the truncated r-process, phenomenological n-capture production models, and α-rich freezeout in a high neutron excess environment; the latter simultaneously provides an attractive explanation of the difference in [Ca, Ti/O, Mg, Si] ratio in HD 134439/134440 compared to in situ dSph stars.

  6. Deep SDSS optical spectroscopy of distant halo stars. I. Atmospheric parameters and stellar metallicity distribution

    NASA Astrophysics Data System (ADS)

    Allende Prieto, C.; Fernández-Alvar, E.; Schlesinger, K. J.; Lee, Y. S.; Morrison, H. L.; Schneider, D. P.; Beers, T. C.; Bizyaev, D.; Ebelke, G.; Malanushenko, E.; Malanushenko, V.; Oravetz, D.; Pan, K.; Simmons, A.; Simmerer, J.; Sobeck, J.; Robin, A. C.

    2014-08-01

    Aims: We analyze a sample of tens of thousands of spectra of halo turnoff stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population "in situ" out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as flux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS). Methods: Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each observed spectrum best. We used an optimization algorithm and evaluate model fluxes by means of interpolation in a precomputed grid. In our analysis, we account for the spectrograph's varying resolution as a function of fiber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars with log g (cgs units) lower than 2.5. Results: An analysis of stars in the globular cluster M 13 reveals a dependence of the inferred metallicity on surface gravity for stars with log g < 2.5, confirming the systematics identified in the comparison with the SSPP. We find that our metallicity estimates are significantly more precise than the SSPP results. We also find excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turnoff stars as flux calibrators efficiently excludes stars with high metallicities, but does not significantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with what is expected from a dual halo system

  7. PRIMORDIAL r-PROCESS DISPERSION IN METAL-POOR GLOBULAR CLUSTERS

    SciTech Connect

    Roederer, Ian U.

    2011-05-01

    Heavy elements, those produced by neutron-capture reactions, have traditionally shown no star-to-star dispersion in all but a handful of metal-poor globular clusters (GCs). Recent detections of low [Pb/Eu] ratios or upper limits in several metal-poor GCs indicate that the heavy elements in these GCs were produced exclusively by an r-process. Re-examining GC heavy element abundances from the literature, we find unmistakable correlations between the [La/Fe] and [Eu/Fe] ratios in four metal-poor GCs (M5, M15, M92, and NGC 3201), only two of which were known previously. This indicates that the total r-process abundances vary from star to star (by factors of 2-6) relative to Fe within each GC. We also identify potential dispersion in two other GCs (M3 and M13). Several GCs (M12, M80, and NGC 6752) show no evidence of r-process dispersion. The r-process dispersion is not correlated with the well-known light element dispersion, indicating that it was present in the gas throughout the duration of star formation. The observations available at present suggest that star-to-star r-process dispersion within metal-poor GCs may be a common but not ubiquitous phenomenon that is neither predicted by nor accounted for in current models of GC formation and evolution.

  8. Hot subdwarf stars in the Galactic halo Tracers of prominent events in late stellar evolution

    NASA Astrophysics Data System (ADS)

    Geier, Stephan; Kupfer, Thomas; Schaffenroth, Veronika; Heber, Ulrich

    2016-08-01

    Hot subdwarf stars (sdO/Bs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. They constitute the dominant population of UV-bright stars in old stellar environments and are most likely formed by binary interactions. We perform the first systematic, spectroscopic analysis of a sample of those stars in the Galactic halo based on data from SDSS. In the course of this project we discovered 177 close binary candidates. A significant fraction of the sdB binaries turned out to have close substellar companions, which shows that brown dwarfs and planets can significantly influence late stellar evolution. Close hot subdwarf binaries with massive white dwarf companions on the other hand are good candidates for the progenitors of type Ia supernovae. We discovered a hypervelocity star, which not only turned out to be the fastest unbound star known in our Galaxy, but also the surviving companion of such a supernova explosion.

  9. The Century Survey Galactic Halo Project III: A Complete 4300 DEG2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo

    NASA Astrophysics Data System (ADS)

    Brown, Warren R.; Beers, Timothy C.; Wilhelm, Ronald; Allende Prieto, Carlos; Geller, Margaret J.; Kenyon, Scott J.; Kurtz, Michael J.

    2008-02-01

    We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg2 of the sky. We identify 655 BHB stars in this non-kinematically selected sample. We calculate the luminosity function of field BHB stars, and find evidence for very few hot BHB stars in the field. The BHB stars located at a distance from the Galactic plane |Z| < 4 kpc trace what is clearly a metal-weak thick disk population, with a mean metallicity of [Fe/H] = -1.7, a rotation velocity gradient of dvrot/d|Z| = -28 ± 3.4 km s-1 in the region |Z| < 6 kpc, and a density scale height of hZ = 1.26 ± 0.1 kpc. The BHB stars located at 5 < |Z| < 9 kpc are a predominantly inner-halo population, with a mean metallicity of [Fe/H] = -2.0 and a mean Galactic rotation of -4 ± 31 km s-1. We infer the density of halo and thick disk BHB stars is 104 ± 37 kpc-3 near the Sun, and the relative normalization of halo to thick-disk BHB stars is 4 ± 1% near the Sun.

  10. STELLAR POPULATION VARIATIONS IN THE MILKY WAY's STELLAR HALO

    SciTech Connect

    Bell, Eric F.; Xue Xiangxiang; Rix, Hans-Walter; Ruhland, Christine; Hogg, David W.

    2010-12-15

    If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of {approx}9000 g < 18 candidates where {approx}70% of them are BHB stars. We map the BHB/MSTO ratio across {approx}1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of the BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e.g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in the BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.

  11. Long GRBs as a tool to investigate star formation in dark matter halos

    NASA Astrophysics Data System (ADS)

    Wei, Jun-Jie; Hao, Jing-Meng; Wu, Xue-Feng; Yuan, Ye-Fei

    2016-03-01

    First stars can only form in structures that are suitably dense, which can be parametrized by the minimum dark matter halo mass Mmin. Mmin must play an important role in star formation. The connection of long gamma-ray bursts (LGRBs) with the collapse of massive stars has provided a good opportunity for probing star formation in dark matter halos. We place some constraints on Mmin using the latest Swift LGRB data. We conservatively consider that LGRB rate is proportional to the cosmic star formation rate (CSFR) and an additional evolution parametrized as (1 + z) α, where the CSFR model is a function of Mmin. Using the χ2 statistic, the contour constraints on the Mmin-α plane show that at the 1σ confidence level, we have Mmin <1010.5M⊙ from 118 LGRBs with redshift z < 4 and luminosity Liso > 1.8 ×1051 ergs-1. We also find that adding 12 high-z (4 < z < 5) LGRBs (consisting of 104 LGRBs with z < 5 and Liso > 3.1 ×1051 ergs-1) could result in much tighter constraints on Mmin, for which, 107.7M⊙ star formation in dark matter halos.

  12. IUE observations of blue halo high luminosity stars

    NASA Technical Reports Server (NTRS)

    Hack, M.; Franco, M. L.; Stalio, R.

    1981-01-01

    Two high luminosity population II blue stars of high galactic latitude, BD+33 deg 2642 and HD 137569 were observed at high resolution. The stellar spectra show the effect of mass loss in BD+33 deg 2642 and abnormally weak metallic lines in HD 137569. The interstellar lines in the direction of BD+33 deg 2642, which lies at a height z greater than or equal to 6.2 kpc from the galactic plane, are split into two components. No high ionization stages are found at the low velocity component; nor can they be detected in the higher velocity clouds because of mixing with the corresponding stellar/circumstellar lines.

  13. EFFECTS OF HOT HALO GAS ON STAR FORMATION AND MASS TRANSFER DURING DISTANT GALAXY–GALAXY ENCOUNTERS

    SciTech Connect

    Hwang, Jeong-Sun; Park, Changbom E-mail: cbp@kias.re.kr

    2015-06-01

    We use N-body/smoothed particle hydrodynamics simulations of encounters between an early-type galaxy (ETG) and a late-type galaxy (LTG) to study the effects of hot halo gas on the evolution for a case with the mass ratio of the ETG to LTG of 2:1 and the closest approach distance of ∼100 kpc. We find that the dynamics of the cold disk gas in the tidal bridge and the amount of the newly formed stars depend strongly on the existence of a gas halo. In the run of interacting galaxies not having a hot gas halo, the gas and stars accreted into the ETG do not include newly formed stars. However, in the run using the ETG with a gas halo and the LTG without a gas halo, a shock forms along the disk gas tidal bridge and induces star formation near the closest approach. The shock front is parallel to a channel along which the cold gas flows toward the center of the ETG. As a result, the ETG can accrete star-forming cold gas and newly born stars at and near its center. When both galaxies have hot gas halos, a shock is formed between the two gas halos somewhat before the closest approach. The shock hinders the growth of the cold gas bridge to the ETG and also ionizes it. Only some of the disk stars transfer through the stellar bridge. We conclude that the hot halo gas can give significant hydrodynamic effects during distant encounters.

  14. NGC 1252: a high altitude, metal poor open cluster remnant

    NASA Astrophysics Data System (ADS)

    de la Fuente Marcos, R.; de la Fuente Marcos, C.; Moni Bidin, C.; Carraro, G.; Costa, E.

    2013-09-01

    If stars form in clusters but most stars belong to the field, understanding the details of the transition from the former to the latter is imperative to explain the observational properties of the field. Aging open clusters are one of the sources of field stars. The disruption rate of open clusters slows down with age but, as an object gets older, the distinction between the remaining cluster or open cluster remnant (OCR) and the surrounding field becomes less and less obvious. As a result, finding good OCR candidates or confirming the OCR nature of some of the best candidates still remain elusive. One of these objects is NGC 1252, a scattered group of about 20 stars in Horologium. Here we use new wide-field photometry in the UBVI passbands, proper motions from the Yale/San Juan SPM 4.0 catalogue and high-resolution spectroscopy concurrently with results from N-body simulations to decipher NGC 1252's enigmatic character. Spectroscopy shows that most of the brightest stars in the studied area are chemically, kinematically and spatially unrelated to each other. However, after analysing proper motions, we find one relevant kinematic group. This sparse object is relatively close (˜1 kpc), metal poor and is probably not only one of the oldest clusters (3 Gyr) within 1.5 kpc from the Sun but also one of the clusters located farthest from the disc, at an altitude of nearly -900 pc. That makes NGC 1252 the first open cluster that can be truly considered a high Galactic altitude OCR: an unusual object that may hint at a star formation event induced on a high Galactic altitude gas cloud. We also conclude that the variable TW Horologii and the blue straggler candidate HD 20286 are unlikely to be part of NGC 1252. NGC 1252 17 is identified as an unrelated, Population II cannonball star moving at about 400 km s-1.

  15. WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

    SciTech Connect

    Fiorentino, Giuliana; Bono, Giuseppe; Monelli, Matteo; Gallart, Carme; Martínez-Vásquez, Clara E.; Tolstoy, Eline; Salaris, Maurizio; Bernard, Edouard J.

    2015-01-01

    We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in 6 dwarf spheroidals (dSphs) and 11 ultra faint dwarf galaxies (∼1300) show a Gaussian period distribution well peaked around a mean period of (Pab) = 0.610 ± 0.001 days (σ = 0.03). The Halo RRL (∼15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dSphs apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P ≲ 0.48 days and A{sub V} ≥ 0.75 mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered 18 globulars covering a broad range in metallicity (–2.3 ≲ [Fe/H] ≲ –1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal-rich than [Fe/H] ∼ –1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of ∼50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.

  16. Chemical Abundances in NGC 5053: A Very Metal-poor and Dynamically Complex Globular Cluster

    NASA Astrophysics Data System (ADS)

    Boberg, Owen M.; Friel, Eileen D.; Vesperini, Enrico

    2015-05-01

    NGC 5053 provides a rich environment to test our understanding of the complex evolution of globular clusters (GCs). Recent studies have found that this cluster has interesting morphological features beyond the typical spherical distribution of GCs, suggesting that external tidal effects have played an important role in its evolution and current properties. Additionally, simulations have shown that NGC 5053 could be a likely candidate to belong to the Sagittarius dwarf galaxy (Sgr dSph) stream. Using the Wisconsin-Indiana-Yale-NOAO-Hydra multi-object spectrograph, we have collected high quality (signal-to-noise ratio ˜ 75-90), medium-resolution spectra for red giant branch stars in NGC 5053. Using these spectra we have measured the Fe, Ca, Ti, Ni, Ba, Na, and O abundances in the cluster. We measure an average cluster [Fe/H] abundance of -2.45 with a standard deviation of 0.04 dex, making NGC 5053 one of the most metal-poor GCs in the Milky Way (MW). The [Ca/Fe], [Ti/Fe], and [Ba/Fe] we measure are consistent with the abundances of MW halo stars at a similar metallicity, with alpha-enhanced ratios and slightly depleted [Ba/Fe]. The Na and O abundances show the Na-O anti-correlation found in most GCs. From our abundance analysis it appears that NGC 5053 is at least chemically similar to other GCs found in the MW. This does not, however, rule out NGC 5053 being associated with the Sgr dSph stream.

  17. The Fraction of Globular Cluster Second-generation Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Vesperini, Enrico; McMillan, Stephen L. W.; D'Antona, Francesca; D'Ercole, Annibale

    2010-08-01

    Many observational studies have revealed the presence of multiple stellar generations in Galactic globular clusters. These studies suggest that second-generation stars make up a significant fraction of the current mass of globular clusters, with the second-generation mass fraction ranging from ~50% to 80% in individual clusters. In this Letter, we carry out hydrodynamical simulations to explore the dependence of the mass of second-generation stars on the initial mass and structural parameters and stellar initial mass function (IMF) of the parent cluster. We then use the results of these simulations to estimate the fraction f SG,H of the mass of the Galactic stellar halo composed of second-generation stars that originated in globular clusters. We study the dependence of f SG,H on the parameters of the IMF of the Galactic globular cluster system. For a broad range of initial conditions, we find that the fraction of mass of the Galactic stellar halo in second-generation stars is always small, f SG,H < 4%-6% for a Kroupa-1993 IMF and f SG,H < 7%-9% for a Kroupa-2001 IMF.

  18. The role of neutron star mergers in the chemical evolution of the Galactic halo

    NASA Astrophysics Data System (ADS)

    Cescutti, G.; Romano, D.; Matteucci, F.; Chiappini, C.; Hirschi, R.

    2015-05-01

    Context. The dominant astrophysical production site of the r-process elements has not yet been unambiguously identified. The suggested main r-process sites are core-collapse supernovae and merging neutron stars. Aims: We explore the problem of the production site of Eu. We also use the information present in the observed spread in the Eu abundances in the early Galaxy, and not only its average trend. Moreover, we extend our investigations to other heavy elements (Ba, Sr, Rb, Zr) to provide additional constraints on our results. Methods: We adopt a stochastic chemical evolution model that takes inhomogeneous mixing into account. The adopted yields of Eu from merging neutron stars and from core-collapse supernovae are those that are able to explain the average [Eu/Fe]-[Fe/H] trend observed for solar neighbourhood stars, the solar abundance of Eu, and the present-day abundance gradient of Eu along the Galactic disc in the framework of a well-tested homogeneous model for the chemical evolution of the Milky Way. Rb, Sr, Zr, and Ba are produced by both the s- and r-processes. The r-process yields were obtained by scaling the Eu yields described above according to the abundance ratios observed in r-process rich stars. The s-process contribution by spinstars is the same as in our previous papers. Results: Neutron star binaries that merge in less than 10 Myr or neutron star mergers combined with a source of r-process generated by massive stars can explain the spread of [Eu/Fe] in the Galactic halo. The combination of r-process production by neutron star mergers and s-process production by spinstars is able to reproduce the available observational data for Sr, Zr, and Ba. We also show the first predictions for Rb in the Galactic halo. Conclusions: We confirm previous results that either neutron star mergers on a very short timescale or both neutron star mergers and at least a fraction of Type II supernovae have contributed to the synthesis of Eu in the Galaxy. The r

  19. The clustering of merging star-forming haloes: dust emission as high frequency arcminute CMB foreground

    NASA Astrophysics Data System (ADS)

    Righi, M.; Hernández-Monteagudo, C.; Sunyaev, R. A.

    2008-02-01

    Context: Future observations of CMB anisotropies will be able to probe high multipole regions of the angular power spectrum, corresponding to a resolution of a few arcminutes. Dust emission from merging haloes is one of the foregrounds that will affect such very small scales. Aims: We estimate the contribution to CMB angular fluctuations from objects that are bright in the sub-millimeter band due to intense star formation bursts following merging episodes. Methods: We base our approach on the Lacey-Cole merger model and on the Kennicutt relation which connects the star formation rate in galaxies with their infrared luminosity. We set the free parameters of the model in order to not exceed the SCUBA source counts, the Madau plot of star formation rate in the universe and COBE/FIRAS data on the intensity of the sub-millimeter cosmic background radiation. Results: We show that the angular power spectrum arising from the distribution of such star-forming haloes will be one of the most significant foregrounds in the high frequency channels of future CMB experiments, such as PLANCK, ACT and SPT. The correlation term, due to the clustering of multiple haloes at redshift z ~ 2-6, is dominant in the broad range of angular scales 200 ⪉ l ⪉ 3000. Poisson fluctuations due to bright sub-millimeter sources are more important at higher l, but since they are generated from the bright sources, such contribution could be strongly reduced if bright sources are excised from the sky maps. The contribution of the correlation term to the angular power spectrum depends strongly on the redshift evolution of the escape fraction of UV photons and the resulting temperature of the dust. The measurement of this signal will therefore give important information about the sub-millimeter emission and the escape fraction of UV photons from galaxies, in the early stage of their evolution.

  20. The SEGUE K Giant Survey. III. Quantifying Galactic Halo Substructure

    NASA Astrophysics Data System (ADS)

    Janesh, William; Morrison, Heather L.; Ma, Zhibo; Rockosi, Constance; Starkenburg, Else; Xue, Xiang Xiang; Rix, Hans-Walter; Harding, Paul; Beers, Timothy C.; Johnson, Jennifer; Lee, Young Sun; Schneider, Donald P.

    2016-01-01

    We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125 kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey’s Sloan Extension for Galactic Understanding and Exploration project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using blue horizontal branch (BHB) stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (˜33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity.

  1. Lithium in halo stars - Constraining the effects of helium diffusion on globular cluster ages and cosmology

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Demarque, Pierre

    1991-01-01

    Stellar evolutionary models with diffusion are used to show that observations of lithium in extreme halo stars provide crucial constraints on the magnitude of the effects of helium diffusion. The flatness of the observed Li-T(eff) relation severely constrains diffusion Li isochrones, which tend to curve downward toward higher T(eff). It is argued that Li observations at the hot edge of the plateau are particularly important in constraining the effects of helium diffusion; yet, they are currently few in number. It is proposed that additional observations are required there, as well as below 5500 K, to define more securely the morphology of the halo Li abundances. Implications for the primordial Li abundance are considered. It is suggested that a conservative upper limit to the initial Li abundance, due to diffusive effects alone, is 2.35.

  2. Spatially resolved dust emission of extremely metal-poor galaxies*

    NASA Astrophysics Data System (ADS)

    Zhou, Luwenjia; Shi, Yong; Diaz-Santos, Taino; Armus, Lee; Helou, George; Stierwalt, Sabrina; Li, Aigen

    2016-05-01

    We present infrared (IR) spectral energy distributions (SEDs) of individual star-forming regions in four extremely metal-poor (EMP) galaxies with metallicity Z ≲ Z⊙/10 as observed by the Herschel Space Observatory. With the good wavelength coverage of the SED, it is found that these EMP star-forming regions show distinct SED shapes as compared to those of grand design Spirals and higher metallicity dwarfs: they have on average much higher f70μm/f160 μm ratios at a given f160 μm/f250 μm ratio; single modified blackbody (MBB) fittings to the SED at λ ≥ 100 μm still reveal higher dust temperatures and lower emissivity indices compared to that of Spirals, while two MBB fittings to the full SED with a fixed emissivity index (β = 2) show that even at 100 μm, about half of the emission comes from warm (50 K) dust, in contrast to the cold (˜20 K) dust component. Our spatially resolved images furthermore reveal that the far-IR colours including f70 μm/f160 μm, f160 μm/f250 μm and f250 μm/f350 μm are all related to the surface densities of young stars as traced by far-UV, 24 μm and star formation rates (SFRs), but not to the stellar mass surface densities. This suggests that the dust emitting at wavelengths from 70 to 350 μm is primarily heated by radiation from young stars.

  3. Lick slit spectra of thirty-eight objective prism quasar candidates and low metallicity halo stars

    NASA Technical Reports Server (NTRS)

    Tytler, David; Fan, Xiao-Ming; Junkkarinen, Vesa T.; Cohen, Ross D.

    1993-01-01

    Lick Observatory slit spectra of 38 objects which were claimed to have pronounced UV excess and emission lines are presented. Eleven QSOs, four galaxies at z of about 0.1, 22 stars, and one unidentified object with a low S/N spectrum were found. Of 11 objects which Zhan and Chen (1987, 1989) suggested were QSO with z(prism) not greater than 2.8; eight are QSOs. Six of the QSOs show absorption systems, including Q0000+027A with a relatively strong associated C IV absorption system, and Q0008+008 with a damped Ly-alpha system with an H I column density of 10 exp 21/sq cm. The equivalent widths of the Ca II K line, the G band, and the Balmer lines in 10 stars with the best spectra are measured, and metallicities are derived. Seven of them are in the range -2.5 to -1.7, while the others are less metal-poor.

  4. Erratum: Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo

    NASA Astrophysics Data System (ADS)

    Capriotti, E. R.; Hawley, S. L.

    1997-07-01

    In § 2 of the recent paper ``Evaporation, Tidal Disruption, and Orbital Decay of Star Clusters in a Galactic Halo'' by E. R. Capriotti and S. L. Hawley (ApJ, 464, 765 [1996]), equation (1) contains a misprint. It should read rt=2r/3 [(Mc)/(AMH(r))]1/3/[1-r/(AMH(r)) (dMH(r))/dr]1/3 , (1)where the difference from the published version is that an A replaces the 3 in the denominator of the last term. The authors regret the error.

  5. Abundances of D, O, and other species towards the Halo Star HD 93521

    NASA Astrophysics Data System (ADS)

    Kruk, J. W.; Oliveira, C.; Sembach, K. R.; Savage, B. D.

    2006-06-01

    FUSE spectra of the halo star HD 93521 have been analyzed to determine column densities of D I, O I, N I, Ar I, Fe II, and H2 in the intermediate velocity cloud (IVC) along the line of sight. Combining these results with those from GHRS and ground-based spectra provides a comprehensive inventory of abundances in the IVC. We find a relatively high value for D/H (17.4 ppm), near solar abundances and low depletions for refractory elements, and a very low molecular fraction.

  6. Constraints of the Origin of the Remarkable Lithium Abundance in the Halo Star BD+23 3912

    NASA Astrophysics Data System (ADS)

    King, Jeremy R.; Deliyannis, Constantine P.; Boesgaard, Ann Merchant

    1996-12-01

    The Li abundance of the halo star BD+23 3912 ([Fe/H]=-1.5) lies a factor of 2 - 3 above the Spite plateau. This remarkable difference could reflect either less-than-average stellar Li depletion from a higher primordial Li abundance (as predicted by the Yale rotational stellar evolutionary models), which may have interesting implications for Big Bang nucleosynthesis, or the extraordinary action of Galactic Li production mechanisms. It is also possible that both processes have acted. We use our high resolution, high S/N Keck HIRES spectrum of BD+23 3912 to determine the s-process element abundances and 6Li/7Li ratio in this star. These values serve as signatures for two possible Li production scenarios: the 7Be transport mechanism in AGB stars, and cosmic ray interactions with the ISM. The unremarkable abundances of Y, Zr, Ba, La, Nd, and Sm that we derive argue against a significant contribution to this star' S excess Li from AGB production mechanisms carrying an s-process signature. Since halo subgiants like BD+23 3912 are expected to be particularly good 6Li preservers, our conservative upper limit of 6Li/7Li≤0.15 (compared to 0.25-0.50 expected from cosmic ray production) argues against cosmic ray + ISM interactions as the source for the excess Li, unless Li depletion from an even higher abundance has occurred with preferential 6Li depletion. Highly speculative RGB production scenarios also seem unlikely given the normal Na and M abundances we find and the normal C and 0 abundances determined by others. The totality of Li data on halo subgiants argues against possible diffusion scenarios, in which all such stars dredge up Li that diffused during the main sequence. While the high Li abundance in BD+23 3912 is consistent with that expected from Yale rotational models having a lower-than-average initial angular momentum, future observations of -process elements (particularly 11B) produced in supernovae should provide additional constraints on any enrichment

  7. A Continued Search for CEMP RR Lyrae Stars

    NASA Astrophysics Data System (ADS)

    Reggiani, H. M.; Kennedy, C. R.; Rossi, S.; Beers, T. C.

    2014-10-01

    Among the stellar populations of the Galactic halo there is a class of stars known as carbon-enhanced metal-poor (CEMP) stars. These are metal-poor ([Fe/H] < 1.0) stars whose atmospheres exhibit large overabundances of carbon ([C/Fe] ≥ +0.7). The frequency of these stars increases with decreasing metallicity, and so by studying their abundance patterns, one can begin to uncover details of the origins of the elements. There exist a number of different classes of CEMP stars (Beers & Christlieb 2005) with specific abundance characteristics; one of them is the CEMP-s class, which exhibit evidence of s-process element enrichment, widely believed to be resultant of mass transfer from a companion low-metallicity asymptotic giant branch (AGB) star, where the production of carbon and s-process elements occurs. Recent spectroscopic observations of metal-poor RR Lyrae stars have revealed that their typical abundance patterns are consistent with very metal-poor (VMP) and extremely metal-poor (EMP) giants and dwarfs studied in the halo system of the Milky Way. Of particular interest is the recent discovery of a VMP RR Lyrae that has large overabundances of carbon and the s-process elements. In this work, we showed results obtained with WiFeS observations 2.3m Siding Spring Observatory telescope of a set of newly-identified CEMP stars that are known RR Lyr stars. We confirmed theses stars as CEMP stars (Kennedy et. al., in prep) and will, eventually, test their abundances against new stellar evolution simulations of CEMP stars.

  8. OXYGEN ABUNDANCES IN NEARBY FGK STARS AND THE GALACTIC CHEMICAL EVOLUTION OF THE LOCAL DISK AND HALO

    SciTech Connect

    Ramirez, I.; Lambert, D. L.; Allende Prieto, C.

    2013-02-10

    Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are derived using high-quality spectra and a non-local thermodynamic equilibrium analysis of the 777 nm O I triplet lines. We assign a kinematic probability for the stars to be thin-disk (P {sub 1}), thick-disk (P {sub 2}), and halo (P {sub 3}) members. We confirm previous findings of enhanced [O/Fe] in thick-disk (P {sub 2} > 0.5) relative to thin-disk (P {sub 1} > 0.5) stars with [Fe/H] {approx}< -0.2, as well as a 'knee' that connects the mean [O/Fe]-[Fe/H] trend of thick-disk stars with that of thin-disk members at [Fe/H] {approx}> -0.2. Nevertheless, we find that the kinematic membership criterion fails at separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very restrictive kinematic separation is employed. Stars with 'intermediate' kinematics (P {sub 1} < 0.7, P {sub 2} < 0.7) do not all populate the region of the [O/Fe]-[Fe/H] plane intermediate between the mean thin-disk and thick-disk trends, but their distribution is not necessarily bimodal. Halo stars (P {sub 3} > 0.5) show a large star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with Galactocentric rotational velocity V < -200 km s{sup -1}; halo stars with V > -200 km s{sup -1} follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early mergers with satellite galaxies explain most of our observations, but the significant fraction of disk stars with 'ambiguous' kinematics and abundances suggests that scattering by molecular clouds and radial migration have both played an important role in determining the kinematic and chemical properties of solar neighborhood stars.

  9. Deep HST/ACS Photometry of an Arc of Young Stars in the Southern Halo of M82

    NASA Astrophysics Data System (ADS)

    Suwannajak, Chutipong

    2016-01-01

    We present deep HST/ACS photometry of an arclike, overdense region of stars in the southern halo of M82, located approximately 5 kpc from its disk. This arc feature was originally identified about a decade ago. The early ground-based studies suggested that it contains young stars with ages and metallicities similar to those that formed in the tidal tails between M81, M82, and NGC3077 during their interactions. The arc is clearly presented in the spatial distribution of stars in our field with significantly higher stellar density than the background M82 halo stars. The location of the tip of the red giant branch (RGB) reveals the arc to have a similar distance to M81 and M82, therefore confirming that it belongs to this interacting system. Combining our data with those from the ACS Nearby Galaxy Survey Treasury (ANGST), we construct a color-magnitude diagram (CMD) for the arc. A sequence of young stars is clearly presented on its CMD. This young main sequence is not seen in other parts of the M82 halo. Single-metallicity isochrones are used to derive the age of the young stars in the arc. We confirm that these stars exhibit ages consistent with young stars found in the HI bridges between M81, M82 and NGC3077. Furthermore, the mean metallicity of the RGB stars is also derived from their metallicity distribution function and found to be similar to that found in the HI bridges.

  10. THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS

    SciTech Connect

    Williams, Benjamin F.; Dalcanton, Julianne J.; Gilbert, Karoline M.; Rosenfield, Philip; Bell, Eric F.; Guhathakurta, Puragra; Seth, Anil C.; Kalirai, Jason S.; Girardi, Leo E-mail: jd@astro.washington.edu E-mail: philrose@astro.washington.edu E-mail: raja@uco.lick.org E-mail: aseth@astro.utah.edu E-mail: lgirardi@pd.astro.it

    2012-11-01

    We attempt to constrain the shape of M31's inner stellar halo by tracing the surface density of blue horizontal branch (BHB) stars at galactocentric distances ranging from 2 kpc to 35 kpc. Our measurements make use of resolved stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury survey, supplemented by several archival Hubble Space Telescope observations. We find that the ratio of BHB to red giant stars is relatively constant outside of 10 kpc, suggesting that the BHB is as reliable a tracer of the halo population as the red giant branch. In the inner halo, we do not expect BHB stars to be produced by the high-metallicity bulge and disk, making BHB stars a good candidate to be a reliable tracer of the stellar halo to much smaller galactocentric distances. If we assume a power-law profile r {sup -{alpha}} for the two-dimensional (2D) projected surface density BHB distribution, we obtain a high-quality fit with a 2D power-law index of {alpha} = 2.6{sup +0.3} {sub -0.2} outside of 3 kpc, which flattens to {alpha} < 1.2 inside of 3 kpc. This slope is consistent with previous measurements but is anchored to a radial baseline that extends much farther inward. Finally, assuming azimuthal symmetry and a constant mass-to-light ratio, the best-fitting profile yields a total halo stellar mass of 2.1{sup +1.7} {sub -0.4} Multiplication-Sign 10{sup 9} M {sub Sun }. These properties are comparable with both simulations of stellar halo formation by satellite disruption alone and simulations that include some in situ formation of halo stars.

  11. The structure of star clusters in the outer halo of M31

    NASA Astrophysics Data System (ADS)

    Tanvir, N. R.; Mackey, A. D.; Ferguson, A. M. N.; Huxor, A.; Read, J. I.; Lewis, G. F.; Irwin, M. J.; Chapman, S.; Ibata, R.; Wilkinson, M. I.; McConnachie, A. W.; Martin, N. F.; Davies, M. B.; Bridges, T. J.

    2012-05-01

    We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, whilst four are from the Huxor et al. population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King profiles with ˜20 parsec core radii and ˜100 parsec photometric tidal radii, or by Sérsic profiles of index ˜1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec; however, the King profile is a less good fit in some cases, particularly at small radii. We find 60 per cent of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sérsic profiles of index ˜2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.

  12. Predicting Galaxy Star Formation Rates via the Co-evolution of Galaxies and Halos

    SciTech Connect

    Watson, Douglas F.; Hearin, Andrew P.; Berlind, Andreas A.; Becker, Matthew R.; Behroozi, Peter S.; Skibba, Ramin A.; Reyes, Reinabelle; Zentner, Andrew R.

    2014-03-06

    In this paper, we test the age matching hypothesis that the star formation rate (SFR) of a galaxy is determined by its dark matter halo formation history, and as such, that more quiescent galaxies reside in older halos. This simple model has been remarkably successful at predicting color-based galaxy statistics at low redshift as measured in the Sloan Digital Sky Survey (SDSS). To further test this method with observations, we present new SDSS measurements of the galaxy two-point correlation function and galaxy-galaxy lensing as a function of stellar mass and SFR, separated into quenched and star forming galaxy samples. We find that our age matching model is in excellent agreement with these new measurements. We also employ a galaxy group finder and show that our model is able to predict: (1) the relative SFRs of central and satellite galaxies, (2) the SFR-dependence of the radial distribution of satellite galaxy populations within galaxy groups, rich groups, and clusters and their surrounding larger scale environments, and (3) the interesting feature that the satellite quenched fraction as a function of projected radial distance from the central galaxy exhibits an approx r-.15 slope, independent of environment. The accurate prediction for the spatial distribution of satellites is intriguing given the fact that we do not explicitly model satellite-specific processes after infall, and that in our model the virial radius does not mark a special transition region in the evolution of a satellite, contrary to most galaxy evolution models. The success of the model suggests that present-day galaxy SFR is strongly correlated with halo mass assembly history.

  13. r-Process Elements as Tracers of Enrichment Processes in the Early Halo

    NASA Astrophysics Data System (ADS)

    Andersen, Johannes; Nordström, Birgitta; Hansen, Terese T.

    2016-08-01

    Significant minorities of extremely metal-poor (EMP) halo stars exhibit dramatic excesses of neutron capture elements. The standard scenario for their origin is mass transfer and dilution in binary systems, but requires them to be binaries. If not, these excesses must have been implanted in them from birth by processes that are not included in current models of SN II chemical enrichment. The binary population of such EMP subgroups is a test of this scenario.

  14. MODELS FOR METAL-POOR STARS WITH ENHANCED ABUNDANCES OF C, N, O, Ne, Na, Mg, Si, S, Ca, AND Ti, IN TURN, AT CONSTANT HELIUM AND IRON ABUNDANCES

    SciTech Connect

    VandenBerg, Don A.; Dotter, Aaron; Bergbusch, Peter A.; Ferguson, Jason W.; Michaud, Georges; Richer, Jacques; Proffitt, Charles R. E-mail: Aaron.Dotter@gmail.com E-mail: proffitt@stsci.edu E-mail: michaudg@astro.umontreal.ca

    2012-08-10

    Recent work has shown that most globular clusters have at least two chemically distinct components, as well as cluster-to-cluster differences in the mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order to investigate the implications of variations in the abundances of these and other metals for H-R diagrams and predicted ages, grids of evolutionary sequences have been computed for scaled solar and enhanced {alpha}-element metal abundances, and for mixtures in which the assumed [m/Fe] value for each of the metals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by 0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages from Almost-Equal-To 5 to 14 Gyr, have been computed for -3.0 {<=} [Fe/H] {<=}-0.6, with helium abundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded versions of the Victoria stellar structure program and the Regina interpolation code, respectively. Turnoff luminosity versus age relations from isochrones are found to depend almost entirely on the importance of the CNO cycle, and thereby mainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do not contribute significantly to the opacities at low temperatures and densities, variations in their abundances do not impact the predicted T{sub eff} scale of red giants. The latter is a strong function of the abundances of only Mg and Si (and Fe, possibly to a lesser extent) because they are so abundant and because they are strong sources of opacity at low temperatures. For these reasons, Mg and Si also have important effects on the temperatures of main-sequence stars. Due to their low abundances, Na, Ca, and Ti are of little consequence for stellar models. The effects of varying the adopted solar metals mixture and the helium abundance at a fixed [Fe/H] are also briefly discussed.

  15. Characterizing stellar halo populations - I. An extended distribution function for halo K giants

    NASA Astrophysics Data System (ADS)

    Das, Payel; Binney, James

    2016-08-01

    We fit an extended distribution function (EDF) to K giants in the Sloan Extension for Galactic Understanding and Exploration survey. These stars are detected to radii ˜80 kpc and span a wide range in [Fe/H]. Our EDF, which depends on [Fe/H] in addition to actions, encodes the entanglement of metallicity with dynamics within the Galaxy's stellar halo. Our maximum-likelihood fit of the EDF to the data allows us to model the survey's selection function. The density profile of the K giants steepens with radius from a slope ˜-2 to ˜-4 at large radii. The halo's axis ratio increases with radius from 0.7 to almost unity. The metal-rich stars are more tightly confined in action space than the metal-poor stars and form a more flattened structure. A weak metallicity gradient ˜-0.001 dex kpc-1, a small gradient in the dispersion in [Fe/H] of ˜0.001 dex kpc-1, and a higher degree of radial anisotropy in metal-richer stars result. Lognormal components with peaks at ˜-1.5 and ˜-2.3 are required to capture the overall metallicity distribution, suggestive of the existence of two populations of K giants. The spherical anisotropy parameter varies between 0.3 in the inner halo to isotropic in the outer halo. If the Sagittarius stream is included, a very similar model is found but with a stronger degree of radial anisotropy throughout.

  16. CARBON-RICH DUST PRODUCTION IN METAL-POOR GALAXIES IN THE LOCAL GROUP

    SciTech Connect

    Sloan, G. C.; Matsuura, M.; Lagadec, E.; Van Loon, J. Th.; Kraemer, K. E.; McDonald, I.; Zijlstra, A. A.; Groenewegen, M. A. T.; Wood, P. R.; Bernard-Salas, J.

    2012-06-20

    We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax, and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. The spectra show significant quantities of dust around the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous comparisons of carbon stars with similar pulsation properties in the Galaxy and the Magellanic Clouds revealed no evidence that metallicity affected the production of dust by carbon stars. However, the more metal-poor stars in the current sample appear to be generating less dust. These data extend two known trends to lower metallicities. In more metal-poor samples, the SiC dust emission weakens, while the acetylene absorption strengthens. The bolometric magnitudes and infrared spectral properties of the carbon stars in Fornax are consistent with metallicities more similar to carbon stars in the Magellanic Clouds than in the other dwarf spheroidals in our sample. A study of the carbon budget in these stars reinforces previous considerations that the dredge-up of sufficient quantities of carbon from the stellar cores may trigger the final superwind phase, ending a star's lifetime on the asymptotic giant branch.

  17. Deep CCD Field Surveys: Numbers of Very Low Mass Stars in the Halo and Disk

    NASA Astrophysics Data System (ADS)

    Boeshaar, Patricia C.; Tyson, Tony; Bernstein, Gary

    1994-12-01

    Deep three band (B_J < 27.5, R < 26.4, I < 25 mag) CCD images of 12 high galactic latitude fields covering a total of 144 arcmin(2) on the sky have been obtained as part of a 4-m survey done at CTIO over the past decade. Together with a single 2048(2) CCD field covering 48 sq. arcmin on the sky obtained at KPNO, these data have been analyzed to search for M dwarfs near the halo and disk hydrogen burning limits. Our color data have been carefully calibrated using stars of different luminosities which have spectroscopically determined metallicities, in order to separate out the different population types. We find no evidence for a population of very low mass M dwarfs sufficient to account for an important fraction of the halo dark matter. For the least luminous halo M subdwarfs (M_V ~ 15) our survey is complete out to 3000 pc, covering a volume of approx. 205,000 pc(3) . We detect 6 objects having colors consistent with M subdwarfs of M_V = 13.5 -- 15, though this sample may be contaminated by 1--2 misclassified compact high redshift galaxies of similar color which appear stellar. Our finding is consistent with the halo luminosity function determined in the solar neighborhood by Dahn and Liebert (1994 Proceedings of the ESO workshop: "The Bottom of the Main Sequence and Beyond"). They predict that we should find 5 +/- 3 of the least luminous subdwarfs within our volume. By comparison, the halo luminosity function of Richer and Fahlman (1992, Nature 358, 383) would predict over five times as many low mass M subdwarfs than we find in our surveys. Moreover, with a completeness limit of 500 pc, we find no excess of the least luminous disk M dwarfs (dM8-9, M_V ~ 18 -- 19) beyond that predicted by the luminosity function determined from a large area CCD Transit Instrument Survey (Kirpatrick et al 1994, ApJS 94, 749). Our data similarly suggest that the latest M dwarfs have a scale height much smaller than the 350 pc. value widely used for earlier M dwarfs.

  18. Li and Be depletion in metal-poor subgiants

    NASA Astrophysics Data System (ADS)

    García Pérez, A. E.; Primas, F.

    2006-02-01

    A sample of metal-poor subgiants has been observed with the UVES spectrograph at the Very Large Telescope and abundances of Li and Be have been determined. Typical signal-to-noise per spectral bin values for the co-added spectra are of the order of 500 for the ion{Li}{i} line (670.78 nm) and 100 for the ion{Be}{ii} doublet lines (313.04 nm). The spectral analysis of the observations was carried out using the Uppsala suite of codes and marcs (1D-LTE) model atmospheres with stellar parameters from photometry, parallaxes, isochrones and Fe ii lines. Abundance estimates of the light elements were corrected for departures from local thermodynamic equilibrium in the line formation. Effective temperatures and Li abundances seem to be correlated and Be abundances correlate with [O/H]. Standard models predict Li and Be abundances approximately one order of magnitude lower than main-sequence values which is in general agreement with the observations. On average, our observed depletions seem to be 0.1 dex smaller and between 0.2 and 0.4 dex larger (depending on which reference is taken) than those predicted for Li and Be, respectively. This is not surprising since the initial Li abundance, as derived from main-sequence stars on the Spite plateau, may be systematically in error by 0.1 dex or more, and uncertainties in the spectrum normalisation and continuum drawing may affect our Be abundances systematically.

  19. Red giants in the outer halo of the elliptical galaxy NGC 5128/Centaurus A

    NASA Astrophysics Data System (ADS)

    Bird, Sarah A.; Flynn, Chris; Harris, William E.; Valtonen, Mauri

    2015-03-01

    We used VIMOS on VLT to perform V and I band imaging of the outermost halo of NGC 5128/Centaurus A ((m - M)0 = 27.91±0.08), 65 kpc from the galaxy's center and along the major axis. The stellar population has been resolved to I0 ≈ 27 with a 50% completeness limit of I0 = 24.7, well below the tip of the red-giant branch (TRGB), which is seen at I0 ≈ 23.9. The surface density of NGC 5128 halo stars in our fields was sufficiently low that dim, unresolved background galaxies were a major contaminant in the source counts. We isolated a clean sample of red-giant-branch (RGB) stars extending to ≈0.8 mag below the TRGB through conservative magnitude and color cuts, to remove the (predominantly blue) unresolved background galaxies. We derived stellar metallicities from colors of the stars via isochrones and measured the density falloff of the halo as a function of metallicity by combining our observations with HST imaging taken of NGC 5128 halo fields closer to the galaxy center. We found both metal-rich and metal-poor stellar populations and found that the falloff of the two follows the same de Vaucouleurs' law profiles from ≈8 kpc out to ≈70 kpc. The metallicity distribution function (MDF) and the density falloff agree with the results of two recent studies of similar outermost halo fields in NGC 5128. We found no evidence of a "transition" in the radial profile of the halo, in which the metal-rich halo density would drop rapidly, leaving the underlying metal-poor halo to dominate by default out to greater radial extent, as has been seen in the outer halo of two other large galaxies. If NGC 5128 has such a transition, it must lie at larger galactocentric distances.

  20. Building Halos by Digesting Satellites

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-05-01

    recently.Dwarfs destroyed and accreted early on are typically low-metallicity as would be expected, since metallicity was lower in the early universe. Dwarfs accreted later in the simulation are typically higher metallicity. So host halos with recent accretion events are not only likely to have accreted more stellar mass, but also probably higher-metallicity stars.Though ultra-faint, low-mass dwarfs have lower average metallicities than the larger classical dwarfs, classical dwarfs contribute more of the very metal-poor stars accreted by host halos (40-80%, compared to the 2-5% from ultra-faint dwarfs).Halos that have relatively quiescent accretion histories tend to have lower-mass surviving dwarfs today.A Transient Fossil?This last point has interesting implications for our own galaxy. The Milky Way is generally though to have a quiescent formation history, and yet it contains two high-mass surviving dwarfs: the Large and Small Magellanic Clouds. The authors suggest that this inconsistency could be resolved if the Milky Way is a transient fossil a halo with a quiescent formation history masked by its recent acquisition of the Large and Small Magellanic Clouds.The outcomes from this suite of simulations provide important clues for better understanding how our own galaxy and galaxies like ours have formed and evolved.CitationAlis J. Deason et al 2016 ApJ 821 5. doi:10.3847/0004-637X/821/1/5

  1. Artificial neural network to search for metal-poor galaxies

    NASA Astrophysics Data System (ADS)

    Shi, Fei; Liu, Yu-Yan; Kong, Xu; Chen, Yang

    2014-02-01

    Aims: To find a fast and reliable method for selecting metal-poor galaxies (MPGs), especially in large surveys and huge databases, an artificial neural network (ANN) method is applied to a sample of star-forming galaxies from the Sloan Digital Sky Survey (SDSS) data release 9 (DR9) provided by the Max Planck Institute and the Johns Hopkins University (MPA/JHU). Methods: A two-step approach is adopted: (i) The ANN network must be trained with a subset of objects that are known to be either MPGs or metal rich galaxies (MRGs), treating the strong emission line flux measurements as input feature vectors in n-dimensional space, where n is the number of strong emission line flux ratios. (ii) After the network is trained on a sample of star-forming galaxies, the remaining galaxies are classified in the automatic test analysis as either MPGs or MRGs. We consider several random divisions of the data into training and testing sets; for instance, for our sample, a total of 70 percent of the data are involved in training the algorithm, 15 percent are involved in validating the algorithm, and the remaining 15 percent are used for blind testing the resulting classifier. Results: For target selection, we have achieved an acquisition rate for MPGs of 96 percent and 92 percent for an MPGs threshold of 12 + log (O/H) = 8.00 and 12 + log (O/H) = 8.39, respectively. Running the code takes minutes in most cases under the Matlab 2013a software environment. The ANN method can easily be extended to any MPGs target selection task when the physical property of the target can be expressed as a quantitative variable. The code in the paper is available on the web (http://fshi5388.blog.163.com).

  2. The Effect of Feedback and Reionization on Star Formation in Low-mass Dwarf Galaxy Halos

    NASA Astrophysics Data System (ADS)

    Simpson, Christine M.; Bryan, G.; Johnston, K. V.; Smith, B. D.; Mac Low, M.; Sharma, S.; Tumlinson, J.

    2013-01-01

    I will present a set of high resolution simulations of a 109 M⊙ dark matter halo in a cosmological setting done with an adaptive-mesh refinement code as a mass analogue to local low-luminosity dwarf spheroidal galaxies. The primary goal of our simulations is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude difference in the final stellar mass of the system. For our full physics run with reionization at z=9, we find a stellar mass of about 105 M⊙ at z=0, and a mass-to-light ratio within the half-light radius of approximately 130 M⊙/L⊙, consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z⊙, considerably larger than observed systems. In addition, we find star formation is truncated between redshifts 4 and 7, at odds with the observed late time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M⊙, and a comoving spatial resolution of 11 pc. This has led us to pursue improvements in our supernova feedback model to include kinetic as well as thermal energy in

  3. An Extremely Fast Halo Hot Subdwarf Star in a Wide Binary System

    NASA Astrophysics Data System (ADS)

    Németh, Péter; Ziegerer, Eva; Irrgang, Andreas; Geier, Stephan; Fürst, Felix; Kupfer, Thomas; Heber, Ulrich

    2016-04-01

    New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 (V = 17.92 mag) revealed a cool companion to the hot subdwarf primary. The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance, confirming the physical nature of the binary, which is the first double-lined hyper-velocity candidate. Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair, analogous to many long-period subdwarf binaries observed in the Galactic disk. We found the subdwarf atmospheric parameters: {T}{{eff}}=30\\600+/- 500 K, {log}g=5.57+/- 0.06 cm s‑2, and He abundance {log}(n{{He}}/n{{H}})=-3.0+/- 0.2. Oxygen is the most abundant metal in the hot subdwarf atmosphere, and Mg and Na lines are the most prominent spectral features of the cool companion, consistent with a metallicity of [{{Fe}}/{{H}}]=-1.3. The non-detection of radial velocity variations suggest the orbital period to be a few hundred days, in agreement with similar binaries observed in the disk. Using the SDSS-III flux calibrated spectrum we measured the distance to the system d=5.5+/- 0.5 {{kpc}}, which is consistent with ultraviolet, optical, and infrared photometric constraints derived from binary spectral energy distributions. Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit cannot be ruled out. On the other hand, a bound orbit requires a massive dark matter halo. We conclude that the binary either formed in the halo or was accreted from the tidal debris of a dwarf galaxy by the Milky Way.

  4. THE TERZAN 5 PUZZLE: DISCOVERY OF A THIRD, METAL-POOR COMPONENT

    SciTech Connect

    Origlia, L.; Massari, D.; Mucciarelli, A.; Ferraro, F. R.; Dalessandro, E.; Lanzoni, B.

    2013-12-10

    We report on the discovery of three metal-poor giant stars in Terzan 5, a complex stellar system in the Galactic bulge, known to have two populations at [Fe/H] = –0.25 and +0.3. For these three stars we present new echelle spectra obtained with NIRSPEC at Keck II, which confirm their radial velocity membership and provide an average [Fe/H] = –0.79 dex iron abundance and [α/Fe] = +0.36 dex enhancement. This new population extends the metallicity range of Terzan 5 to 0.5 dex more metal poor, and it has properties consistent with having formed from a gas polluted by core-collapse supernovae.

  5. On the Origin of the High Lithium Abundance in the Halo Star BD+23{\\ }3912

    NASA Astrophysics Data System (ADS)

    Deliyannis, C. P.; King, J. R.; Boesgaard, A. M.

    1996-09-01

    The Li abundance of the halo star BD+23{\\ }3912 ([Fe/H]=-1.5) lies a factor of 2-3 above the Spite plateau. This remarkable difference could reflect either less-than-average stellar Li depletion from a higher primordial Li abundance (as predicted by the Yale rotational stellar evolutionary models), which may have interesting implications for Big Bang nucleosynthesis, or the extraordinary action of Galactic Li production mechanisms (or both). We use our high resolution, high S/N Keck HIRES spectrum of BD+23{\\ }3912 to determine the s-process element abundances and (6) Li/(7) Li ratio in this star. These values serve as signatures for two possible Li production scenarios: {\\ }the (7) Be transport mechanism in AGB stars, and cosmic ray interactions with the ISM. The unremarkable abundances of Y, Zr, Ba, La, Nd, and Sm that we derive argue against a significant contribution to this star's excess Li from AGB production mechanisms carrying an s-process signature. Our conservative upper limit of (6) Li/(7) Li{<=}0.15 (compared to 0.25-0.50 expected from cosmic ray production) argues against cosmic ray + ISM interactions as the source for the excess Li, unless Li depletion from an even higher abundance has occurred with preferential (6) Li depletion. Highly speculative RGB production scenarios also seem unlikely given the normal Na and Al abundances we find and the normal C and O abundances determined by others. While the high Li abundance in BD+23{\\ }3912 is consistent with that expected from Yale rotational models having a lower-than-average initial angular momentum, future observations of ν-process elements (particularly (11) B) produced in supernovae should provide additional constraints on any enrichment scenarios seeking to explain the large Li abundance of this interesting star.

  6. Constraints on the Origin of the Remarkable Lithium Abundance of the Halo Star BD+23 3912

    NASA Astrophysics Data System (ADS)

    King, Jeremy R.; Deliyannis, Constantine P.; Boesgaard, Ann M.

    1997-02-01

    The Li abundance of the halo star BD+23 3912 ([Fe/(H)] = -1.5) lies a factor of 2 - 3 above the Spite Plateau. This remarkable difference could reflect either less-than-average stellar Li depletion from a higher primordial Li abundance (as predicted by the Yale rotational stellar evolutionary models) having interesting implications for Big Bang nucleosynthesis, or the extraordinary action of Galactic Li production mechanisms. It is also possible that both mechanisms have acted. We use our high resolution, high S/(N) Keck HIRES spectrum of BD+23 3912 to determine the n-capture abundances and 6Li/(7Li) ratio in this star. These values serve as signatures for two possible Li production scenarios: the 7Be transport mechanism in AGB stars and cosmic ray interactions with the ISM. The unremarkable abundances of Y, Zr, Ba, La, Nd, and Sm that we derive argue against a significant contribution to this star's excess Li from AGB production mechanisms carrying an s-process signature. Our conservative upper limit of 6Li/(7Li)<=0.15, compared to 0.25 - 0.50 expected from cosmic ray production, argues against cosmic ray + ISM interactions as the source for the excess Li, unless Li depletion from an even higher abundance has occurred with preferential 6Li depletion. Highly speculative RGB production scenarios also seem unlikely given the normal Na and Al abundances we find and the normal C and O abundances determined by others. While the high Li abundance in BD+23 3912 is consistent with that expected from Yale rotational models having a lower-than-average initial angular momentum, future observations of ν-process elements (particularly 11B) produced in supernovae should provide additional constraints on any enrichment scenarios seeking to explain the large Li abundance of this interesting star.

  7. On the Contribution of Fluorescence to Lyα Halos around Star-Forming Galaxies

    NASA Astrophysics Data System (ADS)

    Mas-Ribas, Lluís; Dijkstra, Mark

    2016-05-01

    We quantify the contribution of Lyα fluorescence to observed spatially extended Lyα halos around Lyα emitters at redshift z = 3.1. The key physical quantities that describe the fluorescent signal include (i) the distribution of cold gas in the circumgalactic medium (CGM); we explore simple analytic models and fitting functions to recent hydrodynamical simulations; and (ii) local variations in the ionizing background due to ionizing sources that cluster around the central galaxy. We account for clustering by boosting the observationally inferred volumetric production rate of ionizing photons, {ɛ }{{LyC}}, by a factor of 1+{ξ }{{LyC}}(r), in which {ξ }{{LyC}}(r) quantifies the clustering of ionizing sources around the central galaxy. We compute {ξ }{{LyC}}(r) by assigning an “effective” bias parameter to the ionizing sources. This novel approach allows us to quantify our ignorance of the population of ionizing sources in a simple parametrized form. We find a maximum enhancement in the local ionizing background in the range 50–200 at r ˜ 10 physical kpc. For spatially uncorrelated ionizing sources and fluorescing clouds we find that fluorescence can contribute up to ˜ 50%–60% of the observed spatially extended Lyα emission. We briefly discuss how future observations can shed light on the nature of Lyα halos around star-forming galaxies.

  8. On the Contribution of Fluorescence to Lyα Halos around Star-Forming Galaxies

    NASA Astrophysics Data System (ADS)

    Mas-Ribas, Lluís; Dijkstra, Mark

    2016-05-01

    We quantify the contribution of Lyα fluorescence to observed spatially extended Lyα halos around Lyα emitters at redshift z = 3.1. The key physical quantities that describe the fluorescent signal include (i) the distribution of cold gas in the circumgalactic medium (CGM); we explore simple analytic models and fitting functions to recent hydrodynamical simulations; and (ii) local variations in the ionizing background due to ionizing sources that cluster around the central galaxy. We account for clustering by boosting the observationally inferred volumetric production rate of ionizing photons, {ɛ }{{LyC}}, by a factor of 1+{ξ }{{LyC}}(r), in which {ξ }{{LyC}}(r) quantifies the clustering of ionizing sources around the central galaxy. We compute {ξ }{{LyC}}(r) by assigning an “effective” bias parameter to the ionizing sources. This novel approach allows us to quantify our ignorance of the population of ionizing sources in a simple parametrized form. We find a maximum enhancement in the local ionizing background in the range 50-200 at r ˜ 10 physical kpc. For spatially uncorrelated ionizing sources and fluorescing clouds we find that fluorescence can contribute up to ˜ 50%-60% of the observed spatially extended Lyα emission. We briefly discuss how future observations can shed light on the nature of Lyα halos around star-forming galaxies.

  9. Population III Stars Around the Milky Way

    NASA Astrophysics Data System (ADS)

    Komiya, Yutaka; Suda, Takuma; Fujimoto, Masayuki Y.

    2016-03-01

    We explore the possibility of observing Population III (Pop III) stars, born of primordial gas. Pop III stars with masses below 0.8 M⊙ should survive to date though are not yet observed, but the existence of stars with low metallicity as [{{Fe}}/{{H}}]\\lt -5 in the Milky Way halo suggests the surface pollution of Pop III stars with accreted metals from the interstellar gas after birth. In this paper, we investigate the runaway of Pop III stars from their host mini-halos, considering the ejection of secondary members from binary systems when their massive primaries explode as supernovae. These stars save them from surface pollution. By computing the star formation and chemical evolution along with the hierarchical structure formation based on the extended Press-Schechter merger trees, we demonstrate that several hundreds to tens of thousands of low-mass Pop III stars escape from the building blocks of the Milky Way. The second and later generations of extremely metal-poor stars also escaped from the mini-halos. We discuss the spatial distributions of these escaped stars by evaluating the distances between the mini-halos in the branches of merger trees under the spherical collapse model of dark matter halos. It is demonstrated that the escaped stars distribute beyond the stellar halo with a density profile close to the dark matter halo, while Pop III stars are slightly more centrally concentrated. 6%-30% of the escaped stars leave the Milky Way and go out into the intergalactic space. Based on the results, we discuss the feasibility of observing the Pop III stars with the pristine surface abundance.

  10. Ruprecht 106 - A young metal-poor Galactic globular cluster

    SciTech Connect

    Buonanno, R.; Buscema, G.; Fusi Pecci, F.; Richer, H.B.; Fahlman, G.G. Bologna Universita British Columbia Univ., Vancouver )

    1990-12-01

    The first CCD photometric survey in the Galactic globular cluster Ruprecht 106 has been performed. The results show that Ruprecht 106 is a metal-poor cluster with (Fe/H) about -2 located at about 25 kpc from the Galactic center. A sizable, high centrally concentrated population of blue stragglers was detected. Significant differences in the positions of the turnoffs in the color-magnitude diagram are found compared to those in metal-poor clusters. The cluster appears younger than other typical metal-poor Galactic globulars by about 4-5 Gyr; if true, this object would represent the first direct proof of the existence of a significant age spread among old, very metal-poor clusters. 51 refs.

  11. EXPLORING THE VARIABLE SKY WITH LINEAR. II. HALO STRUCTURE AND SUBSTRUCTURE TRACED BY RR LYRAE STARS TO 30 kpc

    SciTech Connect

    Sesar, Branimir; Ivezic, Zeljko; Morgan, Dylan M.; Becker, Andrew C.; Stuart, J. Scott; Sharma, Sanjib; Palaversa, Lovro; Juric, Mario; Wozniak, Przemyslaw; Oluseyi, Hakeem

    2013-08-01

    We present a sample of {approx}5000 RR Lyrae stars selected from the recalibrated LINEAR data set and detected at heliocentric distances between 5 kpc and 30 kpc over {approx}8000 deg{sup 2} of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We analyze in detail the light curve properties and Galactic distribution of the subset of {approx}4000 type ab RR Lyrae (RRab) stars, including a search for new halo substructures and the number density distribution as a function of Oosterhoff type. We find evidence for the Oosterhoff dichotomy among field RR Lyrae stars, with the ratio of the type II and I subsamples of about 1:4, but with a weaker separation than for globular cluster stars. The wide sky coverage and depth of this sample allow unique constraints for the number density distribution of halo RRab stars as a function of galactocentric distance: it can be described as an oblate ellipsoid with an axis ratio q = 0.63 and with either a single or a double power law with a power-law index in the range -2 to -3. Consistent with previous studies, we find that the Oosterhoff type II subsample has a steeper number density profile than the Oosterhoff type I subsample. Using the group-finding algorithm EnLink, we detected seven candidate halo groups, only one of which is statistically spurious. Three of these groups are near globular clusters (M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo Stellar Stream); the remaining three groups do not seem to be near any known halo substructures or globular clusters and seem to have a higher ratio of Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the halo. The extended morphology and the position (outside the tidal radius) of some of the groups near globular clusters are suggestive of tidal streams possibly originating from globular clusters. Spectroscopic follow-up of detected halo groups is encouraged.

  12. Probing star formation in the dense environments of z ˜ 1 lensing haloes aligned with dusty star-forming galaxies detected with the South Pole Telescope

    NASA Astrophysics Data System (ADS)

    Welikala, N.; Béthermin, M.; Guery, D.; Strandet, M.; Aird, K. A.; Aravena, M.; Ashby, M. L. N.; Bothwell, M.; Beelen, A.; Bleem, L. E.; de Breuck, C.; Brodwin, M.; Carlstrom, J. E.; Chapman, S. C.; Crawford, T. M.; Dole, H.; Doré, O.; Everett, W.; Flores-Cacho, I.; Gonzalez, A. H.; González-Nuevo, J.; Greve, T. R.; Gullberg, B.; Hezaveh, Y. D.; Holder, G. P.; Holzapfel, W. L.; Keisler, R.; Lagache, G.; Ma, J.; Malkan, M.; Marrone, D. P.; Mocanu, L. M.; Montier, L.; Murphy, E. J.; Nesvadba, N. P. H.; Omont, A.; Pointecouteau, E.; Puget, J. L.; Reichardt, C. L.; Rotermund, K. M.; Scott, D.; Serra, P.; Spilker, J. S.; Stalder, B.; Stark, A. A.; Story, K.; Vanderlinde, K.; Vieira, J. D.; Weiß, A.

    2016-01-01

    We probe star formation in the environments of massive (˜1013 M⊙) dark matter haloes at redshifts of z ˜ 1. This star formation is linked to a submillimetre clustering signal which we detect in maps of the Planck High Frequency Instrument that are stacked at the positions of a sample of high redshift (z > 2) strongly lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope (SPT) 2500 deg2 survey. The clustering signal has submillimetre colours which are consistent with the mean redshift of the foreground lensing haloes (z ˜ 1). We report a mean excess of star formation rate (SFR) compared to the field, of (2700 ± 700) M⊙ yr-1 from all galaxies contributing to this clustering signal within a radius of 3.5 arcmin from the SPT DSFGs. The magnitude of the Planck excess is in broad agreement with predictions of a current model of the cosmic infrared background. The model predicts that 80 per cent of the excess emission measured by Planck originates from galaxies lying in the neighbouring haloes of the lensing halo. Using Herschel maps of the same fields, we find a clear excess, relative to the field, of individual sources which contribute to the Planck excess. The mean excess SFR compared to the field is measured to be (370 ± 40) M⊙ yr-1 per resolved, clustered source. Our findings suggest that the environments around these massive z ˜ 1 lensing haloes host intense star formation out to about 2 Mpc. The flux enhancement due to clustering should also be considered when measuring flux densities of galaxies in Planck data.

  13. Detailed Abundance Analysis of a Metal-poor Giant in the Galactic Center

    NASA Astrophysics Data System (ADS)

    Ryde, N.; Fritz, T. K.; Rich, R. M.; Thorsbro, B.; Schultheis, M.; Origlia, L.; Chatzopoulos, S.

    2016-11-01

    We report the first results from our program to examine the metallicity distribution of the Milky Way nuclear star cluster connected to Sgr A*, with the goal of inferring the star formation and enrichment history of this system, as well as its connection and relationship with the central 100 pc of the bulge/bar system. We present the first high-resolution (R ∼ 24,000), detailed abundance analysis of a K = 10.2 metal-poor, alpha-enhanced red giant projected at 1.5 pc from the Galactic center, using NIRSPEC on Keck II. A careful analysis of the dynamics and color of the star locates it at about {26}-16+54 pc line-of-sight distance in front of the nuclear cluster. It probably belongs to one of the nuclear components (cluster or disk), not to the bar/bulge or classical disk. A detailed spectroscopic synthesis, using a new line list in the K band, finds [Fe/H] ∼ ‑1.0 and [α/Fe] ∼ +0.4, consistent with stars of similar metallicity in the bulge. As known giants with comparable [Fe/H] and alpha enhancement are old, we conclude that this star is most likely to be a representative of the ∼10 Gyr old population. This is also the most metal-poor-confirmed red giant yet discovered in the vicinity of the nuclear cluster of the Galactic center. We consider recent reports in the literature of a surprisingly large number of metal-poor giants in the Galactic center, but the reported gravity of {log}g∼ 4 for these stars calls into question their reported metallicities.

  14. VizieR Online Data Catalog: The SEGUE K giant survey. III. Galactic halo (Janesh+, 2016)

    NASA Astrophysics Data System (ADS)

    Janesh, W.; Morrison, H. L.; Ma, Z.; Rockosi, C.; Starkenburg, E.; Xue, X. X.; Rix, H.-W.; Harding, P.; Beers, T. C.; Johnson, J.; Lee, Y. S.; Schneider, D. P.

    2016-03-01

    We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey's Sloan Extension for Galactic Understanding and Exploration (SEGUE) project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using blue horizontal branch (BHB) stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (~33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity. (2 data files).

  15. GAS REGULATION OF GALAXIES: THE EVOLUTION OF THE COSMIC SPECIFIC STAR FORMATION RATE, THE METALLICITY-MASS-STAR-FORMATION RATE RELATION, AND THE STELLAR CONTENT OF HALOS

    SciTech Connect

    Lilly, Simon J.; Carollo, C. Marcella; Pipino, Antonio; Peng Yingjie; Renzini, Alvio

    2013-08-01

    A very simple physical model of galaxies is one in which the formation of stars is instantaneously regulated by the mass of gas in a reservoir with mass loss scaling with the star-formation rate (SFR). This model links together three different aspects of the evolving galaxy population: (1) the cosmic time evolution of the specific star-formation rate (sSFR) relative to the growth of halos, (2) the gas-phase metallicities across the galaxy population and over cosmic time, and (3) the ratio of the stellar to dark matter mass of halos. The gas regulator is defined by the gas consumption timescale ({epsilon}{sup -1}) and the mass loading {lambda} of the wind outflow {lambda}{center_dot}SFR. The simplest regulator, in which {epsilon} and {lambda} are constant, sets the sSFR equal to exactly the specific accretion rate of the galaxy; more realistic situations lead to an sSFR that is perturbed from this precise relation. Because the gas consumption timescale is shorter than the timescale on which the system evolves, the metallicity Z is set primarily by the instantaneous operation of the regulator system rather than by the past history of the system. The metallicity of the gas reservoir depends on {epsilon}, {lambda}, and sSFR, and the regulator system therefore naturally produces a Z(m{sub star}, SFR) relation if {epsilon} and {lambda} depend on the stellar mass m{sub star}. Furthermore, this relation will be the same at all epochs unless the parameters {epsilon} and {lambda} themselves change with time. A so-called fundamental metallicity relation is naturally produced by these conditions. The overall mass-metallicity relation Z(m{sub star}) directly provides the fraction f{sub star}(m{sub star}) of incoming baryons that are being transformed into stars. The observed Z(m{sub star}) relation of Sloan Digital Sky Survey (SDSS) galaxies implies a strong dependence of stellar mass on halo mass that reconciles the different faint-end slopes of the stellar and halo mass

  16. SEGUE 3: AN OLD, EXTREMELY LOW LUMINOSITY STAR CLUSTER IN THE MILKY WAY's HALO

    SciTech Connect

    Fadely, Ross; Willman, Beth; Geha, Maria; Munoz, Ricardo R.; Vargas, Luis C.; Walsh, Shane

    2011-09-15

    We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g- and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find that the half-light radius of Segue 3 is 26'' {+-} 5'' (2.1 {+-} 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M{sub V} = 0.0 {+-} 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0{sup +1.5}{sub -0.4} Gyr and an [Fe/H] of -1.7{sup +0.07}{sub -0.27}. Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 {+-} 2.6 km s{sup -1}. Photometry of the members indicates that the stellar population has a spread in [Fe/H] of {approx}< 0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the 11 candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii is complicated by the object's spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.

  17. Segue 3: An Old, Extremely Low Luminosity Star Cluster in the Milky Way's Halo

    NASA Astrophysics Data System (ADS)

    Fadely, Ross; Willman, Beth; Geha, Marla; Walsh, Shane; Muñoz, Ricardo R.; Jerjen, Helmut; Vargas, Luis C.; Da Costa, Gary S.

    2011-09-01

    We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g- and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find that the half-light radius of Segue 3 is 26'' ± 5'' (2.1 ± 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere MV = 0.0 ± 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0+1.5 - 0.4 Gyr and an [Fe/H] of -1.7+0.07 - 0.27. Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 ± 2.6 km s-1. Photometry of the members indicates that the stellar population has a spread in [Fe/H] of <~ 0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the 11 candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii is complicated by the object's spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.

  18. A NEW MILKY WAY HALO STAR CLUSTER IN THE SOUTHERN GALACTIC SKY

    SciTech Connect

    Balbinot, E.; Santiago, B. X.; Da Costa, L.; Maia, M. A. G.; Rocha-Pinto, H. J.; Majewski, S. R.; Nidever, D.; Thomas, D.; Wechsler, R. H.; Yanny, B.

    2013-04-20

    We report on the discovery of a new Milky Way (MW) companion stellar system located at ({alpha}{sub J2000,}{delta}{sub J2000}) = (22{sup h}10{sup m}43{sup s}.15, 14 Degree-Sign 56 Prime 58 Double-Prime .8). The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using Canada-France-Hawaii-Telescope/MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of 31.9{sup +1.0}{sub -1.6} kpc, with a half-light radius of r{sub h}= 7.24{sup +1.94}{sub -1.29} pc and a concentration parameter of c = log{sub 10}(r{sub t} /r{sub c} ) = 1.55. A systematic isochrone fit to its color-magnitude diagram resulted in log (age yr{sup -1}) = 10.07{sup +0.05}{sub -0.03} and [Fe/H] = -1.58{sup +0.08}{sub -0.13}. These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 95 {+-} 6 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of M{sub V} = -1.21 {+-} 0.66. The resulting surface brightness is {mu}{sub V} = 25.90 mag arcsec{sup -2}. Its position in the M{sub V} versus r{sub h} diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star cluster-one of the faintest and lowest mass systems yet identified.

  19. Investigating the earliest epochs of the Milky Way halo

    NASA Astrophysics Data System (ADS)

    Starkenburg, Else; Starkenburg

    2016-08-01

    Resolved stellar spectroscopy can obtain knowledge about chemical enrichment processes back to the earliest times, when the oldest stars were formed. In this contribution I will review the early (chemical) evolution of the Milky Way halo from an observational perspective. In particular, I will discuss our understanding of the origin of the peculiar abundance patterns in various subclasses of extremely metal-poor stars, taking into account new data from our abundance and radial velocity monitoring programs, and their implications for our understanding of the formation and early evolution of both the Milky Way halo and the satellite dwarf galaxies therein. I conclude by presenting the ``Pristine'' survey, a program on the Canada-France-Hawaii Telescope to study this intriguing epoch much more efficiently.

  20. Lightcurves of the Dominant Dust Producers in Metal-poor Environments

    NASA Astrophysics Data System (ADS)

    Boyer, Martha; Whitelock, P.; Sonneborn, G.; Sloan, G. C.; Skillman, E. D.; Meixner, M.; McQuinn, K. B. W.; McDonald, I.; Lagadec, E.; Javadi, A.; Groenewegen, M. A. T.; Gehrz, R. D.; Bonanos, A. Z.

    2014-12-01

    DUSTiNGS is a Spitzer Cycle 8 program that has provided the first complete spatially-resolved infrared census of evolved stars in a statistically significant sample of low metallicity, nearby (D<1.5 Mpc) dwarf galaxies. One of the main results of the program to date is the discovery of hundreds of dust-producing AGB stars up to ~1 dex more metal-poor than previously known examples, suggesting that dust forms efficiently at least down to [Fe/H] = -2.2 dex. This discovery not only has implications for our interpretation of the interstellar medium and star formation in nearby, resolved galaxies, but also for our understanding of the dust production and chemical evolution of galaxies in the metal-poor environment of the early Universe. Here, we propose new, multi-epoch IRAC observations of 10 DUSTiNGS galaxies to measure the amplitudes, periods, and dust excesses of the DUSTiNGS AGB stars and thereby gain insight into the basic properties of the underlying dust producing populations. These data will provide a foundation for understanding dust production by AGB stars in chemically un-evolved systems, as well as providing a target list of these enigmatic objects for James Webb Space Telescope (JWST) follow-up.

  1. Core-Halo Structure of a Chemically Homogeneous Massive Star and Bending of the Zero-Age Main Sequence

    NASA Astrophysics Data System (ADS)

    Ishii, Mie; Ueno, Munetaka; Kato, Mariko

    1999-08-01

    We have recalculated the interior structure of very massive stars of uniform chemical composition with the OPAL opacity. Very massive stars are found to develop a core-halo structure with an extended radiative-envelope. With the core-halo structure, because a more massive star has a more extended envelope, the track of the upper zero-age main-sequence (ZAMS) curves redward in the H-R diagram at > 100 MO (Z=0.02), >70 MO (Z=0.05), and > 15 MO for helium ZAMS (X=0, Z=0.02). Therefore, the effective temperatures of very massive ZAMS stars are rather low: e.g., for a 200 MO star, log T_eff=4.75 (Z=0.004), 4.60 (Z=0.02), 4.46 (Z=0.05), and 4.32 (Z=0.10). The effective temperatures of very luminous stars (> 120 MO ) found in the LMC, the SMC, and the Galaxy are discussed in relation to this metal dependence of a curving upper main-sequence.

  2. RADIATIVE AND KINETIC FEEDBACK BY LOW-MASS PRIMORDIAL STARS

    SciTech Connect

    Whalen, Daniel; Hueckstaedt, Robert M.; McConkie, Thomas O.

    2010-03-20

    Ionizing UV radiation and supernova (SN) flows amidst clustered minihalos at high redshift regulated the rise of the first stellar populations in the universe. Previous studies have addressed the effects of very massive primordial stars on the collapse of nearby halos into new stars, but the absence of the odd-even nucleosynthetic signature of pair-instability SNe in ancient metal-poor stars suggests that Population III stars may have been less than 100 M{sub sun}. We extend our earlier survey of local UV feedback on star formation to 25-80 M{sub sun} stars and include kinetic feedback by SNe for 25-40 M{sub sun} stars. We find radiative feedback to be relatively uniform over this mass range, primarily because the larger fluxes of more massive stars are offset by their shorter lifetimes. Our models demonstrate that prior to the rise of global UV backgrounds, Lyman-Werner (LW) photons from nearby stars cannot prevent halos from forming new stars. These calculations also reveal that violent dynamical instabilities can erupt in the UV radiation front enveloping a primordial halo, but that they ultimately have no effect on the formation of a star. Finally, our simulations suggest that relic H II regions surrounding partially evaporated halos may expel LW backgrounds at lower redshifts, allowing stars to form that were previously suppressed. We provide fits to radiative and kinetic feedback on star formation for use in both semianalytic models and numerical simulations.

  3. Extremely metal-poor gas at a redshift of 7.

    PubMed

    Simcoe, Robert A; Sullivan, Peter W; Cooksey, Kathy L; Kao, Melodie M; Matejek, Michael S; Burgasser, Adam J

    2012-12-01

    In typical astrophysical environments, the abundance of heavy elements ranges from 0.001 to 2 times the solar value. Lower abundances have been seen in selected stars in the Milky Way's halo and in two quasar absorption systems at redshift z = 3 (ref. 4). These are widely interpreted as relics from the early Universe, when all gas possessed a primordial chemistry. Before now there have been no direct abundance measurements from the first billion years after the Big Bang, when the earliest stars began synthesizing elements. Here we report observations of hydrogen and heavy-element absorption in a spectrum of a quasar at z =  7.04, when the Universe was just 772 million years old (5.6 per cent of its present age). We detect a large column of neutral hydrogen but no corresponding metals (defined as elements heavier than helium), limiting the chemical abundance to less than 1/10,000 times the solar level if the gas is in a gravitationally bound proto-galaxy, or to less than 1/1,000 times the solar value if it is diffuse and unbound. If the absorption is truly intergalactic, it would imply that the Universe was neither ionized by starlight nor chemically enriched in this neighbourhood at z ≈ 7. If it is gravitationally bound, the inferred abundance is too low to promote efficient cooling, and the system would be a viable site to form the predicted but as yet unobserved massive population III stars. PMID:23222611

  4. The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars

    NASA Astrophysics Data System (ADS)

    Brown, Warren R.; Geller, Margaret J.; Kenyon, Scott J.; Kurtz, Michael J.; Allende Prieto, Carlos; Beers, Timothy C.; Wilhelm, Ronald

    2005-09-01

    We discuss a 175 deg2 spectroscopic survey for blue horizontal branch (BHB) stars in the Galactic halo. We use the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to select BHB candidates, and we find that the 2MASS and SDSS color selection is 38% and 50% efficient, respectively, for BHB stars. Our samples include one likely runaway B7 star 6 kpc below the Galactic plane. The global properties of the BHB samples are consistent with membership in the halo population: the median metallicity is [Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and the mean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is -4+/-30 km s-1. We discuss the theoretical basis of the Preston, Shectman, and Beers MV-color relation for BHB stars and conclude that the intrinsic shape of the BHB MV-color relation results from the physics of stars on the horizontal branch. We calculate the luminosity function for the field BHB star samples using the maximum likelihood method of Efstathiou and coworkers, which is unbiased by density variations. The field BHB luminosity function exhibits a steep rise at bright luminosities, a peak between 0.8stars and BHB stars in globular clusters share a common distribution of luminosities, with the exception of globular clusters with extended BHBs.

  5. The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

    NASA Astrophysics Data System (ADS)

    Santucci, Rafael M.; Placco, Vinicius M.; Rossi, Silvia; Beers, Timothy C.; Reggiani, Henrique M.; Lee, Young Sun; Xue, Xiang-Xiang; Carollo, Daniela

    2015-03-01

    We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, FBSS/BHB, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for FBSS/BHB is ∼ 4.0 in the thick disk (at 3 kpc\\lt |Z|\\lt 4 kpc), declining to on the order of ∼ 1.5-2.0 in the inner-halo region; this ratio continues to decline to ∼1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5% of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.

  6. Dark-matter halo mergers as a fertile environment for low-mass Population III star formation

    NASA Astrophysics Data System (ADS)

    Bovino, S.; Latif, M. A.; Grassi, T.; Schleicher, D. R. G.

    2014-07-01

    While Population III (Pop III) stars are typically thought to be massive, pathways towards lower mass Pop III stars may exist when the cooling of the gas is particularly enhanced. A possible route is enhanced HD cooling during the merging of dark-matter haloes. The mergers can lead to a high ionization degree catalysing the formation of HD molecules and may cool the gas down to the cosmic microwave background temperature. In this paper, we investigate the merging of mini-haloes with masses of a few 105 M⊙ and explore the feasibility of this scenario. We have performed three-dimensional cosmological hydrodynamics calculations with the ENZO code, solving the thermal and chemical evolution of the gas by employing the astrochemistry package KROME. Our results show that the HD abundance is increased by two orders of magnitude compared to the no-merging case and the halo cools down to ˜60 K triggering fragmentation. Based on Jeans estimates, the expected stellar masses are about 10 M⊙. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.

  7. Core-halo age gradients and star formation in the Orion Nebula and NGS 2024 young stellar clusters

    SciTech Connect

    Getman, Konstantin V.; Feigelson, Eric D.; Kuhn, Michael A.

    2014-06-01

    We analyze age distributions of two nearby rich stellar clusters, the NGC 2024 (Flame Nebula) and Orion Nebula cluster (ONC) in the Orion molecular cloud complex. Our analysis is based on samples from the MYStIX survey and a new estimator of pre-main sequence (PMS) stellar ages, Age{sub JX} , derived from X-ray and near-infrared photometric data. To overcome the problem of uncertain individual ages and large spreads of age distributions for entire clusters, we compute median ages and their confidence intervals of stellar samples within annular subregions of the clusters. We find core-halo age gradients in both the NGC 2024 cluster and ONC: PMS stars in cluster cores appear younger and thus were formed later than PMS stars in cluster peripheries. These findings are further supported by the spatial gradients in the disk fraction and K-band excess frequency. Our age analysis is based on Age{sub JX} estimates for PMS stars and is independent of any consideration of OB stars. The result has important implications for the formation of young stellar clusters. One basic implication is that clusters form slowly and the apparent age spreads in young stellar clusters, which are often controversial, are (at least in part) real. The result further implies that simple models where clusters form inside-out are incorrect and more complex models are needed. We provide several star formation scenarios that alone or in combination may lead to the observed core-halo age gradients.

  8. A study of rotating globular clusters. The case of the old, metal-poor globular cluster NGC 4372

    NASA Astrophysics Data System (ADS)

    Kacharov, N.; Bianchini, P.; Koch, A.; Frank, M. J.; Martin, N. F.; van de Ven, G.; Puzia, T. H.; McDonald, I.; Johnson, C. I.; Zijlstra, A. A.

    2014-07-01

    Context. NGC 4372 is a poorly studied old, very metal-poor globular cluster (GC) located in the inner Milky Way halo. Aims: We present the first in-depth study of the kinematic properties and derive the structural parameters of NGC 4372 based on the fit of a Plummer profile and a rotating, physical model. We explore the link between internal rotation to different cluster properties and together with similar studies of more GCs, we put these in the context of globular cluster formation and evolution. Methods: We present radial velocities for 131 cluster member stars measured from high-resolution FLAMES/GIRAFFE observations. Their membership to the GC is additionally confirmed from precise metallicity estimates. We build a velocity dispersion profile and a systemic rotation curve using this kinematic data set. Additionally, we obtain an elliptical number density profile of NGC 4372 based on optical images using a Markov chain Monte Carlo fitting algorithm. From this, we derive the cluster's half-light radius and ellipticity as rh = 3.44' ± 0.04' and ɛ = 0.08 ± 0.01. Finally, we give a physical interpretation of the observed morphological and kinematic properties of this GC by fitting an axisymmetric, differentially rotating, dynamical model. Results: Our results show that NGC 4372 has an unusually high ratio of rotation amplitude to velocity dispersion (1.2 vs. 4.5 km s-1) for its metallicity. This puts it in line, however, with two other exceptional, very metal-poor GCs: M 15 and NGC 4590. We also find a mild flattening of NGC 4372 in the direction of its rotation. Given its old age, this suggests that the flattening is indeed caused by the systemic rotation rather than tidal interactions with the Galaxy. Additionally, we estimate the dynamical mass of the GC Mdyn = 2.0 ± 0.5 × 105M⊙ based on the dynamical model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3 M⊙/L⊙, representative of an old, purely stellar population. Based on

  9. The large, oxygen-rich halos of star-forming galaxies are a major reservoir of galactic metals.

    PubMed

    Tumlinson, J; Thom, C; Werk, J K; Prochaska, J X; Tripp, T M; Weinberg, D H; Peeples, M S; O'Meara, J M; Oppenheimer, B D; Meiring, J D; Katz, N S; Davé, R; Ford, A B; Sembach, K R

    2011-11-18

    The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.

  10. The large, oxygen-rich halos of star-forming galaxies are a major reservoir of galactic metals.

    PubMed

    Tumlinson, J; Thom, C; Werk, J K; Prochaska, J X; Tripp, T M; Weinberg, D H; Peeples, M S; O'Meara, J M; Oppenheimer, B D; Meiring, J D; Katz, N S; Davé, R; Ford, A B; Sembach, K R

    2011-11-18

    The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution. PMID:22096191

  11. A reinterpretation of the Triangulum-Andromeda stellar clouds: a population of halo stars kicked out of the Galactic disc

    NASA Astrophysics Data System (ADS)

    Price-Whelan, Adrian M.; Johnston, Kathryn V.; Sheffield, Allyson A.; Laporte, Chervin F. P.; Sesar, Branimir

    2015-09-01

    The Triangulum-Andromeda stellar clouds (TriAnd1 and TriAnd2) are a pair of concentric ring- or shell-like overdensities at large R (≈30 kpc) and Z (≈-10 kpc) in the Galactic halo that are thought to have been formed from the accretion and disruption of a satellite galaxy. This paper critically reexamines this formation scenario by comparing the number ratio of RR Lyrae to M giant stars associated with the TriAnd clouds with other structures in the Galaxy. The current data suggest a stellar population for these overdensities (fRR: MG < 0.38 at 95 per cent confidence) quite unlike any of the known satellites of the Milky Way (fRR: MG ≈ 0.5 for the very largest and fRR: MG ≫ 1 for the smaller satellites) and more like the population of stars born in the much deeper potential well inhabited by the Galactic disc (fRR: MG < 0.01). N-body simulations of a Milky Way-like galaxy perturbed by the impact of a dwarf galaxy demonstrate that, in the right circumstances, concentric rings propagating outwards from that Galactic disc can plausibly produce similar overdensities. These results provide dramatic support for the recent proposal by Xu et al. that, rather than stars accreted from other galaxies, the TriAnd clouds could represent stars kicked out from our own disc. If so, these would be the first populations of disc stars to be found in the Galactic halo and a clear signature of the importance of this second formation mechanism for stellar haloes more generally. Moreover, their existence at the very extremities of the disc places strong constraints on the nature of the interaction that formed them.

  12. Production and Recycling of Carbon in the Early Galactic Halo

    NASA Astrophysics Data System (ADS)

    Andersen, Johannes; Thidemann Hansen, Terese; Nordström, Birgitta

    2015-08-01

    Extremely metal-poor (EMP) stars - [Fe/H] below ~ -3 - are fossil records of the conditions in the early halo. High-resolution 8m-class spectroscopy has shown that the detailed abundance pattern of EMP giant stars is surprisingly uniform and essentially Solar (e.g. Bonifacio+ 2012), apart from the usual α-enhancement in the halo. In the simplest picture, iron is a proxy for both overall metallicity and time, so the EMP stars should form before the oldest and most metal-poor Galactic globular clusters, notably at the lowest metallicities ([Fe/H] ≲ -3.5).It is thus striking that 20-40% of the EMP giants are strongly enhanced in carbon - the CEMP stars (Lucatello+ 2006). This is conventionally ascribed to mass transfer from a former AGB binary companion, and from a limited compilation of data, Lucatello+ (2005) concluded that most or all CEMP stars are indeed binaries, similar to the classical Ba and CH stars (e.g. Jorissen+ 1998). However, most of the sample was of the inner-halo CEMP-s variety (C and s-process elements both enhanced), while CEMP-no stars dominate the outer halo (Carollo+ 2014). Our precise radial velocity monitoring for CEMP stars over 8 years shed light on this issue.Our data suggest a normal binary frequency for the CEMP-no stars; i.e. the C was not produced in a binary companion, but in sites at interstellar distances, e.g. ‘faint’ SNe, and imprinted on the natal clouds of the low-mass stars we observe. This has immediate implications for the formation of dust in primitive, high-redshift galaxies (Watson+ 2015) and the origin of C-enhanced DLAs (Cooke+ 2011, 2012). The CEMP-s binary orbits are also revealing, with periods up to several decades and generally low amplitudes and eccentricities, suggesting that EMP AGB stars have very large radii, facilitating extensive mass loss. More work on faint SNe and EMP AGB envelopes is needed!

  13. Rotational signature of the Milky Way stellar halo

    NASA Astrophysics Data System (ADS)

    Fermani, Francesco; Schönrich, Ralph

    2013-07-01

    We measure the rotation of the Milky Way stellar halo on two samples of blue horizontal branch (BHB) field halo stars from the Sloan Digital Sky Survey (SDSS) with four different methods. The two samples comprise 1582 and 2563 stars, respectively, and reach out to ˜50 kpc in galactocentric distance. Two of the methods to measure rotation rely exclusively on line-of-sight (l.o.s.) velocities, namely the popular double power-law model and a direct estimate of the de-projected l.o.s. velocity. The other two techniques use the full 3D motions: the radial velocity based rotation estimator of Schönrich et al. and a simple 3D azimuthal velocity mean. In this context we (a) critique the popular model and (b) assess the reliability of the estimators. All four methods agree on a weakly prograde or non-rotating halo. Further, we observe no duality in the rotation of sub-samples with different metallicities or at different radii. We trace the rotation gradient across metallicity measured by Deason et al. on a similar sample of BHB stars back to the inclusion of regions in the apparent magnitude-surface gravity plane known to be contaminated. In the spectroscopically selected sample of Xue et al., we flag ˜500 hot metal-poor stars for their peculiar kinematics w.r.t. to both their cooler metal-poor counterparts and the metal-rich stars in the same sample. They show a seemingly retrograde behaviour in l.o.s. velocities, which is not confirmed by the 3D estimators. Their anomalous vertical motion hints at either a pipeline problem or a stream-like component rather than a smooth retrograde population.

  14. Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31

    NASA Astrophysics Data System (ADS)

    Cunningham, Emily C.; Deason, Alis J.; Guhathakurta, Puragra; Rockosi, Constance M.; van der Marel, Roeland P.; Toloba, Elisa; Gilbert, Karoline M.; Sohn, Sangmo Tony; Dorman, Claire E.

    2016-03-01

    We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions (PMs). The PMs were measured using long baseline (5-7 years) multi-epoch Hubble Space Telescope/Advanced Camera for Surveys photometry, and the LOS velocities were extracted from deep (5-6 hr integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l, b, LOS) coordinate system are {< {v}l2> }1/2={138}-26+43 km s-1, {< {v}b2> }1/2={88}-17+28 {\\text{km s}}-1, and {< {v}{{LOS}}2> }1/2={91}-14+27 {\\text{km s}}-1. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find β =-{0.3}-0.9+0.4, consistent with isotropy and lower than solar neighborhood β measurements by 2σ (βSN ˜ 0.5-0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to β ={0.1}-1.0+0.4, which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r ˜ 25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.

  15. PRODUCTION OF {sup 9}Be THROUGH THE {alpha}-FUSION REACTION OF METAL-POOR COSMIC RAYS AND STELLAR FLARES

    SciTech Connect

    Kusakabe, Motohiko; Kawasaki, Masahiro E-mail: kawasaki@icrr.u-tokyo.ac.jp

    2013-04-10

    Spectroscopic observations of metal-poor stars have indicated possible {sup 6}Li abundances that are much larger than the primordial abundance predicted in the standard big bang nucleosynthesis model. Possible mechanisms of {sup 6}Li production in metal-poor stars include pregalactic and cosmological cosmic-ray (CR) nucleosynthesis and nucleosynthesis by flare-accelerated nuclides. We study {sup 9}Be production via two-step {alpha}-fusion reactions of CR or flare-accelerated {sup 3,4}He through {sup 6}He and {sup 6,7}Li, in pregalactic structure, intergalactic medium, and stellar surfaces. We solve transfer equations of CR or flare particles and calculate nuclear yields of {sup 6}He, {sup 6,7}Li, and {sup 9}Be taking account of probabilities of processing {sup 6}He and {sup 6,7}Li into {sup 9}Be via fusions with {alpha} particles. Yield ratios, i.e., {sup 9}Be/{sup 6}Li, are then calculated for the CR and flare nucleosynthesis models. We suggest that the future observations of {sup 9}Be in metal-poor stars may find enhanced abundances originating from metal-poor CR or flare activities.

  16. THE SPLASH SURVEY: A SPECTROSCOPIC ANALYSIS OF THE METAL-POOR, LOW-LUMINOSITY M31 dSph SATELLITE ANDROMEDA X ,

    SciTech Connect

    Kalirai, Jason S.; Zucker, Daniel B.; Kniazev, Alexei Y.; MartInez-Delgado, David; Bell, Eric F.; Guhathakurta, Puragra; Grebel, Eva K.; Gilbert, Karoline M. E-mail: raja@ucolick.or E-mail: zucker@science.mq.edu.a E-mail: akniazev@saao.ac.z E-mail: grebel@ari.uni-heidelberg.d

    2009-11-01

    Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal galaxy (dSph) found by Zucker et al. in the Sloan Digital Sky Survey (SDSS; York et al.). In this paper, we present the first spectroscopic study of individual red giant branch stars in And X, as a part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) Survey. Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two spectroscopic masks over the face of the galaxy and measure radial velocities for approx100 stars with a median accuracy of sigma {sub v} approx 3 km s{sup -1}. The velocity histogram for this field confirms three populations of stars along the sight line: foreground Milky Way dwarfs at small negative velocities, M31 halo red giants over a broad range of velocities, and a very cold velocity 'spike' consisting of 22 stars belonging to And X with v {sub rad} = -163.8 +- 1.2 km s{sup -1}. By carefully considering both the random and systematic velocity errors of these stars (e.g., through duplicate star measurements), we derive an intrinsic velocity dispersion of just sigma {sub v} = 3.9 +- 1.2 km s{sup -1} for And X, which for its size, implies a minimum mass-to-light ratio of M/L{sub V} = 37{sup +26} {sub -19} assuming that the mass traces the light. Based on the clean sample of member stars, we measure the median metallicity of And X to be [Fe/H] = -1.93 +- 0.11, with a slight radial metallicity gradient. The dispersion in metallicity is large, sigma([Fe/H]{sub phot}) = 0.48, possibly hinting that the galaxy retained much of its chemical enrichment products. And X has a total integrated luminosity (M{sub V} = -8.1 +- 0.5) that straddles the classical Local Group dSphs and the new SDSS ultra-low luminosity galaxies. The galaxy is among the most metal-poor dSphs known, especially relative to those with M{sub V} < -8, and has the second lowest intrinsic velocity dispersion of the entire sample. Our results suggest that And X

  17. Extremely metal-poor galaxies: The H I content

    NASA Astrophysics Data System (ADS)

    Filho, M. E.; Winkel, B.; Sánchez Almeida, J.; Aguerri, J. A.; Amorín, R.; Ascasibar, Y.; Elmegreen, B. G.; Elmegreen, D. M.; Gomes, J. M.; Humphrey, A.; Lagos, P.; Morales-Luis, A. B.; Muñoz-Tuñón, C.; Papaderos, P.; Vílchez, J. M.

    2013-10-01

    Context. Extremely metal-poor (XMP) galaxies are chemically, and possibly dynamically, primordial objects in the local Universe. Aims: Our objective is to characterize the H i content of the XMP galaxies as a class, using as a reference the list of 140 known local XMPs compiled by Morales-Luis et al. (2011). Methods: We have observed 29 XMPs, which had not been observed before at 21 cm, using the Effelsberg radio telescope. This information was complemented with H i data published in literature for a further 53 XMPs. In addition, optical data from the literature provided morphologies, stellar masses, star-formation rates and metallicities. Results: Effelsberg H i integrated flux densities are between 1 and 15 Jy km s-1, while line widths are between 20 and 120 km s-1. H i integrated flux densities and line widths from literature are in the range 0.1-200 Jy km s-1 and 15-150 km s-1, respectively. Of the 10 new Effelsberg detections, two sources show an asymmetric double-horn profile, while the remaining sources show either asymmetric (seven sources) or symmetric (one source) single-peak 21 cm line profiles. An asymmetry in the H i line profile is systematically accompanied by an asymmetry in the optical morphology. Typically, the g-band stellar mass-to-light ratios are ~0.1, whereas the H i gas mass-to-light ratios may be up to two orders of magnitude larger. Moreover, H i gas-to-stellar mass ratios fall typically between 10 and 20, denoting that XMPs are extremely gas-rich. We find an anti-correlation between the H i gas mass-to-light ratio and the luminosity, whereby fainter XMPs are more gas-rich than brighter XMPs, suggesting that brighter sources have converted a larger fraction of their H i gas into stars. The dynamical masses inferred from the H i line widths imply that the stellar mass does not exceed 5% of the dynamical mass, while the H i mass constitutes between 20 and 60% of the dynamical mass. Furthermore, the dark matter mass fraction spans a wide

  18. THE STELLAR METALLICITY DISTRIBUTION FUNCTION OF THE GALACTIC HALO FROM SDSS PHOTOMETRY

    SciTech Connect

    An, Deokkeun; Beers, Timothy C.; Johnson, Jennifer A.; Pinsonneault, Marc H.; Lee, Young Sun; Bovy, Jo; Ivezic, Zeljko; Carollo, Daniela; Newby, Matthew

    2013-01-20

    We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35 Degree-Sign , and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan and Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan and Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] Almost-Equal-To -1.7 and -2.3, the metal-poor component accounts for {approx}20%-35% of the entire halo population in this distance range.

  19. The Stellar Metallicity Distribution Function of the Galactic Halo from SDSS Photometry

    NASA Astrophysics Data System (ADS)

    An, Deokkeun; Beers, Timothy C.; Johnson, Jennifer A.; Pinsonneault, Marc H.; Lee, Young Sun; Bovy, Jo; Ivezić, Željko; Carollo, Daniela; Newby, Matthew

    2013-01-01

    We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35°, and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan & Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan & Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] ≈ -1.7 and -2.3, the metal-poor component accounts for ~20%-35% of the entire halo population in this distance range.

  20. Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars

    NASA Astrophysics Data System (ADS)

    Bruderer, Claudio; Read, Justin I.; Coles, Jonathan P.; Leier, Dominik; Falco, Emilio E.; Ferreras, Ignacio; Saha, Prasenjit

    2016-02-01

    We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile - under the assumption of a Chabrier stellar initial mass function - shows significant variation from galaxy to galaxy. Those with an outermost image beyond ˜10 kpc are very well fit by a projected Navarro-Frenk-White (NFW) profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than edm = 0.2, their stars can extend to e* > 0.2. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear (γ ≳ 0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.

  1. The helium abundance in the metal-poor globular clusters M30 and NGC 6397

    SciTech Connect

    Mucciarelli, A.; Lovisi, L.; Lanzoni, B.; Ferraro, F. R.

    2014-05-01

    We present the helium abundance of the two metal-poor clusters M30 and NGC 6397. Helium estimates have been obtained by using the high-resolution spectrograph FLAMES at the European Southern Observatory Very Large Telescope and by measuring the He I line at 4471 Å in 24 and 35 horizontal branch (HB) stars in M30 and NGC 6397, respectively. This sample represents the largest data set of He abundances collected so far in metal-poor clusters. The He mass fraction turns out to be Y = 0.252 ± 0.003 (σ = 0.021) for M30 and Y = 0.241 ± 0.004 (σ = 0.023) for NGC 6397. These values are fully compatible with the cosmological abundance, thus suggesting that the HB stars are not strongly enriched in He. The small spread of the Y distributions are compatible with those expected from the observed main sequence splitting. Finally, we find a hint of a weak anticorrelation between Y and [O/Fe] in NGC 6397 in agreement with the prediction that O-poor stars are formed by (He-enriched) gas polluted by the products of hot proton-capture reactions.

  2. The Gaia-ESO Survey: Separating disk chemical substructures with cluster models. Evidence of a separate evolution in the metal-poor thin disk

    NASA Astrophysics Data System (ADS)

    Rojas-Arriagada, A.; Recio-Blanco, A.; de Laverny, P.; Schultheis, M.; Guiglion, G.; Mikolaitis, Š.; Kordopatis, G.; Hill, V.; Gilmore, G.; Randich, S.; Alfaro, E. J.; Bensby, T.; Koposov, S. E.; Costado, M. T.; Franciosini, E.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis, J.; Lind, K.; Magrini, L.; Monaco, L.; Morbidelli, L.; Sacco, G. G.; Worley, C. C.; Zaggia, S.; Chiappini, C.

    2016-02-01

    Context. Recent spectroscopic surveys have begun to explore the Galactic disk system on the basis of large data samples, with spatial distributions sampling regions well outside the solar neighborhood. In this way, they provide valuable information for testing spatial and temporal variations of disk structure kinematics and chemical evolution. Aims: The main purposes of this study are to demonstrate the usefulness of a rigorous mathematical approach to separate substructures of a stellar sample in the abundance-metallicity plane, and provide new evidence with which to characterize the nature of the metal-poor end of the thin disk sequence. Methods: We used a Gaussian mixture model algorithm to separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample (essentially at RGC< 10 kpc) from the Gaia-ESO survey (GES) internal data release 2 (iDR2). We aim at decomposing it into data groups highlighting number density and/or slope variations in the abundance-metallicity plane. An independent sample of disk red clump stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) was used to cross-check the identified features. Results: We find that the sample is separated into five groups associated with major Galactic components; the metal-rich end of the halo, the thick disk, and three subgroups for the thin disk sequence. This is confirmed with the sample of red clump stars from APOGEE. The three thin disk groups served to explore this sequence in more detail. The two metal-intermediate and metal-rich groups of the thin disk decomposition ([Fe/H] > -0.25 dex) highlight a change in the slope at solar metallicity. This holds true at different radial regions of the Milky Way. The distribution of Galactocentric radial distances of the metal-poor part of the thin disk ([Fe/H] < -0.25 dex) is shifted to larger distances than those of the more metal-rich parts. Moreover, the metal-poor part of the thin disk presents indications of a scale height

  3. A reservoir of ionized gas in the galactic halo to sustain star formation in the Milky Way.

    PubMed

    Lehner, Nicolas; Howk, J Christopher

    2011-11-18

    Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of galactic stars. We show that iHVCs with 90 ≤ |v(LSR)| ≲ 170 kilometers per second (where v(LSR) is the velocity in the local standard of rest frame) are within one galactic radius of the Sun and have enough mass to maintain star formation, whereas iHVCs with |v(LSR)| ≳ 170 kilometers per second are at larger distances. These may be the next wave of infalling material.

  4. A reservoir of ionized gas in the galactic halo to sustain star formation in the Milky Way.

    PubMed

    Lehner, Nicolas; Howk, J Christopher

    2011-11-18

    Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of galactic stars. We show that iHVCs with 90 ≤ |v(LSR)| ≲ 170 kilometers per second (where v(LSR) is the velocity in the local standard of rest frame) are within one galactic radius of the Sun and have enough mass to maintain star formation, whereas iHVCs with |v(LSR)| ≳ 170 kilometers per second are at larger distances. These may be the next wave of infalling material. PMID:21868626

  5. The Kennicutt-Schmidt Relation in Extremely Metal-Poor Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Filho, M. E.; Sánchez Almeida, J.; Amorín, R.; Muñoz-Tuñón, C.; Elmegreen, B. G.; Elmegreen, D. M.

    2016-04-01

    The Kennicutt-Schmidt (KS) relation between the gas mass and star formation rate (SFR) describes the star formation regulation in disk galaxies. It is a function of gas metallicity, but the low-metallicity regime of the KS diagram is poorly sampled. We have analyzed data for a representative set of extremely metal-poor galaxies (XMPs), as well as auxiliary data, and compared these to empirical and theoretical predictions. The majority of the XMPs possess high specific SFRs, similar to high-redshift star-forming galaxies. On the KS plot, the XMP H i data occupy the same region as dwarfs and extend the relation for low surface brightness galaxies. Considering the H i gas alone, a considerable fraction of the XMPs already fall off the KS law. Significant quantities of “dark” H2 mass (i.e., not traced by CO) would imply that XMPs possess low star formation efficiencies (SFEgas). Low SFEgas in XMPs may be the result of the metal-poor nature of the H i gas. Alternatively, the H i reservoir may be largely inert, the star formation being dominated by cosmological accretion. Time lags between gas accretion and star formation may also reduce the apparent SFEgas, as may galaxy winds, which can expel most of the gas into the intergalactic medium. Hence, on global scales, XMPs could be H i-dominated, high-specific-SFR (≳10-10 yr-1), low-SFEgas (≲10-9 yr-1) systems, in which the total H i mass is likely not a good predictor of the total H2 mass, nor of the SFR.

  6. The role of primary 16O as a neutron poison in AGB stars and fluorine primary production at halo metallicities.

    NASA Astrophysics Data System (ADS)

    Gallino, R.; Bisterzo, S.; Cristallo, S.; Straniero, O.

    The discovery of a historical bug in the s-post-process AGB code obtained so far by the Torino group forced us to reconsider the role of primary 16O in the 13C-pocket, produced by the 13C(alpha , n)16O reaction, as important neutron poison for the build up of the s-elements at Halo metallicities. The effect is noticeable only for the highest 13C-pocket efficiencies (cases ST*2 and ST). For Galactic disc metallicities, the bug effect is negligible. A comparative analysis of the neutron poison effect of other primary isotopes (12C, 22Ne and its progenies) is presented. The effect of proton captures, by 14N(n, p)14C, boosts a primary production of fluorine in halo AGB stars, with [F/Fe] comparable to [C/Fe], without affecting the s-elements production.

  7. V474 Car: A RARE HALO RS CVn BINARY IN RETROGRADE GALACTIC ORBIT

    SciTech Connect

    Bubar, Eric J.; Mamajek, Eric E.; Jensen, Eric L. N.; Walter, Frederick M.

    2011-04-15

    We report the discovery that the star V474 Car is an extremely active, high velocity halo RS CVn system. The star was originally identified as a possible pre-main-sequence star in Carina, given its enhanced stellar activity, rapid rotation (10.3 days), enhanced Li, and absolute magnitude which places it above the main sequence (MS). However, its extreme radial velocity (264 km s{sup -1}) suggested that this system was unlike any previously known pre-MS system. Our detailed spectroscopic analysis of echelle spectra taken with the CTIO 4 m finds that V474 Car is both a spectroscopic binary with an orbital period similar to the photometric rotation period and metal-poor ([Fe/H] {approx_equal}-0.99). The star's Galactic orbit is extremely eccentric (e {approx_equal} 0.93) with a perigalacticon of only {approx}0.3 kpc of the Galactic center-and the eccentricity and smallness of its perigalacticon are surpassed by only {approx}0.05% of local F/G-type field stars. The observed characteristics are consistent with V474 Car being a high-velocity, metal-poor, tidally locked, chromospherically active binary, i.e., a halo RS CVn binary, and one of only a few such specimens known.

  8. Mining the Sloan digital sky survey in search of extremely α-poor stars in the galaxy

    SciTech Connect

    Xing, Q. F.; Zhao, G. E-mail: gzhao@nao.cas.cn

    2014-07-20

    As we know, the majority of metal-poor Galactic halo stars appear to have chemical abundances that were enhanced by α-elements (e.g., O, Mg, Si, Ca, and Ti) during the early stage of the Galaxy. Observed metal-poor halo stars preserved this pattern by exhibiting abundance ratios [α/Fe] ∼+0.4. A few striking exceptions that show severe departures from the general enhanced α-element chemical abundance trends of the halo have been discovered in recent years. They possess relatively low [α/Fe] compared to other comparable-metallicity stars, with abundance ratios over 0.5 dex lower. These stars may have a different chemical enrichment history from the majority of the halo. Similarly, low-α abundances are also displayed by satellite dwarf spheroidal (dSph) galaxies. We present a method to select extremely α-poor (EAP) stars from the SDSS/SEGUE survey. The method consists of a two-step approach. In the first step, we select suspected metal-poor ([Fe/H] <–0.5) and α-poor ([Mg/Fe] <0) stars as our targets. In the second step, we determine [Mg/Fe] from low-resolution (R = 2000) stellar spectra for our targets and select stars with [Mg/Fe] <–0.1 as candidate EAP stars. In a sample of 40,000 stars with atmospheric parameters in the range of T{sub eff} = [4500, 7000] K, log g = [1.0, 5.0], and [Fe/H] = [–4.0, +0.5], 14 candidate stars were identified. Three of these stars are found to have already been confirmed by other research.

  9. Mining the Sloan Digital Sky Survey in Search of Extremely α-poor Stars in the Galaxy

    NASA Astrophysics Data System (ADS)

    Xing, Q. F.; Zhao, G.

    2014-07-01

    As we know, the majority of metal-poor Galactic halo stars appear to have chemical abundances that were enhanced by α-elements (e.g., O, Mg, Si, Ca, and Ti) during the early stage of the Galaxy. Observed metal-poor halo stars preserved this pattern by exhibiting abundance ratios [α/Fe] ~+0.4. A few striking exceptions that show severe departures from the general enhanced α-element chemical abundance trends of the halo have been discovered in recent years. They possess relatively low [α/Fe] compared to other comparable-metallicity stars, with abundance ratios over 0.5 dex lower. These stars may have a different chemical enrichment history from the majority of the halo. Similarly, low-α abundances are also displayed by satellite dwarf spheroidal (dSph) galaxies. We present a method to select extremely α-poor (EAP) stars from the SDSS/SEGUE survey. The method consists of a two-step approach. In the first step, we select suspected metal-poor ([Fe/H] <-0.5) and α-poor ([Mg/Fe] <0) stars as our targets. In the second step, we determine [Mg/Fe] from low-resolution (R = 2000) stellar spectra for our targets and select stars with [Mg/Fe] <-0.1 as candidate EAP stars. In a sample of 40,000 stars with atmospheric parameters in the range of T eff = [4500, 7000] K, log g = [1.0, 5.0], and [Fe/H] = [-4.0, +0.5], 14 candidate stars were identified. Three of these stars are found to have already been confirmed by other research.

  10. Linking Galaxies to Dark Matter Halos at z ~ 1 : Dependence of Galaxy Clustering on Stellar Mass and Specific Star Formation Rate

    NASA Astrophysics Data System (ADS)

    Kim, Jae-Woo; Im, Myungshin; Lee, Seong-Kook; Edge, Alastair C.; Wake, David A.; Merson, Alexander I.; Jeon, Yiseul

    2015-06-01

    We study the dependence of angular two-point correlation functions on stellar mass (M*) and specific star formation rate (sSFR) of {M}*\\gt {10}10{M}ȯ galaxies at z∼ 1. The data from the UK Infrared Telescope Infrared Deep Sky Survey Deep eXtragalactic Survey and Canada–France–Hawaii Telescope Legacy Survey cover 8.2 deg2 sample scales larger than 100 {h}-1 {Mpc} at z∼ 1, allowing us to investigate the correlation between clustering, M*, and star formation through halo modeling. Based on halo occupation distributions (HODs) of M* threshold samples, we derive HODs for M* binned galaxies, and then calculate the {M}*/{M}{halo} ratio. The ratio for central galaxies shows a peak at {M}{halo}∼ {10}12{h}-1{M}ȯ , and satellites predominantly contribute to the total stellar mass in cluster environments with {M}*/{M}{halo} values of 0.01–0.02. Using star-forming galaxies split by sSFR, we find that main sequence galaxies ({log} {sSFR}/{{yr}}-1∼ -9) are mainly central galaxies in ∼ {10}12.5{h}-1{M}ȯ halos with the lowest clustering amplitude, while lower sSFR galaxies consist of a mixture of both central and satellite galaxies where those with the lowest M* are predominantly satellites influenced by their environment. Considering the lowest {M}{halo} samples in each M* bin, massive central galaxies reside in more massive halos with lower sSFRs than low mass ones, indicating star-forming central galaxies evolve from a low M*–high sSFR to a high M*–low sSFR regime. We also find that the most rapidly star-forming galaxies ({log} {sSFR}/{{yr}}-1\\gt -8.5) are in more massive halos than main sequence ones, possibly implying galaxy mergers in dense environments are driving the active star formation. These results support the conclusion that the majority of star-forming galaxies follow secular evolution through the sustained but decreasing formation of stars.

  11. OBSERVATIONAL PROPERTIES OF THE METAL-POOR THICK DISK OF THE MILKY WAY AND INSIGHTS INTO ITS ORIGINS

    SciTech Connect

    Ruchti, Gregory R.; Fulbright, Jon P.; Wyse, Rosemary F. G.; Gilmore, Gerard F.; Bienayme, Olivier; Siebert, Arnaud; Bland-Hawthorn, Joss; Gibson, Brad K.; Grebel, Eva K.; Helmi, Amina; Munari, Ulisse; Navarro, Julio F.; Parker, Quentin A.; Reid, Warren; Seabroke, George M.; Siviero, Alessandro; Steinmetz, Matthias; Williams, Mary; Watson, Fred G.; Zwitter, Tomaz

    2011-08-10

    We have undertaken the study of the elemental abundances and kinematic properties of a metal-poor sample of candidate thick-disk stars selected from the Radial Velocity Experiment spectroscopic survey of bright stars to differentiate among the present scenarios of the formation of the thick disk. In this paper, we report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 main-sequence/sub-giant branch metal-poor stars, which serves to augment our previous sample of only giant stars. We find that the thick disk [{alpha}/Fe] ratios are enhanced and have little variation (<0.1 dex), in agreement with our previous study. The augmented sample further allows, for the first time, investigation of the gradients in the metal-poor thick disk. For stars with [Fe/H] < -1.2, the thick disk shows very small gradients, <0.03 {+-} 0.02 dex kpc{sup -1}, in {alpha}-enhancement, while we find a +0.01 {+-} 0.04 dex kpc{sup -1} radial gradient and a -0.09 {+-} 0.05 dex kpc{sup -1} vertical gradient in iron abundance. In addition, we show that the peak of the distribution of orbital eccentricities for our sample agrees better with models in which the stars that comprise the thick disk were formed primarily in the Galaxy, with direct accretion of stars contributing little. Our results thus disfavor direct accretion of stars from dwarf galaxies into the thick disk as a major contributor to the thick-disk population, but cannot discriminate between alternative models for the thick disk, such as those that invoke high-redshift (gas-rich) mergers, heating of a pre-existing thin stellar disk by a minor merger, or efficient radial migration of stars.

  12. DARK MATTER CORES IN THE FORNAX AND SCULPTOR DWARF GALAXIES: JOINING HALO ASSEMBLY AND DETAILED STAR FORMATION HISTORIES

    SciTech Connect

    Amorisco, N. C.; Zavala, J.; De Boer, T. J. L.

    2014-02-20

    We combine the detailed star formation histories of the Fornax and Sculptor dwarf spheroidals with the mass assembly history of their dark matter (DM) halo progenitors to estimate if the energy deposited by Type II supernovae (SNe II) is sufficient to create a substantial DM core. Assuming the efficiency of energy injection of the SNe II into DM particles is ε{sub gc} = 0.05, we find that a single early episode, z ≳ z {sub infall}, that combines the energy of all SNe II due to explode over 0.5 Gyr is sufficient to create a core of several hundred parsecs in both Sculptor and Fornax. Therefore, our results suggest that it is energetically plausible to form cores in cold dark matter (CDM) halos via early episodic gas outflows triggered by SNe II. Furthermore, based on CDM merger rates and phase-space density considerations, we argue that the probability of a subsequent complete regeneration of the cusp is small for a substantial fraction of dwarf-size halos.

  13. Extremely faint, diffuse satellite systems in the M31 halo: exceptional star clusters or tiny dwarf galaxies?

    NASA Astrophysics Data System (ADS)

    Mackey, Dougal

    2013-10-01

    Recent years have seen the discovery of a variety of low surface brightness, diffuse stellar systems in the Local Group. Of particular prominence are the ultra-faint dwarf satellites of the Milky Way and the extended globular clusters seen in M31, M33, and NGC 6822. As part of the major Pan-Andromeda Archaeological Survey {PAndAS} we have discovered several very faint and diffuse stellar satellites in the M31 halo. In Cycle 19 we obtained ACS/WFC imaging for one of these, PAndAS-48, which has revealed it to be a puzzling and unusual object. On the size-luminosity plane it falls between the extended clusters and ultra-faint dwarfs; however, its characteristics do not allow us to unambiguously class it as either type of system. If PAndAS-48 is an extended cluster then it is the most elliptical, isolated, metal-poor, and lowest-luminosity example yet uncovered. Conversely, while its properties are generally consistent with those observed for the faint dwarf satellites of the Milky Way, it would be a factor 2-3 smaller in spatial extent than its Galactic counterparts at comparable luminosity. Here we propose deep resolved imaging of the remaining five similar objects in our sample, with the aim of probing this hitherto poorly-explored region of parameter space in greater detail. If we are able to confirm any of these objects as faint dwarfs, they will provide the first insight into the behaviour of this class of object in a galaxy other than the Milky Way.

  14. The r- and s-process contributions to heavy-element abundances in the halo star HD 29907

    NASA Astrophysics Data System (ADS)

    Sitnova, T. M.; Mashonkina, L. I.

    2011-07-01

    The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 ( T eff = 5500 K, log g = 4.64) with [Fe/H] = -1.55 using high-quality VLT/UVES spectra (ESO, Chile). The star has a moderate enhancement of r-process elements (Eu-Tm) with [ r/Fe] = 0.63. In the range from Ba to Yb, the derived abundance pattern agrees well with those for strongly r-process enhanced stars (r-II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0), such as CS 22892-052 and CS 31082-001, as well as with the scaled solar r-process curve and the r-process model HEW. Thus, Ba-Yb in HD 29907 originate in the r-process. Just as other moderately r-process enhanced stars studied in the literature, HD 29907 exhibits higher Sr, Y, and Zr abundances than those for r-II stars. These results confirm the assumption by other authors about the existence of an additional Sr-Zr synthesis mechanism in the early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible for the enhancement of Mo-Ag in the star being investigated compared to r-II stars. There are no grounds to suggest the presence of s-nuclei of lead in the material of the star being investigated, because its measured abundance ratio log ɛ(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log ɛ(Pb/Eu) = 0.17 (CS 31082-001) to < 1.55 (HE 1219-0312). Thus, even if there was a contribution of AGB stars to the heavy-element enrichment of the interstellar medium at the epoch with [Fe/H] = -1.55, it was small, at the level of the abundance error.

  15. Kinematics of the Stellar Halo and the Mass Distribution of the Milky Way Using Blue Horizontal Branch Stars

    NASA Astrophysics Data System (ADS)

    Kafle, Prajwal R.; Sharma, Sanjib; Lewis, Geraint F.; Bland-Hawthorn, Joss

    2012-12-01

    Here, we present a kinematic study of the Galactic halo out to a radius of ~60 kpc, using 4664 blue horizontal branch stars selected from the SDSS/SEGUE survey to determine key dynamical properties. Using a maximum likelihood analysis, we determine the velocity dispersion profiles in spherical coordinates (σ r , σθ, σphi) and the anisotropy profile (β). The radial velocity dispersion profile (σ r ) is measured out to a galactocentric radius of r ~ 60 kpc, but due to the lack of proper-motion information, σθ, σphi, and β could only be derived directly out to r ~ 25 kpc. From a starting value of β ≈ 0.5 in the inner parts (9 < r/kpc < 12), the profile falls sharply in the range r ≈ 13-18 kpc, with a minimum value of β = -1.2 at r = 17 kpc, rising sharply at larger radius. In the outer parts, in the range 25 < r/kpc < 56, we predict the profile to be roughly constant with a value of β ≈ 0.5. The newly discovered kinematic anomalies are shown not to arise from halo substructures. We also studied the anisotropy profile of simulated stellar halos formed purely by accretion and found that they cannot reproduce the sharp dip seen in the data. From the Jeans equation, we compute the stellar rotation curve (v circ) of the Galaxy out to r ~ 25 kpc. The mass of the Galaxy within r <~ 25 kpc is determined to be 2.1 × 1011 M ⊙, and with a three-component fit to v circ(r), we determine the virial mass of the Milky Way dark matter halo to be M vir = 0.9+0.4 -0.3 × 1012 M ⊙ (R vir = 249+34 -31 kpc).

  16. Very metal-poor galaxies and the primordial helium abundance.

    NASA Astrophysics Data System (ADS)

    Terlevich, E.; Skillman, E.; Terlevich, R.

    Critical to the understanding of several fundamental problems in astronomy (among which the determination of the primordial helium is of foremost importance), extremely metal-poor galaxies have been almost impossible to find. In the past few years the authors have been successful in discovering them. They are embarked on a programme for obtaining with linear detectors, very high S/N spectra of these objects, in order to derive He abundances to better than the 5% per object needed to constrain the Big Bang model of the origin of the universe. The authors discuss some results and problems encountered in this quest.

  17. Ultraviolet photometry with the Astronomical Netherlands Satellite /ANS/ - Faint blue stars in the halo

    NASA Technical Reports Server (NTRS)

    De Boer, K. S.; Wesselius, P. R.

    1980-01-01

    Blue stars at high galactic latitudes have been observed with the UV telescope on board ANS. In this paper a subset of the collected data pertaining to the cooler stars is discussed. Most of them have energy distributions in general agreement with the visual spectral type. One star is exceptionally blue, and of seven possible horizontal-branch stars, two have UV energy distributions distinct from main-sequence stars in the sense that they have an excess at 1550 A and a large Balmer jump.

  18. SEGUE-2 LIMITS ON METAL-RICH OLD-POPULATION HYPERVELOCITY STARS IN THE GALACTIC HALO

    SciTech Connect

    Kollmeier, Juna A.; Gould, Andrew; Johnson, Jennifer A.; Rockosi, Constance; Beers, Timothy C.; Lee, Young Sun; Knapp, Gillian; Morrison, Heather; Harding, Paul; Weaver, Benjamin A.

    2010-11-01

    We present new limits on the ejection of metal-rich old-population hypervelocity stars (HVSs) from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on the combination of the GC mass function and the ejection mechanism for HVSs. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B stars compared to low-mass F/G stars.

  19. The extended stellar substructures of four metal-poor globular clusters in the Galactic bulge

    NASA Astrophysics Data System (ADS)

    Chun, Sang-Hyun; Sohn, Young-Jong

    2016-08-01

    We investigated the stellar density substructures around four metal-poor globular clusters (NGC 6266, NGC 6626, NGC 6642, and NGC 6723) in the Galactic bulge. Wide-field near-infrared (JHK s ) imaging data were obtained from WFCAM of UKIRT telescope. Field stars contamination around the globular clusters was reduced by using a statistical weighted filtering algorithm. Tidal stripping stellar substructures in the form of tidal tail (NGC 6266 and NGC 6626) or small density lobes/chunk (NGC 6642 and NGC 6723) were found around the four globular clusters in the two-dimensional density contour maps. We also find the overdensity features, which deviate from the theoretical models, in the outer region of radial density profiles. The observed results imply that the four globular clusters have experienced a strong tidal force or the bulge/disk shock effect of the Galaxy.

  20. BVRI CCD photometry of the metal-poor globular cluster NGC 4372

    SciTech Connect

    Alcaino, G.; Liller, W.; Alvarado, F.; Wenderoth, E. )

    1991-07-01

    BVRI CCD photometry is presented in two overlapping fields in the metal-poor globular cluster NGC 4372. The observations extend approximately 2 mag below the main-sequence turnoff to V about 21. By comparing the color-magnitude diagram (CMD) with those of clusters with similar metallicities, it is found that E(B-V) = 0.50 {plus minus} 0.03, and (m-M)v = 14.75 {plus minus} 0.06. Comparison with theoretical isochrones leads to a value E(B-V) = 0.53 {plus minus} 0.03. Comparison of the CMD with that of bright stars published by other authors yields a value for Delta V(TO-HB) = 3.3 {plus minus} 0.3. The weighted mean value of the age of the cluster, derived from the four colors, is 15 {plus minus} 4 Gyr (estimated external uncertainty). 17 refs.

  1. KECK ECHELLETTE SPECTROGRAPH AND IMAGER OBSERVATIONS OF METAL-POOR DAMPED Ly{alpha} SYSTEMS

    SciTech Connect

    Penprase, Bryan E.; Toro-Martinez, Irene; Beeler, Daniel J.; Prochaska, J. Xavier

    2010-09-20

    We present the first results from a survey of SDSS quasars selected for strong H I damped Ly{alpha} (DLA) absorption with corresponding low equivalent width absorption from strong low-ion transitions (e.g., C II {lambda}1334 and Si II {lambda}1260). These metal-poor DLA candidates were selected from the SDSS fifth release quasar spectroscopic database, and comprise a large new sample for probing low-metallicity galaxies. Medium-resolution echellette spectra from the Keck Echellette Spectrograph and Imager spectrograph for an initial sample of 35 systems were obtained to explore the metal-poor tail of the DLA distribution and to investigate the nucleosynthetic patterns at these metallicities. We have estimated saturation corrections for the moderately underresolved spectra, and systems with very narrow Doppler parameters (b {<=} 5 km s{sup -1}) will likely have underestimated abundances. For those systems with Doppler parameters b > 5 km s{sup -1}, we have measured low-metallicity DLA gas with [X/H] <-2.4 for at least one of C, O, Si, or Fe. Assuming non-saturated components, we estimate that several DLA systems have [X/H] <-2.8, including five DLA systems with both low equivalent widths and low metallicity in transitions of both C II and O I. All of the measured DLA metallicities, however, exceed or are consistent with a metallicity of at least 1/1000 of solar, regardless of the effects of saturation in our spectra. Our results indicate that the metal-poor tail of galaxies at z {approx} 3 drops exponentially at [X/H] {approx}<-3. If the distribution of metallicity is Gaussian, the probability of identifying interstellar medium gas with lower abundance is extremely small, and our results suggest that DLA systems with [X/H] < -4.0 are extremely rare, and could comprise only 8 x 10{sup -7} of DLA systems. The relative abundances of species within these low-metallicity DLA systems are compared with stellar nucleosynthesis models, and are consistent with stars having

  2. A HIGH-VELOCITY BULGE RR LYRAE VARIABLE ON A HALO-LIKE ORBIT

    SciTech Connect

    Kunder, Andrea; Storm, J.; Rich, R. M.; Hawkins, K.; Poleski, R.; Johnson, C. I.; Shen, J.; Li, Z.-Y.; Cordero, M. J.; Nataf, D. M.; Bono, G.; Walker, A. R.; Koch, A.; De Propris, R.; Udalski, A.; Szymanski, M. K.; Soszynski, I.; Pietrzynski, G.; Ulaczyk, K.; Wyrzykowski, Ł.; and others

    2015-07-20

    We report on the RR Lyrae variable star, MACHO 176.18833.411, located toward the Galactic bulge and observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay, which has the unusual radial velocity of −372 ± 8 km s{sup −1} and true space velocity of −482 ± 22 km s{sup −1} relative to the Galactic rest frame. Located less than 1 kpc from the Galactic center and toward a field at (l, b) = (3, −2.5), this pulsating star has properties suggesting it belongs to the bulge RR Lyrae star population, yet a velocity indicating it is abnormal, at least with respect to bulge giants and red clump stars. We show that this star is most likely a halo interloper and therefore suggest that halo contamination is not insignificant when studying metal-poor stars found within the bulge area, even for stars within 1 kpc of the Galactic center. We discuss the possibility that MACHO 176.18833.411 is on the extreme edge of the bulge RR Lyrae radial velocity distribution, and also consider a more exotic scenario in which it is a runaway star moving through the Galaxy.

  3. On the formation of dwarf galaxies and stellar haloes

    NASA Astrophysics Data System (ADS)

    Read, J. I.; Pontzen, A. P.; Viel, M.

    2006-09-01

    Using analytic arguments and a suite of very high resolution (~103Msolar per particle) cosmological hydrodynamical simulations, we argue that high-redshift, z ~ 10, M ~ 108Msolar haloes, form the smallest `baryonic building block' (BBB) for galaxy formation. These haloes are just massive enough to efficiently form stars through atomic line cooling and to hold on to their gas in the presence of supernova (SN) winds and reionization. These combined effects, in particular that of the SN feedback, create a sharp transition: over the mass range 3-10 × 107Msolar, the BBBs drop two orders of magnitude in stellar mass. Below ~2 × 107Msolar, galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB. We show that the BBBs have stellar distributions which are spheroidal, of low rotational velocity, old and metal poor: they resemble the dwarf spheroidal galaxies (dSphs) of the Local Group (LG). Unlike the LG dSphs, however, they contain significant gas fractions. We connect these high-redshift BBBs to the smallest dwarf galaxies observed at z = 0 using linear theory. A small fraction (~100) of these gas-rich BBBs at high redshift fall in to a galaxy the size of the Milky Way (MW). We suggest that 10 per cent of these survive to become the observed LG dwarf galaxies at the present epoch. This is consistent with recent numerical estimates. Those infalling haloes on benign orbits which keep them far away from the MW or Andromeda manage to retain their gas and slowly form stars - these become the smallest dwarf irregular galaxies; those on more severe orbits lose their gas faster than they can form stars and become the dwarf spheroidals. The remaining 90 per cent of the BBBs will be accreted. We show that this gives a metallicity and total stellar mass consistent with the MW old stellar halo.

  4. SMC west halo: a slice of the galaxy that is being tidally stripped?. Star clusters trace age and metallicity gradients

    NASA Astrophysics Data System (ADS)

    Dias, B.; Kerber, L.; Barbuy, B.; Bica, E.; Ortolani, S.

    2016-06-01

    Context. The evolution and structure of the Magellanic Clouds is currently under debate. The classical scenario in which both the Large and Small Magellanic Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an alternative in which the LMC and SMC are in their first close passage to our Galaxy. The clouds are close enough to us to allow spatially resolved observation of their stars, and detailed studies of stellar populations in the galaxies are expected to be able to constrain the proposed scenarios. In particular, the west halo (WH) of the SMC was recently characterized with radial trends in age and metallicity that indicate tidal disruption. Aims: We intend to increase the sample of star clusters in the west halo of the SMC with homogeneous age, metallicity, and distance derivations to allow a better determination of age and metallicity gradients in this region. Positions are compared with the orbital plane of the SMC from models. Methods: Comparisons of observed and synthetic V(B-V) colour-magnitude diagrams were used to derive age, metallicity, distance, and reddening for star clusters in the SMC west halo. Observations were carried out using the 4.1 m SOAR telescope. Photometric completeness was determined through artificial star tests, and the members were selected by statistical comparison with a control field. Results: We derived an age of 1.23 ± 0.07 Gyr and [Fe/H] = -0.87 ± 0.07 for the reference cluster NGC 152, compatible with literature parameters. Age and metallicity gradients are confirmed in the WH: 2.6 ± 0.6 Gyr/° and -0.19 ± 0.09 dex/°, respectively. The age-metallicity relation for the WH has a low dispersion in metallicity and is compatible with a burst model of chemical enrichment. All WH clusters seem to follow the same stellar distribution predicted by dynamical models, with the exception of AM-3, which should belong to the counter-bridge. Brück 6 is the youngest cluster in our sample. It is only 130 ± 40 Myr old and

  5. WFPC2 Imaging of Dust Structures and Star Formation in the Disk-Halo Interface of Spiral Galaxies

    NASA Astrophysics Data System (ADS)

    Savage, Blair

    1999-07-01

    WFPC2 images of five edge-on spirals to study star formation and dusty interstellar clouds in the disk-halo interface of these galaxies. Ground-based and HST images of the nearby {9 Mpc} edge-on spiral NGC 891 show an unexpected web of hundreds of dust structures at heights 0.4 <= z <= 1.7 kpc {Howk & Savage 1997}. With masses >10^5-10^6 M{sun}, the more prominent extraplanar dust complexes may be sites of star formation at high-z, and there is evidence for H II regions associated with unresolved continuum sources far above the plane of NGC 891. We have established that such high-z dust features and H II regions are not unique to NGC 891. We propose to image five edge-on spiral galaxies {D 17 - 70 Mpc} with the WFPC2. The proposed BVI images will be used to identify sites of on- going star formation in the thick disks of these galaxies, all of which show evidence for high-z dust complexes, and with ground-based H Alpha images will be used to study the stellar content o f any such regions. The resolution and point-source sensitivity of the WFPC2 are crucial for studying these star-forming regions. We will also use these images to study interstellar matter in the thick disks of these galaxies with unprecedented detail and derive the fundamental properties of high-z dusty clouds-including sizes, extinctions, column densities, masses, and gravitational potential energies.

  6. HALO7D: Disentangling the Milky Way Accretion History with Observations in 7 Dimensions

    NASA Astrophysics Data System (ADS)

    Cunningham, Emily C.; Deason, Alis; Guhathakurta, Puragra; Rockosi, Constance M.; Van Der Marel, Roeland P.; Sohn, S. Tony; HSTPROMO, HALO7D

    2016-01-01

    The Milky Way (MW) is shrouded in a faint metal-poor stellar halo. Its structure and kinematics provide a unique archaeological record of the MW's formation, past evolution, and accretion history. These data also help us constrain the dark matter mass out to large radii (50 to 100 kpc). However, studies of the MW stellar halo are hindered by observational constraints. Beyond D~10 kpc, our knowledge of the MW halo is limited to line of sight velocities and rare tracer populations (blue horizontal branch and red giant branch stars). We aim to address these limitations using highly accurate HST-measured proper motions and very deep (8-24 hour integrations) Keck DEIMOS spectroscopy of MW main sequence turn-off stars in the CANDELS fields. By combining these two datasets, we can obtain 6D phase-space information plus chemical abundances (7 "Dimensions") for our halo stars. This survey, which will be unique even in the era of Gaia, will vastly improve our understanding of the Milky Way structure, evolution and mass in a way that neither the HST proper motions nor Keck spectroscopy can do on their own.

  7. The Outer Galactic Halo As Probed By RR Lyr Stars From the Palomar Transient Facility + Keck

    NASA Astrophysics Data System (ADS)

    Cohen, Judith; Sesar, Branimir; Banholzer, Sophianna

    2016-08-01

    We present initial results from our study of the outer halo of the Milky Way using a large sample of RR Lyr(ab) variables datamined from the archives of the Palomar Transient Facility. Of the 464 RR Lyr in our sample with distances exceeding 50 kpc, 62 have been observed spectroscopically at the Keck Observatory. vr and σ(vr ) are given as a function of distance between 50 and 110 kpc, and a very preliminary rather low total mass for the Milky Way out to 110 kpc of ~7+/-1.5×1011 M ⊙ is derived from our data.

  8. The rotation of the halo of NGC 6822 from the radial velocities of carbon stars

    NASA Astrophysics Data System (ADS)

    Thompson, Graham P.; Ryan, Sean G.; Sibbons, Lisette F.

    2016-11-01

    Using spectra taken with the AAOmega spectrograph, we measure the radial velocities of over 100 stars, many of which are intermediate age carbon stars, in the direction of the dwarf irregular galaxy NGC 6822. Kinematic analysis suggests that the carbon stars in the sample are associated with NGC 6822, and estimates of its radial velocity and galactic rotation are made from a star-by-star analysis of its carbon star population. We calculate a heliocentric radial velocity for NGC 6822 of -51 ± 3 km s-1 and show that the population rotates with a mean rotation speed of 11.2 ± 2.1 km s-1 at a mean distance of 1.1 kpc from the galactic centre, about a rotation axis with a position angle of 26° ± 13°, as projected on the sky. This is close to the rotation axis of the H I gas disc and suggests that NGC 6822 is not a polar ring galaxy, but is dynamically closer to a late-type galaxy. However, the rotation axis is not aligned with the minor axis of the AGB isodensity profiles and this remains a mystery.

  9. The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars.

    PubMed

    Dauphas, Nicolas

    2005-06-30

    Some heavy elements (with atomic number A > 69) are produced by the 'rapid' (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from approximately 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way, to determine the U/Th production ratio very precisely (0.57(+0.037)(-0.031). This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations, to help determine the source of Galactic cosmic rays, and to date circumstellar grains. I also estimate the age of the Milky Way (14.5(+2.8)(-2.2)Gyr in a way that is independent of the uncertainties associated with fluctuations in the microwave background or models of stellar evolution.

  10. The Evolving Mixture of Barium Isotopes in Milky Way Halo Stars

    NASA Astrophysics Data System (ADS)

    Choudhury, Zareen; Kirby, E. N.; Guhathakurta, P.

    2014-01-01

    Heavy metals in stars form through one of two types of neutron capture processes: the rapid r-process or slower s-process. The fraction of odd and even barium isotopes in stars can indicate which process predominantly contributed to a star’s heavy metals, since odd barium isotopes predominantly form through the r-process and even barium isotopes through the s-process. The “stellar model” predicts that older stars contain comparable amounts of odd and even barium isotopes, while the “classical model” states that they almost exclusively contain odd isotopes. This study investigated these competing models by analyzing high-resolution spectra of twelve Milky Way stars. These spectra were analyzed for the first time in this study. To quantify r- and s-process enrichment, we measured the odd barium isotope fraction in the stars by fitting models to the stars’ spectra. Generating models involved measuring the stars’ Doppler shift, resolution, and barium abundance. To reduce error margins we optimized resolution and barium abundance measurements by enhancing existing techniques through several rounds of revisions. Our results support the stellar model of heavy metal enrichment, and our proposed optimizations will enable future researchers to obtain a deeper understanding of chemical enrichment in the Universe. This research was supported by the Science Internship Program at the University of California Santa Cruz, Lick Observatory, and the National Science Foundation.

  11. The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties

    SciTech Connect

    Ibata, Rodrigo A.; Martin, Nicolas F.; Lewis, Geraint F.; McConnachie, Alan W.; Irwin, Michael J.; Ferguson, Annette M. N.; Bernard, Edouard J.; Peñarrubia, Jorge; Babul, Arif; Navarro, Julio; Chapman, Scott C.; Collins, Michelle; Fardal, Mark; Mackey, A. D.; Rich, R. Michael; Tanvir, Nial; Widrow, Lawrence

    2014-01-10

    We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H]<−1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f {sub stream} = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f {sub stream} becoming as high as 86% for [Fe/H]>−0.6. The space density of the smooth metal-poor component has a global power-law slope of γ = –3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ∼300 kpc. The total stellar mass in the halo at distances beyond 2° is ∼1.1 × 10{sup 10} M {sub ☉}, while that of the smooth component is ∼3 × 10{sup 9} M {sub ☉}. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ∼8 × 10{sup 9} M {sub ☉}. We detect a substantial metallicity gradient, which declines from ([Fe/H]) = –0.7 at R = 30 kpc to ([Fe/H]) = –1.5 at R = 150 kpc for the full sample, with the smooth halo being ∼0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as

  12. The Large-scale Structure of the Halo of the Andromeda Galaxy. I. Global Stellar Density, Morphology and Metallicity Properties

    NASA Astrophysics Data System (ADS)

    Ibata, Rodrigo A.; Lewis, Geraint F.; McConnachie, Alan W.; Martin, Nicolas F.; Irwin, Michael J.; Ferguson, Annette M. N.; Babul, Arif; Bernard, Edouard J.; Chapman, Scott C.; Collins, Michelle; Fardal, Mark; Mackey, A. D.; Navarro, Julio; Peñarrubia, Jorge; Rich, R. Michael; Tanvir, Nial; Widrow, Lawrence

    2014-01-01

    We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([{{Fe/H}] \\lt -1.7}) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for [{Fe/H] \\gt -0.6}. The space density of the smooth metal-poor component has a global power-law slope of γ = -3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ~300 kpc. The total stellar mass in the halo at distances beyond 2° is ~1.1 × 1010 M ⊙, while that of the smooth component is ~3 × 109 M ⊙. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ~8 × 109 M ⊙. We detect a substantial metallicity gradient, which declines from lang[Fe/H]rang = -0.7 at R = 30 kpc to lang[Fe/H]rang = -1.5 at R = 150 kpc for the full sample, with the smooth halo being ~0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that

  13. Gas phase abundances and conditions along the sight line to the low-halo, inner galaxy star HD 167756

    NASA Technical Reports Server (NTRS)

    Cardelli, Jason A.; Sembach, Kenneth R.; Savage, Blair D.

    1995-01-01

    We present high-resolution (3.5 km/s) Goddard High Resolution Spectrograph (GHRS) measurements of the Mg II, Si II, Cr II, Fe II, and Zn II lines toward HD 167756, a low-latitude halo star at a distance of 4 kpc in the direction l = 351.5 deg, b = -12.3 and at a Galactic altitude of z = -0.85 kpc. Supplemental Na I, Ca II, and H I data are also presented for comparison with the UV lines. Our analysis centers on converting the observed absoprtion-line data into measures of the apparent column density per unit velocity. N(sub a)(v), over the velocity range -25 less than or = v(sub lsr) less than 30 km/s for each species observed. We use these N(sub a)(v) profiles to construct logarithmic abundance ratios of Mg II, Si II, Cr II, Fe II, and Ca II relative to Zn II, normalized to solar abundances, as a function of velocity. Compared to Zn, these species show an underabundance relative to their solar values, with the largest underabundances occurring in the v(sub lsr) approximately equals 5 km/s component(s), for which we find logarithmic abundances A(sub Si/Zn) greater than -0.38, A(Mg/Zn) = -0.82, A(sub Cr/Zn) = -1.18, and A(sub Fe/Zn) greater than 1.40 dex. We show that ionization effects, abundance gradients, or intrinsic abundance variability cannot be significant sources for the underabundances observed. The most likely explanation is gas phase depletion of elements onto dust grains. Comparisons with the gas phase abundances along other diffuse, warm gas sight lines, like the halo sight line to HD 93521, support this interpretation as do the derived physical properties of the sight line.

  14. Triangulum II: A Very Metal-poor and Dynamically Hot Stellar System

    NASA Astrophysics Data System (ADS)

    Martin, Nicolas F.; Ibata, Rodrigo A.; Collins, Michelle L. M.; Rich, R. Michael; Bell, Eric F.; Ferguson, Annette M. N.; Laevens, Benjamin P. M.; Rix, Hans-Walter; Chapman, Scott C.; Koch, Andreas

    2016-02-01

    We present a study of the recently discovered compact stellar system Triangulum II. From observations conducted with the DEIMOS spectrograph on Keck II, we obtained spectra for 13 member stars that follow the CMD features of this very faint stellar system and include two bright red giant branch stars. Tri II has a very negative radial velocity (< {v}{{r}}> =-{383.7}-3.3+3.0 {km} {{{s}}}-1) that translates to < {v}{{r},{gsr}}> ≃ -264 {km} {{{s}}}-1 and confirms it is a Milky Way satellite. We show that, despite the small data set, there is evidence that Tri II has complex internal kinematics. Its radial velocity dispersion increases from {4.4}-2.0+2.8 {km} {{{s}}}-1 in the central 2\\prime to {14.1}-4.2+5.8 {km} {{{s}}}-1 outwards. The velocity dispersion of the full sample is inferred to be {σ }{vr}={9.9}-2.2+3.2 {km} {{{s}}}-1. From the two bright RGB member stars we measure an average metallicity < {{[Fe/H]}}> =-2.6+/- 0.2, placing Tri II among the most metal-poor Milky Way dwarf galaxies. In addition, the spectra of the fainter member stars exhibit differences in their line widths that could be the indication of a metallicity dispersion in the system. All these properties paint a complex picture for Tri II, whose nature and current state are largely speculative. The inferred metallicity properties of the system however lead us to favor a scenario in which Tri II is a dwarf galaxy that is either disrupting or embedded in a stellar stream.

  15. Searching for chemical relics of first stars with LAMOST and Subaru

    NASA Astrophysics Data System (ADS)

    Li, Haining; Aoki, Wako; Zhao, Gang; Honda, Satoshi; Christlieb, Norbert; Suda, Takuma

    2016-08-01

    We report progresses of a joint project on searching for extremely metal-poor (EMP) stars based on LAMOST survey and Subaru follow-up observation. Follow-up high-resolution snapshot spectra have been obtained for 70 objects, resulting in 42 EMP stars. A number of chemically interesting objects have already been identified, including (1) Two UMP (ultra metal-poor) stars with [Fe/H] ~ -4.0. One of them is the second UMP turnoff star with Li detection. (2) A super Li-rich (A(Li) ~ 3.1) EMP giant. This is the most metal-poor and extreme example of Li enhancement in giants known to date, and will shed light on Li production during the evolution of red giants. (3) A few EMP stars showing extreme overabundance in heavy elements. Detailed abundances of these extreme objects and statistics obtained by the large sample of EMP stars will provide important constraints on the Galactic halo formation.

  16. HALO7D: Investigating the Structure and Accretion History of the Milky Way Stellar Halo with HST Proper Motions and Keck Spectra

    NASA Astrophysics Data System (ADS)

    Cunningham, Emily Clifford; Deason, Alis; Guhathakurta, Puragra; Rockosi, Constance; Kirby, Evan; van der marel, roeland p.; Sohn, Sangmo Tony

    2015-08-01

    The Milky Way (MW) is shrouded in a faint metal-poor stellar halo. Its structure and kinematics provide a unique archaeological record of the MW's formation, past evolution, and accretion history. These data also help us constrain the dark matter mass out to large radii (50 to 100 kpc). However, studies of the MW stellar halo are hindered by observational constraints. Beyond D~10 kpc, our knowledge of the MWhalo is limited to line of sight velocities and rare tracer populations (blue horizontal branch and red giant branch stars). We aim to address these limitations using highly accurate HST-measured proper motions and very deep (8-24 hour integrations) Keck DEIMOS spectroscopy of MW main sequence turn-off stars in the CANDELS fields. By combining these two datasets, we can obtain 6D phase-space information plus chemical abundances for our halo stars. This survey, which will be unique even in the era of Gaia, will vastly improve our understanding of the Milky Way structure, evolution and mass in a way that neither the HST proper motions nor Keck spectroscopy can do on their own.

  17. The extreme chemistry of multiple stellar populations in the metal-poor globular cluster NGC 4833

    NASA Astrophysics Data System (ADS)

    Carretta, E.; Bragaglia, A.; Gratton, R. G.; D'Orazi, V.; Lucatello, S.; Momany, Y.; Sollima, A.; Bellazzini, M.; Catanzaro, G.; Leone, F.

    2014-04-01

    Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, and Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is [Fe/H] = -2.015 ± 0.004 ± 0.084 dex (rms = 0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme [O/Na] ratios. More striking is the finding that large star-to-star variations are also seen for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. These extreme changes are also seen in giants of the much more massive GCs M 54 and ω Cen, and our conclusion is that NGC 4833 has probably lost a conspicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit. Based on observations collected at ESO telescopes under programmes 083.D-0208 and 68.D-0265.Full Tables 2, 6-11 are only available at the CDS via anonymous ftp to http

  18. THE ACS NEARBY GALAXY SURVEY TREASURY. IX. CONSTRAINING ASYMPTOTIC GIANT BRANCH EVOLUTION WITH OLD METAL-POOR GALAXIES

    SciTech Connect

    Girardi, Leo; Williams, Benjamin F.; Gilbert, Karoline M.; Rosenfield, Philip; Dalcanton, Julianne J.; Marigo, Paola; Boyer, Martha L.; Dolphin, Andrew; Weisz, Daniel R.; Skillman, Evan; Melbourne, Jason; Olsen, Knut A. G.; Seth, Anil C.

    2010-12-01

    In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metal-poor stars (with M < 1.5 M{sub sun}, [Fe/H]{approx}< -1.0). Indeed, those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses between 0.51 and 0.55 M{sub sun}. This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.

  19. Lithium abundance in the metal-poor open cluster NGC 2243

    NASA Astrophysics Data System (ADS)

    François, P.; Pasquini, L.; Biazzo, K.; Bonifacio, P.; Palsa, R.

    2013-04-01

    Context. Lithium is a fundamental element for studying the mixing mechanisms acting in the stellar interiors, for understanding the chemical evolution of the Galaxy and the Big Bang nucleosynthesis. The study of Li in stars of open clusters (OC) allows a detailed comparison with stellar evolutionary models and permits us to trace its galactic evolution. The OC NGC 2243 is particularly interesting because of its low metallicity ([Fe/H] = -0.54 ± 0.10 dex). Aims: We measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243. The first aim is to determine whether the Li dip extends to such low metallicities, the second is to compare the results of our Li analysis in this OC with those present in 47 Tuc, a globular cluster of similar metallicity. Methods: We performed a detailed analysis of high-resolution spectra obtained with the multi-object facility FLAMES at the ESO VLT 8.2 m telescope. Lithium abundance was derived through line equivalent widths and the OSMARCS atmosphere models. Iron abundances from Fe i and Fe ii lines have also been measured and used to check the atmospheric model parameters. Results: The Li line is detected in 27 stars confirmed as likely cluster members by repeated radial velocity measurements. We determine a Li dip center of 1.06 M⊙, which is much smaller than that observed in solar metallicity and metal-rich clusters. This finding confirms and strengthens the conclusion that the mass of the stars in the Li dip strongly depends on stellar metallicity. The mean Li abundance of the cluster is log n(Li) = 2.70 dex, which is substantially higher than that observed in 47 Tuc. We estimated an iron abundance of [Fe/H] = -0.54 ± 0.10 dex for NGC 2243, which is similar (within the errors) to previous findings. The [α/Fe] content ranges from 0.00 ± 0.14 for Ca to 0.20 ± 0.22 for Ti, which is low when compared to thick disk stars and to Pop II stars, but compatible with thin disk objects. We found a mean radial velocity of 61

  20. Simulating extremely metal-poor gas and DLA metal content at redshift z ≃ 7

    NASA Astrophysics Data System (ADS)

    Maio, Umberto; Ciardi, Benedetta; Müller, Volker

    2013-10-01

    We present the first theoretical study of metals in damped-Lyα (DLA) systems at redshift z ≃ 7. The features of cold, primordial gas are studied by means of N-body, hydro, chemistry simulations, including atomic and molecular non-equilibrium chemistry, cooling, star formation for Population III and Population II-I regimes, stellar evolution, cosmic metal spreading according to proper yields (for He, C, O, Si, Fe, Mg, S, etc.) and lifetimes and feedback effects. Theoretical expectations are then compared to recently available constraints from DLA observations. We find that DLA galaxies at z ≃ 7 account for ˜10 per cent of the whole galaxy population and for most of the metal-poor galaxies at these epochs. About 7 per cent of these DLA galaxies contain purely pristine material and ˜34 per cent of them consist of very weakly polluted gas, being, therefore, suitable candidates as Population III sites. The remaining ˜59 per cent are enriched above ˜10-4 Z⊙. Additionally, DLA candidates appear to have: gas masses ≲ 2 × 108 M⊙; very low star formation rate, ˜ 10- 3 - 10- 2 M⊙ yr- 1 (significantly weaker than late-time counterparts); mean molecular fractions covering a fairly wide range, xmol ˜ 10- 3-10- 6; typical metallicities Z ≲ 10-3 Z⊙ and H I column densities N_{H I}≳ 3× 10^{20} cm^{-2} (in agreement with recent observations). They present no or weak correlations between their gas mass and Z, N_{H I}, or xmol; a moderate correlation between xmol and Z, linked to the ongoing molecular-driven star formation and metal pollution processes; a mild anticorrelation between N_{H I} and xmol, due to H depletion into molecules; and a chemical content that is subject to environmental dependences.

  1. Tidal stripping stellar substructures around four metal-poor globular clusters in the galactic bulge

    SciTech Connect

    Chun, Sang-Hyun; Kang, Minhee; Jung, DooSeok; Sohn, Young-Jong

    2015-01-01

    We investigate the spatial density configuration of stars around four metal-poor globular clusters (NGC 6266, NGC 6626, NGC 6642, and NGC 6723) in the Galactic bulge region using wide-field deep J, H, and K imaging data obtained with the Wide Field Camera near-infrared array on the United Kingdom Infrared Telescope. A statistical weighted filtering algorithm for the stars on the color–magnitude diagram is applied in order to sort cluster member candidates from the field star contamination. In two-dimensional isodensity contour maps of the clusters, we find that all four of the globular clusters exhibit strong evidence of tidally stripped stellar features beyond the tidal radius in the form of tidal tails or small density lobes/chunks. The orientations of the extended stellar substructures are likely to be associated with the effect of dynamic interaction with the Galaxy and the cluster's space motion. The observed radial density profiles of the four globular clusters also describe the extended substructures; they depart from theoretical King and Wilson models and have an overdensity feature with a break in the slope of the profile at the outer region of clusters. The observed results could imply that four globular clusters in the Galactic bulge region have experienced strong environmental effects such as tidal forces or bulge/disk shocks of the Galaxy during the dynamical evolution of globular clusters. These observational results provide further details which add to our understanding of the evolution of clusters in the Galactic bulge region as well as the formation of the Galaxy.

  2. Herschel's View of LITTLE THINGS Metal-Poor Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Cigan, Phil; Young, Lisa; Cormier, Diane; Lebouteiller, Vianney; Hunter, Deidre Ann; Madden, Suzanne; Little Things

    2015-01-01

    Dwarf galaxies present interesting challenges for the studies of various galaxy properties, due in part to their faintness and their typically low metal content. Low metallicity can lead to quite different physical conditions in the ISM of these systems, which can affect star formation and other processes. To determine the structure of star-forming molecular clouds at low metallicity and moderate star formation rates, far infrared (FIR) fine-structure lines were mapped with Herschel in selected regions of five dwarf irregular galaxies with metal abundances ranging from 13% down to 5% of solar. Abundances of [C II] 158, [O I] 63, [N II] 122, and [O III] 88 microns - the major FIR cooling lines - help to probe the conditions in the gas, and allow us to put these dwarfs in context with spirals and other galaxy types. We report our integrated fluxes and line ratios, and discuss the results: [C II] is the dominant FIR coolant in these systems, and it mostly originates in PDRs instead of the more diffuse phase. Funding for this project was provided by NASA JPL RSA grant 1433776.

  3. Mapping the local galactic halo and an image motion compensation system for the multi-object double spectrograph

    NASA Astrophysics Data System (ADS)

    Marshall, Jennifer L.

    In the first part of this dissertation I describe the results of a photometric and spectroscopic survey of a sample of cool, metal-poor subdwarfs in the solar neighborhood. These metal-poor stars are of interest because, as members of the Galactic halo, they give clues about the history of the Galaxy and its formation mechanisms, and may enable us to study satellites of the Milky Way and the Galactic merger history. A sample of halo subdwarfs have been selected using a reduced proper motion (RPM) diagram. Accurate and precise photometric measurements of 635 stars selected in this manner allow better definition of the RPM diagram and determination of its usefulness as a selection method. Accurate spectrophotometry yields radial velocities of the candidates as well as metallicity and temperature estimates for 288 subdwarfs. Of special interest in this sample are the ten newly discovered extremely metal-poor stars, as well as four carbon-enhanced metal-poor stars. I use these new observations to search the local Galactic halo for structure due to merger remnants and moving groups; there is some evidence for both. I also discuss the metallicity distribution function of the sample and compare it to previous work on this subject. No astronomical observations of any sort are possible without appropriate, well-calibrated instrumentation with which to perform the measurements. In the second part of this dissertation, I describe the Image Motion Compensation System (IMCS) for the Multi-Object Double Spectrograph (MODS), an optical spectrograph for the Large Binocular Telescope. The system performs closed-loop image motion compensation, actively correcting for image motion in the spectrograph's focal plane caused by large scale structural bending due to gravity as well as other effects such as temperature fluctuation and mechanism flexure within the instrument. The system is currently installed in the MODS instrument and controls instrumental flexure to within specifications

  4. The Extremely Metal-Poor Dwarf Galaxy AGC 198691

    NASA Astrophysics Data System (ADS)

    Hirschauer, Alec S.; Salzer, John Joseph; Cannon, John M.; Skillman, Evan D.; SHIELD II Team

    2016-01-01

    We present spectroscopic observations of the nearby dwarf irregular galaxy AGC 198691. This object is part of the Survey of HI in Extremely Low-Mass Dwarfs (SHIELD) sample, which consists of ultra-low HI mass galaxies discovered by the Arecibo Legacy Fast-Acting ALFA (ALFALFA) survey. SHIELD is a multi-configuration Expanded Very Large Array (EVLA) study of the neutral gas content and dynamics of galaxies with HI masses in the range of 106-107 M⊙. Our spectral data were obtained using the new high-throughput KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m telescope as part of a systematic study of the nebular abundances in the SHIELD galaxy sample. These observations enable measurement of the temperature sensitive [OIII]λ4363 line and hence the determination of a "direct" oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) galaxy with an oxygen abundance comparable to such objects as I Zw 18, SBS 0335-052W, Leo P, and DDO 68 - the lowest metallicity star-forming systems known. It is worth noting that two of the five lowest-abundance galaxies currently recognized were discovered via the ALFALFA blind HI survey. These XMD galaxies are potential analogues to the first star-forming systems, which through hierarchical accretion processes built up the large galaxies we observe today in the local Universe. Detailed analysis of such XMD systems offers observational constraint to models of galactic evolution and star formation histories to allow a better understanding of the processes that govern the chemical evolution of low-mass galaxies.

  5. ALFALFA Discovery of the Most Metal-poor Gas-rich Galaxy Known: AGC 198691

    NASA Astrophysics Data System (ADS)

    Hirschauer, Alec S.; Salzer, John J.; Skillman, Evan D.; Berg, Danielle; McQuinn, Kristen B. W.; Cannon, John M.; Gordon, Alex J. R.; Haynes, Martha P.; Giovanelli, Riccardo; Adams, Elizabeth A. K.; Janowiecki, Steven; Rhode, Katherine L.; Pogge, Richard W.; Croxall, Kevin V.; Aver, Erik

    2016-05-01

    We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part of the Survey of H i in Extremely Low-Mass Dwarfs project, which is a multi-wavelength study of galaxies with H i masses in the range of 106–107.2 M ⊙, discovered by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We have obtained spectra of the lone H ii region in AGC 198691 with the new high-throughput KPNO Ohio State Multi-Object Spectrograph on the Mayall 4 m, as well as with the Blue Channel spectrograph on the MMT 6.5 m telescope. These observations enable the measurement of the temperature-sensitive [O iii]λ4363 line and hence the determination of a “direct” oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) system with an oxygen abundance of 12+log(O/H) = 7.02 ± 0.03, making AGC 198691 the lowest-abundance star-forming galaxy known in the local universe. Two of the five lowest-abundance galaxies known have been discovered by the ALFALFA blind H i survey; this high yield of XMD galaxies represents a paradigm shift in the search for extremely metal-poor galaxies.

  6. Main sequence of the metal-poor globular cluster M30 (NGC 7099)

    SciTech Connect

    Alcaino, G.; Liller, W.

    1980-10-01

    We present photographic photometry for 673 stars in the metal-poor globular cluster M30 (NGC 7099). The Racine wedge was used with the CTIO 1-m Yale telescope (..delta..m=3/sup m/.60), the CTIO 4-m telescope (..delta..m=6/sup m/.83), and the ESO 3.6-m telescope (..delta..m=4/sup m/.12) to extend the photoelectric limit from Vapprox. =16.3 to Vapprox. =20.4. For the main-sequence turn-off, we have determined its position to lie at V=18.4 +- 0.1 (m.e.) and B-V=0.49 +- 0.03 (m.e.). From these values, we calculate the intrinsic values M/sub v/ =3.87 and (B-V)/sub 0/=0.47. For the cluster as a whole, we derive a distance modulus (m-M)/sub V/=14.53 +- 0.15 and reddening E(B-V)=0.02 +- 0.02. Using the models of Iben and Rood (Astrophys. J. 159, 605 (1970)) and the isochrones of Demarque and McClure ((1977), in Evolution of Galaxies and Stellar Populations, edited by B. Tinsley and R. B. Larson (Yale University Observatory, New Haven), p. 199), we deduce the cluster's age to be 14.5( +- 4.0) x 10/sup 9/ yr. The large uncertainty in this value emphasizes the dire need for more work on cluster evolution.

  7. Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey

    SciTech Connect

    Miceli, A; Rest, A; Stubbs, C W; Hawley, S L; Cook, K H; Magnier, E A; Krisciunas, K; Bowell, E; Koehn, B

    2007-02-23

    We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg{sup 2} and span distances ranging from 3-30kpc from the Galactic Center. Object selection is based on phased, photometric data with 28-50 epochs. We use this large sample to explore the bulk properties of the stellar halo, including the spatial distribution. The period-amplitude distribution of this sample shows that the majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a significant fraction (26%) which have longer periods and appear to be Oosterhoff type II. We find that the radial distributions of these two populations have significantly different profiles ({rho}{sub OoI} {approx} R{sup -2.26{+-}0.07} and {rho}{sub OoII} {approx} R{sup -2.88{+-}0.11}). This suggests that the stellar halo was formed by at least two distinct accretion processes and supports dual-halo models.

  8. Using chemical tagging to redefine the interface of the Galactic disc and halo

    NASA Astrophysics Data System (ADS)

    Hawkins, K.; Jofré, P.; Masseron, T.; Gilmore, G.

    2015-10-01

    We present a chemical abundance distribution study in 14 α, odd-Z, even-Z, light, and Fe-peak elements of approximately 3200 intermediate-metallicity giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The main aim of our analysis is to explore the Galactic disc-halo transition region within -1.20 < [Fe/H] < -0.55 as a means to study chemical difference (and similarities) between these components. In this paper, we show that there is an α-poor and α-rich sequence within both the metal-poor and intermediate-metallicity regions. Using the Galactic rest-frame radial velocity and spatial positions, we further separate our sample into the canonical Galactic components. We then studied the abundances ratios of Mg, Ti, Si, Ca, O, S, Al, C+N, Na, Ni, Mn, V, and K for each of the components and found the following: (1) the α-poor halo subgroup is chemically distinct in the α-elements, particularly O, Mg, S, Al, C+N, and Ni, from the α-rich halo, consistent with the literature confirming the existence of an α-poor accreted halo population; (2) the canonical thick disc and halo are not chemically distinct in all elements indicating a smooth transition between the thick disc and halo; (3) a subsample of the α-poor stars at metallicities as low as [Fe/H] ˜ -0.85 dex are chemically and dynamically consistent with the thin disc indicating that the thin disc may extend to lower metallicities than previously thought; and (4) the locations of the most metal-poor thin disc stars are consistent with a negative radial metallicity gradient. Finally, we used our analysis to suggest a new set of chemical abundance planes ([α/Fe], [C+N/Fe], [Al/Fe], and [Mg/Mn]) that may be able to chemically label the Galactic components in a clean and efficient way independent of kinematics.

  9. The rarity of dust in metal-poor galaxies.

    PubMed

    Fisher, David B; Bolatto, Alberto D; Herrera-Camus, Rodrigo; Draine, Bruce T; Donaldson, Jessica; Walter, Fabian; Sandstrom, Karin M; Leroy, Adam K; Cannon, John; Gordon, Karl

    2014-01-01

    Galaxies observed at redshift z > 6, when the Universe was less than a billion years old, thus far very rarely show evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko (z = 6.6), which--a mere 840 million years after the Big Bang--is forming stars at a rate of 30-100 solar masses per year, yielding a mass assembly time of about 150 × 10(6) years. Himiko is thought to have a low fraction (2-3 per cent of the Sun's) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun's and is forming stars less rapidly (assembly time about 1.6 × 10(9) years) than Himiko but still vigorously for its mass, is also very dust deficient and is perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18, from which we determine its dust mass to be 450-1,800 solar masses, yielding a dust-to-stellar mass ratio of about 10(-6) to 10(-5) and a dust-to-gas mass ratio of 3.2-13 × 10(-6). If I Zw 18 is a reasonable analogue of Himiko, then Himiko's dust mass must be around 50,000 solar masses, a factor of 100 below the current upper limit. These numbers are quite uncertain, but if most high-z galaxies are more like Himiko than like the very-high-dust-mass galaxy SDSS J114816.64 + 525150.3 at z ≈ 6, which hosts a quasar, then our prospects for detecting the gas and dust inside such galaxies are much poorer than hitherto anticipated.

  10. The rarity of dust in metal-poor galaxies.

    PubMed

    Fisher, David B; Bolatto, Alberto D; Herrera-Camus, Rodrigo; Draine, Bruce T; Donaldson, Jessica; Walter, Fabian; Sandstrom, Karin M; Leroy, Adam K; Cannon, John; Gordon, Karl

    2014-01-01

    Galaxies observed at redshift z > 6, when the Universe was less than a billion years old, thus far very rarely show evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko (z = 6.6), which--a mere 840 million years after the Big Bang--is forming stars at a rate of 30-100 solar masses per year, yielding a mass assembly time of about 150 × 10(6) years. Himiko is thought to have a low fraction (2-3 per cent of the Sun's) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun's and is forming stars less rapidly (assembly time about 1.6 × 10(9) years) than Himiko but still vigorously for its mass, is also very dust deficient and is perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18, from which we determine its dust mass to be 450-1,800 solar masses, yielding a dust-to-stellar mass ratio of about 10(-6) to 10(-5) and a dust-to-gas mass ratio of 3.2-13 × 10(-6). If I Zw 18 is a reasonable analogue of Himiko, then Himiko's dust mass must be around 50,000 solar masses, a factor of 100 below the current upper limit. These numbers are quite uncertain, but if most high-z galaxies are more like Himiko than like the very-high-dust-mass galaxy SDSS J114816.64 + 525150.3 at z ≈ 6, which hosts a quasar, then our prospects for detecting the gas and dust inside such galaxies are much poorer than hitherto anticipated. PMID:24317694

  11. Detailed chemical abundances in NGC 5824: another metal-poor globular cluster with internal heavy element abundance variations

    NASA Astrophysics Data System (ADS)

    Roederer, Ian U.; Mateo, Mario; Bailey, John I.; Spencer, Meghin; Crane, Jeffrey D.; Shectman, Stephen A.

    2016-01-01

    We present radial velocities, stellar parameters, and detailed abundances of 39 elements derived from high-resolution spectroscopic observations of red giant stars in the luminous, metal-poor globular cluster NGC 5824. We observe 26 stars in NGC 5824 using the Michigan/Magellan Fiber System (M2FS) and two stars using the Magellan Inamori Kyocera Echelle spectrograph. We derive a mean metallicity of [Fe/H] = -1.94 ± 0.02 (statistical) ±0.10 (systematic). The metallicity dispersion of this sample of stars, 0.08 dex, is in agreement with previous work and does not exceed the expected observational errors. Previous work suggested an internal metallicity spread only when fainter samples of stars were considered, so we cannot exclude the possibility of an intrinsic metallicity dispersion in NGC 5824. The M2FS spectra reveal a large internal dispersion in [Mg/Fe], 0.28 dex, which is found in a few other luminous, metal-poor clusters. [Mg/Fe] is correlated with [O/Fe] and anticorrelated with [Na/Fe] and [Al/Fe]. There is no evidence for internal dispersion among the other α- or Fe-group abundance ratios. 25 of the 26 stars exhibit a n-capture enrichment pattern dominated by r-process nucleosynthesis (<[Eu/Fe]> = +0.11 ± 0.12; <[Ba/Eu]> = -0.66 ± 0.05). Only one star shows evidence of substantial s-process enhancement ([Ba/Fe] = +0.56 ± 0.12; [Ba/Eu] = +0.38 ± 0.14), but this star does not exhibit other characteristics associated with s-process enhancement via mass transfer from a binary companion. The Pb and other heavy elements produced by the s-process suggest a time-scale of no more than a few hundred Myr for star formation and chemical enrichment, like the complex globular clusters M2, M22, and NGC 5286.

  12. A support vector machine to search for metal-poor galaxies

    NASA Astrophysics Data System (ADS)

    Shi, Fei; Liu, Yu-Yan; Kong, Xu; Chen, Yang; Li, Zhong-Hua; Zhi, Shu-Teng

    2014-10-01

    To develop a fast and reliable method for selecting metal-poor galaxies (MPGs), especially in large surveys and huge data bases, a support vector machine (SVM) supervized learning algorithms is applied to a sample of star-forming galaxies from the Sloan Digital Sky Survey data release 9 provided by the Max Planck Institute and the Johns Hopkins University (http://www.sdss3.org/dr9/spectro/spectroaccess.php). A two-step approach is adopted: (i) the SVM must be trained with a subset of objects that are known to be either MPGs or metal-rich galaxies (MRGs), treating the strong emission line flux measurements as input feature vectors in n-dimensional space, where n is the number of strong emission line flux ratios. (ii) After training on a sample of star-forming galaxies, the remaining galaxies are classified in the automatic test analysis as either MPGs or MRGs using a 10-fold cross-validation technique. For target selection, we have achieved an acquisition accuracy for MPGs of ˜96 and ˜95 per cent for an MPG threshold of 12 + log(O/H) = 8.00 and 12 + log(O/H) = 8.39, respectively. Running the code takes minutes in most cases under the MATLAB 2013a software environment. The code in the Letter is available on the web (http://fshi5388.blog.163.com). The SVM method can easily be extended to any MPGs target selection task and can be regarded as an efficient classification method particularly suitable for modern large surveys.

  13. RELICS OF GALAXY MERGING: OBSERVATIONAL PREDICTIONS FOR A WANDERING MASSIVE BLACK HOLE AND ACCOMPANYING STAR CLUSTER IN THE HALO OF M31

    SciTech Connect

    Kawaguchi, Toshihiro; Saito, Yuriko; Miki, Yohei; Mori, Masao

    2014-07-01

    Galaxies and massive black holes (BHs) presumably grow via galactic merging events and subsequent BH coalescence. As a case study, we investigate the merging event between the Andromeda galaxy (M31) and a satellite galaxy. We compute the expected observational appearance of the massive BH that was at the center of the satellite galaxy prior to the merger and is currently wandering in the M31 halo. We demonstrate that a radiatively inefficient accretion flow with a bolometric luminosity of a few tens of solar luminosities develops when Hoyle-Lyttleton accretion onto the BH is assumed. We compute the associated broadband spectrum and show that the radio band (observable with EVLA, ALMA, and the Square Kilometre Array) is the best frequency range in which to detect the emission. We also evaluate the mass and the luminosity of the stars bound by the wandering BH and find that such a star cluster is sufficiently luminous that it could correspond to one of the star clusters found by the PAndAS survey. The discovery of a relic massive BH wandering in a galactic halo will provide a direct means of investigating in detail the coevolution of galaxies and BHs. It also means a new population of BHs (off-center massive BHs) and offers targets for clean BH imaging that avoid strong interstellar scattering in the centers of galaxies.

  14. Relics of Galaxy Merging: Observational Predictions for a Wandering Massive Black Hole and Accompanying Star Cluster in the Halo of M31

    NASA Astrophysics Data System (ADS)

    Kawaguchi, Toshihiro; Saito, Yuriko; Miki, Yohei; Mori, Masao

    2014-07-01

    Galaxies and massive black holes (BHs) presumably grow via galactic merging events and subsequent BH coalescence. As a case study, we investigate the merging event between the Andromeda galaxy (M31) and a satellite galaxy. We compute the expected observational appearance of the massive BH that was at the center of the satellite galaxy prior to the merger and is currently wandering in the M31 halo. We demonstrate that a radiatively inefficient accretion flow with a bolometric luminosity of a few tens of solar luminosities develops when Hoyle-Lyttleton accretion onto the BH is assumed. We compute the associated broadband spectrum and show that the radio band (observable with EVLA, ALMA, and the Square Kilometre Array) is the best frequency range in which to detect the emission. We also evaluate the mass and the luminosity of the stars bound by the wandering BH and find that such a star cluster is sufficiently luminous that it could correspond to one of the star clusters found by the PAndAS survey. The discovery of a relic massive BH wandering in a galactic halo will provide a direct means of investigating in detail the coevolution of galaxies and BHs. It also means a new population of BHs (off-center massive BHs) and offers targets for clean BH imaging that avoid strong interstellar scattering in the centers of galaxies.

  15. High-dispersion spectroscopy of giants in metal-poor globular clusters. I - Iron abundances

    NASA Technical Reports Server (NTRS)

    Minniti, Dante; Geisler, Doug; Peterson, Ruth C.; Claria, Juan J.

    1993-01-01

    High-resolution, high-SNR CCD spectra have been obtained for 16 giants in eight metal-poor Galactic globular clusters. Fe abundances accurate to 0.15 dex have been determined by a fully consistent set of model atmospheres and spectrum synthesis techniques. A metallicity scale is presented for metal-poor clusters that should prove useful for calibrating a wide variety of photometric and low-resolution spectroscopic metallicity indicators.

  16. The Surface Brightness Profile of the Bulge and Halo of the Andromeda Spiral Galaxy (M31) from R = 10 to 165 kiloparsecs

    NASA Astrophysics Data System (ADS)

    Guhathakurta, Puragra; Gilbert, K.; Kalirai, J.; Ostheimer, J.; Majewski, S.; Patterson, R.; Geha, M.; Cooper, M.; Reitzel, D.; Rich, R.

    2006-12-01

    Understanding the formation of galaxies and their structural subcomponents is a key goal of modern cosmology. Large spiral galaxies like our own consist of a flattened rotating disk, a centrally concentrated bulge whose density decreases exponentially with increasing radius, and an extended halo whose density scales as an inverse power law in radius. Our internal vantage point is disadvantageous for investigating the structure of our own Galaxy. By contrast, the Andromeda spiral galaxy (M31), the Milky Ways neighbour, offers us a global external perspective and yet is close enough for individual stars to be resolved. Over several decades, structural studies of M31 have generally concluded that its outer spheroid is an extension of its inner bulge, displaying the characteristic exponential cut-off out to a distance of about 20 kpc from the center, and/or that its halo is undetected or absent. We report here on the discovery of a halo of red giant stars in M31 extending beyond a radius of 150 kpc. Our finding shows that previous studies of the spheroid of M31 spanning the last few decades have been sampling its extended bulge instead of the pristine metal-poor halo. Characterizing the dynamics, metallicity, substructure, and age of M31's halo will provide unique tests of galaxy formation theories. This research was supported by funds from the NSF and NASA/STScI.

  17. The Light Side and The Dark Side of the Milky Way Halo

    NASA Astrophysics Data System (ADS)

    Kafle, Prajwal R.; Sharma, Sanjib; Lewis, Geraint F.; Bland-Hawthorn, Joss

    2014-01-01

    We present our recent measurement of the kinematics of the Milky Way stellar halo (Light Side) and the derived mass of the dark matter halo (Dark Side) using the Jeans analysis. A tangential dip in the velocity anisotropy profile at r ~ 17 kpc (Kafle et al. 2012), and a distinct difference of ~65 kms-1 in the mean azimuthal velocity and the r.m.s dispersion of the most metal-rich and the metal-poor Blue Horizontal Branch stars we find (Kafle et al. 2013) are reported. The implications of this on the current controversial issue of an existence of the two-components in the halo are also discussed. Aided with the kinematic measurements of the light side, we demonstrate how we infer the dynamical property of the dark side. Considering a realistic three component galaxy model (Hernquist bulge, Miyamoto-Nagai disk and NFW halo), we estimate the virial mass of the Galaxy to be M vir = 1.2+0.5 -0.4 × 1012 M ⊙ (Kafle et al. 2012). We also show that the rotation curve of the Galaxy has undulations similar to what have also been seen in the studies of the HI gas (Sofue et al. 2009).

  18. The Outer Halo -- Halo Origins and Mass of the Galaxy

    NASA Astrophysics Data System (ADS)

    Morrison, Heather; Arabadjis, John; Dohm-Palmer, Robbie; Freeman, Ken; Harding, Paul; Mateo, Mario; Norris, John; Olszewski, Ed; Sneden, Chris

    2000-02-01

    Through our detection of distant halo stars, we are now well placed to map the regions of the Galactic halo where previously only satellite galaxies and a few globular clusters were known. Mapping this region is crucial for answering questions like: How and over what timescales was the Milky Way's stellar halo assembled? What is the total mass and shape of its dark halo? The Sagittarius dwarf has demonstrated that at least some of the stellar halo was accreted. But, HOW MUCH of the halo was accreted? Our previous efforts have proven that the Washington photometric system, in conjuction with spectroscopy, is capable of efficiently and unambiguously identifying halo stars out to 100 kpc or more. We require followup spectroscopy to map velocity substructure, which is more likely visible in the outer halo because of the long dynamical timescales, and to identify the rare objects in the extreme outer halo which will constrain the shape and size of its dark halo. We are applying for 4m/RCSP time at both CTIO and KPNO to observe faint outer-halo giant and BHB candidates.

  19. The Metal-poor Knee in the Fornax Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Hendricks, Benjamin; Koch, Andreas; Lanfranchi, Gustavo A.; Boeche, Corrado; Walker, Matthew; Johnson, Christian I.; Peñarrubia, Jorge; Gilmore, Gerard

    2014-04-01

    We present α-element abundances of Mg, Si, and Ti for a large sample of field stars in two outer fields of the Fornax dwarf spheroidal (dSph) galaxy, obtained with Very Large Telescope/GIRAFFE (R ~ 16, 000). Due to the large fraction of metal-poor (MP) stars in our sample, we are able to follow the α-element evolution from [Fe/H] ≈ -2.5 continuously to [Fe/H] ≈ -0.7. For the first time we are able to resolve the turnover from the Type II supernovae (SNe) dominated, α-enhanced plateau down to subsolar [α/Fe] values, due to the onset of SNe Ia, and thus to trace the chemical enrichment efficiency of the galaxy. Our data support the general concept of an α-enhanced plateau at early epochs, followed by a well-defined "knee" caused by the onset of SNe Ia, and finally a second plateau with sub-solar [α/Fe] values. We find the position of this knee to be at [Fe/H] ≈ -1.9 and therefore significantly more MP than expected from comparison with other dSphs and standard evolutionary models. Surprisingly, this value is rather comparable to the knee in Sculptor, a dSph ~10 times less luminous than Fornax. Using chemical evolution models, we find that the position of the knee and the subsequent plateau at the sub-solar level can hardly be explained unless the galaxy experienced several discrete star formation (SF) events with a drastic variation in SF efficiency, while a uniform SF can be ruled out. One possible evolutionary scenario is that Fornax experienced one or several major accretion events from gas-rich systems in the past, so that its current stellar mass is not indicative of the chemical evolution environment at ancient times. If Fornax is the product of several smaller buildings blocks, this may also have implications for the understanding of the formation process of dSphs in general. This article is based on observations made with ESO Telescopes at the Paranal Observatory under program 082.B-0940(A).

  20. The metal-poor knee in the Fornax dwarf spheroidal galaxy

    SciTech Connect

    Hendricks, Benjamin; Koch, Andreas; Lanfranchi, Gustavo A.; Boeche, Corrado; Walker, Matthew; Johnson, Christian I.; Peñarrubia, Jorge; Gilmore, Gerard

    2014-04-20

    We present α-element abundances of Mg, Si, and Ti for a large sample of field stars in two outer fields of the Fornax dwarf spheroidal (dSph) galaxy, obtained with Very Large Telescope/GIRAFFE (R ∼ 16, 000). Due to the large fraction of metal-poor (MP) stars in our sample, we are able to follow the α-element evolution from [Fe/H] ≈ –2.5 continuously to [Fe/H] ≈ –0.7. For the first time we are able to resolve the turnover from the Type II supernovae (SNe) dominated, α-enhanced plateau down to subsolar [α/Fe] values, due to the onset of SNe Ia, and thus to trace the chemical enrichment efficiency of the galaxy. Our data support the general concept of an α-enhanced plateau at early epochs, followed by a well-defined 'knee' caused by the onset of SNe Ia, and finally a second plateau with sub-solar [α/Fe] values. We find the position of this knee to be at [Fe/H] ≈ –1.9 and therefore significantly more MP than expected from comparison with other dSphs and standard evolutionary models. Surprisingly, this value is rather comparable to the knee in Sculptor, a dSph ∼10 times less luminous than Fornax. Using chemical evolution models, we find that the position of the knee and the subsequent plateau at the sub-solar level can hardly be explained unless the galaxy experienced several discrete star formation (SF) events with a drastic variation in SF efficiency, while a uniform SF can be ruled out. One possible evolutionary scenario is that Fornax experienced one or several major accretion events from gas-rich systems in the past, so that its current stellar mass is not indicative of the chemical evolution environment at ancient times. If Fornax is the product of several smaller buildings blocks, this may also have implications for the understanding of the formation process of dSphs in general.

  1. Formation of Globular Clusters in Atomic-cooling Halos Via Rapid Gas Condensation and Fragmentation during the Epoch of Reionization

    NASA Astrophysics Data System (ADS)

    Kimm, Taysun; Cen, Renyue; Rosdahl, Joakim; Yi, Sukyoung K.

    2016-05-01

    We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with {M}{{halo}}˜ 4× {10}7 {M}⊙ at z\\gt 10 using cosmological radiation-hydrodynamics simulations. We find that very compact (≲1 pc) and massive (˜ 6× {10}5 {M}⊙ ) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Lyα emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short (\\ll 1 {{Myr}}), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.

  2. Probing the galactic disk and halo. 2: Hot interstellar gas toward the inner galaxy star HD 156359

    NASA Technical Reports Server (NTRS)

    Sembach, Kenneth R.; Savage, Blair D.; Lu, Limin

    1995-01-01

    We present Goddard High Resolution Spectrograph intermediate-resolution measurements of the 1233-1256 A spectral region of HD 156396, a halo star at l = 328.7 deg, b = -14.5 deg in the inner Galaxy with a line-of sight distance of 11.1 kpc and a z-distance of -2.8 kpc. The data have a resolution of 18 km/s Full Width at Half Maximum (FWHM) and a signal-to-noise ratio of approximately 50:1. We detect interstellar lines of Mg II, S II, S II, Ge II, and N V and determine log N/(Mg II) = 15.78 +0.25, -0.27, log N(Si II) greater than 13.70, log N(S II) greater than 15.76, log N(Ge II) = 12.20 +0.09,-0.11, and log N(N v) = 14.06 +/- 0.02. Assuming solar reference abundances, the diffuse clouds containing Mg, S, and Ge along the sight line have average logarithmic depletions D(Mg) = -0.6 +/- 0.3 dex, D(S) greater than -0.2 dex, and D(Ge) = -0.2 +/- 0.2 dex. The Mg and Ge depletions are approximately 2 times smaller than is typical of diffuse clouds in the solar vicinity. Galactic rotational modeling of the N v profiles indicates that the highly ionized gas traced by this ion has a scale height of approximately 1 kpc if gas at large z-distances corotates with the underlying disk gas. Rotational modeling of the Si iv and C iv profiles measured by the IUE satellite yields similar scale height estimates. The scale height results contrast with previous studies of highly ionized gas in the outer Milky Way that reveal a more extended gas distribtion with h approximately equals 3-4 kpc. We detect a high-velocity feature in N v and Si II v(sub LSR) approximately equals + 125 km/s) that is probably created in an interface between warm and hot gas.

  3. The Distribution of Carbon Abundances in Stars in the Milky Way’s Satellite Galaxies

    NASA Astrophysics Data System (ADS)

    Guo, Michelle; Zhang, A.

    2013-01-01

    There is evidence that the Milky Way halo is comprised in part of disrupted dwarf satellite galaxies; however, the extent to which they contribute to the halo’s formation is unclear. To further examine the role of dwarf galaxies in building the halo, we compared the degrees of carbon enhancement of the dwarf spheroidal (dSph) galaxies and field halo populations. We generated a grid of high-resolution synthetic spectra for hypothetical stars of specific effective temperature, surface gravity, metallicity, alpha element abundance, and carbon abundance for comparison with medium-resolution observed spectra of dSph stars of unmeasured [C/Fe] but otherwise known properties. After smoothing, rebinning, and normalizing the two data sets, we varied carbon abundance to find the best carbon abundance by determining the synthetic spectrum that gave the minimal deviation. We found a lower Carbon-Enhanced Metal-Poor (CEMP) fraction in the dSph galaxies, which suggests that they have evolved over time. Whereas star formation and chemical evolution stopped for accreted galaxies, the surviving galaxies evolved to became less carbon enhanced and more metal rich. The variation in carbon abundances supports prior knowledge of dSph stars and provide a deeper understanding the formation of stars such as those of the Milky Way halo. We thank the US National Science Foundation, the UCSC Science Internship Program, and the W. M. Keck Observatory where the spectra were obtained.

  4. DISCOVERY OF A GAS-RICH COMPANION TO THE EXTREMELY METAL-POOR GALAXY DDO 68

    SciTech Connect

    Cannon, John M.; Alfvin, Erik D.; Johnson, Megan; Koribalski, Baerbel; McQuinn, Kristen B. W.; Skillman, Evan D.; Bailin, Jeremy; Ford, H. Alyson; Girardi, Léo; Hirschauer, Alec S.; Janowiecki, Steven; Salzer, John J.; Van Sistine, Angela; Dolphin, Andrew; Elson, E. C.; Marigo, Paola; Rosenfield, Philip; Rosenberg, Jessica L.; Venkatesan, Aparna; Warren, Steven R.

    2014-05-20

    We present H I spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only ∼3% Z {sub ☉}, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival H I spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M{sub H} {sub I} = 2.8 × 10{sup 7} M {sub ☉}), recently star-forming (SFR{sub FUV} = 1.4 × 10{sup –3} M {sub ☉} yr{sup –1}, SFR{sub Hα} < 7 × 10{sup –5} M {sub ☉} yr{sup –1}) companion has the same systemic velocity as DDO 68 (V {sub sys} = 506 km s{sup –1}; D = 12.74 ± 0.27 Mpc) and is located at a projected distance of ∼42 kpc. New H I maps obtained with the 100 m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness H I gas forms a bridge between these objects.

  5. Quantitative spectroscopy of blue supergiants in metal-poor dwarf galaxy NGC 3109

    SciTech Connect

    Hosek, Matthew W. Jr.; Kudritzki, Rolf-Peter; Bresolin, Fabio; Urbaneja, Miguel A.; Przybilla, Norbert; Evans, Christopher J.; Pietrzyński, Grzegorz; Gieren, Wolfgang; Carraro, Giovanni E-mail: kud@ifa.hawaii.edu E-mail: Miguel.Urbaneja-Perez@uibk.ac.at E-mail: chris.evans@stfc.ac.uk E-mail: wgieren@astro-udec.cl

    2014-04-20

    We present a quantitative analysis of the low-resolution (∼4.5 Å) spectra of 12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy NGC 3109. A modified method of analysis is presented which does not require use of the Balmer jump as an independent T {sub eff} indicator, as used in previous studies. We determine stellar effective temperatures, gravities, metallicities, reddening, and luminosities, and combine our sample with the early-B-type BSGs analyzed by Evans et al. to derive the distance to NGC 3109 using the flux-weighted gravity-luminosity relation (FGLR). Using primarily Fe-group elements, we find an average metallicity of [ Z-bar ] = –0.67 ± 0.13, and no evidence of a metallicity gradient in the galaxy. Our metallicities are higher than those found by Evans et al. based on the oxygen abundances of early-B supergiants ([ Z-bar ] = –0.93 ± 0.07), suggesting a low α/Fe ratio for the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy mass-metallicity relation accordingly and compare it to metallicity studies of H II regions in star-forming galaxies. We derive an FGLR distance modulus of 25.55 ± 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red giant branch distances. The FGLR itself is consistent with those found in other galaxies, demonstrating the reliability of this method as a measure of extragalactic distances.

  6. Integrated Light Chemical Abundance Analyses of 7 M31 Outer Halo Globular Clusters from the Pan-Andromeda Archaeological Survey

    NASA Astrophysics Data System (ADS)

    Sakari, Charli; Venn, Kim; Mackey, Dougal; Shetrone, Matthew D.; Dotter, Aaron L.; Wallerstein, George

    2015-01-01

    Detailed chemical abundances of globular clusters provide insight into the formation and evolution of galaxies and their globular cluster systems. This talk presents detailed chemical abundances for seven M31 outer halo globular clusters (with projected radii greater than 30 kpc), as derived from high resolution integrated light spectra. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS). The integrated abundances show that 4 of these clusters are metal-poor ([Fe/H] < -1.5) while the other 3 are more metal-rich. The most metal-poor globular clusters are α-enhanced, though 3 of the 4 are possibly less α-enhanced than MW stars (at the 1σ level). Other chemical abundance ratios ([Ba/Eu], [Eu/Ca], and [Ni/Fe]) are consistent with origins in low mass dwarf galaxies (similar to Fornax). The most metal-rich cluster ([Fe/H] ~ -1) stands out as being chemically distinct from Milky Way field stars of the same metallicity---its chemical abundance ratios agree best with the stars and clusters in the Large Magellanic Cloud (LMC) and the Sagittarius dwarf spheroidal (Sgr) than with the Milky Way field stars. The other metal-rich clusters, H10 and H23, look similar to the LMC and Milky Way field stars in all abundance ratios. These results indicate that M31's outer halo is being at least partially built up by the accretion of dwarf satellites, in agreement with previous observations.

  7. Kinematic and Chemical Constraints on the Formation of M31's Inner and Outer Halo

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Rich, R. Michael; Reitzel, David B.; Martin, Nicolas F.; Ibata, Rodrigo A.; Chapman, Scott C.; Majewski, Steven R.; Mori, Masao; Loh, Yeong-Shang; Ostheimer, James C.; Tanaka, Mikito

    2008-12-01

    The halo of M31 shows a wealth of substructures, some of which are consistent with assembly from satellite accretion. Here we report on kinematic and abundance results from Keck DEIMOS spectroscopy in the near-infrared calcium triplet region of over 3500 red giant star candidates along the minor axis and in off-axis spheroid fields of M31. These data reach out to large radial distances of about 160 kpc. The derived radial velocity distributions show an indication of a kinematically cold substructure around ~17 kpc, which has been reported before. We devise a new and improved method to measure spectroscopic metallicities from the calcium triplet in low signal-to-noise ratio spectra using a weighted co-addition of the individual lines. The resulting distribution (accurate to ~0.3 dex down to signal-to-noise ratios of 5) leads us to note an even stronger gradient in the abundance distribution along M31's minor axis and in particular toward the outer halo fields than previously detected. The mean metallicity in the outer fields reaches below -2 dex, with individual values as low as lesssim-2.6 dex. This is the first time such a metal-poor halo has been detected in M31. In the fields toward the inner spheroid, we find a sharp decline of ~0.5 dex in metallicity in a region at ~20 kpc, which roughly coincides with the edge of an extended disk, previously detected from star count maps. A large fraction of red giants that we detect in the most distant fields are likely members of M33's overlapping halo. A comparison of our velocities with those predicted by new N-body simulations argues that the event responsible for the Giant Stream is most likely not responsible for the full population of the inner halo. We show further that the abundance distribution of the Stream is different from that of the inner halo, from which it becomes evident, in turn, that the merger event that formed the Stream and the outer halo cannot have contributed any significant material to the inner

  8. Formation of the Galactic Stellar Halo: Origin of the Metallicity-Eccentricity Relation.

    PubMed

    Bekki; Chiba

    2000-05-01

    Motivated by the recently improved knowledge on the kinematic and chemical properties of the Galactic metal-poor stars, we present the numerical simulation for the formation of the Galactic stellar halo to interpret the observational results. As a model for the Galaxy contraction, we adopt the currently standard theory of galaxy formation based on the hierarchical assembly of the cold dark matter fluctuations. We find, for the simulated stars with &sqbl0;Fe&solm0;H&sqbr0;stars is reproduced correctly for &sqbl0;Fe&solm0;H&sqbr0;halo is a natural consequence of the hierarchical evolution of the subgalactic clumps seeded from the cold dark matter density fluctuations.

  9. The outer halo of the nearest giant elliptical: a VLT/VIMOS survey of the resolved stellar populations in Centaurus A to 85 kpc

    NASA Astrophysics Data System (ADS)

    Crnojević, D.; Ferguson, A. M. N.; Irwin, M. J.; Bernard, E. J.; Arimoto, N.; Jablonka, P.; Kobayashi, C.

    2013-06-01

    We present the first survey of resolved stellar populations in the remote outer halo of our nearest giant elliptical (gE), Centaurus A (D = 3.8 Mpc). Using the VIsible Multi Object Spectrograph (VIMOS)/Very Large Telescope (VLT) optical camera, we obtained deep photometry for four fields along the major and minor axes at projected elliptical radii of ˜30-85 kpc (corresponding to ˜5-14Reff). We use resolved star counts to map the spatial and colour distribution of red giant branch (RGB) stars down to ˜2 mag below the RGB tip. We detect an extended halo out to the furthermost elliptical radius probed (˜85 kpc or ˜14Reff), demonstrating the vast extent of this system. We detect a localized substructure in these parts, visible in both (old) RGB and (intermediate-age) luminous asymptotic giant branch stars, and there is some evidence that the outer halo becomes more elliptical and has a shallower surface brightness profile. We derive photometric metallicity distribution functions for halo RGB stars and find relatively high median metallicity values (<[Fe/H]>med ˜ -0.9 to -1.0 dex) that change very little with radius over the extent of our survey. Radial metallicity gradients are measured to be ≈ -0.002-0.004 dex kpc-1, and the fraction of metal-poor stars (defined as [Fe/H] < -1.0) is ≈40-50 per cent at all radii. We discuss these findings in the context of galaxy formation models for the buildup of gE haloes.

  10. Spectroscopy of the Ca II Line in Metal-Poor Field Red Giants. II. Northern Hemisphere Observations

    NASA Astrophysics Data System (ADS)

    Dupree, A. K.; Smith, Graeme H.

    1995-07-01

    Echelle spectra of the Ca II K line (λ3933.7) for a sample of 53 metal-deficient field giants, including first-ascent red giant branch stars and asymptotic giant branch stars, demonstrate that chromospheres are ubiquitous among these objects. The atmospheres of the metal-deficient giants are found to be quite dynamic; motions in the deeper regions of the chromosphere appear to be irregular, and evidence for both outflows and infall has been found. An absolute magnitude of MV≍-1.7 corresponds to a critical brightness for metal-deficient giants; as stars evolve beyond this magnitude the atmospheric structure and dynamics affecting the Hα and Ca II K profiles undergo a change. Metal-poor giants brighter than this magnitude exhibit evidence of systematic outflows. However, no circumstellar component is seen in any of the spectra of the Ca II K line acquired in this project. The winds of bright Population II red giants are inferred to be hotter than those of Population I giants. The classic Wilson-Bappu effect is found to depend on metal abundance.

  11. A MegaCam Survey of Outer Halo Satellites. VI. The Spatially Resolved Star-formation History of the Carina Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Santana, Felipe A.; Muñoz, Ricardo R.; de Boer, T. J. L.; Simon, Joshua D.; Geha, Marla; Côté, Patrick; Guzmán, Andrés E.; Stetson, Peter; Djorgovski, S. G.

    2016-10-01

    We present the spatially resolved star-formation history (SFH) of the Carina dwarf spheroidal galaxy, obtained from deep, wide-field g and r imaging and a metallicity distribution from the literature. Our photometry covers ˜2 deg2, reaching up to ˜10 times the half-light radius of Carina with a completeness higher than 50% at g ˜ 24.5, more than one magnitude fainter than the oldest turnoff. This is the first time a combination of depth and coverage of this quality has been used to derive the SFH of Carina, enabling us to trace its different populations with unprecedented accuracy. We find that Carina’s SFH consists of two episodes well separated by a star-formation temporal gap. These episodes occurred at old (\\gt 10 Gyr) and intermediate (2-8 Gyr) ages. Our measurements show that the old episode comprises the majority of the population, accounting for 54 ± 5% of the stellar mass within 1.3 times the King tidal radius, while the total stellar mass derived for Carina is 1.60+/- 0.09× {10}6 {M}⊙ , and the stellar mass-to-light ratio is 1.8 ± 0.2. The SFH derived is consistent with no recent star formation, which hints that the observed blue plume is due to blue stragglers. We conclude that the SFH of Carina evolved independently of the tidal field of the Milky Way, since the frequency and duration of its star-formation events do not correlate with its orbital parameters. This result is supported by the age-metallicity relation observed in Carina and the gradients calculated indicating that outer regions are older and more metal-poor. Based on observations obtained with the MegaCam imager on the Magellan II-Clay telescope at Las Campanas Observatory in the Atacama Region, Chile. This telescope is operated by a consortium consisting of the Carnegie Institution of Washington, Harvard University, MIT, the University of Michigan, and the University of Arizona.

  12. Globular cluster clustering around ultra compact dwarf galaxies in the halo of NGC 1399

    NASA Astrophysics Data System (ADS)

    Voggel, Karina; Hilker, Michael; Richtler, Tom

    2016-08-01

    We tested the spatial distribution of UCDs and GCs in the halo of NGC 1399 in the Fornax cluster. In particular we tried to find out if globular clusters are more abundant in the vicinity of UCDs than what is expected from their global distribution. A local overabundance of globular clusters was found around UCDs on a scale of 1 kpc compared to what is expected from the large scale distribution of globulars in the host galaxy. This effect is stronger for the metal-poor blue GCs and weaker for the red GCs. An explanation for these clustered globulars is either that they are the remains of a GC system of an ancestor dwarf galaxy before it was stripped to its nucleus, which appears as UCD today. Alternatively these clustered GCs could have been originally part of a super star cluster complex.

  13. VizieR Online Data Catalog: Abundances of metal-poor star HD 94028 (Roederer+, 2016)

    NASA Astrophysics Data System (ADS)

    Roederer, I. U.; Karakas, A. I.; Pignatari, M.; Herwig, F.

    2016-06-01<