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1

An Infrared and Optical View of Young Eruptive Stars  

NASA Astrophysics Data System (ADS)

Young eruptive stars (FU Orionis- and EX Lupi-type objects) form a small, but remarkable subgroup of pre-main sequence stars. They are characterized by eruptions which may increase the brightness of the system by as much as a factor of 100. Outbursts probably correspond to increased accretion from the circumstellar disk to the star. In my dissertation I present the detailed analysis of three young eruptive stars using ground-based and space-borne observations at optical and infrared wavelengths. The outburst of OO Serpentis between 1995 and 2006. OO Ser is a deeply embedded young star that produced an eruption in 1995. Using archival (ISO, Spitzer) and new observations, I constructed the light curves of the star at ten different infrared wavelengths. My results show that the eruption caused brightening in the whole infrared regime, which is followed by a slow, wavelength-independent fading. The fading is still ongoing and the star will probably not return to quiescence before 2011. The timescale of the outburst is shorter than that of typical FU Ori-type objects, but longer than that of EX Lupi-type stars. Based on the spectral energy distribution, OO Ser seems to be an early Class I object with an age of <10^5 yr. In accordance with outburst models, the star is probably surrounded by an accretion disk and a dense envelope. Due to the shorter timescales, outburst models developed for FU Ori-type stars can only work for OO Ser if the viscosity of the circumstellar disk is set to an order of magnitude larger value than usual for FU Ori objects. The 2004-2006 outburst and environment of V1647 Orionis. V1647 Ori is a pre-main sequence star that suddenly brightened up in less than 4 months, reaching its peak brightness in 2004 February. I participated in a monitoring program, in which we obtained V R_C I_C J H K_s-band observations. After reaching its peak brightness, the star was fading very slowly. Then, in 2005 October it suddenly faded back to its quiescence level. The timescale of the outburst and the moderate increase in bolometric luminosity suggest that V1647 Ori differs both from FU Ori- and EX Lupi-type objects. By calculating the time delay between the brightness variations of the star and a nebular position, I deduced the geometry of the system. The near-infrared color maps of the nebula suggest that the star is surrounded by a disk and an extended nebula. High-resolution polarimetry of Parsamian 21. Parsamian 21 is an FU Orionis-type object consisting of a central star and an extended reflection nebula. We obtained high spatial resolution adaptive optics assisted near-infrared direct and polarimetric observations with the NACO instrument at ESO's Very Large Telescope. I complemented these measurements with archival Hubble, Spitzer and ISO data. The results revealed that (1) the star is probably indeed an FU Orionis-type object; (2) the star is not associated with any known rich cluster of young stars; (3) the star is surrounded by an edge-on disk, and a circumstellar envelope with a polar cavity; (4) the disk seems to be flat and extends from at least 48 to 360 AU from the star; (5) the spectral energy distribution can be successfully modeled with a circumstellar disk and an envelope; (6) within the framework of an evolutionary sequence of FU Ori-type stars, Parsamian 21 seems to be an intermediate-aged object.

Kóspál, Á.

2009-01-01

2

The outburst and nature of young eruptive low mass stars in dark clouds  

NASA Astrophysics Data System (ADS)

The FU Orionis (FUor) or EX Orionis (EXor) phenomenon has attracted increasing attention in recent years and is now accepted as a crucial element in the early evolution of low-mass stars. FUor and EXor eruptions of young stellar objects (YSOs) are caused by strongly enhanced accretion from the surrounding disk. FUors display optical outbursts of ˜ 4 mag or more and last for several decades, whereas EXors show smaller outbursts (?m ˜ 2 - 3 mag) that last from a few months to a few years and may occur repeatedly. Therefore, FUor/EXor eruptions represent a rare but very important phenomenon in early stellar evolution, during which a young low-mass YSO brightens by up to several optical magnitudes. Hence, long-term observations of this class of eruptive variables are important to design theoretical models of low-mass star formation. In this paper, we present recent results from our long-term monitoring observations of three rare types of eruptive young variables with the 2-m Himalayan Chandra Telescope (HCT) and the 2-m IUCAA Girawali Observatory (IGO) telescope.

Ninan, J. P.; Ojha, D. K.; Bhatt, B. C.; Mallick, K. K.; Tej, A.; Sahu, D. K.; Ghosh, S. K.; Mohan, V.

3

A PECULIAR YOUNG ERUPTIVE STAR IN THE DARK CLOUD LYNDS 1340  

SciTech Connect

We conducted a long-term optical photometric and spectroscopic monitoring of the strongly variable, accreting young sun-like star [KOS94] HA11, associated with the dark cloud Lynds 1340 that exhibited large amplitude (5-6 mag in the I{sub C} band) brightness variations on 2-3 years timescales, flat spectral energy distribution (SED), and extremely strong (300{approx}< EW/A {approx}< 900) H{alpha} emission. In this Letter we describe the basic properties of the star, derived from our observations between 1999 and 2011, and put into context the observed phenomena. The observed variations in the emission spectra, near-infrared colors, and SED suggest that [KOS94] HA11 (spectral type: K7-M0) is an eruptive young star, possibly similar in nature to V1647 Ori: its large-scale photometric variations are governed by variable accretion rate, associated with variations in the inner disk structure. The star recently has undergone strong and rapid brightness variations, thus its further observations may offer a rare opportunity for studying structural and chemical rearrangements of the inner disk, induced by variable central luminosity.

Kun, M.; Szegedi-Elek, E.; Moor, A.; Abraham, P.; Kelemen, J.; Pal, A.; Racz, M.; Regaly, Zs.; Szalai, N.; Szing, A. [Konkoly Observatory, H-1121 Budapest, Konkoly Thege ut 15-17 (Hungary); Acosta-Pulido, J. A. [Instituto de Astrofisica de Canarias, E-38200, La Laguna, Tenerife, Canary Islands (Spain); Apai, D. [Steward Observatory and Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 (United States); Szakats, R., E-mail: kun@konkoly.hu [Baja Astronomical Observatory of Bacs-Kiskun County, P.O. Box 766, 6500 Baja (Hungary)

2011-05-20

4

Second outburst phase of a young eruptive star V1647 Orionis (McNeil's nebula)  

NASA Astrophysics Data System (ADS)

Young low mass stars still embedded in dust and molecular gas pass through a stage of outbursts. These outbursts are due to sudden increase in accretion rate from the inner disc. V1647 Orionis underwent an FU Ori kind of outburst in 2004 and returned to its pre-outburst phase in early 2006. Within just 2 years it again underwent a second outburst in 2008; such an event is rarely seen in FU Ori type of outburst. We therefore followed the source in its second outburst phase from 2-m Himalayan Chandra Telescope (HCT) and 2-m IUCAA Girawali Observatory (IGO) Telescope. Our optical and near-infrared (NIR) photometric data show that the source is undergoing a slow but steady dimming of ~ 0.3 - 0.5 mag since the recent second outburst. It seems that the observed properties of the outburst of V1647 Ori are different from both the EX Ori and FU Ori type of outbursts, and suggest that this star probably represents a new type of eruptive young star, to be different from both FU Ori and EX Ori classes.

Ninan, J. P.; Ojha, D. K.; Mallick, K. K.; Ghosh, S. K.; Joshi, J. S.

2013-06-01

5

The 2008 Extreme Outburst of the Young Eruptive Variable Star EX Lupi  

NASA Astrophysics Data System (ADS)

In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over 5 mag for a period of seven months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter, we present new optical and near-IR high-resolution spectroscopy of EX Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the observed energetics and kinematics. Emission line strengths, widths, and profiles significantly changed between the two observations. Also, modeling of the 2.2935 ?m CO overtone band head emission suggests that an inner gap in the circumstellar gas disk around the star may be present and that it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ~2 ± 0.5 L sun and ~(2 ± 0.5) × 10-7 M sun yr-1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.

Aspin, Colin; Reipurth, Bo; Herczeg, Gregory J.; Capak, Peter

2010-08-01

6

The outburst of the eruptive young star OO Serpentis between 1995 and 2006  

NASA Astrophysics Data System (ADS)

Aims:OO Serpentis is a deeply embedded pre-main sequence star in the Serpens NW star-forming region. The star went into outburst in 1995 and gradually faded afterwards. In many respects its eruption resembled the well-known FU Orionis-type (FUor) or EX Lupi-type (EXor) outbursts. Since very few such events have ever been documented at infrared wavelengths, our aim is to study the temporal evolution of OO Ser in the infrared. Methods: OO Ser was monitored with the Infrared Space Observatory in the 3.6-100 ?m wavelength range, starting 4 months after peak brightness and covering a period of 20 months. Eight years later, in 2004-2006 we again observed OO Ser at 2.2 and 12 ?m from the ground and complemented this dataset with archival Spitzer observations also from 2004. We analysed these data with special attention to source confusion and constructed light curves at 10 different wavelengths as well as spectral energy distributions. Results: The outburst caused brightening in the whole infrared regime. According to the infrared light curves, OO Ser started a wavelength-independent fading after the peak brightness. Later the flux decay became slower but stayed practically wavelength-independent. The fading is still ongoing, and current fading rates indicate that OO Ser will not return to quiescent state before 2011. The outburst timescale of OO Ser seems to be shorter than that of FUors, but longer than that of EXors. Conclusions: The outburst timescale and the moderate luminosity suggest that OO Ser is different from both FUors and EXors, and shows some similarities to the recently erupted young star V1647 Ori. Based on its SED and bolometric temperature, OO Ser seems to be an early class I object, with an age of <105 yr. As proposed by outburst models, the object is probably surrounded by an accretion disc and a dense envelope. This picture is also supported by the wavelength-independence of the fading. Due to the shorter outburst timescales, models developed for FUors can only work for OO Ser if the viscosity parameter in the circumstellar disc, ?, is set to an order of magnitude higher value than usual for FUors. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries France, Germany, the Netherlands and the UK) with participation of ISAS and NASA. Tables 1 and 2 are only available in electronic form at http://www.aanda.org

Kóspál, Á.; Ábrahám, P.; Prusti, T.; Acosta-Pulido, J.; Hony, S.; Moór, A.; Siebenmorgen, R.

2007-07-01

7

Dynamics during outburst. VLTI observations of the young eruptive star V1647 Orionis during its 2003-2006 outburst  

NASA Astrophysics Data System (ADS)

Context. It is hypothesized that low-mass young stellar objects undergo eruptive phases during their early evolution. These eruptions are thought to be caused by highly increased mass accretion from the disk onto the star, and therefore play an important role in the early evolution of Sun-like stars, of their circumstellar disks (structure, dust composition), and in the formation of their planetary systems. The outburst of V1647 Ori between 2003 and 2006 offered a rare opportunity to investigate such an accretion event. Aims: By means of our interferometry observing campaign during this outburst, supplemented by other observations, we investigate the temporal evolution of the inner circumstellar structure of V1647 Ori, the region where Earth-like planets could be born. We also study the role of the changing extinction in the brightening of the object and separate it from the accretional brightening. Methods: We observed V1647 Ori with MIDI on the VLTI at two epochs in this outburst. First, during the slowly fading plateau phase (2005 March) and second, just before the rapid fading of the object (2005 September), which ended the outburst. We used the radiative transfer code MC3D to fit the interferometry data and the spectral energy distributions from five different epochs at different stages of the outburst. The comparison of these models allowed us to trace structural changes in the system on AU-scales. We also considered qualitative alternatives for the interpretation of our data. Results: We found that the disk and the envelope are similar to those of non-eruptive young stars and that the accretion rate varied during the outburst. We also found evidence for the increase of the inner radii of the circumstellar disk and envelope at the beginning of the outburst. Furthermore, the change of the interferometric visibilities indicates structural changes in the circumstellar material. We test a few scenarios to interpret these data. We also speculate that the changes are caused by the fading of the central source, which is not immediately followed by the fading of the outer regions. Conclusions: We found that most of our results fit in the canonical picture of young eruptive stars. Our study provided dynamical information from the regions of the innermost few AU of the system: changes of the inner radii of the disk and envelope. However, if the delay in the fading of the disk is responsible for the changes seen in the MIDI data, the effect should be confirmed by dynamical modeling. Based on observations made with ESO telescopes at the Paranal Observatory under program IDs 274.C-5026 and 076.C-0736. In addition, this work is based in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

Mosoni, L.; Sipos, N.; Ábrahám, P.; Moór, A.; Kóspál, Á.; Henning, Th.; Juhász, A.; Kun, M.; Leinert, Ch.; Quanz, S. P.; Ratzka, Th.; Schegerer, A. A.; van Boekel, R.; Wolf, S.

2013-04-01

8

A young star's hectic months  

NASA Astrophysics Data System (ADS)

We present preliminary results ol the optical photometric and spectroscopic monitoring ol the young eruptive variable star PV Cephei, performed during its recent conspicuous lading in 2008-2009, and try to find the reasons of the observed variations.

Elek, Elza; Kun, Mária

2010-03-01

9

Activity on young stars.  

NASA Astrophysics Data System (ADS)

Simultaneous photometry and spectroscopy were made of 6 young stars during two observing periods mainly to study short-term variability on time-scales from minutes to a few hours. The material includes two classical T Tauri stars (CTTS): SY Ori and VW Cha; three T Tauri stars with weak emission line spectra (WTTS): San 1, SZ Cha and ADA 481 and one post-T Tauri candidate: HD 70309B. Both UBV and Stroemgren photometry was made. In the visible spectral region we resolved rapid fluctuations - events - with total amplitudes of about 5% (0.05 magnitudes). In the ultraviolet, the corresponding limit of detection was usually <=10%. On the basis of totally about 100 hours of monitoring we conclude that the normal state of these stars is that they are completely constant in brightness or that they vary only slowly with small amplitudes over several hours. Only a few percent of the time, on the average, is a given star caught at brightness changes >=0.2mag. during one night. No event reached a total amplitude of >=0.3mag. VW Cha is the most active star, but no events were seen on SY Ori and HD 70309B. This confirms earlier indications that powerful "flaring" on T Tauri stars is not frequent. We make a detailed study of all events and find two types of slow events, usually with d(U or u)/dt<=0.1mag/hour. One is caused by changes in the continuous emission (the veiling) superimposed on the stellar photospheric spectrum and operates mainly on VW Cha. These events have nothing to do with stellar surface flares of the type observed on flare stars and we suggest that they originate from inhomogeneous mass accretion from a circumstellar disk to the stellar surface. The time-scales support models with magnetically controlled accretion along the stellar dipole field to rings or spots at the stellar surface. The other type of event appears to originate from relatively rapid changes in the opacity of circumstellar dust in the line-of-sight to the star. This effect dominates on SZ Cha, a WTTS surrounded by a substantial dust reservoir. Also for the rapid events we distinguish two types. On two WTTS we detected a few flare-like events produced by a sudden increase in emission in the Balmer continuum and the Balmer lines and no detectable change of the continuum long-ward of the Balmer jump. With only UBV photometry the Balmer flares could erronously been interpreted as very hot blackbody radiators. We suggest that these events are genuine surface flares with total energies of 10^33^ to 10^34^erg, and discuss the implication of energy supply. On ADA 481 we detected 2 flare-like events in white light. If these are due to the ignition of a source of blackbody radiation, the inferred temperature of the flare is low compared to what is normally observed for flare stars. Even though the events are rare and have small total amplitudes in UV, they are extremely powerful, with the same total energies as the largest flares seen on flare stars. The flare stars may show much larger changes in UV, but the difference comes from the lower contrast of the flares on the TTS. If all TTS have surface magnetic activity similar to the flare stars, only the radii being larger, then we conclude that the frequency distribution of the flare-like events on WTTS are similar to flare stars in the field, but much higher than for the dwarfs in the Pleiades. No flare-like event was seen on the CTTS and we discuss possible implications. For the long-term changes (over days) we conclude that very dark spots on the rotating surfaces of SY Ori and San 1 dominates, while VW Cha varies because of variable veiling, but with an uncertain period. For SZ Cha variable circumstellar extinction operates, also in phase with the hydrogen line absorption. The situation for ADA 481 is still unclear. HD 70309B did not vary.

Gahm, G. F.; Loden, K.; Gullbring, E.; Hartstein, D.

1995-09-01

10

Modeling Giant Eruptions and Inflated Envelopes of LBV Stars  

NASA Astrophysics Data System (ADS)

The Luminous Blue Variable (LBV) stage of massive stars can refer to both S-Doradus variations, characterized by blue-red excursions at nearly constant bolometric luminosity, and eta-Carinae type giant eruptions, characterized by a significant increase in Lbol accompanied by large mass ejection. This talk will discuss how mass-loss-limited envelope inflation might account for S-Dor variations, while eta-Car-type giant eruptions can result from super-Eddington, continuum-driven mass loss that is triggered and moderated by porosity of the stellar envelope.

Owocki, Stan

2013-06-01

11

Silicate Crystals Form in Disk of Erupting Star  

NASA Video Gallery

This artist's animation illustrates how silicate crystals like those found in comets can be created by an outburst from a growing star. In April 2008, NASA's Spitzer Space Telescope detected evidence of this process taking place on the disk of a young sun-like star called EX Lupi.

NASA

2010-05-05

12

Eruptions at Lone Star Geyser, Yellowstone National Park, USA: 1. Energetics and eruption dynamics  

NASA Astrophysics Data System (ADS)

Geysers provide a natural laboratory to study multiphase eruptive processes. We present results from a 4 day experiment at Lone Star Geyser in Yellowstone National Park, USA. We simultaneously measured water discharge, acoustic emissions, infrared intensity, and visible and infrared video to quantify the energetics and dynamics of eruptions, occurring approximately every 3 h. We define four phases in the eruption cycle (1) a 28±3 min phase with liquid and steam fountaining, with maximum jet velocities of 16-28 m s-1, steam mass fraction of less than ˜0.01. Intermittently choked flow and flow oscillations with periods increasing from 20 to 40 s are coincident with a decrease in jet velocity and an increase of steam fraction; (2) a 26±8 min posteruption relaxation phase with no discharge from the vent, infrared (IR), and acoustic power oscillations gliding between 30 and 40 s; (3) a 59±13 min recharge period during which the geyser is quiescent and progressively refills, and (4) a 69±14 min preplay period characterized by a series of 5-10 min long pulses of steam, small volumes of liquid water discharge, and 50-70 s flow oscillations. The erupted waters ascend from a 160-170°C reservoir, and the volume discharged during the entire eruptive cycle is 20.8±4.1 m3. Assuming isentropic expansion, we calculate a heat output from the geyser of 1.4-1.5 MW, which is <0.1% of the total heat output from Yellowstone Caldera.

Karlstrom, Leif; Hurwitz, Shaul; Sohn, Robert; Vandemeulebrouck, Jean; Murphy, Fred; Rudolph, Maxwell L.; Johnston, Malcolm J. S.; Manga, Michael; McCleskey, R. Blaine

2013-08-01

13

Young Stars in  

Microsoft Academic Search

The \\u000a r<\\/font\\u000a>\\\\rho\\u000a Ophiuchi molecular cloud hosts the most nearby embedded cluster with more than 150 young stars. Intensively studied since\\u000a roughly twenty years, a significant fraction of the very low mass cluster members is still to be identified. The high level\\u000a of dust extinction (\\u000a AVA_{\\\\rm V}\\u000a up to 40mag), affecting at most wavelengths the emission arising from the

Sylvain Bontemps

2002-01-01

14

Candidates for Young Super Star Clusters in the Milky Way  

NASA Astrophysics Data System (ADS)

Massive Star Clusters (M > 104 M?) have been known to exist throughout the local Universe, but few such objects have been found within our own Galaxy. These clusters the majority of the galactic OB star formation, and thus dramatically alter their surroundings through winds, ionizing flux and radiation pressure, and supernovae, eventually destroying their natal clouds and inflating superbubbles which will erupt from the Galactic plane. We search for the young stellar clusters within the star forming complexes identified by Rahman & Murray (2010) using the WMAP free-free and Spitzer GLIMPSE 8 micron observations. Located far across the Galactic plane, these clusters are highly extinguished and crowded by field stars. Using the 2MASS catalogue, we have developed a method of identifying overdensities of sources with colours consistent with the extinguished upper main sequence coincident with the star forming complexes. The difficulty in this method comes from the large number of overlapping foreground sources in comparison to the expected number of cluster sources in any given candidate cluster. We identify a candidate for the most massive young cluster in the Galaxy (M 105 M?), which we have dubbed the Dragonfish Cluster. The candidate cluster is at a distance of 9.7 kpc and has a total ionizing luminosity of 7×1051 photons s-1. We identify nearly 400 OB star candidates associated with the cluster, to be confirmed with near-infrared spectroscopy.

Rahman, Mubdi; Matzner, C. D.; Moon, D.

2011-01-01

15

Accretion Disks around Young Stars  

NASA Astrophysics Data System (ADS)

A method to calculate the structure and brightness distribution of accretion disks surrounding low and intermediate mass young stars is introduced and discussed. The method includes a realistic treatment of the energy transport mechanisms and disk heating by radiation from external sources. The disk is assumed steady, geometrically thin and in vertical hydrostatic equilibrium. The turbulent viscosity coefficient is expressed using the ? prescription and the ? parameter and the mass accretion rate are assumed to be constant through the disk. Energy is transported in the vertical direction by: (a) a turbulent flux, computed self-consistently with the viscosity coefficient used to describe the viscous energy dissipation, (b) radiation, using the first moments of the transfer equation, the Eddington approximation, and the Rosseland and Planck Mean Opacities, and (c) convection, taking into account that the convective elements, not necessarily optically thick, lose energy by radiation and turbulent flux. This treatment of the energy transport mechanisms differs from previous work in this field, allowing one to extend, with confidence, the calculation of the disk structure to optically thin regimes. The heating mechanisms considered, which affect the disk's structure and emission, are stellar radiation and a circumstellar envelope which reprocesses and scatters radiation from the star and from the disk itself. In addition to a detailed numerical calculation, an analytical self-consistent formulation of the irradiation of the disk is given. This analytical formulation allows one to understand and extend the numerical results. To evaluate the potential of the method presented in this thesis, a set of models of viscous non-irradiated and irradiated disks are computed. Their predictions are compared with observations of young stellar sources likely to have disks. Given the disk structure and specifying its orientation with respect to the line of sight, the specific intensity distribution is calculated on the plane of the sky, integrating the radiative transfer equation along rays parallel to the line of sight. To this end, monochromatic opacities are used, which also allow us to construct tables of the Rosseland and Planck Mean Opacities. The disk structure and brightness distribution thus obtained are self-consistent with respect to the abundances and optical properties of the gas and the dust. With the disk intensity distribution, its spectrum is constructed and its colors are calculated in different spectral ranges. These are compared to observations of low mass young stars reported in the literature, for which the disk parameters are then inferred. It is found that the observed properties of a large fraction of classical T Tauri stars can be explained as emission from viscous disks irradiated by the central star or by a thin envelope and that the emission in the long wavelength range from a flat spectrum source like HL Tau is consistent with the predictions of a model in which a viscous disk is irradiated by an optically thick infalling envelope.

D'Alessio, Paola

1996-04-01

16

Searching for Young Stars in Northern Orion  

NASA Astrophysics Data System (ADS)

The Orion Molecular Cloud contains many known star-forming regions mostly located in the southern parts of the constellation. However, northern Orion is largely unsurveyed outside of a few well-established clusters meaning there could be more sites of ongoing star formation. We have conducted a search for young stars in northern Orion to find new star-forming regions. Using the MG1 Variable Star Survey we identified 2118 variable stars spanning a region of 30 deg2 from R.A.=4h 00m to 6h 30m and Dec=2.9 to 3.7 degrees. These stars’ variability could result from accretion or spots, which are common characteristics of young stars. We use several methods to detect candidate young stars from these data: selection cuts with color-magnitude diagrams (CMDs), measurement of proper motions and visual inspection of the source images. We make cuts to only include stars that have CMD positions consistent with the Orion sequence, have proper motions within 3 sigma of known Orion members, and are not contaminated by other nearby sources. These cuts identify an area between 5h 20m and 5h 52m in R.A. with a significant overdensity of 74 young star candidates. We will discuss in detail our selection cuts and the implication of these discoveries. This work was conducted by a Research Experience for Undergraduates (REU) position at the University of Hawai'i's Institute for Astronomy and funded by the NSF.

Urban, Laurie; Kraus, A.

2011-01-01

17

Dynamics in Young Star Clusters: From Planets to Massive Stars  

NASA Astrophysics Data System (ADS)

The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the conditions under which star clusters form and the impact of these often harsh environments on the evolution of their stellar and substellar members. We demonstrate the vital role numerical simulations play to uncover both key issues. Using dynamical models of different star cluster environments we show the variety of effects stellar interactions potentially have. Moreover, our significantly improved measure of mass segregation reveals that it can occur rapidly even for star clusters without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions.

Olczak, C.; Spurzem, R.; Henning, Th.; Kaczmarek, T.; Pfalzner, S.; Harfst, S.; Portegies Zwart, S.

18

Variability of Young Stars: the Importance of Keeping an Eye on Children  

NASA Astrophysics Data System (ADS)

(Abstract only) I will review the state of our understanding of young stars with an emphasis on how and why they vary in brightness. The main causes of the variations will be reviewed, including the rotation of spotted weak-lined T Tauri stars, accretion onto classical T Tauri stars, the eruptive behavior of FUors, and the enigmatic variations of the UXors. The important role that amateurs have and will continue to play in these studies is highlighted. I will also discuss the latest results on two unusual young binaries, BM Orionis in the Trapezium asterism and KH 15D in NGC 2264.

Herbst, W.

2013-06-01

19

Young Star Clusters: Lighthouses in the Dark  

Microsoft Academic Search

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. Young clusters are observed to form in a variety of such galaxies a substantial number resembling the well-studied globular clusters in mass and size. Due to the high angular resolution instruments available at present and in the near future such

Peter Anders; Richard de Grijs; Uta Fritze-v. Alvensleben

2003-01-01

20

Young Massive Star Clusters in Spiral Galaxies  

Microsoft Academic Search

Abstract. Using ground-based UBVRIH,CCD photometry we have been carrying out a search for young massive star clus- ters (YMCs) in a sample consisting of 21 nearby spiral galaxies. We find a large variety concerning the richness of the cluster sys- tems, with some galaxies containing no YMCs at all and others hosting very large numbers of YMCs. Examples of galaxies

Soeren S. Larsen; T. Richtler

1999-01-01

21

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula.  

PubMed

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor approximately 50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeil's nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star. PMID:15269761

Kastner, J H; Richmond, M; Grosso, N; Weintraub, D A; Simon, T; Frank, A; Hamaguchi, K; Ozawa, H; Henden, A

2004-07-22

22

Periodic flow instabilities during Lone Star Geyser (YNP) eruptions, as deduced from acoustic measurements  

NASA Astrophysics Data System (ADS)

We performed continuous acoustic measurements during four days at Lone Star Geyser, Yellowstone National Park, USA. The microphone was located at 10 meters from the geyser's cone, and the acoustic signal was sampled at 1000 Hz. The 3-hour-long eruptive cycle at Lone Star Geyser contains several water fountaining episodes followed by the main eruption, which generally lasts 25 minutes. During the 30 main eruptions that we studied, the acoustic signal patterns are very similar, and indicate the flow is unstable and clearly follows a pulsating regime. The period of the acoustic pulses drastically increases during the liquid to steam transition in the flow. This abrupt change in the flow regime corresponds to the start of the ground deflation recorded by tiltmeters, and could be due to a transition from hydro-static to vapor-static conditions in the vent.

Vandemeulebrouck, J.; Hurwitz, S.; Johnston, M. J.; Rudolph, M. L.; Karlstrom, L.; Sohn, R. A.; Murphy, F.; McPhee, D. K.; Glen, J. M.; Soule, S. A.; Meertens, C. M.

2011-12-01

23

Young Star Clusters: Lighthouses in the Dark  

NASA Astrophysics Data System (ADS)

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. Young clusters are observed to form in a variety of such galaxies a substantial number resembling the well-studied globular clusters in mass and size. Due to the high angular resolution instruments available at present and in the near future such young clusters can be observed up to much further distances and in more detail than previously possible. Clusters in the inner parts of bright galaxies and along tidal tails are studied (e.g. ""Tadpole"" galaxy). The determination of precise radii and masses is crucial for survivability estimates. By studying young clusters in merger remnants and peculiar galaxies we can learn about the violent star formation history of these galaxies and about galaxy formation and evolution. We will present a new set of evolutionary synthesis models specially developed to account for the gaseous emission of presently forming star clusters and a new tool to compare large model grids with multi-color observations. First results of newly-born clusters in mergers and starburst galaxies are presented and compared to the well-studied old globulars and interpreted in the framework of galaxy formation / evolution.

Anders, Peter; de Grijs, Richard; Fritze-v. Alvensleben, Uta

24

Young Star Probably Ejected From Triple System  

NASA Astrophysics Data System (ADS)

Astronomers analyzing nearly 20 years of data from the National Science Foundation's Very Large Array radio telescope have discovered that a small star in a multiple-star system in the constellation Taurus probably has been ejected from the system after a close encounter with one of the system's more-massive components, presumed to be a compact double star. This is the first time any such event has been observed. Path of Small Star, 1983-2001 "Our analysis shows a drastic change in the orbit of this young star after it made a close approach to another object in the system," said Luis Rodriguez of the Institute of Astronomy of the National Autonomous University of Mexico (UNAM). "The young star was accelerated to a large velocity by the close approach, and certainly now is in a very different, more remote orbit, and may even completely escape its companions," said Laurent Loinard, leader of the research team that also included Monica Rodriguez in addition to Luis Rodriguez. The UNAM astronomers presented their findings at the American Astronomical Society's meeting in Seattle, WA. The discovery of this chaotic event will be important for advancing our understanding of classical dynamic astronomy and of how stars evolve, including possibly providing an explanation for the production of the mysterious "brown dwarfs," the astronomers said. The scientists analyzed VLA observations of T Tauri, a multiple system of young stars some 450 light-years from Earth. The observations were made from 1983 to 2001. The T Tauri system includes a "Northern" star, the famous star that gives its name to the class of young visible stars, and a "Southern" system of stars, all orbiting each other. The VLA data were used to track the orbit of the smaller Southern star around the larger Southern object, presumed to be a pair of stars orbiting each other closely. The astronomers' plot of the smaller star's orbit shows that it followed an apparently elliptical orbit around its twin companions, moving at about 6 miles per second. Then, between 1995 and 1998, it came within about 200 million miles (about two times the distance between the Sun and the Earth) of its companions. Following that encounter, it changed its path, moving away from its companion at about 12 miles per second, double its previous speed. "We clearly see that this star's orbit has changed dramatically after the encounter with its larger companions," said Luis Rodriguez. "By watching over the next five years or so, we should be able to tell if it will escape completely," he added. "We are very lucky to have been able to observe this event," said Loinard. Though studies with computer simulations long have shown that such close approaches and stellar ejections are likely, the time scales for these events in the real Universe are long -- thousands of years. The chance to study an actual ejection of a star from a multiple system can provide a critical test for the dynamical theories. If a young star is ejected from the system in which it was born, it would be cut off from the supply of gas and dust it needs to gain more mass, and thus its development would be abruptly halted. This process, the astronomers explain, could provide an explanation for the very-low-mass "failed stars" called brown dwarfs. "A brown dwarf could have had its growth stopped by being ejected from its parent system," Loinard said. The VLA observations were made at radio frequencies of 8 and 15 GHz. T Tauri, the "Northern" star in this system, is a famous variable star, discovered in October of 1852 by J.R. Hind, a London astronomer using a 7-inch diameter telescope. At its brightest, it is some 40 times brighter than when at its faintest. It has been studied extensively as a nearby example of a young stellar system. While readily accessible with a small telescope, it is not visible to the naked eye. The observed orbital changes took place in the southern components of the system, displaced from the visible

2003-01-01

25

A Young Man with a Persistent Skin Eruption  

Microsoft Academic Search

There are many types of skin disease that fit into the classification of cutaneous lymphoma, but mycosis fungoides is by far the most common of this group. It is a non-Hodgkin's lymphoma of T-cell origin that presents in the skin. Mycosis fungoides often evolves for years without a specific diagnosis because it can present as an eczematous or psoriasiform eruption.

Christina G. Kendrick; Michelle S. Gerdes; Fred A. Lopez; Elizabeth I. McBurney

26

Early Earth closely surrounded by young stars  

Microsoft Academic Search

Knowledge of the physical and chemical conditions on the primeval Earth is important for the study of the origin of the biosphere.\\u000a This paper discusses the latest modification of the theory of the origin of the Earth and other planets. Possible consequences\\u000a of the formation of the Sun in the area of the star formation closely surrounded by neighboring young

A. V. Vityazev; G. V. Pechernikova

2010-01-01

27

On the properties of young multiple stars  

NASA Astrophysics Data System (ADS)

We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N-body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ~1-103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ~60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (~80 per cent) and that this fraction is an increasing function of primary mass. After N-body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour-magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4-1 Msolar range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and ? Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour-magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations.

Delgado-Donate, E. J.; Clarke, C. J.; Bate, M. R.; Hodgkin, S. T.

2004-06-01

28

Dust and Gas Around Young Stars  

NASA Astrophysics Data System (ADS)

Asteroids, planets, and comets are believed to form within circumstellar disks of gas and dust around young stars (ages < 100 Myr). We have obtained 10 - 20 ? m images, using the Keck I telescope and FUSE spectra of Herbig Ae and main sequence stars to characterize the dust and gas in these systems. We report the following results: (1) For zeta Lep, we find that the majority of dust is located within 6 AU from the star. Since the Poytning-Robertson drag lifetime of grains around this star is 104 years, significantly shorter than the 300 Myr year age of the system, we infer the presence of parent bodies with a total mass 200 times that of the main asteroid belt in our solar system. (2) For the binary system sigma Herculis, which possess a Vega-like infrared excess, we observe circumstellar C 2*, N 2* and N 2**, blueshifted by as much as 40 km/sec. We propose that there is a radiatively driven wind, generated by sigma Her's high luminosity. In this model, the material in the wind is created through collisions between parent bodies at 20 AU from the star, the approximate distance at which blackbodies are in radiative equilibrium with the star and at which 3-body orbits become unstable. (3) For the pre main sequence star AB Aur, we measure a dust mass 6x10-9 M? , significantly less than the 10-4 M? inferred from millimeter photometry, suggesting the presence of a cold optically thick disk. We find that models which include dust envelopes fit the data somewhat better than models which incorporate flared circumstellar disks.

Chen, C. H.

2002-12-01

29

Characterisation of young, nearby stars - The Ursa-Major group  

Microsoft Academic Search

The Ursa Major group is composed of the Ursa Major open star cluster in the Big Dipper constellation and of many co-moving stars spread over the whole sky. The stars of the Ursa Major group have a relatively young age of 200-600 million years. Taking into account their proximity and their common motion, the youth makes these stars interesting targets

M. Ammler

2007-01-01

30

New Young Star Candidates in CG4 and Sa101  

NASA Astrophysics Data System (ADS)

The CG4 and Sa101 regions together cover a region of ~0.5 deg2 in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6-70 ?m) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two Micron All Sky Survey. We find infrared excesses in all six of the previously identified young stars in our maps and identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.

Rebull, L. M.; Johnson, C. H.; Hoette, V.; Kim, J. S.; Laine, S.; Foster, M.; Laher, R.; Legassie, M.; Mallory, C. R.; McCarron, K.; Sherry, W. H.

2011-07-01

31

Many faces of young neutron stars  

NASA Astrophysics Data System (ADS)

The hardware aspect of this thesis consists in the design, fabrication and assembly of twin analog Flexible Filter Banks at Caltech. These are user-friendly, workhorse, radio-pulsar search and timing instruments. Novel features include the flexibility in configuring channel center-frequencies and widths, the rapid sampling down to 25 ?s and a total instrument bandwidth ranging from a narrow 0.2 MHz to a mammoth 100 MHz. Frequency synthesis is used to downconvert, detect and sample the telescope receiver bandpass as 32 separate time-series in each polarization. The collected data are later subjected to standard pulsar search and timing algorithms in software.The vital scientific issue addressed here is the nature of young neutron stars. In the standard picture, young neutron stars are rapidly spinning radio-luminous pulsars, which may also display pulsed emission at high X-ray and ?-ray energies. However there is no evidence that all neutron stars are born according to this standard picture. We present radio or X-ray investigations of steady nebular emission produced by three clearly non-standard and ill-understood objects. In all likelihood, these are young neutron stars, a notion upheld by their association with young Galactic supernova remnants.Based on its display of high energy transients, the soft ?-ray repeater SGR 1806-20 is posited to be a seismically active "magnetar", i.e., a neutron star with a super-strong magnetic field (10[superscript 15] G) nearly three orders of magnitude greater than pulsar dipolar fields. Our VLA observations of fleeting small-scale structure around SGR 1806-20 provide intriguing, although preliminary, support for the magnetar model. In time, similar observations could unravel the riddle of soft ?-ray repeaters and possibly establish the reality of magnetars.X-ray observations of the remnant of the historical supernova of 386 A.D., SNR G 11.2-0.3 are presented. The nature of an embedded underlumnious plerion discovered in these observations argues for a central neutron star very different from the prototypical Crab pulsar. The urgency to undertake a large scale study of young and hollow Galactic shells in broadband X-rays with fine spatial resolution is elucidated.X-ray spectroscopy of the object 1E 1207.4-5209 at the core of the large remnant PKS 1209-51/52 has revealed a non-thermal source with a very steep spectrum. After considering various scenarios for lE 1207.4-5209, we conclude that its spectral signature, its lack of optical emission and its position at the center of a supernova remnant make it a source similar to the mysterious anomalous X-ray pulsars.A large and sensitive search for radio pulsar companions of massive stars was undertaken. Primary motivation stems from the recent discovery of binary radio pulsar B 1259-63 as the first member of such a population and a "missing link" in the current models of evolution. Prevalent expectations, based on binary evolution scenarios, suggested that many more such systems should exist and would be uncovered in sensitive targeted searches. Together with other smaller searches, this survey uncovered no pulsars orbiting early-type stars. We conclude that such binary systems must be rare.

Vasisht, Gautam

32

X-ray emission of young solar type stars  

Microsoft Academic Search

T Tauri Stars (TTS) are young (<= 107 yrs) low mass (<= 2 Modot) stars. They have been originally characterized by strong emission lines (CTTS), and by IR excesses interpreted in terms of circumstellar disks. Ten years ago, the ``Einstein\\

Sophie Casanova

1994-01-01

33

A-Train Observations of Young Volcanic Eruption Clouds  

NASA Astrophysics Data System (ADS)

NASA's A-Train satellite constellation (including Aqua, CloudSat, CALIPSO, and Aura) has been flying in formation since 2006, providing unprecedented synergistic observations of numerous volcanic eruption clouds in various stages of development. Measurements made by A-Train sensors include total column SO2 by the Ozone Monitoring Instrument (OMI) on Aura, upper tropospheric and stratospheric (UTLS) SO2 column by the Atmospheric Infrared Sounder (AIRS) on Aqua and Microwave Limb Sounder (MLS) on Aura, ash mass loading from AIRS and the Moderate resolution Imaging Spectroradiometer (MODIS) on Aqua, UTLS HCl columns and ice water content (IWC) from MLS, aerosol vertical profiles from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument aboard CALIPSO, and hydrometeor profiles from the Cloud Profiling Radar (CPR) on CloudSat. The active vertical profiling capability of CALIPSO, CloudSat and MLS sychronized with synoptic passive sensing of trace gases and aerosols by OMI, AIRS and MODIS provides a unique perspective on the structure and composition of volcanic clouds. A-Train observations during the first hours of atmospheric residence are particularly valuable, as the fallout, segregation and stratification of material in this period determines the concentration and altitude of constituents that remain to be advected downwind. This represents the eruption 'source term' essential for dispersion modeling, and hence for aviation hazard mitigation. In this presentation we show examples of A-Train data collected during recent eruptions including Chaitén (May 2008), Kasatochi (August 2008), Redoubt (March 2009), Eyjafjallajökull (April 2010) and Cordón Caulle (June 2011). We interpret the observations using the canonical three-stage view of volcanic cloud development [e.g., Rose et al., 2000] from initial rapid ash fallout to far-field dispersion of fine ash, gas and aerosol, and results from numerical modeling of volcanic plumes [e.g., Textor et al., 2003] and discuss the degree to which the observations validate existing theory and models. We also describe plans for advanced SO2 and ash retrieval algorithms that will exploit the synergy between UV and IR sensors in the A-Train for improved quantification of ash and SO2 loading by volcanic eruptions.

Carn, S. A.; Prata, F.; Yang, K.; Rose, W. I.

2011-12-01

34

New Young Star Candidates in BRC 27  

NASA Astrophysics Data System (ADS)

All stars originate from clouds of interstellar gas that collapse either under their own gravity or with external help. In triggered star formation, the collapse of a cloud is initiated by pressure, e.g., from nearby star(s). When the external source is bright stars, it can illuminate the rims of the cloud, creating bright-rimmed clouds (BRCs) to be visible at optical and infrared (IR) wavelengths. We searched for new candidate young stellar objects (YSOs) primarily using the March 2012 all-sky release of Wide-field Infrared Survey Explorer (WISE) data in BRC 27, which is part of CMa R1, a region of known star formation. Spitzer data of a 5’x5’ region centered on BRC 27 were presented by Johnson et al. 2012 and Rebull et al. 2012. We investigated WISE data within a 20 arcminute radius of BRC 27 0.35 sq. deg), combining it with Spitzer data serendipitously obtained in this region, 2MASS data, and optical data. We started from nearly 4000 WISE sources and identified about 200 candidate YSOs via a series of color cuts (Koenig et al. 2012) to identify objects with WISE colors consistent with other YSOs, e.g., having an apparent IR excess. There are about 100 objects in this region already identified in the literature as possible YSOs, about 40 of which we recovered with the color cuts. We investigated these literature YSOs and YSO candidates in all available images, and created spectral energy distributions (SEDs) and color-magnitude diagrams for further analysis of each object. We will present an analysis of our selected sub-sample of YSO candidates. This research was made possible through the NASA/IPAC Teacher Archive Research Project (NITARP) and was funded by NASA Astrophysics Data Program and Archive Outreach funds. Our education results are described in a companion education poster, Bonadurer et al.

Novatne, Lauren J.; Mattrocce, G.; Milan, T.; Quinonez, A.; Rebull, L. M.; Barge, J.; Amayo, R.; Bieber, H.; Block, L.; Cheung, E.; Cruz, A.; Elkin, D.; Figueroa, A.; Jakus, M.; Kelo, A.; Larson, O.; Lemma, B.; Li, Y.; Loe, C.; Maciag, V.; Moreno, N.; Nevels, M.; Pezanoski-Cohen, G.; Short, M.; Skatchke, K.; Tur-Kaspa, A.; Zegeye, D.; Armstrong, J.; Bonadurer, R.; French, D.; Free, B.; Miller, C.; Scherich, H.; Willis, T.; Koenig, X.; Laher, R.; Padgett, D.; Piper, M.; Pavlak, A.; Piper, M.; Venezio, E.; Ali, B.

2013-01-01

35

Embedded young stars in northern NGC 3372  

NASA Astrophysics Data System (ADS)

Deep narrow-band (Br? at 2.17?m and H_2 at 2.12?m) and broad-band (JHKs) near-infrared images are presented of the young regions Tr14-N4 and Car I. The observations were made with PANIC, attached to the Baade 6.5 m Magellan Telescope at Las Campanas Observatory. These were supplemented by archive Spitzer/IRAC images. Evidence is given of an embedded young compact cluster (r=23'') of medium-to-low mass stars in Tr14-N4 but that includes at least one high mass protostar. Its properties are derived from the 1.2 to 11?m photometry. The presence of outflows is evinced by knotty H_2 emission near the edge of a cavity. We also presented updates of near-IR light-curves of Car I-136 and 125 embedded in the dark cloud associated with Car I.

Tapia, M.; Roth, M.; Persi, P.

2011-10-01

36

The Astrophysical Implications of Dust Formation during the Eruptions of Hot, Massive Stars  

NASA Astrophysics Data System (ADS)

Dust formation in the winds of hot stars is inextricably linked to the classic eruptive state of luminous blue variables because it requires very high mass-loss rates, \\dot{M}\\gtrsim 10^{-2.5}\\,M_\\odot year-1, for grains to grow and for the non-dust optical depth of the wind to shield the dust formation region from the true stellar photosphere. Thus, dusty shells around hot stars trace the history of "great" eruptions, and the statistics of such shells in the Galaxy indicate that these eruptions are likely the dominant mass-loss mechanism for evolved, M ZAMS >~ 40 M ? stars. Dust formation at such high \\dot{M} also explains why very large grains (a max >~ 1 ?m) are frequently found in these shells, since a_{max}\\propto \\dot{M}. The statistics of these shells (numbers, ages, masses, and grain properties such as a max) provide an archaeological record of this mass-loss process. In particular, the velocities v shell, transient durations (where known), and ejected masses M shell of the Galactic shells and the supernova (SN) "impostors" proposed as their extragalactic counterparts are very different. While much of the difference is a selection effect created by shell lifetimes \\propto (v_{shell}\\sqrt{M_{shell}})^{-1}, more complete Galactic and extragalactic surveys are needed to demonstrate that the two phenomena share a common origin given that their observed properties are essentially disjoint. If even small fractions (1%) of SNe show interactions with such dense shells of ejecta, as is currently believed, then the driving mechanism of the eruptions must be associated with the very final phases of stellar evolution, suggestive of some underlying nuclear burning instability.

Kochanek, C. S.

2011-12-01

37

THE ASTROPHYSICAL IMPLICATIONS OF DUST FORMATION DURING THE ERUPTIONS OF HOT, MASSIVE STARS  

SciTech Connect

Dust formation in the winds of hot stars is inextricably linked to the classic eruptive state of luminous blue variables because it requires very high mass-loss rates, M-dot {approx}>10{sup -2.5} M{sub sun} year{sup -1}, for grains to grow and for the non-dust optical depth of the wind to shield the dust formation region from the true stellar photosphere. Thus, dusty shells around hot stars trace the history of 'great' eruptions, and the statistics of such shells in the Galaxy indicate that these eruptions are likely the dominant mass-loss mechanism for evolved, M{sub ZAMS} {approx}> 40 M{sub Sun} stars. Dust formation at such high M-dot also explains why very large grains (a{sub max} {approx}> 1 {mu}m) are frequently found in these shells, since a{sub max}{proportional_to} M-dot . The statistics of these shells (numbers, ages, masses, and grain properties such as a{sub max}) provide an archaeological record of this mass-loss process. In particular, the velocities v{sub shell}, transient durations (where known), and ejected masses M{sub shell} of the Galactic shells and the supernova (SN) 'impostors' proposed as their extragalactic counterparts are very different. While much of the difference is a selection effect created by shell lifetimes {proportional_to}(v{sub shell}{radical}(M{sub shell})){sup -1}, more complete Galactic and extragalactic surveys are needed to demonstrate that the two phenomena share a common origin given that their observed properties are essentially disjoint. If even small fractions (1%) of SNe show interactions with such dense shells of ejecta, as is currently believed, then the driving mechanism of the eruptions must be associated with the very final phases of stellar evolution, suggestive of some underlying nuclear burning instability.

Kochanek, C. S. [Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States)

2011-12-10

38

Accretion, winds and outflows in young stars  

NASA Astrophysics Data System (ADS)

Young stars and planetary systems form in molecular clouds. After the initial radial infall an accretion disk develops. For classical T Tauri stars (CTTS, F-K type precursors) the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius by the stellar magnetic field. The inner edge of the disk is ionized by the stellar radiation, so that the accretion stream is funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observed over a wide wavelength range as X-ray emission, UV excess, optical veiling and optical and IR emission lines. Some of the accretion tracers, e.g. H?, can be calibrated to measure the accretion rate. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many (if not all) accreting systems also drive strong outflows which are ultimately powered by accretion. However, the exact driving mechanism is still unclear. Several components could contribute to the outflows: slow, wide-angle disk winds, X-winds launched close to the inner disk rim, and thermally driven stellar winds. In any case, the outflows contain material of very different temperatures and speeds. The disk wind is cool and can have a molecular component with just a few tens of km s-1, while the central component of the outflow can reach a few 100 km s-1. In some cases the inner part of the outflow is collimated to a small-angle jet. These jets have an onion-like structure, where the inner components are consecutively hotter and faster. The jets can contain working surfaces, which show up as Herbig-Haro knots. Accretion and outflows in the CTTS phase do not only determine stellar parameters like the rotation rate on the main-sequence, they also can have a profound impact on the environment of young stars. This review concentrates on CTTS in near-by star forming regions where observations of high spatial and spectral resolution are available.

Günther, H. M.

2013-02-01

39

Differential Radial Velocities and Stellar Parameters of Nearby Young Stars  

NASA Astrophysics Data System (ADS)

Radial velocity searches for substellar-mass companions have focused primarily on stars older than 1 Gyr. Increased levels of stellar activity in young stars hinders the detection of solar system analogs, and therefore until recently there has been a prejudice against inclusion of young stars in radial velocity surveys. Adaptive optics surveys of young stars have given us insight into the multiplicity of young stars, but only for massive, distant companions. Understanding the limit of the radial velocity technique, restricted to high-mass, close-orbiting planets and brown dwarfs, we began a survey of young stars of various ages. While the number of stars needed to carry out full analysis of the problems of planetary and brown dwarf population and evolution is large, the beginning of such a sample is included here. We report on 61 young stars ranging in age from the ? Pictoris association (~12 Myr) to the Ursa Major association (~300 Myr). This initial search resulted in no stars showing evidence of companions larger than ~1MJup-2MJup in short-period orbits at the 3 ? level. We also present derived stellar parameters, as most have unpublished values. The chemical homogeneity of a cluster, and presumably of an association, may help to constrain true membership, so we present [Fe/H] abundances for the stars in our sample.

Paulson, Diane B.; Yelda, Sylvana

2006-05-01

40

Evolution of the binary population in young dense star clusters  

NASA Astrophysics Data System (ADS)

Context. Field stars are not always single stars, but can often be found in bound double systems. Since binary frequencies in the birth places of stars, young embedded clusters, are sometimes even higher than on average the question arises of how binary stars form in young dense star clusters and how their properties evolve to those observed in the field population. Aims: We assess, the influence of stellar dynamical interactions on the primordial binary population in young dense cluster environments. Methods: We perform numerical N-body simulations of the Orion nebula cluster like star cluster models including primordial binary populations using the simulation code nbody6++. Results: We find two remarkable results that have yet not been reported: The first is that the evolution of the binary frequency in young dense star clusters is independent predictably of its initial value. The time evolution of the normalized number of binary systems has a fundamental shape. The second main result is that the mass of the primary star is of vital importance to the evolution of the binary. The more massive a primary star, the lower the probability that the binary is destroyed by gravitational interactions. This results in a higher binary frequency for stars more massive than 2 M? compared to the binary frequency of lower mass stars. The observed increase in the binary frequency with primary mass is therefore most likely not due to differences in the formation process but can be entirely explained as a dynamical effect. Conclusions: Our results allow us to draw conclusions about the past and the future number of binary systems in young dense star clusters and demonstrate that the present field stellar population has been influenced significantly by its natal environments.

Kaczmarek, T.; Olczak, C.; Pfalzner, S.

2011-04-01

41

Explosions triggered by violent binary-star collisions: application to Eta Carinae and other eruptive transients  

NASA Astrophysics Data System (ADS)

This paper discusses a scenario where a violent periastron collision of stars in an eccentric binary system induces an eruption or explosion seen as a brief transient source, attributed to luminous blue variables (LBVs), supernova (SN) impostors or other transients. The key ingredient is that an evolved primary increases its photospheric radius on relatively short (year to decade) time-scales, to a point where the radius is comparable to or larger than the periastron separation in an eccentric binary. In such a configuration, a violent and sudden collision would ensue, possibly leading to substantial mass ejection instead of a merger. Sudden energy deposition during the encounter could drive expansion of the optically thick envelope, causing a luminous transient source. Repeated periastral grazings in an eccentric system could quickly escalate to a catastrophic encounter. Outbursts triggered by tidal disturbances or powered by secondary accretion of the primary star's wind have been suggested previously. Instead, this paper proposes a much more violent encounter where the companion star plunges deep inside the photosphere of a bloated primary during periastron, as a result of the primary star increasing its own radius. This is motivated by the case of Eta Carinae, where such a collision must have occurred if conventional estimates of the present-day orbit are correct and where peaks in the light curve coincide with times of periastron. Stellar collisions may explain brief recurring LBV outbursts, such as SN 2000ch and SN 2009ip, and perhaps outbursts from intermediate-mass progenitor stars (i.e. collisions are not necessarily the exclusive domain of very luminous stars), but they cannot explain all non-SN transients. Finally, mass ejections induced repeatedly at periastron cause orbital evolution; this may explain the origin of eccentric Wolf-Rayet binaries such as WR 140.

Smith, Nathan

2011-08-01

42

Observation of Young Stars at the University Observatory Jena  

NASA Astrophysics Data System (ADS)

We report on observation and determination of rotational and orbital periods of young stars and eclipsing binaries in the young open cluster Trumpler 37. Observations were carried out with the "Schmidt-Teleskop-Kamera" (STK) at University Observatory Jena in 2009 and 2010.

Berndt, A.; Errmann, R.; Maciejewski, G.; Raetz, St.; Marka, C.; Ginski, Ch.; Mugrauer, M.; Schmidt, T. O. B.; Neuhäuser, R.; Seeliger, M.; Moualla, M.; Pribulla, T.; Hohle, M. M.; Tetzlaff, N.; Adam, Ch.; Eisenbeiss, T.; YETI Team

2011-12-01

43

Young stars in the CO Cepheus void and its surrounding  

NASA Astrophysics Data System (ADS)

After mixing in the Galactic plane, young stars are barely discernible from old ones. In the RasTyc sample we have identified several stars whose lithium content is higher than that of the Pleiades cluster members. Furthermore, four of them are concentrated, within a few degrees, inside the CO Cepheus void. They likely constitute a new young association. To discover further additional members, we used multivariate analysis methods for selecting optical and infrared counterparts of ROSAT All-Sky Survey/XMM-Newton X-ray sources cross-identified with late-type stars. From our spectroscopic observations, we distinguished two populations of lithium-rich stars that are spatially and kinematically separated. While the sources having the same lithium content as the members of the Pleiades cluster are mostly projected towards the Galactic plane, the youngest stars are mainly located in the sky area surrounding the CO Cepheus void and have an age of about 5 - 15 Myr. The latter stars have properties (age, proper motions, radial velocity, dots) rather similar to those derived for the four comoving T Tauri stars already found in this region. Thus, they form the first young association of the northern sky. The discovery of young field stars is of great importance to give new insight into the process of stellar formation outside standard star-forming regions. Further data, such as those from the future Gaia mission, will certainly shed light on this issue and on the origin of this group that could be related to the Cepheus-Cassiopeia complex, which is the closest active star-forming region.

Klutsch, A.; Montes, D.; Guillout, P.; Frasca, A.; Pineau, F.-X.; Grosso, N.; Marilli, E.; López-Santiago, J.

2013-05-01

44

Observational diagnostics of accretion on young stars and brown dwarfs  

NASA Astrophysics Data System (ADS)

I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

Stelzer, Beate; Argiroffi, Costanza

45

CANDIDATE CORONAGRAPHIC DETECTIONS OF PROTOPLANETARY DISKS AROUND FOUR YOUNG STARS  

SciTech Connect

We present potential detections of H-band scattered light emission around four young stars, selected from a total sample of 45 young stars observed with the Coronagraphic Imager with Adaptive Optics coronagraph of the Subaru telescope. Two Classical T Tauri stars, CI Tau and DI Cep, and two weak-lined T Tauri stars, LkCa 14 and RXJ 0338.3+1020, were detected. In all the four cases, the extended emission is within the area of the residual point-spread function halo, and is revealed only through careful data reduction. We compare the observed extended emission with simulations of the scattered light emission to evaluate the plausibility and nature of the detected emission.

Karr, J. L.; Ohashi, N. [Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617, Taiwan (China); Kudo, T.; Tamura, M. [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

2010-03-15

46

A Photometric Survey of 93 Young Planetary-Candidate Stars  

NASA Astrophysics Data System (ADS)

Nearly all of the 100 plus extrasolar planets discovered to date have been detected via precise (3--6 m s-1) radial velocity measurements of solar-type stars. The majority of candidate stars surveyed so far were selected to avoid young, active stars because of the associated high levels (10--100 m s-1) of radial velocity jitter in these stars. Persistent starspot activity, lasting for several stellar rotations, can produce periodic radial velocity signals that mimic the presence of orbiting planets. Thus, the extrasolar planetary systems detected so far represent mature systems that have completed most of their planetary orbital evolution. Finding much younger systems and gaining insight into their formation and early evolution requires extending extrasolar planet search surveys to much younger stars, in spite of the radial velocity complications caused by stellar activity. Precise photometry of planetary candidate stars has proven very useful in interpreting the results of radial velocity observations by helping to distinguish between the effects of true reflex motion caused by planetary companions and the radial velocity effects of starspot activity. Several examples of periodic radial velocity variations caused by surface activity have been found. Therefore, we are conducting a photometric survey of 93 young solar-type stars (ages 3 Myr to 3 Gyr) with our automatic photometric telescopes (APTs) at Fairborn Observatory. These stars were selected from a list of targets to be observed by the SIRTF satellite as part of the Legacy Science Project "The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context". SIRTF will study the evolution of planetary systems around these stars by observing their dust disks. Also, the 10-meter Keck telescope will make precise radial velocity observations of these 93 stars to search for young planetary systems. Our APT observations complement the SIRTF and Keck observations by measuring the level of starspot activity in these stars and, in many cases, determining their rotation periods directly. This information will allow the detection of young planetary systems around these stars to be made with greater confidence. We present preliminary results for several stars, including new rotation periods.

Alston, F.; Henry, G. W.; Burks, G. S.

2003-12-01

47

The Blueing Effect in Massive Young Stars  

NASA Astrophysics Data System (ADS)

The remarkable photometric effect known as "blueing" or "color-reversal", in which when the star fades the color index (B-V) becomes bluer instead of redder, as predicted for normal main sequence stars, has been found to be present in several massive pre-main sequence or Herbig Ae/Be stars (HAEBE). This obscuration of the central source in many cases of up to 3 magnitudes, due to probably large thick clouds orbiting in the outer parts of the circumstellar disks, gives us the opportunity of obtaining the spectral signature of bipolar flows, winds normal to the disk, and of the incident scattering. No general consensus exits in the explanation of this phenomenon, which occurs mostly in objects later than A0 and possible edgeon systems. Two case studies, UX Ori and HD 45677, are discussed here, which led us to propose that the blueing effect is the signature of the accretion and bipolar winds in these stars. Therefore, in this proposal we would like to observe a larger sample of massive PMS stars at deep minima in order to properly assess the UV signature of the blueing effect. Our current interpretation of this phenomenon is radically different from the explanations derived from optical photometry. Up to now, the blueing effect has only be observed and interpreted using optical photometry; our goal is to describe this phenomenon in term of UV continuum and emission fluxes in a larger sample of HAEBE stars.

Perez, Mario R.

48

First Results from the Galex-Wise Young Star Program  

NASA Astrophysics Data System (ADS)

The recent release of the all-sky WISE catalog has opened up a new frontier in the search for nearby, young, low-mass stars. Over the last few decades, many ~10-100 Myr-old stars have been identified in moving groups located closer than 100 parsecs of the Earth. Initial searches relied on optical identification (such as with Tycho and Hipparcos) and X-ray detection with ROSAT. Recent work has shown that near-IR surveys, like 2MASS, combined with ultraviolet data from GALEX can be used to identify additional members in these moving groups. In particular, this methodology is well suited to searching for low-mass stars, which are generally lacking in moving group member statistics. We present the first results of our work to tie-in the WISE catalog to young, low-mass star searches. We have carried out an all-sky cross correlation between the GALEX and WISE/2MASS databases and identified hundreds of candidate young, low-mass stars. Early spectroscopic results confirm the youthful nature of our candidate stars. This work is supported by NASA Astrophysics Data Analysis Program award NNX12AH37G to RIT and UCLA and Chilean BASAL and FONDECYT grants to Universidad de Chile.

Rodriguez, David; Zuckerman, B. M.; Kastner, J. H.; Bessell, M. S.; Faherty, J.; Murphy, S. J.; Vican, L.

2013-01-01

49

Characterisation of young, nearby stars - The Ursa-Major group  

NASA Astrophysics Data System (ADS)

The Ursa Major group is composed of the Ursa Major open star cluster in the Big Dipper constellation and of many co-moving stars spread over the whole sky. The stars of the Ursa Major group have a relatively young age of 200-600 million years. Taking into account their proximity and their common motion, the youth makes these stars interesting targets for various astrophysical studies. However the list of members is not complete. A few dozens of assured members face hundreds of additional candidates. Therefore the author considers the spectroscopic properties of the Ursa Major group on grounds of a precise analysis and indeed confirms that the stars have more features in common than just the motion.

Ammler, M.

2007-01-01

50

Anne S. Young: Professor and Variable Star Observer Extraordinaire  

NASA Astrophysics Data System (ADS)

One of the original eight members of the AAVSO, but not well known today, was Professor Anne Sewell Young of Mount Holyoke College. Miss Young taught there for thirty-seven years, and trained many women astronomers during the first third of the 20th century. This paper will attempt to present her life as an inspiring teacher, as well as a contributor of more than 6,500 variable star observations to the AAVSO.

Bracher, K.

2012-06-01

51

X-Ray Outburst from Young Star in McNeil's Nebula  

NASA Astrophysics Data System (ADS)

Observations with NASA's Chandra X-ray Observatory captured an X-ray outburst from a young star, revealing a probable scenario for the intermittent brightening of the recently discovered McNeil's Nebula. It appears the interaction between the young star's magnetic field and an orbiting disk of gas can cause dramatic, episodic increases in the light from the star and disk, illuminating the surrounding gas. "The story of McNeil's Nebula is a wonderful example of the importance of serendipity in science," said Joel Kastner of the Rochester Institute of Technology in Rochester, New York, lead author of a paper in the July 22 issue of Nature describing the X-ray results. "Visible-light images were made of this region several months before Jay McNeil made his discovery, so it could be determined approximately when and by how much the star flared up to produce McNeil's Nebula." The small nebula, which lies in the constellation Orion about 1300 light years from Earth, was discovered with a 3-inch telescope by McNeil, an amateur astronomer from Paducah, Kentucky, in January 2004. In November 2002, a team led by Ted Simon of the Institute for Astronomy in Hawaii had observed the star-rich region with Chandra in search of young, X-ray emitting stars, and had detected several objects. Optical and infrared astronomers had, as part of independent surveys, also observed the region about a year later, in 2003. After the announcement of McNeil's discovery, optical, infrared and X-ray astronomers rushed to observe the region again. They found that a young star buried in the nebula had flared up, and was illuminating the nebula. This star was coincident with one of the X-ray sources discovered earlier by Simon. Chandra observations obtained by Kastner's group just after the optical outburst showed that the source had brightened fifty-fold in X-rays when compared to Simon's earlier observation. The visible-light eruption provides evidence that the cause of the X-ray outburst is the sudden infall of matter onto the surface of the star from an orbiting disk of gas. In general, the coupling of the magnetic field of the star and the magnetic field of its circumstellar disk regulates the inflow of gas from the disk onto the star. This slow, steady inflow suddenly can become much more rapid if a large amount of gas accumulates in the disk, and the disk and the star are rotating at different rates. The differing rotation rates would twist and shear the magnetic field, storing up energy. This energy is eventually released in an energetic, X-ray producing outburst as the magnetic field violently rearranges back to a more stable state. During this period, a large amount of gas can fall onto the star, producing the observed optical and infrared outburst. A new buildup of gas in the disk could lead to a new outburst in the future. Such a scenario may explain why the brightness of McNeil's Nebula appears to vary with time. It is faintly present in surveys of this region of Orion in images taken in the 1960s, but absent from images taken in the 1950s and 1990s. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass. Additional information and images are available at: http://chandra.harvard.edu and http://chandra.nasa.gov

2004-07-01

52

Instabilities in Very Young Neutron Stars: Temperature  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the temperature structure for 20 milliseconds after the shock stalls. The minimum temperature is approximately 1.35 MeV. The maximum temperature varies from 6 MeV at the beginning of the calculation to 10 MeV at the later times.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

53

Instabilities in Very Young Neutron Stars: Density  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the density evolution for 20 milliseconds after the shock stalls. The density is plotted on a log scale. Values range from 10^9 gm-cm^3 at the outer boundary to 1.4 x 10^12 gm-cm^3 at the inner boundary.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

54

Magnetic activity of 6 young solar stars. II (Messina+, 2003)  

NASA Astrophysics Data System (ADS)

The following tables contain the results of the periodogram analysis performed on the photometric data of six young solar analogues plus the additional LQ Hya star. The following data are listed: first and last observing HJD, number of observing nights, rotational period, uncertainty on the rotational period, False Alarm Probability (FAP) (7 data files).

Messina, S.; Guinan, E. F.

2003-09-01

55

Beta Pictoris Phenomenon in Young Stars With Accreting Gas.  

National Technical Information Service (NTIS)

Program Hae2BPIC resulted in usable ISO spectra of three young, Herbig Ae stars: HR 5999 (A7e, t=0.6 Myr), SV Cep (al-2e, t=1-3 Myr), and MW Vul (Al-2e, t=1-3 Myr). While too small a sample to pursue our original goal of surveying the silicate emission in...

C. A. Grady

1999-01-01

56

Realistic limitations of detecting planets around young active stars  

NASA Astrophysics Data System (ADS)

Current planet hunting methods using the radial velocity method are limited to observing middle-aged main-sequence stars where the signatures of stellar activity are much less than on young stars that have just arrived on the main-sequence. In this work we apply our knowledge from the surface imaging of these young stars to place realistic limitations on the possibility of detecting orbiting planets. In general we find that the magnitude of the stellar jitter is directly proportional to the stellar vsini. For G and K dwarfs, we find that it is possible, for models with high stellar activity and low stellar vsini, to be able to detect a 1 MJupiter mass planet within 50 epochs of observations and for the M dwarfs it is possible to detect a habitable zone Earth-like planet in 10s of observational epochs.

Jeffers, S. V.; Barnes, J. R.; Jones, H.; Pinfield, D.

2013-04-01

57

The Beta Pictoris Phenomenon in Young Stars With Accreting Gas  

NASA Astrophysics Data System (ADS)

Program Hae2BPIC resulted in usable ISO spectra of three young, Herbig Ae stars: HR 5999 (A7e, t=0.6 Myr), SV Cep (al-2e, t=1-3 Myr), and MW Vul (Al-2e, t=1-3 Myr). While too small a sample to pursue our original goal of surveying the silicate emission in these young, protoplanetary disk systems, comparison of these data with ground-based IR spectra, and published ISO observations of other HAe stars (especially the posters at PPIV) reveals the following: The known binary stars in the sample show signatures of partially crystal line silicate features by t=0.6 Myr, at an epoch when ostensibly single Herbig Ae stars have substantially stronger silicate emission dominated by amorphous grains. The known binary stars also show deficits in the optically thick continuum flux relative to coeval single stars. Comparison of ISO spectra indicates that the flux deficit seen in WD 163296 over 10-100 microns relative to AB Aur reflects a real deficit of material interior to 300.

Grady, Carol A.

1999-09-01

58

Thermal Evolution and Light Curves of Young Bare Strange Stars  

NASA Astrophysics Data System (ADS)

We study numerically the cooling of a young bare strange star and show that its thermal luminosity, mostly due to e(sup +)e(sup -) pair production from the quark surface, may be much higher than the Eddington limit. The mean energy of photons far from the strange star is [approx]10(sup 2) keV or even more. This differs both qualitatively and quantitatively from the thermal emission from neutron stars and provides a definite observational signature for bare strange stars. It is shown that the energy gap of superconducting quark matter may be estimated from the light curves if it is in the range from [approx]0.5 MeV to a few MeV.

Page, Dany; Usov, Vladimir V.

2002-09-01

59

A multiwavelength study of young stars in the Elephant Trunk  

NASA Astrophysics Data System (ADS)

We present the results of a multiwavelength study of young stars in IC 1396A, ``the Elephant Trunk Nebula''. Our targets are selected combining optical, near-infrared and mid-infrared photometry. Near-infrared and optical spectroscopy are used to confirm their youth and to derive spectral types for these objects, showing that they are early to mid-M stars, and that our sample includes some of the lowest-mass objects reported so far in the region. The photometric and spectroscopic information is used to construct the spectral energy distributions and to study the properties of the stars (mass, age, accretion, disks, spatial location). The implications for the triggered star formation picture are discussed.

López Martí, B.; Bayo, A.; Morales Calderón, M.; Barrado, D.

2013-05-01

60

The young star cluster in NGC 1275 - H-alpha linewidth and star-formation properties  

Microsoft Academic Search

High spectral resolution observations of the massive young star cluster in the filaments of NGC 1275 reveal that the associated H-alpha emission line has a velocity width comparable to, or somewhat less than, that of H-alpha in typical giant H II regions of similar emission-line luminosity. If this linewidth reflects virial motions of matter within the star-forming region, as has

Joseph C. Shields; Alexei V. Filippenko; Gibor Basri

1990-01-01

61

Southern near-infrared photometric monitoring of Galactic young star clusters (NIP of Stars)  

NASA Astrophysics Data System (ADS)

We have performed a near-infrared photometric monitoring of 39 galactic young star clusters and star-forming regions, known as NIP of Stars, be- tween the years 2009-2011, using the Swope telescope at Las Campanas Observatory (Chile) and the RetroCam camera. The primary objective of the campaign is to perform a census of photometric variability of such clus- ters and to discover massive eclipsing binary stars. In this work, we describe the general idea, the implementation of the survey, and the first preliminary results of some of the observed clusters. This monitoring program is com- plementary to the Vista Variables in the Vía Láctea (VVV), as the brightest sources observed in NIP of Stars are saturated in VVV.

Barbá, R.; Morrell, N. I.; Gunthardt, G.; Torres Robledo, S.; Jaque, M.; Soto, M.; Ferrero, G.; Arias, J. I.; Roman-Lopes, A.; Gamen, R. C.; Astudillo Hormazabal, J.

62

Finding exoplanets orbiting young active stars - I. Technique  

NASA Astrophysics Data System (ADS)

Stellar activity, such as starspots, can induce radial velocity (RV) variations that can mask or even mimic the RV signature of orbiting exoplanets. For this reason RV exoplanet surveys have been unsuccessful when searching for planets around young, active stars and are therefore failing to explore an important regime which can help to reveal how planets form and migrate. This paper describes a new technique to remove spot signatures from the stellar line-profiles of moderately rotating, active stars (v sin i ranging from 10 to 50 km s-1). By doing so it allows planetary RV signals to be uncovered. We used simulated models of a G5V type star with differing dark spots on its surface along with archive data of the known active star HD 49933 to validate our method. The results showed that starspots could be effectively cleaned from the line-profiles so that the stellar RV jitter was reduced by more than 80 per cent. Applying this procedure to the same models and HD 49933 data, but with fake planets injected, enabled the effective removal of starspots so that Jupiter mass planets on short orbital periods were successfully recovered. These results show that this approach can be useful in the search for hot-Jupiter planets that orbit around young, active stars with a v sin i of ˜10-50 km s-1.

Moulds, V. E.; Watson, C. A.; Bonfils, X.; Littlefair, S. P.; Simpson, E. K.

2013-04-01

63

A Coronagraphic Search for Substellar Companions to Young Stars  

NASA Astrophysics Data System (ADS)

We are using NICMOS to search for very low-mass stellar and sub-stellar companions around nearby, young, main-sequence stars. With the coronagraph on Camera 2 and the F160W filter (1.4-1.8 micron), we search in the region 0.4" - 4" from the target star. To greatly reduce the scattered light near the hole, we image at two roll angles centered on the star and separated by 30 degrees. The lower mass limit of a detectable companion depends on the mass, age, and distance of the target star as well as the angular separation between star and companion. For several of our targets that are 10-50 Myr old, this lower mass limit could be as low as 10 Jupiter masses. We display preliminary results using this technique. A stellar-like object with Delta H=5.3 was discovered 0".9 from HD 102982. From the distance and age of the primary, this companion is most likely a low-mass M star. Recent observations demonstrate that we can obtain a S/N = 5 (clear detection) on a secondary with Delta H=12 at a separation of 1''.

Lowrance, P. J.; Becklin, E. E.; Schneider, G.; Hines, D.; Kirkpatrick, J. D.; Koerner, D.; Low, F.; McCarthy, D.; Meier, R.; Rieke, M.; Smith, B. A.; Terrile, R.; Thompson, R.; Zuckerman, B.

1998-07-01

64

Environments of star formation - Relationship between molecular clouds, dense cores and young stars  

Microsoft Academic Search

In this chapter, the star-forming properties of three molecular cloud complexes, Taurus-Auriga, Ophiuchus and Orion, are discussed. In particular, recent observational knowledge of the distribution and properties of dense molecular gas and young stellar objects within each complex is reviewed. Studies of the two nearest molecular cloud complexes, Taurus and Ophiuchus, have shown that at least two modes or environments

Elizabeth A. Lada; Karen M. Strom; Philip C. Myers

1993-01-01

65

Early type stars at high galactic latitudes. I. Ten young massive B-type stars  

NASA Astrophysics Data System (ADS)

We present the results of quantitative spectral analyses of ten apparently normal B-type stars. These stars were found to be young massive B-type stars at distances of z=2.6 to 7.6 kpc from the galactic plane based on their positions in the (Teff, log g) diagram, normal abundance patterns and/or large projected rotational velocities. We discuss formation scenarios (runaway star scenarios versus a scenario for star formation in the halo) by comparing times-of-flight and evolutionary time scales. For all stars (except SB 357 and HS 1914+7139) both the scales are similar indicating that the stars could have formed in the galactic disk and been ejected from there soon after their birth. Derived ejection velocities range from 130 km s-1 to 440 km s-1 and may be used to constrain models for ejection mechanisms. Using new proper motion measurements we show that PHL 346, which was considered the most likely candidate for a young B-type star born in the halo, can be explained as a runaway star from the galactic plane. Based on observations obtained at the W. M. Keck Observatory, which is operated by the Californian Association for Research in Astronomy for the California Institute of Technology and the University of California. Based on observations collected at the German-Spanish Astronomical Center (DSAZ), Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy. Based on observations collected at the European Southern Observatory (ESO proposal No. 65.H-0341(A)).

Ramspeck, M.; Heber, U.; Moehler, S.

2001-11-01

66

The galactocentric radius dependent upper mass limit of young star clusters: stochastic star formation ruled out  

NASA Astrophysics Data System (ADS)

It is widely accepted that the distribution function of the masses of young star clusters is universal and can be purely interpreted as a probability density distribution function with a constant upper mass limit. As a result of this picture the masses of the most massive objects are exclusively determined by the size of the sample. Here we show, with very high confidence, that the masses of the most massive young star clusters in M33 decrease with increasing galactocentric radius in contradiction to the expectations from a model of a randomly sampled constant cluster mass function with a constant upper mass limit. Pure stochastic star formation is thereby ruled out. We use this example to elucidate how naive analysis of data can lead to unphysical conclusions.

Pflamm-Altenburg, Jan; González-Lópezlira, Rosa A.; Kroupa, Pavel

2013-09-01

67

The galactocentric radius dependent upper mass limit of young star clusters: stochastic star formation ruled out  

NASA Astrophysics Data System (ADS)

It is widely accepted that the distribution function of the masses of young star clusters is universal and can be purely interpreted as a probability density distribution function with a constant upper mass limit. As a result of this picture the masses of the most massive objects are exclusively determined by the size of the sample. Here we show, with very high confidence, that the masses of the most massive young star clusters in M33 decrease with increasing galactocentric radius in contradiction to the expectations from a model of a randomly sampled constant cluster mass function with a constant upper mass limit. Pure stochastic star formation is thereby ruled out. We use this example to elucidate how naive analysis of data can lead to unphysical conclusions.

Pflamm-Altenburg, Jan; González-Lópezlira, Rosa A.; Kroupa, Pavel

2013-11-01

68

Unveiling hidden properties of young star clusters: differential reddening, star-formation spread, and binary fraction  

NASA Astrophysics Data System (ADS)

Context. Usually, important parameters of young, low-mass star clusters are very difficult to obtain by means of photometry, especially when differential reddening and/or binaries occur in large amounts. Aims: We present a semi-analytical approach (ASAmin) that, when applied to the Hess diagram of a young star cluster, is able to retrieve the values of mass, age, star-formation spread, distance modulus, foreground and differential reddening, and binary fraction. Methods: The global optimisation method known as adaptive simulated annealing (ASA) is used to minimise the residuals between the observed and simulated Hess diagrams of a star cluster. The simulations are realistic and take the most relevant parameters of young clusters into account. Important features of the simulations are a normal (Gaussian) differential reddening distribution, a time-decreasing star-formation rate, the unresolved binaries, and the smearing effect produced by photometric uncertainties on Hess diagrams. Free parameters are cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and binary fraction. Results: Tests with model clusters built with parameters spanning a broad range of values show that ASAmin retrieves the input values with a high precision for cluster mass, distance modulus, and foreground reddening, but they are somewhat lower for the remaining parameters. Given the statistical nature of the simulations, several runs should be performed to obtain significant convergence patterns. Specifically, we find that the retrieved (absolute minimum) parameters converge to mean values with a low dispersion as the Hess residuals decrease. When applied to actual young clusters, the retrieved parameters follow convergence patterns similar to the models. We show how the stochasticity associated with the early phases may affect the results, especially in low-mass clusters. This effect can be minimised by averaging out several twin clusters in the simulated Hess diagrams. Conclusions: Even for low-mass star clusters, ASAmin is sensitive to the values of cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and to a lesser degree, binary fraction. Compared with simpler approaches, including binaries, a decaying star-formation rate, and a normally distributed differential reddening appears to yield more constrained parameters, especially the mass, age, and distance from the Sun. A robust determination of cluster parameters may have a positive impact on many fields. For instance, age, mass, and binary fraction are important for establishing the dynamical state of a cluster or for deriving a more precise star-formation rate in the Galaxy.

Bonatto, C.; Lima, E. F.; Bica, E.

2012-04-01

69

Metallicities of Solar-type Stars in Young Open Clusters  

NASA Astrophysics Data System (ADS)

We present chemical abundances of six nearby young open clusters determined from moderate-resolution WIYN-Hydra and CTIO-Hydra spectra of cluster F and G pre-main sequence and main sequence stars. The selection of these particular clusters, with ages of 10-150 Myr, was aimed at examining whether the massive planet-metallicity correlation extends to younger planetary systems, namely debris disks, which are thought to be the observational signatures of planet formation. The relationship between cluster metallicity and frequency of debris disks around early-type cluster stars is explored using complementary Spitzer-MIPS 24 ?m data. Preliminary results show no strong correlation between debris disk frequency and metallicity, consistent with previous studies of older FGK stars with debris disks.

Monroe, T. R.; Pilachowski, C. A.; Rebull, L.

2013-04-01

70

Lick adaptive optics survey searching for low-mass companions to young, nearby stars  

Microsoft Academic Search

Using the Shane 3 meter telescope at Lick Observatory, we surveyed 102 nearby young stars with the near-infrared adaptive optics system. Young stars were targetted in order to increase the sensitivity to low-mass companions. Of these 102 stars, 44 had at least one other source in the field of view with separations ranging from 0.4\\

Q. M. Konopacky; B. A. Macintosh; A. M. Ghez; B. Zuckerman; D. Kaisler; I. Song

2002-01-01

71

The Evolution of X-Ray Emission in Young Stars  

NASA Astrophysics Data System (ADS)

The evolution of magnetic activity in late-type stars is part of the intertwined rotation-age-activity relation, which provides an empirical foundation to the theory of magnetic dynamos. We study the age-activity relation in the pre-main-sequence (PMS) regime, for the first time using mass-stratified subsamples. The effort is based on the Chandra Orion Ultradeep Project (COUP), which provides very sensitive and homogenous X-ray data on a uniquely large sample of 481 optically well-characterized low-extinction low-mass members of the Orion Nebula Cluster, for which individual stellar masses and ages could be determined. More than 98% of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range ~0.1-10 Myr, we establish a mild decay in activity with stellar age ? roughly as LX~?-1/3. On longer timescales, when the Orion stars are compared to main-sequence stars, the X-ray luminosity decay law for stars in the 0.5 Msolarstars. The magnetic activity history for M stars with masses 0.1 Msolarstars, we find that the activity-age decay is strong across the entire history of solar-type stars but is not attributable to rotational deceleration during the early epochs. A combination of tachocline and distributed convective dynamos may be operative in young solar-type stars. The results for the lowest mass stars are most easily understood by the dominance of convective dynamos during both the PMS and main-sequence phases.

Preibisch, Thomas; Feigelson, Eric D.

2005-10-01

72

The young active star SAO 51891 (V383 Lacertae)  

NASA Astrophysics Data System (ADS)

Aims: The aim of this work is to investigate the surface inhomogeneities of a young, late-type star, SAO 51891, at different atmospheric levels, from the photosphere to the upper chromosphere, analyzing contemporaneous optical high-resolution spectra and broad-band photometry. Methods: The full spectral range of FOCES@CAHA (R ? 40 000) is used to perform the spectral classification and to determine the rotational and radial velocities. The lithium abundance is measured to obtain an age estimate. The {BVRIJHK}s photometric bands are used to construct the spectral energy distribution (SED). The variations in the observed BV fluxes and effective temperature are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to H?, Ca ii H & K, H?, and Ca ii IRT lines. Results: We find SAO 51891 to be a young K0-1V star with a lithium abundance close to the Pleiades upper envelope, confirming its youth ( 100 Myr), which is also inferred from its kinematical membership of the Local Association. No infrared excess is detected from analysis of its SED, limiting the amount of remaining circumstellar dust. We detect a rotational modulation of the luminosity, effective temperature, Ca ii H & K, H?, and Ca ii IRT total fluxes. A simple spot model with two main active regions, about 240 K cooler than the surrounding photosphere, fits the observed light and temperature curves very well. The small-amplitude radial velocity variations are also well reproduced by our spot model. The anti-correlation of light curves and chromospheric diagnostics indicates chromospheric plages spatially associated with the spots. The largest modulation amplitude is observed for the H? flux suggesting that this line is very sensitive to the presence of chromospheric plages. Conclusions: SAO 51891 is a young active star, lacking significant amounts of circumstellar dust or any evidence of low mass companions, and displays the typical phenomena produced by magnetic activity. The spots are larger and warmer than those in less active main-sequence stars. If some debris is still present around the star, it will only be detectable by future far-infrared and sub-mm observations (e.g., Herschel or ALMA). The RV variation produced by the starspots has an amplitude comparable with those induced by Jupiter-mass planets orbiting close to the host star. SAO 51891 is another good example of an active star in which the detection of planets may be hampered by the high activity level. Based on observations collected at Calar Alto Astronomical Observatory (Spain) and Catania Astrophysical Observatory (Italy).

Biazzo, K.; Frasca, A.; Marilli, E.; Covino, E.; Alcalã, J. M.; ?akirli, Ö.; Klutsch, A.; Meyer, M. R.

2009-05-01

73

Fast Star, Slow Star; Old Star, Young Star: Subgiant Rotation as a Population and Stellar Physics Diagnostic  

NASA Astrophysics Data System (ADS)

Stellar rotation is a strong function of both mass and evolutionary state. Missions such as Kepler and CoRoT provide tens of thousands of rotation periods, drawn from stellar populations that contain objects at a range of masses, ages, and evolutionary states. Given a set of reasonable starting conditions and a prescription for angular momentum loss, we address the expected range of rotation periods for cool field stellar populations (~0.4-2.0 M ?). We find that cool stars fall into three distinct regimes in rotation. Rapid rotators with surface periods less than 10 days are either young low-mass main sequence (MS) stars, or higher mass subgiants which leave the MS with high rotation rates. Intermediate rotators (10-40 days) can be either cool MS dwarfs, suitable for gyrochronology, or crossing subgiants at a range of masses. Gyrochronology relations must therefore be applied cautiously, since there is an abundant population of subgiant contaminants. The slowest rotators, at periods greater than 40 days, are lower mass subgiants undergoing envelope expansion. We identify additional diagnostic uses of rotation periods. There exists a period-age relation for subgiants distinct from the MS period-age relations. There is also a period-radius relation that can be used as a constraint on the stellar radius, particularly in the interesting case of planet host stars. The high-mass/low-mass break in the rotation distribution on the MS persists onto the subgiant branch, and has potential as a diagnostic of stellar mass. Finally, this set of theoretical predictions can be compared to extensive datasets to motivate improved modeling.

van Saders, Jennifer L.; Pinsonneault, Marc H.

2013-10-01

74

Multiplicity among Young Brown Dwarfs and Very Low Mass Stars  

NASA Astrophysics Data System (ADS)

We report on a near-infrared adaptive optics imaging survey of 31 young brown dwarfs and very low mass (VLM) stars, 28 of which are in the Chamaeleon I star-forming region, using the ESO Very Large Telescope. We resolve the suspected 0.16'' (~26 AU) binary Cha H? 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13'' (~20 AU) and 0.30'' (~50 AU), respectively; the latter is one of the widest VLM systems known. We find a binary frequency of 11+9-6%, thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLM objects (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLM objects in the field. Furthermore, we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few megayears to several gigayears. Instead, the observations so far suggest that VLM objects are either less likely to be born in fragile multiple systems than solar-mass stars or such systems are disrupted very early. We dedicate this paper to the memory of our coauthor, Eduardo Delgado-Donate, who died in a hiking accident in Tenerife earlier this year.

Ahmic, Mirza; Jayawardhana, Ray; Brandeker, Alexis; Scholz, Alexander; van Kerkwijk, Marten H.; Delgado-Donate, Eduardo; Froebrich, Dirk

2007-12-01

75

Cooling of young neutron stars in GRB associated to supernovae  

NASA Astrophysics Data System (ADS)

Context. The traditional study of neutron star cooling has been generally applied to quite old objects such as the Crab Pulsar (957 years) or the central compact object in Cassiopeia A (330 years) with an observed surface temperature ~106 K. However, recent observations of the late (t = 108-109 s) emission of the supernovae (SNe) associated to GRBs (GRB-SN) show a distinctive emission in the X-ray regime consistent with temperatures ~107-108 K. Similar features have been also observed in two Type Ic SNe SN 2002ap and SN 1994I that are not associated to GRBs. Aims: We advance the possibility that the late X-ray emission observed in GRB-SN and in isolated SN is associated to a hot neutron star just formed in the SN event, here defined as a neo-neutron star. Methods: We discuss the thermal evolution of neo-neutron stars in the age regime that spans from ~1 min (just after the proto-neutron star phase) all the way up to ages <10-100 yr. We examine critically the key factor governing the neo-neutron star cooling with special emphasis on the neutrino emission. We introduce a phenomenological heating source, as well as new boundary conditions, in order to mimic the high temperature of the atmosphere for young neutron stars. In this way we match the neo-neutron star luminosity to the observed late X-ray emission of the GRB-SN events: URCA-1 in GRB980425-SN1998bw, URCA-2 in GRB030329-SN2003dh, and URCA-3 in GRB031203-SN2003lw. Results: We identify the major role played by the neutrino emissivity in the thermal evolution of neo-neutron stars. By calibrating our additional heating source at early times to ~1012-1015 erg/g/s, we find a striking agreement of the luminosity obtained from the cooling of a neo-neutron stars with the prolonged (t = 108-109 s) X-ray emission observed in GRB associated with SN. It is therefore appropriate a revision of the boundary conditions usually used in the thermal cooling theory of neutron stars, to match the proper conditions of the atmosphere at young ages. The traditional thermal processes taking place in the crust might be enhanced by the extreme high-temperature conditions of a neo-neutron star. Additional heating processes that are still not studied within this context, such as e+e- pair creation by overcritical fields, nuclear fusion, and fission energy release, might also take place under such conditions and deserve further analysis. Conclusions: Observation of GRB-SN has shown the possibility of witnessing the thermal evolution of neo-neutron stars. A new campaign of dedicated observations is recommended both of GRB-SN and of isolated Type Ic SN.

Negreiros, R.; Ruffini, R.; Bianco, C. L.; Rueda, J. A.

2012-04-01

76

Direct Measurement of the Volume of Liquid Water Emitted During Eruptions of Lone Star Geyser, Yellowstone National Park, Wyoming  

NASA Astrophysics Data System (ADS)

In September, 2010 a comprehensive series of instrumental observations was carried out at Lone Star Geyser in Yellowstone National Park to measure changes in the geyser and its surroundings during eruptions. That project included measurements of flow in the streams that drain the geyser area. Three small streams convey liquid water from the geyser and many of the surrounding hot springs to the Firehole River, about 75 m south of the geyser cone. We developed rating curves for two of these streams by measuring channel cross-sections and timing floating markers (using stopwatches and video recordings) while simultaneously recording stream depth at two-second intervals at two locations using pressure transducers and dataloggers. We estimated the flow in the third (ungaged) stream to be 0.15 of the flow in the easternmost stream, with which it shares a source area and part of its channel. The eruption cycle takes about 3 hours, and a total of nine eruption cycles were observed. During these 3-hour cycles the geyser and the nearby hot springs deliver a total of between 15 and 28 m3 of water to the Firehole River. During the 10-20 minutes of the main phase of an eruption, the geyser delivered between 8 and 11 m3 of water to the three streams. The volume of water emitted during eruptions appears to display a significant diurnal variation which strongly correlates with air temperature, with significantly more flow during early afternoon hours. There were also significant variations in the distribution of flow between the different channels. Our calculations suggest that losses due to evaporation along the flow channels are negligible, and losses due to infiltration appear to be small. The calculated volumes of water discharge do not account for the volume of erupted steam or evaporation of liquid water from the jet. Steam discharge will be assessed using image analysis of high speed video. The calculated volumes provide accurate and important constraint for models of geyser jet dynamics, energetics, and mass balance.

Murphy, F.; Hurwitz, S.; Johnston, M. J.; Vandemeulebrouck, J.; Pontbriand, C.; Sohn, R. A.; Karlstrom, L.; Rudolph, M. L.

2011-12-01

77

Large and kinematically unbiased samples of G- and K-type stars. II. Observations of evolved stars in the Bright Star sample. III. Evolved young disk stars in the Bright Star sample  

SciTech Connect

Four color and IR observations were obtained for a large sample of G-type and K-type stars in the Bright Star Catalogue. Data are first presented for 110 evolved stars. Photometry of evolved young disk population stars have then been calibrated for luminosity, reddening, and metallicity on the basis of results for members of the Hyades and Sirius superclusters. New DDO results are given for 120 stars. 115 references.

Eggen, O.J.

1989-01-01

78

Molecular tracers of photo-evaporating disks around young stars  

NASA Astrophysics Data System (ADS)

Disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations, are eroded by photo-evaporation. In the latter case, this erosion may be an important factor in planet formation. As Johnstone et al. (\\cite{johnstone98a}) have shown, photo-evaporating material is gravitationally retained within a critical radius from the star, and constitutes an envelope similar to a Photon-Dominated Region (PDR) that normally arises at the edge of a molecular cloud irradiated by a massive star. We explore the chemistry of such a PDR/disk system to examine the contribution that it may make to the molecular species that may be observed. The model is in two phases; firstly, a collapse from low density to a high density appropriate for a disk; and, secondly, a 2D calculation of the irradiation of disk material by the radiation field of the central massive star or nearby OB association. The model follows the chemistry self-consistently through both phases. We compute the column densities of species through the PDR/disk system, averaged over the disk. We validate our model by comparing predicted averaged molecular column densities with those of several species detected in the disk around the 10 solar mass star GL 2591, currently the sole example known of this kind of object. Results are in good agreement for a model in which the outer part of the PDR is hot while the inner part is cool, and in which the local ionization rate is comparable with that caused by cosmic rays in the local interstellar medium. We show that in addition to the four detected species, there should be many others also detectable in this system, including HCN, NH3 and CS. Similar conclusions should apply to other disks around massive stars. Disks around low-mass stars are much more common; our models show that when irradiated by a nearby OB association such disks with their attendant PDRs also generate a rich chemistry. No detections of molecules in such objects have yet been reported. However, the models suggest that averaged molecular column densities should be comparable to those detected in disks around massive stars (see references listed in Table \\ref{restab} for molecules in GL 2591). Potential tracers of irradiated disks around low-mass stars include OH, CH3 and C2H. We note that the detection in a disk of PDR-type chemistry is a clear signature that the disk is undergoing erosion. Its duration is therefore limited, with consequences for planet formation.

Nguyen, T. K.; Viti, S.; Williams, D. A.

2002-06-01

79

Galactic kinematics from a sample of young massive stars  

NASA Astrophysics Data System (ADS)

Based on published sources, we have created a kinematic database on 220 massive (> 10 M ?) young Galactic star systems located within ?3 kpc of the Sun. Out of them, ?100 objects are spectroscopic binary and multiple star systems whose components are massive OB stars; the remaining objects are massive Hipparcos B stars with parallax errors of no more than 10%. Based on the entire sample, we have constructed the Galactic rotation curve, determined the circular rotation velocity of the solar neighborhood around the Galactic center at R 0 = 8kpc, V 0 = 259±16 km s-1, and obtained the following spiral density wave parameters: the amplitudes of the radial and azimuthal velocity perturbations f R = -10.8 ± 1.2 km s-1 and f ? = 7.9 ± 1.3 km s-1, respectively; the pitch angle for a two-armed spiral pattern i = -6.0° ± 0.4°, with the wavelength of the spiral density wave near the Sun being ? = 2.6 ± 0.2 kpc; and the radial phase of the Sun in ? ? = -120° ± 4°. We show that such peculiarities of the Gould Belt as the local expansion of the system, the velocity ellipsoid vertex deviation, and the significant additional rotation can be explained in terms of the density wave theory. All these effects decrease noticeably once the influence of the spiral density wave on the velocities of nearby stars has been taken into account. The influence of Gould Belt stars on the Galactic parameter estimates has also been revealed. Eliminating them from the kinematic equations has led to the following new values of the spiral density wave parameters: f ? = 2.9 ± 2.1 km s-1 and ? ? = -104° ± 6°.

Bobylev, V. V.; Bajkova, A. T.

2013-08-01

80

Multiplicity study of young pre-main sequence stars in the Lupus star-forming Region  

NASA Astrophysics Data System (ADS)

We have conducted a high contrast imaging search for (sub)stellar companions among 63 young pre-main sequence stars in the Lupus star forming region, using the adaptive optics imager NACO at UT4 of the ESO Paranal observatory. We detected faint co-moving companions around our targets at angular separations between about 0.1 up to several arc seconds (binaries and triple systems). Some of these companions are in the sub stellar mass regime, according to their apparent near infrared photometry at the distance of the Lupus star forming region (about 140pc). We give a progress report to our long-term project, still in execution with the follow-up spectroscopy of detected substellar companion-candidates, and present some first results.

Vogt, Nikolaus; Mugrauer, Markus; Schmidt, Tobias O. B.; Neuhaeuser, Ralph; Ginski, Christian

2013-07-01

81

YOUNG, ULTRAVIOLET-BRIGHT STARS DOMINATE DUST HEATING IN STAR-FORMING GALAXIES  

SciTech Connect

In star-forming galaxies, dust plays a significant role in shaping the ultraviolet (UV) through infrared (IR) spectrum. Dust attenuates the radiation from stars, and re-radiates the energy through equilibrium and non-equilibrium emission. Polycyclic aromatic hydrocarbons (PAHs), graphite, and silicates contribute to different features in the spectral energy distribution; however, they are all highly opaque in the same spectral region-the UV. Compared to old stellar populations, young populations release a higher fraction of their total luminosity in the UV, making them a good source of the energetic UV photons that can power dust emission. However, given their relative abundance, the question of whether young or old stellar populations provide most of these photons that power the IR emission is an interesting question. Using three samples of galaxies observed with the Spitzer Space Telescope and our dusty radiative transfer model, we find that young stellar populations (on the order of 100 million years old) dominate the dust heating in star-forming galaxies, and old stellar populations (13 billion years old) generally contribute less than 20% of the far-IR luminosity.

Law, Ka-Hei [Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States); Gordon, Karl D. [STScI, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Misselt, K. A., E-mail: klaw@pha.jhu.edu, E-mail: kgordon@stsci.edu, E-mail: misselt@as.arizona.edu [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)

2011-09-10

82

Unraveling the 1974 eruption of Fuego volcano (Guatemala) with small crystals and their young melt inclusions  

Microsoft Academic Search

This study demonstrates that crystal size may be used to temporally constrain melt inclusion data. This approach is applied to the 1974 Vulcanian-type eruption of Volcan Fuego in Guatemala to study magma conditions immediately prior to eruption. The 1974 ash deposit shows significant variations in matrix glass composition, crystal size, crystal content, and volatile (H2O and CO2) content in each

Kurt Roggensack

2001-01-01

83

Evolution of Young Stars and Their Disks in Serpens  

NASA Astrophysics Data System (ADS)

Unbiased, flux-limited surveys of protoplanetary disks and their parent stars currently exist for only a few clouds, primarily Taurus and IC 348, selected primarily by optical and near-IR data. Such surveys are essential to address questions of disk evolution as a function of stellar parameters such as spectral type, age, accretion activity and environment. Using the ‘Cores to Disks’ (c2d) Spitzer Legacy Program, we discovered a new population of young stellar objects (YSOs) in a region of only 0.8 deg2 in the Serpens Molecular Cloud. This sample contains 150 mid-IR bright (? 3 mJy at 8 ?m) YSOs with infrared excess, having a broad range of SED types and luminosities. Serpens is therefore a unique target region for obtaining a complete, well-defined sample of multi-wavelength observations of young stars in a possible evolutionary sequence. Compared with other clouds such as Taurus and Chamaeleon, Serpens has an exceptionally high star-formation rate (5.7 × 10-5 M? yr-1). Follow-up complimentary observations in the optical, near- and mid-infrared (Spitzer/IRS GO3) have allowed us to characterize both the central stars and the surrounding disks. The shape and slope of the mid-infrared excess provide information on the flaring geometry of the disks. The spectral features give constraints on grain growth and mineralogy, which in turn probes heating and radial mixing. The presence of PAH features traces UV radiation, whereas H? and Br? are used as diagnostics of accretion. Assuming that all stars within a sufficiently small region are nearly coeval, this provides direct constraints on the importance of environment and initial conditions on disk evolution. In this meeting, we have presented our latest results on this rich populations of YSOs, as detailed in Oliveira et al. (2009, 2010). We have discussed connections between the evolution of the disks and that of their harboring stars, and the processes that determine the evolutionary sequence of protoplanetary disks.

Oliveira, Isa; Merín, Bruno; Pontoppidan, Klaus; van Dishoeck, Ewine

2010-11-01

84

Young Star May Be Belching Spheres of Gas, Astronomers Say  

NASA Astrophysics Data System (ADS)

A young star more than 2,000 light-years away in the constellation Cepheus may be belching out spheres of gas, say astronomers who observed it with the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope. Not only is the star ejecting spheres of gas, the researchers say, but it also may be ejecting them repeatedly, phenomena not predicted by current theories of how young stars shed matter. Cepheus A star-forming region with blowups of detail In order to remain stable while accumulating matter, young stars have to throw off some of the infalling material to avoid "spinning up" so fast they would break apart, according to current theories. Infalling matter forms a thin spinning disk around the core of the new star, and material is ejected in twin "jets" perpendicular to the plane of the disk. "Twin jets have been seen emerging from many young stars, so we are quite surprised to see evidence that this object may be ejecting not jets, but spheres of gas," said Paul T.P. Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics. The research is reported in the May 17 edition of the scientific journal Nature. The astronomers observed a complex star-forming region in Cepheus and found an arc of water molecules that act like giant celestial amplifiers to boost the strength of radio signals at a frequency of 22 GHz. Such radio-wave amplifiers, called masers, show up as bright spots readily observed with radio telescopes. "With the great ability of the VLBA to show fine detail, we could track the motions of these maser spots over a period of weeks, and saw that this arc of water molecules is expanding at nearly 20,000 miles per hour," said Ho. "This was possible because we could detect detail equivalent to seeing Lincoln's nose on a penny in Los Angeles from the distance of New York," Ho added. "These observations pushed the tremendous capabilities of the VLBA and of modern computing power to their limits. This is an extremely complex observational project," said Luis F. Rodriguez, of Mexico's National Autonomous University. The arc of water masers can be fit to a nearly-perfect circle to within one part in a thousand. That, the researchers say, means that the water vapor in the arc most likely is part of a complete sphere. "The arc we see fits a circle so well that it is unlikely that any geometry other than that of a sphere would produce it," Ho said. The sphere would be about 1.5 times the size of the Solar System. Because the arc, and presumably the sphere of which it is part, is so thin and so uniform, the researchers say that it came from a single, short-lived ejection. In addition, other evidence suggests that the sphere from an earlier ejection now is being overtaken by a newer spherical bubble that took only about 33 years after being ejected to reach its observed size. "We now have at least one case, we believe, in which a young star has repeatedly ejected mass spherically in short bursts," Guillem Anglada, of the Institute of Astrophysics of Andalucia (CSIC), in Granada, Spain, said. "In light of our current understanding of star formation, we don't yet understand how this can happen, so we have an exciting new scientific challenge. It is surprising that nature can maintain such perfect symmetry, especially since the environment around the young star must be so varied. This appears to be a triumph of order over chaos," he added. The researchers, in addition to Rodriguez, Ho and Anglada, are: Jose M. Torrelles, Institute for Space Studies of Catalonia (IEEC)-Spanish Research Council (CSIC), Spain; Nimesh A. Patel and Lincoln Greenhill, of the Harvard-Smithsonian Center for Astrophysics; Jose F. Gomez, Laboratory for Space Astrophysics and Theoretical Physics of the National Institute for Aerospace Technology, Madrid, Spain; Salvador Curiel and Jorge Canto, of Mexico's National Autonomous University; and Guido Garay, Department of Astronomy of the University of Chile. The VLBA is part of the Natio

2001-05-01

85

JET FORMATION FROM MASSIVE YOUNG STARS: MAGNETOHYDRODYNAMICS VERSUS RADIATION PRESSURE  

SciTech Connect

Observations indicate that outflows from massive young stars are more collimated during their early evolution compared to later stages. Our paper investigates various physical processes that impact the outflow dynamics, i.e., its acceleration and collimation. We perform axisymmetric magnetohydrodynamic (MHD) simulations particularly considering the radiation pressure exerted by the star and the disk. We have modified the PLUTO code to include radiative forces in the line-driving approximation. We launch the outflow from the innermost disk region (r < 50 AU) by magnetocentrifugal acceleration. In order to disentangle MHD effects from radiative forces, we start the simulation in pure MHD and later switch on the radiation force. We perform a parameter study considering different stellar masses (thus luminosity), magnetic flux, and line-force strength. For our reference simulation-assuming a 30 M{sub Sun} star-we find substantial de-collimation of 35% due to radiation forces. The opening angle increases from 20 Degree-Sign to 32 Degree-Sign for stellar masses from 20 M{sub Sun} to 60 M{sub Sun }. A small change in the line-force parameter {alpha} from 0.60 to 0.55 changes the opening angle by {approx}8 Degree-Sign . We find that it is mainly the stellar radiation that affects the jet dynamics. Unless the disk extends very close to the star, its force is too small to have much impact. Essentially, our parameter runs with different stellar masses can be understood as a proxy for the time evolution of the star-outflow system. Thus, we have shown that when the stellar mass (thus luminosity) increases with age, the outflows become less collimated.

Vaidya, Bhargav; Porth, Oliver [Member of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD) and the Heidelberg Graduate School of Fundamental Physics (HGSFP). (Germany); Fendt, Christian; Beuther, Henrik, E-mail: vaidya@mpia.de, E-mail: fendt@mpia.de [Max Planck Institute for Astronomy, Koenigstuhl 17, D-69117 Heidelberg (Germany)

2011-11-20

86

SPIN EVOLUTION OF ACCRETING YOUNG STARS. I. EFFECT OF MAGNETIC STAR-DISK COUPLING  

SciTech Connect

We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and the disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, we neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3 Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than {approx}3 days, in the age range of 1-3 Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3 Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately 1-10 days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.

Matt, Sean P.; Greene, Thomas P. [NASA Ames Research Center, M.S. 245-6, Moffett Field, CA 94035-1000 (United States); Pinzon, Giovanni [Observatorio Astronomico Nacional, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota (Colombia); De la Reza, Ramiro, E-mail: sean.p.matt@nasa.go, E-mail: thomas.p.greene@nasa.go, E-mail: gapinzone@unal.edu.c, E-mail: delareza@on.b [Observatorio Nacional, Rio de Janeiro (Brazil)

2010-05-10

87

NEW YOUNG STAR CANDIDATES IN BRC 27 AND BRC 34  

SciTech Connect

We used archival Spitzer Space Telescope mid-infrared data to search for young stellar objects (YSOs) in the immediate vicinity of two bright-rimmed clouds, BRC 27 (part of CMa R1) and BRC 34 (part of the IC 1396 complex). These regions both appear to be actively forming young stars, perhaps triggered by the proximate OB stars. In BRC 27, we find clear infrared excesses around 22 of the 26 YSOs or YSO candidates identified in the literature, and identify 16 new YSO candidates that appear to have IR excesses. In BRC 34, the one literature-identified YSO has an IR excess, and we suggest 13 new YSO candidates in this region, including a new Class I object. Considering the entire ensemble, both BRCs are likely of comparable ages, within the uncertainties of small number statistics and without spectroscopy to confirm or refute the YSO candidates. Similarly, no clear conclusions can yet be drawn about any possible age gradients that may be present across the BRCs.

Rebull, L. M.; Laher, R.; Legassie, M. [Spitzer Science Center/Caltech, M/S 220-6, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Johnson, C. H.; Carlson, S.; Clark, M.; Killingstad, N.; Koop, S. [Breck School, 123 Ottawa Avenue N., Golden Valley, MN 55422 (United States); Gibbs, J. C.; Aryal, S.; Canakapalli, T. S. [Glencoe High School, 2700 NW Glencoe Rd., Hillsboro, OR 97124 (United States); Linahan, M.; Ezyk, N.; Fagan, J. [Carmel Catholic High School, One Carmel Parkway, Mundelein, IL 60060 (United States); Sartore, D.; Badura, K. S. [Pine Ridge High School, 926 Howland Blvd., Deltona, FL 32738 (United States); Armstrong, J. D. [Las Cumbres Observatory Global Telescope (LCOGT) Network, Inc., 6740 Cortona Dr. Suite 102, Goleta, CA 93117 (United States); Allen, L. E. [National Optical Astronomy Observatory (NOAO), Tucson, AZ 85719 (United States); McGehee, P. [Infrared Processing and Analysis Center (IPAC), M/S 220-6, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Padgett, D. L., E-mail: rebull@ipac.caltech.edu [NASA's Goddard Space Flight Center (GSFC), 8800 Greenbelt Rd., Greenbelt, MD 20771 (United States); and others

2013-01-01

88

A massive planet to the young disc star HD 81040  

NASA Astrophysics Data System (ADS)

We report the discovery of a massive planetary companion orbiting the young disc star HD 81040. Based on five years of precise radial-velocity measurements with the HIRES and ELODIE spectrographs, we derive a spectroscopic orbit with a period P =1001.0 days and eccentricity e = 0.53. The inferred minimum mass for the companion of m_2 sin i = 6.86 M_Jup places it in the high-mass tail of the extrasolar planet mass distribution. From the ELODIE spectra we derive a Lithium abundance of log?(Li) = 1.90, and from the HIRES spectra of the cores of the Ca II H and K lines we derive an activity index of < log R'_HK> = -4.48, suggesting an age of about 0.8 Gyr. The radial-velocity residuals exhibit a scatter significantly larger than the typical internal measurement precision of the instruments. We attribute this excess velocity jitter to activity on the surface of the moderately young host star. However, the amplitude of the jitter is much too small and the expected period of rotation is much too short to explain the observed orbital motion, which we conclude is due to a massive planetary companion.

Sozzetti, A.; Udry, S.; Zucker, S.; Torres, G.; Beuzit, J. L.; Latham, D. W.; Mayor, M.; Mazeh, T.; Naef, D.; Perrier, C.; Queloz, D.; Sivan, J.-P.

2006-04-01

89

New Young Star Candidates in BRC 27 and BRC 34  

NASA Astrophysics Data System (ADS)

We used archival Spitzer Space Telescope mid-infrared data to search for young stellar objects (YSOs) in the immediate vicinity of two bright-rimmed clouds, BRC 27 (part of CMa R1) and BRC 34 (part of the IC 1396 complex). These regions both appear to be actively forming young stars, perhaps triggered by the proximate OB stars. In BRC 27, we find clear infrared excesses around 22 of the 26 YSOs or YSO candidates identified in the literature, and identify 16 new YSO candidates that appear to have IR excesses. In BRC 34, the one literature-identified YSO has an IR excess, and we suggest 13 new YSO candidates in this region, including a new Class I object. Considering the entire ensemble, both BRCs are likely of comparable ages, within the uncertainties of small number statistics and without spectroscopy to confirm or refute the YSO candidates. Similarly, no clear conclusions can yet be drawn about any possible age gradients that may be present across the BRCs.

Rebull, L. M.; Johnson, C. H.; Gibbs, J. C.; Linahan, M.; Sartore, D.; Laher, R.; Legassie, M.; Armstrong, J. D.; Allen, L. E.; McGehee, P.; Padgett, D. L.; Aryal, S.; Badura, K. S.; Canakapalli, T. S.; Carlson, S.; Clark, M.; Ezyk, N.; Fagan, J.; Killingstad, N.; Koop, S.; McCanna, T.; Nishida, M. M.; Nuthmann, T. R.; O'Bryan, A.; Pullinger, A.; Rameswaram, A.; Ravelomanantsoa, T.; Sprow, H.; Tilley, C. M.

2013-01-01

90

Investigating Young Stars Near Earth with GALEX, WISE & 2MASS  

NASA Astrophysics Data System (ADS)

During the past dozen years the existence within 100 pc of Earth of various youthful moving groups has become evident. The ages of these groups span the range 10-200 million years, an era of stellar evolution not easy to study with more distant stars. This era is also a critical time in the formation of rocky planets. Probably most interesting from an observational perspective is the unique role such stars play now and will play in the future in the direct imaging of gas-rich planets, such as those that orbit HR 8799 and beta Pictoris. Most nearby stellar group members with spectral types as late as early-M have been identified or confirmed by virtue of their coronal activity and, hence, relatively large X-ray fluxes in the ROSAT all-sky survey. The survey at UV wavelengths by the GALEX satellite -- in tandem with the infrared 2MASS and WISE catalogs -- now enables us to push on down to discovery of young, chromospherically active, mid-M type stars; these are very numerous and play an important role in all of the stellar and planetary research mentioned above. This work is supported by NASA Astrophysics Data Analysis Program award NNX12AH37G to RIT and UCLA.

Zuckerman, Ben M.; Rodriguez, D.; Kastner, J. H.; Darling, S.; Principe, D.; Montez, R.; Diaz, M.; Vican, L.

2013-01-01

91

Photometric monitoring of the young star Par 1724 in Orion  

NASA Astrophysics Data System (ADS)

We report new photometric observations of the ˜ 200 000 year old naked weak-line run-away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ˜ 5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long-term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University; a telescope of the University Observatory Munich on Mount Wendelstein, the 0.9m ESO-Dutch telescope on La Silla, Chile, and with the All Sky Automated Survey (ASAS) project (www.astrouw.edu.pl/asas).

Neuhäuser, R.; Koeltzsch, A.; Raetz, St.; Schmidt, T. O. B.; Mugrauer, M.; Young, N.; Bertoldi, F.; Roell, T.; Eisenbeiss, T.; Hohle, M. M.; Va?ko, M.; Ginski, C.; Rammo, W.; Moualla, M.; Broeg, C.

2009-05-01

92

High resolution spectroscopy of old stars and young disks  

NASA Astrophysics Data System (ADS)

In Chapter 2, we present the details of LinBrod, a program that calculates synthetic spectra of cool Roche-lobe-filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of non- rotating single stars to yield approximate synthetic spectra. We use the line- broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars. In Chapter 3, we discuss the dwarf nova SS Cygni, a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg. Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K K = 162.5 ± 1.0 km s -1 and q = M K /M wd = 0.685 ± 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 ± 0.075 of the total flux at 5500 Å. Taking the weighted average of our results with previously published results obtained using similar techniques, we find [left angle bracket]K K [right angle bracket] = 163.7 ± 0.7 km s -1 and [left angle bracket] q [right angle bracket] = 0.683 ± 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45° <= i <= 56°. The derived masses of the K star and white dwarf are M K = 0.55 ± 0.13 [Special characters omitted.] and M wd = 0.81 ± 0.19[Special characters omitted.] , where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main- Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted. In Chapters 4 and 5, we report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H 2 emission in the S(1), S(2), and S (4) transitions. Keck NIRSPEC observations of the H2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H 2 emission from 6 of 28 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. The detected emission lines are narrow (~10 km s -1 ), centered at the stellar velocity, and spatially unresolved at scales of 0.4 inches, which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we

Bitner, Martin Allan, Jr.

93

Attacking the X-ray emission properties of young stars with the sword of Orion  

NASA Astrophysics Data System (ADS)

We present a survey of X-ray emission from young stars in the Sword of Orion star-formation region using XMM-Newton's EPIC detectors. We find over 850 X-ray sources, of which more than 700 have near-infrared counterparts consistent with being young stars. The survey enables statistical investigation of the dependence of X-ray emission properties of young stars on fundamental stellar parameters (mass, rotation, age) and environmental features (circumstellar disk, active accretion, circumstellar absorption), and study of structure size in individual coronae through analysis of large flares.

Briggs, K. R.; Güdel, M.

2005-03-01

94

Cosmogenic 36Cl dating of a young basaltic eruption complex, Lathrop Wells, Nevada  

Microsoft Academic Search

It has been proposed that the Lathrop Wells volcanic center, a late Quaternary basaltic complex in southern Nevada, has erupted more than once. In common with most Quaternary basalts, this volcanic center has proved difficult to date by K\\/Ar and other commonly employed methods. We have measured the accumulation of 36Cl in 11 samples from lava flows and volcanic bombs

Marek G. Zreda; Fred M. Phillips; Peter W. Kubik; Pankaj Sharma; David Elmore

1993-01-01

95

Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries  

NASA Astrophysics Data System (ADS)

We study the first ~100 Myr of the evolution of isolated star clusters initially containing 144179 stars, including 13107 (10 per cent) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high-precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (<~100Myr) star clusters in the large Magellanic Cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (~10 per cent). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as ? = -1.8 in the central part of the cluster (where the initial Salpeter mass function had ? = -2.35). About 6-12 per cent of the neutron stars were retained in our simulations; the fraction of retained black holes is 40-70 per cent. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an X-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find ~11 potential X-ray binaries containing a black hole. Binaries consisting of two white dwarfs are quite common, but few (20-30) are sufficiently close that they will merge within a Hubble time due to the emission of gravitational radiation. Clusters with shorter relaxation times tend to produce fewer merging white dwarf binaries. The white dwarf binaries that do merge are all sufficiently massive to produce a Type Ia supernova. The densest cluster produces about twice as many blue stragglers as a field population containing the same number of binaries, and these blue stragglers are more massive, bluer and brighter than in less dense clusters.

Portegies Zwart, Simon F.; McMillan, Stephen L. W.; Makino, Junichiro

2007-01-01

96

Understanding Star Formation in the Rosette Molecular Complex: A Chandra View of the Embedded Young Clusters  

NASA Astrophysics Data System (ADS)

The Rosette Molecular Cloud (RMC) complex represents an ideal laboratory for understanding star formation processes, and we have carried out a Chandra imaging survey of the embedded clusters to characterize the young stellar populations. We present new X-ray results on the embedded young star cluster RMC PL6 and in particular the massive young stellar object AFGL 961, which is previously resolved in the near-infrared (the Rosette Eye, Li et al. 2008).

Wang, Junfeng

2012-05-01

97

Jets from young stars and z-pinch machines  

NASA Astrophysics Data System (ADS)

Outflows and jets are intimately related to the formation of stars, and play a central role in redistributing mass, energy and angular momentum within the core, disk and parent cloud. The interplay between magnetic field and rotation is widely thought to be responsible for launching and collimating these outflows. Shear induced by differential rotation along initially poloidal field lines results in an azimuthal component of the magnetic field being generated; the magnetic pressure gradient then accelerates the plasma, and inflates bipolar magnetic cavities within the circumstellar matter. However, the resulting winding of the magnetic field can be potentially disrupted by magneto-hydrodynamic instabilities. To better understand the role of magnetic fields in shaping these ouflows, a series of experiments on pulsed-power z-pinch machines have been developed. In this talk I will present results related to the formation of jets in young stellar objects and in the laboratory, and draw a parallel between the two systems.

Ciardi, A.

2012-02-01

98

Interactions of flares, accretion and wind in young stars  

NASA Astrophysics Data System (ADS)

Young stars can produce X-rays by different mechanisms: Coronal activity, accretion shocks and shocks in outflows. We propose to observe two targets within one field-of-view (FOV): SU Aur is accreting mass from a disk and shows violent coronal activity. We will study the time evolution of stellar flares on time scales from seconds to minutes using the high count rate in the EPIC and the OM to discover interactions with the accretion streams: The signature of the flare onset will differ in X-rays and UV between flares on coronal loops and flares on those field lines, which carry the accretion stream from the disk. AB Aur is more massive, it shows a tentative modulation in the X-ray luminosity apparently without spectral changes and contrary to model expectations.

Moritz Guenther, Hans

2010-10-01

99

X-ray Emission from YSOs, Protostellar Jets, and Accretion Eruptive Variables  

NASA Astrophysics Data System (ADS)

Imaging in X-rays has become an extremely useful tool to identify YSOs residing in star forming regions. X-ray emission is also being measured in eruptive young stars, the FUOr-EXOr type stars, and in protostellar jets. Recent deep near-IR imaging of the North American and Pelican nebulae in JHKs and narrowband emission lines of H2 and [FeII] have revealed one of the most active, richest star forming regions in the Galaxy. Within a single EPIC FOV lies dozens of resolved outflows, jets, clusters of YSOs, and even eruptive FUOR-EXOr stars currently undergoing outbursts. I propose to obtain XMM-Newton imaging of three regions rich in all three types of objects to render x-ray detections to assist with confirming the YSOs, and to measure the x-ray flux of the eruptive stars and shocked outflows.

Stringfellow, Guy

2010-10-01

100

SpS1-Measuring magnetic fields on young stars  

NASA Astrophysics Data System (ADS)

T Tauri stars (TTSs) are young (~few Myr) late type stars that have only recently emerged from their natal molecular cloud material to become visible at optical wavelengths. It is now generally accepted that accretion of circumstellar disk material onto the surface of a TTS is controlled by a strong stellar magnetic field (e.g. see review by Bouvier et al. 2007). The stellar field appears critical for explaining the rotational properties of TTSs (Bouvier et al. 2007, Herbst et al. 2007) and may also play a critical role in driving the outflows seen from many of these sources (e.g. Shang et al. 2007, Mohanty & Shu 2008). As a result, there is a great deal of interest in measuring the magnetic field properties of TTSs (e.g. Johns-Krull 2007, Donati et al. 2008). In particular, disk locking theories predict that an equilibrium is established where the disk is trunctated at or close to corotation and the stellar rotation rate depends only on the (assumed) dipolar magnetic field strength, the stellar mass, radius, and the mass accretion rate in the disk (see Bouvier et al. 2007).

Johns-Krull, Christopher M.; Valenti, Jeff A.

2010-11-01

101

Young Stars in NGC 6231 and the Sco OB1 Association  

NASA Astrophysics Data System (ADS)

NGC 6231 is a young cluster in the southern sky, around 3-5 Myr old, located at a distance of about 1.6 kpc at the near side of the Sagittarius spiral arm. It forms the nucleus of the extended Sco OB1 association. The cluster is very rich, with more than 100 massive stars, among them 15 O-stars. Radial velocity studies have revealed a very large binary fraction among these OB stars. The young low-mass population has recently been identified using deep X-ray observations. Within the large HII region Gum 55 that surrounds NGC 6231 there exists a major elephant trunk, which shows evidence for recent second-generation star formation in the form of young B-stars surrounded by reflection nebulae and a number of low-mass H? emission stars.

Reipurth, B.

2008-12-01

102

Evidence for Significant Reddening of Young Stars in IC 4182  

NASA Astrophysics Data System (ADS)

The Sd galaxy IC 4182 was host to the Type Ia supernova, SN 1937C. Being relatively nearby and well observed, this supernova has been used over the years to set the absolute magnitude, M_B, for the Type Ia class. Until recently, the best distance modulus estimate was given by Sandage and Tammann (1982) as 28.2+/-0.2 from photographic B and V photometry of the brightest red stars. This result was challenged by Pierce, Ressler, and Shure (1992) who derived (m-M)=27.0+/-0.2 using a similar technique (photometry of brightest red stars), but with CCD and IR arrays in the I and K bands where the effects of extinction are much reduced. Even more recently, Sandage et al. (1992) determined the apparent distance modulus to IC 4182 with unprecented precision by identifying 27 Cepheid variables with HST. They find (m-M)=28.47+/-0.05. In order to assess the importance of internal extinction in IC 4182, we have obtained UBVRI images at the KPNO 2.1m and 4m, and CFH 3.6m telescopes. By comparing the color-magnitude diagrams for IC 4182 with those for Local Group galaxies (M33, IC 1613, and NGC 6822) where the reddening has been studied in detail, we find that the young stars in IC 4182 are reddened by E(B-V) ~ 0.32 mag. This comparison eliminates the effects of stellar evolution to first order. The effects of crowding, which can also artificially redden the blue stars, are evaluated by comparing the high resolution CFHT data with the moderate resolution KPNO data. While we find that the true distance modulus for IC 4182 must be reduced from 28.47 to ~ 27.4, the net effect on M_B for SN 1937C cannot be addressed directly since its colors are not well determined. The Type Ia absolute calibration is a very strong function of the extinction because it affects both the distance and the luminosity correction (R_B=4 E_B-V). Thus, even a color uncertainty of 0.1 in B-V for either the Cepheids or SN 1937C is highly significant.

Jacoby, G. H.; Pierce, M. J.; Massey, P.

1992-09-01

103

Structure, stratigraphy, and eruption dynamics of a young tuff ring: Hanauma Bay, O'ahu, Hawai'i  

NASA Astrophysics Data System (ADS)

The Hanauma Bay-Koko Head complex is one of several young volcanic landforms along the Koko fissure, in southeastern O'ahu. The Hanauma Bay region of the complex comprises two nested tuff rings, inner and outer Hanauma Bay, and multiple smaller vents. The internal structure of the inner tuff ring, well exposed due to subsequent breaching by the ocean and wave erosion, indicates that it formed during a minimum of five distinct phases of deposition that produced five mappable units. Significant inward collapses generated major unconformities that separate the units exposed in the inner wall. The planes of failure are cut by narrow steep-walled, locally overhung channels and gullies, suggesting that the collapse events were each followed by short time breaks during which the deposits were eroded by rainfall runoff. Within each pyroclastic unit, there are many local slump scars and unconformities, suggesting that minor instability of the inner wall was a near-constant feature. From bedding sags and surge bed forms, it is apparent that the vent shifted at least twice during tuff ring growth. Ballistic blocks in the youngest unit indicate that the eruption overlapped in time with a separate eruption to the north, most likely to be that of the Kahauloa tuff ring 880 m away.

Rottas, K. M.; Houghton, B. F.

2012-09-01

104

The X-Ray View of Young Stellar Objects  

NASA Astrophysics Data System (ADS)

X-rays offer ideal access to high-energy phenomena in young, accreting stars. The energy released in magnetic flares has profound effects on the stellar environment. Star-disk magnetic reconnection has been suggested as a possible origin of bipolar jets. Such jets from have been detected at X-ray wavelengths, offering new diagnostics for the energy release and jet shock physics. Finally, eruptive phenomena of FU Ori and EX Lup-type stars have been monitored in X-rays. I will discuss observations and suggest simple models for high-energy eruptive phenomena in young stars.

Guedel, Manuel

2007-08-01

105

Masses and Distance to the Young Triple Star Elias 12  

NASA Astrophysics Data System (ADS)

Elias 12 (V807 Tau) is a pre-main-sequence triple system in the Taurus star forming region. We have been mapping the orbital motion of the components using adaptive optics imaging at the Keck Observatory and previously, with the Fine Guidance Sensors on board the Hubble Space Telescope. Our measurements of the close pair, Elias 12 Na-Nb, now cover nearly a complete orbital period and we present the parameters from the orbital fit. We also measure the center of mass motion of the close pair relative to the wide component, Elias 12S, yielding the mass ratio in addition to the total system mass. Combined with a parallax measured using VLBA radio interferometry, we can determine precise masses for the components in the system. We also present high resolution spectroscopy showing the rotational properties of the northern and southern sources. Results from this program will provide key empirical data for testing pre-main-sequence theoretical models, determining binary formation mechanisms, and investigating the dynamical evolution of young multiple systems.

Schaefer, Gail; Prato, L.; Simon, M.; Zavala, R. T.

2010-01-01

106

SPITZER INFRARED SPECTROGRAPH SURVEY OF YOUNG STARS IN THE CHAMAELEON I STAR-FORMING REGION  

SciTech Connect

We present 5-36 {mu}m mid-infrared spectra of 82 young stars in the {approx}2 Myr old Chamaeleon I star-forming region, obtained with the Spitzer Infrared Spectrograph (IRS). We have classified these objects into various evolutionary classes based on their spectral energy distributions and the spectral features seen in the IRS spectra. We have analyzed the mid-IR spectra of Class II objects in Chamaeleon I in detail, in order to study the vertical and radial structure of the protoplanetary disks surrounding these stars. We find evidence for substantial dust settling in most protoplanetary disks in Chamaeleon I. We have identified several disks with altered radial structures in Chamaeleon I, among them transitional disk candidates which have holes or gaps in their disks. Analysis of the silicate emission features in the IRS spectra of Class II objects in Cha I shows that the dust grains in these disks have undergone significant processing (grain growth and crystallization). However, disks with radial holes/gaps appear to have relatively unprocessed grains. We further find the crystalline dust content in the inner ({approx}<1-2 AU) and the intermediate ({approx}<10 AU) regions of the protoplanetary disks to be tightly correlated. We also investigate the effects of accretion and stellar multiplicity on the disk structure and dust properties. Finally, we compare the observed properties of protoplanetary disks in Cha I with those in slightly younger Taurus and Ophiuchus regions and discuss the effects of disk evolution in the first 1-2 Myr.

Manoj, P.; Kim, K. H.; Watson, Dan M.; Forrest, W. J.; Bohac, C.; Arnold, L. A. [Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627 (United States); Furlan, E. [JPL, Caltech, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States); McClure, M. K.; Calvet, N. [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States); Luhman, K. L. [Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 (United States); Espaillat, C. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-78 Cambridge, MA 02138 (United States); Najita, J. R. [National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States); D'Alessio, P.; Adame, L. [Instituto de AstronomIa, UNAM, Apartado Postal 70-264, Ciudad Universitaria, 04510, Mexico DF (Mexico); Sargent, B. A. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Green, J. D., E-mail: manoj@pas.rochester.edu [Department of Astronomy, University of Texas at Austin, Austin, TX 78712-0259 (United States)

2011-03-15

107

SPECTRAL ENERGY DISTRIBUTIONS OF YOUNG STARS IN IC 348: THE ROLE OF DISKS IN ANGULAR MOMENTUM EVOLUTION OF YOUNG, LOW-MASS STARS  

SciTech Connect

Theoretical work suggests that a young star's angular momentum content and rotation rate may be strongly influenced by magnetic interactions with its circumstellar disk. A generic prediction of these 'disk-locking' theories is that a disk-locked star will be forced to co-rotate with the Keplerian angular velocity of the inner edge of the disk; that is, the disk's inner-truncation radius should equal its co-rotation radius. These theories have also been interpreted to suggest a gross correlation between young stars' rotation periods and the structural properties of their circumstellar disks, such that slowly rotating stars possess close-in disks that enforce the star's slow rotation, whereas rapidly rotating stars possess anemic or evacuated inner disks that are unable to brake the stars and instead the stars spin up as they contract. To test these expectations, we model the spectral energy distributions (SEDs) of 33 young stars in IC 348 with known rotation periods and infrared excesses indicating the presence of circumstellar disks. For each star, we match the observed SED, typically sampling 0.6-8.0 {mu}m, to a grid of 200,000 pre-computed star+disk radiative transfer models, from which we infer the disk's inner-truncation radius. We then compare this truncation radius to the disk's co-rotation radius, calculated from the star's measured rotation period. We do not find obvious differences in the disk truncation radii of slow rotators versus rapid rotators. This holds true both at the level of whether close-in disk material is present at all, and in analyzing the precise location of the inner disk edge relative to the co-rotation radius among the subset of stars with close-in disk material. One interpretation is that disk locking is unimportant for the IC 348 stars in our sample. Alternatively, if disk locking does operate, then it must operate on both the slow and rapid rotators, potentially producing both spin-up and spin-down torques, and the transition from the disk-locked state to the disk-released state must occur more rapidly than the stellar contraction timescale.

Le Blanc, Thompson S.; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235 (United States); Covey, Kevin R. [Department of Astronomy, Cornell University, 226 Space Sciences Building, Ithaca, NY 14853 (United States)

2011-08-15

108

Spectral Energy Distributions of Young Stars in IC 348: The Role of Disks in Angular Momentum Evolution of Young, Low-mass Stars  

NASA Astrophysics Data System (ADS)

Theoretical work suggests that a young star's angular momentum content and rotation rate may be strongly influenced by magnetic interactions with its circumstellar disk. A generic prediction of these "disk-locking" theories is that a disk-locked star will be forced to co-rotate with the Keplerian angular velocity of the inner edge of the disk; that is, the disk's inner-truncation radius should equal its co-rotation radius. These theories have also been interpreted to suggest a gross correlation between young stars' rotation periods and the structural properties of their circumstellar disks, such that slowly rotating stars possess close-in disks that enforce the star's slow rotation, whereas rapidly rotating stars possess anemic or evacuated inner disks that are unable to brake the stars and instead the stars spin up as they contract. To test these expectations, we model the spectral energy distributions (SEDs) of 33 young stars in IC 348 with known rotation periods and infrared excesses indicating the presence of circumstellar disks. For each star, we match the observed SED, typically sampling 0.6-8.0 ?m, to a grid of 200,000 pre-computed star+disk radiative transfer models, from which we infer the disk's inner-truncation radius. We then compare this truncation radius to the disk's co-rotation radius, calculated from the star's measured rotation period. We do not find obvious differences in the disk truncation radii of slow rotators versus rapid rotators. This holds true both at the level of whether close-in disk material is present at all, and in analyzing the precise location of the inner disk edge relative to the co-rotation radius among the subset of stars with close-in disk material. One interpretation is that disk locking is unimportant for the IC 348 stars in our sample. Alternatively, if disk locking does operate, then it must operate on both the slow and rapid rotators, potentially producing both spin-up and spin-down torques, and the transition from the disk-locked state to the disk-released state must occur more rapidly than the stellar contraction timescale.

Le Blanc, Thompson S.; Covey, Kevin R.; Stassun, Keivan G.

2011-08-01

109

Magellanic Cloud stars with TiO bands in emission: binary post-RGB/AGB stars or young stellar objects?  

NASA Astrophysics Data System (ADS)

14 stars from a sample of Magellanic Cloud objects selected to have a mid-infrared flux excess have been found to also show TiO bands in emission. The mid-infrared dust emission and the TiO band emission indicate that these stars have large amounts of hot circumstellar dust and gas in close proximity to the central star. The luminosities of the sources are typically several thousand L?, while the effective temperatures are ˜4000-8000 K which puts them bluewards of the giant branch. Such stars could be post-asymptotic giant branch (post-AGB) stars of mass ˜0.4-0.8 M? or pre-main-sequence stars (young stellar objects) with masses in the range ˜7-19 M?. If the stars are pre-main-sequence stars, they are substantially cooler and younger than stars at the birth line where Galactic protostars are first supposed to become optically visible out of their molecular clouds. They should therefore be hidden in their present evolutionary state, although this problem may be overcome if asymmetries are invoked or if the reduced metallicity of the Small Magellanic Cloud and Large Magellanic Cloud compared to the Galaxy makes the circumstellar material more transparent. The second explanation for these stars is that they are post-AGB or post-red giant branch stars that have recently undergone a binary interaction when the red giant of the binary system filled its Roche lobe. Being oxygen-rich, they have gone through this process before becoming carbon stars. Most of the stars vary slowly on time-scales of 1000 d or more, suggesting a changing circumstellar environment. Apart from the slow variations, most stars also show variability with periods of tens to hundreds of days. One star shows a period that is rapidly decreasing and we speculate that this star may have accreted a large blob of gas and dust on to a disc whose orbital radius is shrinking rapidly. Another star has Cepheid-like pulsations of rapidly increasing amplitude, suggesting a rapid rate of evolution. Seven stars show quasi-periodic variability and one star has a light curve similar to that of an eclipsing binary.

Wood, P. R.; Kamath, D.; Van Winckel, H.

2013-10-01

110

Extreme Star Formation in the Massive Young Cluster Westerlund 1  

NASA Astrophysics Data System (ADS)

We propose to extend Spitzer's study of massive star formation to the massive cluster Westerlund 1, which at ~4 kpc is the closest and most massive 'Super Star Cluster' known in the Galaxy. Star formation may have proceeded differently in this region, having created a higher overall density of coeval massive stars. The proposed observations will allow us to compare star formation in this region to that seen near the Sun, in the massive Cygnus-X complex, and in the outer Galaxy (coming from the studies of W5, the Cycle-5 SMOG project, and GLIMPSE360), and therefore to complete a more representative view of star formation in the Galaxy.

Hora, Joseph; Kraemer, Kathleen; Megeath, Tom; Gutermuth, Rob; Smith, Howard; Martinez Galarza, Juan Rafael; Guzman Fernandez, Andres; Carey, Sean; Koenig, Xavier; Schneider, Nicola; Motte, Frederique; Bontemps, Sylvain; Adams, Joseph; Simon, Robert; Nguyen-Luong, Quang; Schilke, Peter; Keto, Eric; Fazio, Giovanni; Allen, Lori

2012-12-01

111

Long-slit spectroscopic investigations of the forbidden emission line regions of young stars  

Microsoft Academic Search

Our poor knowledge about the physical properties of outflows from young stars (e.g. outflow velocity, collimation, excitation) on scales of a few 100 AU distance from the source was the reason for conducting a long-slit spectroscopic survey of the forbidden emission lines from 38 classical T Tauri stars in the optical. The forbidden emission lines which are thought to be

Gerhard A. Hirth

1994-01-01

112

Near-infrared images of IC 348 and the luminosity functions of young embedded star clusters  

Microsoft Academic Search

We present the results of a sensitive near-infrared (JHK) imaging survey of the young cluster IC 348 and a nearby control field. From comparison of the cluster and control field observations we estimate that 380 sources, the majority of the stars observed in the cluster field, are members of the cluster. The spatial density of these stars is found to

Elizabeth A. Lada; Charles J. Lada

1995-01-01

113

The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters  

Microsoft Academic Search

We introduce a method to relate a possible truncation of the star cluster mass function at the high mass end to the shape of the cluster luminosity function (LF). We compare the observed LFs of five galaxies containing young star clusters with synthetic cluster population models with varying initial conditions. The LF of the SMC, the LMC and NGC 5236

Mark Gieles; Soeren Larsen; Nate Bastian; I. T. Stein

2006-01-01

114

A Multiwavelength Study of the Young Star Clusters and Interstellar Medium in the Antennae Galaxies  

Microsoft Academic Search

We report on a multi-wavelength study of the relationship between young star clus- ters in the Antennae galaxies (NGC 4038\\/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X- rays to radio wavelengths) have become available

Qing Zhang; S. Michael Fall; Bradley C. Whitmore

2001-01-01

115

A Multiwavelength Study of the Young Star Clusters and Interstellar Medium in the Antennae Galaxies  

Microsoft Academic Search

We report on a multiwavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038\\/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the

Qing Zhang; S. Michael Fall; Bradley C. Whitmore

2001-01-01

116

Age and Mass Studies for Young Star Clusters in M31 from SEDS-FIT  

NASA Astrophysics Data System (ADS)

In this paper, we present photometry for young star clusters in M31, which are selected from Caldwell et al. These star clusters have been observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2008 March. The BATC images including these star clusters are taken with 15 intermediate-band filters covering 3000-10000 Å. Combined with photometry in the GALEX far- and near-ultraviolet, broadband UBV RI, SDSS ugriz, and infrared JHK s of Two Micron All Sky Survey, we obtain their accurate spectral energy distributions (SEDs) from 1538 to 20000 Å. We derive these star clusters' ages and masses by comparing their SEDs with stellar population synthesis models. Our results are in good agreement with previous determinations. The mean value of age and mass of young clusters (<2 Gyr) is about 385 Myr and 2 × 104 M ?, respectively. There are two distinct peaks in the age distribution, a highest peak at age ~60 Myr and a secondary peak around 250 Myr, while the mass distribution shows a single peak around 104 M ?. A few young star clusters have two-body relaxation times greater than their ages, indicating that those clusters have not been well dynamically relaxed and therefore have not established the thermal equilibrium. There are several regions showing aggregations of young star clusters around the 10 kpc ring and the outer ring, indicating that the distribution of the young star clusters is well correlated with M31's star-forming regions. The young massive star clusters (age <=100 Myr and mass >=104 M ?) show apparent concentration around the ring splitting region, suggesting a recent passage of a satellite galaxy (M32) through M31 disk.

Wang, Song; Ma, Jun; Fan, Zhou; Wu, Zhenyu; Zhang, Tianmeng; Zou, Hu; Zhou, Xu

2012-12-01

117

X-Rays from Young Stars and Eggs in the Eagle Nebula (M16)  

NASA Astrophysics Data System (ADS)

The Chandra X-ray Observatory observed the Eagle Nebula (M16) a young star forming region containing the dark columns of dust and cold molecular gas known as the ""Pillars of Creation"" or ""elephant trunks"". We identify more than 1000 X-ray sources coincident with K-band stars that are premain sequence stars ranging in spectral type from O to M. A handful of the hard X-ray sources in the pillars are spatially coincident with deeply embedded young stellar objects seen in JHK images. However none of the X-ray sources are associated with the evaporating gaseous globules (EGGs) first observed by Hester et al. (1996).

Linsky, Jeffrey L.; Gagne, Marc; Mytyk, Anna

118

Kinematics of exoplanet host stars: membership in young moving groups and the thin/thick disc  

NASA Astrophysics Data System (ADS)

We present a detailed study of the kinematics of known exoplanets host stars with known parallactic distance and precise proper motion and radial velocity measurements, from where the Galactic space motions (U, V, W) were computed. For the stars with U and V velocity components inside or near the boundaries that determine the young disc population, we have analyzed the possible membership in the classical moving groups and nearby loose associations with ages between 10 and 600 Ma. For the candidate members, we have compiled the information available in the literature in order to constrain their membership by applying age-dating methods for late-type stars. We identify several dozen young exoplanet host star candidates, many of which were considered to have solar-like ages. We also look for old exoplanet host stars in the Galactic thick disc and the thin-thick transition.

Montes, D.; Caballero, J. A.; Rojas-Peña, I.; Sánchez de Miguel, A.; Alloza, L. J.; Bertrán de Lis, S.; Fernández Rodríguez, C. J.; Garrido Rubio, A.; Greciano, R.; Herranz Luque, J. E.; Juárez-Martínez, I.; Manjavacas, E.; Ocaña, F.; Pila Díez, B.; Tapia Ayuga, C. E.

2013-05-01

119

Dust and young embedded stars in the Lagoon Nebula - a near-IR imaging survey.  

NASA Astrophysics Data System (ADS)

A large HII region, the Lagoon Nebula is one of the brightest sources of CO emission in the sky, containing at least two OB stars, and is surrounded by fairly warm (25-30 K) molecular gas and dust. It may be the site of ongoing star formation triggered by the OB stars. We have taken near infra-red broad band data (J,H and Ks) to hunt for deeply embedded young stars and to estimate the dust opacity in the dense gas, by comparing the near-IR extinction to the submillimeter continuum emission mapped with SCUBA. We present initial imaging and photometry over a 30 by 20 arcminute field.

Kenworthy, M. A.; Tothill, N. F. H.

2001-12-01

120

An X-ray Survey of FU Orionis Stars andUnusual X-ray Emission from Embedded YoungStars in NGC 2071  

Microsoft Academic Search

We present new results from the first X-ray survey of accreting FU Orionis stars (FUors) and a pointed X-ray observation of the infrared cluster near the reflection nebula NGC 2071 in the Orion B cloud. Both observations reveal unusual X-ray spectra that challenge interpretive models. FUors are low-mass pre-main sequence (PMS) stars that have undergone optical eruptions attributed to a

Steve L. Skinner; A. E. Simmons; M. Audard; K. R. Briggs; M. Guedel; M. R. Meyer

2006-01-01

121

Young stars of low mass in the Gum nebula  

SciTech Connect

Observations are presented for four recently formed stars in the vicinity of the Gum nebula which are heavily obscured by surrounding dust and are associated with small reflection nebulae. HH46 is the only currently active star of the sample, and it is found to have a spectral type in the range of late G-early K, with superimposed emission lines of H-alpha, Ca II, Fe I, Fe II, and weak He I at near zero velocities. It is suggested that the observed scenario of low-mass stars in an older massive star environment may be analogous to the circumstances surrounding the birth of the sun. 53 refs.

Graham, J.A.; Heyer, M.H. (Carnegie Institution of Washington, Washington, DC (USA))

1989-06-01

122

The Gemini NICI Planet-Finding Campaign: the Frequency of Giant Planets Around Young A Stars  

NASA Astrophysics Data System (ADS)

The Gemini NICI Planet-Finding Campaign is a 3-year direct imaging survey for planets around over 200 young, nearby stars, using the Near Infrared Coronagraphic Imager at Gemini South, a specialized instrument for finding planets. We present here the results from the NICI Campaign's subsample of 69 A and B stars, which is the largest, deepest survey for planets around high mass stars conducted to date. In addition to our previously reported discovery that HD 1160 is a triple system made up of an A star, an M star, and an L-dwarf, the Campaign has found a tight binary brown dwarf around a nearby A star which will be useful for future dynamical mass measurements. We develop a new Bayesian approach to determining ages for A and B stars that yields more realistic age probability distributions for these stars. Monte Carlo simulations are used to determine the completeness of our observations to giant planets, and we find that, consistent with previous studies of solar-type stars, large- and medium-separation (>20 AU) giant planets are relatively rare around A and B stars. At 95% confidence, fewer than 10% of A stars can have a planet with the mass and separation of HR 8799 b.

Nielsen, Eric L.; Liu, M. C.; Wahhaj, Z.; Biller, B. A.; Hayward, T. L.; Close, L. M.; Chun, M. R.; Ftaclas, C.; Toomey, D. W.; Gemini NICI Planet-Finding Campaign Team

2013-01-01

123

The Beta Pictoris phenomenon among young stars. 1: The case of the Herbig AE star UX Orionis  

NASA Astrophysics Data System (ADS)

In this paper we present the first results of the study of very young stars having non-periodic Algol type brightness minimal. In August-September, 1992, a very deep (delta V = 2.5), long lasting minimum of the light of the isolated Herbig Ae star UX Ori occurred. At this event the star was observed photometrically (UBVRI) and polarimetrically at the CAO, and spectroscopically (high resolution: H-alpha and Na I D) at the ESO. The spectroscopic observations were continued at the NSO with the McMath solar/stellar telescope in October-December, 1992, when the star returned to maximum brightness and again at the ESO in July and October, 1993, when the star was bright. The main results of our observations can be briefly summarized as follows: (1) The photometric and polarimetric results are in agreement with the model according to which UX Ori is surrounded by an edge-on circumstellar disk-like envelope, and its variability is caused by variable obscuration of the star by opaque circumstellar dust clouds; (2) The double-peaked H-alpha profile observed at maximum light changed to single-peaked at deep minimum; obscuration of a part of the circumstellar gas by an optically thick dust cloud is causing this variation; (3) The inverse P Cygni profiles and variable redshifted absorption components have been observed in the Na I D lines indicating the infall of cool gas onto the star.

Grinin, V. P.; The, P. S.; de Winter, D.; Giampapa, M.; Rostopchina, A. N.; Tambovtseva, L. V.; van den Ancker, M. E.

1994-12-01

124

The Spatial Structure of Young Stellar Clusters in High-Mass Star-Forming Regions  

NASA Astrophysics Data System (ADS)

The spatial distributions of young stars in star-forming regions can be linked to the theory of clustered star formation using spatial statistical methods. The MYStIX project (Massive Young stellar clusters Study in Infrared and X-rays) provides rich samples of young stars from the nearest high-mass star-forming regions. Maps of stellar surface density reveal diverse structure and subclustering. Young stellar clusters and subclusters are fit with isothermal spheres and ellipsoids using the Bayesian Information Criterion to estimate the number of subclusters. We investigate the relation between (sub)cluster size and density and examine evidence for dynamical relaxation in some of the (sub)clusters. Clustering properties are also measured using Cartwright and Whitworth's Q statistic and the inhomogeneous two-point correlation function, which can be used to distinguish between centrally concentrated and fractal distributions. Mass segregation is detected in several cases, in both centrally concentrated and fractally structured star clusters, but a few clusters are not mass segregated. We discuss the astrophysical implications of the comparison of cluster structures in the different regions.

Kuhn, Michael

2013-01-01

125

Star Formation History of a Young Super-Star Cluster in NGC 4038/39: Direct Detection of Low-Mass Pre-Main Sequence Stars  

NASA Astrophysics Data System (ADS)

We present an analysis of the near-infrared spectrum of a young massive star cluster in the overlap region of the interacting galaxies NGC 4038/39 using population synthesis models. Our goal is to model the cluster population as well as provide rough constraints on its initial mass function (IMF). The cluster shows signs of youth, such as thermal radio emission and strong hydrogen emission lines in the near-infrared. Late-type absorption lines are also present which are indicative of late-type stars in the cluster. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence (PMS) stars or red supergiants alone. Thus, we interpret the spectrum as a superposition of two star clusters of different ages, which is feasible since the 1'' spectrum encompasses a physical region of ?90 pc and radii of super-star clusters (SSCs) are generally measured to be a few parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr-18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. Both are required to accurately reproduce the near-infrared spectrum of the object. Thus, we have directly detected PMS objects in an unresolved SSC for the first time using a combination of population synthesis models and PMS tracks. This analysis serves as a testbed of our technique to constrain the low-mass IMF in young SSCs as well as an exploration of the star formation history of young UC H II regions.

Greissl, Julia; Meyer, Michael R.; Christopher, Micol H.; Scoville, Nick Z.

2010-02-01

126

Candidates for Young Super Star Clusters in the Milky Way  

Microsoft Academic Search

Massive Star Clusters (M > 104 M&sun;) have been known to exist throughout the local Universe, but few such objects have been found within our own Galaxy. These clusters the majority of the galactic OB star formation, and thus dramatically alter their surroundings through winds, ionizing flux and radiation pressure, and supernovae, eventually destroying their natal clouds and inflating superbubbles

Mubdi Rahman; C. D. Matzner; D. Moon

2011-01-01

127

Proper Motion Measurements of Jets from Young Stars  

NASA Astrophysics Data System (ADS)

The first extended proper motion measurements in jets from young stars are presented. In five well-known jet systems (HH 34, HH 46/47, DG Tau, DG Tau B, Haro 6-5B), consisting of a total of eight optically observable jets and counterjets, as well as of six bow shocks, proper motions of the knots (condensations) have been measured. These measurements are based on deep CCD images, taken through narrow-band filters, which are centred on the H? or the [SII]??6716,6731 lines. They have been taken on at least three different epochs with a total epoch difference of at least four years. For several observing runs the seeing conditions were excellent (? 0.''6 -- 0.''9). The proper motion measurements presented allow further principal insights into the physics of the jets and their bow shocks. Among the main results are the following: In all eight investigated jets and counterjets the knots are clearly moving. Their tangential velocities reach values from close to the detection limit (? 20 km s-1) up to 400 km s-1, with most of the measured values in the range from 100 to 350 km s-1. Please note that the knots are generally interpreted as the cooling regions of oblique, crossing shock waves within the jets. For the first time the ratio ? of the pattern speed of the knots to the fluid speed in the jets is investigated. The measured ?-values range from 0.2 to 1.0, with most values between 0.4 and 1.0. Values larger than 1.0 are not observed. The proper motion measurements put important constraints on the various hitherto proposed jet models: obviously they are in contrary with models of stationary jets, which interpret the jet knots as standing shock waves. Also non-stationary models, in which the knots arise from the interaction of jet material of different velocity, are in most cases not supported by the measured ?-values. The measurements are, however, in agreement with jet models, which interpret the knots as running shock waves, excited by Kelvin-Helmholtz instabilities along the jet channel. In the four Herbig-Haro objects HH 34N, HH 34S, HH 47C and HH 47D, which have been interpreted as bow shocks of jets, the internal flow patterns of the condensations have been investigated. They yield important data for detailed investigations of the complex flow motions in the jet head. The measured flow patterns support the idea, that these Herbig-Haro objects are indeed the bow shocks of the jets.

Eislöffel, Jochen

1992-02-01

128

STAR FORMATION AND YOUNG STELLAR CONTENT IN THE W3 GIANT MOLECULAR CLOUD  

SciTech Connect

In this work, we have carried out an in-depth analysis of the young stellar content in the W3 giant molecular cloud (GMC). The young stellar object (YSO) population was identified and classified in the Infrared Array Camera/Multiband Imaging Photometer color-magnitude space according to the 'Class' scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low-/intermediate-mass pre-main-sequence (PMS) stars selected through their colors and magnitudes in the Two Micron All Sky Survey. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate-mass objects can be more reliably identified. By means of the minimum spanning tree algorithm and our YSO spatial distribution and age maps, we investigated the YSO groups and the star formation history in W3. We find signatures of clustered and distributed star formation in both triggered and quiescent environments. The central/western parts of the GMC are dominated by large-scale turbulence likely powered by isolated bursts of star formation that triggered secondary star formation events. Star formation in the eastern high-density layer (HDL) also shows signs of quiescent and triggered stellar activity, as well as extended periods of star formation. While our findings support triggering as a key factor for inducing and enhancing some of the major star-forming activity in the HDL (e.g., W3 Main/W3(OH)), we argue that some degree of quiescent or spontaneous star formation is required to explain the observed YSO population. Our results also support previous studies claiming a spontaneous origin for the isolated massive star(s) powering KR 140.

Rivera-Ingraham, Alana [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada); Martin, Peter G. [Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8 (Canada); Polychroni, Danae [INAF-IFSI, via Fosso del Cavaliere 100, 00133 Roma (Italy); Moore, Toby J. T. [Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD (United Kingdom)

2011-12-10

129

Star Formation and Young Stellar Content in the W3 Giant Molecular Cloud  

NASA Astrophysics Data System (ADS)

In this work, we have carried out an in-depth analysis of the young stellar content in the W3 giant molecular cloud (GMC). The young stellar object (YSO) population was identified and classified in the Infrared Array Camera/Multiband Imaging Photometer color-magnitude space according to the "Class" scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low-/intermediate-mass pre-main-sequence (PMS) stars selected through their colors and magnitudes in the Two Micron All Sky Survey. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate-mass objects can be more reliably identified. By means of the minimum spanning tree algorithm and our YSO spatial distribution and age maps, we investigated the YSO groups and the star formation history in W3. We find signatures of clustered and distributed star formation in both triggered and quiescent environments. The central/western parts of the GMC are dominated by large-scale turbulence likely powered by isolated bursts of star formation that triggered secondary star formation events. Star formation in the eastern high-density layer (HDL) also shows signs of quiescent and triggered stellar activity, as well as extended periods of star formation. While our findings support triggering as a key factor for inducing and enhancing some of the major star-forming activity in the HDL (e.g., W3 Main/W3(OH)), we argue that some degree of quiescent or spontaneous star formation is required to explain the observed YSO population. Our results also support previous studies claiming a spontaneous origin for the isolated massive star(s) powering KR 140.

Rivera-Ingraham, Alana; Martin, Peter G.; Polychroni, Danae; Moore, Toby J. T.

2011-12-01

130

APOD: Companion of a Young, Sun-like Star Confirmed  

NSDL National Science Digital Library

This APOD page features the first confirmed image of an extrasolar planet orbiting a sun-like star. Text and numerous links provide additional information; one link goes to the press release of the confirmation.

2012-01-21

131

Characterisation of young nearby stars : the Ursa Major group  

Microsoft Academic Search

This work presents a homogeneous set of stellar parameters for a larger sample of kinematic Ursa Major (UMa) group members with spectral types late-F to early-K. The UMa group is comprised of the stars in the UMa cluster in the Big Dipper constellation and of many co-moving stars spread over the whole sky. The definition of kinematic membership criteria has

M. Ammler

2006-01-01

132

The Velocity Field of Young Stars in the Solar Neighbourhood  

Microsoft Academic Search

A sample of O- and B-type stars with Hipparcos astrometric data, ages computed from Strmgren photometry and radial velocities,\\u000a has been used to characterize the structure, age and kinematics of the Gould Belt system. The local spiral structure of our\\u000a galaxy is determined from this sample, and also from a sample of Hipparcos Cepheid stars. The Gould Belt, with an

J. Torra; D. Fernández; F. Figueras; F. Comerón

2000-01-01

133

M20: Star Formation in a Young HII Region  

Microsoft Academic Search

The Trifid Nebula (M20) has a remarkable optical appearance with a large, reddish nebula of gas ionized by an O7 star (HD 164492) and trisected by obscuring dust lanes, with a blue reflection nebula in the north. During the last two decades, M20 has generated considerable interest because of multi-wavelength identifications of sites of low- and high-mass star formation. M20

J. Rho; B. Lefloch; W. T. Reach; J. Cernicharo

2008-01-01

134

IDENTIFYING THE YOUNG LOW-MASS STARS WITHIN 25 pc. II. DISTANCES, KINEMATICS, AND GROUP MEMBERSHIP  

SciTech Connect

We have conducted a kinematic study of 165 young M dwarfs with ages of {approx}<300 Myr. Our sample is composed of stars and brown dwarfs with spectral types ranging from K7 to L0, detected by ROSAT and with photometric distances of {approx}<25 pc assuming that the stars are single and on the main sequence. In order to find stars kinematically linked to known young moving groups (YMGs), we measured radial velocities for the complete sample with Keck and CFHT optical spectroscopy and trigonometric parallaxes for 75 of the M dwarfs with the CAPSCam instrument on the du Pont 2.5 m Telescope. Due to their youthful overluminosity and unresolved binarity, the original photometric distances for our sample underestimated the distances by 70% on average, excluding two extremely young ({approx}<3 Myr) objects found to have distances beyond a few hundred parsecs. We searched for kinematic matches to 14 reported YMGs and identified 10 new members of the AB Dor YMG and 2 of the Ursa Majoris group. Additional possible candidates include six Castor, four Ursa Majoris, two AB Dor members, and one member each of the Her-Lyr and {beta} Pic groups. Our sample also contains 27 young low-mass stars and 4 brown dwarfs with ages {approx}<150 Myr that are not associated with any known YMG. We identified an additional 15 stars that are kinematic matches to one of the YMGs, but the ages from spectroscopic diagnostics and/or the positions on the sky do not match. These warn against grouping stars together based only on kinematics and that a confluence of evidence is required to claim that a group of stars originated from the same star-forming event.

Shkolnik, Evgenya L. [Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001 (United States); Anglada-Escude, Guillem [Institut fuer Astrophysik, Universitaet Goettingen, Friedrich-Hund-Platz 1, D-37077 Goettingen (Germany); Liu, Michael C.; Bowler, Brendan P. [Institute for Astronomy, University of Hawaii at Manoa 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Weinberger, Alycia J.; Boss, Alan P. [Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015 (United States); Reid, I. Neill [Space Telescope Science Institute, Baltimore, MD 21218 (United States); Tamura, Motohide, E-mail: shkolnik@lowell.edu [National Astronomical Observatory of Japan, Tokyo (Japan)

2012-10-10

135

Gravitational Slingshot of Young Massive Stars in Orion  

NASA Astrophysics Data System (ADS)

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest among the ONC's ~1000 members are: ?1 Ori C, the most massive binary in the cluster with stars of masses 38 and 9 M ? the Becklin-Neugebauer (BN) object, a 30 km s-1 runaway star of ~8 M ? and the Kleinmann-Low (KL) nebula protostar, a highly obscured, ~15 M ? object still accreting gas while also driving a powerful, apparently "explosive" outflow. The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here, we report the results of a systematic survey using ~107 numerical experiments of gravitational interactions of the ?1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for ?1C. Five other observed properties of ?1C are also consistent with it having ejected BN and altogether we estimate that there is only a <~ 10-5 probability that ?1C has these properties by chance. We conclude that BN was dynamically ejected from the ?1C system about 4500 years ago. BN then plowed through the KL massive star-forming core within the last 1000 years causing its recently enhanced accretion and outflow activity.

Chatterjee, Sourav; Tan, Jonathan C.

2012-08-01

136

The Dispersal of Young Stars and the Greater Sco-Cen Association  

NASA Astrophysics Data System (ADS)

We review topics related to the dispersal of young stars from their birth-sites, and focus in particular on the entourage of young stars related to the ongoing star-formation event in the Sco-Cen OB association. We conduct a follow-up kinematic study to that presented in Mamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby, isolated, young stars. In addition to the eta Cha and TW Hya groups, we find several more intriguing Sco-Cen outlier candidates: most notably ? Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connection between Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds, and in particular, Corona Australis. The kinematic evidence suggests that many of the nearby, isolated ~10 Myr-old stars were born near Sco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesize that these stars inherited 5-10 km/s velocities moving away from Sco-Cen, either through molecular cloud turbulence, or through formation in molecular clouds associated with the expanding Sco-Cen superbubbles (e.g. Loop I).

Mamajek, E. E.; Feigelson, E. D.

137

Constraints on the low-mass IMF in young super-star clusters in starburst galaxies  

NASA Astrophysics Data System (ADS)

As evidence for variations in the initial mass function (IMF) in nearby star forming regions remains elusive we are forced to expand our search to more extreme regions of star formation. Starburst galaxies, which contain massive young clusters have in the past been reported to have IMFs different than that characterizing the field star IMF. In this thesis we use high signal-to-noise near-infrared spectra to place constraints on the shape of the IMF in extreme regions of extragalactic star formation and also try to understand the star formation history in these regions. Through high signal-to-noise near-infrared spectra it is possible to directly detect low-mass PMS stars in unresolved young super-star clusters, using absorption features that trace cool stars. Combining Starburst99 and available PMS tracks it is then possible to constrain the IMF in young super-star clusters using a combination of absorption lines each tracing different ranges of stellar masses and comparing observed spectra to models. Our technique can provide a direct test of the universality of the IMF compared to the Milky Way. We have obtained high signal-to-noise H- and K-band spectra of two young super-star clusters in the starburst galaxies NGC 4039/39 and NGC 253 in order to constrain the low-mass IMF and star formation history in the clusters. The cluster in NGC 4038/39 shows signs of youth such as thermal radio emission and strong hydrogen emission lines as well as late-type absorption lines indicative of cool stars. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence stars or red supergiants alone. We interpret the spectrum as a superposition of two star clusters of different ages over the physical region of 90 pc our spectrum represents. One cluster is young (? 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr - 18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. While the superposition of clusters does not allow us to place stringent constraints on the IMF there is no evidence of a low-mass cutoff in the cluster and the IMF is consistent with a Chabrier and Kroupa IMF typical of the field. The cluster in NGC 253 shows the same signs of youth as the cluster in NGC 4038/39 and sits in front of a background population of older stars. The background population has an age of ? 12 Myr and thus contains red supergiants. After carefully subtracting this background we model the spectrum of the young cluster. We find that its IMF is consistent with a Chabrier and Kroupa IMF with a best-fit power-law slope of 1.0 in linear units. Slopes of 0.0 - 1.5 are also formally consistent with the cluster spectrum. We conclude that there is no strong evidence for an unusual IMF or a lack of low-mass stars (? 1 M? ) in either of these galaxies.

Greissl, Julia Jennifer

2010-12-01

138

Constraining globular cluster formation through studies of young massive clusters - I. A lack of ongoing star formation within young clusters  

NASA Astrophysics Data System (ADS)

We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 104 < M/M? < 108, 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star formation within the clusters. Such episodes of secondary (or continuous) star formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star formation histories within Large/Small Magellanic Cloud intermediate-age clusters (1-2 Gyr), also imply that many YMCs should show ongoing star formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar initial mass function variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.

Bastian, N.; Cabrera-Ziri, I.; Davies, B.; Larsen, S. S.

2013-10-01

139

CoRot observation of a young Sun-like star  

NASA Astrophysics Data System (ADS)

The present study reports measurements of the rotation period of a young solar analogue, estimates of its surface coverage by photospheric starspots and of its chromospheric activity level, and derivations of its evolutionary status. It compares the chromospheric activity level of this young star with a model of chromospheric activity evolution established by combining relationships between the R' HK index and the Rossby number with a recent model of stellar rotation evolution on the main-sequence.

Gondoin, P.; Gandolfi, D.; Fridlund, M.; Guenther, E. W.; Hatzes, A.

2013-07-01

140

High molecular gas fractions in normal massive star-forming galaxies in the young Universe.  

PubMed

Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today's massive spiral galaxies. The slow decrease between z approximately 2 and z approximately 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies. PMID:20148033

Tacconi, L J; Genzel, R; Neri, R; Cox, P; Cooper, M C; Shapiro, K; Bolatto, A; Bouché, N; Bournaud, F; Burkert, A; Combes, F; Comerford, J; Davis, M; Schreiber, N M Förster; Garcia-Burillo, S; Gracia-Carpio, J; Lutz, D; Naab, T; Omont, A; Shapley, A; Sternberg, A; Weiner, B

2010-02-11

141

SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS  

SciTech Connect

We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

Matt, Sean P. [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette (France); Pinzon, Giovanni [Observatorio Astronomico Nacional, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota (Colombia); Greene, Thomas P. [NASA Ames Research Center, M.S. 245-6, Moffett Field, CA 94035-1000 (United States); Pudritz, Ralph E., E-mail: sean.matt@cea.fr, E-mail: thomas.p.greene@nasa.gov, E-mail: gapinzone@unal.edu.co, E-mail: pudritz@physics.mcmaster.ca [Physics and Astronomy Department, McMaster University, Hamilton, ON L8S 4M1 (Canada)

2012-01-20

142

Gravitational Slingshot of Young Massive Stars in Orion  

NASA Astrophysics Data System (ADS)

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest amongst the ONC's 1000 members are: theta1C, the most massive binary in the cluster with stars of masses 38 and 9 Msun; the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of 8 Msun; and the Kleinmann-Low (KL) nebula protostar, a highly-obscured, 15 Msun object still accreting gas while also driving a powerful, apparently "explosive" outflow. The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here we report the results of a systematic survey using 10^7 numerical experiments of gravitational interactions of the theta1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for theta1C. Several other observed properties of theta1C are also consistent with it having ejected BN and altogether we estimate there is only a 10^-5 probability that theta1C has these properties by chance. Our results suggest that after being launched from theta1C 4,500 years ago, BN has plowed through the KL massive-star-forming core within the last 1,000 years causing its recently-enhanced accretion and outflow activity.

Tan, Jonathan; Chatterjee, S.

2012-05-01

143

Structure of young star clusters in the Large Magellanic Cloud  

Microsoft Academic Search

The surface brightness profiles presented for 10 rich star clusters in the LMC extend over 8-10 mag in surface brightness, and to radii of 4 arcmin. The crossing times are shorter than the ages of the clusters, and the two-body relaxation times, except in a few of the cores, are noted to be significantly longer than the ages; the clusters

Rebecca A. W. Elson; S. Michael Fall; Kenneth C. Freeman

1987-01-01

144

Instabilities in Very Young Neutron Stars: Electron Fraction  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the mixing of composition which results from the convective motions. The variable plotted is the electron fraction Ye, which ranges from 0.2 to 0.5.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

145

Young and intermediate-age massive star clusters.  

PubMed

An overview of our current understanding of the formation and evolution of star clusters is given, with the main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a small percentage of star formation occurs in clusters that remain bound, although it is not yet clear whether this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on time scales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (>10(5) M(o)) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, dN/dM proportional to M(-2), but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M > 2 x 10(5) M(o). In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several x10(6) M(o). The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation. PMID:20083510

Larsen, Søren S

2010-02-28

146

A Dynamical Origin for Early Mass Segregation in Young Star Clusters  

Microsoft Academic Search

Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. We show that mergers of small clumps that

Steve McMillan; Enrico Vesperini; Simon Portegies Zwart

2008-01-01

147

Numerical simulations of steady and pulsed non-adiabatic magnetised jets from young stars  

Microsoft Academic Search

In contrast to jets from radio galaxies, energy losses due to radiation effects, atomic hydrogen ionisa- tion\\/recombination and molecular hydrogen dissociation are important in jets from young stars. Moreover there is now gen- eral agreement that magnetic fields may play a very important role not only in the formation of these jets but also their subse- quent collimation. With these

S. O'Sullivan; T. P. Ray

148

The relevance of the IUE results on young stars for Earth's paleoatmosphere  

Microsoft Academic Search

Using the latest IUE results for seven T Tauri stars, which are believed to represent the young Sun and a detailed photochemical chemical model of the paleoatmosphere, the vertical distribution of Oxygen and Ozone in the early atmosphere was calculated. The calculations indicate that the surface Oxygen mixing ratio is as much as six orders of magnitude larger than previously

V. M. Canuto; J. S. Levine; T. R. Augustsson; C. L. Imhoff; M. S. Giampapa

1982-01-01

149

The Dust and Gas Content of a Disk around the Young Star HR 4796A  

Microsoft Academic Search

We have used the James Clerk Maxwell Telescope in Hawaii to search at submillimeter wavelengths for continuum emission from dust, and spectral line emission from carbon monoxide (CO) gas, in the neighborhood of HR 4796A. This young star has a dusty disk with a central cavity, where planets may have formed. We detect the dust component at a wavelength of

J. S. Greaves; V. Mannings; W. S. Holland

2000-01-01

150

Features of Sources of Radiation in the Environment of Young Stars  

NASA Astrophysics Data System (ADS)

Using the results of broad-band ground-based and extra-atmospheric infrared photometric observations, we studied the spectral energy distributions of 87 young stars in the 0.36-100 ?m range. Only 5 types of SED curves are met among the objects of our sample. We suggest to append the photometric classification scheme based on the IR spectra of young stars, earlier introduced by other authors. For some stars, we observe up to three wavelength intervals with excess radiation in the spectrum, which can be explained with the presence of additional thermal radiation sources in the system. The most probable temperatures of these sources are {˜}1500{-}2500 and 90{-}120 K.

Ismailov, N. Z.; Alimardanova, F. N.

2010-02-01

151

The Nearby, Young, Isolated, Dusty Star HD 166191  

NASA Astrophysics Data System (ADS)

We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 ?m is exceptionally high (L IR/L bol ~ 10%). Near-diffraction-limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at ~10 ?m. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building.

Schneider, Adam; Song, Inseok; Melis, Carl; Zuckerman, B.; Bessell, Mike; Hufford, Tara; Hinkley, Sasha

2013-11-01

152

The expansion of massive young star clusters - observation meets theory  

NASA Astrophysics Data System (ADS)

Context. Most stars form as part of a star cluster. The most massive clusters in the Milky Way exist in two groups - loose and compact clusters - with significantly different sizes at the end of the star formation process. After their formation, both types of clusters expand up to a factor 10-20 within the first 20 Myr of their development. Gas expulsion at the end of the star formation process is usually regarded as the only possible process that can lead to such an expansion during this early period of development. Aims: We investigate the effect of gas expulsion by a direct comparison between numerical models and observed clusters and concentrate on clusters with masses >103M?. For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation efficiency (SFE) are investigated. Methods: We performed N-body simulations of the cluster expansion process after gas expulsion and compared the results with observations. Results: We find that the expansion processes of the observed loose and compact massive clusters are driven by completely different physical processes. As expected, the expansion of loose massive clusters is largely driven by the gas loss due to the low SFE of ~30%. One new revelation is that all the observed massive clusters of this group seem to have a very similar size of 1-3 pc at the onset of expansion. It is demonstrated that compact clusters have a much higher effective SFE of 60-70% and are as a result much less affected by gas expulsion. Their expansion is mainly driven by stellar ejections caused by interactions between the cluster members. The reason ejections are so efficient in driving cluster expansion is that they occur dominantly from the cluster centre and over an extended period of time. During the first 10 Myr the internal dynamics of loose and compact clusters thus differ fundamentally. Appendices are available in electronic form at http://www.aanda.org

Pfalzner, Susanne; Kaczmarek, Thomas

2013-11-01

153

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model  

NASA Astrophysics Data System (ADS)

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = Rx, where the Keplerian angular speed of rotation Omegax equals the angular speed of the star Omega*. For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, Rx exceeds the radius of the star R* by a factor of a few, and the inner disk is effectively truncated at a radius Rt somewhat smaller than Rx. Where the closed field lines between Rt and Rx bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as Rx remains significantly greater than R*. Exterior to Rx field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate Mw equal to a definite fraction f of the disk accretion rate MD. For high disk accretion rates, Rx is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Shu, Frank; Najita, Joan; Ostriker, Eve; Wilkin, Frank; Ruden, Steven; Lizano, Susana

1994-07-01

154

A Multiwavelength Study of the Young Star Clusters and Interstellar Medium in the Antennae Galaxies  

NASA Astrophysics Data System (ADS)

We report on a multiwavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the past several years. Quantitative comparisons are made between the positions of the star clusters (broken into three age groups) and the properties of the interstellar medium by calculating the two-point correlation functions. We find that young star clusters are distributed in a clustered fashion, demonstrated by power-law angular autocorrelation functions with slopes in the range -0.8 to -1.0. The young embedded clusters (ages ~5 Myr) are found to be more associated with long-wavelength radiation (mid-infrared and longer), while clusters with ages ~10 Myr or older are more associated with short-wavelength radiation (e.g., far-UV and X-ray). The youngest star clusters are associated with molecular cloud complexes with characteristic radii of about 1 kpc. In addition, there is a weak tendency for them to be found in regions with higher H I velocity dispersions. There is some evidence that both cloud-cloud collisions and shocks from recent star formation can trigger star cluster formation, but no dominant triggering mechanism is identified for the majority of the clusters in the Antennae. Feedback from young bright cluster complexes reveals itself in the form of large H? bubbles and H? velocity gradients in shells around the complexes. We estimate the current star formation rate to be ~20 Msolar yr-1 and the gas consumption timescale to be ~700 Myr. The latter is comparable to the merging timescale and indicates that star formation has been enhanced by the merger event. Finally, we find that the Schmidt law, with index N~-1.4, is also a good description of the cluster formation triggered by merging in the Antennae. There is some evidence that feedback effects may modify the Schmidt law at scales below 1 kpc.

Zhang, Qing; Fall, S. Michael; Whitmore, Bradley C.

2001-11-01

155

Discovering young stars in the Gum 31 region with infrared observations  

NASA Astrophysics Data System (ADS)

Context. The Gum 31 bubble, which contains the stellar cluster NGC 3324, is a poorly studied young region close to the Carina Nebula. Aims: We are aiming to characterise the young stellar and protostellar population in and around Gum 31 and to investigate the star-formation process in this region. Methods: We identified candidate young stellar objects from Spitzer, WISE, and Herschel data. Combining these, we analysed the spectral energy distributions of the candidate young stellar objects. With density and temperature maps obtained from Herschel data and comparisons to a collect-and-collapse scenario for the region we are able to further constrain the characteristics of the region as a whole. Results: We find 661 candidate young stellar objects from WISE data; 91 protostar candidates are detected through Herschel observations in a 1.0° × 1.1° area. Most of these objects are found in small clusters or are well aligned with the H II bubble. We also identify the sources of Herbig-Haro jets. The infrared morphology of the region suggests that it is part of the larger Carina Nebula complex. Conclusions: The location of the candidate young stellar objects on the rim of the H II bubble is suggestive of their being triggered according to a collect-and-collapse scenario, which agrees well with the observed parameters of the region. Some candidate young stellar objects are found in the heads of pillars, which indicates radiative triggering of star formation. All in all, we find evidence that in the region different mechanisms of triggered star formation are at work. Correcting the number of candidate young stellar objects for contamination, we find ~600 young stellar objects in Gum 31 above our completeness limit of about 1 M?. Extrapolating the initial mass function down to 0.1 M?, we estimate a total population of ~5000 young stars for the region. This work is based in part on data collected by Herschel, an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA, and on data observed by VISTA (ESO run number 088.C-0117(A)), an ESO survey telescope developed by a consortium of 18 universities in the United Kingdom, led by Queen Mary, University of London.Tables 1-3 are available in electronic form at http://www.aanda.org

Ohlendorf, H.; Preibisch, T.; Gaczkowski, B.; Ratzka, T.; Ngoumou, J.; Roccatagliata, V.; Grellmann, R.

2013-04-01

156

StarChild: A Learning Center for Young Astronomers  

NSDL National Science Digital Library

NASA's well known High Energy Astrophysics Learning Center has created an astronomy site just for kids. StarChild, still under construction at this time, contains sections on the solar system, the universe, and "space stuff" (astronauts, space travel, and the Hubble Space Telescope, among others). It also includes a glossary with brief explanations of astronomy-related terms. The site is intensively graphical, includes QuickTime and .avi movies, and is intended for the 4-14 year age group. Content is written at two reading levels, with level two available now, and level one due by December.

Dejoie, Joyce.; Truelove, Elizabeth.

1996-01-01

157

The mass function of young star clusters in spiral galaxies  

Microsoft Academic Search

Aims: The initial cluster mass function (ICMF) in spiral discs is constrained and compared with data for old globular clusters and young clusters in starbursts. Methods: For a given absolute magnitude, the cluster age distribution depends on the ICMF. Here, the behaviour of the median age-magnitude relation is analysed in detail for Schechter ICMFs with various cut-off masses, M_c. The

Søren S. Larsen

2009-01-01

158

Proper motions of young stars in Chamaeleon (Lopez Marti+, 2013)  

NASA Astrophysics Data System (ADS)

We present the results of a search for new candidate members of the Chamaeleon I and II star forming regions based on proper motions and multiwavelength photometry. Kinematic candidate members are initially selected in an area of 3 degrees around each cloud on the basis of proper motions and colours using the UCAC4 catalogue. The SEDs of the objects are constructed using photometry retrieved from the Virtual Observatory and other resources, and fitted to models of stellar photospheres in order to derive effective temperatures, gravity values and luminosities. Masses and ages are estimated by comparison with theoretical evolutionary tracks in a Hertzprung-Russell diagram. Objects with ages <=20Myr are selected as probable members of the moving groups. The properties of our candidates are compared with those of the previously known members of the clouds. (5 data files).

Lopez Marti, B.; Jimenez-Esteban, F.; Bayo, A.; Barrado, D.; Solano, E.; Bouy, H.; Rodrigo, C.

2013-07-01

159

A dusty torus around the luminous young star LkH alpha101.  

PubMed

A star forms when a cloud of dust and gas collapses. It is generally believed that this collapse first produces a flattened rotating disk, through which matter is fed onto the embryonic star at the centre of the disk. When the temperature and density at the centre of the star pass a critical threshold, thermonuclear fusion begins. The remaining disk, which can still contain up to 0.3 times the mass of the star, is then sculpted and eventually dissipated by the radiation and wind from the newborn star. But this picture of the structure and evolution of the disk remains speculative because of the lack of morphological data of sufficient resolution and uncertainties regarding the underlying physical processes. Here we present images of a young star, LkH alpha101, in which the structure of the inner accretion disk is resolved. We find that the disk is almost face-on, with a central gap (or cavity) and a hot inner edge. The cavity is bigger than previous theoretical predictions, and we infer that the position of the inner edge is probably determined by sublimation of dust grains by direct stellar radiation, rather than by disk-reprocessing or viscous-heating processes as usually assumed. PMID:11234003

Tuthill, P G; Monnier, J D; Danchi, W C

2001-02-22

160

Search for associations containing young stars: chemical tagging IC 2391 and the Argus association  

NASA Astrophysics Data System (ADS)

We explore the possible connection between the open cluster IC 2391 and the unbound Argus association identified by the search for associations containing young stars survey. In addition to common kinematics and ages between these two systems, here we explore their chemical abundance patterns to confirm if the two substructures shared a common origin. We carry out a homogenous high-resolution elemental abundance study of eight confirmed members of IC 2391 as well as six members of the Argus association using UVES spectra. We derive spectroscopic stellar parameters and abundances for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba. All stars in the open cluster and Argus association were found to share similar abundances with the scatter well within the uncertainties, where [Fe/H] = -0.04 ± 0.03 for cluster stars and [Fe/H] = -0.06 ± 0.05 for Argus stars. Effects of overionization/excitation were seen for stars cooler than roughly 5200 K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6 dex were observed in both systems. The common ages, kinematics and chemical abundances strongly support the fact that the Argus association stars originated from the open cluster IC 2391. Simple modelling of this system finds this dissolution to be consistent with two-body interactions.

De Silva, G. M.; D'Orazi, V.; Melo, C.; Torres, C. A. O.; Gieles, M.; Quast, G. R.; Sterzik, M.

2013-05-01

161

A search for young stars in the halo of M31  

NASA Astrophysics Data System (ADS)

Previous studies have identified young hot stars that appear to have been formed in the halo of our Galaxy. Here we present preliminary results of a spectroscopic search for similar objects in the halo of M31. Radial velocities have been measured for 23 apparently blue objects in a B(v) magnitude range of about 17-21. For one of these targets, AND 0029+413, the radial velocity suggests that the object is gravitationally bound to M31 and hence that it may be in its halo. However, recent CCD photometry for this object indicates that it is redder than implied by the original photographic observations. A number of possible scenarios are presented to explain this object, including one consistent with it being a young star in the halo of M31.

McCausland, R. J. H.; Conlon, E. S.; Dufton, P. L.; Fitzsimmons, A.; Irwin, M. J.; Keenan, F. P.

1993-07-01

162

Gas and stars in compact (young) radio sources  

NASA Astrophysics Data System (ADS)

Gas can be used to trace the formation and evolution of galaxies as well as the impact that the nuclear activity has on the surrounding medium. For nearby compact radio sources, we have used observations of neutral hydrogen - that we detected in emission distributed over very large scales - combined with the study of the stellar population and deep optical images to investigate the history of the formation of their host galaxy and the triggering of the activity. For more distant and more powerful compact radio sources, we have used optical spectra and H I - in absorption - to investigate the presence of fast outflows that support the idea that compact radio sources are young radio loud AGN observed during the early stages of their evolution and currently shredding their natal cocoons through extreme circumnuclear outflows. We will review the most recent results obtained from these projects.

Morganti, B.; Emonts, R.; Holt, J.; Tadhunter, C.; Oosterloo, T.; Struve, C.

2009-02-01

163

Identifying Planet-Forming Disks Around Young Stars  

NASA Astrophysics Data System (ADS)

In the past few years, several disks with inner holes that are relatively empty of small dust grains have been detected and are known as transitional disks. Spitzer identified a new class of “pre-transitional disks” with gaps; these objects have an optically thick inner disk separated from an optically thick outer disk by an optically thin disk gap. Here we review spectral observations which provided the first confirmations of gaps in the pre-transitional disks of LkCa 15 and UX Tau A. We also review the results of a Spitzer IRS study of variability in transitional and pre-transitional objects. The structure and behavior of pre-transitional and transitional disks may be a sign of young planets forming in these disks and future studies of these disks will provide constraints to aid in theoretical modeling of planet formation.

Espaillat, C.

2013-04-01

164

Formation of massive black holes through runaway collisions in dense young star clusters  

Microsoft Academic Search

A luminous X-ray source is associated with MGG 11-a cluster of young stars ~200pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350Msolar), which is intermediate between stellar-mass and supermassive black holes. A nearby

Simon F. Portegies Zwart; Holger Baumgardt; Piet Hut; Junichiro Makino; Stephen L. W. McMillan

2004-01-01

165

Dynamical masses of young star clusters in NGC 4038\\/4039  

Microsoft Academic Search

In order to estimate the masses of the compact, young star clusters in the merging galaxy pair, NGC 4038\\/4039 (``the Antennae''), we have obtained medium and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of five such clusters. The velocity dispersions were estimated using the stellar absorption features of CO at 2.29 mu m and metal absorption

Sabine Mengel; M. D. Lehnert; Niranjan Thatte; Reinhard Genzel

2002-01-01

166

A dynamical origin for early mass segregation in young star clusters  

Microsoft Academic Search

Some young star clusters show a degree of mass segregation that is\\u000ainconsistent with the effects of standard two-body relaxation from an initially\\u000aunsegregated system without substructure, in virial equilibrium, and it is\\u000aunclear whether current cluster formation models can account for this degree of\\u000ainitial segregation in clusters of significant mass. In this Letter we\\u000ademonstrate that mergers of

Stephen L. W. McMillan; Enrico Vesperini; Simon F. Portegies Zwart

2006-01-01

167

The Mass Function of Young Star Clusters in the ``Antennae'' Galaxies  

Microsoft Academic Search

We determine the mass function of young star clusters in the merging galaxies the ``Antennae'' (NGC 4038\\/39) from deep images taken with the WFPC2 on the refurbished Hubble Space Telescope (HST). This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age and hence the mass of each

Qing Zhang; S. Michael Fall

1999-01-01

168

Magnetic activity and differential rotation in the very young star KIC 8429280  

Microsoft Academic Search

Aims: We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine its activity level, spot distribution, and differential rotation. Methods: We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (vsini, spectral type,

A. Frasca; H.-E. Fröhlich; A. Bonanno; G. Catanzaro; K. Biazzo; J. Molenda-Zakowicz

2011-01-01

169

The influence of grain growth in circumstellar dust envelopes on observed colors and polarization of some eruptive stars.  

NASA Astrophysics Data System (ADS)

The existence of gas and dust envelopes for many types of stars is well known. R CrB stars are classical examples of stars where dust envelope formation takes place. Dust envelope formation was detected around the Kuwano-Honda object (PU Vul) in 1980 - 1981 when the star's brightness fell to 8m. The explanation of all the observed features is difficult. One source of confusion may be due to the ignoring of change of optical parameters of dust particles as they grow during dust envelope formation from a thousandth part of a micrometer at the beginning to submicrometer size at the end of the process. In fact, all optical properties of dust grains are strongly dependent on the composition, shape, and size of the grains. Hence, for particles of any material and constant geometry, size variations lead to variations in the optical characteristics of particles. This implies that optical parameters of dust in the circumstellar envelope will change too.

Efimov, Yu. S.

1989-12-01

170

Background X-ray Radiation Fields Produced by Young Embedded Star Clusters  

NASA Astrophysics Data System (ADS)

Most star formation in our galaxy occurs within embedded clusters, and these background environments can affect the star and planet formation processes occurring within them. In turn, young stellar members can shape the background environment and thereby provide a feedback mechanism. This work explores one aspect of stellar feedback by quantifying the background X-ray radiation fields produced by young stellar objects. Specifically, the distributions of X-ray luminosities and X-ray fluxes produced by cluster environments are constructed as a function of cluster membership size N. Composite flux distributions, for given distributions of cluster sizes N, are also constructed. The resulting distributions are wide and the X-ray radiation fields are moderately intense, with the expected flux levels exceeding the cosmic and galactic X-ray backgrounds by factors of ˜10-1000 (for energies 0.2--15 keV). For circumstellar disks that are geometrically thin and optically thick, the X-ray flux from the background cluster dominates that provided by a typical central star in the outer disk where r ? 9-14 AU. In addition, the expectation value of the ionization rate provided by the cluster X-ray background is ?X ˜ 8 × 10-17 s-1, about 4-8 times larger than the canonical value of the ionization rate from cosmic rays. These elevated flux levels in clusters indicate that X-rays can affect ionization, chemistry, and heating in circumstellar disks and in the material between young stellar objects.

Adams, Fred C.; Fatuzzo, Marco; Holden, Lisa

2012-09-01

171

The spectrum of the young star HD 100546 observed with the Infrared Space Observatory  

NASA Astrophysics Data System (ADS)

It is generally assumed that planets form in the dusty disks that surround young stars (Beckwith & Sargent 1996). The Infrared Space Observatory (Kessler et al. 1996) now enables us to determine the characteristics of these disks with unprecedented spectral resolution and signal-to-noise. We present here ISO spectra of the disk that surrounds the young star HD 100546. A remarkable variety of emission features of carbon- and oxygen-rich dust occurs. Most prominent are a series of emission features that can be attributed to silicates in crystalline form, mostly forsterite. In the interstellar medium and HII regions the silicate dust is mostly amorphous, but crystalline silicates are found in comets, meteorites and interplanetary dust particles. The forsterite features of HD 100546 are astonishingly similar to those observed in the ISO spectrum of Comet Hale-Bopp (Crovisier et al. 1997), strengthening the hypothesis that the disk around HD 100546 contains a huge swarm of comets (Grady et al. 1997). We argue that the crystallisation process occurs during the early evolution of the circumstellar disks of young stars and speculate about the formation of an Oort cloud around HD 100546. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

Malfait, K.; Waelkens, C.; Waters, L. B. F. M.; Vandenbussche, B.; Huygen, E.; de Graauw, M. S.

1998-04-01

172

Signatures of Multiple Stellar Populations in Unresolved Extragalactic Globular/Young Massive Star Clusters  

NASA Astrophysics Data System (ADS)

We present an investigation of potential signatures of the formation of multiple stellar populations in recently formed extragalactic star clusters. All of the Galactic globular clusters for which good samples of individual stellar abundances are available show evidence for multiple populations. This appears to require that multiple episodes of star formation and light element enrichment are the norm in the history of a globular cluster. We show that there are detectable observational signatures of multiple formation events in the unresolved spectra of massive, young extragalactic star clusters. We present the results of a pilot program to search for one of the cleanest signatures that we identify—the combined presence of emission lines from a very recently formed population and absorption lines from a somewhat older population. A possible example of such a system is identified in the Antennae galaxies. This source's spectrum shows evidence of two stellar populations with ages of 8 Myr and 80 Myr. Further investigation shows that these populations are in fact physically separated, but only by a projected distance of 59 pc. We show that the clusters are consistent with being bound and discuss the possibility that their coalescence could result in a single globular cluster hosting multiple stellar populations. While not the prototypical system proposed by most theories of the formation of multiple populations in clusters, the detection of this system in a small sample is both encouraging and interesting. Our investigation suggests that expanded surveys of massive young star clusters should detect more clusters with such signatures.

Peacock, Mark B.; Zepf, Stephen E.; Finzell, Thomas

2013-06-01

173

Search for Young Planetary Systems And the Evolution of Young Stars.  

National Technical Information Service (NTIS)

The Space Interferometer Mission (SIM) will provide a census of planetary systems by con- ducting a broad survey of 2,000 stars that will be sensitive to the presence of planets with masses as small as approx. 15 Earth masses (1 Uranus mass) and a deep su...

C. A. Beichman A. Boden A. Ghez L. W. Hartman L. Hillenbrand J. I. Lunine M. J. Simon J. R. Stauffer T. Velusamy

2004-01-01

174

Precise High-cadence Time Series Observations of Five Variable Young Stars in Auriga with MOST  

NASA Astrophysics Data System (ADS)

To explore young star variability on a large range of timescales, we have used the MOST satellite to obtain 24 days of continuous, sub-minute cadence, high-precision optical photometry on a field of classical and weak-lined T Tauri stars (TTSs) in the Taurus-Auriga star formation complex. Observations of AB Aurigae, SU Aurigae, V396 Aurigae, V397 Aurigae, and HD 31305 reveal brightness fluctuations at the 1%-10% level on timescales of hours to weeks. We have further assessed the variability properties with Fourier, wavelet, and autocorrelation techniques, identifying one significant period per star. We present spot models in an attempt to fit the periodicities, but find that we cannot fully account for the observed variability. Rather, all stars exhibit a mixture of periodic and aperiodic behavior, with the latter dominating stochastically on timescales less than several days. After removal of the main periodicity, periodograms for each light curve display power-law trends consistent with those seen for other young accreting stars. Several of our targets exhibited unusual variability patterns not anticipated by prior studies, and we propose that this behavior originates with the circumstellar disks. The MOST observations underscore the need for investigation of TTS light variations on a wide range of timescales in order to elucidate the physical processes responsible; we provide guidelines for future time series observations. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Systems Canada Inc. (MSCI), formerly part of Dynacon, Inc., the University of Toronto Institute for Aerospace Studies, and the University of British Columbia with the assistance of the University of Vienna.

Cody, Ann Marie; Tayar, Jamie; Hillenbrand, Lynne A.; Matthews, Jaymie M.; Kallinger, Thomas

2013-03-01

175

SINFONI View of Young Massive Stars in Galactic Mini-Starbursts W31 and W43  

NASA Astrophysics Data System (ADS)

Star formation frequently takes place in high concentrations of massive stars at intense levels, a phenomenon referred to as starbursts. While starbursts are the most prominent features in a galaxy, its detailed properties cannot be easily studied: in distant galaxies, the stellar and gas contents are not resolved, and in the Milky Way the distances and association among stars can only be established with observationally-expensive spectroscopic data. As part of the GLOSTAR (A Global View of Star Formation in the Milky Way) project, we use high-resolution SINFONI near-IR integral field spectra of central clusters in mini-starbursts W31 and W43 as well as complementary 2MASS+UKIDSS near-IR and Spitzer GLIMPSE+MIPSGAL mid-IR imaging surveys to obtain a comprehensive census of the underlying massive stellar and young stellar object (YSO) content. These two young, obscured mini-starbursts, one in the inner arm and the other at one end of the Galactic Bar, have similar IR luminosities and column density of molecular gas but show distinct cluster morphologies. We have used the resolved massive stellar and YSO content to determine the intensity and propagation of star formation in these two mini-starbursts in the last 30 Myr, and compare to properties of their natal molecular environment and Galactic environments, in order to investigate environmental effect on cluster formation and the resultant starburst phenomenon. We have also compared the star formation rate (SFR) determined from resolved massive stellar content and from the integrated IR luminosities and found that the latter underestimated the SFR by a factor of 10, stressing the importance of using high-resolution data to understand the starburst phenomenon.

Chen, C.-H. Rosie; Messineo, Maria; Menten, Karl; Wyrowski, Fredrich

2013-06-01

176

The Collimation and Propagation of Jets from Young Stars  

NASA Astrophysics Data System (ADS)

It is over ten years since the discovery of young stellar object (YSO) jets. Although clearly there is an association between these jets and YSO disks, their collimation mechanism is poorly understood. Moreover while radiative shocks are known to be the source of their emission, the origin of these shocks is uncertain. To address these problems, we propose to study the outflows in the vicinity of HL Tau, particularly the jets associated with HL Tau itself. With these images it should be possible to test, for the first time, two rival scenarios for the origin of the jet emission (oblique shocks versus internal working surfaces). We have found that jet opening angles visibly decrease with distance from their source. This argues strongly for models in which external pressure gradients (over scales less than several hundred AU) confine the jet. We wish now to examine the importance of such gradients even closer to the source (i.e. within a few arcseconds) to see whether they are the primary focussing mechanism. Our HST observations will obtain as a byproduct unprecedented information on the circumstellar environs of HL Tau, a YSO which is thought to possess a protoplanetary disk. The study will be carried out using the [SII]6717,6731, H ALPHA, [OI]6300 and F547M filters. This proposal was initially accepted for Cycle 1 but not retained after reassessment.

Ray, Thomas

1994-07-01

177

Chandra Study of Young Stellar Objects in the NGC 1333 Star-forming Cloud  

NASA Astrophysics Data System (ADS)

NGC 1333, a highly active star formation region within the Perseus molecular cloud complex, has been observed with the ACIS-I detector on board the Chandra X-Ray Observatory. In our image with a sensitivity limit of ~1028 ergs s-1, we detect 127 X-ray sources, most with subarcsecond positional accuracy. While 32 of these sources appear to be foreground stars and extragalactic background, 95 X-ray sources are identified with known cluster members. The X-ray luminosity function of the discovered young stellar object (YSO) population spans a range of logLX~=28.0-31.5 ergs s-1 in the 0.5-8 keV band, and absorption ranges from logNH~=20 to 23 cm-2. Most of the sources have plasma energies between 0.6 and 3 keV, but a few sources show higher energies up to ~7 keV. Comparison with K-band source counts indicates that we detect all of the known cluster members with K<12 and about half of the members with K>12. K~=11, the peak of the K-band luminosity function, corresponds to 0.2-0.4 Msolar stars for a cluster age of ~1 Myr. We detect seven of the 20 known YSOs in NGC 1333 producing jets or molecular outflows as well as one deeply embedded object without outflows. No evident difference in X-ray emission of young stars with and without outflows is found. Based on the complete subsample of T Tauri stars, we also find no difference in X-ray properties and X-ray production mechanism of stars with and without K-band excess disks. Several other results are obtained. We suggest that the X-ray emission from two late B stars that illuminate the reflection nebula originates from unresolved late-type companions. Two T Tauri stars are discovered in the ACIS images as previously unknown components of visual binaries. A good correlation LX~J is seen, which confirms the well-known relation LX~Lbol found in many star-forming regions. Based on spectral analysis for the X-ray counterpart of SVS 16, we establish that the column density NH is much lower than that expected from near-IR photometry so that its X-ray luminosity, logLX~=30.6 ergs s-1, is not unusually high.

Getman, Konstantin V.; Feigelson, Eric D.; Townsley, Leisa; Bally, John; Lada, Charles J.; Reipurth, Bo

2002-08-01

178

Eruption Variability  

NSDL National Science Digital Library

This resource provides information about variances in size and explosiveness of volcanoes that have erupted during the past 10,000 years. Included is a table showing that small eruptions are more frequent than larger ones. A feature of this site is the introduction of a scheme for estimating the magnitude of historic eruptions, called the Volcanic Explosivity Index (VEI). Historical eruptions can be assigned a VEI number on a scale of 0 to 8, using one or more of the criteria which include volume of ejecta, height of the eruptive column, qualitative descriptions, style of past activity, and height of spreading of the eruptive plume head. A chart shows an example for each VEI from 0 through 8 along with the plume height, volume, and eruption type classification. Each volcano name provides a link to detailed information about it.

Camp, Victor

179

Neutron Stars From Young Nearby Associations: The Origin of RX J1605.3+3249  

NASA Astrophysics Data System (ADS)

Many neutron stars (NSs) and runaway stars apparently come from the same regions on the sky. This suggests that they share the same birth places, namely associations and clusters of young massive stars. To identify NS birth places, we attempt to find NS-runaway pairs that could be former companions that were disrupted in a supernova (SN). The remains of recent (stars HIP 68228 and HIP 89394 - as well as the appearance of a feature in the ?-ray emission from 26Al decay at the predicted SN place. Both, the progenitor masses estimated by comparison with theoretical 26Al yields as well as derived from the life-time of the progenitor star, are found to be ~11Msolar.

Tetzlaff, N.; Schmidt, J. G.; Hohle, M. M.; Neuhäuser, R.

2012-03-01

180

The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga  

NASA Astrophysics Data System (ADS)

We present an astrometric study of the candidates of T Tauri stars (TTS) and non-TTS X-ray sources around Tau-Aur, based on the Hipparcos Catalogue and the ACT Reference Catalogue. The ROSAT selected X-ray sources are found to be a mixed population. A few of them are associated with the Tau-Aur or Orion Star Forming Regions (SFR). Some, with distances similar to that of Tau-Aur but with discrepant proper motions, are probable or sure Pleiades super-cluster members or other late type young active stars with unresolved nature, more likely to originate in rapidly moving cloudlets, or else having originated from different sites other than Tau-Aur and moved to the present locations. A good many of the non-TTS X-ray sources are considered as Hyades cluster members. Some TTS candidates could be foreground pre-main sequence stars or actually young dwarfs not yet depleted of their Lithium. Under the hypothesis that the sources we studied are representative of the ROSAT selected TTS candidates discovered in the outskirts of the Tau-Aur region, we conclude that only up to one third of the weak-line TTS candidates could be expected to be physically associated with the Tau-Aur association. Along with the parallax and proper motion analysis of the non-TTS X-ray sources around the Tau-Aur SFR, our result suggests that the vast majority of the young active X-ray sources within an angular diameter of about 30 ° of the Tau-Aur SFR, belong to four main subgroups that are spatially separate.

Li, Jin-Zeng

2004-06-01

181

Relations between the Luminosity, Mass, and Age Distributions of Young Star Clusters  

NASA Astrophysics Data System (ADS)

We derive and interpret some relations between the luminosity, mass, and age distributions of star clusters, denoted here by ?(L), ?(M), and ?(?), respectively. Of these, ?(L) is the easiest to determine observationally, whereas ?(M) and ?(?) are more informative about formation and disruption processes. For a population of young clusters, with a relatively wide range of ages, ?(L) depends on both ?(M) and ?(?) and thus cannot serve as a proxy for ?(M) in general. We demonstrate this explicitly by four illustrative examples with specific forms for either ?(M) or ?(?). In the special case in which ?(M) is a power law and is independent of ?(?), however, ?(L) is also a power law with the same exponent as ?(M). We conclude that this accounts for the observed similarity between ?(L) and ?(M) for the young clusters in the Antennae galaxies. This result reinforces our picture in which clusters form with ?(M)~M-2 and are then disrupted rapidly at a rate roughly independent of their masses. The most likely disruptive process in this first stage is the removal of interstellar matter by the energy and momentum input from young stars (by photoionization, winds, jets, and supernovae). The few clusters that avoid this ``infant mortality'' are eventually disrupted in a second stage by the evaporation of stars driven by two-body relaxation, a process with a strong dependence on mass. We suspect this picture may apply to many, if not all, populations of star clusters, but this needs to be verified observationally by determinations of ?(M) and ?(?) in more galaxies.

Fall, S. Michael

2006-12-01

182

ASSESSMENT OF STELLAR STRATIFICATION IN THREE YOUNG STAR CLUSTERS IN THE LARGE MAGELLANIC CLOUD  

SciTech Connect

We present a comprehensive study of stellar stratification in young star clusters in the Large Magellanic Cloud (LMC). We apply our recently developed effective radius method for the assessment of stellar stratification on imaging data obtained with the Advanced Camera for Surveys of three young LMC clusters to characterize the phenomenon and develop a comparative scheme for its assessment in such clusters. The clusters of our sample, NGC 1983, NGC 2002, and NGC 2010, are selected on the basis of their youthfulness, and their variety in appearance, structure, stellar content, and surrounding stellar ambient. Our photometry is complete for magnitudes down to m{sub 814} approx = 23 mag, allowing the calculation of the structural parameters of the clusters, the estimation of their ages, and the determination of their stellar content. Our study shows that each cluster in our sample demonstrates stellar stratification in a quite different manner and at different degree from the others. Specifically, NGC 1983 shows partial segregation, with the effective radius increasing with fainter magnitudes only for the faintest stars of the cluster. Our method on NGC 2002 provides evidence of strong stellar stratification for both bright and faint stars; the cluster demonstrates the phenomenon with the highest degree in the sample. Finally, NGC 2010 is not segregated, as its bright stellar content is not centrally concentrated, the relation of effective radius to magnitude for stars of intermediate brightness is rather flat, and we find no evidence of stratification for its faintest stars. For the parameterization of the phenomenon of stellar stratification and its quantitative comparison among these clusters, we propose the slope derived from the change in the effective radius over the corresponding magnitude range as indicative parameter of the degree of stratification in the clusters. A positive value of this slope indicates mass segregation in the cluster, while a negative or zero value signifies the lack of the phenomenon.

Gouliermis, Dimitrios A.; Rochau, Boyke [Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg (Germany); Mackey, Dougal [Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ (United Kingdom); Xin Yu, E-mail: dgoulier@mpia-hd.mpg.d, E-mail: rochau@mpia-hd.mpg.d, E-mail: dmy@roe.ac.u, E-mail: yxin@astro.uni-bonn.d [Argelander Institut fuer Astronomie, Rheinische Friedrich-Wilhelms-Universitaet Bonn, Auf dem Huegel 71, 53121 Bonn (Germany)

2010-01-20

183

Near-infrared Variability in Young Stars in Cygnus OB7  

NASA Astrophysics Data System (ADS)

We present the first results from a 124 night J, H, K near-infrared monitoring campaign of the dark cloud L 1003 in Cygnus OB7, an active star-forming region. Using three seasons of UKIRT observations spanning 1.5 years, we obtained high-quality photometry on 9200 stars down to J = 17 mag, with photometric uncertainty better than 0.04 mag. On the basis of near-infrared excesses from disks, we identify 30 pre-main-sequence stars, including 24 which are newly discovered. We analyze those stars and find that the NIR excesses are significantly variable. All 9200 stars were monitored for photometric variability; among the field star population, ~160 exhibited near-infrared variability (1.7% of the sample). Of the 30 young stellar objects (YSOs), 28 of them (93%) are variable at a significant level. Of the 30 YSOs, twenty-five have near-infrared excess consistent with simple disk-plus-star classical T Tauri models. Nine of these (36%) drift in color space over the course of these observations and/or since Two Micron All Sky Survey observations such that they cross the boundary defining the NIR excess criteria; effectively, they have a transient near-infrared excess. Thus, time-series JHK observations can be used to obtain a more complete sample of disk-bearing stars than single-epoch JHK observations. About half of the YSOs have color-space variations parallel to either the classical T Tauri star locus or a hybrid track which includes the dust reddening trajectory. This indicates that the NIR variability in YSOs that possess accretion disks arises from a combination of variable extinction and changes in the inner accretion disk: either in accretion rate, central hole size, and/or the inclination of the inner disk. While some variability may be due to stellar rotation, the level of variability on the individual stars can exceed a magnitude. This is a strong empirical suggestion that protoplanetary disks are quite dynamic and exhibit more complex activity on short timescales than is attributable to rotation alone or captured in static disk models.

Rice, Thomas S.; Wolk, Scott J.; Aspin, Colin

2012-08-01

184

Dynamical oscillation and propulsion of magnetic fields in the convective zone of a star. IV - Eruption to the surface  

NASA Astrophysics Data System (ADS)

It was shown in the previous paper that the heat accumulating beneath the azimuthal field in the convective zone of the sun initiates a Rayleight-Taylor instability, causing tongues of gas to intrude upward into the field. The present paper works out the conditions in an intruding tongue of gas, showing that it penetrates all the way through the field, emerging into the atmosphere above with a specific entropy significantly in excess of the ambient value. The entropy is large enough that the gas rises to the visible surface in 10 to the 6th s or less, where it produces a bipolar magnetic region. Once the eruption to the surface has carried away the accumulated hot gas below the field, there remains only the general downdraft which retracts the magnetic flux from the surface in a characteristic time of 10 to the 7th s.

Parker, E. N.

1988-02-01

185

Photometric properties of stars clusters with young or mixed age stellar populations  

NASA Astrophysics Data System (ADS)

The main goal of this work is to present and discuss the synthetic photometrical properties of stellar clusters resulting from the PopStar code. Colors in Johnson and SDSS systems, H? and H? luminosities and equivalent widths, and ionizing region size, have been computed for a wide range of metallicities Z = 0.0001, 0.0004, 0.004,0.008,0.02 and 0.05, and ages, from 0.1 Myr to 20 Gyr in Mollá, Garc{í}a-Vargas, & Bressan (2009, MNRAS, 398, 451). Emission lines are shown in Mart{í}n-Manj{ó}n et al. (2010, MNRAS, 403, 2012). Now we calculate colors with the emission lines contribution to the broad band color, so colors include stellar and nebular components, plus the emission lines following the evolution of the cluster and the region geometry in a consistent way. We compare the Single Stellar Populations contaminated and uncontaminated colors (in both Johnson and SDSS systems) and show the importance of emission lines contribution when photometry is used as a tool to characterize stellar populations. With these models we may determine the physical properties of young ionizing clusters when only photometrical observations are available and these correspond to the isolated star forming regions, subtracted the contribution of the underlying population In most cases, however, the ionizing population is usually embedded in a large and complex system, and the observed photometrical properties are the result of the combination of both the young star-forming burst and the host-underlying older population. The second objective of our work is therefore to provide a grid of models for nearby galaxies able to interpret mixed regions where the separation of young and old population is not possible or reliable enough. We obtain a set of PopStar Spectral Energy Distributions (available at PopStar site and also in VO) and derived colors for mixed populations where an underlying host population is combined in different mass ratios with a recent, metal-rich ionizing burst. These colors, together with other photometrical parameters, like H? radius of the ionized region, and Balmer lines equivalent width and luminosity allow to infer the physical properties of star-forming regions without any spectroscopic information. For details and a complete set of tables and figures see Mollá, García-Vargas, & Martín-Manjón (2012, MNRAS, submitted).

Mollá, M.; García-Vargas, M. L.; Martín-Manjón, M. L.

2013-05-01

186

Young Star Cluster Found Aglow With Mysterious X-Ray Cloud  

NASA Astrophysics Data System (ADS)

A mysterious cloud of high-energy electrons enveloping a young cluster of stars has been discovered by astronomers using NASA's Chandra X-ray Observatory. These extremely high-energy particles could cause dramatic changes in the chemistry of the disks that will eventually form planets around stars in the cluster. Known as RCW 38, the star cluster covers a region about 5 light years across. It contains thousands of stars formed less than a million years ago and appears to be forming new stars even today. The crowded environment of a star cluster is thought to be conducive to the production of hot gas, but not high-energy particles. Such particles are typically produced by exploding stars, or in the strong magnetic fields around neutron stars or black holes, none of which is evident in RCW 38. "The RCW 38 observation doesn't agree with the conventional picture," said Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, lead author of an Astrophysical Journal Letters paper describing the Chandra observation. "The data show that somehow extremely high-energy electrons are being produced there, although it is not clear how." RCW 38 RCW 38 X-ray, Radio, Infrared Composite Electrons accelerated to energies of trillions of volts are required to account for the observed X-ray spectrum of the gas cloud surrounding the ensemble of stars, which shows an excess of high-energy X-rays. As these electrons move in the magnetic field that threads the cluster, they produce X-rays. One possible origin for the high-energy electrons is a previously undetected supernova that occurred in the cluster. Although direct evidence for the supernova could have faded away thousands of years ago, a shock wave or a rapidly rotating neutron star produced by the outburst could be acting in concert with stellar winds to produce the high-energy electrons. "Regardless of the origin of the energetic electrons," said Wolk, "their presence would change the chemistry of proto-stellar disks in ways that could still be manifest billions of years later." For example, in our own solar system, we find evidence of certain short-lived radioactive nuclides (Aluminum 26 being the most well known). This implies the existence of a high-energy process late in the evolution of our solar system. If our solar system was immersed for a time in a sea of energetic particles, this could explain the rare nuclides present in meteorites found on the Earth today. RCW 38, at a distance of 6,000 light years from Earth, is one of the nearest star-forming regions with very young, hot stars. Other authors of the paper, which appeared in the 2002 December 1 issue of the Astrophysical Journal Letters, are Tyler Bourke, Randall Smith and Bradley Spitzbart of the Harvard-Smithsonian Center for Astrophysics, and Joao Alves of the European Southern Observatory in Garching, Germany. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science in Washington. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.

2002-12-01

187

Identifying nearby, Young, Late-type Stars by Means of their Circumstellar Disks  

NASA Astrophysics Data System (ADS)

It has recently been shown that a significant fraction of late-type members of nearby, very young associations (age lsim10 Myr) display excess emission at mid-IR wavelengths indicative of dusty circumstellar disks. We demonstrate that the detection of mid-IR excess emission can be utilized to identify new nearby, young, late-type stars including two definite new members ("TWA 33" and "TWA 34") of the TW Hydrae Association (TWA). Both new TWA members display mid-IR excess emission in the Wide-field Infrared Survey Explorer catalog and they show proper motion and youthful spectroscopic characteristics—namely, H? emission, strong lithium absorption, and low surface gravity features consistent with known TWA members. We also detect mid-IR excess—the first unambiguous evidence of a dusty circumstellar disk—around a previously identified UV-bright, young, accreting star (2M1337) that is a likely member of the Lower-Centaurus Crux region of the Scorpius-Centaurus Complex.

Schneider, Adam; Song, Inseok; Melis, Carl; Zuckerman, B.; Bessell, Mike

2012-10-01

188

VVV Study of the Young Milky Way Star Clusters: Mercer 35, 69 and 70  

NASA Astrophysics Data System (ADS)

Young massive clusters are usually deeply embedded in dust and gas. They represent excellent astrophysical laboratories for revealing the enigma of star formation processes and the early stages of stellar evolution. Wide-field infrared surveys are a modern tool for studying hidden clusters. "Vista variables in Vía Láctea - VVV" (Minniti et al. New Astron. 15:433, 2010) is an ESO/Chile/VISTA public deep near-IR survey, covering the Galactic Bulge and Southern Galactic disk. This is the next step of our systematic study of the cluster content of the inner Milky Way. Here we present our first analysis of the young hidden clusters Mercer 35, 69 and 70. It is based on VVV and SOFI/NTT { JHK} S photometry and follow-up NIR spectroscopy. All of the investigated clusters are young (t ? 10 Myr). The basic cluster parameters are obtained from photometry. The follow-up spectroscopy of probable cluster members shows that the brightest star in Mercer 35 presents characteristics of a red supergiant!

Kurtev, Radostin; Borissova, Jura; Bonatto, Charles; Peñaloza, Francisco; Ivanov, Valentin; Artigau, Étienne; Folkes, Stuart; Geisler, Douglas; Minniti, Dante; Lucas, Philip; Sale, Stuart

189

IDENTIFYING NEARBY, YOUNG, LATE-TYPE STARS BY MEANS OF THEIR CIRCUMSTELLAR DISKS  

SciTech Connect

It has recently been shown that a significant fraction of late-type members of nearby, very young associations (age {approx}<10 Myr) display excess emission at mid-IR wavelengths indicative of dusty circumstellar disks. We demonstrate that the detection of mid-IR excess emission can be utilized to identify new nearby, young, late-type stars including two definite new members ('TWA 33' and 'TWA 34') of the TW Hydrae Association (TWA). Both new TWA members display mid-IR excess emission in the Wide-field Infrared Survey Explorer catalog and they show proper motion and youthful spectroscopic characteristics-namely, H{alpha} emission, strong lithium absorption, and low surface gravity features consistent with known TWA members. We also detect mid-IR excess-the first unambiguous evidence of a dusty circumstellar disk-around a previously identified UV-bright, young, accreting star (2M1337) that is a likely member of the Lower-Centaurus Crux region of the Scorpius-Centaurus Complex.

Schneider, Adam; Song, Inseok [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States); Melis, Carl [Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093 (United States); Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA, 90095 (United States); Bessell, Mike, E-mail: aschneid@physast.uga.edu, E-mail: song@physast.uga.edu, E-mail: cmelis@ucsd.edu, E-mail: ben@astro.ucla.edu, E-mail: bessell@mso.anu.edu.au [Research School of Astronomy and Astrophysics, The Australian National University, Weston Creek, ACT 2611 (Australia)

2012-10-01

190

Inner disc rearrangement revealed by dramatic brightness variations in the young star PV Cep  

NASA Astrophysics Data System (ADS)

Young Sun-like stars at the beginning of the pre-main-sequence (PMS) evolution are surrounded by accretion discs and remnant protostellar envelopes. Photometric and spectroscopic variations of these stars are driven by interactions of the star with the disc. Time-scales and wavelength dependence of the variability carry information on the physical mechanisms behind these interactions. We conducted multi-epoch, multiwavelength study of PV Cep, a strongly variable, accreting PMS star. By combining our own observations from 2004 to 2010 with archival and literature data, we show that PV Cep started a spectacular fading in 2005, reaching an IC-band amplitude of 4 mag. Analysis of variation of the optical and infrared fluxes, colour indices and emission line fluxes suggests that the photometric decline in 2005-2009 resulted from an interplay between variable accretion and circumstellar extinction: since the central luminosity of the system is dominated by accretion, a modest drop in the accretion rate could induce the drastic restructuring of the inner disc. Dust condensation in the inner disc region might have resulted in the enhancement of the circumstellar extinction.

Kun, M.; Szegedi-Elek, E.; Moór, A.; Kóspál, Á.; Ábrahám, P.; Apai, D.; Kiss, Z. T.; Klagyivik, P.; Magakian, T. Yu.; Mez?, Gy.; Movsessian, T. A.; Pál, A.; Rácz, M.; Rogers, J.

2011-06-01

191

A general catalogue of molecular hydrogen emission-line objects (MHOs) in outflows from young stars  

NASA Astrophysics Data System (ADS)

We present a catalogue of Molecular Hydrogen emission-line Objects (MHOs) in outflows from young stars, most of which are deeply embedded. All objects are identified in the near-infrared lines of molecular hydrogen, all reside in the Milky Way, and all are associated with jets or molecular outflows. Objects in both low and high-mass star forming regions are included. This catalogue complements the existing database of Herbig-Haro objects; indeed, for completeness, HH objects that are detected in H2 emission are included in the MHO catalogue. http://www.jach.hawaii.edu/UKIRT/MHCat/ Tables A.1 to A.15 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A24

Davis, C. J.; Gell, R.; Khanzadyan, T.; Smith, M. D.; Jenness, T.

2010-02-01

192

Search for Substellar Objects Around Young and Nearby Stars: The SACY Sample  

NASA Astrophysics Data System (ADS)

We have started a systematic survey to detect substellar (brown dwarfs and planetary mass) companions around young and nearby stars discovered in the course of the SACY survey. In this paper we present first epoch observations of 17 new identified members of three nearby associations. As a result, we have detected substellar companion candidates to twelve stars from the ? Cha and ? Pic associations. If bound to the primaries, 37 of these objects are detected at projected separations ? 800 AU, that is, well within the separation range where confirmed companion candidates have been detected so far. Second epoch imaging observations, together with infrared spectroscopy, are necessary to unveil the true nature of the detected companion candidates.

Huélamo, N.; Barrado Y Navascués, D.; Bouy, H.; Torres, C. A. O.; Quast, G.; Melo, C. H. F.; Sterzik, M. F.; Chauvin, G.

2009-08-01

193

Long-slit spectroscopic investigations of the forbidden emission line regions of young stars  

NASA Astrophysics Data System (ADS)

Our poor knowledge about the physical properties of outflows from young stars (e.g. outflow velocity, collimation, excitation) on scales of a few 100 AU distance from the source was the reason for conducting a long-slit spectroscopic survey of the forbidden emission lines from 38 classical T Tauri stars in the optical. The forbidden emission lines which are thought to be excited by internal shocks in the outflowing gas are often the only diagnostic mean to investigate these outflows since these stars have no extended jet or Herbig-Haro objects. More than half of the T Tauri stars of the survey are located within nearby star formation regions (d <= 200 pc) and for 14 objects spatial information on the properties of the forbidden emission line regions have been obtained by taking long-slit spectra at four to six slit positions which allowed an approximate determination of the outflow direction. As a surprising result of this survey all but one of these 14 T Tauri stars show spatially extended forbidden emission line regions. In most cases this is probably the result of a bipolar jet. The typical spatial extent in the outflow direction is of the order of 50 -- 500 AU. The opening angles of the outflows derived from radial velocity and velocity dispersion data suggest values between 10 and 20 degrees at distances between a few 100 and 1000 AU and values up to 2 -- 3 times higher at scales below 200 AU. This means that extended and partially well collimated outflows are a typical property of presumably most of the classical T Tauri stars. Interestingly in 50% of all bipolar outflows from T Tauri stars and embedded sources the outflow velocity of the blueshifted and redshifted part differs about a factor of 1.4 to 2.6. Probably the flow is accelerated to intrinsically different velocities on opposite sides of the star. However, an asymmetric pressure and density distribution of the surrounding circumstellar matter could also cause similar effects. A detailed study of the double-peaked profile of the forbidden emission lines gives strong evidence that the two velocity components originate in spatially different regions. The high-velocity component is probably formed in a well collimated jet whereas there are indications that the low-velocity component originates within a rotating line formation region presumably associated with a disk wind. For seven young stars of the survey spectra of at least three epochs are available. From these observations and from data in the literature it was possible to show that temporal variations in the forbidden emission lines are probably a typical property of many T Tauri stars. Studying such variations gives an additional interesting diagnostic mean for these outflows. Due to the relatively small extent of the emission regions of some forbidden emission lines (r <= 30 AU) and the high outflow velocities (? 200 km s-1) it is in principle possible that strong variations in the emission properties of the outflows occur within a few months. Note that so far radial velocity studies along extended jets and Herbig-Haro objects provided information only on time scales of the order of 10 to 1000 years. In studying position-velocity maps obtained from spectra with epoch differences of a few months up to a few years significant changes in the forbidden emission line flux, radial velocity, velocity dispersion and in the spatial properties of the high- and low-velocity component have been observed in seven TTSs. The different temporal variation of the two velocity components gives additional support to the idea that the two components in the double-peaked profile of the forbidden emission lines are formed in two physically separated regions.

Hirth, Gerhard A.

1994-10-01

194

Chemical analysis of giant stars in the young open cluster NGC 3114  

NASA Astrophysics Data System (ADS)

Context. Open clusters are very useful targets for examining possible trends in galactocentric distance and age, especially when young and old open clusters are compared. Aims: We carried out a detailed spectroscopic analysis to derive the chemical composition of seven red giants in the young open cluster NGC 3114. Abundances of C, N, O, Li, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd were obtained, as well as the carbon isotopic ratio. Methods: The atmospheric parameters of the studied stars and their chemical abundances were determined using high-resolution optical spectroscopy. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code MOOG. The abundances of the light elements were derived using the spectral synthesis technique. Results: We found that NGC 3114 has a mean metallicity of [Fe/H] = -0.01 ± 0.03. The isochrone fit yielded a turn-off mass of 4.2 M?. The [N/C] ratio is in good agreement with the models predicted by first dredge-up. We found that two stars, HD 87479 and HD 304864, have high rotational velocities of 15.0 km s-1 and 11.0 km s-1; HD 87526 is a halo star and is not a member of NGC 3114. Conclusions: The carbon and nitrogen abundance in NGC 3114 agree with the field and cluster giants. The oxygen abundance in NGC 3114 is lower compared to the field giants. The [O/Fe] ratio is similar to the giants in young clusters. We detected sodium enrichment in the analyzed cluster giants. As far as the other elements are concerned, their [X/Fe] ratios follow the same trend seen in giants with the same metallicity. Based on observations made with the 2.2 m telescope at the European Southern Observatory (La Silla, Chile).Tables 2 and 5 are available in electronic form at http://www.aanda.org

Santrich, O. J. Katime; Pereira, C. B.; Drake, N. A.

2013-06-01

195

Photometric/spectroscopic analyses and magnetic activity in young late-type stars  

NASA Astrophysics Data System (ADS)

We present preliminary results of a study in progress based on photometric and spectroscopic observations of young weak-line T Tauri and post-T Tauri stars just arriving on the Zero Age Main Sequence. The study is part of a project based on high-resolution spectra obtained with FOCESatCAHA (Spain) and SARGatTNG (Spain) and contemporaneous photometry performed at Catania (Italy) and TUBITAK (Turkey) observatories. The main aim is to investigate the topology of magnetic active regions at photospheric and chromospheric levels in young single stars. Since our targets are slow rotators, with rotation periods longer than about 2 days, we are able to apply the spectroscopic technique based on line-depth ratio for the measure of the photospheric temperature modulation. These stars, possible members of Stellar Kinematic Groups, display emission cores in the CaII H&K and IRT lines, as well as a conspicuous filling-in of the H? core. Moreover, we derive lithium abundance, and measure rotational and radial velocities. In several cases we detect a clear rotational modulation of brightness. In this work we briefly mention some preliminary results obtained for SAO 51891. We have also developed a spot/plage model to be applied to the data deriving the spot parameters (filling factor and temperature) and recovering information about the chromospheric inhomogeneities (flux contrast and filling factor). This study will contribute to explore the correlations between global stellar parameters and spot/plage characteristics in stars with different activity level and evolutionary stage.

Biazzo, K.; Frasca, A.; Marilli, E.; Covino, E.; Alcalà, J. M.; Çakirli, Ö.

2009-02-01

196

Spectral evolution and dust mixing in the recently erupted disk of EXLup  

NASA Astrophysics Data System (ADS)

This proposal is a follow-up of our previous Spitzer DDT PID 477, where we observed the young eruptive star EX Lup during its outburst, and detected on-going silicate crystal formation for the first time in a young eruptive star. We argue that spectral evolution at mid-infrared wavelengths takes place also in the fading phase of the outburst, and we request two new IRS observations to complete our spectral monitoring program and document the spectral changes. The data will help to characterize the newly formed crystal population, and provide important insight into the mixing processes in the disk. Spitzer/IRS is the only instrument we could use for our program. We request 0.5 hours observing time. Since the propose date of the first observation is early October 2008, this is a time-critical program.

Abraham, Peter; Bouwman, Jeroen; Dullemond, Cornelis; Henning, Thomas; Juhasz, Attila; Kospal, Agnes; Moor, Attila; Mosoni, Laszlo; Sicilia Aguilar, Aurora; Sipos, Nikoletta

2008-09-01

197

Young stars in old galaxies - surprising discovery with the world's leading telescopes  

NASA Astrophysics Data System (ADS)

Do elliptical galaxies only contain old stars? One of the challenges of modern astronomy is to understand how galaxies - large systems of stars, gas and dust - form and evolve. When did most of the stars in the Universe form? Did this happen at a very early stage, within a few billion years of the Big Bang? Have a significant number of the stars we now observe formed much more recently? Spectacular collisions between galaxies take place all the time, triggering the formation of thousands or even millions of stars. However, when looking at the Universe as a whole, most of its stars are found in elliptical galaxies whose overall appearance has so far led us to believe that they, and their stars and as well, are old. These elliptical galaxies do shine with the diffuse, reddish glow normally associated with stars that are many thousand million years old. However, what is the underlying mix of stars that produces this elderly appearance? Could a significant number of much younger stars be 'hiding' among the older ones? Detailed observations with the world's premier telescopes have now cast new light on this central question about the behaviour of some of the major building blocks of the Universe. Cosmic paleonthology To break the stellar 'cocktail' in elliptical galaxies down into its different constituents, a team of European and American astronomers observed massive stellar clusters in and around nearby galaxies. These "globular" clusters, so called because of their shape, exist in large numbers around all observed galaxies and form a kind of 'skeleton' within their host galaxies. These 'bones' receive an imprint for every episode of star formation they undergo. By reading the ages of the globular clusters in a galaxy, it is possible to identify the past epoch(s) of active star formation in a galaxy. Reading the imprints and deducing the distribution of ages of the globular clusters, astronomers can reveal when many of the stars in elliptical galaxies formed. This is similar to the way a palaeontologist uses the skeletons of dinosaurs to deduce information about the era in which they lived. A surprising discovery The team combined images of a number of galaxies from Hubble's Wide Field and Planetary Camera 2 with infrared images obtained from the multi-mode ISAAC instrument on the 8.2m VLT Antu telescope at the ESO Paranal Observatory (Chile). To their great surprise, they discovered that many of the globular clusters in one of these galaxies, NGC 4365, a member of the large Virgo cluster of galaxies, were only a few thousand million years old, much younger than most of the other stars in this galaxy (roughly 12 thousand million years old). The astronomers were able to identify three major groups of stellar clusters. There is an old population of clusters of metal-poor stars, some clusters of old but metal-rich stars and now, seen for the first time, a population of clusters with young and metal-rich stars. These results have been fully confirmed by spectroscopic observations made with another of the world's giant telescopes, the 10-metre Keck on Hawaii. "It is a great pleasure to see two projects wholly or partly funded by Europe - VLT and Hubble - work in concert to produce such an important scientific result", says Piero Benvenuti, ESA Hubble Project Scientist. "The synergy between the most advanced ground and space telescopes continues to prove its effectiveness, paving the way to impressive new discoveries that would not otherwise be possible." The discovery of young globular clusters within old galaxies is surprising since the stars in the giant elliptical galaxies were until now believed to have formed during a single period early in the history of the Universe. It is now clear that some of the galaxies may be hiding their true nature and have indeed experienced much more recent periods of major star formation. Notes for editors This press release is issued in coordination between ESA and ESO. The Hubble Space Telescope project is an international cooperation between ESA and NASA. The te

2002-06-01

198

POPSTAR evolutionary synthesis models - III. Photometric properties of young star clusters and mixed populations  

NASA Astrophysics Data System (ADS)

This is the third paper of a series reporting the results from the POPSTAR evolutionary synthesis models. The main goal of this work is to present and discuss the synthetic photometric properties of single stellar populations resulting from our POPSTAR code. Colours in the Johnson and Sloan Digital Sky Survey (SDSS) systems, H? and H? luminosities and equivalent widths, and ionizing region size, have been computed for a wide range of metallicity (Z = 0.0001-0.05) and age (0.1 Myr to 20 Gyr). We calculate the evolution of the cluster and the region geometry in a consistent manner. We demonstrate the importance of the contribution of emission lines to broader band photometry when characterizing stellar populations, through the presentation of both contaminated and non-contaminated colours (in both the Johnson and SDSS systems). The tabulated colours include stellar and nebular components, in addition to line emission. The main application of these models is the determination of physical properties of a given young ionizing cluster, when only photometric observations are available; for an isolated star-forming region, the young star cluster models can be used, free from the contamination of any underlying background stellar population. In most cases, however, the ionizing population is usually embedded in a large and complex system, and the observed photometric properties result from the combination of a young star-forming burst and the underlying older population of the host. Therefore, the second objective of this paper is to provide a grid of models useful in the interpretation of mixed regions where the separation of young and old populations is not sufficiently reliable. We describe the set of POPSTAR spectral energy distributions (SEDs), and the derived colours for mixed populations where an underlying host population is combined in different mass-ratios with a recent ionizing burst. These colours, together with other common photometric parameters, such as the H? radius of the ionized region, and Balmer line equivalent widths and luminosities, allow one to infer the physical properties of star-forming regions even in the absence of spectroscopic information.

García-Vargas, M. L.; Mollá, M.; Martín-Manjón, M. L.

2013-07-01

199

The long-term survival chances of young massive star clusters  

NASA Astrophysics Data System (ADS)

I review the long-term survival chances of young massive star clusters (YMCs), hallmarks of intense starburst episodes often associated with violent galaxy interactions. In particular, I address the key question as to whether at least some of these YMCs can be considered proto-globular clusters (GCs). In the absence of significant external perturbations, the key factor determining a cluster’s long-term survival chances is the shape of its stellar initial mass function. I conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.

de Grijs, Richard

2009-12-01

200

The relevance of the IUE results on young stars for Earth's paleoatmosphere  

NASA Astrophysics Data System (ADS)

Using the latest IUE results for seven T Tauri stars, which are believed to represent the young Sun and a detailed photochemical chemical model of the paleoatmosphere, the vertical distribution of Oxygen and Ozone in the early atmosphere was calculated. The calculations indicate that the surface Oxygen mixing ratio is as much as six orders of magnitude larger than previously estimated, but appears low enough for the formation of amino acids via the Urey-Miller type of experiments. It is believed that the quantification of the oxygen level in the Earth's paleoatmosphere presented can reconcile the demands of both biological and geological considerations.

Canuto, V. M.; Levine, J. S.; Augustsson, T. R.; Imhoff, C. L.; Giampapa, M. S.

201

Disk structure and planet formation around young low-mass stars and brown dwarfs  

NASA Astrophysics Data System (ADS)

We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a sample of 41 young low-mass stars and brown dwarfs, to study the characteristics of their disks and to search for evidence of ongoing planet formation. We also propose to use IRAC to observe an additional 43 newly-identified brown dwarfs in Taurus and Chamaeleon, for future IRS spectroscopic followup. This guaranteed-time observing program has a total of 50.1 hours, with 14 hours provided by IRAC PI Giovanni Fazio, and 36.1 hours by IRS PI Jim Houck.

Houck, James R.; Luhman, Kevin; Allen, Lori; Calvet, Nuria; Chen, Christine; Fazio, Giovanni; Forrest, Bill; Furlan, Elise; Hartmann, Lee; Jura, Mike; Keller, Luke; Mainzer, Amy; Markwick-Kemper, Ciska; Marley, Mark; Megeath, Tom; Muzerolle, James; Najita, Joan; Roellig, Tom; Sloan, Greg; Uchida, Keven; Watson, Dan; D'Alessio, Paola

2006-05-01

202

A Comprehensive GALEX Ultraviolet Catalog of Star Clusters in M31 and a Study of the Young Clusters  

NASA Astrophysics Data System (ADS)

We present a comprehensive catalog of 700 confirmed star clusters in the field of M31 compiled from three major existing catalogs. We detect 418 and 257 star clusters in Galaxy Evolution Explorer near-ultraviolet and far-ultraviolet (FUV) imaging, respectively. Our final catalog includes photometry of star clusters in up to 16 passbands ranging from FUV to NIR as well as ancillary information such as reddening, metallicity, and radial velocities. In particular, this is the most extensive and updated catalog of UV-integrated photometry for M31 star clusters. Ages and masses of star clusters are derived by fitting the multi-band photometry with model spectral energy distribution (SED); UV photometry enables more accurate age estimation of young clusters. Our catalog includes 182 young clusters with ages less than 1 Gyr. Our estimated ages and masses of young clusters are in good agreement with previously determined values in the literature. The mean age and mass of young clusters are about 300 Myr and 104 M ?, respectively. We found that the compiled [Fe/H] values of young clusters included in our catalog are systematically lower (by more than 1 dex) than those from recent high-quality spectroscopic data and our SED-fitting result. We confirm that most of the young clusters' kinematics shows systematic rotation around the minor axis and association with the thin disk of M31. The young cluster distribution exhibits a distinct peak in the M31 disk around 10-12 kpc from the center and follows a spatial distributions similar to other tracers of disk structure such as OB stars, UV star-forming regions, and dust. Some young clusters also show concentration around the ring splitting regions found in the southern part of the M31 disk and most of them have systematically younger (<100 Myr) ages. Considering the kinematical properties and spatial distribution of young clusters, they might be associated with the well-known 10 kpc star formation ring structure in the M31 disk. Consequently, we suggest that various properties of young clusters in M31 might be in line with the scenarios that a satellite galaxy had passed through the disk of M31 less than few hundred million years ago.

Kang, Yongbeom; Rey, Soo-Chang; Bianchi, Luciana; Lee, Kyungsook; Kim, YoungKwang; Sohn, Sangmo Tony

2012-04-01

203

The Massive Disk around the Young B2-B3 Star AFGL 490  

NASA Astrophysics Data System (ADS)

The formation of massive stars is one of the unresolved mysteries of present research. Whether the dominant formation process for massive stars is disk accretion or coalescence is still an open question. In this respect, we studied the object AFGL 490 in much more detail. This object is a key target of the class of deeply embedded young stellar objects with a mass of 8-10 Msun (spectral type B2-B3, D=1kpc) being in a transition stage to Herbig Be stars. Our new observational data confirm our previous suggestion of the presence of a massive gas torus or disk around this object. Thus, our optically thin C17O 2-1 data point to a clumpy gas ring inclined to our line of sight with an angle of 50-80 deg; in which the 230GHz-continuum point source is centered. The velocity distribution of this gas exhibits the signature of rotation. In addition, our VLA 7mm data show an extension of the "continuum point" source in the same direction as the elongated gas ring. Based on our previous Plateau de Bure data in CS 2-1, we estimated a gas mass inside a radius of 4000 AU of about 8 M which is similar to the mass of the central star. In addition, the variability of the source extension in the NIR and in the VLA-2cm continuum (Hoare et al., Campbell et al.) in the same orientation as the disk is interpreted as a variability in the accretion onto the star which supports well the idea that such a massive disk cannot exist for a very long time without disrupting due to its own self-gravity. Thus, we can speculate that the outer gas torus feeds the inner gravitationally unstable gas disk until the torus is depleted. Then, the gas torus and the disk disappear in 10^3 to 10^4 years. This would be in a good agreement with the observations that Herbig Be stars (and more massive stars) have no disks anymore, and this would be a possible procedure to build up stars with masses -- maybe -- until 15 to 20Msun. Here, more theoretical work and high-resolution observations are needed. P. H. acknowledges partial support from the Research Corporation grant No CC4996, as well as from NSF grant AST-0098524.

Schreyer, Katharina; Hofner, Peter; Araya, Esteban; Linz, Hendrik; Stecklum, Bringfried; Henning, Thomas

2004-08-01

204

TRIGGERED STAR FORMATION AND YOUNG STELLAR POPULATION IN BRIGHT-RIMMED CLOUD SFO 38  

SciTech Connect

We have investigated the young stellar population in and around SFO 38, one of the massive globules located in the northern part of the Galactic H II region IC 1396, using the Spitzer IRAC and MIPS observations (3.6-24 {mu}m), and followed up with ground-based optical photometric and spectroscopic observations. Based on the IRAC and MIPS colors and H{alpha} emission, we identify {approx}45 young stellar objects (Classes 0/I/II) and 13 probable pre-main-sequence candidates. We derive the spectral types (mostly K- and M-type stars), effective temperatures, and individual extinction of the relatively bright and optically visible Class II objects. Most of the Class II objects show variable H{alpha} emission as well as optical and near-infrared photometric variability, which confirm their 'youth'. Based on optical photometry and theoretical isochrones, we estimate the spread in stellar ages to be between 1 and 8 Myr with a median age of 3 Myr and a mass distribution of 0.3-2.2 M{sub sun} with a median value around 0.5 M{sub sun}. Using the width of the H{alpha} emission line measured at 10% peak intensity, we derive the mass accretion rates of individual objects to be between 10{sup -10} and 10{sup -8} M{sub sun} yr{sup -1}. From the continuum-subtracted H{alpha} line image, we find that the H{alpha} emission of the globule is not spatially symmetric with respect to the O-type ionizing star HD 206267, and the interstellar extinction toward the globule is also anomalous. We clearly detect an enhanced concentration of YSOs closer to the southern rim of SFO 38 and identify an evolutionary sequence of YSOs from the rim to the dense core of the cloud, with most of the Class II objects located at the bright rim. The YSOs appear to be aligned along two different directions toward the O6.5V type star HD 206267 and the B0V type star HD 206773. This is consistent with the Radiation Driven Implosion (RDI) model for triggered star formation. Further, the apparent speed of sequential star formation is consistent with the speed of propagation of shocks in dense globules as derived from numerical simulations of RDI.

Choudhury, Rumpa; Bhatt, H. C. [Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034 (India); Mookerjea, Bhaswati, E-mail: rumpa@iiap.res.i, E-mail: hcbhatt@iiap.res.i, E-mail: bhaswati@tifr.res.i [Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005 (India)

2010-07-10

205

Variable stars  

SciTech Connect

This book covers the following topics: pulsating variables; eruptive variables; eclipsing stars; supplement to the classification; the discovery of variable stars; the significance of variable stars for research on the structure of the galaxy and stellar evolution; and observational methods and organizations.

Hoffmeister, C.; Richter, G.; Wenzel, W.

1985-01-01

206

Spitzer View of Young Massive Stars in the Large Magellanic Cloud H II Complex N 44  

NASA Astrophysics Data System (ADS)

The H II complex N 44 in the Large Magellanic Cloud (LMC) provides an excellent site to perform a detailed study of star formation in a mild starburst, as it hosts three regions of star formation at different evolutionary stages, and it is not as complicated and confusing as the 30 Doradus giant H II region. We have obtained Spitzer Space Telescope observations and complementary ground-based 4 m uBVIJK observations of N 44 to identify candidate massive young stellar objects (YSOs). We further classify the YSOs into Types I, II, and III, according to their spectral energy distributions (SEDs). In our sample of 60 YSO candidates, ~65% of them are resolved into multiple components or extended sources in high-resolution ground-based images. We have modeled the SEDs of 36 YSOs that appear single or dominant within a group. We find good fits for Types I and I/II YSOs, but Types II and II/III YSOs show deviations between their observed SEDs and models that do not include PAH emission. We have also found that some Type III YSOs have central holes in their disk components. YSO counterparts are found in four ultracompact H II regions and their stellar masses determined from SED model fits agree well with those estimated from the ionization requirements of the H II regions. The distribution of YSOs is compared with those of the underlying stellar population and interstellar gas conditions to illustrate a correlation between the current formation of O-type stars and previous formation of massive stars. Evidence of triggered star formation is also presented.

Chen, C.-H. Rosie; Chu, You-Hua; Gruendl, Robert A.; Gordon, Karl D.; Heitsch, Fabian

2009-04-01

207

Young stars in ? Chamaleontis and their disks: disk evolution in sparse associations  

NASA Astrophysics Data System (ADS)

Context. The nearby young stellar association ? Cha has an estimated age of 3-5 Myr, making it an ideal laboratory to study the disk dissipation process and provide empirical constraints on the timescale of planet formation. Aims: We wish to complement existing optical and near-infrared data of the ? Cha association, which provide the stellar properties of its members, with mid-infrared data that probe the presence, geometry, and mineralogical composition of protoplanetary disks around individual stars. Methods: We combine the available literature data with our Spitzer/IRS spectroscopy and VLT/VISIR imaging data. We use proper motions to refine the membership of ? Cha. Masses and ages of individual stars are estimated by fitting model atmospheres to the optical and near-infrared photometry, followed by placement in the Hertzsprung-Russell diagram. The Spitzer/IRS spectra are analyzed using the two-layer temperature distribution spectral decomposition method. Results: Two stars previously identified as members, CXOU J120152.8 and 2MASS J12074597, have proper motions that are very different from those of the other stars. But other observations suggest that the two stars are still young and thus might still be related to ? Cha. HD 104237C is the lowest mass member of ? Cha with an estimated mass of ~13-15 Jupiter masses. The very low mass stars USNO-B120144.7 and 2MASS J12005517 show globally depleted spectral energy distributions, pointing at strong dust settling. 2MASS J12014343 may have a disk with a very specific inclination, where the central star is effectively screened by the cold outer parts of a flared disk, but the 10 ?m radiation of the warm inner disk can still reach us. We find that the disks in sparse stellar associations are dissipated more slowly than those in denser (cluster) environments. We detect C2H2 rovibrational band around 13.7 ?m on the IRS spectrum of USNO-B120144.7. We find strong signatures of grain growth and crystallization in all ? Cha members with 10 ?m features detected in their IRS spectra. We combine the dust properties derived in the ? Cha sample with those found using identical or similar methods in the MBM 12, Coronet, ? Cha associations, and in the cores-to-disks legacy program. We find that disks around low-mass young stars show a negative radial gradient in the mass-averaged grain size and mass fraction of crystalline silicates. A positive correlation exists between the mass-averaged grain sizes of amorphous silicates and the accretion rates if the latter is above ~10-9 M? yr-1, possibly indicating that those disks are sufficiently turbulent to prevent grains of several microns in size to sink into the disk interior. Based on observations performed at ESO's La Silla-Paranal observatory under programme 076.C-0470.

Fang, M.; van Boekel, R.; Bouwman, J.; Henning, Th.; Lawson, W. A.; Sicilia-Aguilar, A.

2013-01-01

208

Young Stars in Old Galaxies - a Cosmic Hide and Seek Game  

NASA Astrophysics Data System (ADS)

Surprise Discovery with World's Leading Telescopes [1] Summary Combining data from the NASA/ESA Hubble Space Telescope (HST) and the ESO Very Large Telescope (VLT) , a group of European and American astronomers [2] have made an unexpected, major discovery. They have identified a huge number of "young" stellar clusters , only a few billion years old [3], inside an "old" elliptical galaxy (NGC 4365), probably aged some 12 billion years. For the first time, it has been possible to identify several distinct periods of star-formation in a galaxy as old as this one . Elliptical galaxies like NGC 4365 have until now been considered to have undergone one early star-forming period and thereafter to be devoid of any star formation. However, the combination of the best and largest telescopes in space and on the ground has now clearly shown that there is more than meets the eye. This important new information will help to understand the early history of galaxies and the general theory of star formation in the Universe . PR Photo 15a/02 : Combined HST+VLT image of elliptical galaxy NGC 4365 PR Photo 15b/02 : Same image, with "old" and "young" stellar clusters indicated PR Photo 15c/02 : Animated GIF image, showing the three cluster populations observed in NGC 4365 Do elliptical galaxies only contain old stars? One of the challenges of modern astronomy is to understand how galaxies, those large systems of stars, gas and dust, form and evolve. In this connection, a central question has always been to learn when most of the stars in the Universe formed. Did this happen at a very early stage, within a few billion years after the Big Bang? Or were a significant number of the stars we now observe formed much more recently? Spectacular collisions between galaxies take place all the time, triggering the formation of thousands or even millions of stars, cf. ESO PR Photo 29b/99 of the dramatic encounter between NGC 6872 and IC 4970. However, when looking at the Universe as a whole, most of its stars are found in large elliptical galaxies (this refers to their form) whose overall appearance has so far led us to believe that they, and their stars as well, are very old, indeed among the oldest objects in the Universe. These elliptical galaxies do shine with the diffuse, reddish glow normally associated with stars that are many billions of years old. However, what is really the underlying mix of stars that produces this elderly appearance? Could perhaps a significant number of much younger stars be "hiding" among the older ones? Whatever the case, this question must obviously be looked into, before it is possible to claim understanding of the evolution of these old galaxies. It is a very challenging investigation and it is only now that new and more detailed observations with the world's premier telescopes have been obtained that cast more light on this central question and thus on the true behaviour of some of the major building blocks of the Universe. Cosmic archaeology In order to identify the constitutents of the stellar "cocktail" in elliptical galaxies, a team of European and American astronomers [2] observed massive stellar clusters in and around several nearby galaxies. These clusters, referred to as "globular" because of their shape, are present in large numbers around most galaxies and together they form a kind of "skeleton" within their host galaxies. These "bones" receive an imprint for every episode of star formation they undergo. Thus, by reading the ages of the globular clusters in a galaxy, it is possible to identify the past epoch(s) of active star formation in that galaxy. This is like digging into the ruins of an ancient archaeological city site and to find those layers and establish those times when the city underwent bursts of building activity. In this way, by the study of the distribution and ages of the globular clusters in an elliptical galaxy, astronomers can reveal when many of its stars were formed. A surprise discovery ESO PR Phot

2002-05-01

209

High Angular Resolution Mid-Infrared Imaging of Young Stars in Orion BN/KL  

SciTech Connect

The authors present Keck LWS images of the Orion BN/KL star forming region obtained in the first multi-wavelength study to have 0.3--0.5 resolution from 4.7 {micro}m to 22 {micro}m. The young stellar objects designed infrared source n and radio source I are believed to dominate the BN/KL region. They have detected extended emission from a probable accretion disk around source n but infer a stellar luminosity on the order of only 2000 L{sub {center_dot}}. Although source I is believed to be more luminous, they do not detect an infrared counterpart even at the longest wavelengths. However, they resolve the closeby infrared source, IRc2, into an arc of knots {approx} 10{sup 3} AU long at all wavelengths. Although the physical relation of source I to IRc2 remains ambiguous, they suggest these sources mark a high density core (10{sup 7}-10{sup 8} pc{sup -3} over {approx} 10{sup 3} AU) within the larger BN/KL star forming cluster. The high density may be a consequence of the core being young and heavily embedded. The authors suggest the energetics of the BN/KL region may be dominated by this cluster core rather than one or two individual sources.

greenhill, l

2004-06-25

210

IS THE YOUNG STAR CLUSTER NGC 376 DISSOLVING IN THE FIELD OF THE SMALL MAGELLANIC CLOUD?  

SciTech Connect

We use deep images acquired with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope in the filters F555W and F814W to characterize the properties of NGC 376, a young star cluster located in the wing of the Small Magellanic Cloud. Using isochrone fitting we derive for NGC 376 an age of 28 {+-} 7 Myr, in good agreement with previous studies. The high spatial resolution ACS data allow us to determine the center of gravity of the cluster and to construct extended surface brightness and radial density profiles. Neither of these profiles can be fitted with a theoretical model, suggesting that the cluster is not in virial equilibrium. Considering the young age of the cluster, we speculate that the distortion of the radial profiles may be the result of the rapid gas dispersal that follows the initial phase of star formation (SF). The cluster shows clear evidence of dynamical mass segregation. From the properties of the radial profiles and the present-day mass function we conclude that NGC 376 appears to have already lost nearly 90% of its initial stellar mass, probably as a consequence of the sudden gas dispersal that follows the early phase of SF.

Sabbi, E.; Nota, A.; Smith, L. J. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Tosi, M.; Cignoni, M. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127, Bologna (Italy); Gallagher, J., E-mail: sabbi@stsci.edu [Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706 (United States)

2011-09-20

211

Orion in a New Light - VISTA exposes high-speed antics of young stars  

NASA Astrophysics Data System (ADS)

The Orion Nebula reveals many of its hidden secrets in a dramatic image taken by ESO's new VISTA survey telescope. The telescope's huge field of view can show the full splendour of the whole nebula and VISTA's infrared vision also allows it to peer deeply into dusty regions that are normally hidden and expose the curious behaviour of the very active young stars buried there. VISTA - the Visible and Infrared Survey Telescope for Astronomy - is the latest addition to ESO's Paranal Observatory (eso0949). It is the largest survey telescope in the world and is dedicated to mapping the sky at infrared wavelengths. The large (4.1-metre) mirror, wide field of view and very sensitive detectors make VISTA a unique instrument. This dramatic new image of the Orion Nebula illustrates VISTA's remarkable powers. The Orion Nebula [1] is a vast stellar nursery lying about 1350 light-years from Earth. Although the nebula is spectacular when seen through an ordinary telescope, what can be seen using visible light is only a small part of a cloud of gas in which stars are forming. Most of the action is deeply embedded in dust clouds and to see what is really happening astronomers need to use telescopes with detectors sensitive to the longer wavelength radiation that can penetrate the dust. VISTA has imaged the Orion Nebula at wavelengths about twice as long as can be detected by the human eye. As in the many visible light pictures of this object, the new wide field VISTA image shows the familiar bat-like form of the nebula in the centre of the picture as well as the fascinating surrounding area. At the very heart of this region lie the four bright stars forming the Trapezium, a group of very hot young stars pumping out fierce ultraviolet radiation that is clearing the surrounding region and making the gas glow. However, observing in the infrared allows VISTA to reveal many other young stars in this central region that cannot be seen in visible light. Looking to the region above the centre of the picture, curious red features appear that are completely invisible except in the infrared. Many of these are very young stars that are still growing and are seen through the dusty clouds from which they form. These youthful stars eject streams of gas with typical speeds of 700 000 km/hour and many of the red features highlight the places where these gas streams collide with the surrounding gas, causing emission from excited molecules and atoms in the gas. There are also a few faint, red features below the Orion Nebula in the image, showing that stars form there too, but with much less vigour. These strange features are of great interest to astronomers studying the birth and youth of stars. This new image shows the power of the VISTA telescope to image wide areas of sky quickly and deeply in the near-infrared part of the spectrum. The telescope is just starting to survey the sky and astronomers are anticipating a rich harvest of science from this unique ESO facility. Notes [1] The Orion Nebula lies in the sword of the famous celestial hunter and is a favourite target both for casual sky watchers and astrophysicists alike. It is faintly visible to the unaided eye and appeared to early telescopic observers as a small cluster of blue-white stars surrounded by a mysterious grey-green mist. The object was first described in the early seventeenth century although the identity of the discoverer is uncertain. The French comet-hunter Messier made an accurate sketch of its main features in the mid-eighteenth century and gave it the number 42 in his famous catalogue. He also allocated the number 43 to the smaller detached region just above the main part of the nebula. Later William Herschel speculated that the nebula might be "the chaotic material of future suns" and astronomers have since discovered that the mist is indeed gas glowing under the fierce ultraviolet light from young hot stars that have recently formed there. More information ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the

2010-02-01

212

The Galactic Center Cloud G2—a Young Low-mass Star with a Stellar Wind  

NASA Astrophysics Data System (ADS)

We explore the possibility that the G2 gas cloud falling in toward SgrA* is the mass-loss envelope of a young T Tauri star. As the star plunges to smaller radius at 1000-6000 km s-1, a strong bow shock forms where the stellar wind is impacted by the hot X-ray emitting gas in the vicinity of SgrA*. For a stellar mass-loss rate of 4 × 10-8 M ? yr-1 and wind velocity 100 km s-1, the bow shock will have an emission measure (EM = n 2 vol) at a distance ~1016 cm, similar to that inferred from the IR emission lines. The ionization of the dense bow shock gas is potentially provided by collisional ionization at the shock front and cooling radiation (X-ray and UV) from the post shock gas. The former would predict a constant line flux as a function of distance from SgrA*, while the latter will have increasing emission at lesser distances. In this model, the star and its mass-loss wind should survive pericenter passage since the wind is likely launched at 0.2 AU and this is much less than the Roche radius at pericenter (~3 AU for a stellar mass of 2 M ?). In this model, the emission cloud will probably survive pericenter passage, discriminating this scenario from others.

Scoville, N.; Burkert, A.

2013-05-01

213

Exo-comet Detection in Debris Disks Around Young A-type Stars  

NASA Astrophysics Data System (ADS)

We present details of the successful search for comet-like bodies (i.e. exo-comets) in orbit around several nearby stars. These objects have been found in young stellar systems that are in the transitional stage of evolution between possession of a gaseous protoplanetary disk to that of a dust-rich debris disk. During this period it is thought that large planetesimals of ~ 1000 km diameter may cause dynamical perturbations in the population of smaller bodies (such as asteroids and comets), such that they are sent on highly eccentric orbits towards their parent star resulting in the liberation of large amounts of evaporating gas and dust. By observing the varying spectral absorption signature of the CaII K-line at 3933Å due to this liberated gas, we have been able to track the trajectory of these exo-comets over a time-frame of several nights as they approach (and sometimes pass around) the central star. The youngest debris disks (1 - 50 Myr) are thought to represent the last stage in the formation of planetary systems and they may resemble our solar system’s own debris disk at the time of the Late Heavy Bombardment when the terrestrial worlds were subject to frequent collisions with asteroids and comets. Collisions with water-rich comets from the outer regions of our solar system may have delivered water to thee Earth’s oceans.

Welsh, Barry; Montgomery, S. L.

2013-01-01

214

Spitzer IRS Spectra of Young Stars Near the Hydrogen Burning Mass Limit  

NASA Astrophysics Data System (ADS)

We present Spitzer Infrared Spectrograph* (IRS) measurements for two young stars in the Taurus star-forming region, with masses near the hydrogen-burning mass limit . While one of the objects displays no mid-infrared excess, the other one shows a large excess at wavelengths longward of 10 microns. The latter spectrum resembles in detail the spectra of flared, optically-thick accretion disks around more massive classical T Tauri stars. A bright and complex 10 micron silicate emission feature in this spectrum indicates the presence in the disk photosphere of small dust grains that contain significant amounts of crystalline silicates. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through Contract Number 1257184 issued by JPL/Caltech, and Cornell subcontracts 31419-5714 to the University of Rochester. * The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.

Furlan, E.; Calvet, N.; D'Alessio, P.; Hartmann, L.; Forrest, W. J.; Uchida, K. I.; Watson, D. M.; Luhman, K. L.; Green, J. D.; Sargent, B.; Najita, J.; Sloan, G. C.; Keller, L. D.; Herter, T. L.

2004-12-01

215

Star formation in grand-design, spiral galaxies. Young, massive clusters in the near-infrared  

NASA Astrophysics Data System (ADS)

Aims: Spiral structure is a prominent feature in many disk galaxies and is often outlined by bright, young objects. We study the distribution of young stellar clusters in grand-design spiral galaxies and thereby determine whether strong spiral perturbations can influence star formation. Methods: Deep, near-infrared JHK-maps were observed for ten nearby, grand-design, spiral galaxies using HAWK-I at the Very Large Telescope. Complete, magnitude-limited candidate lists of star-forming complexes were obtained by searching within the K-band maps. The properties of the complexes were derived from (H - K) - (J - H) diagrams including the identification of the youngest complexes (i.e. ?7 Myr) and the estimation of their extinction. Results: Young stellar clusters with ages ?7 Myr have significant internal extinction in the range of AV = 3-7m, while older ones typically have AV < 1m. The cluster luminosity function (CLF) is well-fitted by a power law with an exponent of around -2 and displays no evidence of a high luminosity cut-off. The brightest cluster complexes in the disk reach luminosities of MK = -15.5m or estimated masses of 106 M?. At radii with a strong, two-armed spiral pattern, the star formation rate in the arms is higher by a factor of 2-5 than in the inter-arm regions. The CLF in the arms is also shifted towards brighter MK by at least 0.4m. We also detect clusters with colors compatible with Large Magellanic Cloud intermediate age clusters and Milky Way globular clusters. The (J - K) - MK diagram of several galaxies shows, for the brightest clusters, a clear separation between young clusters that are highly attenuated by dust and older ones with low extinction. Conclusions: The gap in the (J - K) - MK diagrams implies that there has been a rapid expulsion of dust at an age around 7 Myr, possibly triggered by supernovae. Strong spiral perturbations concentrate the formation of clusters in the arm regions and shifts their CLF towards brighter magnitudes. Based on observations collected at the European Southern Observatory, Chile; program: ESO 82.B-0331.Appendices A-C are available in electronic form at http://www.aanda.orgThe photometric data are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/542/A39

Grosbøl, P.; Dottori, H.

2012-06-01

216

3-D Structure of Outflows from Young Stars: GMOS IFU Observations of HH 32  

NASA Astrophysics Data System (ADS)

To better understand the 3-D physics and kinematics of collimated outflows from young stars, we have observed two knots in the HH 32 outflow with the GMOS Integral Field Unit (IFU) at the The Frederick C. Gillett Gemini Telescope. These observations were obtained for System Verification of the dithered IFU observations of a purely emission line object. HH 32 is a collimated Herbig-Haro outflow from the active young binary AS 353. In one spectral setting we were easily able to detect the H alpha, [SII], [NII] and [OI] emission lines in the spectra of knots A and B. In this poster we present the 6275-6785 Angstrom spectra of HH 32 averaged over the A and B knots, images of the knots in each emisson line, and several images of the H alpha emission with velocities ranging from -15 km/s to 415km/s. These observations reveal a complicated 3-D velocity structure that could not be discerned with longslit spectroscopy alone, and thus demonstrate the success of observing outflows from active young stars with optical integral field technology. We will be collaborating closely with with theorists in this field to intepret this data within the confines of current HH flow excitation and kinematic models. This study is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina)

Beck, T. L.; Aspin, C.

2002-12-01

217

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. III. The Antennae  

NASA Astrophysics Data System (ADS)

We present optical spectroscopy of 16 star clusters in the merging galaxies NGC 4038/39 ("the Antennae") and supplement this data set with Hubble Space Telescope imaging. The age and metallicity of each cluster is derived through a comparison between the observed Balmer and metal line strengths with simple stellar population models. We then estimate extinctions and masses using the photometry. We find that all but three clusters have ages between ~3 and 200 Myr, consistent with the expected increase in the star formation rate (SFR) due to the merger. Most of the clusters have velocities in agreement with nearby molecular and H I gas that has been previously shown to be rotating within the progenitor galaxies, hence star/cluster formation is still taking place within the galactic disks. However, three clusters have radial velocities that are inconsistent with being part of the rotating gas disks, which is surprising given their young (200-500 Myr) ages. Interestingly, we find a stellar association with the same colors (V - I) near one of these three clusters, suggesting that the cluster and association were formed concurrently and have remained spatially correlated. We find evidence for spatially distributed cluster formation throughout the duration of the merger. The impact of various assumptions about the SFR/cluster formation rate on the interpretation of the cluster age distribution is explored, and we do not find evidence for long-term "infant mortality" as has been previously suggested. Models of galaxy mergers that include a prescription for star formation can provide an overall good fit to the observed cluster age distribution.

Bastian, Nate; Trancho, Gelys; Konstantopoulos, Iraklis S.; Miller, Bryan W.

2009-08-01

218

GEMINI SPECTROSCOPIC SURVEY OF YOUNG STAR CLUSTERS IN MERGING/INTERACTING GALAXIES. III. THE ANTENNAE  

SciTech Connect

We present optical spectroscopy of 16 star clusters in the merging galaxies NGC 4038/39 ('the Antennae') and supplement this data set with Hubble Space Telescope imaging. The age and metallicity of each cluster is derived through a comparison between the observed Balmer and metal line strengths with simple stellar population models. We then estimate extinctions and masses using the photometry. We find that all but three clusters have ages between {approx}3 and 200 Myr, consistent with the expected increase in the star formation rate (SFR) due to the merger. Most of the clusters have velocities in agreement with nearby molecular and H I gas that has been previously shown to be rotating within the progenitor galaxies, hence star/cluster formation is still taking place within the galactic disks. However, three clusters have radial velocities that are inconsistent with being part of the rotating gas disks, which is surprising given their young (200-500 Myr) ages. Interestingly, we find a stellar association with the same colors (V - I) near one of these three clusters, suggesting that the cluster and association were formed concurrently and have remained spatially correlated. We find evidence for spatially distributed cluster formation throughout the duration of the merger. The impact of various assumptions about the SFR/cluster formation rate on the interpretation of the cluster age distribution is explored, and we do not find evidence for long-term 'infant mortality' as has been previously suggested. Models of galaxy mergers that include a prescription for star formation can provide an overall good fit to the observed cluster age distribution.

Bastian, Nate [Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom); Trancho, Gelys; Konstantopoulos, Iraklis S.; Miller, Bryan W. [Gemini Observatory, Casilla 603, La Serena (Chile)], E-mail: bastian@ast.cam.ac.uk, E-mail: gtrancho@gemini.edu

2009-08-10

219

New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope  

NASA Astrophysics Data System (ADS)

We present 24 ?m and 70 ?m Multiband Imaging Photometer for Spitzer (MIPS) observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program, including a re-analysis of some previously published source photometry. Our sample is selected from stars with common youth indicators such as lithium abundance, X-ray activity, chromospheric activity, and rapid rotation. We compare our MIPS observations to empirically derived Ks -[24] colors as a function of the stellar effective temperature to identify 24 ?m and 70 ?m excesses. We place constraints or upper limits on dust temperatures and fractional infrared luminosities with a simple blackbody dust model. We confirm the previously published 70 ?m excesses for HD 92945, HD 112429, and AU Mic, and provide updated flux density measurements for these sources. We present the discovery of 70 ?m excesses for five stars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599 is also a known Spitzer IRS (InfraRed Spectrograph) excess source, and we confirm the excess at 24 ?m. We also present the detection of 24 ?m excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD 123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find that large 70 ?m excesses are less common around stars with effective temperatures of less than 5000 K (3.7+7.6 -1.1%) than around stars with effective temperatures between 5000 K and 6000 K (21.4+9.5 -5.7%), despite the cooler stars having a younger median age in our sample (12 Myr vs. 340 Myr). We find that the previously reported excess for TWA 13A at 70 ?m is due to a nearby background galaxy, and the previously reported excess for HD 177724 is due to saturation of the near-infrared photometry used to predict the mid-infrared stellar flux contribution. In the Appendix, we present an updated analysis of dust grain removal timescales due to grain-grain collisions and radiation pressure, Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dust dynamical interaction. We find that drag forces can be important for disk dynamics relative to grain-grain collisions for L IR/L * < 10-4, and that stellar wind drag is more important than P-R drag for K and M dwarfs, and possibly for young (<1 Gyr) G dwarfs as well.

Plavchan, Peter; Werner, M. W.; Chen, C. H.; Stapelfeldt, K. R.; Su, K. Y. L.; Stauffer, J. R.; Song, I.

2009-06-01

220

The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars  

NASA Astrophysics Data System (ADS)

We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (?1.5-2.5 M ?) conducted to date and includes the planet hosts ? Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58^{+21}_{-20} M Jup and 55^{+20}_{-19} M Jup, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M ? stars can have giant planets greater than 4 M Jup between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M Jup between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M Jup, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties.

Nielsen, Eric L.; Liu, Michael C.; Wahhaj, Zahed; Biller, Beth A.; Hayward, Thomas L.; Close, Laird M.; Males, Jared R.; Skemer, Andrew J.; Chun, Mark; Ftaclas, Christ; Alencar, Silvia H. P.; Artymowicz, Pawel; Boss, Alan; Clarke, Fraser; de Gouveia Dal Pino, Elisabete; Gregorio-Hetem, Jane; Hartung, Markus; Ida, Shigeru; Kuchner, Marc; Lin, Douglas N. C.; Reid, I. Neill; Shkolnik, Evgenya L.; Tecza, Matthias; Thatte, Niranjan; Toomey, Douglas W.

2013-10-01

221

A Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129  

NASA Astrophysics Data System (ADS)

Context: The reflection nebula NGC 7129 has long been known to be a site of recent star formation as shown, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. A major step forward has been made with recent Spitzer imaging of the region. Aims: This study represents the next step towards a systematic assessment of the pre-main sequence population in NGC 7129. Completeness of the pre-main sequence sample is necessary for studying key features that allow the star-forming process to be understood, such as disk evolution, dynamical evolution, and mass function. At a presumed age of 3 Myr, NGC 7129 is in the critical range where disks around young stars disappear. Methods: We make use of X-ray and IR imaging observations to identify the pre-main sequence stars in NGC 7129. We define a sample of young stellar objects based on color-color diagrams composed from IR photometry between 1.6 and 8 ?m, from 2 MASS and Spitzer, and based on X-ray detected sources from a Chandra observation. Results: This sample is composed of 26 Class II and 25 Class III candidates. It has been selected from infrared sources in the Chandra field (287 objects with photometry in all four Spitzer/IRAC bands, 811 objects with near-IR photometry) and the 59 X-ray sources detected with Chandra. The sample is estimated to be complete down to 0.5 {M_?}. The most restricted and least biased subsample of pre-main sequence stars is composed of lightly absorbed (AV < 5 mag) stars in the cluster core. This sample comprises 7 Class II and 14 Class III sources and has a disk fraction of 33^+24-19% and a median X-ray luminosity of log{L_x} [erg/s] = 30.3. Conclusions: Despite the various uncertainties related to the sample selection, absorption, mass distribution, distance, and, consequently in the computation of disk fraction and X-ray luminosities, the data yield consistent results. In particular, we confirm the age of ˜ 3 Myr for the NGC 7129 cluster. The derived disk fraction is similar to that of ? Orionis, smaller than for Cha I (˜ 2 Myr), and larger than for Upper Sco (5 Myr). The X-ray luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than for the Orion Nebula Cluster (1 Myr). The pre-main sequence census should be further refined and extended with optical photometric and spectroscopic searches for cluster members. Tables 2-6 are only available in electronic form at http://www.aanda.org

Stelzer, B.; Scholz, A.

2009-11-01

222

Photospheric and chromospheric activity on the young solar-type star HD 171488 (V889 Herculis)  

NASA Astrophysics Data System (ADS)

Aims: We present the results of contemporaneous spectroscopic and photometric monitoring of the young solar-type star HD 171488 (Prot ? 1.337 days) aimed at studying surface inhomogeneities at both photospheric and chromospheric levels. Methods: Echelle FOCES spectra (R ? 40 000) and Johnson BV photometry have been performed in August 2006, with a good coverage of rotational phases. Spectral type, rotational velocity, metalicity, and gravity were determined with a code developed by us (Rotfit) and a library of high-resolution spectra of slowly-rotating reference stars. The metalicity was measured from the analysis of iron lines with the Moog code. The spectral subtraction technique was applied to the most relevant chromospheric diagnostics included in the FOCES spectral range, namely Ca II IRT, H?, He I D3, H?, and Ca II H&K lines. Results: A simple model with two large high-latitude spots is sufficient to reproduce the B and V light curves as well as the radial velocity modulation if a temperature difference between photosphere and spots of about 1500 K is used. A Doppler-imaging analysis of photospheric lines basically confirms a similar spot distribution. With the help of an analogous geometric two-spot model, we are able to reproduce the observed modulations in the residual chromospheric emissions adopting different values of ratios between the flux of plages and the quiet chromosphere (about 5 for H? and 3 for Ca II diagnostics). Facular regions of solar type appear to be the main responsible features for the modulations of chromospheric diagnostics. Both the spot/plage model and the cross-correlation between the light curve and the chromospheric line fluxes display a significant lead effect of plages with respect to spots (from 20° to 40° in longitude), as already observed in some active solar-type stars and RS CVn systems. Conclusions: The contemporaneous monitoring of photospheric and chromospheric diagnostics in the young and rapidly rotating solar-type star HD 171488 allowed us to detect active regions which have nearly the same location at both atmospheric layers, with plages slightly leading spots in longitudes. These active regions are similar to the solar ones in some respects, because the spot temperature is close to that of sunspot umbrae and the plage flux-contrast is consistent with the average solar values. The main differences with respect to the Sun are larger sizes and higher latitudes. Based on observations collected at Calar Alto Astronomical Observatory (Spain) and Catania Astrophysical Observatory (Italy).Table 5 and Fig. 7 are only available in electronic form at http://www.aanda.org

Frasca, A.; Biazzo, K.; K?vári, Zs.; Marilli, E.; Çak?rl?, Ö.

2010-07-01

223

On the time dependence of differential rotation in young late-type stars  

NASA Astrophysics Data System (ADS)

A model for the angular momentum transfer within the convection zone of a rapidly rotating star is introduced and applied to the analysis of recent observations of temporal fluctuations of the differential rotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae (LQ Hya). Under the hypothesis that the mean magnetic field produced by the stellar dynamo rules the angular momentum exchanges and that the angular velocity depends only on the distance s from the rotation axis and the time, the minimum azimuthal Maxwell stress |BsB?|, averaged over the convection zone, is found to range from ~0.04 to ~0.14T2. If the poloidal mean magnetic field Bs is of the order of 0.01T, as indicated by the Zeeman-Doppler imaging maps of those stars, then the azimuthal mean field B? can reach an intensity of several teslas, which significantly exceeds equipartition with the turbulent kinetic energy. Such strong fields can account also for the orbital period modulation observed in cataclysmic variables and RS Canum Venaticorum systems with a main-sequence secondary component. Moreover, the model allows us to compute the kinetic energy dissipation rate during the maintenance of the differential rotation. Only in the case of the largest surface shear observed on LQ Hya may the dissipated power exceed the stellar luminosity, but the lack of a sufficient statistic on the occurrence of such episodes of large shear does not allow us to estimate their impact on the energy budget of the convection zone.

Lanza, Antonino F.

2006-12-01

224

A new Herschel view of the young star T54: not a transitional disk?  

NASA Astrophysics Data System (ADS)

Context. Observations of transitional disks give us an understanding of the formation of planets and planetary systems such as our own. But care must be taken in identifying such sources: the higher spatial resolution of the Herschel Space Observatory provides a new view of the origin of the far-infrared and submillimeter excesses observed. Aims: We review the nature of previously known transitional disks in the Chamaeleon I star-forming region with Herschel data. Methods: We analyze Herschel PACS and SPIRE images of the young star T54, together with ancillary images. We also analyze its spectral energy distribution and indications from optical and mid-infrared spectroscopy. Results: We detect extended emission in the PACS 70 ?m image ~6'' off source at a position angle of 196° from T54. The emission detected at longer wavelengths (PACS 100, 160, SPIRE 250 and 350 ?m) is also offset from the position of the star. This suggests that the excess observed in the far-infrared part of the SED is not fully associated with T54. Conclusions.Herschel images show that the far-infrared excess seen in T54 is not due to a transitional disk but to extended emission southwest of the source. The object still shows point-like and now downscaled excess at mid-infrared wavelengths, but its origin cannot be constrained without higher spatial resolution data. However, different indications point towards an evolved disk or extended unresolved emission close to the source. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Matrà, L.; Merín, B.; Alves de Oliveira, C.; Huélamo, N.; Kóspál, A.; Cox, N. L. J.; Ribas, Á.; Puga, E.; Vavrek, R.; Royer, P.; Prusti, T.; Pilbratt, G. L.; André, P.

2012-12-01

225

How Do Astronomers Know That? Educating Teachers, Students & the Public on HOW You Discover Young Stars  

NASA Astrophysics Data System (ADS)

Every day amazing astronomical facts are taught to thousands of curious people. Students learn them in the classroom. Museum visitors hear them in a Planetarium show or lecture. When it’s time for questions, many intuitively ask, “how do you know that?” NITARP helps close this gap in astronomy education. NITARP stands for NASA/IPAC Teacher Archive Research Program. NITARP brings together an astronomer with a small group of teachers and students to do real astronomical research. After the year long program is completed, the education and experiences gained the teachers are brought back to their classrooms and museums across America. Our NITARP group researched apparent infrared (IR) excesses to identify Young Stellar Objects (YSOs). Our science results are presented in a companion paper, Novatne et al, at this AAS conference. We concentrated our search in the Bright Rimmed Cloud (BRC) 27, located in the constellation Canis Major. Our main focus was to use data from the Wide-field Infrared Survey Explorer (WISE), along with other archived infrared data such as Spitzer and 2MASS. Thus, our NITARP group was called C-WAYS—standing for Cool, WISE and Young Stars. In this poster, we present our educational plan to connect real science by astronomers to educators, students, and ultimately our communities.

Bonadurer, Robert; Piper, M.; French, D.; Barge, J.; Novatne, L. J.; Rebull, L. M.; Ali, B.; Laher, R.; Armstrong, J.

2013-01-01

226

Young stars and reflection nebulae near the lower ``edge'' of the Galactic molecular disc  

NASA Astrophysics Data System (ADS)

Aims: We investigate the star formation occurring in a region well below the Galactic plane towards the optical reflection nebula ESO 368-8 (IRAS 07383-3325). We confirm the presence of a small young stellar cluster (or aggregate of tens of YSOs) identified earlier, embedded in a molecular cloud located near the lower “edge” of the Galactic disc, and characterise the young stellar population. We report the discovery of a near-infrared nebula, and present a CO map revealing a new dense, dynamic cloud core. Methods: We used near-infrared {JHK}_S images obtained with VLT/ISAAC, millimetre CO spectra obtained with the SEST telescope, and optical V-band images from the YALO telescope. Results: This star formation region displays an optical reflection nebula (ESO 368-8) and a near-infrared nebula located about 46'' (1.1 pc) from each other. The two nebulae are likely to be coeval and to represent two manifestations of the same single star formation episode with about 1 Myr age. The near-IR nebula reveals an embedded, optically and near-IR invisible source whose light scatters off a cavity carved by previous stellar jets or molecular outflows and into our line-of-sight. The molecular cloud is fully covered by our CO(J = 1-0) maps and, traced by this line, extends over a region of ˜7.8 ×7.8 pc2, exhibiting an angular size ˜5.4' ×5.4' and shape (close to circular) similar to spherical (or slightly cometary) globules. Towards the direction of the near-IR nebula, the molecular cloud contains a dense core where the molecular gas exhibits large line widths indicative of a very dynamical state, with stirred gas and supersonic motions. Our estimates of the mass of the molecular gas in this region range from 600 to 1600 M?. The extinction AV towards the positions of the optical reflection nebula and of the near-IR nebula was found to be AV ? 3-4 mag and AV ? 12-15 mag, respectively. Based on observations collected at the ESO 8.2-m VLT-UT1 Antu telescope (programme 66.C-0015A).

Palmeirim, P. M.; Yun, J. L.

2010-02-01

227

GEMINI SPECTROSCOPIC SURVEY OF YOUNG STAR CLUSTERS IN MERGING/INTERACTING GALAXIES. IV. STEPHAN's QUINTET  

SciTech Connect

We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy (TDG) candidate in Stephan's Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through Hubble Space Telescope photometry by Fedotov et al., as well as their modeled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age ({approx}125 Myr), high mass ({approx}2 Multiplication-Sign 10{sup 6} M{sub Sun }), and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (R{sub eff} {approx} 12-15 pc) compared to clusters observed in galaxy disks (R{sub eff} {approx} 3-4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least {approx}150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population ({approx}7 Myr), and, assuming a single age for the entire region, has a mass of at least 10{sup 6} M{sub Sun }.

Trancho, Gelys [Gemini Observatory, Casilla 603, La Serena (Chile); Konstantopoulos, Iraklis S.; Mullan, Brendan; Charlton, Jane C. [Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802 (United States); Bastian, Nate [Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching (Germany); Fedotov, Konstantin; Gallagher, Sarah, E-mail: gtrancho@gmto.org [Physics and Astronomy Department, University of Western Ontario, London, ON N6A 3K7 (Canada)

2012-04-01

228

Young Galaxy Surrounded by Material Needed to Make Stars, VLA Reveals  

NASA Astrophysics Data System (ADS)

Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have discovered a massive reservoir of cold gas from which a primeval galaxy formed its first stars. Looking more than 12 billion years into the past, the scientists found that the young galaxy experiencing a "burst" of star formation was surrounded by enough cold molecular gas to make 100 billion suns. Optical and Radio Images of APM 08279+5255 at About the Same Scale "This is the first time anyone has seen the massive reservoir of cold gas required for these incredible 'starbursts' to produce a galaxy," said Chris Carilli, an astronomer at the NSF's National Radio Astronomy Observatory (NRAO) in Socorro, NM. "There is much more gas here than we anticipated," Carilli added. The research team was led by Padeli Papadoupoulos of Leiden Observatory in the Netherlands and also included Rob Ivison of University College London and Geraint Lewis of the Anglo-Australian Observatory in Australia. The scientists reported their findings in the January 4 edition of the journal Nature. The astronomers found the gas when studying a quasar called APM 08279+5255, discovered in 1998. Observations with optical and infrared telescopes revealed that the quasar, a young galaxy with a voracious black hole at its center, was forming new stars rapidly in a starburst. At a distance of more than 12 billion light-years, the quasar is seen as it was more than 12 billion years ago, just a billion or so years after the Big Bang. "This thing is at the edge of the dark ages," before the first stars in the universe were born, said Carilli. The year after its discovery, APM 08279+5255 was found to have warm carbon monoxide (CO) gas near its center, heated by the energy released as the galaxy's black hole devours material. The VLA observations revealed cold CO gas much more widely distributed than its warmer counterpart. Based on observations of closer objects, the astronomers presume the CO gas is accompanied by large amounts of molecular hydrogen gas (H2). Cold CO gas never has been detected before in such a distant object. Though APM 08279+5255 is a young galaxy undergoing its first massive burst of star formation, the CO gas indicates that very massive stars formed quickly, lived through their short lifetimes, and exploded as supernovae. Carbon and Oxygen, the component elements of CO, are formed in the cores of stars, so their presence in the cold gas tells the astronomers that massive, short-lived stars had to have exploded already, spreading these elements throughout the galaxy's interstellar gas. "The original discovery of this quasar was quite a surprise, as observations revealed it is among the most luminous objects known in the universe. The discovery of this massive reservoir of cold gas is equally surprising. It provides vital clues to the birth of galaxies, such as our own Milky Way," Lewis said. Discovery of the gas was made possible by the galaxy's great distance. The expansion of the universe "stretches" light and radio waves to longer wavelengths -- the more distant the object, the more stretching is seen. Radio waves emitted by the cold CO gas originally had wavelengths of about 1.3 and 2.6 millimeters, but were "redshifted" to wavelengths of 7 and 13 millimeters -- wavelengths the VLA can receive. "It took eight years to refine this technique, but the effort has been worthwhile. This is the golden age of cosmology. We are learning more and more about our universe, from the smallest planets to the largest galaxy clusters. This new result is a crucial piece in the jigsaw and may help resolve many misconceptions about how galaxies form and evolve" Ivison said. "Because of its sensitivity and its ability to make detailed images, the VLA is the only telescope able to unveil these large reservoirs of cold molecular gas in the distant universe," Carilli said. "In addition, as we expand the technical capabilities of the VLA in the coming years, making it even m

2001-01-01

229

SPITZER VIEW OF YOUNG MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD H II COMPLEXES. II. N 159  

SciTech Connect

The H II complex N 159 in the Large Magellanic Cloud is used to study massive star formation in different environments, as it contains three giant molecular clouds (GMCs) that have similar sizes and masses but exhibit different intensities of star formation. We identify candidate massive young stellar objects (YSOs) using infrared photometry, and model their spectral energy distributions to constrain mass and evolutionary state. Good fits are obtained for less evolved Type I, I/II, and II sources. Our analysis suggests that there are massive embedded YSOs in N 159B, a maser source, and several ultracompact H II regions. Massive O-type YSOs are found in GMCs N 159-E and N 159-W, which are associated with ionized gas, i.e., where massive stars formed a few Myr ago. The third GMC, N 159-S, has neither O-type YSOs nor evidence of previous massive star formation. This correlation between current and antecedent formation of massive stars suggests that energy feedback is relevant. We present evidence that N 159-W is forming YSOs spontaneously, while collapse in N 159-E may be triggered. Finally, we compare star formation rates determined from YSO counts with those from integrated H{alpha} and 24 {mu}m luminosities and expected from gas surface densities. Detailed dissection of extragalactic GMCs like the one presented here is key to revealing the physics underlying commonly used star formation scaling laws.

Chen, C.-H. Rosie; Indebetouw, Remy [Department of Astronomy, University of Virginia, Charlottesville, VA 22904 (United States); Chu, You-Hua; Gruendl, Robert A.; Seale, Jonathan P. [Department of Astronomy, University of Illinois, Urbana, IL 61801 (United States); Testor, Gerard [Observatoire de Paris, 92195 Meudon (France); Heitsch, Fabian [Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 (United States); Meixner, Margaret; Sewilo, Marta [Space Telescope Science Institute, Baltimore, MD 21218 (United States)

2010-10-01

230

Solar Eruption  

NASA Website

A solar eruption gracefully rose up from the sun on Dec. 31, 2012, twisting and turning. Magnetic forces drove the flow of plasma, but without sufficient force to overcome the sun's gravity much of the plasma fell back into the sun. The length of ...

231

DYNAMICAL EVOLUTION OF THE YOUNG STARS IN THE GALACTIC CENTER: N-BODY SIMULATIONS OF THE S-STARS  

SciTech Connect

We use Newtonian N-body simulations to study the evolution of the orbital eccentricities of stars deposited near ({approx}<0.05 pc) the Milky Way massive black hole (MBH), starting from initial conditions motivated by two competing models for their origin: formation in a disk followed by inward migration and exchange interactions involving a binary star. The first model predicts modest eccentricities, lower than those observed in the S-star cluster, while the second model predicts higher eccentricities than observed. The Newtonian N-body simulations include a dense cluster of 10 M{sub sun} stellar-mass black holes (SBHs), expected to accumulate near the MBH by mass segregation. Perturbations from the SBHs tend to randomize the stellar orbits, partially erasing the dynamical signatures of their origin. The eccentricities of the initially highly eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that is consistent with the observed eccentricity distribution. In contrast, the eccentricities of the initially more circular orbits fail to evolve to the observed values in 20 Myr, arguing against the disk migration scenario. We find that 20%-30% of the S-stars are tidally disrupted by the MBH over their lifetimes, and that the S-stars are not likely to be ejected as hypervelocity stars outside the central 0.05 pc by close encounters with SBHs.

Perets, Hagai B.; Kupi, Gabor; Alexander, Tal [Weizmann Institute of Science, P.O. Box 26, Rehovot 76100 (Israel); Gualandris, Alessia; Merritt, David [Department of Physics and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623 (United States)], E-mail: hagai.perets@weizmann.ac.il

2009-09-10

232

Galactic Globular Cluster NGC1261: "Young", Low-alpha and Star-to-Star Abundance Variations In Na-O?  

NASA Astrophysics Data System (ADS)

We present the first high-resolution (R 40,000) chemical abundance analysis of the relatively young (8-9 Gyr), outer halo Galactic globular cluster NGC 1261. Stellar parameters for three stars were derived using spectroscopic constraints. Abundances were deduced using a combination of EWs and fitting synthetic spectra. Our derived metallicity ([Fe/H] = -1.19 ± 0.02) is in excellent agreement with the metallicity scale of Kraft & Ivans (2003, 2004). However, the Na-O anticorrelation spanning 1.1 ± 0.1 dex in sodium, is as large as any other cluster reported to date. In the light element group, we report -0.9 < [C/Fe] < -0.6 with C12/C13 = 4, (for which three regions in the G-band were synthesized); +0.5 < [N/Fe] < +1.1 from the CN band head near 8004A; -0.25 < [O/Fe] < +0.2 from the forbidden lines; -0.3 < [Al/Fe] < +0.15 from the aluminum doublet near 6696A. We also present abundances for the alpha-elements [Mg/Fe] = +0.2 ± 0.1; [Si/Fe] = +0.1 ± 0.1; [Ca/Fe] = +0.15 ± 0.1; [Ti/Fe] = +0.15 ± 0.15. NGC 1261 is distinguished from other clusters of comparable metallicity due to low-alpha abundances. We will discuss the nucleosynthetic histories that may have given rise to these abundances including the possibility of enrichment by Type 1a supernovae. We also report abundances for the iron-peak elements [Sc/Fe] = +0.0 ± 0.2; [V/Fe] = -0.1 ± 0.1; [Cr/Fe] = -0.1 ± 0.15; [Mn/Fe] = -0.1 ± 0.15; [Co/Fe] = +0.2 ± 0.1; [Ni/Fe] = +0.0 ± 0.1; the light neutron-capture elements [Y/Fe] = +0.1 ± 0.1; [Zr/Fe] = +0.35 ± 0.1; the s-process elements [Ba/Fe] = +0.15 ± 0.1; [La/Fe] = +0.1 ± 0.1; [Nd/Fe] = +0.15 ± 0.1; and the r-process element [Eu/Fe] = 0.6 ± 0.2.

Filler, Dan; Ivans, I. I.; Simmerer, J.

2012-01-01

233

OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE  

SciTech Connect

The idea that non-steady accretion during the embedded phase of protostar evolution can produce the observed luminosity spread in the Herzsprung-Russell diagram (HRD) of young clusters has recently been called into question. Observations of FU Ori, for instance, suggest an expansion of the star during strong accretion events, whereas the luminosity spread implies a contraction of the accreting objects, decreasing their radiating surface. In this paper, we present a global scenario based on calculations coupling episodic accretion histories derived from numerical simulations of collapsing cloud prestellar cores of various masses and subsequent protostar evolution. Our calculations show that, assuming an initial protostar mass M{sub i} {approx} 1 M{sub Jup}, typical of the second Larson's core, both the luminosity spread in the HRD and the inferred properties of FU Ori events (mass, radius, accretion rate) can be explained by this scenario, providing two conditions. First, there must be some variation within the fraction of accretion energy absorbed by the protostar during the accretion process. Second, the range of this variation should increase with increasing accretion burst intensity and thus with the initial core mass and final star mass. The numerical hydrodynamics simulations of collapsing cloud prestellar cores indeed show that the intensity of the accretion bursts correlates with the mass and initial angular momentum of the prestellar core. Massive prestellar cores with high initial angular momentum are found to produce intense bursts characteristic of FU Ori-like events. Our results thus suggest a link between the burst intensities and the fraction of accretion energy absorbed by the protostar, with some threshold in the accretion rate, of the order of 10{sup -5} M{sub Sun} yr{sup -1}, delimitating the transition from 'cold' to 'hot' accretion. Such a transition might reflect a change in the accretion geometry with increasing accretion rate, i.e., a transition from magnetospheric or thin-disk to thick-disk accretion, or in the magnetospheric interaction between the star and the disk. Conversely, the luminosity spread can also be explained by a variation of the initial protostar mass within the {approx}1-5 M{sub Jup} range, although it is unclear for now whether such a spread among the second Larson's core can be produced during the prestellar core second collapse. This unified picture confirms the idea that early accretion during protostar and proto-brown dwarf formation/evolution can explain the observed luminosity spread in young clusters without invoking any significant age spread, and that the concept of a well-defined birthline does not apply for low-mass objects. Finally, we examine the impact of accretion on the determination of the initial mass function in young clusters.

Baraffe, I.; Chabrier, G. [Astrophysics Group, University of Exeter, EX4 4QL Exeter (United Kingdom); Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr [Institute of Astrophysics, University of Vienna, Vienna A-1180 (Austria)

2012-09-10

234

Near-Infrared Polarimetric Imaging of Disks around Young Intermediate-mass Stars in SEEDS  

NASA Astrophysics Data System (ADS)

We present our recent results to directly image circumstellar disks around Herbig Fe/Ae/Be stars in scattered light with Subaru. Observations of such young disks are critically important to understand how disks evolve possibly under the mutual interaction with new-born planets. One of the observational approaches is direct imaging in scattered light, and the progress in this field since PPV can be found in the ability to prove inner regions of disks. This improvement largely owes to the technique of polarization differential imaging (PDI) which provides higher contrast by extracting scattered light from the disk while suppressing unpolarized stellar light. Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS) is the project dedicated to exoplanet hunting and study of circumstellar disks by direct imaging. Since its beginning in 2009, thirteen Herbig Fe/Ae/Be stars have been observed primarily in H band (1.6 micron). The PDI method has been employed with adaptive optics, enabling us to look into the inner region as close as 0.2 arcsec (˜30 AU) in radius with the typical angular resolution of 0.06 arcsec (˜8 AU). As a result, the SEEDS imagery has newly uncovered rich structures such as spiral arms, inner holes, and gaps for (pre-)transitional disks while suggested the variably illuminated disks for primordial systems. The highlight is the discovery of two spiral arms each for SAO 206462 and MWC 758. The spiral feature has been uniquely found toward Herbig Fe/Aes so far, which might be due to their warmer disks producing arms loosely wound and more easily detected. The observed morphology can be interpreted by the density-wave model, and those disks are implied to harbor Jupiter-mass companions as the exciting sources of the spiral structures according to these models.

Fukagawa, Misato; Hashimoto, Jun; Grady, C. A.; Momose, Munetake; Wisniewski, J. P.; Okamoto, Yoshiko; Muto, Takayuki; Kusakabe, Nobuhiko; Bonnefoy, Mickael; Kotani, Takayuki; Maruta, Yayoi; Tamura, Motohide; Seeds/Hiciao/Ao188 Collaboration,

2013-07-01

235

THE FIRST X-SHOOTER OBSERVATIONS OF JETS FROM YOUNG STARS  

SciTech Connect

We present the first pilot study of jets from young stars conducted with X-shooter, on the ESO/Very Large Telescope. As it offers simultaneous, high-quality spectra in the range 300-2500 nm, X-shooter is uniquely important for spectral diagnostics in jet studies. We chose to probe the accretion/ejection mechanisms at low stellar masses examining two targets with well-resolved continuous jets lying on the plane of the sky: ESO-HA 574 in Chameleon I and Par-Lup3-4 in Lupus III. The mass of the latter is close to the sub-stellar boundary (M{sub *} = 0.13 M{sub sun}). A large number of emission lines probing regions of different excitation are identified, position-velocity diagrams are presented, and mass outflow/accretion rates are estimated. Comparison between the two objects is striking. ESO-HA 574 is a weakly accreting star for which we estimate a mass accretion rate of log ( M-dot{sub acc}) = -10.8{+-}0.5 (in M{sub sun} yr{sup -1}), yet it drives a powerful jet with M-dot{sub out} {approx} 1.5-2.7 x 10{sup -9} M{sub sun} yr{sup -1}. These values can be reconciled with a magneto-centrifugal jet acceleration mechanism assuming that the presence of the edge-on disk severely depresses the luminosity of the accretion tracers. In comparison, Par-Lup3-4, with stronger mass accretion ( log ( M-dot{sub acc}) = -9.1{+-}0.4 M{sub sun} yr{sup -1}), drives a low-excitation jet with about M-dot{sub out} {approx} 3.2 x 10{sup -10} M{sub sun} yr{sup -1} in both lobes. Despite the low stellar mass, M-dot{sub out}/ M-dot{sub acc} for Par-Lup3-4 is at the upper limit of the range usually measured for young objects, but still compatible with a steady magneto-centrifugal wind scenario if all uncertainties are considered.

Bacciotti, F.; Randich, S. [INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze (Italy); Whelan, E. T. [Laboratoire d'Astrophysique de l'Observatoire de Grenoble, UMR 5521 du CNRS, 38041 Grenoble Cedex (France); Alcala, J. M. [INAF-Osservatorio di Capodimonte, Via Moiariello 16, 80131 Napoli (Italy); Nisini, B. [INAF-Osservatorio di Roma, Via di Frascati 33, 00040, Monteporzio Catone (Italy); Podio, L. [Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen (Netherlands); Stelzer, B. [INAF-Osservatorio di Palermo, Piazza del Parlamento 1, 90134 Palermo (Italy); Cupani, G. [INAF-Osservatorio di Trieste, Via Tiepolo 11, 34143 Trieste (Italy)

2011-08-20

236

Properties of young star cluster systems: the age signature from near-infrared integrated colours  

NASA Astrophysics Data System (ADS)

Context. A recent JHKs study of several grand-design spiral galaxies, including NGC 2997, shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J - H) vs. (H - Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H - Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram MK vs. (J - Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H - Ks) - 0.884 (J - H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. Aims: We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. In particular, we investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. In addition, the whole set of homogeneous measurements will serve as a template for analyses of the populations belonging to distant galaxies that are unresolved clusters or complexes. Methods: Surface photometry was carried out for 2MASS images of populous clusters younger than ~100 Myr whose ages were available. The integrated magnitude and colours were measured to a limiting radius and combined to generate the photometric diagrams. Some clusters, particularly the embedded ones, were studied for the first time using this method. Results: The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group show the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7 Myr from older clusters.

Santos, J. F. C.; Dottori, H.; Grosbøl, P.

2013-05-01

237

CLOSE COMPANIONS TO YOUNG STARS. I. A LARGE SPECTROSCOPIC SURVEY IN CHAMAELEON I AND TAURUS-AURIGA  

SciTech Connect

We present the results of a multiplicity survey of 212 T Tauri stars in the Chamaeleon I and Taurus-Auriga star-forming regions, based on high-resolution spectra from the Magellan Clay 6.5 m telescope. From these data, we achieved a typical radial velocity (RV) precision of {approx}80 m s{sup -1} with slower rotators yielding better precision, in general. For 174 of these stars, we obtained multi-epoch data with sufficient time baselines to identify binaries based on RV variations. We identified eight close binaries and four close triples, of which three and two, respectively, are new discoveries. The spectroscopic multiplicity fractions we find for Chamaeleon I (7%) and Taurus-Auriga (6%) are similar to each other, and to the results of field star surveys in the same mass and period regime. However, unlike the results from imaging surveys, the frequency of systems with close companions in our sample is not seen to depend on primary mass. Additionally, we do not find a strong correlation between accretion and close multiplicity. This implies that close companions are not likely the main source of the accretion shut down observed in weak-lined T Tauri stars. Our results also suggest that sufficient RV precision can be achieved for at least a subset of slowly rotating young stars to search for hot Jupiter planets.

Nguyen, Duy Cuong; Brandeker, Alexis; Van Kerkwijk, Marten H.; Jayawardhana, Ray, E-mail: nguyen@astro.utoronto.ca, E-mail: mhvk@astro.utoronto.ca, E-mail: rayjay@astro.utoronto.ca, E-mail: dcnguyen@pas.rochester.edu, E-mail: duy.nguyen@astro.su.se, E-mail: alexis@astro.su.se [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada)

2012-02-01

238

Magnetic activity and differential rotation in the very young star KIC 8429280  

NASA Astrophysics Data System (ADS)

Aims: We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine its activity level, spot distribution, and differential rotation. Methods: We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]), and we adopt a spectral subtraction technique to highlight the strong chromospheric emission in the cores of hydrogen H? and Ca ii H&K and infrared triplet (IRT) lines. We then fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. Results: We find that KIC 8429280 is a cool (K2 V) star with an age of about 50 Myr, based on its lithium content, that has passed its T Tau phase and is spinning up approaching the ZAMS on its radiative track. Its high level of chromospheric activity is clearly indicated by the strong radiative losses in Ca ii H&K and IRT, H?, and H? lines. Furthermore, its Balmer decrement and the flux ratio of Ca ii IRT lines imply that these lines are mainly formed in optically-thick regions similar to solar plages. The analysis of the Kepler data uncovers evidence of at least seven enduring spots. Since the star's inclination is rather high - nearly 70° - the assignment of the spots to either the northern or southern hemisphere is not unambiguous. We find at least three solutions with nearly the same level of residuals. Even in the case of seven spots, the fit is far from being perfect. Owing to the exceptional precision of the Kepler photometry, it is not possible to reach the noise floor without strongly enhancing the degrees of freedom and, consequently, the non-uniqueness of the solution. The distribution of the active regions is such that the spots are located around three latitude belts, i.e. around the star's equator and around ± (50°-60°), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation d? ? 0.27 rad d-1 is at variance with some recent mean-field models of differential rotation in rapidly rotating main-sequence stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle, as proposed by other models. Based on public Kepler data, on observations made with the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundación Galileo Galilei of INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque del los Muchachos of the Instituto de Astrofisica de Canarias, and on observations collected at the Catania Astrophysical Observatory (Italy).

Frasca, A.; Fröhlich, H.-E.; Bonanno, A.; Catanzaro, G.; Biazzo, K.; Molenda-?akowicz, J.

2011-08-01

239

IRS 16SW - A New Comoving Group of Young Stars in the Central Parsec of the Milky Way  

Microsoft Academic Search

One of the most perplexing problems associated with the supermassive black\\u000ahole at the center of our Galaxy is the origin of the young stars in its close\\u000avicinity. Using proper motion measurements and stellar number density counts\\u000abased on 9 years of diffraction-limited K(2.2 micron)-band speckle imaging at\\u000athe W. M. Keck 10-meter telescopes, we have identified a new

J. R. Lu; A. M. Ghez; S. D. Hornstein; M. Morris; E. E. Becklin

2005-01-01

240

PMS objects in the star formation region Cep OB3. II. Young stellar objects in the H? nebula Cep B  

NASA Astrophysics Data System (ADS)

Models for the spectral energy distributions of four stellar objects in the bright compact H? nebula Cep B are constructed. With a high probability, three of them are found to be very young stellar objects of evolutionary class 0/I with ages of 104-105 years, comparable to the kinematic age of the ionization front of the nebula itself. The IRAS 22551+6221 source associated with Cep B is initiated by heated dust. An intermediate-mass (B2-B3) star of evolutionary class III lies at the center of the ionization front. The local density of PMS stars in the immediate neighborhood of the Cep B nebula exceeds that for the cluster as a whole. It is highly probable that this zone is a local source of a new star-formation stage.

Nikoghosyan, E. H.

2013-06-01

241

THE FORMATION AND EVOLUTION OF YOUNG LOW-MASS STARS WITHIN HALOS WITH HIGH CONCENTRATION OF DARK MATTER PARTICLES  

SciTech Connect

The formation and evolution of low-mass stars within dense halos of dark matter (DM) leads to evolution scenarios quite different from the classical stellar evolution. As a result of our detailed numerical work, we describe these new scenarios for a range of DM densities on the host halo, for a range of scattering cross sections of the DM particles considered, and for stellar masses from 0.7 to 3 M {sub sun}. For the first time, we also computed the evolution of young low-mass stars in their Hayashi track in the pre-main-sequence phase and found that, for high DM densities, these stars stop their gravitational collapse before reaching the main sequence, in agreement with similar studies on first stars. Such stars remain indefinitely in an equilibrium state with lower effective temperatures (|DELTAT{sub eff}|>10{sup 3} K for a star of one solar mass), the annihilation of captured DM particles in their core being the only source of energy. In the case of lower DM densities, these protostars continue their collapse and progress through the main-sequence burning hydrogen at a lower rate. A star of 1 M{sub sun} will spend a time period greater than the current age of the universe consuming all the hydrogen in its core if it evolves in a halo with DM density rho{sub c}hi = 10{sup 9} GeV cm{sup -3}. We also show the strong dependence of the effective temperature and luminosity of these stars on the characteristics of the DM particles and how this can be used as an alternative method for DM research.

Casanellas, Jordi; Lopes, IlIDio, E-mail: jordicasanellas@ist.utl.p, E-mail: ilidio.lopes@ist.utl.p [Centro Multidisciplinar de Astrofisica, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa (Portugal)

2009-11-01

242

THE HIDDEN MAGNETIC FIELD OF THE YOUNG NEUTRON STAR IN KESTEVEN 79  

SciTech Connect

Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few Multiplication-Sign 10{sup 10} G) but an anomalously large ({approx}64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed 'Temperature Template with Full Transport' (TTFT), that computes the synthetic pulse profile of surface X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few Multiplication-Sign 10{sup 14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the 'visible' dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.

Shabaltas, Natalia; Lai Dong [Center for Space Research, Department of Astronomy, Cornell University, Ithaca, NY 14853 (United States)

2012-04-01

243

Broad-band optical polarimetric studies towards the Galactic young star cluster Berkeley 59  

NASA Astrophysics Data System (ADS)

We present multiwavelength optical linear polarimetric observations of 69 stars towards the young open cluster Berkeley 59. The observations reveal the presence of three dust layers located at distances of ˜300, ˜500 and ˜700 pc. The dust layers produce a total polarization PV˜ 5.5 per cent. The mean values of polarization and polarization angles due to the dust layers are found to increase systematically with distance. We show that polarimetry in combination with the (U-B)-(B-V) colour-colour diagram yields a better identification of cluster members. The polarization measurements suggest that the polarization due the intracluster medium is ˜2.2 per cent. An anomalous reddening law exists for the cluster region, indicating a relatively larger grain size than that in the diffuse interstellar medium. The spatial variation of polarization and colour excess E(B-V) are found to increase with radial distance from the cluster centre, whereas ?V and ?max are found to decrease with increasing radial distance from the cluster centre. About 40 per cent of cluster members show the signatures of either intrinsic polarization or rotation in their polarization angles. There is an indication that the starlight of the cluster members might have been depolarized because of non-uniform alignment of dust grains in the foreground dust layers and in the intracluster medium.

Eswaraiah, C.; Pandey, A. K.; Maheswar, G.; Chen, W. P.; Ojha, D. K.; Chandola, H. C.

2012-01-01

244

GAS SURFACE DENSITY, STAR FORMATION RATE SURFACE DENSITY, AND THE MAXIMUM MASS OF YOUNG STAR CLUSTERS IN A DISK GALAXY. I. THE FLOCCULENT GALAXY M 33  

SciTech Connect

We analyze the relationship between maximum cluster mass M{sub max} and surface densities of total gas ({Sigma}{sub gas}), molecular gas ({Sigma}{sub H{sub 2}}), and star formation rate ({Sigma}{sub SFR}) in the flocculent galaxy M 33, using published gas data and a catalog of more than 600 young star clusters in its disk. By comparing the radial distributions of gas and most massive cluster masses, we find that M{sub max}{proportional_to}{Sigma}{sup 4.7{+-}0.4}{sub gas}, M{sub max}{proportional_to}{Sigma}{sup 1.3{+-}0.1}{sub H{sub 2}}, and M{sub max}{proportional_to}{Sigma}{sup 1.0{+-}0.1}{sub SFR}. We rule out that these correlations result from the size of the sample; hence, the change of the maximum cluster mass must be due to physical causes.

Gonzalez-Lopezlira, Rosa A.; Pflamm-Altenburg, Jan; Kroupa, Pavel, E-mail: r.gonzalez@crya.unam.mx [Argelander Institut fuer Astronomie, Universitaet Bonn, Auf dem Huegel 71, D-53121 Bonn (Germany)

2012-12-20

245

Gas Surface Density, Star Formation Rate Surface Density, and the Maximum Mass of Young Star Clusters in a Disk Galaxy. I. The Flocculent Galaxy M 33  

NASA Astrophysics Data System (ADS)

We analyze the relationship between maximum cluster mass M max and surface densities of total gas (?gas), molecular gas (\\Sigma _H_2), and star formation rate (?SFR) in the flocculent galaxy M 33, using published gas data and a catalog of more than 600 young star clusters in its disk. By comparing the radial distributions of gas and most massive cluster masses, we find that M maxvprop?4.7 ± 0.4 gas, M maxvprop?1.3 ± 0.1 H2 , and M maxvprop?1.0 ± 0.1 SFR. We rule out that these correlations result from the size of the sample; hence, the change of the maximum cluster mass must be due to physical causes.

González-Lópezlira, Rosa A.; Pflamm-Altenburg, Jan; Kroupa, Pavel

2012-12-01

246

Extreme Stars  

Microsoft Academic Search

Over the past 200 years, our knowledge of stars has expanded enormously. From seeing myriad dots of different brightnesses, we haved moved on to measure their distances, temperatures, sizes, chemical compositions, and even ages, finding both young and ancient stars that dwarf our Sun and are dwarfed by it. Unique in its approach, Extreme Stars describes the lives of stars

James B. Kaler

2001-01-01

247

Evolution of Dynamo-Generated Magnetic Fields in Accretion Disks around Compact and Young Stars.  

National Technical Information Service (NTIS)

Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized obje...

T. F. Stepinski

1994-01-01

248

Young Stars near Earth: The Octans-Near Association and Castor Moving Group  

NASA Astrophysics Data System (ADS)

All cataloged stellar moving groups and associations with ages <=100 Myr and within 100 pc of Earth have Galactic space motions (UVW) situated in a "good box" with dimensions ~20 km s–1 on a side. Torres et al. defined the Octans Association as a group of 15 stars with age "20 Myr?" and located ~140 pc from Earth, but with average V space velocity –3.6 km s–1 that is well outside of the good box. We present a list of 14 Hipparcos star systems within 100 pc of Earth that we call "Octans-Near"; these systems have UVW similar to those of the much more distant Octans Association. The Octans-Near stars have apparent ages between about 30 and 100 Myr and their relationship to the Octans Association stars is unclear. Six additional star systems have UVW similar to those of Octans-Near stars and likely ages <=200 Myr. These six systems include the late-type binary star EQ Peg—6.2 pc from Earth with likely age <=100 Myr and thus likely to be the nearest known pre-main sequence star system. The UVW of stars in a previously proposed ~200 Myr old Castor moving group are not too dissimilar from the UVW of Octans-Near stars. However, stars in the Castor group—if it exists at all—are mostly substantially older than 200 Myr and thus generally can readily be distinguished from the much younger Octans-Near stars.

Zuckerman, B.; Vican, Laura; Song, Inseok; Schneider, Adam

2013-11-01

249

CCD photometry and proper motions of late-type stars in the young open cluster Stock 2  

NASA Astrophysics Data System (ADS)

We present the results of the first CCD BVRI photometric and proper motion study of late-type stars in the young open cluster Stock 2. Twenty-one fields of approximately 5'x 5' size have been observed photometrically, from which we have identified 118 candidate members based on their positions on colour-magnitude diagrams relative to theoretical isochrones. From a comparison of the known star density of the Pleiades, we estimate the contamination of this selection process due to background stars to be as large as ~ 50%. However, only 22 of those 118 candidate members have proper motions consistent with membership, suggesting that the contamination is of the order of 80%. Additional candidate members were found by means of a proper motion analysis of Schmidt plate material for a 3degx 3deg field containing the cluster. The cluster proper motion allows us to separate members from background and foreground stars. We have found 634 stars with a membership probability >= 50% down to a limiting magnitude of B ~ 20, corresponding to late-M dwarfs at the distance of Stock 2. Table 5 and the Table Appendix are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.

Foster, D. C.; Theissen, A.; Butler, C. J.; Rolleston, W. R. J.; Byrne, P. B.; Hawley, S. L.

2000-05-01

250

MAGNETIC CYCLES IN A CONVECTIVE DYNAMO SIMULATION OF A YOUNG SOLAR-TYPE STAR  

SciTech Connect

Young solar-type stars rotate rapidly and many are magnetically active. Some appear to undergo magnetic cycles similar to the 22 yr solar activity cycle. We conduct simulations of dynamo action in rapidly rotating suns with the three-dimensional magnetohydrodynamic anelastic spherical harmonic (ASH) code to explore dynamo action achieved in the convective envelope of a solar-type star rotating at five times the current solar rotation rate. We find that dynamo action builds substantial organized global-scale magnetic fields in the midst of the convection zone. Striking magnetic wreaths span the convection zone and coexist with the turbulent convection. A surprising feature of this wreath-building dynamo is its rich time dependence. The dynamo exhibits cyclic activity and undergoes quasi-periodic polarity reversals where both the global-scale poloidal and toroidal fields change in sense on a roughly 1500 day timescale. These magnetic activity patterns emerge spontaneously from the turbulent flow and are more organized temporally and spatially than those realized in our previous simulations of the solar dynamo. We assess in detail the competing processes of magnetic field creation and destruction within our simulations that contribute to the global-scale reversals. We find that the mean toroidal fields are built primarily through an {Omega}-effect, while the mean poloidal fields are built by turbulent correlations which are not well represented by a simple {alpha}-effect. During a reversal the magnetic wreaths propagate toward the polar regions, and this appears to arise from a poleward propagating dynamo wave. As the magnetic fields wax and wane in strength and flip in polarity, the primary response in the convective flows involves the axisymmetric differential rotation which varies on similar timescales. Bands of relatively fast and slow fluid propagate toward the poles on timescales of roughly 500 days and are associated with the magnetic structures that propagate in the same fashion. In the Sun, similar patterns are observed in the poleward branch of the torsional oscillations, and these may represent poleward propagating magnetic fields deep below the solar surface.

Brown, Benjamin P. [Department of Astronomy, University of Wisconsin, Madison, WI 53706-1582 (United States); Miesch, Mark S. [High Altitude Observatory, NCAR, Boulder, CO 80307-3000 (United States); Browning, Matthew K. [Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S3H8 (Canada); Brun, Allan Sacha [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette Cedex (France); Toomre, Juri, E-mail: bpbrown@astro.wisc.edu [JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440 (United States)

2011-04-10

251

Variability of young stars: Determination of rotational periods of weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region  

NASA Astrophysics Data System (ADS)

We report on observation and determination of rotational periods of ten weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region. Observations were carried out with the Cassegrain-Teleskop-Kamera (CTK) at University Observatory Jena between 2007 June and 2008 May. The periods obtained range between 0.49 d and 5.7 d, typical for weak-line and post T Tauri stars. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University.

Koeltzsch, A.; Mugrauer, M.; Raetz, St.; Schmidt, T. O. B.; Roell, T.; Eisenbeiss, T.; Hohle, M. M.; Va?ko, M.; Ginski, Ch.; Marka, C.; Moualla, M.; Schreyer, K.; Broeg, Ch.; Neuhäuser, R.

2009-05-01

252

Stars  

NSDL National Science Digital Library

This section of the Windows to the Universe web site provides information and images about stars including star statistics, and a star gallery. Windows to the Universe is a user-friendly learning system pertaining to the Earth and Space sciences. The objective of this project is to develop an innovative and engaging web site that spans the Earth and Space sciences and includes a rich array of documents, including images, movies, animations, and data sets that explore the Earth and Space sciences and the historical and cultural ties between science, exploration and the human experience. Links at the top of each page allow users to navigate between beginner, intermediate and advanced levels.

Johnson, Roberta

2000-07-01

253

Spectrophotometric Evolution of Eta Carinae's Great Eruption  

NASA Astrophysics Data System (ADS)

Eta Carinae is one of the most massive binaries in the Milky Way, and its expanding circumstellar nebula has been studied in detail. It was seen as the second brightest star in the sky during its 1800s "Great Eruption", but only visual estimates of its brightness were recorded. We discovered light echoes of the Great Eruption, which allowed us to obtain a spectrum of this event now, 150 years after it was first observed. We will present our new follow-up observations with which we have started to retrace its spectrophotometric evolution during and before the eruption.

Rest, Armin; Prieto, Jose Luis; Bianco, Federica; Matheson, Thomas; Smith, Nathan; Smith, Chris; Chornock, Ryan; Sinnott, Brendan; Welch, Douglas; Walborn, Nolan

2013-06-01

254

ON THE DYNAMICAL FORMATION OF VERY YOUNG, X-RAY EMITTING BLACK HOLE BINARIES IN DENSE STAR CLUSTERS  

SciTech Connect

We recently discovered a population of very young ({tau} {approx}< 6-8 Myr), X-ray emitting black hole binaries (BHBs) in the nearby starburst galaxy NGC 4449. These BHBs are located within or near to very young star clusters, indicating that they form within the clusters, but that some fraction are dynamically ejected. Here we present results from a suite of N-body simulations of N = 16,384 ({approx}6000 M{sub Sun }) star clusters, similar to the masses of BHB hosts in NGC 4449, through the first 10 Myr of their lives. Our goal is to determine whether dynamical interactions are responsible for the observed population of BHBs in NGC 4449. Our simulations span a wide range of initial size and density profiles, both with and without primordial mass segregation, testing both realistic initial conditions and extreme ones. We find that clusters without primordial mass segregation only dynamically produce BHBs within 10 Myr when they are extremely compact and centrally concentrated. Preliminary results that include primordial binaries support this conclusion. The introduction of strong primordial mass segregation, however, greatly increases the rapidity with which the binaries form, although these are still not tight enough that they will emit X-rays. We conclude that X-ray emitting BHBs are unlikely to form dynamically in clusters of this mass under realistic conditions. Instead, they probably originate from binaries that contain two massive stars with small orbital separations, which are present from the cluster's birth.

Garofali, Kristen [Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 (United States); Converse, Joseph M.; Chandar, Rupali; Rangelov, Blagoy, E-mail: garofal4@msu.edu [Department of Physics and Astronomy, University of Toledo, 2801 W Bancroft Street, Toledo, OH 43606 (United States)

2012-08-10

255

THE GEMINI NICI PLANET-FINDING CAMPAIGN: DISCOVERY OF A MULTIPLE SYSTEM ORBITING THE YOUNG A STAR HD 1160  

SciTech Connect

We report the discovery of two low-mass companions to the young A0V star HD 1160 at projected separations of 81 {+-} 5 AU (HD 1160 B) and 533 {+-} 25 AU (HD 1160 C) by the Gemini NICI Planet-Finding Campaign. Very Large Telescope images of the system taken over a decade for the purpose of using HD 1160 A as a photometric calibrator confirm that both companions are physically associated. By comparing the system to members of young moving groups and open clusters with well-established ages, we estimate an age of 50{sup +50}{sub -40} Myr for HD 1160 ABC. While the UVW motion of the system does not match any known moving group, the small magnitude of the space velocity is consistent with youth. Near-IR spectroscopy shows HD 1160 C to be an M3.5 {+-} 0.5 star with an estimated mass of 0.22{sup +0.03}{sub -0.04} M{sub Sun }, while NIR photometry of HD 1160 B suggests a brown dwarf with a mass of 33{sup +12}{sub -9} M{sub Jup}. The very small mass ratio (0.014) between the A and B components of the system is rare for A star binaries, and would represent a planetary-mass companion were HD 1160 A to be slightly less massive than the Sun.

Nielsen, Eric L.; Liu, Michael C.; Wahhaj, Zahed; Bowler, Brendan; Kraus, Adam; Chun, Mark; Ftaclas, Christ [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Biller, Beth A. [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg (Germany); Hayward, Thomas L. [Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena (Chile); Boss, Alan [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 (United States); Shkolnik, Evgenya L. [Lowell Observatory, 1400 West Mars Road, Flagstaff, AZ 86001 (United States); Tecza, Matthias; Clarke, Fraser [Department of Astronomy, University of Oxford, DWB, Keble Road, Oxford OX1 3RH (United Kingdom); Close, Laird M.; Hartung, Markus; Males, Jared R.; Skemer, Andrew J. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Reid, I. Neill [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Alencar, Silvia H. P. [Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, 30270-901, Belo Horizonte, MG (Brazil); Burrows, Adam [Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States); and others

2012-05-01

256

Erupting Volcanoes!  

NSDL National Science Digital Library

This lesson presents volcanoes through the making of volcano models. While students are constructing their physical representations of volcanoes, they will be filled with questions about volcanoes as well as how to build their models. This process will provide students with a tangible reference for learning about volcanoes and give them a chance to problem-solve as they build their models. Students will be able to observe how the eruption changes the original form of their volcano model. In this way, students see first hand how this type of phenomenon creates physical change. While students at this level may struggle to understand larger and more abstract geographical concepts, they will work directly with material that will help them build a foundation for understanding concepts of phenomena that sculpt the Earth.

257

Embedded Young Stellar Object Candidates in the Active Star-Forming Complex W51: Mass Function and Spatial Distribution  

NASA Astrophysics Data System (ADS)

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 1fdg25 × 1fdg00 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M sun (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M sun and 18 M sun. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

Kang, Miju; Bieging, John H.; Povich, Matthew S.; Lee, Youngung

2009-11-01

258

THE PRESENT-DAY STAR FORMATION RATE OF THE MILKY WAY DETERMINED FROM SPITZER-DETECTED YOUNG STELLAR OBJECTS  

SciTech Connect

We present initial results from a population synthesis model aimed at determining the star formation rate (SFR) of the Milky Way. We find that a total SFR of 0.68-1.45 M {sub sun} yr{sup -1} is able to reproduce the observed number of young stellar objects (YSOs) in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the three-dimensional Galactic distributions of YSOs and interstellar dust, and using model spectral energy distributions to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, H{alpha}, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the SFR estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic SFR, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

Robitaille, Thomas P. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138 (United States); Whitney, Barbara A. [Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States)], E-mail: trobitaille@cfa.harvard.edu

2010-02-10

259

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION  

SciTech Connect

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 1.{sup 0}25 x 1.{sup 0}00 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M{sub sun} (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M{sub sun} and 18 M{sub sun}. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

Kang, Miju; Lee, Youngung [International Center for Astrophysics, Korea Astronomy and Space Science Institute, Hwaam 61-1, Yuseong, Daejeon 305-348 (Korea, Republic of); Bieging, John H. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Povich, Matthew S., E-mail: mjkang@kasi.re.k [Department of Astronomy, University of Wisconsin at Madison, 475 N. Charter Street, Madison, WI 53706 (United States)

2009-11-20

260

The Present-Day Star Formation Rate of the Milky Way Determined from Spitzer-Detected Young Stellar Objects  

NASA Astrophysics Data System (ADS)

We present initial results from a population synthesis model aimed at determining the star formation rate (SFR) of the Milky Way. We find that a total SFR of 0.68-1.45 M sun yr-1 is able to reproduce the observed number of young stellar objects (YSOs) in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the three-dimensional Galactic distributions of YSOs and interstellar dust, and using model spectral energy distributions to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, H?, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the SFR estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic SFR, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

Robitaille, Thomas P.; Whitney, Barbara A.

2010-02-01

261

X-ray Variability of Young Stars in Taurus-Auriga-Perseus  

Microsoft Academic Search

Late-type stars are characterized by strong and variable emission from all parts of the stellar atmosphere. Their activity is explained in terms of a magnetic field which is supported by a dynamo mechanism involving rotation and convective motions. The magnetic structure of the coronae of active stars is dominated by loops filled with an hot, X-ray emitting thermal plasma. The

B. Stelzer

2001-01-01

262

A TREASURY STUDY OF STAR-FORMING REGIONS IN THE LOCAL GROUP. I. HST PHOTOMETRY OF YOUNG POPULATIONS IN SIX DWARF GALAXIES  

SciTech Connect

We present a comprehensive study of young stellar populations in six dwarf galaxies in or near the Local Group: Phoenix, Pegasus, Sextans A, Sextans B, WLM, and NGC 6822. Their star-forming regions, selected from GALEX wide-field far-UV imaging, were imaged (at sub-pc resolution) with the WFPC2 camera on board the Hubble Space Telescope (HST) in six bandpasses from far-UV to I to detect and characterize their hot massive star content. This study is part of HST treasury survey program HST-GO-11079; the general data characteristics and reduction procedures are detailed in this paper and results are presented for the first six galaxies. From a total of 180 HST images, we provide catalogs of the multi-band stellar photometry and derive the physical parameters of massive stars by analyzing it with model-atmosphere colors. We use the results to infer ages, number of massive stars, extinction, and spatial characteristics of the young stellar populations. The hot massive star content varies largely across our galaxy sample, from an inconspicuous presence in Phoenix and Pegasus to the highest relative abundance of young massive stars in Sextans A and WLM. Albeit to a largely varying extent, most galaxies show a very young population (a few Myrs, except for Phoenix), and older ones (a few 10{sup 7} years in Sextans A, Sextans B, NGC 6822, and WLM, {approx}10{sup 8}yr in Phoenix and Pegasus), suggesting discrete bursts of recent star formation in the mapped regions. The hot massive star content (indicative of the young populations) broadly correlates with the total galaxy stellar mass represented by the integrated optical magnitude, although it varies by a factor of {approx}3 between Sextans A, WLM, and Sextans B, which have similar M{sub V}. Extinction properties are also derived.

Bianchi, Luciana; Efremova, Boryana [Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States); Hodge, Paul [Astronomy Department, University of Washington, Seattle, WA (United States); Massey, Philip [Lowell Observatory, Flagstaff, AZ (United States); Olsen, K. A. G., E-mail: bianchi@pha.jhu.edu [National Optical Astronomy Observatory, Tucson, AZ 85719 (United States)

2012-03-15

263

Population Synthesis of Young Isolated Neutron Stars: The Effect of Fallback Disk Accretion and Magnetic Field Evolution  

NASA Astrophysics Data System (ADS)

The spin evolution of isolated neutron stars (NSs) is dominated by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out a Monte Carlo simulation of the evolution of young NSs, and show that most of the pulsars have braking indices ranging from –1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range.

Fu, Lei; Li, Xiang-Dong

2013-10-01

264

An icy Kuiper belt around the young solar-type star HD 181327  

NASA Astrophysics Data System (ADS)

Context. HD 181327 is a young main sequence F5/F6 V star belonging to the ? Pictoris moving group (age ~12 Myr). It harbors an optically thin belt of circumstellar material at radius ~90 AU, presumed to result from collisions in a population of unseen planetesimals. Aims: We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. Methods: We obtained far-infrared photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory, complemented by new 3.2 mm observations carried with the ATCA array. The geometry of the belt is constrained with newly reduced HST/NICMOS scattered light images that allow the degeneracy between the disk geometry and the dust properties to be broken. We then use the radiative transfer code GRaTeR to compute a large grid of models, and we identify the grain models that best reproduce the spectral energy distribution (SED) through a Bayesian analysis. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo. Results: The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the [OI] and [CII] line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of ~0.05 M? (in grains up to 1 mm). We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than ~17 M?. Conclusions: Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.

Lebreton, J.; Augereau, J.-C.; Thi, W.-F.; Roberge, A.; Donaldson, J.; Schneider, G.; Maddison, S. T.; Ménard, F.; Riviere-Marichalar, P.; Mathews, G. S.; Kamp, I.; Pinte, C.; Dent, W. R. F.; Barrado, D.; Duchêne, G.; Gonzalez, J.-F.; Grady, C. A.; Meeus, G.; Pantin, E.; Williams, J. P.; Woitke, P.

2012-03-01

265

Deep Imaging Surveys of Star-forming Clouds. IV. The Meek and the Mighty: Outflows from Young Stars in Chamaeleon I  

NASA Astrophysics Data System (ADS)

We present a survey of shocks and outflows in the Chamaeleon I star-forming complex using H?, [S II], and SDSS i' images obtained from the ground, an i' image obtained with the Hubble Space Telescope, and 4.5 ?m images obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We find new Herbig-Haro (HH) objects and extensions to the previously cataloged shocks that trace parts of at least 20 distinct outflows from young stars. Some HH objects mark the presence of giant outflows, the largest of which is powered by Cha-MMS1 and associated with HH 49/50 near the Ced 110 region. Other large flows are powered by Cha-MMS2 in the Ced 112 region and the IRN in the Ced 111 region. Although some shocks exhibit infrared emission in the IRAC bands, most notably HH 49/50 (the ``tornado''), most outflows in the Cha I clouds are not detected in the Spitzer IRAC bands. This result is consistent with the general lack of extensive 2.12 ?m H2 emission from Cha I.

Bally, John; Walawender, Josh; Luhman, Kevin L.; Fazio, Giovanni

2006-11-01

266

Regular frequency patterns in the young ? Scuti star HD 261711 observed by the CoRoT and MOST satellites  

NASA Astrophysics Data System (ADS)

Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims: We concentrate on an asteroseismological study of HD 261711, a rather hot ? Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD 261711 was discovered to be a PMS ? Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods: High-precision, time-series photometry of HD 261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results: With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the ? Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new ? Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (Teff) was derived to be 8600 ± 200 K, log g is 4.1 ± 0.2, and the projected rotational velocity (?sini) is 53 ± 1 km s-1. Using our Teff value and the radius of 1.8 ± 0.5 R? derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L? of 1.20 ± 0.14 which agrees well to the seismologically determined values of 1.65 R? and, hence, a log L/L? of 1.13. The radial velocity of 14 ± 2 km s-1 we derived for HD 261711, confirms the star's membership to NGC 2264. Conclusions: Our asteroseismic models suggest that HD 261711 is a ? Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M?. With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Microsatellite Systems Canada Inc. (MSCI), formerly part of Dynacon, Inc., the University of Toronto Institute for Aerospace Studies and the University of British Columbia with the assistance of the University of Vienna.Reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A68

Zwintz, K.; Fossati, L.; Guenther, D. B.; Ryabchikova, T.; Baglin, A.; Themessl, N.; Barnes, T. G.; Matthews, J. M.; Auvergne, M.; Bohlender, D.; Chaintreuil, S.; Kuschnig, R.; Moffat, A. F. J.; Rowe, J. F.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.

2013-04-01

267

Star warriors: The young scientists who are inventing the weaponry of space  

SciTech Connect

Broad's account of a week spent at Livermore nuclear lab in California offers a picture of the breakthrough weaponry-their theoretical origins and technology-that gave rise to the President's ''Star Wars'' proposal.

Broad, W.J.

1985-01-01

268

STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS  

SciTech Connect

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 {mu}m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 {mu}m excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from {approx}10 to {approx}20 M{sub sun}. Within the central 400 x 50 pc (|l| < 1.{sup 0}3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M{sub sun} yr{sup -1}. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10{sup 5} yr. This suggestion is also consistent with estimates of SFRs within the last {approx}10{sup 7} yr showing a peak around 10{sup 5} yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.

Yusef-Zadeh, F.; Hewitt, J. W. [Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208 (United States); Arendt, R. G. [Science Systems and Applications, Inc., Baltimore County, GSFC, Code 665, Greenbelt, MD 20771 (United States); Whitney, B. [Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States); Rieke, G.; Hinz, J. L. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Wardle, M. [Department of Physics and Engineering, Macquarie University, Sydney, NSW 2109 (Australia); Stolovy, S.; Ramirez, S. [Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Lang, C. C. [Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245 (United States); Burton, M. G. [School of Physics, University of New South Wales, Sydney, NSW 2052 (Australia)], E-mail: zadeh@northwestern.edu

2009-09-01

269

Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive Young Stellar Objects  

NASA Astrophysics Data System (ADS)

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 ?m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 ?m excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from ~10 to ~20 M sun. Within the central 400 × 50 pc (|l| < 1fdg3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M sun yr-1. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 105 yr. This suggestion is also consistent with estimates of SFRs within the last ~107 yr showing a peak around 105 yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.

Yusef-Zadeh, F.; Hewitt, J. W.; Arendt, R. G.; Whitney, B.; Rieke, G.; Wardle, M.; Hinz, J. L.; Stolovy, S.; Lang, C. C.; Burton, M. G.; Ramirez, S.

2009-09-01

270

Magnetic activity and differential rotation in the young Sun-like stars KIC 7985370 and KIC 7765135  

NASA Astrophysics Data System (ADS)

Aims: We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, to determine their activity level, spot distribution, and differential rotation. Both stars were previously discovered by us to be young stars and were observed by the NASA Kepler mission. Methods: The fundamental stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]) were derived from optical spectroscopy by comparison with both standard-star and synthetic spectra. The spectra of the targets allowed us to study the chromospheric activity based on the emission in the core of hydrogen H? and Ca ii infrared triplet (IRT) lines, which was revealed by the subtraction of inactive templates. The high-precision Kepler photometric data spanning over 229 days were then fitted with a robust spot model. Model selection and parameter estimation were performed in a Bayesian manner, using a Markov chain Monte Carlo method. Results: We find that both stars are Sun-like (of G1.5 V spectral type) and have an age of about 100-200 Myr, based on their lithium content and kinematics. Their youth is confirmed by their high level of chromospheric activity, which is comparable to that displayed by the early G-type stars in the Pleiades cluster. The Balmer decrement and flux ratio of their Ca ii-IRT lines suggest that the formation of the core of these lines occurs mainly in optically thick regions that are analogous to solar plages. The spot model applied to the Kepler photometry requires at least seven persistent spots in the case of KIC 7985370 and nine spots in the case of KIC 7765135 to provide a satisfactory fit to the data. The assumption of the longevity of the star spots, whose area is allowed to evolve with time, is at the heart of our spot-modelling approach. On both stars, the surface differential rotation is Sun-like, with the high-latitude spots rotating slower than the low-latitude ones. We found, for both stars, a rather high value of the equator-to-pole differential rotation (d? ? 0.18 rad d-1), which disagrees with the predictions of some mean-field models of differential rotation for rapidly rotating stars. Our results agree instead with previous works on solar-type stars and other models that predict a higher latitudinal shear, increasing with equatorial angular velocity, that can vary during the magnetic cycle. Based on public Kepler data, on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei - INAF at the Observatorio del Roque del los Muchachos, La Palma (Canary Islands), on observations collected at the 2.2-m telescope of the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Max-Planck-Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC), and on observations collected at the Catania Astrophysical Observatory (Italy).

Fröhlich, H.-E.; Frasca, A.; Catanzaro, G.; Bonanno, A.; Corsaro, E.; Molenda-?akowicz, J.; Klutsch, A.; Montes, D.

2012-07-01

271

The VLTI/MIDI survey of massive young stellar objects . Sounding the inner regions around intermediate- and high-mass young stars using mid-infrared interferometry  

NASA Astrophysics Data System (ADS)

Context. Because of inherent difficulties involved in observations and numerical simulations of the formation of massive stars, an understanding of the early evolutionary phases of these objects remains elusive. In particular, observationally probing circumstellar material at distances ?100 AU from the central star is exceedingly difficult, as such objects are rare (and thus, on average, far away) and typically deeply embedded. Long-baseline mid-infrared interferometry provides one way of obtaining the necessary spatial resolution at appropriate wavelengths for studying this class of objects; however, interpreting such observations is often difficult due to sparse spatial-frequency coverage. Aims: We aim to characterize the distribution and composition of circumstellar material around young massive stars and to investigate exactly which physical structures in these objects are probed by long-baseline mid-infrared interferometric observations. Methods: We used the two-telescope interferometric instrument MIDI of the Very Large Telescope Interferometer of the European Southern Observatory to observe a sample of 24 intermediate- and high-mass young stellar objects in the N band (8-13 ?m). We had successful fringe detections for 20 objects and present spectrally-resolved correlated fluxes and visibility levels for projected baselines of up to 128 m. We fit the visibilities with geometric models to derive the sizes of the emitting regions, as well as the orientation and elongation of the circumstellar material. Fourteen objects in the sample show the 10 ?m silicate feature in absorption in the total and correlated flux spectra. For 13 of these objects, we were able to fit the correlated flux spectra with a simple absorption model, allowing us to constrain the composition and absorptive properties of the circumstellar material. Results: Nearly all of the massive young stellar objects observed show significant deviations from spherical symmetry at mid-infrared wavelengths. In general, the mid-infrared emission can trace both disks and outflows, and in many cases it may be difficult to disentangle these components on the basis of interferometric data alone, because of the sparse spatial frequency coverage normally provided by current long-baseline interferometers. For the majority of the objects in this sample, the absorption occurs on spatial scales larger than those probed by MIDI. Finally, the physical extent of the mid-infrared emission around these sources is correlated with the total luminosity, albeit with significant scatter. Conclusions: Circumstellar material is ubiquitous at distances ?100 AU around young massive stars. Long-baseline mid-infrared interferometry provides the resolving power necessary for observing this material directly. However, in particular for deeply-embedded sources, caution must be used when attempting to attribute mid-infrared emission to specific physical structures, such as a circumstellar disk or an outflow. Appendix A is available in electronic form at http://www.aanda.orgBased on observations with the Very Large Telescope Interferometer of the European Southern Observatory.The reduced interferometric data presented here are 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/558/A24

Boley, Paul A.; Linz, Hendrik; van Boekel, Roy; Henning, Thomas; Feldt, Markus; Kaper, Lex; Leinert, Christoph; Müller, André; Pascucci, Ilaria; Robberto, Massimo; Stecklum, Bringfried; Waters, L. B. F. M.; Zinnecker, Hans

2013-10-01

272

A Multi-Wavelength Study of the Young Star Clusters and Interstellar Medium in the Antennae Galaxies (NGC 4038\\/9)  

Microsoft Academic Search

We report on a multi-wavelength study of the relationship between young star\\u000aclusters in the Antennae galaxies (NGC 4038\\/9) and their interstellar\\u000aenvironment, with the goal of understanding the formation and feedback effects\\u000aof star clusters in merging galaxies. This is possible for the first time\\u000abecause various new observations (from X-rays to radio wavelengths) have become\\u000aavailable in the

Qing Zhang; S. Michael Fall; Bradley C. Whitmore

2001-01-01

273

Triggering eruptive mass ejection in luminous blue variables  

NASA Astrophysics Data System (ADS)

We study the runaway mass loss process of major eruptions of luminous blue variables (LBVs) stars, such as the 1837-1856 Great Eruption of ? Carinae. We follow the evolution of a massive star with a spherical stellar evolution numerical code. After the star exhausted most of the hydrogen in the core and had developed a large envelope, we remove mass at a rate of 1M?year from the outer envelope for 20 years. We find that after removing a small amount of mass at a high rate, the star contracts and releases a huge amount of gravitational energy. We suggest that this energy can sustain the high mass loss rate. The triggering of this runaway mass loss process might be a close stellar companion or internal structural changes. We show that a strong magnetic field region can be built in the radiative zone above the convective core of the evolved massive star. When this magnetic energy is released it might trigger a fast removal of mass, and by that trigger an eruption. Namely, LBV major eruptions might be triggered by magnetic activity cycles. The prediction is that LBV stars that experience major eruptions should be found to have a close companion and/or have signatures of strong magnetic activity during or after the eruption.

Harpaz, Amos; Soker, Noam

2009-08-01

274

An XMM-Newton view of the young open cluster NGC 6231 - II. The OB star population  

NASA Astrophysics Data System (ADS)

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30 per cent can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. In the 0.5-10.0keV band, their X-ray luminosities scale with their bolometric luminosities as logLX - logLbol = -6.912 +/- 0.153. Such a scaling law holds in the soft (0.5-1.0keV) and intermediate (1.0-2.5keV) bands but breaks down in the hard band. While the two colliding wind binaries in our sample clearly deviate from this scheme, the remaining O-type objects show a very limited dispersion (40 or 20 per cent according to whether `cool' dwarfs are included or not), much smaller than that obtained from previous studies. At our detection threshold and with our sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. We thus propose that the intrinsic X-ray emission of non-peculiar O-type stars can be considered as constant for a given star. In addition, the level of X-ray emission is accurately related to the star luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25 per cent in the subtype range B0-B4 and remains mostly uniform throughout the different subpopulations while it drops significantly at later subtypes. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the detected emission being associated with a physical (in a multiple system) PMS companion. However, we still observe a correlation between the bolometric luminosity of the B stars and the measured X-ray luminosity. The best-fitting power law in the 0.5-10.0keV band yields logLX = 0.22(+/-0.06) logLbol + 22.8(+/-2.4). Based on observations collected with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). An extra table and extra figures are available from the CDS: http://cdsweb.u-strasbg.fr. E-mail: hsana@eso.org ‡ FNRS Research Associate (Belgium). § FNRS Postdoctoral Researcher (Belgium).

Sana, H.; Rauw, G.; Nazé, Y.; Gosset, E.; Vreux, J.-M.

2006-10-01

275

Cooling of young neutron stars and the Einstein x-ray observations  

SciTech Connect

Cooling of neutron stars is calculated using an exact stellar evolution code. The full general relativistic version of the stellar structure equations are solved, with the best physical input available. For neutron stars with a stiff equation of state, it is found that the deviation from the isothermality in the interior is significant and that it takes at least a few thousand years to reach the isothermal state. By comparing theoretical and observational results, it is concluded that for Cas A, SN1006, and probably Tycho, standard cooling is inconsistent with the results from the Einstein Observatory, if neutron stars are assumed to be present in these objects. On the other hand, the detection points for RCW103 and the Crab are consistent with these theoretical results.

Nomoto, K.; Tsuruta, S.

1981-06-01

276

Cooling of young neutron stars and the Einstein x-ray observations  

SciTech Connect

Cooling of neutron stars is calculated using an exact stellar evolution code. The fully general relativistic version of the stellar structure equations are solved with the best physical input currently available. For neutron stars with a stiff equation of state, we find that the derivation from isothermally in the interior is significant and that it takes at least a few thousand years to reach the isothermal state. By comparing the most recent theoretical and observational results, we conclude that for Cas A, SN 1006, and probably Tycho, ''standard'' cooling is inconsistent with the results from the Einstein Observatory, if neutron stars are assumed to be present in these objects. On the other hand, the ''detection'' points for RCW 103 and the Crab are consistent with these theoretical results.

Nomoto, K.; Tsuruta, S.

1981-11-01

277

VVV study of the young Milky Way star clusters: Mercer 35, 69 and 70  

NASA Astrophysics Data System (ADS)

We present here our first analysis of the young hidden clusters Mercer 35, 69 and 70 based on JHKs photometry and near-IR spectroscopy, as part of our systematic study of the inner Milky Way cluster content.

Peñaloza, F.; Kurtev, R.; Borissova, J.; Bonatto, C.; Ivanov, V. D.; Artigau, E.; Folkes, S.; Geisler, D.; Minniti, D.; Lucas, P.; Sale, S.

2011-10-01

278

The Young, Massive, Star Cluster Sandage-96 After the Explosion of Supernova 2004dj in NGC 2403  

NASA Astrophysics Data System (ADS)

The bright Type II-plateau supernova (SN) 2004dj occurred within the young, massive stellar cluster Sandage-96 in a spiral arm of NGC 2403. New multiwavelength observations obtained with several ground-based and space-based telescopes were combined to study the radiation from Sandage-96 after SN 2004dj faded away. Sandage-96 started to dominate the flux in the optical bands starting from 2006 September (~800 days after explosion). The optical fluxes are equal to the pre-explosion ones within the observational uncertainties. An optical Keck spectrum obtained ~900 days after explosion shows the dominant blue continuum from the cluster stars shortward of 6000 Å as well as strong SN nebular emission lines redward. The integrated spectral energy distribution (SED) of the cluster has been extended into the ultraviolet region by archival XMM-Newton and new Swift observations, and compared with theoretical models. The outer parts of the cluster have been resolved by the Hubble Space Telescope, allowing the construction of a color-magnitude diagram (CMD). The fitting of the cluster SED with theoretical isochrones results in cluster ages distributed between 10 and 40 Myr, depending on the assumed metallicity and the theoretical model family. The isochrone fitting of the CMDs indicates that the resolved part of the cluster consists of stars having a bimodal age distribution: a younger population at ~10-16 Myr and an older one at ~32-100 Myr. The older population has an age distribution similar to that of the other nearby field stars. This may be explained with the hypothesis that the outskirts of Sandage-96 are contaminated by stars captured from the field during cluster formation. The young age of Sandage-96 and the comparison of its pre and postexplosion SEDs suggest 12 lsim M prog lsim 20 M sun as the most probable mass range for the progenitor of SN 2004dj. This is consistent with, but perhaps slightly higher than, most of the other Type II-plateau SN progenitor masses determined so far.

Vinkó, J.; Sárneczky, K.; Balog, Z.; Immler, S.; Sugerman, B. E. K.; Brown, P. J.; Misselt, K.; Szabó, Gy. M.; Csizmadia, Sz.; Kun, M.; Klagyivik, P.; Foley, R. J.; Filippenko, A. V.; Csák, B.; Kiss, L. L.

2009-04-01

279

Volcanic Eruptions and Hazards  

NSDL National Science Digital Library

This interactive resource adapted from the National Park Service explains the difference between explosive or effusive (lava pours from a vent onto the ground) volcanic eruptions, and describes the hazards associated with each type of eruption.

2010-10-15

280

Current star formation in young, compact clusters in the Small Magellanic Cloud  

Microsoft Academic Search

The Small Magellanic Cloud {SMC} offers a deep, resolved stellar population that leverages fundamental parameters {metallicity, dust content} with respect to the Milky Way and to its most studied counterpart, the LMC. Its subsolar metallicity makes it the best analog to the large majority of dwarf irregulars, and gives us the possibility to study star formation and evolution in an

Antonella Nota

2004-01-01

281

Villains, fools or heroes? Sports stars as role models for young people  

Microsoft Academic Search

Sporting texts are designed to prioritize, personalize and sensationalize characters in an attempt to capture audience attention. The sporting hero has traditionally been perceived of as epitomizing social ideals and masculine virtues, and as embodying values that learnt on the playing fields will readily transfer into everyday life. However, growing media intrusion signifies the contemporary sports star as a ‘damaged

Gill Lines

2001-01-01

282

Hot and cold: A study of molecular hydrogen jets and carbon monoxide molecular outflows from young stars  

NASA Astrophysics Data System (ADS)

This thesis examines outflows from young low-mass accreting protostars. The outflow phenomenon appears to be ubiquitous to all newborn stars, and is often the first observational evidence for embedded young stars. Outflows may be crucial to removing angular momentum from the accretion disk, thereby allowing material to accrete onto the stellar core. In addition, since they can transport supersonic gas over parsec-scale distances, outflows probably contribute to changing the chemistry of the molecular cloud, adding to the cloud's overall turbulence, and may even affect the efficiency of star formation by physically disrupting the infall environments around other protostars. Because young stellar objects (YSOs) are usually still embedded within their parent molecular clouds, they are shrouded by the gas and dust that they are born from, and hence are invisible at optical wavelengths. Outflows are however often energetic enough to punch out of the densest parts of the molecular clouds into less extincted regions. A proper study of all the aspects of protostellar flows [including optical Herbig-Haro (HH) objects, near- infrared (NIR) jets in the ? = 1-0 S(1) H2 line at 2.12 ?m, and molecular maps of outflows in various transitions of 12CO] thus requires observations at a variety of lines that are populated under different excitation conditions, and which are detected at, a wide range of wavelengths. This thesis consists of new observations and interpretations of two regions: the Barnard 5 cloud containing the IRS 1 flow, and the OMC-2 and OMC-3 cloud cores, containing a rich cluster of young stars and criss-crossing H 2 jets. 12CO J = 2-1 mapping, H2 and optical emission line imaging, and high resolution optical and NIR spectroscopy have revealed the locations of the H2 emission with respect to the molecular gas; provided support for bow shock entrainment models for the acceleration of CO bearing gas; showed evidence for H2 heating by a magnetic precursor or HH-object-induced fluorescence; and showed that many H2 knots can be described as bow shocks with forward and reverse shock line profiles. In addition, comparisons of CO, H?, and H2 structures within 20'' of the Barnard 5 IRS 1 source supports an outflow acceleration model where both a jet and a wide angle wind emanate from the YSO or accretion disk. CO filaments found parallel to the IRS 1 flow may trace magnetosonic perturbations excited by major mass loss episodes of IRS 1. Luminosity masses of the outflow lobes are derived using a new technique which corrects for the optical depth of the line at each velocity channel. A comparison of mass spectra of flows from this work as well as from other authors reveals possible differences in how low and high mass YSOs entrain ambient gas into outflows as they evolve. Finally the numbers of flows found in the observed clouds are consistent with the hypothesis that outflows can provide part of the turbulent support within molecular clouds. The efficiency of this pressure support however drops in the case of OMC-2/3 where the main cloud is a narrow ridge.

Yu, Ka Chun

2000-10-01

283

An Aladin-Based Search for Proper-Motion Companions to Young Stars in the Local Association, Tucana-Horologium and Beta Pictoris  

NASA Astrophysics Data System (ADS)

We have used the Aladin sky atlas of the Virtual Observatory to look for new common proper-motion pairs in three young stellar kinematic groups: Local Association (Tau ˜ 10-120 Myr), Tucana-Horologium (Tau ˜ 30 Myr) and Beta Pictoris (Tau ˜ 12 Myr). We have found 9 new and 14 known common proper-motion companions to the 210 investigated stars. With the CAFOS instrument at the 2.2 m Calar Alto telescope, we have investigated in detail one of the new pairs, the HD 143809 AB system, which is formed by a bright G0V primary star and a previously unknown young M1.0-1.5Ve star.

Alonso-Floriano, F. J.; Caballero, J. A.; Montes, D.

284

The binary fractions in the massive young Large Magellanic Cloud star clusters NGC 1805 and NGC 1818  

NASA Astrophysics Data System (ADS)

Using high-resolution data sets obtained with the Hubble Space Telescope, we investigate the radial distributions of the F-type main-sequence binary fractions in the massive young Large Magellanic Cloud star clusters NGC 1805 and NGC 1818. We apply both an isochrone-fitting approach and ?2 minimization using Monte Carlo simulations, for different mass-ratio cut-offs, q, and present a detailed comparison of the methods' performance. Both methods yield the same radial binary fraction profile for the same cluster, which therefore supports the robustness and applicability of either method to young star clusters which are as yet unaffected by the presence of multiple stellar populations. The binary fractions in these two clusters are characterized by opposite trends in their radial profiles. NGC 1805 exhibits a decreasing trend with increasing radius in the central region, followed by a slow increase to the field's binary-fraction level, while NGC 1818 shows a monotonically increasing trend. This may indicate dominance of a more complicated physical mechanism in the cluster's central region than expected a priori. Time-scale arguments imply that early dynamical mass segregation should be very efficient and, hence, likely dominates the dynamical processes in the core of NGC 1805. Meanwhile, in NGC 1818 the behaviour in the core is probably dominated by disruption of soft binary systems. We speculate that this may be owing to the higher velocity dispersion in the NGC 1818 core, which creates an environment in which the efficiency of binary disruption is high compared with that in the NGC 1805 core.

Li, Chengyuan; de Grijs, Richard; Deng, Licai

2013-09-01

285

FIRST KECK NULLING OBSERVATIONS OF A YOUNG STELLAR OBJECT: PROBING THE CIRCUMSTELLAR ENVIRONMENT OF THE HERBIG Ae STAR MWC 325  

SciTech Connect

We present the first N-band nulling plus K- and L-band V{sup 2} observations of a young stellar object, MWC 325, taken with the 85 m baseline Keck Interferometer. The Keck nuller was designed for the study of faint dust signatures associated with debris disks, but it also has a unique capability for studying the temperature and density distribution of denser disks found around young stellar objects. Interferometric observations of MWC 325 at K, L, and N encompass a factor of five in spectral range and thus, especially when spectrally dispersed within each band, enable characterization of the structure of the inner disk regions where planets form. Fitting our observations with geometric models such as a uniform disk or a Gaussian disk show that the apparent size increases monotonically with wavelength in the 2-12 {mu}m wavelength region, confirming the widely held assumption based on radiative transfer models, now with spatially resolved measurements over a broad wavelength range, that disks are extended with a temperature gradient. The effective size is a factor of about 1.4 and 2.2 larger in the L band and N band, respectively, compared to that in the K band. The existing interferometric measurements and the spectral energy distribution can be reproduced by a flat disk or a weakly shadowed nearly flat disk model, with only slight flaring in the outer regions of the disk, consisting of representative 'sub-micron' (0.1 {mu}m) and 'micron' (2 {mu}m) grains of a 50:50 ratio of silicate and graphite. This is in marked contrast to the disks previously found in other Herbig Ae/Be stars, suggesting a wide variety in the disk properties among Herbig Ae/Be stars.

Ragland, S.; Hrynevich, M. [W. M. Keck Observatory, Kamuela, HI 96743 (United States); Ohnaka, K. [Max-Planck-Institut fuer Radioastronomie, 53121 Bonn (Germany); Hillenbrand, L. [Astrophysics Department, California Institute of Technology, Pasadena, CA 91125 (United States); Ridgway, S. T. [National Optical Astronomy Observatories, Tucson, AZ 85726-6732 (United States); Colavita, M. M.; Traub, W. A. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, 91109 (United States); Akeson, R. L.; Millan-Gabet, R. [NExScI, California Institute of Technology, Pasadena, CA 91125 (United States); Cotton, W. [National Radio Astronomy Observatory, Charlottesville, VA 22903-2475 (United States); Danchi, W. C., E-mail: sragland@keck.hawaii.edu [NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics, Greenbelt, MD 20771 (United States)

2012-02-20

286

Gas, Dust, and Young Stars in the Outer Disk of M31  

NASA Astrophysics Data System (ADS)

Using the Canada-France-Hawaii Telescope, we have obtained deep high-resolution CCD images in V and I of a 28'×28' field in the outer disk of M31 at ~116' from the center along the major axis to the southwest and covering a range of projected galactocentric distance from about 23 to 33 kpc. The field was chosen to correspond with extended H I features recorded near the H I edge of the galaxy. The many tens of thousands of objects detected in this large field have been classified using an automatic algorithm that distinguishes unresolved from resolved structures and provides photometry on them. For the most part the unresolved objects are stars in M31. The V-I colors of these stars are highly correlated with the column density of H I in the field. Assuming a Galactic extinction law, this yields a minimum extinction/atomic gas ratio about one-third of that in the solar neighborhood. The interstellar medium (ISM) in this outer disk of M31 therefore contains substantial amounts of dust. We have identified a population of B stars in the field whose distribution is also well correlated with the extended H I distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M31. According to the current view of the star formation process, molecular gas is therefore also expected to be present. The objects classified as ``resolved'' turn out to be a mix of background galaxies and overlapping images of foreground stars in M31. The counts and colors of the slightly resolved objects in these ground-based CCD images therefore cannot be used for a reliable determination of the total extinction and reddening by the ISM in M31. However, the larger background galaxies are easily recognizable, and their surface density above a specific magnitude limit is anticorrelated with the H I column density, confirming that a relatively large amount of extinction is closely associated with the H I gas.

Cuillandre, Jean-Charles; Lequeux, James; Allen, Ronald J.; Mellier, Yannick; Bertin, Emmanuel

2001-06-01

287

Rapid photometric and spectroscopic evolution of the young planetary nebula Hen 3-1357 and its central star SAO 244567  

NASA Astrophysics Data System (ADS)

We present the results of spectroscopic and photometric observations for the young compact planetary nebula Hen 3-1357 and its central star SAO 244567. High-resolution spectroscopy has allowed the expansion velocity of the nebula, V exp = 8.4 ± 1.5 km s-1, and the heliocentric velocity of the object, V r = +12.6 ± 1.7 km s-1, to be determined. The gas shell parameters ( N e , T e ), the extinction in the H ? line, and the O, N, Ne, Ar, S, Cl, He, and C abundances have been determined from low-resolution spectra taken in 1992 and 2011. We have found significant changes in the relative intensities of forbidden lines in the spectrum of Hen 3-1357 within the last 20 years: the low-excitation [O I], [O II], and [N II] lines became stronger relative to H ? by a factor of ˜2, while the [O III] lines weakened by a factor of ˜ 2, suggesting a decrease in the excitation class of the nebula. The V-band photometry performed under the ASAS-3 program revealed a decline in the yearly mean brightness of SAO 244 567 from 2001 to 2009 by 0_.^m 5 and rapid variability with an amplitude of a few tenths of a magnitude. Published observational data in a wide spectral range, from the near ultraviolet to the radio band, suggest an appreciable weakening of the flux from the star and the nebula.

Arkhipova, V. P.; Ikonnikova, N. P.; Kniazev, A. Yu.; Rajoelimanana, Andry

2013-03-01

288

EVIDENCE FOR AN EDGE-ON DISK AROUND THE YOUNG STAR MWC 778 FROM INFRARED IMAGING AND POLARIMETRY  

SciTech Connect

MWC 778 is an unusual and little-studied young stellar object located in the IC 2144 nebula. Recent spectroscopy by Herbig and Vacca suggested the presence of an edge-on circumstellar disk around it. We present near-infrared adaptive optics imaging polarimetry and mid-infrared imaging which directly confirm the suspected nearly edge-on disk around MWC 778 (i {approx} 70{sup 0}-80{sup 0}) plus reveal a more extensive envelope pierced by bipolar outflow cavities. In addition, our mid-infrared images and near-infrared polarization maps detect a spiral-shaped structure surrounding MWC 778, with arms that extend beyond 6'' on either side of the star. Although MWC 778 has previously been classified as a Herbig Ae/Be star, the properties of its central source (including its spectral type) remain fairly uncertain. Herbig and Vacca suggested an F or G spectral type based on the presence of metallic absorption lines in the optical spectrum, which implies that MWC 778 may belong to the fairly rare class of intermediate-mass T Tauri Stars (IMTTSs) which are the evolutionary precursors to Herbig Ae/Be objects. Yet its integrated bolometric luminosity, {approx}>750 L {sub sun} (for an assumed distance of 1 kpc) is surprisingly high for an F or G spectral type, even for an IMTTS. We speculate on several possible explanations for this discrepancy, including its true distance being much closer than 1 kpc, the presence of a binary companion, and/or a nonstellar origin for the observed absorption lines.

Perrin, Marshall D. [Division of Astronomy, University of California, Los Angeles, CA 90095 (United States); Vacca, William D. [SOFIA-USRA, NASA Ames Research Center, Moffett Field, CA 94035 (United States); Graham, James R. [Department of Astronomy, University of California, Berkeley, CA 94720-3411 (United States)], E-mail: mperrin@ucla.edu, E-mail: wvacca@sofia.usra.edu, E-mail: jrg@berkeley.edu

2009-05-15

289

Measuring Spectra of B-type Stars to Study Young Open Clusters  

NASA Astrophysics Data System (ADS)

We used spectra of the open clusters NGC 1960, NGC 1502, NGC 7160, NGC 2384, NGC 2244, NGC 2422, and IC 1590 to analyze their B-type stellar populations. The spectral lines He I ??4388, 4713, 4471, and H? at 4340 Å were analyzed to measure the stars’ projected rotational velocity, temperature, and surface gravity. Then by comparing our measurements to the evolutionary tracks of Schaller et al., we determined the masses of the stars. We determined the ages of the clusters by comparing the luminosity and temperature measurements to isochrones by Lejeune & Schaerer. Finally, we calculate the interstellar reddening and distance to each cluster. This work is supported by the NSF REU site grant PHY-0849416, NASA DPR No. NNX09AP86G, and Lehigh University.

Chiw, Charisee; McSwain, M. V.

2011-01-01

290

The Luminosity Function of Young Star Clusters in ``the Antennae'' Galaxies (NGC 4038-4039)  

Microsoft Academic Search

The Wide Field Planetary Camera 2 of the Hubble Space Telescope has been used to obtain high-resolution images of NGC 4038\\/4039 that go roughly 3 mag deeper in V than previous observations made during cycle 2. These new images allow us to measure the luminosity functions (LFs) of clusters and stars over a range of 8 mag (-14

Bradley C. Whitmore; Qing Zhang; Claus Leitherer; S. Michael Fall; François Schweizer; Bryan W. Miller

1999-01-01

291

New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope  

Microsoft Academic Search

We present 24 mum and 70 mum Multiband Imaging Photometer for Spitzer (MIPS) observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program, including a re-analysis of some previously published source photometry. Our sample is selected from stars with common youth indicators such as lithium abundance,

Peter Plavchan; M. W. Werner; C. H. Chen; K. R. Stapelfeldt; K. Y. L. Su; J. R. Stauffer; I. Song

2009-01-01

292

The massive star binary fraction in young open clusters - II. NGC6611 (Eagle Nebula)  

NASA Astrophysics Data System (ADS)

Based on a set of over 100 medium- to high-resolution optical spectra collected from 2003 to 2009, we investigate the properties of the O-type star population in NGC6611 in the core of the Eagle Nebula (M16). Using a much more extended data set than previously available, we revise the spectral classification and multiplicity status of the nine O-type stars in our sample. We confirm two suspected binaries and derive the first SB2 orbital solutions for two systems. We further report that two other objects are displaying a composite spectrum, suggesting possible long-period binaries. Our analysis is supported by a set of Monte Carlo simulations, allowing us to estimate the detection biases of our campaign and showing that the latter do not affect our conclusions. The absolute minimal binary fraction in our sample is fmin = 0.44 but could be as high as 0.67 if all the binary candidates are confirmed. As in NGC6231 (see Paper I), up to 75 per cent of the O star population in NGC6611 are found in an O+OB system, thus implicitly excluding random pairing from a classical IMF as a process to describe the companion association in massive binaries. No statistical difference could be further identified in the binary fraction, mass-ratio and period distributions between NGC6231 and NGC 6611, despite the difference in age and environment of the two clusters.

Sana, H.; Gosset, E.; Evans, C. J.

2009-12-01

293

Production of gamma ray bursts from asymmetric core combustion of magnetized young neutron stars  

NASA Astrophysics Data System (ADS)

Many works in the past have explored the idea that the conversion of hadronic matter into strange quark matter in neutron stars may be an energy source for GRBs (see references in Lugones et al. 2002, Lugones and Horvath 2003). These models addressed essentially spherically symmetric conversions of the whole neutron star rendering isotropic gamma emission. Accumulating observational evidence suggests that at least ''long'' GRBs are strongly asymmetric, jet-like outflows. The ''short'' burst subclass is not obviously asymmetric, and they may actually be spherically symmetric if the sources are close enough. A new potentially important feature recently recognized (Lugones et al. 2002) is that if a conversion to strange quark matter actually begins near the center of a neutron star, the presence of a magnetic field with intensity B ˜ 1013 G (see also Ghezi, de Gouveia Dal Pino & Horvath 2004) will originate a prompt collimated gamma emission, which may be observed as a short, beamed GRB after the recovery of a fraction of the neutrino energy via ? {bar?} ? e+e- ? ??. The calculations show that the neutrino luminosity is ˜ 1053 erg/sec and that the e+e- luminosity is about two orders of magnitude smaller ( tet{Lugones2002grb}). We find that 90 % of the e+e- pairs are injected inside small cylinders located just above the polar caps (with radius ? and height 0.4 R) in a timescale of ?i ? 0.2 s almost independently of the initial temperature. This provides an interesting suitable explanation for the inner engine of short gamma ray bursts.

de Gouveia dal Pino, E. M.; Lugones, G.; Horvath, J. E.; Ghezzi, C. R.

2005-09-01

294

Dynamics of Molecular Clouds Under the Influence of Ionizing Radiation from Young Massive Stars  

NASA Astrophysics Data System (ADS)

We present numerical simulations treating the ra-di-a-tive dri-ven implosion of density enhancements in molecular cloud cores under the influence of ionizing radiation from massive stars. We performed SPH calculations with initial density perturbations described by Gaussian random fields. We found that large scale perturbations with index -3 in their power spectrum cannot prevent gravitational collapse in the compressed cloud cores, whereas small scale perturbations (power index -1) can at least delay the onset of gravitational instability for a significant fraction of the compression time.

Kessel-Deynet, O.; Burkert, A.

2002-02-01

295

ON THE ORIGIN OF THE RED EXCESS IN VERY YOUNG SUPER STAR CLUSTERS: THE CASE OF SBS 0335-052E  

SciTech Connect

The spectral energy distribution analysis of very young unresolved star clusters challenges our understanding of the cluster formation process. Studies of resolved massive clusters in the Milky Way and in the nearby Magellanic Clouds show us that the contribution from photoionized gas is very important during the first Myr of cluster evolution. We present our models which include both a self-consistent treatment of the photoionized gas and the stellar continuum and quantify the impact of such a nebular component on the total flux of young unresolved star clusters. A comparison with other available models is considered. The very young star clusters in the SBS 0335-052E dwarf starburst galaxy are used as a test for our models. Due to the low metallicity of the galactic medium our models predict a longer lasted nebular phase which contributes between 10% and 40% of the total near-infrared (NIR) fluxes at around 10 Myr. We thus propose a possible solution for the observed flux excess in the six bright super star clusters (SSCs) of SBS 0335-052E. Reines et al. showed that the observed cluster fluxes, in the red-optical and NIR range, sit irreconcilably above the stellar continuum models provided. We find that in the age range estimated from the H{alpha} emission we can explain the red excess in all six SSCs as due to nebular emission, which at cluster ages around 10 Myr still affects the NIR wavebands substantially.

Adamo, A.; Zackrisson, E.; Oestlin, G. [Department of Astronomy, Stockholm University, Oscar Klein Center, AlbaNova, Stockholm SE-106 91 (Sweden); Hayes, M., E-mail: adamo@astro.su.s [Observatoire Astronomique de l'Universite de Geneve, 51, ch des Maillettes, CH-1290 Sauverny (Switzerland)

2010-12-20

296

On the Origin of the Red Excess in Very Young Super Star Clusters: The Case of SBS 0335-052E  

NASA Astrophysics Data System (ADS)

The spectral energy distribution analysis of very young unresolved star clusters challenges our understanding of the cluster formation process. Studies of resolved massive clusters in the Milky Way and in the nearby Magellanic Clouds show us that the contribution from photoionized gas is very important during the first Myr of cluster evolution. We present our models which include both a self-consistent treatment of the photoionized gas and the stellar continuum and quantify the impact of such a nebular component on the total flux of young unresolved star clusters. A comparison with other available models is considered. The very young star clusters in the SBS 0335-052E dwarf starburst galaxy are used as a test for our models. Due to the low metallicity of the galactic medium our models predict a longer lasted nebular phase which contributes between 10% and 40% of the total near-infrared (NIR) fluxes at around 10 Myr. We thus propose a possible solution for the observed flux excess in the six bright super star clusters (SSCs) of SBS 0335-052E. Reines et al. showed that the observed cluster fluxes, in the red-optical and NIR range, sit irreconcilably above the stellar continuum models provided. We find that in the age range estimated from the H? emission we can explain the red excess in all six SSCs as due to nebular emission, which at cluster ages around 10 Myr still affects the NIR wavebands substantially.

Adamo, A.; Zackrisson, E.; Östlin, G.; Hayes, M.

2010-12-01

297

On the Heating Efficiency Derived from Observations of Young Super Star Clusters in M82  

NASA Astrophysics Data System (ADS)

Here, we discuss the mechanical feedback that massive stellar clusters provide to the interstellar medium of their host galaxy. We apply an analytic theory developed in a previous study for M82-A1 to a sample of 10 clusters located in the central zone of the starburst galaxy M82, all surrounded by compact and dense H II regions. We claim that the only way that such H II regions can survive around the selected clusters, is if they are embedded into a high-pressure ISM and if the majority of their mechanical energy is lost within the star cluster volume via strong radiative cooling. The latter implies that these clusters have a low heating efficiency, ?, and evolve in the bimodal hydrodynamic regime. In this regime, the shock-heated plasma in the central zones of a cluster becomes thermally unstable, loses its pressure and is accumulated there, whereas the matter injected by supernovae and stellar winds outside this volume forms a high-velocity outflow—the star cluster wind. We calculated the heating efficiency for each of the selected clusters and found that in all cases it does not exceed 10%. Such low heating efficiency values imply a low mechanical energy output and the impact that the selected clusters provide to the ISM of M82 is thus much smaller than what one would expect using stellar cluster synthetic models.

Silich, Sergiy; Tenorio-Tagle, Guillermo; Torres-Campos, Ana; Muñoz-Tuñón, Casiana; Monreal-Ibero, Ana; Melo, Veronica

2009-08-01

298

Magnetic Cycles in a Wreath-Building Dynamo Simulation of a Young Solar-type Star  

NASA Astrophysics Data System (ADS)

Stars like the Sun build global-scale magnetic fields though dynamo processes in their convection zones. There, global-scale plasma motions couple with rotation and likely drive cycles of magnetic activity, though the exact processes at work in solar and stellar dynamos remain elusive. Observations of younger suns indicate that they rotate quite rapidly, have strong magnetic fields at their surfaces, and show signs of cyclic activity. Here we explore recent 3-D MHD simulations of younger, more rapidly rotating solar-type stars conducted with the anelastic spherical harmonic (ASH) code. These simulations of global-scale convection and dynamo action produce strikingly organized magnetic structures in the bulk of their convection zones. Wreaths of magnetic field fill the convection zone and can undergo regular cycles of polarity reversal. Indeed, we find that cyclic behavior is a common feature throughout the parameter space we have explored. Though these magnetic wreaths can coexist with tachoclines of penetration and shear, they do not rely on that internal boundary layer for their formation or persistence. Tachoclines may play a less critical role in the stellar dynamos of younger Suns than has been supposed in solar dynamo theory.

Brown, Benjamin; Miesch, M. S.; Browning, M. K.; Brun, A. S.; Nelson, N. J.; Toomre, J.

2011-01-01

299

Chandra HETGS Multiphase Spectroscopy of the Young Magnetic O Star ?1 Orionis C  

NASA Astrophysics Data System (ADS)

We report on four Chandra grating observations of the oblique magnetic rotator ?1 Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star's 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray-emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hot plasma. We investigate these diagnostics as a function of viewing angle and analyze them in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on ?1 Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray-emitting plasma with rotation phase.

Gagné, Marc; Oksala, Mary E.; Cohen, David H.; Tonnesen, Stephanie K.; ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard H. D.; MacFarlane, Joseph J.

2005-08-01

300

A Deep Chandra X-ray Observation of Embedded Star Formation in the Young Cluster NGC 2024  

NASA Astrophysics Data System (ADS)

We present results of a 75 ksec Chandra observation of the young (0.3 Myr) stellar cluster NGC 2024 in the Orion region. Previous infrared (IR) studies have detected a large population of embedded young stars in this cluster, most of which are still surrounded by circumstellar disks. Very few of the IR members are optically visible, so X-ray observations provide a crucial check on the completeness of IR surveys. Chandra detected 283 X-ray sources in the 17 x 17 arc-minute ACIS-I field-of-view, of which 204 were identified with previously known counterparts. Several of the X-ray sources without known counterparts exhibited X-ray flares, suggesting that they are heavily obscured cluster members that have escaped IR detection. The Chandra sources in NGC 2024 are characterized by variable emission and hard, heavily-absorbed X-ray spectra. More than one-half of the detections showed some form of variability, including rapid impulsive flares indicative of magnetic activity as well as more mysterious slow variability. Spectral analysis of more than 100 of the brightest detections yields an average absorption column density NH = 2.3E22 cm-2 (Av 10.5 mag), but a few sources are much more heavily absorbed (Av > 45 mag). Typical X-ray temperatures are kT 3 keV, but flare temperatures in several objects exceeded 5 keV. We present Chandra images and representative X-ray spectra and light curves, and compare properties of the X-ray and IR populations of NGC 2024. This work was supported by SAO grant GO1-2009A.

Skinner, S.; Belzer, E.; Gagne, M.

2002-12-01

301

Multifrequency observations of AB Doradus. X-ray flaring and rotational modulation of a young star  

NASA Astrophysics Data System (ADS)

X-ray observations of AB Doradus, performed by the Large Area Counter (LAC) instrument of the GINGA satellite on January 1990, are reported. The observations covered 5 rotations of the star (2.6 days) during which 4 flares were detected. When added to the previously observed EINSTEIN and EXOSAT flares, a total of 7 X-ray flares in AB Dor have been observed so far. The flares seem to cluster around rotational phases 0.1-0.25 and 0.6-0.75 although the statistics are poor. The mean flare energies were around (1-3) x 1034 erg with peak luminosities (4-6) x 1030 ergs/s. The flaring loops were compact (ne = 1012/cu cm) and extended (1-2) x 1010 cm above the surface. Flare masses (1018 g) and frequencies (two per day) are similar to prominence-like cloud formations discovered previously in the star. The flare spectra can be best-fitted either by thermal Bremstrahlung with kT = 3-6 keV or with a power-law, with photon index gamma = 2.2-2.6. During the strongest flare peaks AB Dor is a 10 mCrab source with a Crab-like spectrum. The 3 sigma upper limit for the 6.7 keV iron line during the flares is somewhat smaller than predicted by thin plasma models. We discuss the possibility of lowering the equivalent width by an extra non-thermal continuum due to mildly relativistic electrons. Simultaneous 8.4 GHz observations during flare No. 1 gave only a marginal detection, constraining the magnetic field strength to less than 50 Gauss if the total X-ray continuum is non-thermal in origin. The sensitivity was not good enough to detect any clear modulation in the X-ray light curve, folded over the 0.514 d rotation period. Simultaneous 8.4 GHz observations were performed with the 64 m antenna of the Australia Telescope National Facility at Parkes and reveal a clear variability with two maxima at phases 0.0 (spot A) and 0.5 (spot B). Nearly simultaneous optical photometry can be modeled by a cool extended photospheric spot at the phase 0.0 (spot A). Simultaneous H-alpha photometry revealed a transient H-alpha absorption feature above the spot A, trapped in co-rotation with the stellar magnetic field. All these multifrequency data can be understood by a geometrical two spot model. The geometry seems to have been permanent during the past 10 years, although the relative contributions of the two poles vary and the spots are not necessarily fixed into 180 degrees apart all the time. stars.

Vilhu, O.; Tsuru, T.; Collier Cameron, A.; Budding, E.; Banks, T.; Slee, B.; Ehrenfreund, P.; Foing, B. H.

1993-11-01

302

Gas and Dust Environments of Young Stars in the Taurus Molecular Cloud: A Synthesis from the XMM-Newton and Spitzer Surveys  

Microsoft Academic Search

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) and the Spitzer Space Telescope Taurus Survey both provide unique databases to study the environments of the young stellar population in Taurus. We are using both surveys together to characterize the stellar gas and dust environments of Class I protostars, classical and weak-lined T Tauri stars. The X-ray measurements characterize

Manuel Guedel; D. Padgett; M. Audard; K. Briggs; T. Brooke; S. Carey; N. Evans; M. Fukagawa; A. Glauser; N. Grosso; S. Guieu; L. Hillenbrand; D. Hines; T. Huard; J. Knapp; C. McCabe; J. Monin; A. Noriega-Crespo; F. Palla; L. Rebull; L. Scelsi; S. L. Skinner; K. Stapelfeldt; A. Telleschi; S. Tereby

2007-01-01

303

THE INITIAL MASS FUNCTION AND YOUNG BROWN DWARF CANDIDATES IN NGC 2264. IV. THE INITIAL MASS FUNCTION AND STAR FORMATION HISTORY  

SciTech Connect

We have studied the star formation history and the initial mass function (IMF) using the age and mass derived from spectral energy distribution (SED) fitting and from color-magnitude diagrams (CMDs). We also examined the physical and structural parameters of more than 1000 pre-main-sequence stars in NGC 2264 using the online SED fitting tool (SED fitter) of Robitaille et al. We have compared the physical parameters of central stars from SED fitter and other methods. The temperature of the central star is, in many cases, much higher than that expected from its spectral type. The mass and age from SED fitter are not well matched with those from CMDs. We have made some suggestions to improve the accuracy of temperature estimates in the SED fitter. In most cases, these parameters of individual stars from the SED fitter in a star-forming region (SFR) or in the whole cluster showed nearly no systematic variation with age or with any other relevant parameter. On the other hand, the median properties of stars in NGC 2264 SFRs showed an evident evolutionary effect and were interrelated to each other. Such differences are caused by a larger age spread within an SFR than between them. The cumulative distribution of stellar ages showed a distinct difference among SFRs. A Kolmogorov-Smirnov test gave a very low probability of them being from the same population. The results indicate that star formation in NGC 2264 started at the surface region (Halo and Field regions) about 6-7 Myr ago, propagated into the molecular cloud and finally triggered the recent star formation in the Spokes cluster. The kind of sequential star formation that started in the low-density surface region (Halo and Field regions) implies that star formation in NGC 2264 was triggered by an external source. The IMF of NGC 2264 was determined in two different ways. One method used the stellar mass from the SED fitting tool, the other used the stellar mass from CMDs. The first IMF showed a distinct peak at m {approx} 2 M{sub sun}, but the second did not. We attribute the peak as an artifact of the SED fitting tool because there is no such IMF with a peak at m {approx} 2 M{sub sun}. The slope of the IMF of NGC 2264 for massive stars (log m {>=} 0.5) is -1.7 {+-} 0.1, which is somewhat steeper than the so-called standard Salpeter-Kroupa IMF. We also present data for 79 young brown dwarf candidates.

Sung, Hwankyung [Department of Astronomy and Space Science, Sejong University, 98, Kunja-dong, Kwangjin-gu, Seoul 143-747 (Korea, Republic of); Bessell, Michael S., E-mail: sungh@sejong.ac.k, E-mail: bessell@mso.anu.edu.a [Research School of Astronomy and Astrophysics, Australian National University, MSO, Cotter Road, Weston, ACT 2611 (Australia)

2010-12-15

304

Mid-infrared diagnostics of metal-rich HII regions from VLT and Spitzer spectroscopy of young massive stars in W31  

NASA Astrophysics Data System (ADS)

We present near-infrared Very Large Telescope/Infrared Spectrograph and Array Camera and mid-infrared (mid-IR) Spitzer/Infrared Spectrograph spectroscopy of the young massive cluster in the W31 star-forming region. H-band spectroscopy provides refined classifications for four cluster member O stars with respect to Blum et al. In addition, photospheric features are detected in the massive young stellar object (massive YSO) #26. Spectroscopy permits estimates of stellar temperatures and masses, from which a cluster age of ~0.6 Myr and distance of 3.3kpc are obtained, in excellent agreement with Blum et al. IRS spectroscopy reveals mid-IR fine structure line fluxes of [NeII-III] and [SIII-IV] for four O stars and five massive YSOs. In common with previous studies, stellar temperatures of individual stars are severely underestimated from the observed ratios of fine-structure lines, despite the use of contemporary stellar atmosphere and photoionization models. We construct empirical temperature calibrations based upon the W31 cluster stars of known spectral type, supplemented by two inner Milky Way ultracompact (UC) HII regions whose ionizing star properties are established. Calibrations involving [NeIII] 15.5 ?m/[NeII] 12.8 ?m, [SIV] 10.5 ?m/[NeII] 12.8 ?m or [ArIII] 9.0 ?m/[NeII] 12.8 ?m have application in deducing the spectral types of early to mid O stars for other inner Milky Way compact and UC HII regions. Finally, evolutionary phases and time-scales for the massive stellar content in W31 are discussed, due to the presence of numerous young massive stars at different formation phases in a `coeval' cluster. Based on observations made with ESO telescopes at the Paranal Observatory under programme ID 077.C-0550(A) and the Spitzer Space Telescope which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. E-mail: Paul.Crowther@shef.ac.uk

Furness, J. P.; Crowther, P. A.; Morris, P. W.; Barbosa, C. L.; Blum, R. D.; Conti, P. S.; van Dyk, S. D.

2010-04-01

305

Seasonality of volcanic eruptions  

Microsoft Academic Search

An analysis of volcanic activity during the last three hundred years reveals that volcanic eruptions exhibit seasonality to a statistically significant degree. This remarkable pattern is observed primarily along the Pacific ``Ring of Fire'' and locally at some individual volcanoes. Globally, seasonal fluctuations amount to 18% of the historical average monthly eruption rate. In some regions, seasonal fluctuations amount to

B. G. Mason; D. M. Pyle; W. B. Dade; T. Jupp

2004-01-01

306

Seasonality of volcanic eruptions  

Microsoft Academic Search

An analysis of volcanic activity in the last three hundred years reveals that the frequency of onset of volcanic eruptions varies systematically with the time of year. We analysed the Smithsonian catalogue of more than 3200 subaerial eruptions recorded during the last 300 years. We also investigated continuous records, which are not part of the general catalogue, of individual explosions

B. Mason; D. Pyle; B. Dade; T. Jupp

2003-01-01

307

Seasonality of Volcanic Eruptions  

Microsoft Academic Search

An analysis of volcanic activity in the last three hundred years reveals that the frequency of onset of volcanic eruptions varies systematically with the time of year. We analysed the Smithsonian catalogue of more than 3200 subaerial eruptions recorded during the last 300 years. We also investigated continuous records, which are not part of the general catalogue, of individual explosions

B. Mason; D. M. Pyle; W. B. Dade; T. Jupp

2001-01-01

308

Triple Solar Eruption  

NASA Video Gallery

Solar activity surged on the morning of Dec 12, 2010 when the sun erupted three times in quick succession, hurling a trio of bright coronal mass ejections (CMEs) into space. Coronagraphs onboard the Solar and Heliospheric Observatory recorded the action. In all three cases a magnetic filament erupted.

Holly Zell

2010-12-14

309

Volcanic eruptions and climate  

Microsoft Academic Search

Volcanic eruptions are an important natural cause of climate change on many timescales. A new capability to predict the climatic response to a large tropical eruption for the succeeding 2 years will prove valuable to society. In addition, to detect and attribute anthropogenic influences on climate, including effects of greenhouse gases, aerosols, and ozone-depleting chemicals, it is crucial to quantify

Alan Robock

2000-01-01

310

Ready to Erupt!  

NSDL National Science Digital Library

Students observe an in-classroom visual representation of a volcanic eruption. The water-powered volcano demonstration is made in advance, using sand, hoses and a waterballoon, representing the main components of all volcanoes. During the activity, students observe, measure and sketch the volcano, seeing how its behavior provides engineers with indicators used to predict an eruption.

Integrated Teaching And Learning Program

311

The mysterious eruption of V838 Mon  

Microsoft Academic Search

V838 Mon is marking one of the most mysterious stellar outbursts on record. The spectral energy distribution of the progenitor resembles an under-luminous F main sequence star (at V=15.6 mag), that erupted into a cool supergiant following a complex and multi-maxima lightcurve (peaking at V=6.7 mag). The outburst spectrum show BaII, LiI and lines of several s-elements, with wide P-Cyg

U. Munari; A. Henden; S. Kiyota; D. Laney; F. Marang; T. Zwitter; R. L. M. Corradi; S. Desidera; P. M. Marrese; E. Giro; F. Boschi; M. B. Schwartz

2002-01-01

312

Eruption of Alaska volcano breaks historic pattern  

USGS Publications Warehouse

In the late morning of 12 July 2008, the Alaska Volcano Observatory (AVO) received an unexpected call from the U.S. Coast Guard, reporting an explosive volcanic eruption in the central Aleutians in the vicinity of Okmok volcano, a relatively young (~2000-year-old) caldera. The Coast Guard had received an emergency call requesting assistance from a family living at a cattle ranch on the flanks of the volcano, who reported loud "thunder," lightning, and noontime darkness due to ashfall. AVO staff immediately confirmed the report by observing a strong eruption signal recorded on the Okmok seismic network and the presence of a large dark ash cloud above Okmok in satellite imagery. Within 5 minutes of the call, AVO declared the volcano at aviation code red, signifying that a highly explosive, ash-rich eruption was under way.

Larsen, Jessica; Neal, Christina; Webley, Peter; Freymueller, Jeff; Haney, Matthew; McNutt, Stephen; Schneider, David; Prejean, Stephanie; Schaefer, Janet; Wessels, Rick

2009-01-01

313

New Young Star Candidates in the Taurus-Auriga Region as Selected from WISE  

NASA Astrophysics Data System (ADS)

The Taurus Molecular Cloud subtends a large solid angle on the sky, in excess of 250 square degrees. The search for legitimate Taurus members to date has been limited by sky coverage as well as the challenge of distinguishing members from field interlopers. The Wide-field Infrared Survey Explorer (WISE) has recently observed the entire sky, and we take advantage of the opportunity to search for young stellar object (YSO) candidate Taurus members from a 260 square degree region designed to encompass previously-identified Taurus members. We use near- and mid-infrared colors to select objects with apparent infrared excesses and incorporate other catalogs of ancillary data to present: a list of rediscovered Taurus YSOs with infrared excesses (taken to be due to circumstellar disks), a list of rejected YSO candidates (largely galaxies), and a list of 94 surviving candidate new YSO-like Taurus members. There is likely to be contamination lingering in this candidate list, and follow-up spectra are warranted.

Rebull, Luisa M.; Koenig, X.; Padgett, D.; Terebey, S.; McGehee, P.; Hillenbrand, L.; Knapp, G.; Leisawitz, D.; Liu, W.; Noriega-Crespo, A.; Ressler, M.; Stapelfeldt, K.; Fajardo-Acosta, S.; Mainzer, A.

2012-01-01

314

Stars do not Eat Their Young Migrating Planets: Empirical Constraints on Planet Migration Halting Mechanisms  

NASA Astrophysics Data System (ADS)

The discovery of "hot Jupiters" very close to their parent stars confirmed that Jovian planets migrate inward via several potential mechanisms. We present empirical constraints on planet migration halting mechanisms. We compute model density functions of close-in exoplanets in the orbital semi-major axis-stellar mass plane to represent planet migration that is halted via several mechanisms, including the interior 1:2 resonance with the magnetospheric disk truncation radius, the interior 1:2 resonance with the dust sublimation radius, and several scenarios for tidal halting. The models differ in the predicted power-law dependence of the exoplanet orbital semi-major axis as a function of stellar mass, and thus we also include a power-law model with the exponent as a free parameter. We use a Bayesian analysis to assess the model success in reproducing empirical distributions of confirmed exoplanets and Kepler candidates that orbit interior to 0.1 AU. Our results confirm a correlation of the halting distance with stellar mass. Tidal halting provides the best fit to the empirical distribution of confirmed Jovian exoplanets at a statistically robust level, consistent with the Kozai mechanism and the spin-orbit misalignment of a substantial fraction of hot Jupiters. We can rule out migration halting at the interior 1:2 resonances with the magnetospheric disk truncation radius and the interior 1:2 resonance with the dust disk sublimation radius, a uniform random distribution, and a distribution with no dependence on stellar mass. Note that our results do not rule out Type-II migration, but rather eliminate the role of a circumstellar disk in stopping exoplanet migration. For Kepler candidates, which have a more restricted range in stellar mass compared to confirmed planets, we are unable to discern between the tidal dissipation and magnetospheric disk truncation braking mechanisms at a statistically significant level. The power-law model favors exponents in the range of 0.38-0.9. This is larger than that predicted for tidal halting (0.23-0.33), which suggests that additional physics may be missing in the tidal halting theory.

Plavchan, Peter; Bilinski, Christopher

2013-06-01

315

Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions  

NASA Astrophysics Data System (ADS)

One of the largest explosive eruptions in the past several thousand years occurred at Tianchi volcano, also known as Changbaishan, on the China-North Korea border. This historically active polygenetic central volcano consists of three parts: a lower basaltic shield, an upper trachytic composite cone, and young comendite ash flows. The Millennium Eruption occurred between 938 and 946 ad, and was preceded by two smaller and chemically different rhyolitic pumice deposits. There has been at least one additional, small eruption in the last three centuries. From 2002 to 2005, seismicity, deformation, and the helium and hydrogen gas contents of spring waters all increased markedly, causing regional concern. We attribute this event to magma recharge or volatile exhalation or both at depth, followed by two episodes of addition of magmatic fluids into the overlying aquifer without a phreatic eruption. The estimated present magma accumulation rate is too low by itself to account for the 2002-2005 unrest. The most serious volcanic hazards are ash eruption and flows, and lahars. The available geological information and volcano monitoring data provide a baseline for comprehensive assessment of future episodes of unrest and possible eruptive activity.

Wei, Haiquan; Liu, Guoming; Gill, James

2013-04-01

316

A Hubble Space Telescope Treasury Study of Star-forming Regions in the Local Group. II. Young Stellar Populations in M31  

NASA Astrophysics Data System (ADS)

We studied the young stellar populations of 22 star-forming regions in the Andromeda galaxy (M31), with Hubble Space Telescope (HST) multi-band imaging from far-UV to I. The regions were selected from Galaxy Evolution Explorer (GALEX) wide-field far-UV imaging; they sample different environments and galactocentric distances from 6 to 22 kpc. They were imaged with 30 HST fields (360 distinct images, in six bandpasses), with a pixel scale of 0.38 pc projected on the sky, at the distance of M31. This study is part of HST treasury survey program HST-GO-11079, which includes star-forming regions in eight Local Group galaxies. We provide a merged catalog of six-band stellar photometry in the 30 M31 fields, containing 118,036 sources brighter than V and B ~ 23 mag. Each HST field covers about 0.3 kpc2 in M31, and contains up to ~7000 stars, of which the number varies by a factor of >7 among the target regions; a large fraction of the sample are hot massive stars, due to our choice of filters and exposures. We derived stellar physical parameters and interstellar extinction for individual sources by spectral energy distribution analysis with model-atmosphere colors, and used the results to infer ages, massive stars content, and extinction of the star-forming regions. Reddening is up to E(B - V) <~ 0.6 mag in some OB associations, and lowest in the outermost regions (average of lsim0.12 mag in OB184 at 21.9 kpc). We examined the spatial distribution (clustering) of the hot massive stars, and defined OB associations on various spatial scales from compact to wider, more spread out ones. A hierarchical structuring is observed, with small compact groups arranged within large complexes. Their areas vary from less than 10 to 105 pc2, and masses are up to ?105 M ?, in the scales sampled by our analysis. Their cumulative mass distribution follows a power law, at least in part of the sampled regime. Hot-star counts in the young regions compare very well with integrated measurements of UV flux from GALEX.

Bianchi, Luciana; Efremova, Boryana; Hodge, Paul; Kang, Yongbeom

2012-11-01

317

Volcanic Eruptions and Climate  

NSDL National Science Digital Library

This slide presentation provides an overview of the effects of volcanic eruptions on climate. The presentation includes 155 slides, too many for a single class presentation; teachers may select from these to create their own, shorter presentations. Topics include what volcanic gases and aerosols do to the atmosphere, how they migrate with atmospheric circulation, how soon the effects occur, and how long they last. Some famous historic eruptions are discussed, along with the climate changes that accompanied them. Several slides are devoted to the effects of the 1991 eruption of Mount Pinatubo in the Phillipines.

Robock, Alan

2005-07-30

318

Dynamics of Lake Eruptions and Possible Ocean Eruptions  

Microsoft Academic Search

Dissolved gas in liquid is able to power violent eruptions. Two kinds of such gas-driven eruptions are known in nature: explosive volcanic eruptions driven by dissolved H2O in magma at high temperatures and lake eruptions driven by dissolved CO2 in water at low temperatures. There are two known occurrences of lake eruptions, one in 1984 (Lake Monoun) and one in

Youxue Zhang; George W. Kling

2006-01-01

319

A Giant Herbig-Haro Flow from a Massive Young Star in G192.16-3.82  

NASA Astrophysics Data System (ADS)

We report the discovery of a 10 pc-long Herbig-Haro (HH) flow powered by a moderately massive young star associated with the compact H II region G192.16-3.82. At a distance of 2 kpc, the luminosity of G192.16-3.82 is about 3000 L_solar. The HH 396/397 complex consists of a network of filamentary emission-line objects tracing a pair of collimated and limb-brightened outflow lobes that emerge from an opaque cloud core. The H?- and [S II]-bright nebulosity has a large area covering factor, extending well beyond the mapped extent of the associated high-velocity molecular outflow. The HH 396/397 complex contains some shocks with a surface brightness comparable to HH 168 in Cepheus A, placing these among the brightest known HH objects. Furthermore, a large fraction of a 1' by 18' region surrounding G192.16-3.82 is laced with lower surface brightness emission-line features, making HH 396/397 one of the most spatially extended HH complexes studied so far. The dynamical age of this outflow is in the range 10^4 to 10^5 yr. We discuss plausible evolutionary scenarios for outflows powered by high-mass protostars and argue that the G192.16-3.82 outflow complex is relatively evolved. Despite being very elongated, this HH flow is most likely powered by a moderately collimated wind, rather than a highly collimated jet.

Devine, David; Bally, John; Reipurth, Bo; Shepherd, Debra; Watson, Alan

1999-06-01

320

DETECTION OF A COOL, ACCRETION-SHOCK-GENERATED X-RAY PLASMA IN EX LUPI DURING THE 2008 OPTICAL ERUPTION  

SciTech Connect

EX Lupi is the prototype for a class of young, pre-main-sequence stars which are observed to undergo irregular, presumably accretion-generated, optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra ACIS Target of Opportunity observations shortly thereafter. We find very strong evidence that most of the X-ray emission in the first few months after the optical outburst is generated by accretion of circumstellar material onto the stellar photosphere. Specifically, we find a strong correlation between the decreasing optical and X-ray fluxes following the peak of the outburst in the optical, which suggests that these observed declines in both the optical and X-ray fluxes are the result of declining accretion rate. In addition, in our models of the X-ray spectrum, we find strong evidence for a {approx}0.4 keV plasma component, as expected for accretion shocks on low-mass, pre-main-sequence stars. From 2008 March through October, this cool plasma component appeared to fade as EX Lupi returned to its quiescent level in the optical, consistent with a decrease in the overall emission measure of accretion-shock-generated plasma. The overall small increase of the X-ray flux during the optical outburst of EX Lupi is similar to what was observed in previous X-ray observations of the 2005 optical outburst of the EX Lupi-type star V1118 Ori but contrasts with the large increase of the X-ray flux from the erupting young star V1647 Ori during its 2003 and 2008 optical outbursts.

Teets, William K.; Weintraub, David A. [Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States); Kastner, Joel H.; Richmond, Michael [Rochester Institute of Technology, Rochester, NY 14623-5604 (United States); Grosso, Nicolas [Observatoire Astronomique de Strasbourg, Universite de Strasbourg, CNRS, UMR 7550, 11 rue de l'Universite, F-67000 Strasbourg (France); Hamaguchi, Kenji [Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

2012-11-20

321

How Volcanoes Work: Eruption Types  

NSDL National Science Digital Library

This site describes the variability of eruption types, from quiescent lava emissions to extremely violent and explosive events. Eruption variability is largely related to magma composition and the amount of water present. Types of eruptions discussed include fissure and Hawaiian eruptions, which are characterized by fire fountains and lava lakes, strombolian eruptions as exemplified by Paricutin in Mexico, and eruptions that are often associated with growing lava domes, such as that at Mt. Pelee in 1902, and with the generation of pyroclastic flows from dome collapse, known as vulcanian eruptions. Numerous active links throughout the site lead to more examples and detailed explanations.

Camp, Victor

322

Post200-ka Pyroclastic Eruptions of the Yellowstone Plateau  

Microsoft Academic Search

Pyroclastic deposits intercalated in post-Yellowstone-caldera rhyolitic lava flows form a minor component of the total volume of high-silica rhyolites erupted between 200 and 70 ka. Such events produced significant volumes of ash, fast-moving pyroclastic flows, and volcanic gases during young eruptions on the Plateau. Thus, while these were less common events, it is important to know the details of these

L. A. Morgan; W. C. Shanks

2010-01-01

323

Polymorphic light eruption  

MedlinePLUS

Polymorphic light eruption is a common reaction to sunlight (ultraviolet light) that occurs in light-sensitive individuals. ... skin within 1 - 4 days after exposure to sunlight. The lesions may also appear as scaly skin.

324

Mount Etna Eruption  

NSDL National Science Digital Library

This animation is a zoom into the Ongoing Mount Etna Eruption. The data was aquired from the MODIS instrument at 9:40 UTC on July 24, 2001. The Ash Plume and lava streaming from the volcano are clearly visible.

Perkins, Lori; Salomonson, Vincent

2001-07-25

325

Mid-Infrared Imaging of the Young Binary Star HEN 3-600: Evidence for a Dust Disk around the Primary  

NASA Astrophysics Data System (ADS)

We present high-resolution mid-infrared observations of the nearby late-type young binary system Hen 3-600. The binary, at a distance of ~50 pc, could be a member of the TW Hydrae association, the nearest known group of young stars, with an age of a few million years. Our images make it possible for the first time to determine which star in the pair, separated by 1.4", harbors the mid-infrared excess detected by IRAS. In the near-infrared, where the radiation is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux ratio of 1.6. At 4.8, 10.8, and 18.2 mum, the primary becomes increasingly dominant over the secondary, suggesting that most of the circumstellar dust in the system resides around Hen 3-600A. Comparison of the spectral energy distribution (SED) of Hen 3-600A to the median SED of classical T Tauri stars suggests that its disk may be truncated by the secondary and provides tentative evidence for a central disk hole. The distribution of dust in the Hen 3-600 system may provide important clues to the formation and evolution of protoplanetary disks in close binaries.

Jayawardhana, Ray; Hartmann, Lee; Fazio, Giovanni; Fisher, R. Scott; Telesco, Charles M.; Piña, Robert K.

1999-07-01

326

Double Prominence Eruptions  

NASA Video Gallery

The Sun erupted with two prominence eruptions, one after the other over a four-hour period (Nov. 16, 2012). The action was captured in the 304 Angstrom wavelength of extreme ultraviolet light. It seems possible that the disruption to the Sun’s magnetic field might have triggered the second event since they were in relatively close proximity to each other. The expanding particle clouds heading into space do not appear to be Earth-directed. Credit: NASA/SDO

Holly Zell

2012-11-16

327

Seismic Eruption Teaching Modules  

NSDL National Science Digital Library

This site presents educational modules for teaching about earthquakes, volcano eruptions and related plate tectonic concepts using an interactive computer program for mapping called Seismic/Eruption (also called SeisVolE). The program includes up-to-date earthquake and volcanic eruption catalogs and allows the user to display earthquake and volcanic eruption activity in "speeded up real time" on global, regional or local maps that also show the topography of the area in a shaded relief map image. SeisVolE is an interactive program that includes a number of tools that allow the user to analyze earthquake and volcanic eruption data and produce effective displays to illustrate seismicity and volcano patterns. The program can be used to sort data and provide results for statistical analysis, to generate detailed earthquake and volcano activity maps of specific areas or for specific purposes, to investigate earthquake sequences such as foreshocks and aftershocks, and to produce cross section or 3-D perspective views of earthquake locations. The Seismic/Eruption program can be a powerful and effective tool for teaching about plate tectonics and geologic hazards using earthquake and volcano locations, and for learning (or practicing) fundamental science skills such as statistical analysis, graphing, and map skills. The teaching modules describe and illustrate how to use the Seismic/Eruption program effectively in demonstrations, classroom presentations and interactive presentations, and independent study/research. Because the program has many useful options and can be used to examine earthquake activity and volcanic eruption data, the modules provide instructions and examples of quantitative analysis, graphing of results, creating useful maps and cross section diagrams, and performing in-depth exploration and research. The examples are intended to illustrate the features and capabilities of the program and stimulate interest in using the program for discovery learning in Earth science, especially earthquakes, volcanoes and plate tectonics.

Braile, Lawrence

328

Seasonality of volcanic eruptions  

NASA Astrophysics Data System (ADS)

An analysis of volcanic activity during the last three hundred years reveals that volcanic eruptions exhibit seasonality to a statistically significant degree. This remarkable pattern is observed primarily along the Pacific "Ring of Fire" and locally at some individual volcanoes. Globally, seasonal fluctuations amount to 18% of the historical average monthly eruption rate. In some regions, seasonal fluctuations amount to as much as 50% of the average eruption rate. Seasonality principally reflects the temporal distribution of the smaller, dated eruptions (volcanic explosivity index of 0-2) that dominate the eruption catalog. We suggest that the pattern of seasonality correlates with the annual Earth surface deformation that accompanies the movement of surface water mass during the annual hydrological cycle and illustrate this with respect to global models of surface deformation and regional measurements of annual sea level change. For example, seasonal peaks in the eruption rate of volcanoes in Central America, the Alaskan Peninsula, and Kamchatka coincide with periods of falling regional sea level. In Melanesia, in contrast, peak numbers of volcanic eruptions occur during months of maximal regional sea level and falling regional atmospheric pressure. We suggest that the well-documented slow deformation of Earth's surface that accompanies the annual movements of water mass from oceans to continents acts to impose a fluctuating boundary condition on volcanoes, such that volcanic eruptions tend to be concentrated during periods of local or regional surface change rather than simply being distributed randomly throughout the year. Our findings have important ramifications for volcanic risk assessment and volcanoclimate feedback mechanisms.

Mason, B. G.; Pyle, D. M.; Dade, W. B.; Jupp, T.

2004-04-01

329

GENERALIZED ERUPTIVE SYRINGOMAS  

PubMed Central

Generalized eruptive syringoma is a rare clinical presentation of a benign adnexal tumor that derives from the intraepidermal portion of the eccrine sweat ducts. It presents as successive crops of small flesh-colored papules on the anterior body surfaces. It generally occurs in the peripubertal period. Treatment of this benign condition is cosmetic only. A case of a 28-year-old female with an eight-year history of eruptive syringoma is presented.

Jamalipour, Mahnaz; Heidarpour, Mitra; Rajabi, Parvin

2009-01-01

330

Modeling Transient Eruptive Processes  

NASA Astrophysics Data System (ADS)

Computer models of volcanic eruptions are essential for understanding the physical processes occurring within a volcanic conduit. We are building upon previous modeling efforts in order to simulate two-phase homogeneous flow prior to fragmentation. Our code is designed to track changing pressure, velocity, density, and volatile concentrations within the conduit. Its primary emphasis is the efficient numerical modeling of transient eruptive events. As such, it is possible to model eruption triggers associated with sudden decompression, more gradual diffusive bubble growth, and injection of volatile-rich magma. We alter initial conditions, including volatile distribution, conduit geometry, and magma recharge to quantify how these parameters affect eruption evolution. Our model complements existing numerical approaches for eruption dynamics and incorporates results from experimentalists and observational volcanologists. This enables us to assess the relative impact of various solubility laws, diffusion rates, as well as nucleation and fragmentation criteria. In the process, we address other important questions within the modeling community, such as: When and where does nucleation occur? What kind of boundary conditions should be imposed at the fragmentation front / gas-particle-jet boundary? How do turbulent flow, wall friction, and erosion affect flow? Modeling results are to be compared with observations from contemporaneous eruptions at Colima, Mexico, Santiaguito, Guatemala, and Tungurahua, Ecuador.

Johnson, J. B.; Proussevitch, A.; Sahagian, D.

2004-05-01

331

A Survey of H2O, CO2, and CO Ice Features toward Background Stars and low-mass Young Stellar Objects Using AKARI  

NASA Astrophysics Data System (ADS)

We present near-infrared spectroscopic observations of 19 molecular clouds made using the AKARI satellite, and the data reduction pipeline written to analyze those observations. The 2.5-5 ?m spectra of 30 objects—22 field stars behind quiescent molecular clouds and 8 low-mass young stellar objects in cores—were successfully extracted using the pipeline. Those spectra are further analyzed to calculate the column densities of key solid phase molecular species, including H2O, CO2, CO, and OCN–. The profile of the H2O ice band is seen to vary across the objects observed and we suggest that the extended red wing may be an evolutionary indicator of both dust and ice mantle properties. The observation of 22 spectra with fluxes as low as < 5 mJy toward background stars, including 15 where the column densities of H2O, CO, and CO2 were calculated, provides valuable data that could help to benchmark the initial conditions in star-forming regions prior to the onset of star formation. Based on observations with AKARI, a JAXA project with the participation of ESA.

Noble, J. A.; Fraser, H. J.; Aikawa, Y.; Pontoppidan, K. M.; Sakon, I.

2013-10-01

332

Molecular Shocks Associated with Massive Young Stars: CO Line Images with a New Far-Infrared Spectroscopic Camera on the Kuiper Airborne Observatory  

NASA Astrophysics Data System (ADS)

Under the terms of our contract with NASA Ames Research Center, the University of Rochester (UR) offers the following final technical report on grant NAG 2-958, Molecular shocks associated with massive young stars: CO line images with a new far-infrared spectroscopic camera, given for implementation of the UR Far-Infrared Spectroscopic Camera (FISC) on the Kuiper Airborne Observatory (KAO), and use of this camera for observations of star-formation regions 1. Two KAO flights in FY 1995, the final year of KAO operations, were awarded to this program, conditional upon a technical readiness confirmation which was given in January 1995. The funding period covered in this report is 1 October 1994 - 30 September 1996. The project was supported with $30,000, and no funds remained at the conclusion of the project.

Watson, Dan M.

1997-07-01

333

Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess  

NASA Astrophysics Data System (ADS)

We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16?m by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5Myr). A fraction of low-mass (below 104 Msolar), very young (1-3Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 Msolar, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ~38 per cent of the galactic star formation rate.

Adamo, A.; Östlin, G.; Zackrisson, E.; Hayes, M.; Cumming, R. J.; Micheva, G.

2010-09-01

334

The Gemini/NICI Planet-Finding Campaign: The Frequency of Planets around Young Moving Group Stars  

NASA Astrophysics Data System (ADS)

We report results of a direct imaging survey for giant planets around 80 members of the ? Pic, TW Hya, Tucana-Horologium, AB Dor, and Hercules-Lyra moving groups, observed as part of the Gemini/NICI Planet-Finding Campaign. For this sample, we obtained median contrasts of ?H = 13.9 mag at 1'' in combined CH4 narrowband ADI+SDI mode and median contrasts of ?H = 15.1 mag at 2'' in H-band ADI mode. We found numerous (>70) candidate companions in our survey images. Some of these candidates were rejected as common-proper motion companions using archival data; we reobserved with Near-Infrared Coronagraphic Imager (NICI) all other candidates that lay within 400 AU of the star and were not in dense stellar fields. The vast majority of candidate companions were confirmed as background objects from archival observations and/or dedicated NICI Campaign followup. Four co-moving companions of brown dwarf or stellar mass were discovered in this moving group sample: PZ Tel B (36 ± 6 M Jup, 16.4 ± 1.0 AU), CD–35 2722B (31 ± 8 M Jup, 67 ± 4 AU), HD 12894B (0.46 ± 0.08 M ?, 15.7 ± 1.0 AU), and BD+07 1919C (0.20 ± 0.03 M ?, 12.5 ± 1.4 AU). From a Bayesian analysis of the achieved H band ADI and ASDI contrasts, using power-law models of planet distributions and hot-start evolutionary models, we restrict the frequency of 1-20 M Jup companions at semi-major axes from 10-150 AU to <18% at a 95.4% confidence level using DUSTY models and to <6% at a 95.4% using COND models. Our results strongly constrain the frequency of planets within semi-major axes of 50 AU as well. We restrict the frequency of 1-20 M Jup companions at semi-major axes from 10-50 AU to <21% at a 95.4% confidence level using DUSTY models and to <7% at a 95.4% using COND models. This survey is the deepest search to date for giant planets around young moving group stars. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

Biller, Beth A.; Liu, Michael C.; Wahhaj, Zahed; Nielsen, Eric L.; Hayward, Thomas L.; Males, Jared R.; Skemer, Andrew; Close, Laird M.; Chun, Mark; Ftaclas, Christ; Clarke, Fraser; Thatte, Niranjan; Shkolnik, Evgenya L.; Reid, I. Neill; Hartung, Markus; Boss, Alan; Lin, Douglas; Alencar, Silvia H. P.; de Gouveia Dal Pino, Elisabete; Gregorio-Hetem, Jane; Toomey, Douglas

2013-11-01

335

A panchromatic view of star-forming regions in the Magellanic clouds: Characterizing physical and evolutionary parameters of young stellar objects  

NASA Astrophysics Data System (ADS)

Many factors affect the formation of stars, and none of them is well-understood. Metal abundance, environment, turbulence, and multiplicity all have roles to play in how quickly Young Stellar Objects (YSOs) form and evolve, what masses they achieve, and the rate at which interstellar medium is converted into stars. The first step in understanding star formation must be to construct a representative catalog of YSOs, including a variety of evolutionary stages, masses, and environments. Here, we examine clustered star formation in 10 H II regions in the Large and Small Magellanic Clouds (LMC and SMC). These two galaxies have different sub-Solar metallicities, and we choose star-forming regions with different sizes, morphologies, locations, and surrounding environments. By combining photometric data across a range of wavelengths to characterize diverse stellar and proto-stellar populations, we refine a method to find and characterize YSO candidates, identifying ˜ 1200 new YSOs in the two Clouds. We construct initial lists of YSO candidates based on 10 new color-selection criteria and then leverage data from the SAGE Spitzer Space Telescope Legacy Programs along with ancillary data from ground-based surveys. We fit photometric measurements to Spectral Energy Distributions (SEDs) of model YSOs and select which are well-fit based on their reduced chi 2 values. Where deep Hubble Space Telescope (HST) observations are available, we note that most (˜ 70%) single infrared sources actually correspond to several optical sources in apparent compact proto-clusters. Preliminary imaging from the HERITAGE Herschel Space Telescope photometric survey of the Magellanic Clouds even affords us a qualitative glimpse of the earliest, most embedded stages of star formation, which we compare visually to our YSO candidates. We outline an entirely new technique to determine the lifetimes of YSO evolutionary stages, comparing the spatial locations of stellar populations, pre-main sequence stars, YSOs, and nebulosity to describe age gradients and possible triggering. Through this work, we discover more than 1150 new YSO candidates in 9 LMC H II regions, an increase of nearly 65% in the total known LMC YSO candidates, and lay the groundwork for innovative new studies of star formation timescales. This research has been completed under the supervision of Margaret Meixner (STScI) and Colin Norman (JHU).

Carlson, Lynn Redding

2010-07-01

336

PLANETS AROUND LOW-MASS STARS (PALMS). I. A SUBSTELLAR COMPANION TO THE YOUNG M DWARF 1RXS J235133.3+312720  

SciTech Connect

We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.''4 ({approx}120 AU) pair is confirmed to be comoving from two epochs of high-resolution imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0{sup +2}{sub -1}. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of {approx}10-100 Myr. UVW kinematics based the measured radial velocity of the primary and the system's photometric distance (50 {+-} 10 pc) indicate it is likely a member of the {approx}50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the {approx}200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 {+-} 6 M{sub Jup} for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages.

Bowler, Brendan P.; Liu, Michael C.; Cieza, Lucas A.; Kraus, Adam L. [Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Shkolnik, Evgenya L. [Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001 (United States); Dupuy, Trent J. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Tamura, Motohide, E-mail: bpbowler@ifa.hawaii.edu [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

2012-07-10

337

Evolution of variable stars  

SciTech Connect

Throughout the domain of the H R diagram lie groupings of stars whose luminosity varies with time. These variable stars can be classified based on their observed properties into distinct types such as ..beta.. Cephei stars, delta Cephei stars, and Miras, as well as many other categories. The underlying mechanism for the variability is generally felt to be due to four different causes: geometric effects, rotation, eruptive processes, and pulsation. In this review the focus will be on pulsation variables and how the theory of stellar evolution can be used to explain how the various regions of variability on the H R diagram are populated. To this end a generalized discussion of the evolutionary behavior of a massive star, an intermediate mass star, and a low mass star will be presented. 19 refs., 1 fig., 1 tab.

Becker, S.A.

1986-08-01

338

Storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of Augustine Volcano, Alaska  

Microsoft Academic Search

The 1986 eruption of Augustine Volcano, Alaska, was remarkable for its sustained compositional heterogeneity. Magmas with 56-64 wt % bulk-rock SiO2 were erupted throughout the six-month eruption in both early explosive and later effusive phases, raising questions as to the nature of magma storage and interaction beneath this young and frequently active volcano. Linear trends in major element variation diagrams

D. C. Roman; K. V. Cashman; C. A. Gardner; P. J. Wallace

2006-01-01

339

Seasonality of Volcanic Eruptions  

NASA Astrophysics Data System (ADS)

An analysis of volcanic activity in the last three hundred years reveals that the frequency of onset of volcanic eruptions varies systematically with the time of year. We analysed the Smithsonian catalogue of more than 3200 subaerial eruptions recorded during the last 300 years. We also investigated continuous records, which are not part of the general catalogue, of individual explosions at Sakurajima volcano (Japan, 150 events per year since 1955) and Semeru (Indonesia, 100,000 events during the period 1997-2000). A higher proportion (as much as 18 percent of the average monthly rate) of eruptions occur worldwide between December and March. This observation is statistically significant at above the 99 percent level. This pattern is independent of the time interval considered, and emerges whether individual eruptions are counted with equal weight or with weights proportional to event explosivity. Elevated rates of eruption onset in boreal winter months are observed in northern and southern hemispheres alike, as well as in most volcanically-active regions including, most prominently, the 'Ring of Fire' surrounding the Pacific basin. Key contributors to this regional pattern include volcanoes in Central and South America, the volcanic provinces of the northwest Pacific rim, Indonesia and the southwest Pacific basin. On the smallest spatial scales, some individual volcanoes for which detailed histories exist exhibit peak levels in eruption activity during November-January. Seasonality is attributed to one or more mechanisms associated with the annual hydrological cycle, and may correspond to the smallest time-scale over which fluctuations in stress due to the redistribution of water-masses are felt by the Earth's crust. Our findings have important ramifications for volcanic risk assessment, and offer new insight into possible changes in volcanic activity during periods of long-term changes in global sea level.

Mason, B.; Pyle, D. M.; Dade, W. B.; Jupp, T.

2001-12-01

340

Seasonality of volcanic eruptions  

NASA Astrophysics Data System (ADS)

An analysis of volcanic activity in the last three hundred years reveals that the frequency of onset of volcanic eruptions varies systematically with the time of year. We analysed the Smithsonian catalogue of more than 3200 subaerial eruptions recorded during the last 300 years. We also investigated continuous records, which are not part of the general catalogue, of individual explosions at Sakurajima volcano (Japan, 150 events per year since 1955) and Semeru (Indonesia, 100,000 events during the period 1997-2000). A higher proportion (as much as 18 percent of the average monthly rate) of eruptions occur worldwide between December and March. This observation is statistically significant at above the 99 percent level. This pattern is independent of the time interval considered, and emerges whether individual eruptions are counted with equal weight or with weights proportional to event explosivity. Elevated rates of eruption onset in boreal winter months are observed in northern and southern hemispheres alike, as well as in most volcanically-active regions including, most prominently, the 'Ring of Fire' surrounding the Pacific basin. Key contributors to this regional pattern include volcanoes in Central and South America, the volcanic provinces of the northwest Pacific rim, Indonesia and the southwest Pacific basin. On the smallest spatial scales, some individual volcanoes for which detailed histories exist exhibit peak levels in eruption activity during November-January. Seasonality is attributed to one or more mechanisms associated with the annual hydrological cycle, and may correspond to the smallest time-scale over which fluctuations in stress due to the redistribution of water-masses are felt by the Earth's crust. Our findings have important ramifications for volcanic risk assessment, and offer new insight into possible changes in volcanic activity during periods of long-term changes in global sea level.

Mason, B.; Pyle, D.; Dade, B.; Jupp, T.

2003-04-01

341

The young stellar population of IC 1613. III. New O-type stars unveiled by GTC-OSIRIS  

NASA Astrophysics Data System (ADS)

Context. Very low-metallicity massive stars are key to understanding the reionization epoch. Radiation-driven winds, chief agents in the evolution of massive stars, are consequently an important ingredient in our models of the early-Universe. Recent findings hint that the winds of massive stars with poorer metallicity than the SMC may be stronger than predicted by theory. Besides calling the paradigm of radiation-driven winds into question, this result would affect the calculated ionizing radiation and mechanical feedback of massive stars, as well as the role these objects play at different stages of the Universe. Aims: The field needs a systematic study of the winds of a large sample of very metal-poor massive stars. The sampling of spectral types is particularly poor in the very early types. This paper's goal is to increase the list of known O-type stars in the dwarf irregular galaxy IC 1613, whose metallicity is lower than the SMC's roughly by a factor 2. Methods: Using the reddening-free Q pseudo-colour, evolutionary masses, and GALEX photometry, we built a list of very likely O-type stars. We obtained low-resolution (R ~ 1000) GTC-OSIRIS spectra for a fraction of them and performed spectral classification, the only way to unequivocally confirm candidate OB-stars. Results: We have discovered 8 new O-type stars in IC 1613, increasing the list of 7 known O-type stars in this galaxy by a factor of 2. The best quality spectra were analysed with the model atmosphere code FASTWIND to derive stellar parameters. We present the first spectral type - effective temperature scale for O-stars beyond the SMC. Conclusions: The target selection method is successful. From the pre-selected list of 13 OB star candidates, we have found 8 new O-stars and 4 early-B stars and provided a similar type for a formerly known early-O star. Further tests are needed, but the presented procedure can eventually make preliminary low-resolution spectroscopy to confirm candidates unnecessary. The derived effective temperature calibration for IC 1613 is about 1000 K hotter than the scale at the SMC. The analysis of an increased list of O-type stars will be crucial for studies of the winds and feedback of massive stars at all ages of the Universe. Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. Programme ID GTC59-11B.Figures 4, 6 and Appendix A are available in electronic form at http://www.aanda.orgSpectra as FITS files are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/551/A74

Garcia, M.; Herrero, A.

2013-03-01

342

A NEW METHOD FOR THE ASSESSMENT OF AGE AND AGE SPREAD OF PRE-MAIN-SEQUENCE STARS IN YOUNG STELLAR ASSOCIATIONS OF THE MAGELLANIC CLOUDS  

SciTech Connect

We present a new method for the evaluation of the age and age spread among pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion, and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams (CMDs) using isochrones from two families of PMS evolutionary models. We convert each isochrone into two-dimensional probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age as well as the best age for the entire cluster. We apply our method to the photometric catalog of {approx}2000 PMS stars in the young association LH 95 in the Large Magellanic Cloud, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V {approx} 28, corresponding to M {approx} 0.2 M o-dot. We assume the initial mass function and reddening distribution for the system, as they have been previously derived by us. Our treatment shows that the age determination is very sensitive to the considered grid of evolutionary models and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr to 4.4 Myr, depending on these factors. We evaluate the accuracy of our age estimation and find that the method is fairly accurate in the PMS regime, while the precision of the measurement of the age is lower at higher luminosities. Our analysis allows us to disentangle a real age spread from the apparent CMD broadening caused by the physical and observational biases. We find that LH 95 hosts an age spread that is represented well by a Gaussian distribution with an FWHM of the order of 2.8-4.4 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with the stellar mass. This dependence does not appear to have any physical meaning, being rather due to imperfections of the PMS evolutionary models, which tend to predict lower ages for the intermediate-mass and higher ages for the low-mass stars.

Da Rio, Nicola; Gennaro, Mario [Member of IMPRS for Astronomy, and Cosmic Physics at the University of Heidelberg. (Germany); Gouliermis, Dimitrios A., E-mail: dario@mpia-hd.mpg.d, E-mail: dgoulier@mpia-hd.mpg.d, E-mail: gennaro@mpia-hd.mpg.d [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)

2010-11-01

343

A New Method for the Assessment of Age and Age Spread of Pre-main-sequence Stars in Young Stellar Associations of the Magellanic Clouds  

NASA Astrophysics Data System (ADS)

We present a new method for the evaluation of the age and age spread among pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion, and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams (CMDs) using isochrones from two families of PMS evolutionary models. We convert each isochrone into two-dimensional probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age as well as the best age for the entire cluster. We apply our method to the photometric catalog of ~2000 PMS stars in the young association LH 95 in the Large Magellanic Cloud, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V ~ 28, corresponding to M ~ 0.2 M _{?}. We assume the initial mass function and reddening distribution for the system, as they have been previously derived by us. Our treatment shows that the age determination is very sensitive to the considered grid of evolutionary models and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr to 4.4 Myr, depending on these factors. We evaluate the accuracy of our age estimation and find that the method is fairly accurate in the PMS regime, while the precision of the measurement of the age is lower at higher luminosities. Our analysis allows us to disentangle a real age spread from the apparent CMD broadening caused by the physical and observational biases. We find that LH 95 hosts an age spread that is represented well by a Gaussian distribution with an FWHM of the order of 2.8-4.4 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with the stellar mass. This dependence does not appear to have any physical meaning, being rather due to imperfections of the PMS evolutionary models, which tend to predict lower ages for the intermediate-mass and higher ages for the low-mass stars.

Da Rio, Nicola; Gouliermis, Dimitrios A.; Gennaro, Mario

2010-11-01

344

Evidence for a deficit of young and old stars in the Milky Way inner in-plane disc  

Microsoft Academic Search

We give independent proof of the deficit of stars in the in-plane central disc (2star density along the line of sight; 2) the

M. López-Corredoira; A. Cabrera-Lavers; O. E. Gerhard; F. Garzón

2004-01-01

345

SPECTRAL TYPING OF LATE-TYPE STELLAR COMPANIONS TO YOUNG STARS FROM LOW-DISPERSION NEAR-INFRARED INTEGRAL FIELD UNIT DATA  

SciTech Connect

We used the Project 1640 near-infrared coronagraph and integral field spectrograph to observe 19 young solar-type stars. Five of these stars are known binary stars and we detected the late-type secondaries and were able to measure their JH spectra with a resolution of R {approx} 30. The reduced, extracted, and calibrated spectra were compared to template spectra from the IRTF spectral library. With this comparison, we test the accuracy and consistency of spectral-type determination with the low-resolution near-infrared spectra from P1640. Additionally, we determine effective temperature and surface gravity of the companions by fitting synthetic spectra calculated with the PHOENIX model atmosphere code. We also present several new epochs of astrometry of each of the systems. Together, these data increase our knowledge and understanding of the stellar make up of these systems. In addition to the astronomical results, the analysis presented helps validate the Project 1640 data reduction and spectral extraction processes and the utility of low-resolution, near-infrared spectra for characterizing late-type companions in multiple systems.

Roberts, Lewis C.; Beichman, Charles A.; Burruss, Rick; Ligon, E. Robert; Lockhart, Thomas G.; Roberts, Jennifer E.; Shao, Michael [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States); Rice, Emily L.; Brenner, Douglas; Oppenheimer, Ben R. [American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States); Crepp, Justin R.; Dekany, Richard G.; Hillenbrand, Lynne A.; Hinkley, Sasha [California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125 (United States); King, David; Parry, Ian R. [Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA (United Kingdom); Metchev, Stanimir [Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800 (United States); Pueyo, Laurent; Sivaramakrishnan, Anand; Soummer, Remi, E-mail: lewis.c.roberts@jpl.nasa.gov [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); and others

2012-07-15

346

Simultaneous Monitoring of the Photometric and Polarimetric Activity of the Young Star PV Cep in the Optical/Near-infrared Bands  

NASA Astrophysics Data System (ADS)

We present the results of a simultaneous monitoring, lasting more than two years, of the optical and near-infrared photometric and polarimetric activity of the variable protostar PV Cep. During the monitoring period, an outburst with declining phase (?J ? 3 mag) lasting about 120 days occurred in all the photometric bands. A time lag of ~30 days between optical and infrared light curves has been measured and interpreted in the framework of an accretion event. This latter is directly recognizable in the significant variations of the near-infrared colors, which appear bluer in the outburst phase, when the star dominates the emission, and redder in the declining phase, when the disk emission prevails. All the observational data have been combined to derive a coherent picture of the complex morphology of the whole PV Cep system that is composed of, in addition to the star and the accretion disk, a variable bi-conical nebula. The mutual interaction between all these components is the cause of the high value of the polarization (? 20%) and of its fluctuations. The observational data concur to indicate that PV Cep is not a genuine EXor star, but rather a more complex object; moreover, the case of PV Cep leads to arguments about the classification of other recently discovered young sources in outburst that have been considered, perhaps oversimplifying, as EXor. Based on observations collected at AZT-24 telescope (Campo Imperatore, Italy), AZT-8 (Crimea, Ukraine), and LX-200 (St. Petersburg, Russia).

Lorenzetti, D.; Giannini, T.; Larionov, V. M.; Arkharov, A. A.; Antoniucci, S.; Di Paola, A.; Konstantinova, T. S.; Kopatskaya, E. N.; Li Causi, G.; Nisini, B.

2011-05-01

347

SIMULTANEOUS MONITORING OF THE PHOTOMETRIC AND POLARIMETRIC ACTIVITY OF THE YOUNG STAR PV Cep IN THE OPTICAL/NEAR-INFRARED BANDS  

SciTech Connect

We present the results of a simultaneous monitoring, lasting more than two years, of the optical and near-infrared photometric and polarimetric activity of the variable protostar PV Cep. During the monitoring period, an outburst with declining phase ({Delta}J {approx} 3 mag) lasting about 120 days occurred in all the photometric bands. A time lag of {approx}30 days between optical and infrared light curves has been measured and interpreted in the framework of an accretion event. This latter is directly recognizable in the significant variations of the near-infrared colors, which appear bluer in the outburst phase, when the star dominates the emission, and redder in the declining phase, when the disk emission prevails. All the observational data have been combined to derive a coherent picture of the complex morphology of the whole PV Cep system that is composed of, in addition to the star and the accretion disk, a variable bi-conical nebula. The mutual interaction between all these components is the cause of the high value of the polarization ({approx} 20%) and of its fluctuations. The observational data concur to indicate that PV Cep is not a genuine EXor star, but rather a more complex object; moreover, the case of PV Cep leads to arguments about the classification of other recently discovered young sources in outburst that have been considered, perhaps oversimplifying, as EXor.

Lorenzetti, D.; Giannini, T.; Antoniucci, S.; Di Paola, A.; Li Causi, G.; Nisini, B. [INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio (Italy); Larionov, V. M.; Konstantinova, T. S.; Kopatskaya, E. N. [Astronomical Institute of St. Petersburg University (Russian Federation); Arkharov, A. A., E-mail: lorenzetti@oa-roma.inaf.it, E-mail: giannini@oa-roma.inaf.it, E-mail: antoniucci@oa-roma.inaf.it, E-mail: dipaola@oa-roma.inaf.it, E-mail: licausi@oa-roma.inaf.it, E-mail: nisini@oa-roma.inaf.it, E-mail: vlar2@yandex.ru, E-mail: azt8@mail.ru, E-mail: enik1346@rambler.ru, E-mail: arkadi@arharov.ru [Central Astronomical Observatory of Pulkovo, Pulkovskoe Shosse 65, 196140 St. Petersburg (Russian Federation)

2011-05-10

348

Modeling lunar volcanic eruptions  

Microsoft Academic Search

Simple physical arguments are used to show that basaltic volcanos on different planetary bodies would fountain to the same height if the mole fraction of gas in the magma scaled with the acceleration of gravity. It is suggested that the actual eruption velocities and fountain heights are controlled by the velocities of sound in the two phase gas\\/liquid flows. These

R. M. Housley

1978-01-01

349

Complex Solar Eruption  

NASA Video Gallery

On August 1, 2010 around 0855 UT, Earth orbiting satellites detected a C3-class solar flare. The origin of the blast was sunspot 1092. At about the same time, an enormous magnetic filament stretching across the sun's northern hemisphere erupted. NASA's Solar Dynamics Observatory recorded the action.

Holly Zell

2010-08-02

350

Lichenoid eruptions in children  

Microsoft Academic Search

Lichenoid eruptions are quite common in children and can result from many different origins. In most instances the precise mechanism of disease is not known, although it is usually believed to be immunologic in nature. Certain disorders are common in children, whereas others more often affect the adult population. Lichen striatus, lichen nitidus, Gianotti-Crosti syndrome, and lichen spinulosus are examples

Joline J. Tilly; Beth A. Drolet; Nancy B. Esterly

2004-01-01

351

High-resolution imaging of young M-type stars of the solar neighbourhood: probing for companions down to the mass of Jupiter  

NASA Astrophysics Data System (ADS)

Context. High-contrast imaging is a powerful technique when searching for gas giant planets and brown dwarfs orbiting at separations greater than several AU. Around solar-type stars, giant planets are expected to form by core accretion or by gravitational instability, but since core accretion is increasingly difficult as the primary star becomes lighter, gravitational instability would be a probable formation scenario for still-to-find distant giant planets around a low-mass star. A systematic survey for such planets around M dwarfs would therefore provide a direct test of the efficiency of gravitational instability. Aims: We search for gas giant planets orbiting late-type stars and brown dwarfs of the solar neighbourhood. Methods: We obtained deep high-resolution images of 16 targets with the adaptive optic system of VLT-NACO in the L' band, using direct imaging and angular differential imaging. This is currently the largest and deepest survey for Jupiter-mass planets around M-dwarfs. We developed and used an integrated reduction and analysis pipeline to reduce the images and derive our 2D detection limits for each target. The typical contrast achieved is about 9 mag at 0.5? and 11 mag beyond 1?. For each target we also determine the probability of detecting a planet of a given mass at a given separation in our images. Results: We derived accurate detection probabilities for planetary companions, taking orbital projection effects into account, with in average more than 50% probability to detect a 3 MJup companion at 10 AU and a 1.5 MJup companion at 20 AU, bringing strong constraints on the existence of Jupiter-mass planets around this sample of young M-dwarfs. Based on observations made with the NACO at VLT UT-4 at the Paranal Observatory under programme IDs 084.C-0739, 085.C-0675(A), 087.C-0413(A) and 087.C-0450(B).

Delorme, P.; Lagrange, A. M.; Chauvin, G.; Bonavita, M.; Lacour, S.; Bonnefoy, M.; Ehrenreich, D.; Beust, H.

2012-03-01

352

LOCAL LYMAN BREAK GALAXY ANALOGS: THE IMPACT OF MASSIVE STAR-FORMING CLUMPS ON THE INTERSTELLAR MEDIUM AND THE GLOBAL STRUCTURE OF YOUNG, FORMING GALAXIES  

SciTech Connect

We report on the results of Hubble Space Telescope optical and UV imaging, Spitzer mid-IR photometry, and optical spectroscopy of a sample of 30 low-redshift (z approx 0.1 to 0.3) galaxies chosen from the Sloan Digital Sky Survey and Galaxy Evolution Explorer surveys to be accurate local analogs of the high-redshift Lyman break galaxies. The Lyman break analogs (LBAs) are similar in stellar mass, metallicity, dust extinction, star formation rate (SFR), physical size, and gas velocity dispersion, thus enabling a detailed investigation of many processes that are important in star-forming galaxies at high redshift. The main optical emission-line properties of LBAs, including evidence for outflows, are also similar to those typically found at high redshift. This indicates that the conditions in their interstellar medium are comparable. In the UV, LBAs are characterized by complexes of massive clumps of star formation, while in the optical they most often show evidence for (post-)mergers and interactions. In six cases, we find a single extremely massive (up to several x10{sup 9} M{sub sun}) compact (radius approx10{sup 2} pc) dominant central object (DCO). The DCOs are preferentially found in LBAs with the highest mid-IR luminosities (L{sub 24m}u{sub m} = 10{sup 10.3}-10{sup 11.2} L{sub sun}) and correspondingly high SFRs (15-100 M{sub sun} yr{sup -1}). We show that the massive star-forming clumps (including the DCOs) have masses much larger than the nuclear super star clusters seen in normal late-type galaxies. However, the DCOs do have masses, sizes, and densities similar to the excess light/central cusps seen in typical elliptical galaxies with masses similar to the LBA galaxies. We suggest that the DCOs form in the present-day examples of the dissipative mergers at high redshift that are believed to have produced the central cusps in local ellipticals (consistent with the disturbed optical morphologies of the LBAs). More generally, the properties of the LBAs are consistent with the idea that instabilities in a gas-rich disk lead to very massive star-forming clumps that eventually coalesce to form a spheroid. Finally, we comment on the apparent lack of energetically significant active galactic nuclei in the DCOs. We speculate that the DCOs are too young at present to grow a supermassive black hole because they are still in a supernova-dominated outflow phase (age less than 50 Myr).

Overzier, Roderik A. [Max-Planck-Institut for Astrophysics, D-85748 Garching (Germany); Heckman, Timothy M.; Ptak, Andy; Ford, Holland C. [Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States); Tremonti, Christy [Max-Planck Institute for Astronomy, Koenigstuhl 17, D-69117, Heidelberg (Germany); Armus, Lee [Spitzer Science Center, California Institute of Technology, Pasadena, CA (United States); Basu-Zych, Antara [NASA Goddard Space Flight Center, Laboratory for X-ray Astrophysics, Greenbelt, MD 20771 (United States); Goncalves, Thiago; Martin, D. Christopher [California Institute of Technology, MC 405-47, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Rich, R. Michael [Deptartment of Physics and Astronomy, Division of Astronomy and Astrophysics, University of California, Los Angeles, CA 90095-1562 (United States); Schiminovich, David [Department of Astronomy, Columbia University, MC 2457, 550 West 120th Street, New York, NY 10027 (United States); Madore, Barry; Seibert, Mark, E-mail: overzier@mpa-garching.mpg.d [Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)

2009-11-20

353

X-raying the Heart of Trumpler 37: Chandra and Spitzer Observations Reveal a Population of Young Stars  

NASA Astrophysics Data System (ADS)

We present Chandra archival observations of an 8' x 8' region in the 4Myr old open cluster Trumpler 37. Thirty seven X-ray sources were detected in this area; they all match spatially to bright IR sources identified in Spitzer IRAC data. The X-ray data reveal a larger population of pre-main sequence cluster members, not previously cataloged by optical and infrared surveys. The X-ray spectra of three stars permit spectral fits that reveal two distinct plasma temperatures, as has been seen in some of the brightest nearby T Tauri stars. Using measurements of X-ray and IR fluxes of these sources, we examine the influence of accretion on the role of X-ray activity in pre-main sequence stars. Contrary to some recent studies, the data do not suggest that higher accretion rates inhibit X-ray emission.

Mercer, Emily P.; Miller, J. M.; Calvet, N.; Hartmann, L.; Sicilia-Aguilar, A.

2007-12-01

354

THE YOUNG OPEN CLUSTERS KING 12, NGC 7788, AND NGC 7790: PRE-MAIN-SEQUENCE STARS AND EXTENDED STELLAR HALOS  

SciTech Connect

The stellar contents of the open clusters King 12, NGC 7788, and NGC 7790 are investigated using MegaCam images. Comparisons with isochrones yield an age <20 Myr for King 12, 20-40 Myr for NGC 7788, and 60-80 Myr for NGC 7790 based on the properties of stars near the main-sequence turnoff (MSTO) in each cluster. The reddening of NGC 7788 is much larger than previously estimated. The luminosity functions (LFs) of King 12 and NGC 7788 show breaks that are attributed to the onset of pre-main-sequence (PMS) objects, and comparisons with models of PMS evolution yield ages that are consistent with those measured from stars near the MSTO. In contrast, the r' LF of main-sequence stars in NGC 7790 is matched to r' = 20 by a model that is based on the solar neighborhood mass function. The structural properties of all three clusters are investigated by examining the two-point angular correlation function of blue main-sequence stars. King 12 and NGC 7788 are each surrounded by a stellar halo that extends out to a radius of 5 arcmin ({approx}3.4 pc). It is suggested that these halos form in response to large-scale mass ejection early in the evolution of the clusters, as predicted by models. In contrast, blue main-sequence stars in NGC 7790 are traced out to a radius of {approx}7.5 arcmin ({approx}5.5 pc), with no evidence of a halo. It is suggested that all three clusters may have originated in the same star-forming complex, but not in the same giant molecular cloud.

Davidge, T. J. [Dominion Astrophysical Observatory, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7 (Canada)

2012-12-20

355

Gas Surface Density, Star Formation Rate Surface Density, and the Maximum Mass of Young Star Clusters in a Disk Galaxy. II. The Grand-design Galaxy M51  

NASA Astrophysics Data System (ADS)

We analyze the relationship between maximum cluster mass and surface densities of total gas (?gas), molecular gas (\\Sigma _H_2), neutral gas (\\Sigma _{H\\,\\scriptsize{I}}), and star formation rate (?SFR) in the grand-design galaxy M51, using published gas data and a catalog of masses, ages, and reddenings of more than 1800 star clusters in its disk, of which 223 are above the cluster mass distribution function completeness limit. By comparing the two-dimensional distribution of cluster masses and gas surface densities, we find for clusters older than 25 Myr that M_3rd \\propto \\Sigma _H\\,\\scriptsize{I}^{0.4 +/- 0.2}, whereM 3rd is the median of the five most massive clusters. There is no correlation with?gas,?H2, or?SFR. For clusters younger than 10 Myr, M_3rd \\propto \\Sigma _{H\\,\\scriptsize{I}}^{0.6 +/- 0.1} and M_3rd \\propto \\Sigma _gas^{0.5 +/- 0.2}; there is no correlation with either \\Sigma _H_2 or