ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae

NASA Astrophysics Data System (ADS)

McDonald, I.; Zijlstra, A. A.; Lagadec, E.; Sloan, G. C.; Boyer, M. L.; Matsuura, M.; Smith, R. J.; Smith, C. L.; Yates, J. A.; van Loon, J. Th.; Jones, O. C.; Ramstedt, S.; Avison, A.; Justtanont, K.; Olofsson, H.; Blommaert, J. A. D. L.; Goldman, S. R.; Groenewegen, M. A. T.

2015-11-01

Atacama Large Millimetre Array observations show a non-detection of carbon monoxide around the four most luminous asymptotic giant branch (AGB) stars in the globular cluster 47 Tucanae. Stellar evolution models and star counts show that the mass-loss rates from these stars should be ˜1.2-3.5 × 10-7 M⊙ yr-1. We would naïvely expect such stars to be detectable at this distance (4.5 kpc). By modelling the ultraviolet radiation field from post-AGB stars and white dwarfs in 47 Tuc, we conclude that CO should be dissociated abnormally close to the stars. We estimate that the CO envelopes will be truncated at a few hundred stellar radii from their host stars and that the line intensities are about two orders of magnitude below our current detection limits. The truncation of CO envelopes should be important for AGB stars in dense clusters. Observing the CO (3-2) and higher transitions and targeting stars far from the centres of clusters should result in the detections needed to measure the outflow velocities from these stars.

Gas and dust from solar metallicity AGB stars

NASA Astrophysics Data System (ADS)

Ventura, P.; Karakas, A.; Dell'Agli, F.; García-Hernández, D. A.; Guzman-Ramirez, L.

2018-04-01

We study the asymptotic giant branch (AGB) evolution of stars with masses between 1 M⊙and8.5 M⊙. We focus on stars with a solar chemical composition, which allows us to interpret evolved stars in the Galaxy. We present a detailed comparison with models of the same chemistry, calculated with a different evolution code and based on a different set of physical assumptions. We find that stars of mass ≥3.5 M⊙ experience hot bottom burning at the base of the envelope. They have AGB lifetimes shorter than ˜3 × 105 yr and eject into their surroundings gas contaminated by proton-capture nucleosynthesis, at an extent sensitive to the treatment of convection. Low-mass stars with 1.5 M⊙ ≤ M ≤ 3 M⊙ become carbon stars. During the final phases, the C/O ratio grows to ˜3. We find a remarkable agreement between the two codes for the low-mass models and conclude that predictions for the physical and chemical properties of these stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB phase. The dust produced is also dependent on the mass: low-mass stars produce mainly solid carbon and silicon carbide dust, whereas higher mass stars produce silicates and alumina dust. Possible future observations potentially able to add more robustness to the present results are also discussed.

High-resolution spectroscopy of the extremely iron-poor post-AGB star CC Lyr

NASA Astrophysics Data System (ADS)

Aoki, Wako; Matsuno, Tadafumi; Honda, Satoshi; Parthasarathy, Mudumba; Li, Haining; Suda, Takuma

2017-04-01

High-resolution optical spectroscopy was conducted for the metal-poor post-AGB star CC Lyr to determine its chemical abundances and spectral line profiles. Our standard abundance analysis confirms its extremely low metallicity ([Fe/H] < -3.5) and a clear correlation between abundance ratios and the condensation temperature for 11 elements, indicating that dust depletion is the cause of the abundance anomaly of this object. The very low abundances of Sr and Ba, which are detected for the first time for this object, suggest that heavy neutron-capture elements are not significantly enhanced in this object by the s-process during its evolution through the AGB phase. The radial velocity of this object and profiles of some atomic absorption lines show variations depending on pulsation phases, which could be formed by dynamics of the atmosphere rather than by binarity or contributions of circumstellar absorption. On the other hand, the Hα emission with double peaks shows no evident velocity shift, suggesting that the emission is originating from the circumstellar matter, presumably the rotating disk around the object.

Amplitude variations of modulated RV Tauri stars support the dust obscuration model of the RVb phenomenon

NASA Astrophysics Data System (ADS)

Kiss, L. L.; Bódi, A.

2017-12-01

Context. RV Tauri-type variables are pulsating post-asymptotic giant branch (AGB) stars that evolve rapidly through the instability strip after leaving the AGB. Their light variability is dominated by radial pulsations. Members of the RVb subclass show an additional variability in the form of a long-term modulation of the mean brightness, for which the most popular theories all assume binarity and some kind of circumstellar dust. Here we assess whether or not the amplitude modulations are consistent with the dust obscuration model. Aims: We measure and interpret the overall changes of the mean amplitude of the pulsations along the RVb variability. Methods: We compiled long-term photometric data for RVb-type stars, including visual observations of the American Association of Variable Star Observers, ground-based CCD photometry from the OGLE and ASAS projects, and ultra-precise space photometry of one star, DF Cygni, from theKepler space telescope. After converting all the observations to flux units, we measure the cycle-to-cycle variations of the pulsation amplitude and correlate them to the actual mean fluxes. Results: We find a surprisingly uniform correlation between the pulsation amplitude and the mean flux; they scale linearly with each other for a wide range of fluxes and amplitudes. This means that the pulsation amplitude actually remains constant when measured relative to the system flux level. The apparent amplitude decrease in the faint states has long been noted in the literature but it was always claimed to be difficult to explain with the actual models of the RVb phenomenon. Here we show that when fluxes are used instead of magnitudes, the amplitude attenuation is naturally explained by periodic obscuration from a large opaque screen, one most likely corresponding to a circumbinary dusty disk that surrounds the whole system.

Rubidium and Zirconium Production in Massive AGB Stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Raai, M. A. van; Lugaro, M.; Karakas, A. I.

2008-04-06

A recent survey of a large sample of massive Galactic asymptotic giant branch (AGB) stars shows that significant overabundances of rubidium (up to 100 times solar), but merely solar zirconium, are present in these stars. These observations can set constraints on our theoretical notion of the slow neutron capture process (the s process) in AGB stars, as well as on the rates of the neutron capture reactions involved in the production of Rb and Zr. We use the Monash nucleosynthesis code with a recently extended network to try to reproduce these observations. We present results for AGB stars of massesmore » 5, 6, and 6.5 M{center_dot} and solar metallicity. We also show results for different available choices of the neutron capture rates, as well as for the possible inclusion of a partial mixing zone (PMZ), leading to the activation of the {sup 13}C neutron source. We find increasing Rb overabundances with increasing stellar mass, as observed, but we are far from matching the highest observed Rb enhancements. Inclusion of a PMZ increases the Rb abundance, but also produces an overabundance of Zr, contrary to what is observed. Only if the third dredge up efficiency remains as high as before the onset of the superwind phase during the final few pulses of a massive AGB star, can we match the highest [Rb/Fe] ratios observed by Garcia-Hernandez et al. [l]. A better understanding of the third dredge up efficiency with decreasing envelope mass for massive AGB stars is essential for further investigation of this issue.« less

Search for X-ray Emission from AGB Stars in the Coronal Graveyard

NASA Astrophysics Data System (ADS)

Montez, Rodolfo

2013-10-01

Maser observations demonstrate the existence of magnetic fields in the circumstellar envelopes of AGB stars. However, thus far, only 2-3 AGB stars have exhibited evidence for coronal X-ray emission. We have demonstrated that only the sensitivity of modern X-ray telescopes can detect magnetically-induced coronal emission and have identified a sample of AGB stars which are ideal candidates to search for such emission. Specifically, we have selected a sample of AGB stars with SiO maser emission, UV emission in at least one of the GALEX bandpasses, and low mass loss rates. The four selected AGB stars provide a pilot sample that optimally probes for coronal activity beyond the giant phase and that provides valuable tests for the launching and shaping of AGB mass loss.

AGB stars in Leo P and their use as metallicity probes

NASA Astrophysics Data System (ADS)

Lee, Chien-Hsiu

2016-09-01

Leo P is the most metal-poor yet star-forming galaxy in the local volume, and has the potential to serve as a local counterpart to interpret the properties of distant galaxies in the early universe. We present a comprehensive search of asymptotic giant branch (AGB) stars in Leo P using deep infrared imaging. AGB stars are the major dust contributors; the metal poor nature of Leo P can help to shed light on the dust formation process in very low-metallicity environments, similar to the early Universe. We select and classify oxygen-rich and carbon-rich candidate AGB stars using J - K versus K colour-magnitude diagram. To filter out contaminations from background galaxies, we exploit the high-resolution Hubble Space Telescope imaging and identify 9 oxygen-rich AGBs and 13 carbon-rich AGB stars in Leo P. We then use the ratio of carbon-rich and oxygen-rich AGB stars (C/M ratio) as an indicator of on-site metallicity and derive the global metallicity [Fe/H] = -1.8 dex for Leo P, in good agreement with previous studies using isochrone fitting. Follow-up observations of these Leo P AGB stars in the mid-infrared [e.g. Spitzer, James Webb Space Telescope (JWST)] will be invaluable to measure the dust formation rates using Spectral energy distribution (SED) fitting.

Calibration of Post-AGB Supergiants as Standard Extragalactic Candles for HST

NASA Technical Reports Server (NTRS)

Bond, Howard E.

1998-01-01

This report summarizes activities carried out with support from the NASA Ultraviolet, Visible, and Gravitational Astrophysics Research and Analysis Program. The aim of the program is to calibrate the absolute magnitudes of post-asymptotic-giant-branch (post-AGB or PAGB) stars, which we believe will be an excellent new "standard candle" for measuring extragalactic distances. The reason for this belief is that in old populations, the stars that are evolving through the PAGB region of the HR (Hertzsprung-Russell) diagram arise from only a single main-sequence turnoff mass. In addition, the theoretical PAGB evolutionary tracks show that they evolve through this region at constant luminosity; hence the PAGB stars should have an extremely narrow luminosity function. Moreover, as the PAGB stars evolve through spectral types F and A (en route from the AGB to hot stellar remnants and white dwarfs), they have the highest luminosities attained by old stars (both bolometrically and in the visual band). Finally, the PAGB stars of these spectral types are very easily identified, due to their large Balmer jumps, which are due to their very low surface gravities.

The Fate of Super-AGB Stars at Low Metallicity

NASA Astrophysics Data System (ADS)

Decressin, T.; Siess, L.; Charbonnel, C.; Leclair, G.

2005-11-01

Super-AGBs are stars massive enough to ignite carbon off center and to climb the asymptotic giant branch (where they undergo thermal pulses) after core carbon exhaustion. The destiny of these stars, ONe white dwarfs or neutron stars, depends essentially on the ability of the mass loss to peel the stellar envelope before the core reaches the critical mass for electron captures to occur. Here, we focus on a super-AGB stars of 8.5 Msolar at Z=0.0001 and investigate the effects of various mass loss prescriptions. We study the interplay between mass loss and diffusive overshooting below the convective envelope. The former determines the duration of the AGB phase and the final state of the star; the latter affects the increase of the core mass and the nucleosynthesis as well.

Lithium and zirconium abundances in massive Galactic O-rich AGB stars

NASA Astrophysics Data System (ADS)

García-Hernández, D. A.; García-Lario, P.; Plez, B.; Manchado, A.; D'Antona, F.; Lub, J.; Habing, H.

2007-02-01

Lithium and zirconium abundances (the latter taken as representative of s-process enrichment) are determined for a large sample of massive Galactic O-rich AGB stars, for which high-resolution optical spectroscopy has been obtained (R˜ 40 000{-}50 000). This was done by computing synthetic spectra based on classical hydrostatic model atmospheres for cool stars and using extensive line lists. The results are discussed in the framework of "hot bottom burning" (HBB) and nucleosynthesis models. The complete sample is studied for various observational properties such as the position of the stars in the IRAS two-colour diagram ([ 12] - [25] vs. [ 25] - [60] ), Galactic distribution, expansion velocity (derived from the OH maser emission), and period of variability (when available). We conclude that a considerable fraction of these sources are actually massive AGB stars (M>3{-}4 M⊙) experiencing HBB, as deduced from the strong Li overabundances we found. A comparison of our results with similar studies carried out in the past for the Magellanic Clouds (MCs) reveals that, in contrast to MC AGB stars, our Galactic sample does not show any indication of s-process element enrichment. The differences observed are explained as a consequence of metallicity effects. Finally, we discuss the results obtained in the framework of stellar evolution by comparing our results with the data available in the literature for Galactic post-AGB stars and PNe. Based on observations at the 4.2 m William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias. Also based on observations with the ESO 3.6 m telescope at La Silla Observatory (Chile). Tables [see full text]-[see full text] are only available in electronic form at http://www.aanda.org

The nebula around the post-AGB star 89 Herculis

NASA Astrophysics Data System (ADS)

Bujarrabal, V.; van Winckel, H.; Neri, R.; Alcolea, J.; Castro-Carrizo, A.; Deroo, P.

2007-06-01

Aims:We aim to study the structure of the nebula around the post-AGB, binary star 89 Her. The presence of a rotating disk around this star had been proposed but not been yet confirmed by observations. Methods: We present high-resolution PdBI maps of CO J=2-1 and 1-0. Properties of the nebula are directly derived from the data and model fitting. We also present N-band interferometric data on the extent of the hot dust emission, obtained with the VLTI. Results: Two nebular components are found: (a) an extended hour-glass-like structure, with expansion velocities of 7 km s-1 and a total mass 3× 10-3 M{⊙}, and (b) an unresolved very compact component, smaller than 0.4 arcsec and with a low total velocity dispersion of 5 km s-1. We cannot determine the velocity field in the compact component, but we argue that it can hardly be in expansion, since this would require too recent and too sudden an ejection of mass. On the other hand, assuming that this component is a Keplerian disk, we derive disk properties that are compatible with expectations for such a structure; in particular, the size of the rotating gas disk should be very similar to the extent of the hot dust component from our VLTI data. Assuming that the equator of the extended nebula coincides with the binary orbital plane, we provide new results on the companion star mass and orbit. Based on observations carried out with the IRAM Plateau de Bure Interferometer, as well as on observations of the Belgian Guaranteed time on VISA (ESO). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

The COSAS survey I: First results from the IRAM mapping survey of 12CO J=1-0 & J=2-1 emission in AGB and early post-AGB circumstellar envelopes

NASA Astrophysics Data System (ADS)

Alcolea Jimanez, J.; Castr-Carrizo, A.; Quintana-Lacaci, G.; Neri, R.; Bujarrabal, V.; Schoeier, F. L.; Winters, J. M.; Olofsson, H.; Lindqvist, M.; Lucas, R.; Grewing, M.

Here we present the first result from the COSAS (CO Survey of late AGB Stars) program (P.I. A. Castro-Carrizo), a J=1-0 and J=2-1 line emission mapping survey of a statistically representative sample of circumstellar envelopes around AGB and post-AGB stars. This mapping survey has been carried out to investigate the small and large scale morphological and kinematical properties of the molecular environment surrounding stars in the late AGB and early post-AGB phases. For this, COSAS ideally combines the high spatial resolution and sensitivity of the IRAM Plateau de Bure Interferometer, with the IRAM Pico de Veleta 30m-MRT capabilities to map more extended emission. The whole program includes of 45 stars, selected to sample a wide variety in mass loss rate, chemical type (M, S and C types), variability type (regular variables like Miras and OH/IRs, semiregulars, irregulars, and non varying post-AGBs), evolutionary state, and initial mass. By no means it is an unbiased sample, so results must be interpreted with care, and in terms of the different population of sources represented in the sample. COSAS products (at first. maps and velocity fields, and after modeling, excitation and density profiles across the envelopes) provides means to quantify variations in the mass-loss rate history, assess on the prevalence of different morphological and kinematical features, and investigate the appearance of fast aspherical winds in the late-AGB and early post-AGB phases. This paper, which is the first of a series of COSAS papers, presents the results from the final mapping of a sample of 16 selected sources (about 1/3 of the whole list), namely: WX Psc, IK Tau, TX Cam, RX Boo, X Her, CRL 2362, x Cyg, V Cyg, S Cep, OH 104.9+2.4, R Cas, IRAS 19475+3119, IRAS 20028+3910, IRAS 21282+5050, IRAS 23321+6545 and CRL 2477. The envelopes around late AGB stars are found to be mostly spherical, but often presenting features like concentric arcs (R Cas and TX Cam), spiral density patterns

Fluorine Abundances in AGB Carbon Stars: New Results?

NASA Astrophysics Data System (ADS)

Abia, C.; de Laverny, P.; Recio-Blanco, A.; Domínguez, I.; Cristallo, S.; Straniero, O.

2009-09-01

A recent reanalysis of the fluorine abundance in three Galactic Asymptotic Giant Branch (AGB) carbon stars (TX Psc, AQ Sgr and R Scl) by Abia et al. (2009) results in estimates of fluorine abundances systematically lower by ~0.8 dex on average, with respect to the sole previous estimates by Jorissen, Smith & Lambert (1992). The new F abundances are in better agreement with the predictions of full-network stellar models of low-mass (<3 Msolar) AGB stars.

AGB stars as tracers to IC 1613 evolution.

NASA Astrophysics Data System (ADS)

Hashemi, S. A.; Javadi, A.; van Loon, J. Th.

We are going to apply AGB stars to find star formation history for IC 1613 galaxy; this a new and simple method that works well for nearby galaxies. IC 1613 is a Local Group dwarf irregular galaxy that is located at distance of 750 kpc, a gas rich and isolated dwarf galaxy that has a low foreground extinction. We use the long period variable stars (LPVs) that represent the very final stage of evolution of stars with low and intermediate mass at the AGB phase and are very luminous and cool so that they emit maximum brightness in near-infrared bands. Thus near-infrared photometry with using stellar evolutionary models help us to convert brightness to birth mass and age and from this drive star formation history of the galaxy. We will use the luminosity distribution of the LPVs to reconstruct the star formation history-a method we have successfully applied in other Local Group galaxies. Our analysis shows that the IC 1613 has had a nearly constant star formation rate, without any dominant star formation episode.

FUV Emission from AGB Stars: Modeling Accretion Activity Associated with a Binary Companion

NASA Technical Reports Server (NTRS)

Stevens, Alyx Catherine; Sahai, Raghvendra

2012-01-01

It is widely believed that the late stages of evolution for Asymptotic Giant Branch (AGB) stars are influenced by the presence of binary companions. Unfortunately, there is a lack of direct observational evidence of binarity. However, more recently, strong indirect evidence comes from the discovery of UV emission in a subsample of these objects (fuvAGB stars). AGB stars are comparatively cool objects (< or =3000 K), thus their fluxes falls off drastically for wavelengths 3000 Angstroms and shorter. Therefore, ultraviolet observations offer an important, new technique for detecting the binary companions and/or associated accretion activity. We develop new models of UV emission from fuvAGB stars constrained by GALEX photometry and spectroscopy of these objects. We compare the GALEX UV grism spectra of the AGB M7 star EY Hya to predictions using the spectral synthesis code Cloudy, specifically investigating the ultraviolet wavelength range (1344-2831 Angstroms). We investigate models composed of contributions from a photoionized "hot spot" due to accretion activity around the companion, and "chromospheric" emission from collisionally ionized plasma, to fit the UV observations.

AI canis minoris, a pulsating low-mass supergiant at an early transition phase from the AGB to the post-AGB stage of evolution

NASA Astrophysics Data System (ADS)

Arkhipova, V. P.; Ikonnikova, N. P.; Esipov, V. F.; Komissarova, G. V.

2017-06-01

The U BV photometry and low-resolution spectroscopy for the semiregular variable AI CMi, a candidate for post-AGB objects, performed in 1996-2016 and 2000-2013, respectively, are presented. The star showed multiperiodic brightness variations with an amplitude up to 1\\underset{\\cdot}{m} 5 in the V band, a significant (up to 0\\underset{\\cdot}{m} 4) bluing of the B - V and U - B colors as the star faded, and a change of its spectrum from G5 I to K3-5 I, depending on its brightness. A possible long-term fading of AI CMi below 8\\underset{\\cdot}{m} 5 in the period from May 2013 to early 2015 is observed in the light curve. The colors in this episode did not change the pattern of their unusual behavior with brightness. The main feature of the spectrum for AI CMi is the appearance and strengthening of TiO absorption bands as its brightness declines, which are atypical in the spectra of ordinary G5-K3 supergiants. The bluing of the B - V and U - B colors is interpreted as the blanketing of stellar radiation predominantly in V (and to a lesser extent in B) by the TiO absorption bands whose intensity increases dramatically with decreasing brightness. Another cause of the bluing can be the scattering of stellar radiation by small dust particles in the gas-dust shell of AI CMi. The star's continuum-normalized spectra over the period from 2000 to 2013 in the wavelength range 4200 to 7700 or 9200 Å are presented. These were taken at different phases of the pulsation cycle and clearly demonstrate the behavior of the TiO absorption bands depending on the V magnitude and B - V color. The equivalent widths of individual TiO bands weremeasured, and their correlation with the photometric parameters of the star is shown. AI CMi belongs to the O-rich branch of AGB/post-AGB supergiants and has a luminosity of 4000 L ⊙ at a distance of 1500 ± 700 pc. The mass of AI CMi is most likely small and close to the lower mass limit for post-AGB stars. The connection of the star

Super-AGB Stars and their Role as Electron Capture Supernova Progenitors

NASA Astrophysics Data System (ADS)

Doherty, Carolyn L.; Gil-Pons, Pilar; Siess, Lionel; Lattanzio, John C.

2017-11-01

We review the lives, deaths and nucleosynthetic signatures of intermediate-mass stars in the range ≈6-12 M⊙, which form super-AGB stars near the end of their lives. The critical mass boundaries both between different types of massive white dwarfs (CO, CO-Ne, ONe), and between white dwarfs and supernovae, are examined along with the relative fraction of super-AGB stars that end life either as an ONe white dwarf or as a neutron star (or an ONeFe white dwarf), after undergoing an electron capture supernova event. The contribution of the other potential single-star channel to electron-capture supernovae, that of the failed massive stars, is also discussed. The factors that influence these different final fates and mass limits, such as composition, rotation, the efficiency of convection, the nuclear reaction rates, mass-loss rates, and third dredge-up efficiency, are described. We stress the importance of the binary evolution channels for producing electron-capture supernovae. Recent nucleosynthesis calculations and elemental yield results are discussed and a new set of s-process heavy element yields is presented. The contribution of super-AGB star nucleosynthesis is assessed within a Galactic perspective, and the (super-)AGB scenario is considered in the context of the multiple stellar populations seen in globular clusters. A brief summary of recent works on dust production is included. Last, we conclude with a discussion of the observational constraints and potential future advances for study into these stars on the low mass/high mass star boundary.

Dust in AGB Stars: Transparent or Opaque?

NASA Astrophysics Data System (ADS)

Bladh, S.; Höfner, S.; Aringer, B.

2011-09-01

The optical properties of the dust particles that drive the winds of cool giant stars affect the stellar spectra in two ways: (1) indirectly, through their influence on the dynamical structure of the atmosphere/envelope and the resulting molecular features, and (2) directly, by changes of the spectral energy distribution due to absorption and scattering on dust grains. The qualitative differences in the energy distributions of C-type and M-type AGB stars in the visual and near-infrared regions suggest that the dust particles in oxygen rich atmospheres are relatively transparent to radiation. By using detailed dynamical models of gas and radiation combined with a simple description for the dust opacity (which can be adjusted to mimic different wavelength dependences and condensation temperatures) and also by adjusting the fraction of the opacity that is treated as true absorption, we investigate which dust properties produce synthetic photometry consistent with observations. The goal of this study is to narrow down the possible dust species that may be driving the winds in M-type AGB stars.

Abundances of presolar graphite and SiC from supernovae and AGB stars in the Murchison meteorite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Amari, Sachiko; Zinner, Ernst; Gallino, Roberto

2014-05-02

Pesolar graphite grains exhibit a range of densities (1.65 – 2.20 g/cm{sup 3}). We investigated abundances of presolar graphite grains formed in supernovae and in asymptotic giant branch (AGB) stars in the four density fractions KE3, KFA1, KFB1 and KFC1 extracted from the Murchison meteorite to probe dust productions in these stellar sources. Seventy-six and 50% of the grains in the low-density fractions KE3 and KFA1, respectively, are supernova grains, while only 7.2% and 0.9% of the grains in the high-density fractions KFB1 and KFC1 have a supernova origin. Grains of AGB star origin are concentrated in the high-density fractionsmore » KFB1 and KFC1. From the C isotopic distributions of these fractions and the presence of s-process Kr with {sup 86}Kr/{sup 82}Kr = 4.43±0.46 in KFC1, we estimate that 76% and 80% of the grains in KFB1 and KFC1, respectively, formed in AGB stars. From the abundance of graphite grains in the Murchison meteorite, 0.88 ppm, the abundances of graphite from supernovae and AGB stars are 0.24 ppm and 0.44 ppm, respectively: the abundances of graphite in supernovae and AGB stars are comparable. In contrast, it has been known that 1% of SiC grains formed in supernovae and 95% formed in AGB stars in meteorites. Since the abundance of SiC grains is 5.85 ppm in the Murchison meteorite, the abundances of SiC from supernovae and AGB stars are 0.063 ppm and 5.6 ppm, respectively: the dominant source of SiC grains is AGB stars. Since SiC grains are harder and likely to survive better in space than graphite grains, the abundance of supernova graphite grains, which is higher than that of supernova SiC grains, indicates that supernovae proficiently produce graphite grains. Graphite grains from AGB stars are, in contrast, less abundant that SiC grains from AGB stars (0.44 ppm vs. 5.6 ppm). It is difficult to derive firm conclusions for graphite and SiC formation in AGB stars due to the difference in susceptibility to grain destruction

CONSTRAINTS OF THE PHYSICS OF LOW-MASS AGB STARS FROM CH AND CEMP STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cristallo, S.; Piersanti, L.; Gobrecht, D.

2016-12-20

We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis using model atmospheres, and thus they represent the largest homogeneous abundance data available for CH stars to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass asymptotic giant branch (AGB) s.tars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enriched surfaces.more » We discuss the effects induced on the surface AGB s-process distributions by different prescriptions for convection and rotation. Our reference theoretical FRUITY set fits only part of the observations. Moreover, the s-process observational spread for a fixed metallicity cannot be reproduced. At [Fe/H] > −1, a good fit is found when rotation and a different treatment of the inner border of the convective envelope are simultaneously taken into account. In order to increase the statistics at low metallicities, we include in our analysis a selected number of CEMP stars and, therefore, we compute additional AGB models down to [Fe/H] = −2.85. Our theoretical models are unable to attain the large [hs/ls] ratios characterizing the surfaces of those objects. We speculate on the reasons for such a discrepancy, discussing the possibility that the observed distribution is a result of a proton mixing episode leading to a very high neutron density (the so-called i-process).« less

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

NASA Astrophysics Data System (ADS)

Groenewegen, M. A. T.; Sloan, G. C.

2018-01-01

Context. Mass loss is one of the fundamental properties of asymptotic giant branch (AGB) stars, and through the enrichment of the interstellar medium, AGB stars are key players in the life cycle of dust and gas in the universe. However, a quantitative understanding of the mass-loss process is still largely lacking. Aims: We aim to investigate mass loss and luminosity in a large sample of evolved stars in several Local Group galaxies with a variety of metalliticies and star-formation histories: the Small and Large Magellanic Cloud, and the Fornax, Carina, and Sculptor dwarf spheroidal galaxies (dSphs). Methods: Dust radiative transfer models are presented for 225 carbon stars and 171 oxygen-rich evolved stars in several Local Group galaxies for which spectra from the Infrared Spectrograph on Spitzer are available. The spectra are complemented with available optical and infrared photometry to construct spectral energy distributions. A minimization procedure was used to determine luminosity and mass-loss rate (MLR). Pulsation periods were derived for a large fraction of the sample based on a re-analysis of existing data. Results: New deep K-band photometry from the VMC survey and multi-epoch data from IRAC (at 4.5 μm) and AllWISE and NEOWISE have allowed us to derive pulsation periods longer than 1000 days for some of the most heavily obscured and reddened objects. We derive (dust) MLRs and luminosities for the entire sample. The estimated MLRs can differ significantly from estimates for the same objects in the literature due to differences in adopted optical constants (up to factors of several) and details in the radiative transfer modelling. Updated parameters for the super-AGB candidate MSX SMC 055 (IRAS 00483-7347) are presented. Its current mass is estimated to be 8.5 ± 1.6 M⊙, suggesting an initial mass well above 8 M⊙ in agreement with estimates based on its large Rubidium abundance. Using synthetic photometry, we present and discuss colour-colour and

VizieR Online Data Catalog: AGB stars with GALEX observations (Montez+, 2017)

NASA Astrophysics Data System (ADS)

Montez, R.; Ramstedt, S.; Kastner, J. H.; Vlemmings, W.; Sanchez, E.

2018-01-01

Our sample of AGB stars is derived from numerous AGB samples found in the literature and was originally compiled by Ramstedt+ (2012A&A...543A.147R) to search for X-ray detections associated with AGB stars. The GALEX mission performed a two-band survey of the UV sky. Using a dichromatic beam splitter, GALEX simultaneously observed far-UV (FUV; λeff~1528Å; 1344-1786Å) and near-UV (NUV; λeff~2310Å; 1771-2831Å) in surveys with different depths. The spatial resolution is 4.3" in FUV and 5.3" in NUV. As described in further detail in Morrissey+ (2007ApJS..173..682M), spectroscopic observations place a grism into the converging beam of the telescope to simultaneously disperse all sources onto the detector plane. According to Morrissey+ (2007), the usable ranges of the grism spectra are 1300-1820Å and 1820-3000Å in the FUV and NUV, with average resolutions of 8Å and 20Å, respectively. To supplement our study of the GALEX observations of AGB stars, we collected photometric data from across the electromagnetic spectrum for all the AGB stars considered using SIMBAD and VizieR tools. We aslo collected AAVSO light curves that span the GALEX mission lifetime (2003 May 28 to 2013 June 28) from the AAVSO International Database. (3 data files).

Application of the Calculating Formula for the Mean Neutron Exposure in CEPM-s and CEPM-r/s Stars %Kstars: AGB and post-AGB, nuclear reactions, nucleosynthesis, abundances, methods: analytical

NASA Astrophysics Data System (ADS)

Zhang, F. H.; Zhang, L.; Cui, W. Y.; Zhang, B.

2017-09-01

Recent studies have shown that, for the current s-process nucleosynthesis model for the low-mass asymptotic giant branch (AGB) stars with (13C) pocket radiative burning during the interpulse period, the neutron exposure distribution in the nucleosynthesis region can be regarded as an exponential function, and the relation between the mean neutron exposure (τ0) and the model parameters is τ0 = - Δ τ/ln [q/(1 - r + q)]), in which (Δ τ) is the exposure value of each neutron irradiation, (r) is the overlap factor, and (q) is the mass ratio of the (13C) shell to the He intershell. Using the published data resulted from fitting the observed abundances of neutron-capture elements in 20 CEMP (Carbon-Enhanced Metal-Poor)-s and CEMP-s/r stars with the parametric AGB stellar s-process model, the reliability of the derived formula is tested, and further more the application of the formula in the s-process nucleosynthesis study is explored preliminarily. Our results show that, under the radiative s-process nucleosynthesis mechanism, the formula is suitable for CEMP stars experiencing recurrent neutron exposures. Combined with the parametric AGB nucleosynthesis model, the formula could be regarded as an effective tool to screen the CEMP stars with a single neutron exposure or a special type. Considering the uncertainty of the (13C) pocket, the role of this formula in understanding the physical conditions necessary to reproduce the observed s-process abundances in CEMP stars needs further study.

Abundance Patterns in S-type AGB Stars: Setting Constraints on Nucleosynthesis and Stellar Evolution Models

NASA Astrophysics Data System (ADS)

Neyskens, P.; van Eck, S.; Plez, B.; Goriely, S.; Siess, L.; Jorissen, A.

2011-09-01

During evolution on the AGB, stars of type S are the first to experience s-process nucleosynthesis and the third dredge-up, and therefore to exhibit s-process signatures in their atmospheres. Their high mass-loss rates (10-7 to 10-6 M⊙/year) make them major contributors to the AGB nucleosynthesis yields at solar metallicity. Precise abundance determinations in S stars are of the utmost importance for constraining e.g. the third dredge-up luminosity and efficiency (which has been only crudely parameterized in current nucleosynthetic models so far). Here, dedicated S-star model atmospheres are used to determine precise abundances of key s-process elements, and to set constraints on nucleosynthesis and stellar evolution models. Special interest is paid to technetium, an element with no stable isotopes. Its detection is considered the best signature that the star effectively populates the thermally-pulsing AGB phase of evolution. The derived Tc/Zr abundances are compared, as a function of the derived [Zr/Fe] overabundances, with AGB stellar model predictions. The [Zr/Fe] overabundances are in good agreement with model predictions, while the Tc/Zr abundances are slightly overpredicted. This discrepancy can help to set better constraints on nucleosynthesis and stellar evolution models of AGB stars.

The Mass-loss Return from Evolved Stars to the Large Magellanic Cloud. IV. Construction and Validation of a Grid of Models for Oxygen-rich AGB Stars, Red Supergiants, and Extreme AGB Stars

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, S.; Meixner, M.

2011-02-01

To measure the mass loss from dusty oxygen-rich (O-rich) evolved stars in the Large Magellanic Cloud (LMC), we have constructed a grid of models of spherically symmetric dust shells around stars with constant mass-loss rates using 2Dust. These models will constitute the O-rich model part of the "Grid of Red supergiant and Asymptotic giant branch star ModelS" (GRAMS). This model grid explores four parameters—stellar effective temperature from 2100 K to 4700 K luminosity from 103 to 106 L sun; dust shell inner radii of 3, 7, 11, and 15 R star; and 10.0 μm optical depth from 10-4 to 26. From an initial grid of ~1200 2Dust models, we create a larger grid of ~69,000 models by scaling to cover the luminosity range required by the data. These models are available online to the public. The matching in color-magnitude diagrams and color-color diagrams to observed O-rich asymptotic giant branch (AGB) and red supergiant (RSG) candidate stars from the SAGE and SAGE-Spec LMC samples and a small sample of OH/IR stars is generally very good. The extreme AGB star candidates from SAGE are more consistent with carbon-rich (C-rich) than O-rich dust composition. Our model grid suggests lower limits to the mid-infrared colors of the dustiest AGB stars for which the chemistry could be O-rich. Finally, the fitting of GRAMS models to spectral energy distributions of sources fit by other studies provides additional verification of our grid and anticipates future, more expansive efforts.

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

NASA Astrophysics Data System (ADS)

Sahai, R.; Sánchez Contreras, C.; Mangan, A. S.; Sanz-Forcada, J.; Muthumariappan, C.; Claussen, M. J.

2018-06-01

Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on timescales of ≲20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blueshifted by velocities of ∼500 {km} {{{s}}}-1 from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blueshifted features due to the absorption of UV continuum emitted by the disk, whereas the redshifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. that Y Gem’s companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 L ⊙, and thus a mass-accretion rate >5 × 10‑7 M ⊙ yr‑1 we infer that Roche-lobe overflow is the most likely binary accretion mode for Y Gem.

The abundance of HCN in circumstellar envelopes of AGB stars of different chemical type

NASA Astrophysics Data System (ADS)

Schöier, F. L.; Ramstedt, S.; Olofsson, H.; Lindqvist, M.; Bieging, J. H.; Marvel, K. B.

2013-02-01

Aims: A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. Methods: In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. Results: The median values for the derived abundances of HCN (with respect to H2) are 3 × 10-5, 7 × 10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. Conclusions: We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars. This publication is based on data

Common Envelope Evolution: Implications for Post-AGB Stars and Planetary Nebulae

NASA Astrophysics Data System (ADS)

Nordhaus, J.

2017-10-01

Common envelopes (CE) are of broad interest as they represent one method by which binaries with initially long-period orbits of a few years can be converted into short-period orbits of a few hours. Despite their importance, the brief lifetimes of CE phases make them difficult to directly observe. Nevertheless, CE interactions are potentially common, can produce a diverse array of nebular shapes, and can accommodate current post-AGB and planetary nebula outflow constraints. Here, I discuss ongoing theoretical and computational work on CEs and speculate on what lies ahead for determining accurate outcomes of this elusive phase of evolution.

Dust formation and wind acceleration around the aluminum oxide-rich AGB star W Hydrae

NASA Astrophysics Data System (ADS)

Takigawa, Aki; Kamizuka, Takafumi; Tachibana, Shogo; Yamamura, Issei

2017-11-01

Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but 40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide-rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae.

Dust formation and wind acceleration around the aluminum oxide–rich AGB star W Hydrae

PubMed Central

Takigawa, Aki; Kamizuka, Takafumi; Tachibana, Shogo; Yamamura, Issei

2017-01-01

Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but ~40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide–rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae. PMID:29109978

Influence of the outer boundary condition on models of AGB stars

NASA Astrophysics Data System (ADS)

Wagstaff, G.; Weiss, A.

2018-07-01

Current implementations of the stellar atmosphere typically derive boundary conditions for the interior model from either grey plane-parallel atmospheres or scaled solar atmospheres, neither of which can be considered to have appropriate underlying assumptions for the Thermally Pulsing Asymptotic Giant Branch (TP-AGB). This paper discusses the treatment and influence of the outer boundary condition within stellar evolution codes, and the resulting effects on the AGB evolution. The complex interaction of processes, such as the third dredge up and mass-loss, governing the TP-AGB can be affected by varying the treatment of this boundary condition. Presented here are the results from altering the geometry, opacities, and the implementation of a grid of MARCS/COMARCS model atmospheres in order to improve this treatment. Although there are changes in the TP-AGB evolution, observable quantities, such as the final core mass, are not significantly altered as a result of the change of atmospheric treatment. During the course of the investigation, a previously unseen phenomenon in the AGB models was observed and further investigated. This is believed to be physical, although arising from specific conditions which make its presence unlikely. If it were present in stars, this phenomenon would increase the carbon-star lifetime above 10 Myr and increase the final core mass by ˜0.1 M⊙ in the narrow initial-mass range where it was observed (˜2-2.3 M⊙).

Influence of the Outer Boundary Condition on models of AGB stars

NASA Astrophysics Data System (ADS)

Wagstaff, G.; Weiss, A.

2018-04-01

Current implementations of the stellar atmosphere typically derive boundary conditions for the interior model from either grey plane-parallel atmospheres or scaled solar atmospheres, neither of which can be considered to have appropriate underlying assumptions for the Thermally Pulsing Asymptotic Giant Branch (TP-AGB). This paper discusses the treatment and influence of the outer boundary condition within stellar evolution codes, and the resulting effects on the AGB evolution. The complex interaction of processes, such as the third dredge up and mass loss, governing the TP-AGB can be affected by varying the treatment of this boundary condition. Presented here are the results from altering the geometry, opacities and the implementation of a grid of MARCS/COMARCS model atmospheres in order to improve this treatment. Although there are changes in the TP-AGB evolution, observable quantities, such as the final core mass, are not significantly altered as a result of the change of atmospheric treatment. During the course of the investigation, a previously unseen phenomena in the AGB models was observed and further investigated. This is believed to be physical, although arising from specific conditions which make its presence unlikely. If it were present in stars, this phenomenon would increase the carbon-star lifetime above 10Myr and increase the final core mass by ˜0.1M⊙ in the narrow initial-mass range where it was observed (˜2 - 2.3M⊙).

Mass Loss from Dusty AGB and Red Supergiant Stars in the Magellanic Clouds and in the Galaxy

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, Sundar; Meixner, Margaret; Kastner, Joel

2016-01-01

Asymptotic giant branch (AGB) and red supergiant (RSG) stars are evolved stars that eject large parts of their mass in outflows of dust and gas. As part of an ongoing effort to measure mass loss from evolved stars in our Galaxy and in the Magellanic Clouds, we are modeling mass loss from AGB and RSG stars in these galaxies. Our approach is twofold. We pursue radiative transfer modeling of the spectral energy distributions (SEDs) of AGB and RSG stars in the Large Magellanic Cloud (LMC), in the Small Magellanic Cloud (SMC), and in the Galactic bulge and in globular clusters of the Milky Way. We are also constructing detailed dust opacity models of AGB and RSG stars in these galaxies for which we have infrared spectra; e.g., from the Spitzer Space Telescope Infrared Spectrograph (IRS). Our sample of infrared spectra largely comes from Spitzer-IRS observations. The detailed dust modeling of spectra informs our choice of dust properties to use in radiative transfer modeling of SEDs. We seek to determine how mass loss from these evolved stars depends upon the metallicity of their host environments. BAS acknowledges funding from NASA ADAP grant NNX15AF15G.

Stellar Evolution with Rotation: Mixing Processes in AGB Stars

NASA Astrophysics Data System (ADS)

Driebe, T.; Blöcker, T.

We included diffusive angular momentum transport and rotationally induced mixing processes in our stellar evolution code and studied the influence of rotation on the evolution of intermediate mass stars (M*=2dots6 Msolar) towards and along the asymptotic giant branch (AGB). The calculations start in the fully convective pre-main sequence phase and the initial angular momentu m was adjusted such that on the zero-age main sequence vrot=200 km/ s is achieved. The diffusion coefficients for the five rotational instabilities considered (dynamical shear, secular shear, Eddington-Sweet (ES) circulation, Solberg-Høiland-instability and Goldreich-Schubert-Fricke (GSF) instability) were adopted from Heger et al. (2000, ApJ 528, 368). Mixing efficiency and sensitivity of these processes against molecular weight gradients have been determined by calibration of the main sequence width. In this study we focus on the abundance evolution of carbon. On the one hand, the surface abundance ratios of 12C/13C a nd 12C/16O at the base of the AGB were found to be ≈ 7dots 10 and ≈ 0.1, resp., being a factor of two lower than in non-rotating models. This results from the slow but continuously operating rotationally induced mixing due to the ES-circulation and the GSF-instability during the long main sequence phase. On the other hand, 13C serves as neutron source for interior s-process nucleosynthesis in AGB stars vi a 13C(α,n)16O. Herwig et al. (1997, A&A 324, L81) found that a 13C pocket is forme d in the intershell region of 3 Msolar AGB star if diffusive overshoot is considered. Our calculations show, that mixing processes due to rotation open an alternative channel for the formation of a 13C pocket as found by Langer et al. (1999, A&A 346, L37). Again, ES-circulation and GSF-instability are the predominant rotational mixing processes.

An Analytical Approach to the Evolution and Death of AGB Stars

NASA Astrophysics Data System (ADS)

Prager, Henry Alexander; Willson, Lee Anne M.; Marengo, Massimo; Creech-Eakman, Michelle J.

2017-01-01

Pop. I and II stars have a significant amount of metals throughout their structure, In the final stages of their evolution, intermediate mass stars (between 0.7 and 2 solar masses) ascend the Asymptotic Giant Branch (AGB). During their last few hundred thousand years on the AGB, these stars quickly lose their envelopes, recycling their metals as dust into the interstellar medium. The rate at which this happens consequently impacts the formation rate of stars, stellar systems, and the wider distribution of s-process isotopes.At the end of their life cycles, AGB stars experience a steep increase in mass loss rate. We can define the death line in two steps. First we define the critical mass loss rate to be where the mass loss rate equals the initial mass divided by the evolution time. Then the death line is where the rate of change of logMdot equals the rate of change of logL. Most of the stars we observe to be rapidly losing mass appear in the death zone between 0.1 and 10 times the critical mass loss rate.Assuming the mass loss rate increases exponentially with time, or, equivalently, the luminosity increases as a power of a characteristic exponent b, then the width of the death zone is the change in logL. This directly implies time is inversely proportional to b. This can be found for any mass-loss rate formula near the death line. By combining this with what we know about the initial-final mass relation and the core mass-luminosity relation, we can test for b with three observables — duration (width) of the death zone, the amplitude of mass loss variations (when L varies on an observable time scale such as a shell flash), and distributions of luminosity and pulsation period.By applying the initial mass function (IMF) and star formation rate (SFR) of an observed region, we can relate these observables to the characteristic exponent. We will need to look at nearby regions where we can see large numbers of AGB stars, such as the Magellanic clouds. We will show that

Sage Studies Of The Mass Return From AGB And RSG Stars In The Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, S.; Meixner, M.

2011-01-01

The Surveying the Agents of a Galaxy's Evolution (SAGE; PI: M. Meixner) Spitzer Space Telescope Legacy project aims to further our understanding of the life cycle of matter in galaxies by studying this life cycle in our neighboring galaxy, the Large Magellanic Cloud (LMC). Combining SAGE mid-infrared photometry with that at shorter wavelengths from other catalogs, the spectral energy distribution (SED) for each of >25000 Asymptotic Giant Branch (AGB) and Red Supergiant (RSG) stars in the LMC has been assembled. To model mass loss from these stars, my colleagues and I have constructed the grid of RSG and AGB models (GRAMS) using the radiative transfer code 2Dust. I will discuss how GRAMS was constructed, and how we use it to determine the mass-loss rate for each evolved star studied, which gives the total mass-loss return to the LMC from AGB and RSG stars. In my talk, I show how this total mass-loss return is divided into oxygen-rich (O-rich) and carbon-rich (C-rich) dust using SED-fitting to identify O-rich versus C-rich AGB stars. Applications of this work to determining the mass return from evolved stars in other galaxies, including the Milky Way, will also be discussed.

Are We Observing Coronal Mass Ejections in OH/IR AGB Stars?

NASA Astrophysics Data System (ADS)

Heiles, Carl

2017-05-01

Solar Coronal Mass Ejections (CMEs) are magnetic electron clouds that are violently ejected by the same magnetic reconnection events that produce Solar flares. CMEs are the major driving source of the hazardous space weather environments near the Earth. In exoplanet systems, the equivalent of Solar wind and CMEs can affect a planet's atmosphere, and in extreme cases can erode it, as probably happened with Mars, or disrupt the cosmic-ray shielding aspect of the planet's magnetic field.We (Jensen et al. 2013SoPh..285...83J, 2016SoPh..291..465J) have developed a new way to observe the electron column density and magnetic field of CMEs, namely to measure the frequency change and Faraday rotation of a spacecraft downlink carrier produced by propagation effects in the plasma. Surprisingly, this can work on other stars if they have the equivalent of the spacecraft carrier, as do OH/IR stars.OH/IR stars are Asymptotic Giant Branch (AGB) stars, which are red giant stars burning He in their final stages of stellar evolution. They have highly convective surfaces and large mass-ejection rates in the form of expanding dense shells of molecular gas and obscuring dust, which were ejected from the star by chaotic turbulent motions and then accelerated by radiation pressure. OH masers reside in these shells, pumped by the IR emission from the dust. The OH masers on the far side of the star (i.e., the positive-velocity masers) are the surrogate for the Solar-case spacecraft signal.The big question: Can we see CMEs in OH/IR stars? We have observed six OH/IR stars with the Arecibo Observatory for a total of about 150 hours over the past 1.5 years. We see changes in OH maser frequency and in the position angle of linear polarization. Both can be produced by electron clouds moving across the line of sight. We will present statistical summaries of the variability and interpret them in terms of CME models.

Fluorine Abundances of AGB Stars in Stellar Clusters

NASA Astrophysics Data System (ADS)

Hren, A.; Lebzelter, T.; Aringer, B.; Hinkle, K. H.; Nowotny, W.

2015-08-01

We have measured the abundance of fluorine, [F/Fe], in a number of AGB stars in stellar clusters have correlated the results with their C/O ratios. This allows us to investigate the change in the fluorine abundance along the evolution on the giant branch. The target list includes primarily O-rich stars in three LMC globular clusters - NGC 1806, NGC 1846 and NGC 1978 - as well as Rup 106 and 47 Tuc in our Galaxy. The observational data were obtained with the PHOENIX spectrograph, and the COMA code was used for modelling the synthetic spectra. Within individual clusters, we find consistent [F/Fe] values at similar C/O for most of our target stars.

HIRAS images of fossil dust shells around AGB stars

NASA Technical Reports Server (NTRS)

Waters, L. B. F. M.; Kester, Do J. M.; Bontekoe, Tj. Romke; Loup, C.

1994-01-01

We present high resolution HIRAS 60 and 100 micron images of AGB stars surrounded by fossil dust shells. Resolving the extended emission of the circumstellar dust allows a determination of the mass loss history of the star. We show that the geometry of the 60 micron emission surrounding HR 3126 agrees well with that of the optical reflection nebula. The emission around the carbon star U Hya is resolved into a central point source and a ring of dust, and the mass loss rate in the detached shell is 70 times higher than the current mass loss rate.

The FRUITY database on AGB stars: past, present and future

NASA Astrophysics Data System (ADS)

Cristallo, S.; Piersanti, L.; Straniero, O.

2016-01-01

We present and show the features of the FRUITY database, an interactive web- based interface devoted to the nucleosynthesis in AGB stars. We describe the current available set of AGB models (largely expanded with respect to the original one) with masses in the range 1.3≤M/M⊙≤3.0 and metallicities -2.15 ≤[Fe/H]≤+0.15. We illustrate the details of our s-process surface distributions and we compare our results to observations. Moreover, we introduce a new set of models where the effects of rotation are taken into account. Finally, we shortly describe next planned upgrades.

High-resolution observations of IRAS 08544-4431. Detection of a disk orbiting a post-AGB star and of a slow disk wind

NASA Astrophysics Data System (ADS)

Bujarrabal, V.; Castro-Carrizo, A.; Winckel, H. Van; Alcolea, J.; Contreras, C. Sánchez; Santander-García, M.; Hillen, M.

2018-06-01

Context. Aims: In order to study the effects of rotating disks in the post-asymptotic giant branch (post-AGB) evolution, we observe a class of binary post-AGB stars that seem to be systematically surrounded by equatorial disks and slow outflows. Although the rotating dynamics had only been well identified in three cases, the study of such structures is thought to be fundamental to the understanding of the formation of disks in various phases of the late evolution of binary stars and the ejection of planetary nebulae from evolved stars. Methods: We present ALMA maps of 12CO and 13CO J = 3-2 lines in the source IRAS 08544-4431, which belongs to the above mentioned class of objects. We analyzed the data by means of nebula models, which account for the expectedly composite source and can reproduce the data. From our modeling, we estimated the main nebula parameters, including the structure and dynamics and the density and temperature distributions. We discuss the uncertainties of the derived values and, in particular, their dependence on the distance. Results: Our observations reveal the presence of an equatorial disk in rotation; a low-velocity outflow is also found, probably formed of gas expelled from the disk. The main characteristics of our observations and modeling of IRAS 08544-4431 are similar to those of better studied objects, confirming our interpretation. The disk rotation indicates a total central mass of about 1.8 M⊙, for a distance of 1100 pc. The disk is found to be relatively extended and has a typical diameter of 4 × 1016 cm. The total nebular mass is 2 × 10-2 M⊙, of which 90% corresponds to the disk. Assuming that the outflow is due to mass loss from the disk, we derive a disk lifetime of 10 000 yr. The disk angular momentum is found to be comparable to that of the binary system at present. Assuming that the disk angular momentum was transferred from the binary system, as expected, the high values of the disk angular momentum in this and other

Sodium abundances of AGB and RGB stars in Galactic globular clusters. II. Analysis and results of NGC 104, NGC 6121, and NGC 6809

NASA Astrophysics Data System (ADS)

Wang, Y.; Primas, F.; Charbonnel, C.; Van der Swaelmen, M.; Bono, G.; Chantereau, W.; Zhao, G.

2017-11-01

Aims: We investigate the Na abundance distribution of asymptotic giant branch (AGB) stars in Galactic globular clusters (GCs) and its possible dependence on GC global properties, especially age and metallicity. Methods: We analyze high-resolution spectra of a large sample of AGB and red giant branch (RGB) stars in the Galactic GCs NGC 104, NGC 6121, and NGC 6809 obtained with FLAMES/GIRAFFE at ESO/VLT, and determine their Na abundances. This is the first time that the AGB stars in NGC 6809 are targeted. Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters. Results: NGC 104 and NGC 6809 have comparable AGB and RGB Na abundance distributions revealed by the K-S test, while NGC 6121 shows a lack of very Na-rich AGB stars. By analyzing all nine GCs, we find that the Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes. In addition, the fitted relations between fractions of the AGB second population and GC global parameters show that the AGB second-population fraction slightly anticorrelates with GC central concentration, while no robust dependency can be confirmed with other GC parameters. Conclusions: Current data roughly support the prediction of the fast-rotating massive star (FRMS) scenario. However, considering the weak observational and theoretical trends where scatter and exceptions exist, the fraction of second-population AGB stars can be affected by more than one or two factors, and may even be a result of stochasticity. Based on observations made with ESO telescopes at the La Silla Paranal

Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae

NASA Astrophysics Data System (ADS)

Danilovich, T.; Bergman, P.; Justtanont, K.; Lombaert, R.; Maercker, M.; Olofsson, H.; Ramstedt, S.; Royer, P.

2014-09-01

Context. S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. Aims: We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. Methods: We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. Results: We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0 × 10-6 M⊙ yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5 × 10-5, which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3 × 10-6, and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6 × 10-6, and distribution of CN in W Aql's circumstellar envelopeusing ground-based data. We find an SiO abundance of 3 × 10-6, and an NH3 abundance of 1.7 × 10-5, confined to a small envelope. If we include uncertainties

Short-lived Isotopes from a Close-by AGB Star Triggering the Protosolar Nebula

NASA Astrophysics Data System (ADS)

Gallino, R.; Busso, M.; Wasserburg, G. J.; Straniero, O.

The presence of short-lived isotopes in the early solar system, in particular 26Al, 41Ca, 60Fe, and 107Pd, point to a close-by and fresh nucleosynthesis source, possibly triggering the collapse of the protosolar nebula. We present the results of nucleosynthesis calculations based on an AGB polluting hypothesis. A general concordance of the predicted yields of the above radioactivities relative to 26Al can be obtained in the case of an intermediate mass AGB star with hot bottom burning in the envelope (thus producing 26Al), and mixing through a series of third dredge-up episodes a fraction of the C-rich and s-processed material from the He intershell with the extended envelope. Polution of the protosolar nebula with freshly synthesized material may derive from the efficient winds of the AGB star. In AGB stars, the s-process nucleosynthesis occurs both during the maximum phase of every thermal runaway, driven by the partial activation of the 22Ne(alpha,n)25Mg reaction, and in the interpulse phase, where the 13C nuclei are fully consumed in radiative conditions by the activation of the 13C(alpha,n)16O reaction. We have used different prescriptions for the amount of the 13C nuclei present in the intershell. A minimum amount of 13C is naturally expected in the ashes of H-shell burning. Possible formation of an extra "13C-pocket" derives from the injection of a small amount of protons from the envelope into the 12C-rich intershell during any third dredge-up episode, when the H-shell is inactivated. Prediction for other short-lived, 36Cl, 135Cs, and 205Pb, are given. General consequences for the pollution of the protosolar nebula with newly synthesized stable isotopes from the AGB winds are outlined. The origin of other detected short-lived nuclei, in particular 53Mn, 129I, and 182Hf, which cannot come from an AGB source, is analysed. The alternative trigger hypothesis by a close-by Supernova is discussed.

Multi-band polarimetry of post-asymptotic giant branch stars - I. Optical measurements

NASA Astrophysics Data System (ADS)

Akras, S.; Ramírez Vélez, J. C.; Nanouris, N.; Ramos-Larios, G.; López, J. M.; Hiriart, D.; Panoglou, D.

2017-04-01

We present new optical broad-band (UBVRI) aperture polarimetric observations of 53 post-asymptotic giant branch (AGB) stars selected to exhibit a large near-infrared excess. 24 out of the 53 stars (45 per cent of our sample) are presented for the first time. A statistical analysis shows four distinctive groups of polarized post-AGB stars: unpolarized or very lowly polarized (degree of polarization or DoP < 1 per cent), lowly polarized (1 per cent < DoP < 4 per cent), moderately polarized (4 per cent < DoP < 8 per cent) and highly polarized (DoP > 8 per cent). 23 out of the 53 (66 per cent) belong to the first group, 10 (19 per cent) to the second, five (9 per cent) to the third and only three (6 per cent) to the last group. Approximately 34 per cent of our sample was found to be unpolarized objects, which is close to the percentage of round planetary nebulae. On average, the low and moderate groups show a wavelength-dependent polarization that increases towards shorter wavelengths, implying an intrinsic origin of the polarization, which signifies a Rayleigh-like scattering spectrum typical for non-symmetrical envelopes composed principally of small dust grains. The moderately polarized stars exhibit higher K - W3 and W1 - W3 colour indices compared with the group of lowly polarized stars, suggesting a possible relation between DoP and mass-loss rate. Moreover, they are found to be systematically colder (redder in B - V), which may be associated with the condensation process close to these stars that results in a higher degree of polarization. We also provide evidence that multiple scattering in optically thin polar outflows is the mechanism that gives high DoP in post-AGB stars with bipolar or multi-polar envelopes.

Current Mysteries of AGB Stars

NASA Astrophysics Data System (ADS)

Tout, Christopher A.

We pay tribute to the memory of Manuel Forestini by recalling his contributions to astronomy and in particular to our understanding of AGB stars. We critically examine the current status of this understanding amongst the community and deduce that major uncertainties arise in the physics of convection, any form of extra mixing beyond convection and the mass loss from the stellar surface. Coupled with these are numerical difficulties associated with the short and similar timescales for structural changes, nuclear burning and convective mixing. We hope that workshops such as this will promote familiarity amongst our diverse international community young and old and so promote effective dialogue that will ultimately lead to solutions to our problems along with the creation of new ones for the future!

Nucleation Studies under the Conditions of Carbon-rich AGB Star Envelopes: TiC

NASA Astrophysics Data System (ADS)

Patzer, A. B. C.; Wendt, M.; Chang, C.; Sülzle, D.

2011-09-01

Many studies of dust nucleation in winds of carbon-rich AGB stars consider primarily carbon as the dust forming material. But dust grains formed in such circumstellar envelopes are rather a mixture of several chemical elements such as titanium or silicon in addition to carbon, as verified by many investigations of pre-solar grains enclosed in meteorites, for example. In this contribution we focus on the study of the nucleation of titanium carbide particles from the gas phase. The necessary properties of molecular titanium carbide clusters have been estimated by the density functional approach, and the first implications for the homogeneous nucleation of TiC are studied for conditions representative of circumstellar dust shells around carbon-rich AGB stars.

The Contribution of TP-AGB Stars to the Mid-infrared Colors of Nearby Galaxies

NASA Astrophysics Data System (ADS)

Chisari, Nora E.; Kelson, Daniel D.

2012-07-01

We study the mid-infrared color space of 30 galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) survey for which Sloan Digital Sky Survey data are also available. We construct two-color maps for each galaxy and compare them to results obtained from combining Maraston evolutionary synthesis models, galactic thermally pulsating asymptotic giant branch (TP-AGB) colors, and smooth star formation histories. For most of the SINGS sample, the spatially extended mid-IR emission seen by Spitzer in normal galaxies is consistent with our simple model in which circumstellar dust from TP-AGB stars dominates at 8 and 24 μm. There is a handful of exceptions that we identify as galaxies that have high star formation rates presumably with star formation histories that cannot be assumed to be smooth, or anemic galaxies, which were depleted of their H I at some point during their evolution and have very low ongoing star formation rates.

THE CONTRIBUTION OF TP-AGB STARS TO THE MID-INFRARED COLORS OF NEARBY GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chisari, Nora E.; Kelson, Daniel D., E-mail: nchisari@astro.princeton.edu

2012-07-10

We study the mid-infrared color space of 30 galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) survey for which Sloan Digital Sky Survey data are also available. We construct two-color maps for each galaxy and compare them to results obtained from combining Maraston evolutionary synthesis models, galactic thermally pulsating asymptotic giant branch (TP-AGB) colors, and smooth star formation histories. For most of the SINGS sample, the spatially extended mid-IR emission seen by Spitzer in normal galaxies is consistent with our simple model in which circumstellar dust from TP-AGB stars dominates at 8 and 24 {mu}m. There is a handfulmore » of exceptions that we identify as galaxies that have high star formation rates presumably with star formation histories that cannot be assumed to be smooth, or anemic galaxies, which were depleted of their H I at some point during their evolution and have very low ongoing star formation rates.« less

The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High-redshift Galaxies

NASA Astrophysics Data System (ADS)

Melbourne, J.; Williams, Benjamin F.; Dalcanton, Julianne J.; Rosenfield, Philip; Girardi, Léo; Marigo, P.; Weisz, D.; Dolphin, A.; Boyer, Martha L.; Olsen, Knut; Skillman, E.; Seth, Anil C.

2012-03-01

weighted mean factor of 2.3 (2.2 with outliers removed) with a standard deviation of 0.8. This larger offset is driven by the prediction of modest numbers of high-luminosity TP-AGB stars at young (<300 Myr) ages. The best-fit SPS models simultaneously tend to underpredict the numbers and fluxes of stars on the RHeB sequence, typically by a factor of 2.0 ± 0.6 for galaxies with significant numbers of RHeBs. Possible explanations for both the TP-AGB and RHeB model results include (1) difficulties with measuring the SFHs of galaxies especially on the short timescales over which these stars evolve (several Myr), (2) issues with the way the SPS codes populate the color-magnitude diagrams (e.g., how they handle pulsations or self-extinction), and/or (3) lingering issues with the lifetimes of these stars in the stellar evolution codes. Coincidentally these two competing discrepancies—overprediction of the TP-AGB and underprediction of the RHeBs—result in a predicted NIR M/L ratio largely unchanged for a rapid SFR, after correcting for these effects. However, the NIR-to-optical flux ratio of galaxies could be significantly smaller than AGB-rich models would predict, an outcome that has been observed in some intermediate-redshift post-starburst galaxies.

HABITABLE ZONES OF POST-MAIN SEQUENCE STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ramirez, Ramses M.; Kaltenegger, Lisa

Once a star leaves the main sequence and becomes a red giant, its Habitable Zone (HZ) moves outward, promoting detectable habitable conditions at larger orbital distances. We use a one-dimensional radiative-convective climate and stellar evolutionary models to calculate post-MS HZ distances for a grid of stars from 3700 to 10,000 K (∼M1 to A5 stellar types) for different stellar metallicities. The post-MS HZ limits are comparable to the distances of known directly imaged planets. We model the stellar as well as planetary atmospheric mass loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phases for super-Moons tomore » super-Earths. A planet can stay between 200 million years up to 9 Gyr in the post-MS HZ for our hottest and coldest grid stars, respectively, assuming solar metallicity. These numbers increase for increased stellar metallicity. Total atmospheric erosion only occurs for planets in close-in orbits. The post-MS HZ orbital distances are within detection capabilities of direct imaging techniques.« less

A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries

NASA Astrophysics Data System (ADS)

Hillen, M.; Van Winckel, H.; Menu, J.; Manick, R.; Debosscher, J.; Min, M.; de Wit, W.-J.; Verhoelst, T.; Kamath, D.; Waters, L. B. F. M.

2017-03-01

Aims: We present a mid-IR interferometric survey of the circumstellar environment of a specific class of post-asymptotic giant branch (post-AGB) binaries. For this class the presence of a compact dusty disk has been postulated on the basis of various spatially unresolved measurements. The aim is to determine the angular extent of the N-band emission directly and to resolve the compact circumstellar structures. Methods: Our interferometric survey was performed with the MIDI instrument on the VLTI. In total 19 different systems were observed using variable baseline configurations. Combining all the visibilities at a single wavelength at 10.7 μm, we fitted two parametric models to the data: a uniform disk and a ring model mimicking a temperature gradient. We compared our observables of the whole sample, with synthetic data computed from a grid of radiative transfer models of passively irradiated disks in hydrostatic equilibrium. These models are computed with a Monte Carlo code that has been widely applied to describe the structure of protoplanetary disks around young stellar objects (YSO). Results: The spatially resolved observations show that the majority of our targets cluster closely together in the distance-independent size-colour diagram, and have extremely compact N-band emission regions. The typical uniform disk diameter of the N-band emission region is 40 mas, which corresponds to a typical brightness temperature of 400-600 K. The resolved objects display very similar characteristics in the interferometric observables and in the spectral energy distributions. Therefore, the physical properties of the disks around our targets must be similar. Our results are discussed in the light of recently published sample studies of YSOs to compare quantitatively the secondary discs around post-AGB stars to the ones around YSOs. Conclusions: Our high-angular-resolution survey further confirms the disk nature of the circumstellar structures present around wide post-AGB

An Analysis and Classification of Dying AGB Stars Transitioning to Pre-Planetary Nebulae

NASA Technical Reports Server (NTRS)

Blake, Adam C.

2011-01-01

The principal objective of the project is to understand part of the life and death process of a star. During the end of a star's life, it expels its mass at a very rapid rate. We want to understand how these Asymptotic Giant Branch (AGB) stars begin forming asymmetric structures as they start evolving towards the planetary nebula phase and why planetary nebulae show a very large variety of non-round geometrical shapes. To do this, we analyzed images of just-forming pre-planetary nebula from Hubble surveys. These images were run through various image correction processes like saturation correction and cosmic ray removal using in-house software to bring out the circumstellar structure. We classified the visible structure based on qualitative data such as lobe, waist, halo, and other structures. Radial and azimuthal intensity cuts were extracted from the images to quantitatively examine the circumstellar structure and measure departures from the smooth spherical outflow expected during most of the AGB mass-loss phase. By understanding the asymmetrical structure, we hope to understand the mechanisms that drive this stellar evolution.

AGB Sodium Abundances in the Globular Cluster 47 Tucanae (NGC 104)

NASA Astrophysics Data System (ADS)

Johnson, Christian I.; McDonald, Iain; Pilachowski, Catherine A.; Mateo, Mario; Bailey, John I., III; Cordero, Maria J.; Zijlstra, Albert A.; Crane, Jeffrey D.; Olszewski, Edward; Shectman, Stephen A.; Thompson, Ian

2015-02-01

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high-resolution spectra obtained with the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5 m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be < R{{V}helio.}> = -18.56 km s-1 (σ = 10.21 km s-1) and < [Fe/H]> = -0.68 (σ = 0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction (≲20%) of Na-rich stars in 47 Tuc may fail to ascend the AGB, which is a similar result to that observed in M13. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. The contrasting behavior of Na-rich stars in 47 Tuc and NGC 6752 suggests that the RGB [Na/Fe] abundance alone is insufficient for predicting if a star will ascend the AGB.

A Spitzer Space Telescope Survey of Extreme Asymptotic Giant Branch Stars in M32

NASA Technical Reports Server (NTRS)

Jones, O.C.; McDonald, I.; Rich, R.M.; Kemper, F.; Boyer, M.L.; Zijlstra, A.A.; Bendo, G.J.

2014-01-01

We investigate the population of cool, evolved stars in the Local Group dwarf elliptical galaxy M32, using Infrared Array Camera observations from the Spitzer Space Telescope. We construct deep mid-infrared colour-magnitude diagrams for the resolved stellar populations within 3.5 arcminutes of M32's centre, and identify those stars that exhibit infrared excess. Our data is dominated by a population of luminous, dustproducing stars on the asymptotic giant branch (AGB) and extend to approximately 3 magnitudes below the AGB tip. We detect for the first time a sizeable population of 'extreme' AGB stars, highly enshrouded by circumstellar dust and likely completely obscured at optical wavelengths. The total dust-injection rate from the extreme AGB candidates is measured to be 7.5 x 10 (sup -7) solar masses per year, corresponding to a gas mass-loss rate of 1.5 x 10 (sup -4) solar masses per year. These extreme stars may be indicative of an extended star-formation epoch between 0.2 and 5 billion years ago.

Modelling a Set of Carbon-Rich AGB Stars at High-Angular Resolution

NASA Astrophysics Data System (ADS)

Rau, Gioia; Hron, Josef; Paladini, Claudia; Aringer, Bernard; Eriksson, Kjell; Marigo, Paola; Nowotny, Walter; Grellmann, Rebekka

2016-07-01

We compared spectro-photometric and interferometric observations of six carbon-rich AGB stars with a grid of self-consistentmodel atmospheres. The targets are: R Lep, R Vol, Y Pav, AQ Sgr, U Hya and X TrA. Please refer to the publication Rau et al. 2016(subm.) for further details on those findings.

NuSTAR Observations of Heavily Obscured Quasars Selected by WISE

NASA Astrophysics Data System (ADS)

Yan, Wei

2017-08-01

A key goal of the Nuclear Spectroscopic Telescope Array (NuSTAR) program is to find and characterize heavily obscured quasars, luminous accreting supermassive black holes hidden by gas and dust. Based on mid-infrared (IR) photometry from Wide-Field Infrared Survey Explorer (WISE) and optical photometry from the Sloan Digital Sky Surveys, we have selected a large population of obscured quasars; here we report the NuSTAR observations of four WISE-selected heavily obscured quasars for which we have optical spectroscopy from the Southern African Large Telescope and KECK Telescope. Three of four objects are undetected with NuSTAR, while the fourth has only a marginal detection. We confirm our objects have observed hard X-ray (10-40 keV) luminosities at or below ~1043 erg s-1. We compare IR and X-ray luminosities to obtain estimates of hydrogen column NH based on the suppression of the hard X-ray emission. We estimate NH to be at or greater than 1025 cm-2, confirming that WISE and optical selection can identify very heavily obscured quasars that may be missed in X-ray surveys.

Asteroseismology of ZZ Ceti stars with full evolutionary white dwarf models. II. The impact of AGB thermal pulses on the asteroseismic inferences of ZZ Ceti stars

NASA Astrophysics Data System (ADS)

De Gerónimo, F. C.; Althaus, L. G.; Córsico, A. H.; Romero, A. D.; Kepler, S. O.

2018-05-01

Context. The thermally pulsing phase on the asymptotic giant branch (TP-AGB) is the last nuclear burning phase experienced by most low- and intermediate-mass stars. During this phase, the outer chemical stratification above the C/O core of the emerging white dwarf (WD) is built up. The chemical structure resulting from progenitor evolution strongly impacts the whole pulsation spectrum exhibited by ZZ Ceti stars, which are pulsating C/O core white dwarfs located on a narrow instability strip at Teff 12 000 K. Several physical processes occurring during progenitor evolution strongly affect the chemical structure of these stars; those found during the TP-AGB phase are the most relevant for the pulsational properties of ZZ Ceti stars. Aims: We present a study of the impact of the chemical structure built up during the TP-AGB evolution on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Methods: Our analysis is based on a set of carbon-oxygen core white dwarf models with masses from 0.534 to 0.6463 M⊙ derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We computed evolutionary sequences that experience different number of thermal pulses (TP). Results: We find that the occurrence or not of thermal pulses during AGB evolution implies an average deviation in the asteroseimological effective temperature of ZZ Ceti stars of at most 8% and on the order of ≲5% in the stellar mass. For the mass of the hydrogen envelope, however, we find deviations up to 2 orders of magnitude in the case of cool ZZ Ceti stars. Hot and intermediate temperature ZZ Ceti stars show no differences in the hydrogen envelope mass in most cases. Conclusions: Our results show that, in general, the impact of the occurrence or not of thermal pulses in the progenitor stars is not negligible and must be taken into account in asteroseismological studies of ZZ Ceti stars.

AGB sodium abundances in the globular cluster 47 Tucanae (NGC 104)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Christian I.; McDonald, Iain; Zijlstra, Albert A., E-mail: cjohnson@cfa.harvard.edu, E-mail: iain.mcdonald-2@manchester.ac.uk, E-mail: albert.zijlstra@manchester.ac.uk

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globularmore » cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high-resolution spectra obtained with the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan–Clay 6.5 m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be 〈RV{sub helio.}〉 = −18.56 km s{sup −1} (σ = 10.21 km s{sup −1}) and 〈[Fe/H]〉 = −0.68 (σ = 0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction (≲20%) of Na-rich stars in 47 Tuc may fail to ascend the AGB, which is a similar result to that observed in M13. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. The contrasting behavior of Na-rich stars in 47 Tuc and NGC 6752 suggests that the RGB [Na/Fe] abundance alone is insufficient for predicting if a

The circumstellar envelope around the S-type AGB star W Aql. Effects of an eccentric binary orbit

PubMed Central

Ramstedt, S.; Mohamed, S.; Vlemmings, W. H. T.; Danilovich, T.; Brunner, M.; De Beck, E.; Humphreys, E. M. L.; Lindqvist, M.; Maercker, M.; Olofsson, H.; Kerschbaum, F.; Quintana-Lacaci, G.

2017-01-01

Context Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3→2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results The estimated average mass-loss rate of W Aql is Ṁ = 3.0×10−6 M⊙ yr−1 and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10″ of the CSE is asymmetric with arc-like structures at separations of 2-3″ scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions The CO(J = 3→2) emission is dominated by a smooth component overlayed with two weak

The circumstellar envelope around the S-type AGB star W Aql. Effects of an eccentric binary orbit.

PubMed

Ramstedt, S; Mohamed, S; Vlemmings, W H T; Danilovich, T; Brunner, M; De Beck, E; Humphreys, E M L; Lindqvist, M; Maercker, M; Olofsson, H; Kerschbaum, F; Quintana-Lacaci, G

2017-09-21

Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO( J = 3→2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. The estimated average mass-loss rate of W Aql is Ṁ = 3.0×10 -6 M ⊙ yr -1 and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10″ of the CSE is asymmetric with arc-like structures at separations of 2-3″ scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. The CO( J = 3→2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Placco, Vinicius M.; Rossi, Silvia; Frebel, Anna

2013-06-20

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

Barium Stars: Theoretical Interpretation

NASA Astrophysics Data System (ADS)

Husti, Laura; Gallino, Roberto; Bisterzo, Sara; Straniero, Oscar; Cristallo, Sergio

2009-09-01

Barium stars are extrinsic Asymptotic Giant Branch (AGB) stars. They present the s-enhancement characteristic for AGB and post-AGB stars, but are in an earlier evolutionary stage (main sequence dwarfs, subgiants, red giants). They are believed to form in binary systems, where a more massive companion evolved faster, produced the s-elements during its AGB phase, polluted the present barium star through stellar winds and became a white dwarf. The samples of barium stars of Allen & Barbuy (2006) and of Smiljanic et al. (2007) are analysed here. Spectra of both samples were obtained at high-resolution and high S/N. We compare these observations with AGB nucleosynthesis models using different initial masses and a spread of 13C-pocket efficiencies. Once a consistent solution is found for the whole elemental distribution of abundances, a proper dilution factor is applied. This dilution is explained by the fact that the s-rich material transferred from the AGB to the nowadays observed stars is mixed with the envelope of the accretor. We also analyse the mass transfer process, and obtain the wind velocity for giants and subgiants with known orbital period. We find evidence that thermohaline mixing is acting inside main sequence dwarfs and we present a method for estimating its depth.

Observational constraints for C-rich AGB stars

NASA Astrophysics Data System (ADS)

Rau, G.; Hron, J.; Paladini, C.; Aringer, B.; Marigo, P.; Eriksson, K.

We modeled the atmospheres of six carbon-rich Asymptotic Giant Branch stars (R Lep, R Vol, Y Pav, AQ Sgr, U Hya, and X TrA) using VLTI/MIDI interferometric observations, together with spectro-photometric data, we compared them with self-consistent, dynamic model atmospheres. The results show that the models can reproduce the Spectral Energy Distribution (SED) data well at wavelengths longer than 1 mu m, and the interferometric observations between 8 mu m and 10 mu m. We found differences at wavelengths shorter than 1 mu m in the SED, and longer than 10 mu m in the visibilities. The discrepancies observed can be explained in terms of a combination of data- and model-related reasons. We derived some stellar parameters, and our findings agree well with literature values within the uncertainties. Also, when comparing the location of the stars in the H-R diagram, with evolutionary tracks, the results show that the main derived properties (L, Teff, C/O ratios and stellar masses) from the model fitting are in good agreement with TP-AGB evolutionary calculations.

Spectral Analysis of the O(He)-Type Central Stars of the Planetary Nebulae K 1-27 and LoTr 4

NASA Technical Reports Server (NTRS)

Reindl, N.; Ringat, E.; Rauch, T.; Werner, K.; Kruk, J. W.

2011-01-01

The four known O(He) stars are the only amongst the hottest post-AGB stars whose atmospheres are composed of almost pure helium. Thus, their evolution deviates from the hydrogen-defiCient post-AGB evolutionary sequence of carbon-dominated stars like e.g. PG 1159 stars. The origin of the O(He) stars is still not explained. They might be either post-early AGB stars or the progeny of R Coronae Borealis stars. We present preliminary results of a non-LTE spectral analysis based on FUSE and HST/COS observations.

Dust-obscured star-forming galaxies in the early universe

NASA Astrophysics Data System (ADS)

Wilkins, Stephen M.; Feng, Yu; Di Matteo, Tiziana; Croft, Rupert; Lovell, Christopher C.; Thomas, Peter

2018-02-01

Motivated by recent observational constraints on dust reprocessed emission in star-forming galaxies at z ∼ 6 and above, we use the very large cosmological hydrodynamical simulation BLUETIDES to explore predictions for the amount of dust-obscured star formation in the early Universe (z > 8). BLUETIDES matches current observational constraints on both the UV luminosity function and galaxy stellar mass function and predicts that approximately 90 per cent of the star formation in high-mass (M* > 1010 M⊙) galaxies at z = 8 is already obscured by dust. The relationship between dust attenuation and stellar mass predicted by BLUETIDES is consistent with that observed at lower redshift. However, observations of several individual objects at z > 6 are discrepant with the predictions, though it is possible that their uncertainties may have been underestimated. We find that the predicted surface density of z ≥ 8 submm sources is below that accessible to current Herschel, SCUBA-2 and Atacama Large Millimetre Array (ALMA) submm surveys. However, as ALMA continues to accrue an additional surface area the population of z > 8 dust-obscured galaxies may become accessible in the near future.

Sensitivity of the s-process nucleosynthesis in AGB stars to the overshoot model

NASA Astrophysics Data System (ADS)

Goriely, S.; Siess, L.

2018-01-01

Context. S-process elements are observed at the surface of low- and intermediate-mass stars. These observations can be explained empirically by the so-called partial mixing of protons scenario leading to the incomplete operation of the CN cycle and a significant primary production of the neutron source. This scenario has been successful in qualitatively explaining the s-process enrichment in AGB stars. Even so, it remains difficult to describe both physically and numerically the mixing mechanisms taking place at the time of the third dredged-up between the convective envelope and the underlying C-rich radiative layer Aims: We aim to present new calculations of the s-process nucleosynthesis in AGB stars testing two different numerical implementations of chemical transport. These are based on a diffusion equation which depends on the second derivative of the composition and on a numerical algorithm where the transport of species depends linearly on the chemical gradient. Methods: The s-process nucleosynthesis resulting from these different mixing schemes is calculated with our stellar evolution code STAREVOL which has been upgraded to include an extended s-process network of 411 nuclei. Our investigation focuses on a fiducial 2 M⊙, [Fe/H] = -0.5 model star, but also includes four additional stars of different masses and metallicities. Results: We show that for the same set of parameters, the linear mixing approach produces a much larger 13C-pocket and consequently a substantially higher surface s-process enrichment compared to the diffusive prescription. Within the diffusive model, a quite extreme choice of parameters is required to account for surface s-process enrichment of 1-2 dex. These extreme conditions can not, however, be excluded at this stage. Conclusions: Both the diffusive and linear prescriptions of the overshoot mixing are suited to describe the s-process nucleosynthesis in AGB stars provided the profile of the diffusion coefficient below the

ALMA observations of the nearby AGB star L2 Puppis. I. Mass of the central star and detection of a candidate planet

NASA Astrophysics Data System (ADS)

Kervella, P.; Homan, W.; Richards, A. M. S.; Decin, L.; McDonald, I.; Montargès, M.; Ohnaka, K.

2016-12-01

Six billion years from now, while evolving on the asymptotic giant branch (AGB), the Sun will metamorphose from a red giant into a beautiful planetary nebula. This spectacular evolution will impact the solar system planets, but observational confirmations of the predictions of evolution models are still elusive as no planet orbiting an AGB star has yet been discovered. The nearby AGB red giant L2 Puppis (d = 64 pc) is surrounded by an almost edge-on circumstellar dust disk. We report new observations with ALMA at very high angular resolution (18 × 15 mas) in band 7 (ν ≈ 350 GHz) that allow us to resolve the velocity profile of the molecular disk. We establish that the gas velocity profile is Keplerian within the central cavity of the dust disk, allowing us to derive the mass of the central star L2 Pup A, mA = 0.659 ± 0.011 ± 0.041 M⊙ (± 6.6%). From evolutionary models, we determine that L2 Pup A had a near-solar main-sequence mass, and is therefore a close analog of the future Sun in 5 to 6 Gyr. The continuum map reveals a secondary source (B) at a radius of 2 AU contributing fB/fA = 1.3 ± 0.1% of the flux of the AGB star. L2 Pup B is also detected in CO emission lines at a radial velocity of vB = 12.2 ± 1.0 km s-1. The close coincidence of the center of rotation of the gaseous disk with the position of the continuum emission from the AGB star allows us to constrain the mass of the companion to mB = 12 ± 16 MJup. L2 Pup B is most likely a planet or low-mass brown dwarf with an orbital period of about five years. Its continuum brightness and molecular emission suggest that it may be surrounded by an extended molecular atmosphere or an accretion disk. L2 Pup therefore emerges as a promising vantage point on the distant future of our solar system.

Global 3D radiation-hydrodynamics models of AGB stars. Effects of convection and radial pulsations on atmospheric structures

NASA Astrophysics Data System (ADS)

Freytag, B.; Liljegren, S.; Höfner, S.

2017-04-01

Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales. Aims: To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations. Methods: We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters. Results: The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features. Conclusions: Our models self-consistently describe convection, convectively generated acoustic noise, fundamental-mode radial

A mechanism to account for well known peculiarities of low mass AGB star nucleosynthesis

NASA Astrophysics Data System (ADS)

Palmerini, Sara; Trippella, Oscar; Vescovi, Diego; Busso, Maurizio

2018-01-01

We present here the application of a model for a mass circulation mechanism induced by the stellar magnetic field to study peculiar aspects of AGB star nucleosynthesis. The mixing scheme is based on a previously suggested magnetic-buoyancy process [1, 2] and here shown to account adequately for the formation of the 13C neutron source for s-processes. In particular our analysis results are focused on addressing the constrains to AGB nucleosynthesis coming from the isotopic composition of presolar grains recovered in meteorites. It turns out that n-captures driven by the magnetically-induced mixing can account for the isotopic abundance ratios of s-elements recorded.

Detection of CI line emission towards the oxygen-rich AGB star omi Ceti

NASA Astrophysics Data System (ADS)

Saberi, M.; Vlemmings, W. H. T.; De Beck, E.; Montez, R.; Ramstedt, S.

2018-05-01

We present the detection of neutral atomic carbon CI(3P1-3P0) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 × 10-5 predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field. However, the CI emission is shifted by 4 km s-1 from the stellar velocity. Based on this velocity shift, we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion. The velocity shift could, therefore, be the result of the orbital velocity of the binary companion around omi Cet. In this case, the CI column density is estimated to be 1.1 × 1019 cm-2. This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement. However, this hypothesis can be confirmed by high-angular resolution observations.

Detection of HCN and C2H2 in ISO Spectra of Oxygen-Rich AGB Stars

NASA Technical Reports Server (NTRS)

Carbon, Duane F.; Chiar, Jean; Goorvitch, David; Kwak, Dochan (Technical Monitor)

2002-01-01

Cool oxygen-rich AGB stars were not expected to have organic molecules like HCN in either their photospheres or circumstellar envelopes (CSEs). The discovery of HCN and CS microwave emission from the shallowest CSE layers of these stars was a considerable surprise and much theoretical effort has been expended in explaining the presence of such organics. To further explore this problem, we have undertaken a systematic search of oxygen-rich AGB stellar spectra in the Infrared Space Observatory (ISO) data archive. Our purposes are to find evidence regarding critical molecular species that could be of value in choosing among the proposed theoretical models, to locate spectral features which might give clues to conditions deeper in the CSEs, and to lay the groundwork for future SIRTF (Space Infrared Telescope Facility) and SOFIA (Stratospheric Observatory for Infrared Astronomy) observations. Using carefully reduced observations, we have detected weak absorption features arising from HCN and possibly C2H2 in a small number of oxygen-rich AGB stars. The most compelling case is NML Cyg which shows both HCN (14 microns) and CO2 (15 microns). VY CMa, a similar star, shows evidence for HCN, but not CO2. Two S-type stars show evidence for the C-H bending transitions: W Aql at 14 microns (HCN) and both W Aql and S Cas at 13.7 microns (C2H2). Both W Aql and S Cas as well as S Lyr, a SC-type star, show 3 micron absorption which may arise from the C-H stretch of HCN and C2H2. In the case of NML Cyg, we show that the HCN and CO2 spectral features are formed in the CSE at temperatures well above those of the outermost CSE layers and derive approximate column densities. In the case of the S-stars, we discuss the evidence for the organic features and their photospheric origin.

HST/COS Observations of the UV-Bright Star Y453 in the Globular Cluster M4 (NGC 6121)

NASA Astrophysics Data System (ADS)

Dixon, William V.; Chayer, Pierre; Benjamin, Robert A.

2016-01-01

Post-AGB stars represent a short-lived phase of stellar evolution during which stars cross the optical color-magnitude diagram from the cool, red tip of the assymptotic giant branch (AGB) to the hot, blue tip of the white-dwarf cooling curve. Their surface chemistry reflects the nuclear-shell burning, mixing, and mass-loss processes characteristic of AGB stars, and their high effective temperatures allow the detection of elements that are unobservable in cool giants. Post-AGB stars in globular clusters offer the additional advantages of known distance, age, and initial chemistry. To better understand the AGB evolution of low-mass stars, we have observed the post-AGB star Y453 in the globular cluster M4 (NGC 6121) with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. The star, which has an effective temperature of at least 60,000 K, shows absorption from He, C, N, O, Ne, Si, S, Ti, Cr, Mn, Fe, Co, Ni, and Ga. While the star's C and O abundances are consistent with those measured in a sample of nitrogen-poor RGB stars in M4, its N abundance is considerably enhanced. The star's low C abundance suggests that it left the AGB before the onset of third dredge-up.This work was supported by NASA grant HST-GO-13721.001-A to the University of Wisconsin, Whitewater. P.C. is supported by the Canadian Space Agency under a contract with NRC Herzberg Astronomy and Astrophysics.

The Constant Average Relationship Between Dust-obscured Star Formation and Stellar Mass from z=0 to z=2.5

NASA Astrophysics Data System (ADS)

Whitaker, Katherine E.; Pope, Alexandra; Cybulski, Ryan; Casey, Caitlin M.; Popping, Gergo; Yun, Min; 3D-HST Collaboration

2018-01-01

The total star formation budget of galaxies consists of the sum of the unobscured star formation, as observed in the rest-frame ultraviolet (UV), together with the obscured component that is absorbed and re-radiated by dust grains in the infrared. We explore how the fraction of obscured star formation depends (SFR) and stellar mass for mass-complete samples of galaxies at 0 < z < 2.5. We combine GALEX and WISE photometry for SDSS-selected galaxies with the 3D-HST treasury program and Spitzer/MIPS 24μm photometry in the well-studied 5 extragalactic CANDELS fields. We find a strong dependence of the fraction of obscured star formation (f_obscured=SFR_IR/SFR_UV+IR) on stellar mass, with remarkably little evolution in this fraction with redshift out to z=2.5. 50% of star formation is obscured for galaxies with log(M/M⊙)=9.4 although unobscured star formation dominates the budget at lower masses, there exists a tail of low mass extremely obscured star-forming galaxies at z > 1. For log(M/M⊙)>10.5, >90% of star formation is obscured at all redshifts. We also show that at fixed total SFR, f_obscured is lower at higher redshift. At fixed mass, high-redshift galaxies are observed to have more compact sizes and much higher star formation rates, gas fractions and hence surface densities (implying higher dust obscuration), yet we observe no redshift evolution in f_obscured with stellar mass. This poses a challenge to theoretical models to reproduce, where the observed compact sizes at high redshift seem in tension with lower dust obscuration.

The Constant Average Relationship between Dust-obscured Star Formation and Stellar Mass from z = 0 to z = 2.5

NASA Astrophysics Data System (ADS)

Whitaker, Katherine E.; Pope, Alexandra; Cybulski, Ryan; Casey, Caitlin M.; Popping, Gergö; Yun, Min S.

2017-12-01

The total star formation budget of galaxies consists of the sum of the unobscured star formation, as observed in the rest-frame ultraviolet (UV), together with the obscured component that is absorbed and re-radiated by dust grains in the infrared. We explore how the fraction of obscured star formation depends on stellar mass for mass-complete samples of galaxies at 0< z< 2.5. We combine GALEX and WISE photometry for SDSS-selected galaxies with the 3D-HST treasury program and Spitzer/MIPS 24 μm photometry in the well-studied five extragalactic Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) fields. We find a strong dependence of the fraction of obscured star formation (f obscured = SFRIR/SFRUV+IR) on stellar mass, with remarkably little evolution in this fraction with redshift out to z = 2.5. 50% of star formation is obscured for galaxies with log(M/M ⊙) = 9.4 although unobscured star formation dominates the budget at lower masses, there exists a tail of low-mass, extremely obscured star-forming galaxies at z> 1. For log(M/M ⊙) > 10.5, >90% of star formation is obscured at all redshifts. We also show that at fixed total SFR, {f}{obscured} is lower at higher redshift. At fixed mass, high-redshift galaxies are observed to have more compact sizes and much higher star formation rates, gas fractions, and hence surface densities (implying higher dust obscuration), yet we observe no redshift evolution in {f}{obscured} with stellar mass. This poses a challenge to theoretical models, where the observed compact sizes at high redshift seem in tension with lower dust obscuration.

On Driving AGB Mass-Loss from Core-Contraction

NASA Astrophysics Data System (ADS)

Lewis, B. M.

1997-12-01

A bulk movement of mass constitutes a momentum flow. An instructive instance occurs in the radial pulsation of stars and white dwarfs, where a symmetric contraction phase implies the existence of an inwardly- directed radial momentum flow, that is followed during the subsequent expansion by an outwardly-directed flow. The key notion here is that an inward flow is effectively transmitted through the center to become in turn an outward flow: in adiabatic processes the momentum flux is not cancelled simply because it arrives at the center. However, during the radial pulsation of AGB stars momentum is cancelled in atmospheric shock-waves and consumed in work against gravity while mass is lifted far enough away from the star for dust to form, whereon radiation pressure drives it away. These momentum-dissipative conditions at the outer boundary therefore require a stellar source of radially directed momentum if pulsation is to continue in an AGB star. A sufficient source is found in the contraction of the whole of the electron-degenerate core of an AGB star under the addition of He ashes from shell-hydrogen burning. This produces an inwardly- directed radial momentum flow that must reach the center. Lewis quantifies the resulting momentum flux (http://xxx.lanl.gov/ps/astro-ph /9707233), and finds that it easily suffices to support the mass-loss of every AGB star. But it is necessary to assume that most of the inwardly directed flux is transmitted through the center to become in turn an outwardly directed flux. The AGB core maintains its virial equilibrium by exporting its excess momentum flux to the stellar envelope. This mechanism explains the dependence of the mass-loss rate from AGB stars on core mass; its generalization to objects with angular momentum and/or strong magnetic fields suggests a novel explanation for the axial symmetry exhibited by most planetary nebulae and proto planetary nebulae. Gravitational contraction can also account for the momentum flux in the

Modelling a set of C-rich AGB stars: the cases of RU Vir and R Lep

NASA Astrophysics Data System (ADS)

Rau, G.; Paladini, C.; Hron, J.; Aringer, B.; Groenewegen, M. A. T.; Nowotny, W.

We study the atmospheres of a set of carbon-rich asymptotic giant branch AGB stars to improve our understanding of the dynamic processes happening there. We compare in a systematic way spectrometric, photometric and mid-infrared (VLTI/MIDI) interferometric measurements with different types of model atmospheres: (1) hydrostatic models + MOD-dusty models added a posteriori; (2) self-consistent dynamic model atmospheres. These allow us to interpret in a coherent way the dynamic behavior of gas and dust. The results underline that the joint use of different kinds of observations, as photometry, spectroscopy and interferometry, is essential for understanding the atmospheres of pulsating C-rich AGB stars. For our first target, the carbon-rich Mira star RU Vir, the dynamic model atmospheres fit well the ISO/SWS spectrum in the wavelength range lambda = [2.9, 13.0] mu m. However, the object turned out to be somehow ''peculiar''. The other target we present is R Lep. Here the agreement between models and observations is much better although the MIDI data at 11.4 mu m cannot be properly modelled.

AGB subpopulations in the nearby globular cluster NGC 6397

NASA Astrophysics Data System (ADS)

MacLean, B. T.; Campbell, S. W.; De Silva, G. M.; Lattanzio, J.; D'Orazi, V.; Cottrell, P. L.; Momany, Y.; Casagrande, L.

2018-03-01

It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used.

The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity

NASA Astrophysics Data System (ADS)

Goldman, Steven R.; van Loon, Jacco Th.; Zijlstra, Albert A.; Green, James A.; Wood, Peter R.; Nanni, Ambra; Imai, Hiroshi; Whitelock, Patricia A.; Matsuura, Mikako; Groenewegen, Martin A. T.; Gómez, José F.

2017-02-01

We present the results of our survey of 1612-MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC). We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from infrared (IR) stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust-driven winds: vexp ∝ ZL0.4. We compare the subsolar metallicity LMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic bulge. We derive pulsation periods for eight of the bulge stars for the first time by using near-IR photometry from the Vista Variables in the Via Lactea survey. We have modelled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass-loss rates by scaling the models to the results from maser profiles. We have done this also for samples in the Galactic Centre and bulge and derived a new mass-loss prescription which includes luminosity, pulsation period, and gas-to-dust ratio dot{M} = 1.06^{+3.5}_{-0.8} × }10^{-5 (L/10^4 L_{⊙})^{0.9± 0.1}(P/500 {d})^{0.75± 0.3} (r_gd/200)^{-0.03± 0.07} M⊙ yr-1. The tightest correlation is found between mass-loss rate and luminosity. We find that the gas-to-dust ratio has little effect on the mass-loss of oxygen-rich AGB stars and RSGs within the Galaxy and the LMC. This suggests that the mass-loss of oxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a half and twice solar.

Obscured Activity and Stellar Mass in z~0.7 Post-starburst Galaxies

NASA Astrophysics Data System (ADS)

Rieke, George; Diamond-Stanic, Aleks; Moustakas, John; Tremonti, Christy

2008-03-01

We are proposing 3.6-24 micron imaging of a sample of z~0.7 post-starburst galaxies. These galaxies are presumed to be late-stage mergers that have evolved past their ULIRG/quasar phase and are in transition to becoming early-type galaxies. We have detected outflowing winds with velocities ranging from 500-2200 km/s for 2/3 of the sample, so it is tempting to conclude that these spectacular outflows are the result of feedback from an AGN that has expelled cold gas quenched star formation. However, it is not clear that the existing near-UV and optical data are telling the full story. With Spitzer, we can verify whether or not these 'post-starburst' galaxies are truly quiescent by measuring the amount obscured star formation and black hole activity. We will also be able to determine how significant the recent starburst event was by accurately measuring stellar mass using the red end of the stellar SED. If these galaxies do have non-negligible dust emission, we will be able to use broad-band colors as blunt tools to measure spectral features and compare to known star-forming galaxies, AGNs, and LIRGs. If not, we will have strong evidence that the feedback event has been able to halt galaxy-wide star formation.

Tracers of Star Formation in the Near Infrared

NASA Astrophysics Data System (ADS)

Martins, L.; Ardila, A.; Gruenwald, R.; de Souza, R.

2010-04-01

Starburst features in the optical are nowadays well known, but the use of this knowledge is not always possible (e.g. objects heavily obscured). In this case the near-IR is of unprecedented value. Recent models show that TP-AGB stars should dominate the NIR spectra of populations 0.3 to 2 Gyr old. While the optical spectra is insensitive to the presence of these stars, the near-IR changes dramatically. Not only does the absolute flux in the near-IR is affected, but also peculiar absorption features appear. These features can be used as indicators of 1 Gyr stellar population. In this work we used the IRTF Spex to create the first empirical database of NIR spectra of carefully selected starbursts, to test for the first time and in a consistent way the new stellar population models that account for the TP-AGB. The methodology used is to do stellar population synthesis in the optical and in the NIR, and compare the predictions of both spectral regions. We also compare the strength of important features of the TP-AGB stars, like the CN (1.1 microns) and CO (2.3 microns) bands with optical diagnostics.

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

NASA Astrophysics Data System (ADS)

Sacuto, S.; Ramstedt, S.; Höfner, S.; Olofsson, H.; Bladh, S.; Eriksson, K.; Aringer, B.; Klotz, D.; Maercker, M.

2013-03-01

Aims: We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods: We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results: MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 μm. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 μm visibility measurements for all baselines. Conclusions: This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere

The pathways of C: from AGB stars, to the Interstellar Medium, and finally into the protoplanetary disk

NASA Astrophysics Data System (ADS)

Trigo-Rodriguez, J. M.; Garcia-Hernandez, D. A.

2011-05-01

The origin, and role of C in the formation of first solar system aggregates is described. Stellar grains evidence demonstrates that Asymptotic Giant Branch (AGB) stars were nearby to the solar nebula at the time of solar system formation. Such stars continue to burn H and He in shells that surround the C-O core. During their evolution, flashes occur in the He shell and the C, and O produced are eventually dredged up into the star's envelop and then to the stellar surface, and finally masively ejected to the interstellar medium (IM). Once in a molecular cloud, the electrophilicity of C makes this element reactable with the surrounding gas to produce different molecular species. Primitive meteorites, particularly these known as chondrites, preserved primeval materials of the disk. The abundances of short-lived radionuclides (SLN), inferred to have been present in the early solar system (ESS), are a constraint on the birth and early evolution of the solar system as their relatively short half lives do not allow the observed abundances to be explained by galactic chemical evolution processes. We present a model of a 6.5 solar masses star of solar metallicity that simultaneously match the abundances of SLNs inferred to have been present in the ESS by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of SLNs and consolidation of chondrites equal to 0.53 Myr [2]. Such a polluting source does not overproduce 53Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. The AGB stars released O- and C-rich gas with important oxidizing implications to first solar system materials as recently detected in circumstellar environments [3]. REF: [1] Lada C.J. and Lada E.A. 2003. Ann. Rev. A&A. 41: 57; [2] Trigo-Rodriguez J.M. et al. 2009. MAPS 44: 627; [3] Decin L. et al. 2010. Nature 467: 64.

Finding η Car Analogs in Nearby Galaxies Using Spitzer. II. Identification of An Emerging Class of Extragalactic Self-Obscured Stars

NASA Astrophysics Data System (ADS)

Khan, Rubab; Kochanek, C. S.; Stanek, K. Z.; Gerke, Jill

2015-02-01

Understanding the late-stage evolution of the most massive stars such as η Carinae is challenging because no true analogs of η Car have been clearly identified in the Milky Way or other galaxies. In Khan et al., we utilized Spitzer IRAC images of 7 nearby (lsim 4 Mpc) galaxies to search for such analogs, and found 34 candidates with flat or red mid-IR spectral energy distributions. Here, in Paper II, we present our characterization of these candidates using multi-wavelength data from the optical through the far-IR. Our search detected no true analogs of η Car, which implies an eruption rate that is a fraction 0.01 <~ F <~ 0.19 of the core-collapse supernova (ccSN) rate. This is roughly consistent with each M ZAMS >~ 70 M ⊙ star undergoing one or two outbursts in its lifetime. However, we do identify a significant population of 18 lower luminosity (log (L/L ⊙) ~= 5.5-6.0) dusty stars. Stars enter this phase at a rate that is a fraction 0.09 <~ F <~ 0.55 of the ccSN rate, and this is consistent with all 25 < M ZAMS < 60 M ⊙ stars undergoing an obscured phase at most lasting a few thousand years once or twice. These phases constitute a negligible fraction of post-main-sequence lifetimes of massive stars, which implies that these events are likely to be associated with special periods in the evolution of the stars. The mass of the obscuring material is of order ~M ⊙, and we simply do not find enough heavily obscured stars for theses phases to represent more than a modest fraction (~10% not ~50%) of the total mass lost by these stars. In the long term, the sources that we identified will be prime candidates for detailed physical analysis with the James Webb Space Telescope.

The effects of rotation on the surface composition and yields of low mass AGB stars.

NASA Astrophysics Data System (ADS)

Cristallo, S.; Piersanti, L.; Straniero, O.

Over the past 20 years, stellar evolutionary models have been strongly improved in order to reproduce with reasonable accuracy both photometric and spectroscopic observations. Notwithstanding, the majority of these models do not take into account macroscopic phenomena, like rotation and/or magnetic fields. Their explicit treatment could modify stellar physical and chemical properties. One of the most interesting problems related to stellar nucleosynthesis is the behavior of the s-process spectroscopic indexes ([hs/ls] and [Pb/hs]) in Asymptotic Giant Branch (AGB) stars. In this contribution we show that, for a fixed metallicity, rotation can lead to a spread in the [hs/ls] and [Pb/hs] in low-mass AGB stars. In particular, we demonstrate that the Eddington-Sweet and the Goldreich-Schubert-Fricke instabilities may have enough time to smear the 13C-pocket (the major neutron source) and the 14N-pocket (the major neutron poison). In fact, a different overlap between these pockets leads to a different neutrons-to-seeds ratio, with important consequences on the corresponding s-process distributions. Possible consequences on the chemical evolution of Galactic globular clusters are discussed.

Nucleosynthesis Predictions for Intermediate-Mass AGB Stars: Comparison to Observations of Type I Planetary Nebulae

NASA Technical Reports Server (NTRS)

Karakas, Amanda I.; vanRaai, Mark A.; Lugaro, Maria; Sterling, Nicholas C.; Dinerstein, Harriet L.

2008-01-01

Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of approx. 3-8 Stellar Mass. These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a C-13 pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] less than or approx. 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically less than or approx. 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the greater than or approx. 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for more s-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M grester than or approx.5 Stellar Mass) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3-4 Stellar Mass), if these stars are to evolve into Type I PNe.

Dynamical Model Calculations of AGB Star Winds Including Time Dependent Dust Formation and Non-LTE Radiative Cooling

NASA Astrophysics Data System (ADS)

Schirrmacher, V.; Woitke, P.; Sedlmayr, E.

Stars on the Asymptotic Giant Branch (AGB) are pulsating objects in a late evolutionary stage. The stellar pulsation creates sound waves which steepen up to shock waves in the upper atmosphere and lead to a time dependent levitation of the outer atmosphere. Thereby, the stellar pulsation triggers and facilitates the formation of dust close to the star. The dust is accelerated by radiation pressure and drags the gas outwards due to frictional forces which is identified to provide the basic mass loss mechanism. A longstanding problem concerning the modelling of these physical processes is the influence of the propagating shock waves on the temperature structure of the wind, which strongly influences the dust formation. We have therefore improved our numerical models of AGB-star envelopes by including (i) a detailed calculation of non-LTE radiative heating and cooling rates, predominantly arising from atomic and molecular lines and (ii) atomic and molecular exitation aswell as ionisation and dissociation in the equation of state. First results, presented here, show that the cooling time scales behind the shock waves are usually rather short, but the binding energies of molecular hydrogen provide an important energy buffer capable to delay the radiative heating or cooling. Thus considerable deviations from radiative equilibrium may occur in the important inner dust forming layers.

Processing of Presolar Grains around Post-Asymptotic Giant Branch Stars: Silicon Carbide as the Carrier of the 21 Micron Feature

NASA Astrophysics Data System (ADS)

Speck, Angela K.; Hofmeister, Anne M.

2004-01-01

Some proto-planetary nebulae (PPNs) exhibit an enigmatic feature in their infrared spectra at ~21 μm. This feature is not seen in the spectra of either the precursors to PPNs, the asymptotic giant branch (AGB) stars, or the successors of PPNs, ``normal'' planetary nebulae (PNs). However, the 21 μm feature has been seen in the spectra of PNs with Wolf-Rayet central stars. Therefore, the carrier of this feature is unlikely to be a transient species that only exists in the PPN phase. This feature has been attributed to various molecular and solid-state species, none of which satisfy all constraints, although titanium carbide (TiC) and polycyclic aromatic hydrocarbons (PAHs) have seemed the most viable. We present new laboratory data for silicon carbide (SiC) and show that it has a spectral feature that is a good candidate for the carrier of the 21 μm feature. The SiC spectral feature appears at approximately the same wavelength (depending on the polytype/grain size) and has the same asymmetric profile as the observed astronomical feature. We suggest that processing and cooling of the SiC grains known to exist around carbon-rich AGB stars are responsible for the emergence of the enigmatic 21 μm feature. The emergence of this feature in the spectra of post-AGB stars demonstrates the processing of dust due to the changing physical environments around evolving stars.

FINDING η CAR ANALOGS IN NEARBY GALAXIES USING Spitzer. II. IDENTIFICATION OF AN EMERGING CLASS OF EXTRAGALACTIC SELF-OBSCURED STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khan, Rubab; Kochanek, C. S.; Stanek, K. Z.

Understanding the late-stage evolution of the most massive stars such as η Carinae is challenging because no true analogs of η Car have been clearly identified in the Milky Way or other galaxies. In Khan et al., we utilized Spitzer IRAC images of 7 nearby (≲ 4 Mpc) galaxies to search for such analogs, and found 34 candidates with flat or red mid-IR spectral energy distributions. Here, in Paper II, we present our characterization of these candidates using multi-wavelength data from the optical through the far-IR. Our search detected no true analogs of η Car, which implies an eruption rate that is a fraction 0.01more » ≲ F ≲ 0.19 of the core-collapse supernova (ccSN) rate. This is roughly consistent with each M {sub ZAMS} ≳ 70 M {sub ☉} star undergoing one or two outbursts in its lifetime. However, we do identify a significant population of 18 lower luminosity (log (L/L {sub ☉}) ≅ 5.5-6.0) dusty stars. Stars enter this phase at a rate that is a fraction 0.09 ≲ F ≲ 0.55 of the ccSN rate, and this is consistent with all 25 < M {sub ZAMS} < 60 M {sub ☉} stars undergoing an obscured phase at most lasting a few thousand years once or twice. These phases constitute a negligible fraction of post-main-sequence lifetimes of massive stars, which implies that these events are likely to be associated with special periods in the evolution of the stars. The mass of the obscuring material is of order ∼M {sub ☉}, and we simply do not find enough heavily obscured stars for theses phases to represent more than a modest fraction (∼10% not ∼50%) of the total mass lost by these stars. In the long term, the sources that we identified will be prime candidates for detailed physical analysis with the James Webb Space Telescope.« less

Evolution, Nucleosynthesis, and Yields of AGB Stars at Different Metallicities. III. Intermediate-mass Models, Revised Low-mass Models, and the ph-FRUITY Interface

NASA Astrophysics Data System (ADS)

Cristallo, S.; Straniero, O.; Piersanti, L.; Gobrecht, D.

2015-08-01

We present a new set of models for intermediate-mass asymptotic giant branch (AGB) stars (4.0, 5.0, and 6.0 M⊙) at different metallicities (-2.15 ≤ [Fe/H] ≤ +0.15). This set integrates the existing models for low-mass AGB stars (1.3 ≤ M/M⊙ ≤ 3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the main sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. This effect is due to the fact that the interpulse phases are short and, therefore, thermal pulses (TPs) are weak. Moreover, the high temperature at the base of the convective envelope prevents it from deeply penetrating the underlying radiative layers. Depending on the initial stellar mass, the heavy element nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the 22Ne(α,n)25Mg reaction, which is efficiently activated during TPs. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the influence intermediate-mass models have on the carbon star luminosity function. Finally, we present the upgrade of the FRUITY web interface, which now also includes the physical quantities of the TP-AGB phase for all of the models included in the database (ph-FRUITY).

Molecules and dust in the Large Magellanic Cloud: new colour classifications for post-Main-Sequence stars

NASA Astrophysics Data System (ADS)

Markwick-Kemper, Ciska; Leisenring, Jarron; Meixner, Margaret; van Dyk, Schuyler; Szczerba, Ryszard

In the Large Magellanic Cloud (LMC), as in the Milky Way, dust formation predominantly occurs in the circumstellar environments of evolved stars. The process of dust condensation is not fully understood, and investigating the dust condensation sequence in the low metallicity environment of the LMC (about half of the solar metallicity), may yield additional insights in the dust condensation process. Topics to be studied include the final condensation products, the correlation of the condensation sequence with evolutionary status of the star, degree of crystallinity of the silicates and ratio of carbon-rich dust producing stars. The composition and properties of dust are most easily studied using infrared spectroscopy, and using the high sensitivity of the Infrared Spectrograph (IRS) on board of Spitzer, we were able to observe the thermal emission from circumstellar dust of these stars individually. A sample of 63 post-Main-Sequence stars were selected, using their 2MASS/MSX colours (Egan et al. 2001). We aimed to cover all post-Main-Sequence evolutionary stages, to make an inventory of the dust condensation products, while we placed a certain emphasis on oxygen-rich AGB stars with a intermediate mass-loss rate, to study the existence of a threshold mass-loss rate above which crystalline silicates are observed (Kemper et al. 2001). Here we will present the observed spectra, along with their spectral classification. We find that a large fraction of the stars we observed exhibit spectral features of carbon-rich dust and molecules, such as SiC, C2H2 and MgS. In fact we find that many of these stars are previously classified as oxygen-rich AGB stars, or as OH/IR stars based on their NIR/MIR colours (Egan et al. 2001). These colours are determined for Galactic samples, while in the LMC sample, stars with a carbon-rich chemistry in their outflows occupy a much larger region of the various colour-colour diagrams. In addition, a large fraction of the sample show amorphous

Spectral Analysis of PG 1034+001, the Exciting Star of Hewett 1

NASA Technical Reports Server (NTRS)

Kruk, J. W.; Mahsereci, M.; Ringat, E.; Rauch, T.; Werner, K.

2011-01-01

PG 1034+001 is an extremely hot, helium-rich DO-type star that excites the planetary nebula Hewett 1 and large parts of the surrounding interstellar medium. We present preliminary results of an ongoing spectral analysis by means of non-LTE model atmospheres that consider most elements from hydrogen to nickel. This analysis is based on high-resolution ultraviolet (FUSE, IUE) and optical (VLT/UVES, KECK) data. The results are compared with those of PG 1034+001's spectroscopic twin, the DO star PG 0038+ 199. Keywords. stars: abundances, stars: AGB and post-AGB, stars: atmospheres, stars: evolution, stars: individual (PG 1034+001, PG 0038+ 199), planetary nebulae: individual (Hewett 1)

Uncovering Heavily Obscured AGN with WISE and NuSTAR

NASA Astrophysics Data System (ADS)

Hickox, Ryan C.; Carroll, Christopher M.; Yan, Wei; DiPompeo, Michael A.; Hainline, Kevin N.; NuSTAR Obscured AGN Team

2018-01-01

Supermassive black holes gain their mass through accretion as active galactic nuclei (AGN), but it is now clear that a large fraction of this growth is "hidden" behind large columns of gas and dust. Of particular interest are Compton-thick (CT) AGN, with columns NH > 1024 cm-2, that have been difficult to identify using optical or soft X-ray surveys. We will present two studies of heavily obscured AGN that aim to uncover more of the full population of "hidden" growing black holes: (1) Analysis of the spectral energy distributions of millions of galaxies with photometry from WISE (mid-IR), UKIDSS (near-IR), and SDSS (optical), that uncovers large populations of weak or heavily buried AGN, and (2) NuSTAR observations of a sample of candidate highly obscured AGN, selected from WISE and SDSS photometry,and confirmed using SALT and Keck spectroscopy. The NuSTAR data reveal the existence of powerful CT quasars with extremely large columns NH > 1025 cm-2, which may represent a significant fraction of previously hidden black hole growth. This work is supported by NASA grant numbers NNX16AN48G and NNX15AP24G, and the NSF through grant numbers 1515364 and 1554584.

New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme

NASA Astrophysics Data System (ADS)

Danilovich, T.; Teyssier, D.; Justtanont, K.; Olofsson, H.; Cerrigone, L.; Bujarrabal, V.; Alcolea, J.; Cernicharo, J.; Castro-Carrizo, A.; García-Lario, P.; Marston, A.

2015-09-01

Context. Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission. Aims: We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J = 1 → 0 line to the J = 9 → 8 line, and even the J = 14 → 13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates. Methods: We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind. Results: We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (~10-8 to 2 × 10-5 M⊙ yr-1) and gas expansion velocities (2 to 21.5 km s-1) , and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present "bonus" lines detected during our CO observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Based on observations carried out with the IRAM 30 m Telescope. IRAM is

The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars

NASA Astrophysics Data System (ADS)

Rayner, John T.; Cushing, Michael C.; Vacca, William D.

2009-12-01

We present a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using the Two Micron All Sky Survey photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry. The library is available in digital form from the IRTF Web site.

On the missing second generation AGB stars in NGC 6752

NASA Astrophysics Data System (ADS)

Cassisi, Santi; Salaris, Maurizio; Pietrinferni, Adriano; Vink, Jorick S.; Monelli, Matteo

2014-11-01

In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, it is now thought that basically all globular clusters host multiple stellar populations, each with its own chemical abundance pattern and colour-magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the globular cluster NGC 6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first generation stars, and that all second generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second generation horizontal branch stars - all located at the hot side of the blue and extended horizontal branch of this cluster - possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the asymptotic giant branch phase, thus explaining at the same time the low value of the ratio between horizontal branch and asymptotic giant branch stars (the R2 parameter) observed in NGC 6752. We have critically discussed this mass-loss scenario, finding that the required mass-loss rates are of the order of 10-9 M⊙ yr-1, significantly higher than current theoretical and empirical constraints. By making use of synthetic horizontal branch simulations, we demonstrate that our modelling correctly predicts the R2 parameter for NGC 6752, without the need to invoke very efficient mass loss during the core He-burning stage. As a test of our stellar models we show that we can reproduce the observed value of R2 for both M 3, a cluster of approximately the same metallicity and with a redder horizontal branch morphology, and M 13, a cluster with a horizontal branch very similar to NGC 6752. However, our simulations for the NGC 6752 horizontal branch predict however the presence of a significant fraction of second generation stars (about 50%) along

A view of the H-band light-element chemical patterns in globular clusters under the AGB self-enrichment scenario

NASA Astrophysics Data System (ADS)

Dell'Agli, F.; García-Hernández, D. A.; Ventura, P.; Mészáros, Sz; Masseron, T.; Fernández-Trincado, J. G.; Tang, B.; Shetrone, M.; Zamora, O.; Lucatello, S.

2018-04-01

We discuss the self-enrichment scenario by asymptotic giant branch (AGB) stars for the formation of multiple populations in globular clusters (GCs) by analysing data set of giant stars observed in nine Galactic GCs, covering a wide range of metallicities and for which the simultaneous measurements of C, N, O, Mg, Al, and Si are available. To this aim, we calculated six sets of AGB models, with the same chemical composition as the stars belonging to the first generation of each GC. We find that the AGB yields can reproduce the set of observations available, not only in terms of the degree of contamination shown by stars in each GC but, more important, also the observed trend with metallicity, which agrees well with the predictions from AGB evolution modelling. While further observational evidences are required to definitively fix the main actors in the pollution of the interstellar medium from which new generation of stars formed in GCs, the present results confirm that the gas ejected by stars of mass in the range 4 M_{⊙} ≤ M ≤ 8 M_{⊙} during the AGB phase share the same chemical patterns traced by stars in GCs.

Modelling the carbon AGB star R Sculptoris. Constraining the dust properties in the detached shell based on far-infrared and sub-millimeter observations

NASA Astrophysics Data System (ADS)

Brunner, M.; Maercker, M.; Mecina, M.; Khouri, T.; Kerschbaum, F.

2018-06-01

Context. On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars. Aims: We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass. Methods: We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of Herschel/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass. Results: The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 × 10-10 M⊙ yr-1 and a detached shell dust mass of (2.9 ± 0.3) × 10-5 M⊙. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

From Nuclei to Dust Grains: How the AGB Machinery Works

NASA Astrophysics Data System (ADS)

Gobrecht, D.; Cristallo, S.; Piersanti, L.

2015-12-01

With their circumstellar envelopes AGB stars are marvelous laboratories to test our knowledge of microphysics (opacities, equation of state), macrophysics (convection, rotation, stellar pulsations, magnetic fields) and nucleosynthesis (nuclear burnings, slow neutron capture processes, molecules and dust formation). Due to the completely different environments those processes occur, the interplay between stellar interiors (dominated by mixing events like convection and dredge-up episodes) and stellar winds (characterized by dust formation and wind acceleration) is often ignored. We intend to develop a new approach involving a transition region, taking into consideration hydrodynamic processes which may drive AGB mass-loss. Our aim is to describe the process triggering the mass-loss in AGB stars with different masses, metallicities and chemical enrichments, possibly deriving a velocity field of the outflowing matter. Moreover, we intend to construct an homogeneous theoretical database containing detailed abundances of atomic and molecular species produced by these objects. As a long term goal, we will derive dust production rates for silicates, alumina and silicon carbides, in order to explain laboratory measurements of isotopic ratios in AGB dust grains.

ISO observations of obscured Asymptotic Giant Branch stars in the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Trams, N. R.; van Loon, J. Th.; Waters, L. B. F. M.; Zijlstra, A. A.; Loup, C.; Whitelock, P. A.; Groenewegen, M. A. T.; Blommaert, J. A. D. L.; Siebenmorgen, R.; Heske, A.; Feast, M. W.

1999-06-01

We present ISO photometric and spectroscopic observations of a sample of 57 bright Asymptotic Giant Branch stars and red supergiants in the Large Magellanic Cloud, selected on the basis of IRAS colours indicative of high mass-loss rates. PHOT-P and PHOT-C photometry at 12, 25 and 60 mu m and CAM photometry at 12 mu m are used in combination with quasi-simultaneous ground-based near-IR photometry to construct colour-colour diagrams for all stars in our sample. PHOT-S and CAM-CVF spectra in the 3 to 14 mu m region are presented for 23 stars. From the colour-colour diagrams and the spectra, we establish the chemical types of the dust around 49 stars in this sample. Many stars have carbon-rich dust. The most luminous carbon star in the Magellanic Clouds has also a (minor) oxygen-rich component. OH/IR stars have silicate absorption with emission wings. The unique dataset presented here allows a detailed study of a representative sample of thermal-pulsing AGB stars with well-determined luminosities. This paper is based on observations with the Infrared Space Observatory (ISO). ISO is 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.

On Helium-Dominated Stellar Evolution: The Mysterious Role of the O(He)-Type Stars

NASA Technical Reports Server (NTRS)

Reindl, N.; Rauch, T.; Werner, K.; Kruk, J. W.; Todt, H.

2014-01-01

Context. About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are not yet understood. Aims. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. A precise analysis of their photospheric abundances will establish constraints to their evolution. Methods. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. Results. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf (WD) merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. Conclusions. A helium-dominated stellar evolutionary sequence exists that may be fed by different types of mergers or CE scenarios. It appears likely that all these pass through the O(He) phase just before they become WDs.

Full Stokes IQUV spectropolarimetry of AGB and post-AGB stars: probing surface magnetism and atmospheric dynamics

NASA Astrophysics Data System (ADS)

Lèbre, Agnès; Aurière, Michel; Fabas, Nicolas; Gillet, Denis; Josselin, Eric; Mathias, Philippe; Petit, Pascal

2015-10-01

Full Stokes spectropolarimetric observations of a Mira star (χ Cyg) and a RV Tauri star (R Sct) are presented and analyzed comparatively. From their Stokes V data (circular polarization), we report the detection of a weak magnetic field at the surface of these cool and evolved radially pulsating stars. For both stars, we analyse this detection in the framework of their complex atmospheric dynamics, with the possibility that shock waves may imprint an efficient compressive effect on the surface magnetic field. We also report strong Stokes U and Stokes Q signatures associated to metallic lines (as a global trend), those linear polarimetric features appear to be time variable along the pulsating phase. More surprising, in the Stokes U and Stokes Q data, we also detect signatures associated to individual metallic lines (such as Sr i 460.7 nm, Na D2 588.9 nm), that are known (from the solar case) to be easily polarizable in case of a global asymmetry at the photospheric level.

A NEW GENERATION OF PARSEC-COLIBRI STELLAR ISOCHRONES INCLUDING THE TP-AGB PHASE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marigo, Paola; Aringer, Bernhard; Chen, Yang

2017-01-20

We introduce a new generation of PARSEC–COLIBRI stellar isochrones that includes a detailed treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase, covering a wide range of initial metallicities (0.0001 < Z {sub i} < 0.06). Compared to previous releases, the main novelties and improvements are use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in themore » stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first-overtone modes; and new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the “C-star island” that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6-Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.« less

The adventure of carbon stars. Observations and modeling of a set of C-rich AGB stars

NASA Astrophysics Data System (ADS)

Rau, G.; Hron, J.; Paladini, C.; Aringer, B.; Eriksson, K.; Marigo, P.; Nowotny, W.; Grellmann, R.

2017-04-01

Context. Modeling stellar atmospheres is a complex and intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain the models. Aims: We intend to perform self-consistent modeling of the atmospheres of six carbon-rich AGB stars (R Lep, R Vol, Y Pav, AQ Sgr, U Hya, and X TrA) with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres. Methods: We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent, dynamic model atmospheres. Results: We found that the models can reproduce spectral energy distribution (SED) data well at wavelengths longer than 1 μm, and the interferometric observations between 8 μm and 10 μm. Discrepancies observed at wavelengths shorter than 1 μm in the SED, and longer than 10 μm in the visibilities, could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, and have a prominent shell-like structure. On the contrary, the models best fitting the non-Miras are more compact, showing lower average mass loss. The mass loss is of episodic or multi-periodic nature but causes the visual amplitudes to be notably larger than the observed ones. A number of stellar parameters were derived from the model fitting: TRoss, LRoss, M, C/O, and Ṁ. Our findings agree well with literature values within the uncertainties. TRoss, and LRoss are also in good agreement with the temperature derived from the angular diameter T(θ(V-K)) and the bolometric luminosity from the SED fitting Lbol, except for AQ Sgr. The possible reasons are discussed in the text. Finally, θRoss and θ(V-K) agree with one another better for the Miras than for the non-Miras targets, which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary

Jet creation in post-AGB binaries: the circum-companion accretion disk around BD+46°442

NASA Astrophysics Data System (ADS)

Bollen, Dylan; Van Winckel, Hans; Kamath, Devika

2017-11-01

Aims: We aim at describing and understanding binary interaction processes in systems with very evolved companions. Here, we focus on understanding the origin and determining the properties of the high-velocity outflow observed in one such system. Methods: We present a quantitative analysis of BD+46°442, a post-AGB binary that shows active mass transfer that leads to the creation of a disk-driven outflow or jet. We obtained high-resolution optical spectra from the HERMES spectrograph, mounted on the 1.2 m Flemish Mercator Telescope. By performing a time-series analysis of the Hα profile, we identified the different components of the system. We deduced the jet geometry by comparing the orbital phased data with our jet model. In order to image the accretion disk around the companion of BD+46°442, we applied the technique of Doppler tomography. Results: The orbital phase-dependent variations in the Hα profile can be related to an accretion disk around the companion, from which a high-velocity outflow or jet is launched. Our model shows that there is a clear correlation between the inclination angle and the jet opening angle. The latitudinally dependent velocity structure of our jet model shows a good correspondence to the data, with outflow velocities higher than at least 400 km s-1. The intensity peak in the Doppler map might be partly caused by a hot spot in the disk, or by a larger asymmetrical structure in the disk. Conclusions: We show that BD+46°442 is a result of a binary interaction channel. The origin of the fast outflow in this system might be to a gaseous disk around the secondary component, which is most likely a main-sequence star. Our analysis suggests that the outflow has a rather wide opening angle and is not strongly collimated. Our time-resolved spectral monitoring reveals the launching site of the jet in the binary BD+46°442. Similar orbital phase-dependent Hα profiles are commonly observed in post-AGB binaries. Post-AGB binaries provide ideal

To be or not to be Asymmetric? VLTI/MIDI and the Mass-loss Geometry of AGB Stars

NASA Astrophysics Data System (ADS)

Paladini, C.; Klotz, D.; Sacuto, S.; Lagadec, E.; Wittkowski, M.; Richichi, A.; Hron, J.; Jorissen, A.; Groenewegen, M. A. T.; Kerschbaum, F.; Verhoelst, T.; Rau, G.; Olofsson, H.; Zhao-Geisler, R.; Matter, A.

2017-06-01

The Mid-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) has been used to spatially resolve the dust-forming region of 14 asymptotic giant branch (AGB) stars with different chemistry (O-rich and C-rich) and variability types (Miras, semi-regular, and irregular variables). The main goal of the programme was to detect deviations from spherical symmetry in the dust-forming region of these stars. All the stars of the sample are well resolved with the VLTI, and five are asymmetric and O-rich. This finding contrasts with observations in the near-infrared, where the C-rich objects are found to be more asymmetric than the O-rich ones. The nature of the asymmetric structures so far detected (dusty discs versus blobs)remains uncertain and will require imaging on milli-arcsecond scales.

Dusty Mass Loss from Galactic Asymptotic Giant Branch Stars

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, Sundar; Meixner, Margaret; Kastner, Joel H.

2016-06-01

We are probing how mass loss from Asymptotic Giant Branch (AGB) stars depends upon their metallicity. Asymptotic giant branch (AGB) stars are evolved stars that eject large parts of their mass in outflows of dust and gas in the final stages of their lives. Our previous studies focused on mass loss from AGB stars in lower metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). In our present study, we analyze AGB star mass loss in the Galaxy, with special attention to the Bulge, to investigate how mass loss differs in an overall higher metallicity environment. We construct radiative transfer models of the spectral energy distributions (SEDs) of stars in the Galaxy identified as AGB stars from infrared and optical surveys. Our Magellanic Cloud studies found that the AGB stars with the highest mass loss rates tended to have outflows with carbon-rich dust, and that overall more carbon-rich (C-rich) dust than oxygen-rich (O-rich) was produced by AGB stars in both LMC and SMC. Our radiative transfer models have enabled us to determine reliably the dust chemistry of the AGB star from the best-fit model. For our Galactic sample, we are investigating both the dust chemistries of the AGB stars and their mass-loss rates, to compare the balance of C-rich dust to O-rich dust between the Galactic bulge and the Magellanic Clouds. We are also constructing detailed dust opacity models of AGB stars in the Galaxy for which we have infrared spectra; e.g., from the Spitzer Space Telescope Infrared Spectrograph (IRS). This detailed dust modeling of spectra informs our choice of dust properties to use in radiative transfer modeling of SEDs of Galactic AGB stars. BAS acknowledges funding from NASA ADAP grant NNX15AF15G.

Innocent Bystanders and Smoking Guns: Dwarf Carbon Stars

NASA Astrophysics Data System (ADS)

Green, Paul J.

2014-01-01

As far as we know, most carbon throughout the Universe is created and dispersed by AGB stars. So it was at first surprising to find that the carbon stars most prevalent in the Galaxy are in fact dwarfs. We suspect that dC stars are most likely innocent bystanders in post-mass transfer binaries, and may be predominantly metal-poor. Among 1200 C stars found in the SDSS (Green 2013), we confirm 724 dCs, of which a dozen are DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. The dCs likely span absolute magnitudes M_i from about 6.5 to 10.5. G-type dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C_2 bands. Eleven very red C stars with strong red CN bands appear to be N-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Le A. Two such stars within 30arcmin of each other may trace a previously unidentified dwarf galaxy or tidal stream at ~40 kpc. We describe follow-up projects to study the spatial, kinematic, and binary properties of these C-enriched dwarfs.

Are TiC Grains a Carrier of the 21 Micron Emission Band Observed around Post-Asymptotic Giant Branch Objects?

NASA Astrophysics Data System (ADS)

Chigai, Takeshi; Yamamoto, Tetsuo; Kaito, Chihiro; Kimura, Yuki

2003-04-01

The carrier of the 21 μm band observed in post-asymptotic giant branch (post-AGB) stars is examined. We analyze the infrared spectra of the TiC clusters measured by von Helden et al. in 2000 and determine the absorption efficiency Q in the 21 μm band. Using Q, we estimate the Ti/Si abundance ratios needed to realize the flux ratios of the 21 and 11 μm emission observed in the infrared spectra of the post-AGB stars exhibiting both 21 and 11 μm emission. In view of the nature of the TiC condensation by which TiC grains are quickly mantled by graphite, we calculate the emission spectra of the graphite-coated TiC grains and other possible types of core-mantle grains and compare with the observed spectra. Both the abundance and condensation considerations strongly suggest that TiC is an implausible carrier of the observed infrared 21 μm feature around carbon-rich post-AGB stars.

An Imperfectly Passive Nature: Bright Submillimeter Emission from Dust-obscured Star Formation in the z = 3.717 “Passive” System, ZF 20115

NASA Astrophysics Data System (ADS)

Simpson, J. M.; Smail, Ian; Wang, Wei-Hao; Riechers, D.; Dunlop, J. S.; Ao, Y.; Bourne, N.; Bunker, A.; Chapman, S. C.; Chen, Chian-Chou; Dannerbauer, H.; Geach, J. E.; Goto, T.; Harrison, C. M.; Hwang, H. S.; Ivison, R. J.; Kodama, Tadayuki; Lee, C.-H.; Lee, H.-M.; Lee, M.; Lim, C.-F.; Michałowski, M. J.; Rosario, D. J.; Shim, H.; Shu, X. W.; Swinbank, A. M.; Tee, W.-L.; Toba, Y.; Valiante, E.; Wang, Junxian; Zheng, X. Z.

2017-07-01

The identification of high-redshift, massive galaxies with old stellar populations may pose challenges to some models of galaxy formation. However, to securely classify a galaxy as quiescent, it is necessary to exclude significant ongoing star formation, something that can be challenging to achieve at high redshifts. In this Letter, we analyze deep ALMA/870 μm and SCUBA-2/450 μm imaging of the claimed “post-starburst” galaxy ZF 20115 at z = 3.717 that exhibits a strong Balmer break and absorption lines. The rest-frame far-infrared imaging identifies a luminous starburst 0.″4 ± 0.″1 (˜3 kpc in projection) from the position of the ultraviolet/optical emission and is consistent with lying at the redshift of ZF 20115. The star-forming component, with an obscured star formation rate of {100}-70+15 {M}⊙ {{yr}}-1, is undetected in the rest-frame ultraviolet but contributes significantly to the lower angular resolution photometry at rest-frame wavelengths ≳3500 Å. This contribution from the obscured starburst, especially in the Spitzer/IRAC wavebands, significantly complicates the determination of a reliable stellar mass for the ZF 20015 system, and we conclude that this source does not pose a challenge to current models of galaxy formation. The multi-wavelength observations of ZF 20115 unveil a complex system with an intricate and spatially varying star formation history. ZF 20115 demonstrates that understanding high-redshift obscured starbursts will only be possible with multi-wavelength studies that include high-resolution observations, available with the James Webb Space Telescope, at mid-infrared wavelengths.

Galaxy and mass assembly (GAMA): dust obscuration in galaxies and their recent star formation histories

NASA Astrophysics Data System (ADS)

Wijesinghe, D. B.; Hopkins, A. M.; Sharp, R.; Gunawardhana, M.; Brough, S.; Sadler, E. M.; Driver, S.; Baldry, I.; Bamford, S.; Liske, J.; Loveday, J.; Norberg, P.; Peacock, J.; Popescu, C. C.; Tuffs, R. J.; Bland-Hawthorn, J.; Cameron, E.; Croom, S.; Frenk, C.; Hill, D.; Jones, D. H.; van Kampen, E.; Kelvin, L.; Kuijken, K.; Madore, B.; Nichol, B.; Parkinson, H.; Pimbblet, K. A.; Prescott, M.; Robotham, A. S. G.; Seibert, M.; Simmat, E.; Sutherland, W.; Taylor, E.; Thomas, D.

2011-02-01

We present self-consistent star formation rates derived through pan-spectral analysis of galaxies drawn from the Galaxy and Mass Assembly (GAMA) survey. We determine the most appropriate form of dust obscuration correction via application of a range of extinction laws drawn from the literature as applied to Hα, [O II] and UV luminosities. These corrections are applied to a sample of 31 508 galaxies from the GAMA survey at z < 0.35. We consider several different obscuration curves, including those of Milky Way, Calzetti and Fischera & Dopita curves and their effects on the observed luminosities. At the core of this technique is the observed Balmer decrement, and we provide a prescription to apply optimal obscuration corrections using the Balmer decrement. We carry out an analysis of the star formation history (SFH) using stellar population synthesis tools to investigate the evolutionary history of our sample of galaxies as well as to understand the effects of variation in the initial mass function (IMF) and the effects this has on the evolutionary history of galaxies. We find that the Fischera & Dopita obscuration curve with an Rv value of 4.5 gives the best agreement between the different SFR indicators. The 2200 Å feature needed to be removed from this curve to obtain complete consistency between all SFR indicators suggesting that this feature may not be common in the average integrated attenuation of galaxy emission. We also find that the UV dust obscuration is strongly dependent on the SFR.

The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

NASA Astrophysics Data System (ADS)

Lansbury, G. B.; Alexander, D. M.; Aird, J.; Gandhi, P.; Stern, D.; Koss, M.; Lamperti, I.; Ajello, M.; Annuar, A.; Assef, R. J.; Ballantyne, D. R.; Baloković, M.; Bauer, F. E.; Brandt, W. N.; Brightman, M.; Chen, C.-T. J.; Civano, F.; Comastri, A.; Del Moro, A.; Fuentes, C.; Harrison, F. A.; Marchesi, S.; Masini, A.; Mullaney, J. R.; Ricci, C.; Saez, C.; Tomsick, J. A.; Treister, E.; Walton, D. J.; Zappacosta, L.

2017-09-01

We identify sources with extremely hard X-ray spectra (I.e., with photon indices of {{Γ }}≲ 0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at > 10 {keV}. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; {N}{{H}}> 1.5× {10}24 cm-2) AGNs at low redshift (z< 0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (< 10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from ≈ 5× {10}42 to 1045 erg s-1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z< 0.07), where we measure a high CT fraction of {f}{CT}{obs}={30}-12+16 % . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of “normal” AGNs.

First evidence of multiple populations along the AGB from Strömgren photometry

NASA Astrophysics Data System (ADS)

Gruyters, Pieter; Casagrande, Luca; Milone, Antonino P.; Hodgkin, Simon T.; Serenelli, Aldo; Feltzing, Sofia

2017-07-01

Spectroscopic studies have demonstrated that nearly all Galactic globular clusters (GCs) harbour multiple stellar populations with different chemical compositions. Moreover, colour-magnitude diagrams based exclusively on Strömgrem photometry have allowed us to identify and characterise multiple populations along the RGB of a large number of clusters. In this paper we show for the first time that Strömgren photometry is also very efficient at identifying multiple populations along the AGB, and demonstrate that the AGB of M 3, M 92, NGC 362, NGC 1851, and NGC 6752 are not consistent with a single stellar population. We also provide a catalogue of RGB and AGB stars photometrically identified in these clusters for further spectroscopic follow-up studies. We combined photometry and elemental abundances from the literature for RGB and AGB stars in NGC 6752 where the presence of multiple populations along the AGB has been widely debated. We find that, while the MS, SGB, and RGB host three stellar populations with different helium and light element abundances, only two populations of AGB stars are present in the cluster. These results are consistent with standard evolutionary theory. Based on observations made with the Isaac Newton Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.Full Tables B.1 and B.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/603/A37

THE S{sup 4}G PERSPECTIVE ON CIRCUMSTELLAR DUST EXTINCTION OF ASYMPTOTIC GIANT BRANCH STARS IN M100

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meidt, Sharon E.; Schinnerer, Eva; Munoz-Mateos, Juan-Carlos

2012-04-01

We examine the effect of circumstellar dust extinction on the near-IR (NIR) contribution of asymptotic giant branch (AGB) stars in intermediate-age clusters throughout the disk of M100. For our sample of 17 AGB-dominated clusters we extract optical-to-mid-IR spectral energy distributions (SEDs) and find that NIR brightness is coupled to the mid-IR dust emission in such a way that a significant reduction of AGB light, of up to 1 mag in the K band, follows from extinction by the dust shell formed during this stage. Since the dust optical depth varies with AGB chemistry (C-rich or O-rich), our results suggest thatmore » the contribution of AGB stars to the flux from their host clusters will be closely linked to the metallicity and the progenitor mass of the AGB star, to which dust chemistry and mass-loss rate are sensitive. Our sample of clusters-each the analogue of a {approx}1 Gyr old post-starburst galaxy-has implications within the context of mass and age estimation via SED modeling at high-z: we find that the average {approx}0.5 mag extinction estimated here may be sufficient to reduce the AGB contribution in the (rest-frame) K band from {approx}70%, as predicted in the latest generation of synthesis models, to {approx}35%. Our technique for selecting AGB-dominated clusters in nearby galaxies promises to be effective for discriminating the uncertainties associated with AGB stars in intermediate-age populations that plague age and mass estimation in high-z galaxies.« less

On the Evolution of O(He)-Type Stars

NASA Technical Reports Server (NTRS)

Kruk, Jeffrey W.; Reindl, N.; Rauch, T.; Werner, K.

2012-01-01

O(He) stars represent a small group of four very hot post-AGB stars whose atmospheres are composed of almost pure helium. Their evolution deviates from the hydrogen-deficient post-AGO evolutionary sequence of carbon-dominated stars like e.g. PG 1159 or Wolf- Rayet stars. While (very) late thermal pulse evolutionary models can explain the observed He/C/O abundances in these objects, they do not reproduce He-dominated surface abundances. Currently it seems most likely that the O(He) stars originate from a double helium white dwarf merger and so they could be the successors of the luminous helium-rich sdO-stars. An other possibility is that O(He)-stars could be successors of RCB or EHe stars.

Tracking the Obscured Star Formation Along the Complete Evolutionary Merger Sequence of LIRGs

NASA Astrophysics Data System (ADS)

Diaz-Santos, Tanio

2014-10-01

We propose to obtain WFC3 narrow-band Pa-beta imaging of a sample of 24 nearby luminous infrared (IR) galaxies (LIRGs) from the Great Observatories All-sky LIRG survey (GOALS) selected to be in advanced stages of interaction. LIRGs account for half of the obscured star formation of the Universe at z ~ 1-2, and they represent a key population in galaxy formation and evolution. We will use the Pa-beta images to trace the ionized gas in LIRGs and study its spatial distribution from scales of ~ 100 pc to up to several kpc, probing the youngest, massive stars formed in the most buried environments of LIRGs due to the interaction process. This will allow us to measure how the gas in the center of mergers is converted into stars, which eventually leads to the build-up of a nuclear stellar cusp and the "inside-out" growth of bulges. We will also create spatially-resolved Pa-beta equivalent width maps to search for age gradients across the galaxies and correlate the distribution of Pa-beta emission with that of un-obscured star clusters detected in the UV and optical with HST on the same spatial scales. Finally, we will combine our data with previous studies mainly focused on isolated and early-stage interacting LIRG systems to analyze the size and compactness of the starburst along the complete merger sequence of LIRGs. The requested data represent a critical missing piece of information that will allow us to understand both the physics of merger-induced star formation and the applicability of local LIRGs as templates for high-z interacting starburst galaxies.

GRAMS: A Grid of RSG and AGB Models

NASA Astrophysics Data System (ADS)

Srinivasan, S.; Sargent, B. A.; Meixner, M.

2011-09-01

We present a grid of oxygen- and carbon-rich circumstellar dust radiative transfer models for asymptotic giant branch (AGB) and red supergiant (RSG) stars. The grid samples a large region of the relevant parameter space, and it allows for a quick calculation of bolometric fluxes and dust mass-loss rates from multi-wavelength photometry. This method of fitting observed spectral energy distributions (SEDs) is preferred over detailed radiative transfer calculations, especially for large data sets such as the SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Magellanic Clouds. The mass-loss rates calculated for SAGE data will allow us to quantify the dust returned to the interstellar medium (ISM) by the entire AGB population. The total injection rate provides an important constraint for models of galactic chemical evolution. Here, we discuss our carbon star models and compare the results to SAGE observations in the Large Magellanic Cloud (LMC).

High-Speed Bullet Ejections during the AGB to Planetary Nebula Transition: A Study of the Carbon Star V Hydrae

NASA Astrophysics Data System (ADS)

Sahai, Raghvendra

2017-08-01

The carbon star V Hya is experiencing heavy mass loss as it undergoes the transition from an AGB star to a planetary nebula (PN). This is possibly the earliest object known in this brief phase, which is so short that few nearby stars are likely to be caught in the act. Molecular observations reveal that a bipolar nebula has been established even at this early stage. Using STIS, we obtained high spatial-resolution long-slit optical spectra of V Hya spanning 3 epochs spaced apart by a year during each of two periods (2002-2004, 2011-2013). These data reveal high-velocity emission in [SII] lines from compact blobs located both on- and off-source, with the ejection axis executing a flip-flop, both in, and perpendicular to, the sky-plane. We have proposed a detailed model in which V Hya ejects high-speed (200-250 km/s) bullets once every 8.5 yr associated with periastron passage of a binary companion in an eccentric orbit with an 8.5 yr period. We suggest that the jet driver is an accretion disk (produced by gravitational capture of material from the primary) that is warped and precessing. Our model predicts the locations of previously ejected bullets in V Hya and future epochs at which new bullets will emerge. We now propose new STIS observations of these remarkable bullet ejections over two new epochs well separated from previous ones, to robustly test our model. The proposed observations will provide us with an unprecedented opportunity to look on as V Hya's circumstellar envelope is sculpted by these bullets. Our study will help solve the long-standing puzzle of how the spherical mass-loss envelopes of AGB stars evolve into bipolar and multipolar PNe.

IRC +10 216 in 3-D: morphology of a TP-AGB star envelope

PubMed Central

Guélin, M.; Patel, N.A.; Bremer, M.; Cernicharo, J.; Castro-Carrizo, A.; Pety, J.; Fonfría, J.P.; Agúndez, M.; Santander-García, M.; Quintana-Lacaci, G.; Velilla Prieto, L.; Blundell, R.; Thaddeus, P.

2017-01-01

During their late pulsating phase, AGB stars expel most of their mass in the form of massive dusty envelopes, an event that largely controls the composition of interstellar matter. The envelopes, however, are distant and opaque to visible and NIR radiation: their structure remains poorly known and the mass-loss process poorly understood. Millimeter-wave interferometry, which combines the advantages of longer wavelength, high angular resolution and very high spectral resolution is the optimal investigative tool for this purpose. Mm waves pass through dust with almost no attenuation. Their spectrum is rich in molecular lines and hosts the fundamental lines of the ubiquitous CO molecule, allowing a tomographic reconstruction of the envelope structure. The circumstellar envelope IRC +10 216 and its central star, the C-rich TP-AGB star closest to the Sun, are the best objects for such an investigation. Two years ago, we reported the first detailed study of the CO(2-1) line emission in that envelope, made with the IRAM 30-m telescope. It revealed a series of dense gas shells, expanding at a uniform radial velocity. The limited resolution of the telescope (HPBW 11″) did not allow us to resolve the shell structure. We now report much higher angular resolution observations of CO(2-1), CO(1-0), CN(2-1) and C4H(24-23) made with the SMA, PdB and ALMA interferometers (with synthesized half-power beamwidths of 3″, 1″ and 0.3″, respectively). Although the envelope appears much more intricate at high resolution than with an 11″ beam, its prevailing structure remains a pattern of thin, nearly concentric shells. The average separation between the brightest CO shells is 16″ in the outer envelope, where it appears remarkably constant. Closer to the star (< 40″), the shell pattern is denser and less regular, showing intermediary arcs. Outside the small (r < 0.3″) dust formation zone, the gas appears to expand radially at a constant velocity, 14.5 km s−1, with small

Rings in Evolved Stars: Fingerprints of Their Mass-Loss History

NASA Astrophysics Data System (ADS)

Ramos-Larios, Gerardo; Santamaria, Edgar; Sabin, Laurence; Guerrero, Martin; Marquez-Lugo, Alejandro

2015-08-01

The majority of intermediate mass evolved stars i.e. asymptotic giant branch (AGB) stars, post-AGB and pre-planetary nebulae (PPN) are well known for been characterized by external structures such as knots, arcs, ansae, jets, haloes, shells and even annular enhancements in intensity -features which are commonly referred to as rings. These are well described either as spherical bubbles of periodic isotropic nuclear mass pulsations (Balick, Wilson & Hajian 2001) or projections of spherical shells onto the plane of the sky by Kwok (2001).These interesting structures are part of the AGB wind, suggesting that this wind comes in a series of semi periodic lapses, indicating that the outflow has quasi-periodic oscillations.After an extensive analysis in the Hubble Space Telescope (HST) archives we found new ring-like structures in several evolved stars. Following the image analysis procedure described by Corradi et al. (2004), and using unsharp masking techniques it was possible to enhance the ring structures, and to obtain an effective removal of the underlying halo emission.Our new findings will help first to constrain the physical processes responsible for the rings creation and then to better understand the mass loss activity in these evolved stars.

Accurate abundance determinations in S stars

NASA Astrophysics Data System (ADS)

Neyskens, P.; Van Eck, S.; Plez, B.; Goriely, S.; Siess, L.; Jorissen, A.

2011-12-01

S-type stars are thought to be the first objects, during their evolution on the asymptotic giant branch (AGB), to experience s-process nucleosynthesis and third dredge-ups, and therefore to exhibit s-process signatures in their atmospheres. Until present, the modeling of these processes is subject to large uncertainties. Precise abundance determinations in S stars are of extreme importance for constraining e.g., the depth and the formation of the 13C pocket. In this paper a large grid of MARCS model atmospheres for S stars is used to derive precise abundances of key s-process elements and iron. A first estimation of the atmospheric parameters is obtained using a set of well-chosen photometric and spectroscopic indices for selecting the best model atmosphere of each S star. Abundances are derived from spectral line synthesis, using the selected model atmosphere. Special interest is paid to technetium, an element without stable isotopes. Its detection in stars is considered as the best possible signature that the star effectively populates the thermally-pulsing AGB (TP-AGB) phase of evolution. The derived Tc/Zr abundances are compared, as a function of the derived [Zr/Fe] overabundances, with AGB stellar model predictions. The computed [Zr/Fe] overabundances are in good agreement with the AGB stellar evolution model predictions, while the Tc/Zr abundances are slightly over-predicted. This discrepancy can help to set stronger constraints on nucleosynthesis and mixing mechanisms in AGB stars.

Disentangling Dominance: Obscured AGN Activity versus Star Formation in BPT-Composites

NASA Astrophysics Data System (ADS)

Trouille, Laura

2011-11-01

Approximately 20% of SDSS emission-line galaxies (ELG) lie in the BPT-comp regime, between the Kauffmann et al. (2003) empirically determined SF-dominated regime and the Kewley et al. (2001) theoretically predicted AGN-dominated regime. BPT-AGN, on the other hand, make up only 11% of the ELG population. Whether to include the significant number of BPT-comp in samples of AGN or samples of star-forming galaxies is an open question and has important implications for galaxy evolution studies, metallicity studies, etc. Using a large pectroscopic sample of GOODS-N and LH galaxies with deep Chandra imaging, we perform an X-ray stacking analysis of BPT-comp. We find the stacked signal to be X-ray hard. This X-ray hardness can be indicative of obscured AGN activity or the presence of HMXBs associated with ongoing star formation. In order to distinguish between these scenarios, we perform an IR stacking analysis using Spitzer 24 micron data. The stacked BPT-comp lies well above the expected value for L_x/L_IR for pure star-forming galaxies; similarly for the X-ray detected BPT-comp. We also find that the BPT-comp lie in the AGN-dominated regime of our new TBT diagnostic, which uses [NeIII]/[OII] versus rest-frame g-z colour to identify AGN and star forming galaxies out to z=1.4. [NeIII], which has a higher ionisation potential than other commonly used forbidden emission lines, appears to foster a more reliable selection of AGN-dominated galaxies. These findings suggest that both the X-ray and optical signal in BPT-comp are dominated by obscured or low accretion rate AGN activity rather than star formation. This is in contrast to claims by previous optical emission-line studies that the signal in BPT-comp is dominated by star-formation activity. Therefore, we recommend that groups carefully consider the impact of excluding or including BPT-comp on the interpretation of their results. For example, for studies involving determining the bolometric contribution from AGN activity

s-Process in low metallicity Pb stars.

NASA Astrophysics Data System (ADS)

Bisterzo, S.; Gallino, R.; Straniero, O.; Ivans, I. I.; Käppeler, F.; Aoki, W.

We consider a sample of very metal-poor, C-rich, s-rich and lead-rich stars observed at high-resolution spectroscopy, and some recent spectroscopic data of C+s-rich stars obtained at moderate resolution. The spectroscopic data of these stars are interpreted with AGB theoretical models of different 13C-pocket efficiencies, initial mass and initial r-enrichment. When lead is not measured we give our theoretical prediction. The observed stars are not on the AGB phase, but are main sequence or giant stars. They acquired the C and s enrichments by mass transfer in a close binary system from the more massive companion while on the AGB (now a white dwarf). A considerable fraction of the stars show both high s and r enrichments. To explain the s+r enriched stars we assume a parental cloud already enriched in r-elements. The measurement of Nb is an indicator of an extrinsic AGB in a binary system. The intrinsic indicator [hs/ls] constrains the initial mass, while [Pb/hs] and [Pb/ls] are a measure of the s-process efficiency. The apparent discrepancies of C and N abundances may be reconciled by assuming a strong cool bottom process occurring during the AGB. An important primary production of light elements, from Ne to Si, increasing with the star mass, is predicted for AGB models at very low metallicity, induced by n capture on primary 22Ne and its progenies.

ON IRON MONOXIDE NANOPARTICLES AS A CARRIER OF THE MYSTERIOUS 21 μm EMISSION FEATURE IN POST-ASYMPTOTIC GIANT BRANCH STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Aigen; Jiang, B. W.; Liu, J. M., E-mail: lia@missouri.edu, E-mail: bjiang@bnu.edu.cn

2013-11-10

A prominent mysterious emission feature peaking at ∼20.1 μm—historically known as the '21 μm' feature—is seen in over two dozen Galactic and Magellanic Cloud carbon-rich, post-asymptotic giant branch (post-AGB) stars. The nature of its carrier remains unknown since the first detection of the 21 μm feature in 1989. Over a dozen materials have been suggested as possible carrier candidates. However, none of them has been accepted: they either require too much material (compared to what is available in the circumstellar shells around these post-AGB stars), or exhibit additional emission features that are not seen in these 21 μm sources. Recently,more » iron monoxide (FeO) nanoparticles seem to be a promising carrier candidate as Fe is an abundant element and FeO emits exclusively at ∼21 μm. In this work, using the proto-typical protoplanetary nebula HD 56126 as a test case, we examine FeO nanoparticles as a carrier for the 21 μm feature by modeling their infrared emission, with FeO being stochastically heated by single stellar photons. We find that FeO emits too broad a 21 μm feature to explain that observed and the Fe abundance required to be locked up in FeO exceeds what is available in HD 56126. We therefore conclude that FeO nanoparticles are not likely to be responsible for the 21 μm feature.« less

Examining the infrared variable star population discovered in the Small Magellanic Cloud using the SAGE-SMC survey

DOE Office of Scientific and Technical Information (OSTI.GOV)

Polsdofer, Elizabeth; Marengo, M.; Seale, J.

2015-02-01

We present our study on the infrared variability of point sources in the Small Magellanic Cloud (SMC). We use the data from the Spitzer Space Telescope Legacy Program “Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud” (SAGE-SMC) and the “Spitzer Survey of the Small Magellanic Cloud” (S{sup 3}MC) survey, over three different epochs, separated by several months to 3 years. Variability in the thermal infrared is identified using a combination of Spitzer’s InfraRed Array Camera 3.6, 4.5, 5.8, and 8.0 μm bands, and the Multiband Imaging Photometer for Spitzer 24 μm band. Anmore » error-weighted flux difference between each pair of three epochs (“variability index”) is used to assess the variability of each source. A visual source inspection is used to validate the photometry and image quality. Out of ∼2 million sources in the SAGE-SMC catalog, 814 meet our variability criteria. We matched the list of variable star candidates to the catalogs of SMC sources classified with other methods, available in the literature. Carbon-rich Asymptotic Giant Branch (AGB) stars make up the majority (61%) of our variable sources, with about a third of all of our sources being classified as extreme AGB stars. We find a small, but significant population of oxygen-rich (O-rich) AGB (8.6%), Red Supergiant (2.8%), and Red Giant Branch (<1%) stars. Other matches to the literature include Cepheid variable stars (8.6%), early type stars (2.8%), Young-stellar objects (5.8%), and background galaxies (1.2%). We found a candidate OH maser star, SSTISAGE1C J005212.88-730852.8, which is a variable O-rich AGB star, and would be the first OH/IR star in the SMC, if confirmed. We measured the infrared variability of a rare RV Tau variable (a post-AGB star) that has recently left the AGB phase. 59 variable stars from our list remain unclassified.« less

A Method to Estimate the Masses of Asymptotic Giant Branch Variable Stars

NASA Astrophysics Data System (ADS)

Takeuti, Mine; Nakagawa, Akiharu; Kurayama, Tomoharu; Honma, Mareki

2013-06-01

AGB variable stars are at the transient phase between low and high mass-loss rates; estimating the masses of these stars is necessary to study the evolutionary processes and mass-loss processes during the AGB stage. We applied the pulsation constant theoretically derived by Xiong and Deng (2007 MNRAS, 378, 1270) to 15 galactic AGB stars in order to estimate their masses. We found that using the pulsation constant is effective to estimate the mass of a star pulsating with two different pulsation modes, such as S Crt and RX Boo, which provides mass estimates comparable to theoretical results of AGB star evolution. We also extended the use of the pulsation constant to single-mode variables, and analyzed the properties of AGB stars related to their masses.

Probing Collimated Jets and Dusty Waists in Dying Stars with Keck LGSAO

NASA Astrophysics Data System (ADS)

Sahai, R.; Le Mignant, D.; Sanchez Contreras, C.; Stute, M.; Morris, M.

2005-12-01

The shaping of planetary nebulae (PNs) is probably the most exciting yet least understood problem in the late evolution of intermediate mass stars. PNs evolve from the envelopes of AGB stars via a supposedly short ( ˜1000 yr) pre-planetary nebula (PPN) phase. HST imaging of PPNs and PNs has shown the widespread presence of diverse bipolar and multipolar morphologies. In 1998, in a radical departure from the long-standing theoretical paradigm for PN formation, Sahai & Trauger proposed that as most stars evolve off the AGB, they drive collimated fast winds that sweep up and shock the AGB circumstellar envelope, producing the observed dramatic changes in circumstellar geometry and kinematics from the AGB to the PN phase. The search for these collimated jets has proved to be rather elusive, partly because these are most likely episodic and operate only for a few x 100 years in the early PPN phase. During this phase, much of the circumstellar environment, including the central dusty waist of these nebulae, is optically-thick at visible wavelengths. We are therefore carrying out a program of observing PPNs with the LGSAO system on Keck II at near-infrared (1.1-4.7 micron) wavelengths. Our very first attempt met with remarkable success -- observations of the bipolar young PPN, IRAS16342-3814, revealed a remarkable corkscrew-shaped structure apparently etched into the lobe walls -- direct signature of an underlying precessing jet. Here we present results from new high-resolution (55 mas at 2 micron) observations of a small sample of PPNs with the LGSAO system. As in their HST images, our objects display bipolar/multipolar morphologies, but in addition, the bubble-like ``wind-swept" structure of the lobes is clearly revealed. Furthermore, the dusty waists appear much thinner geometrically than in the HST images, but surprisingly, in some PPNs, the central stars still remain obscured, with important implications for the poorly-known physical structure of the waists. We

Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968

NASA Astrophysics Data System (ADS)

LaMassa, Stephanie M.; Yaqoob, Tahir; Levenson, N. A.; Boorman, Peter; Heckman, Timothy M.; Gandhi, Poshak; Rigby, Jane R.; Urry, C. Megan; Ptak, Andrew F.

2017-01-01

We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z = 0.00986) Seyfert 2 galaxy. We discover extended (˜1 kpc) X-ray emission in the soft band (0.5-2 keV) that is neither coincident with the narrow line region nor the extended radio emission. Based on spectral modeling, it is linked to on-going star formation (˜2.6-4 M⊙ yr-1). The soft emission at circumnuclear scales (inner ˜400 pc) originates from hot gas, with kT ˜ 0.7 keV, while the most extended thermal emission is cooler (kT ˜ 0.3 keV). We refine previous measurements of the extreme Fe Kα equivalent width in this source ({EW}={2.5}-1.0+2.6 {keV}), which suggests the central engine is completely embedded within Compton-thick levels of obscuration. Using physically motivated models fit to the Chandra spectrum, we derive a Compton-thick column density (NH > 1.25 × 1024 cm-2) and an intrinsic hard (2-10 keV) X-ray luminosity of ˜3-8 × 1042 erg s-1 (depending on the presumed geometry of the obscurer), which is over two orders of magnitude larger than that observed. The large Fe Kα EW suggests a spherical covering geometry, which could be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other active galaxies that exhibit extreme Fe Kα EWs (I.e., >2 keV) in that they also contain on-going star formation. This work supports the idea that gas associated with nuclear star formation may increase the covering factor of the enshrouding gas and play a role in obscuring active galactic nuclei.

Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968

NASA Technical Reports Server (NTRS)

LaMassa, Stephanie M.; Yaqoob, Tahir; Levenson, N. A.; Boorman, Peter; Heckman, Timothy M.; Gandhi, Poshak; Rigby, Jane R.; Urry, C. Megan; Ptak, Andrew F.

2017-01-01

We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z = 0.00986) Seyfert 2 galaxy. We discover extended (approx. 1 kpc) X-ray emission in the soft band (0.5-2 keV) that is neither coincident with the narrow line region nor the extended radio emission. Based on spectral modeling, it is linked to on-going star formation [approx. 2.6-4 Mass compared to Earth yr(exp.- 1)]. The soft emission at circumnuclear scales (inner approx. 400 pc) originates from hot gas, with kT approx. 0.7 keV, while the most extended thermal emission is cooler (kT approx. 0.3 keV). We refine previous measurements of the extreme Fe K alpha equivalent width in this source (EW 2.5 + 2.6/-1.0 keV), which suggests the central engine is completely embedded within Compton-thick levels of obscuration. Using physically motivated models fit to the Chandra spectrum, we derive a Compton-thick column density [N(sub H) is greater than 1.25× 10(exp 24) cm(exp.- 2)] and an intrinsic hard (2-10 keV) X-ray luminosity of approx. 3-8× 10(exp. 42) erg s(exp. - 1) (depending on the presumed geometry of the obscurer), which is over two orders of magnitude larger than that observed. The large Fe K Alpha EW suggests a spherical covering geometry, which could be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other active galaxies that exhibit extreme Fe K Alpha EWs (i.e., greater than 2 keV) in that they also contain on-going star formation. This work supports the idea that gas associated with nuclear star formation may increase the covering factor of the enshrouding gas and play a role in obscuring active galactic nuclei.

Water isotopologues in the circumstellar envelopes of M-type AGB stars

NASA Astrophysics Data System (ADS)

Danilovich, T.; Lombaert, R.; Decin, L.; Karakas, A.; Maercker, M.; Olofsson, H.

2017-06-01

Aims: In this study we intend to examine rotational emission lines of two isotopologues of water: H217O and H218O. By determining the abundances of these molecules, we aim to use the derived isotopologue - and hence oxygen isotope - ratios to put constraints on the masses of a sample of M-type AGB stars that have not been classified as OH/IR stars. Methods: We have used detailed radiative transfer analysis based on the accelerated lambda iteration method to model the circumstellar molecular line emission of H217O and H218O for IK Tau, R Dor, W Hya, and R Cas. The emission lines used to constrain our models came from Herschel/HIFI and Herschel/PACS observations and are all optically thick, meaning that full radiative transfer analysis is the only viable method of estimating molecular abundance ratios. Results: We find generally low values of the 17O/18O ratio for our sample, ranging from 0.15 to 0.69. This correlates with relatively low initial masses, in the range 1.0 to 1.5 M⊙ for each source, based on stellar evolutionary models. We also find ortho-to-para ratios close to 3, which are expected from warm formation predictions. Conclusions: The 17O/18O ratios found for this sample are at the lower end of the range predicted by stellar evolutionary models, indicating that the sample chosen had relatively low initial masses. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

The AGB star nucleosynthesis in the light of the recent {sup 17}O(p,α){sup 14}N and {sup 18}O(p,α){sup 15}N reaction rate determinations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palmerini, S.; Sergi, M. L.; La Cognata, M.

2015-02-24

Presolar grains form in the cold and dusty envelopes of Asymptotic Giant Branch (AGB) stars. These solides, once that have been ejected by stellar winds, come to us as inclusions in meteorites providing invaluable benchmarks and constraints for our knowledge of low temeperature H-burning in stars. The Trojan Horse Method (THM) has been used to investigate the low-energy cross sections of the {sup 17}O(p,α){sup 14}N and {sup 18}O(p,α){sup 15}N reactions. Moreover, the strength of the 65 keV resonance in the {sup 17}O(p,α){sup 14}N reaction, measured by means of the THM, has been used to renormalize the corresponding resonance strength inmore » the {sup 17}O+p radiative capture channel. The new estimates of the reaction rates have been introduced into calculations of AGB star nucleosynthesis and the results have been compared with geochemical analysis of 'presolar' grains to determine their impact on astrophysical environments.« less

Infrared Studies of the Variability and Mass Loss of Dusty Asymptotic Giant Branch Stars in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Sargent, Benjamin; Groenewegen, M. A. T.

2018-01-01

The asymptotic giant branch (AGB) phase is one of the last phases of a star's life. AGB stars lose mass in an outflow in which dust condenses and is pushed away from the star. Extreme AGB stars are so named because their very red colors suggest very large amounts of dust, which in turn suggests extremely high mass loss rates. AGB stars also vary in brightness, and studies show that extreme AGB stars tend to have longer periods than other AGB stars and are more likely to be fundamental mode pulsators than other AGB stars. Extreme AGB stars are difficult to study, as their colors are so red due to their copious amounts of circumstellar dust that they are often not detected at optical wavelengths. Therefore, they must be observed at infrared wavelengths to explore their variability. Using the Spitzer Space Telescope, my team and I have observed a sample of extreme AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) over Cycles 9 through 12 during the Warm Spitzer mission. For each cycle, we typically observed a set of extreme AGB stars at both 3.6 and 4.5 microns wavelength approximately monthly for most of a year. These observations reveal a wide range of variability properties. I present results from our analysis of the data obtained from these Spitzer variability programs, including light curve analyses and comparison to period-luminosity diagrams. Funding is acknowledged from JPL RSA # 1561703.

Probing Dust Formation Around Evolved Stars with Near-Infrared Interferometry

NASA Astrophysics Data System (ADS)

Sargent, B.; Srinivasan, S.; Riebel, D.; Meixner, M.

2014-09-01

Near-infrared interferometry holds great promise for advancing our understanding of the formation of dust around evolved stars. For example, the Magdalena Ridge Observatory Interferometer (MROI), which will be an optical/near-infrared interferometer with down to submilliarcsecond resolution, includes studying stellar mass loss as being of interest to its Key Science Mission. With facilities like MROI, many questions relating to the formation of dust around evolved stars may be probed. How close to an evolved star such as an asymptotic giant branch (AGB) or red supergiant (RSG) star does a dust grain form? Over what temperature ranges will such dust form? How does dust formation temperature and distance from star change as a function of the dust composition (carbonaceous versus oxygen-rich)? What are the ranges of evolved star dust shell geometries, and does dust shell geometry for AGB and RSG stars correlate with dust composition, similar to the correlation seen for planetary nebula outflows? At what point does the AGB star become a post-AGB star, when dust formation ends and the dust shell detaches? Currently we are conducting studies of evolved star mass loss in the Large Magellanic Cloud using photometry from the Surveying the Agents of a Galaxy's Evolution (SAGE; PI: M. Meixner) Spitzer Space Telescope Legacy program. We model this mass loss using the radiative transfer program 2Dust to create our Grid of Red supergiant and Asymptotic giant branch ModelS (GRAMS). For simplicity, we assume spherical symmetry, but 2Dust does have the capability to model axisymmetric, non-spherically-symmetric dust shell geometries. 2Dust can also generate images of models at specified wavelengths. We discuss possible connections of our GRAMS modeling using 2Dust of SAGE data of evolved stars in the LMC and also other data on evolved stars in the Milky Way's Galactic Bulge to near-infrared interferometric studies of such stars. By understanding the origins of dust around evolved

Kinematics of Hα Emitting Stars in Andromeda

NASA Astrophysics Data System (ADS)

Ilango, Megha; Ilango, Anita; Damon, Gabriel; Prichard, Laura; Guhathakurta, Puragra; PHAT Collaboration; SPLASH Collaboration

2017-01-01

Studying emission line stars helps improve our understanding of stellar evolution, types of stars, and their environments. In this study, we analyzed stars exhibiting Hα emission (Hα stars) in the Andromeda Galaxy. We used a combination of spectroscopic and photometric diagnostic methods to remove a population of foreground Milky Way (MW) star contaminants from our data set. The Hα stars were selected from a sample of 5295 spectra from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH) survey and accompanying photometric data from the Panchromatic Hubble Andromeda Treasury (PHAT) survey. Velocities of two classes of Hα stars, main sequence (MS) stars and asymptotic giant branch (AGB) stars, were analyzed through a novel Age-Velocity Difference Correlation (AVDC) method, which utilizes line-of-sight velocity differences (LOSVDs) in order to estimate the age of a rare stellar population. Histograms, weighted means, and weighted standard deviations of the LOSVDs were used to conclude that MS stars are more kinematically coherent than AGB stars, and that Hα stars are kinematically comparable and thus close in age to their non-Hα counterparts. With these results, it can definitively be inferred that mass loss is important in two stages of stellar evolution: massive MS and intermediate mass AGB. We hypothesized that this mass loss could either occur as a normal part of MS and AGB evolution, or that it could be emitted by only a subpopulation of MS and AGB stars throughout their life cycle. Our use of the novel AVDC method sets a precedent for the use of similar methods in predicting the ages of rare stellar subgroups.This research was supported by NASA and the National Science Foundation. Most of this work was carried out by high school students working under the auspices of the Science Internship Program at UC Santa Cruz.

IRC +10 216 in 3D: morphology of a TP-AGB star envelope

NASA Astrophysics Data System (ADS)

Guélin, M.; Patel, N. A.; Bremer, M.; Cernicharo, J.; Castro-Carrizo, A.; Pety, J.; Fonfría, J. P.; Agúndez, M.; Santander-García, M.; Quintana-Lacaci, G.; Velilla Prieto, L.; Blundell, R.; Thaddeus, P.

2018-02-01

During their late pulsating phase, AGB stars expel most of their mass in the form of massive dusty envelopes, an event that largely controls the composition of interstellar matter. The envelopes, however, are distant and opaque to visible and NIR radiation: their structure remains poorly known and the mass-loss process poorly understood. Millimeter-wave interferometry, which combines the advantages of longer wavelength, high angular resolution and very high spectral resolution is the optimal investigative tool for this purpose. Mm waves pass through dust with almost no attenuation. Their spectrum is rich in molecular lines and hosts the fundamental lines of the ubiquitous CO molecule, allowing a tomographic reconstruction of the envelope structure. The circumstellar envelope IRC +10 216 and its central star, the C-rich TP-AGB star closest to the Sun, are the best objects for such an investigation. Two years ago, we reported the first detailed study of the CO(2-1) line emission in that envelope, made with the IRAM 30-m telescope. It revealed a series of dense gas shells, expanding at a uniform radial velocity. The limited resolution of the telescope (HPBW 11″) did not allow us to resolve the shell structure. We now report much higher angular resolution observations of CO(2-1), CO(1-0), CN(2-1) and C4H(24-23) made with the SMA, PdB and ALMA interferometers (with synthesized half-power beamwidths of 3″, 1″ and 0.3″, respectively). Although the envelope appears much more intricate at high resolution than with an 11″ beam, its prevailing structure remains a pattern of thin, nearly concentric shells. The average separation between the brightest CO shells is 16″ in the outer envelope, where it appears remarkably constant. Closer to the star (<40″), the shell pattern is denser and less regular, showing intermediary arcs. Outside the small (r< 0.3'') dust formation zone, the gas appears to expand radially at a constant velocity, 14.5 km s-1, with small

The circumstellar envelope of the C-rich post-AGB star HD 56126

NASA Astrophysics Data System (ADS)

Hony, S.; Tielens, A. G. G. M.; Waters, L. B. F. M.; de Koter, A.

2003-04-01

We present a detailed study of the circumstellar envelope of the post-asymptotic giant branch ``21 mu m object'' HD 56126. We build a detailed dust radiative transfer model of the circumstellar envelope in order to derive the dust composition and mass, and the mass-loss history of the star. To model the emission of the dust we use amorphous carbon, hydrogenated amorphous carbon, magnesium sulfide and titanium carbide. We present a detailed parametrisation of the optical properties of hydrogenated amorphous carbon as a function of H/C content. The mid-infrared imaging and spectroscopy is best reproduced by a single dust shell from 1.2 to 2.6'' radius around the central star. This shell originates from a short period during which the mass-loss rate exceeded 10-4 Msun/yr. We find that the strength of the ``21'' mu m feature poses a problem for the TiC identification. The low abundance of Ti requires very high absorption cross-sections in the ultraviolet and visible wavelength range to explain the strength of the feature. Other nano-crystalline metal carbides should be considered as well. We find that hydrogenated amorphous carbon in radiative equilibrium with the local radiation field does not reach a high enough temperature to explain the strength of the 3.3-3.4 and 6-9 mu m hydrocarbon features relative to the 11-17 mu m hydrocarbon features. We propose that the carriers of these hydrocarbon features are not in radiative equilibrium but are transiently heated to high temperature. We find that 2 per cent of the dust mass is required to explain the strength of the ``30'' mu m feature, which fits well within the measured atmospheric abundance of Mg and S. This further strengthens the MgS identification of the ``30'' mu m feature. Based on observations taken at the European Southern Observatory, La Silla, Chile and observation obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, The Netherlands and

Is There a Metallicity Ceiling to Form Carbon Stars? - A Novel Technique Reveals a Scarcity of C-Stars in the Inner M31 Disk

NASA Technical Reports Server (NTRS)

Boyer, Martha L.; Girardi, L.; Marigo, P.; Williams, B. F.; Aringer, B.; Nowotny, W.; Rosenfield, P.; Dorman, C. E.; Guhathakurta, P.; Dalcanton, J. J.;

EVIDENCE OF AGB POLLUTION IN GALACTIC GLOBULAR CLUSTERS FROM THE Mg–Al ANTICORRELATIONS OBSERVED BY THE APOGEE SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ventura, P.; Dell’Agli, F.; D’Antona, F.

We study the formation of multiple populations in globular clusters (GCs), under the hypothesis that stars in the second generation formed from the winds of intermediate-mass stars, ejected during the asymptotic giant branch (AGB) phase, possibly diluted with pristine gas, sharing the same chemical composition of first-generation stars. To this aim, we use the recent Apache Point Observatory Galactic Evolution Experiment (APOGEE) data, which provide the surface chemistry of a large sample of giant stars, belonging to clusters that span a wide metallicity range. The APOGEE data set is particularly suitable to discriminate among the various pollution scenarios proposed somore » far, as it provides the surface abundances of Mg and Al, the two elements involved in a nuclear channel extremely sensitive to the temperature, hence to the metallicity of the polluters. The present analysis shows a remarkable agreement between the observations and the theoretical yields from massive AGB stars. In particular, the observed extension of the depletion of Mg and O and the increase in Al is well reproduced by the models and the trend with the metallicity is also fully accounted for. This study further supports the idea that AGB stars were the key players in the pollution of the intra-cluster medium, from which additional generations of stars formed in GCs.« less

Differential Depletion of Mg and Fe in Planetary Nebulae: Implications for the Composition of AGB-Star Dust

NASA Astrophysics Data System (ADS)

Dinerstein, Harriet L.; Prasla, F.; Speck, A. K.

2012-01-01

We have investigated the gas-phase abundances of Mg and Fe, both refractory elements that are potentially major constituents of silicates and other minerals, for 25 planetary nebulae. The Mg abundances are derived from Mg II 4481 A, a recombination line of Mg++; we obtain Fe/H from [Fe III] 4658 A, after correcting for ionization structure. We find strikingly different behavior for the two elements. Fe is deficient by factors of 20-200 relative to solar, presumably due to incorporation into dust that condensed while the star was on the Asymptotic Giant Branch (AGB). On the other hand, Mg/H is virtually solar, implying that Mg is at most minimally depleted. This result is surprising since some of the nebulae display mid-infrared emission features often attributed to forsterite, the pure-Mg form of crystalline olivine. If this identification is correct, there must be only a small mass of Mg-rich crystalline silicate dust, coexisting with a larger amount of Fe-rich amorphous silicates or another Fe-bearing material. Another possibility is that the observed features might actually arise from Fe-rich crystalline silicates such as fayalite, which provide a good fit to the spectra of some AGB stars (Pitman et al. 2010, MNRAS, 406, 460; Guha Nigoya et al. 2011, ApJ, 733, 93). Finally, our Mg abundances are based on an optical recombination line (ORL), and such lines from C, N, O, Ne tend to be anomalously strong in nebulae. Although empirically Mg does not correlate with the ORL abundance discrepancy (Barlow et al. 2003, ASPC, 209, 273; Wang & Liu 2007, MNRAS, 381, 669), solving the origin of the ORL effect would increase our confidence in our Mg/H values. This work was supported by NSF grants AST-0708245 to HLD and CAREER AST-0642991 to AKS, and Big XII Faculty Fellowships to both.

An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-redshift AGB Analogs

NASA Astrophysics Data System (ADS)

Boyer, M. L.; McQuinn, K. B. W.; Groenewegen, M. A. T.; Zijlstra, A. A.; Whitelock, P. A.; van Loon, J. Th.; Sonneborn, G.; Sloan, G. C.; Skillman, E. D.; Meixner, M.; McDonald, I.; Jones, O. C.; Javadi, A.; Gehrz, R. D.; Britavskiy, N.; Bonanos, A. Z.

2017-12-01

The survey for DUST in Nearby Galaxies with Spitzer (DUSTiNGS) identified several candidate Asymptotic Giant Branch (AGB) stars in nearby dwarf galaxies and showed that dust can form even in very metal-poor systems ({\\boldsymbol{Z}}∼ 0.008 {Z}ȯ ). Here, we present a follow-up survey with WFC3/IR on the Hubble Space Telescope (HST), using filters that are capable of distinguishing carbon-rich (C-type) stars from oxygen-rich (M-type) stars: F127M, F139M, and F153M. We include six star-forming DUSTiNGS galaxies (NGC 147, IC 10, Pegasus dIrr, Sextans B, Sextans A, and Sag DIG), all more metal-poor than the Magellanic Clouds and spanning 1 dex in metallicity. We double the number of dusty AGB stars known in these galaxies and find that most are carbon rich. We also find 26 dusty M-type stars, mostly in IC 10. Given the large dust excess and tight spatial distribution of these M-type stars, they are most likely on the upper end of the AGB mass range (stars undergoing Hot Bottom Burning). Theoretical models do not predict significant dust production in metal-poor M-type stars, but we see evidence for dust excess around M-type stars even in the most metal-poor galaxies in our sample (12+{log}({{O}}/{{H}})=7.26{--}7.50). The low metallicities and inferred high stellar masses (up to ∼10 {M}ȯ ) suggest that AGB stars can produce dust very early in the evolution of galaxies (∼30 Myr after they form), and may contribute significantly to the dust reservoirs seen in high-redshift galaxies. Based on observations made with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-14073.

An observational estimate of the probability of encounters between mass-losing evolved stars and molecular clouds

NASA Astrophysics Data System (ADS)

Kastner, Joel H.; Myers, P. C.

1994-02-01

One hypothesis for the elevated abundance of Al-26 present during the formation of the solar system is that an asymptotic giant branch (AGB) star expired within the molecular cloud (MC) containing the protosolar nebula. To test this hypothesis for star-forming clouds at the present epoch, we compared nearly complete lists of rapidly mass-losing AGB stars and MCs in the solar neighborhood and identified those stars which are most likely to encounter a nearby cloud. Roughly 10 stars satisfy our selection criteria. We estimated probabilities of encounter for these stars from the position of each star relative to cloud CO emission and the likely star-cloud distance along the line of sight. Typical encounter probabilities are approximately 1%. The number of potential encounters and the probability for each star-cloud pair to result in an encounter suggests that within 1 kpc of the Sun, there is a approximately 1% chance that a given cloud will be visited by a mass-losing AGB star over the next million years. The estimate is dominated by the possibility of encounters involving the stars IRC +60041 and S Cep. Over a MC lifetime, the probability for AGB encounter may be as high as approximately 70%. We discuss the implications of these results for theories of AL-26 enrichment of processed and unprocessed meteoritic inclusions. If the Al-26 in either type of inclusion arose from AGB-MC interaction, the low probability estimated here seems to require that AGB-MC encounters trigger multiple star formation and/or that the production rate of AGB stars was higher during the epoch of solar system formation than at present. Various lines of evidence suggest only the more massive (5-8 solar mass) AGB stars can produce significant AL-26 enrichment of star-forming clouds.

Dust Production and Mass Loss in Cool Evolved Stars

NASA Technical Reports Server (NTRS)

Boyer, M. L.

2013-01-01

Following the red giant branch phase and the subsequent core He-burning phase, the low- to intermediate-mass stars (0.8AGB). Pulsations levitate material from the stellar surface and provide density enhancements and shocks, which can encourage dust formation and re-processing. The dust composition depends on the atmospheric chemistry (abundance of carbon relative to oxygen), which is altered by dredging up newly formed carbon to the surface of the star. I will briefly review the current status of models that include AGB mass loss and relate them to recent observations of AGB stars from the Surveying the Agents of Galaxy Evolution (SAGE) Spitzer surveys of the Small and Large Magellanic Clouds, including measures of the total dust input to the interstellar medium from AGB stars.

Using a Weak CN Spectral Feature as a Marker for Massive AGB Stars in the Andromeda Galaxy

NASA Astrophysics Data System (ADS)

Guhathakurta, Puragra; Kamath, Anika; Sales, Alyssa; Sarukkai, Atmika; Hays, Jon; PHAT Collaboration; SPLASH Collaboration

2017-01-01

The Panchromatic Hubble Andromeda Treasury (PHAT) survey has produced six-filter photometry at near-ultraviolet, optical and nearly infrared wavelengths (F275W, F336W, F475W, F814W, F110W and F160W) for over 100 million stars in the disk of the of the Andromeda galaxy (M31). As part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey, medium resolution (R ~ 2000) spectra covering the wavelength range 4500-9500A were obtained for over 5000 relatively bright stars from the PHAT source catalog using the Keck II 10-meter telescope and DEIMOS spectrograph. While searching for carbon stars in the spectroscopic data set, we discovered a rare population of stars that show a weak CN spectral absorption feature at ~7900A (much weaker than the CN feature in typical carbon stars) along with other spectral absorption features like TiO and the Ca triplet that are generally not present/visible in carbon star spectra but that are typical for normal stars with oxygen rich atmospheres. These 150 or so "weak CN" stars appear to be fairly localized in six-filter space (i.e., in various color-color and color-magnitude diagrams) but are generally offset from carbon stars. Comparison to PARSEC model stellar tracks indicates that these weak CN stars are probably massive (5-10 Msun) asymptotic giant branch (AGB) stars in a relatively short-lived core helium burning phase of their evolution. Careful spectroscopic analysis indicates that the details of the CN spectral feature are about 3-4x weaker in weak CN stars than in carbon stars. The kinematics of weak CN stars are similar to those of other young stars (e.g., massive main sequence stars) and reflect the well ordered rotation of M31's disk.This research project is funded in part by NASA/STScI and the National Science Foundation. Much of this work was carried out by high school students and undergraduates under the auspices of the Science Internship Program and LAMAT program at the University of

A Search for Nitrogen-enhanced Metal-poor Stars

NASA Astrophysics Data System (ADS)

Johnson, Jennifer A.; Herwig, Falk; Beers, Timothy C.; Christlieb, Norbert

2007-04-01

Theoretical models of very metal-poor intermediate-mass asymptotic giant branch (AGB) stars predict a large overabundance of primary nitrogen. The very metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the polluted companions of now extinct AGB stars, provide direct tests of the predictions of these models. Recent studies of the carbon and nitrogen abundances in metal-poor stars have focused on the most carbon-rich stars, leading to a potential selection bias against stars that have been polluted by AGB stars that produced large amounts of nitrogen and hence have small [C/N] ratios. We call these stars nitrogen-enhanced metal-poor (NEMP) stars and define them as having [N/Fe]>+0.5 and [C/N]<-0.5. In this paper we report on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three of which have [C/Fe]<+2.0. If NEMP stars were made as easily as carbon-enhanced metal-poor (CEMP) stars, then we expected to find between two and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore, this observational bias is not an important contributor to the apparent dearth of N-rich stars. Our [C/N] values are in the same range as values reported previously in the literature (-0.5 to +2.0), and all stars are in disagreement with the predicted [C/N] ratios for both low- and high-mass AGB stars. We suggest that the decrease in [C/N] from the low-mass AGB models is due to enhanced extramixing, while the lack of NEMP stars may be caused by unfavorable mass ratios in binaries or the difficulty of mass transfer in binary systems with large mass ratios. Based on observations obtained at Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory, a division of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

Sublimating comets as the source of nucleation seeds for grain condensation in the gas outflow from AGB stars

NASA Technical Reports Server (NTRS)

Whitmire, D. P.; Matese, John J.; Reynolds, R. T.

1989-01-01

A growing amount of observational and theoretical evidence suggests that most main sequence stars are surrounded by disks of cometary material. The dust production by comets in such disks is investigated when the central stars evolve up the red giant and asymptotic giant branch (AGB). Once released, the dust is ablated and accelerated by the gas outflow and the fragments become the seeds necessary for condensation of the gas. The origin of the requisite seeds has presented a well known problem for classical nucleation theory. This model is consistent with the dust production observed in M giants and supergiants (which have increasing luminosities) and the fact that earlier supergiants and most WR stars (whose luminosities are unchanging) do not have significant dust clouds even though they have significant stellar winds. Another consequence of the model is that the spatial distribution of the dust does not, in general, coincide with that of the gas outflow, in contrast to the conventional condensation model. A further prediction is that the condensation radius is greater that that predicted by conventional theory which is in agreement with IR interferometry measurements of alpha-Ori.

R CORONAE BOREALIS STARS ARE VIABLE FACTORIES OF PRE-SOLAR GRAINS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karakas, Amanda I.; Ruiter, Ashley J.; Hampel, Melanie, E-mail: amanda.karakas@anu.edu.au

2015-08-20

We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB stars to be ≈25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon–oxygen and helium-rich white dwarfs. Our new RCB birthrate (1.8 × 10{sup −3} yr{sup −1}) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180–540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, wemore » calculate the total dust production from RCB stars and compare this rate to dust production from novae and born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.« less

Seven new carbon-enhanced metal-poor RR Lyrae stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kennedy, Catherine R.; Stancliffe, Richard J.; Kuehn, Charles

2014-05-20

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

Determining the forsterite abundance of the dust around asymptotic giant branch stars

NASA Astrophysics Data System (ADS)

de Vries, B. L.; Min, M.; Waters, L. B. F. M.; Blommaert, J. A. D. L.; Kemper, F.

2010-06-01

Aims: We present a diagnostic tool to determine the abundance of the crystalline silicate forsterite in AGB stars surrounded by a thick shell of silicate dust. Using six infrared spectra of high mass-loss oxygen rich AGB stars we obtain the forsterite abundance of their dust shells. Methods: We use a monte carlo radiative transfer code to calculate infrared spectra of dust enshrouded AGB stars. We vary the dust composition, mass-loss rate and outer radius. We focus on the strength of the 11.3 and the 33.6 μm forsterite bands, that probe the most recent (11.3 μm) and older (33.6 μm) mass-loss history of the star. Simple diagnostic diagrams are derived, allowing direct comparison to observed band strengths. Results: Our analysis shows that the 11.3 μm forsterite band is a robust indicator for the forsterite abundance of the current mass-loss period for AGB stars with an optically thick dust shell. The 33.6 μm band of forsterite is sensitive to changes in the density and the geometry of the emitting dust shell, and so a less robust indicator. Applying our method to six high mass-loss rate AGB stars shows that AGB stars can have forsterite abundances of 12% by mass and higher, which is more than the previously found maximum abundance of 5%.

Spitzer-IRS Spectroscopic Studies of Oxygen-Rich Asymptotic Giant Branch Star and Red Supergiant Star Dust Properties

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, Sundar; Speck, Angela; Volk, Kevin; Kemper, Ciska; Reach, William T.; Lagadec, Eric; Bernard, Jean-Philippe; McDonald, Iain; Meixner, Margaret

2015-01-01

We analyze the dust emission features seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of Oxygen-rich (O-rich) asymptotic giant branch (AGB) and red supergiant (RSG) stars. The spectra come from the Spitzer Legacy program SAGE-Spectroscopy (PI: F. Kemper) and other archival Spitzer-IRS programs. The broad 10 and 20 micron emission features attributed to amorphous dust of silicate composition seen in the spectra show evidence for systematic differences in the centroid of both emission features between O-rich AGB and RSG populations. Radiative transfer modeling using the GRAMS grid of models of AGB and RSG stars suggests that the centroid differences are due to differences in dust properties. We present an update of our investigation of differences in dust composition, size, shape, etc that might be responsible for these spectral differences. We explore how these differences may arise from the different circumstellar environments around RSG and O-rich AGB stars. BAS acknowledges funding from NASA ADAP grant NNX13AD54G.

CHEMICAL ANALYSIS OF ASYMPTOTIC GIANT BRANCH STARS IN M62

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lapenna, E.; Mucciarelli, A.; Ferraro, F. R.

2015-11-10

We have collected UVES-FLAMES high-resolution spectra for a sample of 6 asymptotic giant branch (AGB) and 13 red giant branch (RGB) stars in the Galactic globular cluster (GC) M62 (NGC 6266). Here we present the detailed abundance analysis of iron, titanium, and light elements (O, Na, Mg, and Al). For the majority (five out of six) of the AGB targets, we find that the abundances of both iron and titanium determined from neutral lines are significantly underestimated with respect to those obtained from ionized features, the latter being, instead, in agreement with those measured for the RGB targets. This ismore » similar to recent findings in other clusters and may suggest the presence of nonlocal thermodynamic equilibrium (NLTE) effects. In the O–Na, Al–Mg, and Na–Al planes, the RGB stars show the typical correlations observed for GC stars. Instead, all the AGB targets are clumped in the regions where first-generation stars are expected to lie, similar to what was recently found for the AGB population of NGC 6752. While the sodium and aluminum abundances could be underestimated as a consequence of the NLTE bias affecting iron and titanium, the oxygen line used does not suffer from the same effects, and the lack of O-poor AGB stars therefore is a solid result. We can thus conclude that none of the investigated AGB stars belongs to the second stellar generation of M62. We also find an RGB star with extremely high sodium abundance ([Na/Fe] = +1.08 dex)« less

Cool Bottom Processing on the AGB and Presolar Grain Compositions

NASA Technical Reports Server (NTRS)

Nollett, Kenneth M.; Busso, M.; Wasserburg, G. J.

2002-01-01

We describe results from a model of cool bottom processing (CBP) in AGB (asymptotic giant branch) stars. We predict O, Al, C and N isotopic compositions of circumstellar grains. Measured compositions of mainstream SiC grains and many oxide grains are consistent with CBP. Additional information is contained in the original extended abstract.

The LF of TP-AGB stars in the LMC/SMC

NASA Technical Reports Server (NTRS)

Bruzual, Gustavo; Charlot, Stephane; GonzalezLopezlira, Rosa; Srinivasan, Sundar; Boyer, Martha L.

2013-01-01

We show that Monte Carlo simulations of the TP-AGB stellar population in the LMC and SMC galaxies using the CB. models produce LF and color distributions that are in closer agreement with observations than those obtained with the BC03 and CB07 models. This is a progress report of work that will be published elsewhere.

Barium Stars and Thermohaline Mixing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Husti, Laura

2008-01-24

Barium stars are formed in binary systems through mass transfer from the carbon and s-element rich primary in the AGB phase, to the secondary star which is in a less evolved evolutionary stage. The mixing of the accreted material from the AGB donor with the envelope of the secondary results in a dilution of the s-element abundances. Dilution in red giants is explained by the occurence of the first dredge up, while in case of dwarfs thermohaline mixing would determine it. A comparison between the theoretical predictions of the AGB stellar models and the spectroscopical observations of a large samplemore » of barium stars has been made. Dilution due to thermohaline mixing was taken into account when searching for best fits of the observational data. The importance of thermohaline mixing in barium dwarfs is discussed.« less

An interferometric study of the post-AGB binary 89 Herculis. I. Spatially resolving the continuum circumstellar environment at optical and near-IR wavelengths with the VLTI, NPOI, IOTA, PTI, and the CHARA Array

NASA Astrophysics Data System (ADS)

Hillen, M.; Verhoelst, T.; Van Winckel, H.; Chesneau, O.; Hummel, C. A.; Monnier, J. D.; Farrington, C.; Tycner, C.; Mourard, D.; ten Brummelaar, T.; Banerjee, D. P. K.; Zavala, R. T.

2013-11-01

Context. Binary post-asymptotic giant branch (post-AGB) stars are interesting laboratories to study both the evolution of binaries as well as the structure of circumstellar disks. Aims: A multiwavelength high angular resolution study of the prototypical object 89 Herculis is performed with the aim of identifying and locating the different emission components seen in the spectral energy distribution. Methods: A large interferometric data set, collected over the past decade and covering optical and near-infrared wavelengths, is analyzed in combination with the spectral energy distribution and flux-calibrated optical spectra. In this first paper only simple geometric models are applied to fit the interferometric data. Combining the interferometric constraints with the photometry and the optical spectra, we re-assess the energy budget of the post-AGB star and its circumstellar environment. Results: We report the first (direct) detection of a large (35-40%) optical circumstellar flux contribution and spatially resolve its emission region. Given this large amount of reprocessed and/or redistributed optical light, the fitted size of the emission region is rather compact and fits with(in) the inner rim of the circumbinary dust disk. This rim dominates our K band data through thermal emission and is rather compact, emitting significantly already at a radius of twice the orbital separation. We interpret the circumstellar optical flux as due to a scattering process, with the scatterers located in the extremely puffed-up inner rim of the disk and possibly also in a bipolar outflow seen pole-on. A non-local thermodynamic equilibrium gaseous origin in an inner disk cannot be excluded but is considered highly unlikely. Conclusions: This direct detection of a significant amount of circumbinary light at optical wavelengths poses several significant questions regarding our understanding of both post-AGB binaries and the physics in their circumbinary disks. Although the

The Class of Jsolated Stars and Luminous Planetary Nebulae in old stellar populations

NASA Astrophysics Data System (ADS)

Sabach, Efrat; Soker, Noam

2018-06-01

We suggest that stars whose angular momentum (J) does not increase by a companion, star or planet, along their post-main sequence evolution have much lower mass loss rates along their giant branches. Their classification to a separate group can bring insight on their late evolution stages. We here term these Jsolated stars. We argue that the mass loss rate of Jsolated stars is poorly determined because the mass loss rate expressions on the giant branches are empirically based on samples containing stars that experience strong binary interaction, with stellar or sub-stellar companions, e.g., planetary nebula (PN) progenitors. We use our earlier claim for a low mass loss rate of asymptotic giant branch (AGB) stars that are not spun-up by a stellar or substellar companion to show that we can account for the enigmatic finding that the brightest PNe in old stellar populations reach the same luminosity as the brightest PNe in young populations. It is quite likely that the best solution to the existence of bright PNe in old stellar populations is the combination of higher AGB luminosities, as obtained in some new stellar models, and the lower mass loss rates invoked here.

Exceptional Stars Origins, Companions, Masses and Planets

NASA Technical Reports Server (NTRS)

Kulkarni, Shrinivas R.; Hansen, Bradley M. S.; Phinney, Sterl; vanKerkwijk, Martin H.; Vasisht, Gautam

2004-01-01

As SIM Interdisciplinary Scientist, we will study the formation, nature and planetary companions of the exotic endpoints of stellar evolution. Our science begins with stars evolving from asymptotic branch giants into white dwarfs. We will determine the parallax and orbital inclination of several iron-deficient post-AGB stars, who peculiar abundances and infrared excesses are evidence that they are accreting gas depleted of dust from a circumbinary disk. Measurement of the orbital inclination, companion mass arid parallax will provide critical constraints. One of these stars is a prime candidate for trying nulling observations, which should reveal light reflected from both the circumbinary and Roche disks. The circumbinary disks seem favorable sites for planet formation. Next, we will search for planets around white dwarfs, both survivors froni the main-sequence stage, and ones newly formed from the circumbinary disks of post-AGB binaries or in white dwarf mergers. Moving up in mass, we will measure the orbital reflex of OB/Be companions to pulsars, determine natal kicks and presupernova orbits, and expand the sample of well-determined neutron star masses. We will obtain the parallax of a transient X-ray binary, whose quiescent emission may be thermal emission from the neutron star, aiming for precise measurement of the neutron star radius. Finally, black holes. We will measure the reflex motions of the companion of what appear to be the most massive stellar black holes. The visual orbits will determine natal kicks, and test the assumptions underlying mass estimates made from the radial velocity curves, projected rotation, and ellipsoidal variations. In addition, we will attempt to observe the visual orbit of SS 433, as well as the proper motion of the emission line clumps in its relativistic jets. Additional information is included in the original document.

Studies of Evolved Star Mass Loss: GRAMS Modeling of Red Supergiant and Asymptotic Giant Branch Stars in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, S.; Riebel, D.; Boyer, M.; Meixner, M.

2012-01-01

As proposed in our NASA Astrophysics Data Analysis Program (ADAP) proposal, my colleagues and I are studying mass loss from evolved stars. Such stars lose their own mass in their dying stages, and in their expelled winds they form stardust. To model mass loss from these evolved stars, my colleagues and I have constructed GRAMS: the Grid of Red supergiant and Asymptotic giant branch star ModelS. These GRAMS radiative transfer models are fit to optical through mid-infrared photometry of red supergiant (RSG) stars and asymptotic giant branch (AGB) stars. I will discuss our current studies of mass loss from AGB and RSG stars in the Small Magellanic Cloud (SMC), fitting GRAMS models to the photometry of SMC evolved star candidates identified from the SAGE-SMC (PI: K. Gordon) Spitzer Space Telescope Legacy survey. This work will be briefly compared to similar work we have done for the LMC. I will also discuss Spitzer Infrared Spectrograph (IRS) studies of the dust produced by AGB and RSG stars in the LMC. BAS is grateful for support from the NASA-ADAP grant NNX11AB06G.

Infrared Spectroscopic Studies of the Properties of Dust in the Ejecta of Galactic Oxygen-Rich Asymptotic Giant Branch Stars

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Srinivasan, Sundar; Kastner, Joel; Meixner, Margaret; Riley, Allyssa

2018-06-01

We are conducting a series of infrared studies of large samples of mass-losing asymptotic giant branch (AGB) stars to explore the relationship between the composition of evolved star ejecta and host galaxy metallicity. Our previous studies focused on mass loss from evolved stars in the relatively low-metallicity Large and Small Magellanic Clouds. In our present study, we analyze dust in the mass-losing envelopes of AGB stars in the Galaxy, with special focus on the ejecta of oxygen-rich (O-rich) AGB stars. We have constructed detailed dust opacity models of AGB stars in the Galaxy for which we have infrared spectra from, e.g., the Spitzer Space Telescope Infrared Spectrograph (IRS). This detailed modeling of dust features in IRS spectra informs our choice of dust properties to use in radiative transfer modeling of the broadband SEDs of Bulge AGB stars. We investigate the effects of dust grain composition, size, shape, etc. on the AGB stars' infrared spectra, studying both the silicate dust and the opacity source(s) commonly attributed to alumina (Al2O3). BAS acknowledges funding from NASA ADAP grant 80NSSC17K0057.

Applicaton of the Calculating Formula for Mean Neutron Exposure on Barium stars

NASA Astrophysics Data System (ADS)

Zhang, F. H.; Zhang, L.; Cui, W. Y.; Zhang, B.

2017-11-01

Latest studies have shown that, in the s-process nucleosynthesis model for the low-mass asymptotic giant branch (AGB) star with (13C) pocket radiative burning during the interpulse period, the distribution of neutron exposures in the nucleosynthesis region can be regarded as an exponential function, and the relation between the mean neutron exposure (τ0) and the model parameters is (τ0} = - Δ τ/ln [q/(1 - r + q)]), in which (Δ τ) is the exposure value of each neutron irradiation, (r) is the overlap factor, and (q) is the mass ratio of the (13C) shell to the He intershell. In this paper the formula is applied to 26 samples of barium stars to test its reliability, and furthermore the neutron exposure nature in the AGB companion stars of 26 barium stars are analyzed. The results show that, the formula is reliable; in the AGB companion stars of 26 barium stars, at least 8 stars definitely have and 12 stars are highly likely to have exponential distribution of neutron exposures, while 4 stars tend to experience single neutron exposure; most of the AGB companion stars may have experienced fewer times of neutron irradiations before the element abundance distribution of the s-process comes to asymptotic condition.

Yellow evolved stars in open clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sowell, J.R.

1987-05-01

This paper describes a program in which Galactic cluster post-AGB candidates were first identified and then analyzed for cluster membership via radial velocities, monitored for possible photometric variations, examined for evidence of mass loss, and classified as completely as possible in terms of their basic stellar parameters. The intrinsically brightest supergiants are found in the youngest clusters. With increasing cluster age, the absolute luminosities attained by the supergiants decline. It appears that the evolutionary tracks of luminosity class II stars are more similar to those of class I than of class III. Only two superluminous giant star candidates are foundmore » in open clusters. 154 references.« less

On the formation of molecules and solid-state compounds from the AGB to the PN phases

NASA Astrophysics Data System (ADS)

García-Hernández, D. A.; Manchado, A.

2016-07-01

During the asymptoyic giant branch (AGB) phase, different elements are dredge- up to the stellar surface depending on progenitor mass and metallicity. When the mass loss increases at the end of the AGB, a circumstellar dust shell is formed, where different (C-rich or O-rich) molecules and solid-state compounds are formed. These are further processed in the transition phase between AGB stars and planetary nebulae (PNe) to create more complex organic molecules and inorganic solid-state compounds (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene precursors in C-rich environments and oxides and crystalline silicates in O-rich ones). We present an observational review of the different molecules and solid-state materials that are formed from the AGB to the PN phases. We focus on the formation routes of complex fullerene (and fullerene-based) molecules as well as on the level of dust processing depending on metallicity.

The evolution of CNO isotopes: a new window on cosmic star formation history and the stellar IMF in the age of ALMA

NASA Astrophysics Data System (ADS)

Romano, D.; Matteucci, F.; Zhang, Z.-Y.; Papadopoulos, P. P.; Ivison, R. J.

2017-09-01

We use state-of-the-art chemical models to track the cosmic evolution of the CNO isotopes in the interstellar medium of galaxies, yielding powerful constraints on their stellar initial mass function (IMF). We re-assess the relative roles of massive stars, asymptotic giant branch (AGB) stars and novae in the production of rare isotopes such as 13C, 15N, 17O and 18O, along with 12C, 14N and 16O. The CNO isotope yields of super-AGB stars, novae and fast-rotating massive stars are included. Having reproduced the available isotope enrichment data in the solar neighbourhood, and across the Galaxy, and having assessed the sensitivity of our models to the remaining uncertainties, e.g. nova yields and star formation history, we show that we can meaningfully constrain the stellar IMF in galaxies using C, O and N isotope abundance ratios. In starburst galaxies, where data for multiple isotopologue lines are available, we find compelling new evidence for a top-heavy stellar IMF, with profound implications for their star formation rates and efficiencies, perhaps also their stellar masses. Neither chemical fractionation nor selective photodissociation can significantly perturb globally averaged isotopologue abundance ratios away from the corresponding isotope ones, as both these processes will typically affect only small mass fractions of molecular clouds in galaxies. Thus, the Atacama Large Millimeter Array now stands ready to probe the stellar IMF, and even the ages of specific starburst events in star-forming galaxies across cosmic time unaffected by the dust obscuration effects that plague optical/near-infrared studies.

Monash Chemical Yields Project (Monχey) Element production in low- and intermediate-mass stars

NASA Astrophysics Data System (ADS)

Doherty, Carolyn; Lattanzio, John; Angelou, George; Campbell, Simon W.; Church, Ross; Constantino, Thomas; Cristallo, Sergio; Gil-Pons, Pilar; Karakas, Amanda; Lugaro, Maria; Stancliffe, Richard

The Monχey project will provide a large and homogeneous set of stellar yields for the low- and intermediate- mass stars and has applications particularly to galactic chemical evolution modelling. We describe our detailed grid of stellar evolutionary models and corresponding nucleosynthetic yields for stars of initial mass 0.8 M⊙ up to the limit for core collapse supernova (CC-SN) ~ 10 M⊙. Our study covers a broad range of metallicities, ranging from the first, primordial stars (Z = 0) to those of super-solar metallicity (Z = 0.04). The models are evolved from the zero-age main-sequence until the end of the asymptotic giant branch (AGB) and the nucleosynthesis calculations include all elements from H to Bi. A major innovation of our work is the first complete grid of heavy element nucleosynthetic predictions for primordial AGB stars as well as the inclusion of extra-mixing processes (in this case thermohaline) during the red giant branch. We provide a broad overview of our results with implications for galactic chemical evolution as well as highlight interesting results such as heavy element production in dredge-out events of super-AGB stars. We briefly introduce our forthcoming web-based database which provides the evolutionary tracks, structural properties, internal/surface nucleosynthetic compositions and stellar yields. Our web interface includes user- driven plotting capabilities with output available in a range of formats. Our nucleosynthetic results will be available for further use in post processing calculations for dust production yields.

Are some CEMP-s stars the daughters of spinstars?

NASA Astrophysics Data System (ADS)

Choplin, Arthur; Hirschi, Raphael; Meynet, Georges; Ekström, Sylvia

2017-11-01

Carbon-enhanced metal-poor (CEMP)-s stars are long-lived low-mass stars with a very low iron content as well as overabundances of carbon and s-elements. Their peculiar chemical pattern is often explained by pollution from an asymptotic giant branch (AGB) star companion. Recent observations have shown that most CEMP-s stars are in binary systems, providing support to the AGB companion scenario. A few CEMP-s stars, however, appear to be single. We inspect four apparently single CEMP-s stars and discuss the possibility that they formed from the ejecta of a previous-generation massive star, referred to as the "source" star. In order to investigate this scenario, we computed low-metallicity massive-star models with and without rotation and including complete s-process nucleosynthesis. We find that non-rotating source stars cannot explain the observed abundance of any of the four CEMP-s stars. Three out of the four CEMP-s stars can be explained by a 25M⊙ source star with vini 500 km s-1 (spinstar). The fourth CEMP-s star has a high Pb abundance that cannot be explained by any of the models we computed. Since spinstars and AGB predict different ranges of [O/Fe] and [ls/hs], these ratios could be an interesting way to further test these two scenarios.

THE KILOPARSEC-SCALE STAR FORMATION LAW AT REDSHIFT 4: WIDESPREAD, HIGHLY EFFICIENT STAR FORMATION IN THE DUST-OBSCURED STARBURST GALAXY GN20

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hodge, J. A.; Riechers, D.; Decarli, R.

2015-01-01

We present high-resolution observations of the 880 μm (rest-frame FIR) continuum emission in the z = 4.05 submillimeter galaxy GN20 from the IRAM Plateau de Bure Interferometer (PdBI). These data resolve the obscured star formation (SF) in this unlensed galaxy on scales of 0.''3 × 0.''2 (∼2.1 × 1.3 kpc). The observations reveal a bright (16 ± 1 mJy) dusty starburst centered on the cold molecular gas reservoir and showing a bar-like extension along the major axis. The striking anti-correlation with the Hubble Space Telescope/Wide Field Camera 3 imaging suggests that the copious dust surrounding the starburst heavily obscures the rest-frame UV/optical emission. A comparison with 1.2 mm PdBI continuummore » data reveals no evidence for variations in the dust properties across the source within the uncertainties, consistent with extended SF, and the peak star formation rate surface density (119 ± 8 M {sub ☉} yr{sup –1} kpc{sup –2}) implies that the SF in GN20 remains sub-Eddington on scales down to 3 kpc{sup 2}. We find that the SF efficiency (SFE) is highest in the central regions of GN20, leading to a resolved SF law with a power-law slope of Σ{sub SFR} ∼ Σ{sub H{sub 2}{sup 2.1±1.0}}, and that GN20 lies above the sequence of normal star-forming disks, implying that the dispersion in the SF law is not due solely to morphology or choice of conversion factor. These data extend previous evidence for a fixed SFE per free-fall time to include the star-forming medium on ∼kiloparsec scales in a galaxy 12 Gyr ago.« less

Gravitational lensing reveals extreme dust-obscured star formation in quasar host galaxies

NASA Astrophysics Data System (ADS)

Stacey, H. R.; McKean, J. P.; Robertson, N. C.; Ivison, R. J.; Isaak, K. G.; Schleicher, D. R. G.; van der Werf, P. P.; Baan, W. A.; Berciano Alba, A.; Garrett, M. A.; Loenen, A. F.

2018-06-01

We have observed 104 gravitationally lensed quasars at z ˜ 1-4 with Herschel/SPIRE, the largest such sample ever studied. By targeting gravitational lenses, we probe intrinsic far-infrared (FIR) luminosities and star formation rates (SFRs) more typical of the population than the extremely luminous sources that are otherwise accessible. We detect 72 objects with Herschel/SPIRE and find 66 per cent (69 sources) of the sample have spectral energy distributions (SEDs) characteristic of dust emission. For 53 objects with sufficiently constrained SEDs, we find a median effective dust temperature of 38^{+12}_{-5} K. By applying the radio-infrared correlation, we find no evidence for an FIR excess that is consistent with star-formation-heated dust. We derive a median magnification-corrected FIR luminosity of 3.6^{+4.8}_{-2.4} × 10^{11} L_{⊙} and median SFR of 120^{+160}_{-80} M_{⊙} yr^{-1}} for 94 quasars with redshifts. We find ˜10 per cent of our sample have FIR properties similar to typical dusty star-forming galaxies at z ˜ 2-3 and a range of SFRs <20-10 000 M⊙ yr-1 for our sample as a whole. These results are in line with current models of quasar evolution and suggests a coexistence of dust-obscured star formation and AGN activity is typical of most quasars. We do not find a statistically significant difference in the FIR luminosities of quasars in our sample with a radio excess relative to the radio-infrared correlation. Synchrotron emission is found to dominate at FIR wavelengths for <15 per cent of those sources classified as powerful radio galaxies.

High Resolution Optical Spectroscopy of an Intriguing High-Latitude B-Type Star HD119608

NASA Astrophysics Data System (ADS)

Şahin, T.

2018-01-01

We present an LTE analysis of high resolution echelle optical spectra obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph for a B1Ib high galactic latitude supergiant HD119608. A fresh determination of the atmospheric parameters using line-blanketed LTE model atmospheres and spectral synthesis provided T eff = 23 300 ± 1000 K, log g = 3.0 ± 0.3, and the microturbulent velocity ξ = 6.0 ± 1.0 kms-1 and [Fe/H] = 0.16. The rotational velocity of the star was derived fromC, O, N, Al, and Fe lines as v sin i = 55.8 ± 1.3 kms-1. Elemental abundances were obtained for 10 different species. He, Al, and P abundances of the star were determined for the first time. In the spectra, hot post-AGB status as well as the Pop I characteristics of the star were examined. The approximately solar carbon and oxygen abundances, along with mild excess in helium and nitrogen abundances do not stipulate a CNO processed surface composition, hence a hot post-AGB status. The LTE abundances analysis also indicates solar sulphur and moderately enriched magnesium abundances. The average abundances of B dwarfs of well studied OB associations and Population I stars show a striking resemblance to abundances obtained for HD119608 in this study. This may imply a runaway status for the star.

Simultaneous Chandra and NuSTAR Observations of the Highly Obscured AGN Candidate in NGC660.

NASA Astrophysics Data System (ADS)

Annuar, Ady

2014-09-01

We are using NuSTAR to undertake a detailed investigation of the obscured AGN population at D<15Mpc. Our latest target is NGC660 where the presence of an AGN has been ambiguous. However, recently it was observed to undergo a radio outburst which reveals a bright continuum source (Argo et al. 2015), coincident with Chandra 2-8 keV emission from one of the three point sources near the nucleus (<5"). This confirms and pinpoints the X-ray position of the AGN. Comparisons of the Chandra flux with the radio emission and other multiwavelength luminosity indicators indicate that the X-ray flux is suppressed, suggesting that it is absorbed by a high column of gas. A NuSTAR observation for this object has been scheduled as part of our program. The requested Chandra observation is essential to unambiguously constrain the AGN and isolate it from other sources at <8 keV. When combined with NuSTAR, we will then be able to accurately characterise the 0.5-30 keV spectrum of the AGN for the first time.

Infra-Red Characteristics of Faint Galactic Carbon Stars from the First Byurakan Spectral Sky Survey

NASA Astrophysics Data System (ADS)

Kostandyan, G. R.; Gigoyan, K. S.

2017-07-01

Infra-Red (IR) astronomical databases, namely, IRAS, 2MASS, WISE, and Spitzer, are used to analyze photometric data of 126 carbon (C) stars whose spectra are visible in the First Byurakan Survey (FBS) (Markarian et al. 1989) low-resolution (lr) spectral plates. In this work several IR color-color diagrams are studied. Early and late-type C stars are separated in the JHK Near-Infra-Red (NIR) color-color plots, as well as in the WISE W3-W4 versus W1-W2 diagram. Late N-type Asymptotic Giant Branch (AGB) stars are redder in W1-W2, while early-types (CH and R giants) are redder in W3-W4 as expected. Objects with W2-W3 > 1.0m show double-peaked spectral energy distribution (SED), indicating the existence of the circumstellar envelopes around them. 26 N-type stars have IRAS Point Source Catalog (PSC) associations. The reddest object among the targets is N-type C star FBS 2213+421, which belong to the group of the cold post-AGB R Coronae Borealis (R CrB) variables (Rossi et al. 2016).

The puzzle of the CNO isotope ratios in asymptotic giant branch carbon stars

NASA Astrophysics Data System (ADS)

Abia, C.; Hedrosa, R. P.; Domínguez, I.; Straniero, O.

2017-03-01

Context. The abundance ratios of the main isotopes of carbon, nitrogen and oxygen are modified by the CNO-cycle in the stellar interiors. When the different dredge-up events mix the burning material with the envelope, valuable information on the nucleosynthesis and mixing processes can be extracted by measuring these isotope ratios. Aims: Previous determinations of the oxygen isotopic ratios in asymptotic giant branch (AGB) carbon stars were at odds with the existing theoretical predictions. We aim to redetermine the oxygen ratios in these stars using new spectral analysis tools and further develop discussions on the carbon and nitrogen isotopic ratios in order to elucidate this problem. Methods: Oxygen isotopic ratios were derived from spectra in the K-band in a sample of galactic AGB carbon stars of different spectral types and near solar metallicity. Synthetic spectra calculated in local thermodynamic equillibrium (LTE) with spherical carbon-rich atmosphere models and updated molecular line lists were used. The CNO isotope ratios derived in a homogeneous way, were compared with theoretical predictions for low-mass (1.5-3 M⊙) AGB stars computed with the FUNS code assuming extra mixing both during the RGB and AGB phases. Results: For most of the stars the 16O/17O/18O ratios derived are in good agreement with theoretical predictions confirming that, for AGB stars, are established using the values reached after the first dredge-up (FDU) according to the initial stellar mass. This fact, as far as the oxygen isotopic ratios are concerned, leaves little space for the operation of any extra mixing mechanism during the AGB phase. Nevertheless, for a few stars with large 16O/17O/18O, the operation of such a mechanism might be required, although their observed 12C/13C and 14N/15N ratios would be difficult to reconcile within this scenario. Furthermore, J-type stars tend to have lower 16O/17O ratios than the normal carbon stars, as already indicated in previous studies

Study of the aluminium content in AGB winds using ALMA. Indications for the presence of gas-phase (Al2O3)n clusters

NASA Astrophysics Data System (ADS)

Decin, L.; Richards, A. M. S.; Waters, L. B. F. M.; Danilovich, T.; Gobrecht, D.; Khouri, T.; Homan, W.; Bakker, J. M.; Van de Sande, M.; Nuth, J. A.; De Beck, E.

2017-12-01

Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O < 1) asymptotic giant branch (AGB) winds. Aluminium oxides and iron-free silicates are often put forward as promising candidates for the first dust seeds. Aims: We aim to constrain the dust formation histories in the winds of oxygen-rich AGB stars. Methods: We obtained Atacama Large Millimeter/sub-millimeter array (ALMA) observations with a spatial resolution of 120 × 150 mas tracing the dust formation region of the low mass-loss rate AGB star, R Dor, and the high mass-loss rate AGB star, IK Tau. We detected emission line profiles of AlO, AlOH, and AlCl in the ALMA data and used these line profiles to derive a lower limit of atomic aluminium incorporated in molecules. This constrains the aluminium budget that can condense into grains. Results: Radiative transfer models constrain the fractional abundances of AlO, AlOH, and AlCl in IK Tau and R Dor. We show that the gas-phase aluminium chemistry is completely different in both stars with a remarkable difference in the AlO and AlOH abundance stratification. The amount of aluminium locked up in these three molecules is small, ≤1.1 × 10-7 w.r.t. H2, for both stars, i.e. only ≤2% of the total aluminium budget. An important result is that AlO and AlOH, which are the direct precursors of alumina (Al2O3) grains, are detected well beyond the onset of the dust condensation, which proves that the aluminium oxide condensation cycle is not fully efficient. The ALMA observations allow us to quantitatively assess the current generation of theoretical dynamical-chemical models for AGB winds. We discuss how the current proposed scenario of aluminium dust condensation for low mass-loss rate AGB stars within a few stellar radii from the star, in particular for R Dor and W Hya, poses a challenge if one wishes to explain both

Clear Evidence for the Presence of Second-generation Asymptotic Giant Branch Stars in Metal-poor Galactic Globular Clusters

NASA Astrophysics Data System (ADS)

García-Hernández, D. A.; Mészáros, Sz.; Monelli, M.; Cassisi, S.; Stetson, P. B.; Zamora, O.; Shetrone, M.; Lucatello, S.

2015-12-01

Galactic globular clusters (GCs) are known to host multiple stellar populations: a first generation (FG) with a chemical pattern typical of halo field stars and a second generation (SG) enriched in Na and Al and depleted in O and Mg. Both stellar generations are found at different evolutionary stages (e.g., the main-sequence turnoff, the subgiant branch, and the red giant branch (RGB)). The non detection of SG asymptotic giant branch (AGB) stars in several metal-poor ([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and that failed AGB stars may be very common in metal-poor GCs. This observation represents a serious problem for stellar evolution and GC formation/evolution theories. We report fourteen SG-AGB stars in four metal-poor GCs (M13, M5, M3, and M2) with different observational properties: horizontal branch (HB) morphology, metallicity, and age. By combining the H-band Al abundances obtained by the Apache Point Observatory Galactic Evolution Experiment survey with ground-based optical photometry, we identify SG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC stars should be Na-rich. Our observations provide strong support for present, standard stellar models, i.e., without including a strong mass-loss efficiency, for low-mass HB stars. In fact, current empirical evidence is in agreement with the predicted distribution of FG and SG stars during the He-burning stages based on these standard stellar models.

The s-Process Nucleosynthesis in Extremely Metal-Poor Stars as the Generating Mechanism of Carbon Enhanced Metal-Poor Stars

NASA Astrophysics Data System (ADS)

Suda, Takuma; Yamada, Shimako; Fujimoto, Masayuki Y.

The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-metal-poor (EMP) stars with [Fe/H] ≤ -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] ≲ -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.

A SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arneson, R. A.; Gehrz, R. D.; Woodward, C. E.

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust in a sample of post-asymptotic giant branch (post-AGB) yellow supergiants that are believed to be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon-rich and oxygen-rich dust species—contrary to simple dredge-up models—with a majority of the dust in the form of amorphous carbon and graphite. The oxygen-rich dust is primarily in the form of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests thatmore » the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. These results help to determine the physical properties of the post-AGB circumstellar environment and to constrain models of post-AGB mass loss and planetary nebula formation.« less

Low-Metallicity Lead Stars: Comparison between Theory and Observations

NASA Astrophysics Data System (ADS)

Bisterzo, S.; Gallino, R.; Straniero, O.; Aoki, W.; Ryan, S.; Beers, T. C.

2006-07-01

We compare AGB theoretical models with spectroscopic abundances of a sample of very metal-poor, C-rich, s-rich and lead-rich stars observed at high-resolution spectroscopy. Fits are obtained for AGB models with different 13C-pocket efficiencies and initial masses. The two intrinsic indicators, [hs/ls] and [Pb/hs] versus [Fe/H], are analyzed. An extended analysis of all the observed elements is made, outlining apparent discrepancies for a few elements. The analysis of C and N abundances strengthen the need of a strong cool bottom process occurring during the AGB. A significant number of these stars are both s-enriched and r-enriched. For them, the envelope abundances are predicted by mass transfer from the more massive AGB companion in a binary system from a parental cloud already enriched in r-elements.

Innocent Bystanders: Carbon Stars from the Sloan Digital Sky Survey

NASA Astrophysics Data System (ADS)

Green, Paul

2013-03-01

Among stars showing carbon molecular bands (C stars), the main-sequence dwarfs, likely in post-mass transfer binaries, are numerically dominant in the Galaxy. Via spectroscopic selection from the Sloan Digital Sky Survey, we retrieve 1220 high galactic latitude C stars, ~5 times more than previously known, including a wider variety than past techniques such as color or grism selection have netted, and additionally yielding 167 DQ white dwarfs. Of the C stars with proper motion measurements, we identify 69% clearly as dwarfs (dCs), while ~7% are giants. The dCs likely span absolute magnitudes Mi from ~6.5 to 10.5. "G-type" dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C2 bands. We report Balmer emission in 22 dCs, none of which are G-types. We find 8 new DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. Eleven very red C stars with strong red CN bands appear to be "N"-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Leo A. Two such stars within 30' of each other may trace a previously unidentified dwarf galaxy or tidal stream at ~40 kpc. We explore the multiwavelength properties of the sample and report the first X-ray detection of a dC star, which shows strong Balmer emission. Our own spectroscopic survey additionally provides the dC surface density from a complete sample of dwarfs limited by magnitude, color, and proper motion.

Galaxy gas as obscurer - II. Separating the galaxy-scale and nuclear obscurers of active galactic nuclei

NASA Astrophysics Data System (ADS)

Buchner, Johannes; Bauer, Franz E.

2017-03-01

The 'torus' obscurer of active galactic nuclei (AGN) is poorly understood in terms of its density, sub-structure and physical mechanisms. Large X-ray surveys provide model boundary constraints, for both Compton-thin and Compton-thick levels of obscuration, as obscured fractions are mean covering factors fcov. However, a major remaining uncertainty is host-galaxy obscuration. In Paper I, we discovered a relation of {NH} ∝ M_{star }^{1/3} for the obscuration of galaxy-scale gas. Here, we apply this observational relation to the AGN population, and find that galaxy-scale gas is responsible for a luminosity-independent fraction of Compton-thin AGN, but does not produce Compton-thick columns. With the host-galaxy obscuration understood, we present a model of the remaining nuclear obscurer, which is consistent with a range of observations. Our radiation-lifted torus model consists of a Compton-thick component (fcov ∼ 35 per cent) and a Compton-thin component (fcov ∼ 40 per cent), which depends on both black hole mass and luminosity. This provides a useful summary of observational constraints for torus modellers who attempt to reproduce this behaviour. It can also be employed as a sub-grid recipe in cosmological simulations that do not resolve the torus. We also investigate host-galaxy X-ray obscuration inside cosmological, hydrodynamic simulations (Evolution and Assembly of Galaxies and their Environment; Illustris). The obscuration from ray-traced galaxy gas can agree with observations, but is highly sensitive to the chosen feedback assumptions.

The Detection of 6.9 μm Emission Features in the Infrared Spectra of IRAS 04296+3429, IRAS 05341+0852, and IRAS 22272+5435: Evidence for the Presence of Hn-PAHs in Post-AGB Stars

NASA Astrophysics Data System (ADS)

Materese, Christopher K.; Bregman, Jesse D.; Sandford, Scott A.

2017-12-01

Polycyclic aromatic hydrocarbons (PAHs) are generally believed to be ubiquitous in space and responsible for numerous telltale interstellar infrared emission bands. In Sandford et al., we suggested that PAHs with excess hydrogenation at their periphery ({{{H}}}{{n}}-PAHs) may be an important subclass of these molecules in some astrophysical environments. These molecules are candidates to explain objects with anomalously large 3.4 μm features, which are presumed to be associated with the aliphatic C-H stretching vibrations of the excess hydrogen. In that work, we suggest that for Hn-PAHs to be a viable candidate as the source for this 3.4 μm feature, we must also expect to observe methylene scissoring modes at 6.9 μm. In this work, we continue to develop the {{{H}}}{{n}} - {PAH} hypothesis with a focus on the 6.9 μm feature. We also present some new observations of three post-asymptotic giant branch (post-AGB) objects with abnormally large 3.4 μm features, IRAS 04296+3429, IRAS 05341+0852, and IRAS 22272+5435, in addition to one post-AGB object with normal PAH emissions, IRAS 20000+3239. These observations were made using the FORCAST instrument in grism mode on the Stratospheric Observatory for Infrared Astronomy aircraft and demonstrate the presence of a 6.9 μm feature for the three objects with abnormally large 3.4 μm features and no detectable 6.9 μm feature for the normal PAH emitter. These results are consistent with the hypothesis that Hn-PAHs are a possible source of these infrared emission bands.

Rings and arcs around evolved stars - I. Fingerprints of the last gasps in the formation process of planetary nebulae

NASA Astrophysics Data System (ADS)

Ramos-Larios, G.; Santamaría, E.; Guerrero, M. A.; Marquez-Lugo, R. A.; Sabin, L.; Toalá, J. A.

2016-10-01

Evolved stars such as asymptotic giant branch stars (AGB), post-AGB stars, proto-planetary nebulae (proto-PNe), and planetary nebulae (PNe) show rings and arcs around them and their nebular shells. We have searched for these morphological features in optical Hubble Space Telescope and mid-infrared Spitzer Space Telescope images of ˜650 proto-PNe and PNe and discovered them in 29 new sources. Adding those to previous detections, we derive a frequency of occurrence ≃8 per cent. All images have been processed to remove the underlying envelope emission and enhance outer faint structures to investigate the spacing between rings and arcs and their number. The averaged time lapse between consecutive rings and arcs is estimated to be in the range 500-1200 yr. The spacing between them is found to be basically constant for each source, suggesting that the mechanism responsible for the formation of these structures in the final stages of evolved stars is stable during time periods of the order of the total duration of the ejection. In our sample, this period of time spans ≤4500 yr.

DEEP MIXING IN EVOLVED STARS. II. INTERPRETING Li ABUNDANCES IN RED GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palmerini, S.; Busso, M.; Maiorca, E.

2011-11-01

We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After outlining the problems affecting our knowledge of the Li content in low-mass stars (M {<=} 3 M{sub sun}), we discuss deep-mixing models for the red giant branch stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M{sub sun}. Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich asymptotic giant branch (AGB) stars. Here, the constraints onmore » extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) {identical_to} log {epsilon}(Li) {approx_equal} 1.5 (and sometimes more). Also, their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, and the C/O and {sup 12}C/{sup 13}C ratios) can be explained. This requires generally moderate efficiencies (M-dot < or approx. 0.3-0.5 x 10{sup -6} M{sub sun} yr{sup -1}) for non-convective mass transport. At such rates, slow extra mixing does not remarkably modify Li abundances in early AGB phases; on the other hand, faster mixing encounters a physical limit in destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be produced; therefore, its destruction on the AGB is modest. Li is then significantly produced by the third dredge up. We also show that effective circulation episodes, while not destroying Li, would easily bring the {sup 12}C/{sup 13}C ratios to equilibrium, contrary to the evidence in most AGB stars, and would burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the common idea that efficient extra mixing drastically reduces the Li content of C stars with respect

THE INSIDIOUS BOOSTING OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS IN INTERMEDIATE-AGE MAGELLANIC CLOUD CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Girardi, Léo; Marigo, Paola; Bressan, Alessandro

2013-11-10

In the recent controversy about the role of thermally pulsing asymptotic giant branch (TP-AGB) stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incorporated into EPS models, are found to overestimate, to various extents, the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect, linked to the physics of stellar interiors, that in all probability is the main cause of this conundrum.more » As soon as stellar populations intercept the ages at which red giant branch stars first appear, a sudden and abrupt change in the lifetime of the core He-burning phase causes a temporary 'boost' in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of ∼2. The boost occurs for turn-off masses of ∼1.75 M{sub ☉}, just in the proximity of the expected peak in the TP-AGB lifetimes (for MC metallicities), and for ages of ∼1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. As we discuss in this paper, the identification of this evolutionary effect brings about three main consequences. First, we claim that present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters are biased toward too large values. Second, the relative TP-AGB contribution of single-burst populations

The Mass-loss Return from Evolved Stars to the Large Magellanic Cloud. VI. Luminosities and Mass-loss Rates on Population Scales

NASA Astrophysics Data System (ADS)

Riebel, D.; Srinivasan, S.; Sargent, B.; Meixner, M.

2012-07-01

We present results from the first application of the Grid of Red Supergiant and Asymptotic Giant Branch ModelS (GRAMS) model grid to the entire evolved stellar population of the Large Magellanic Cloud (LMC). GRAMS is a pre-computed grid of 80,843 radiative transfer models of evolved stars and circumstellar dust shells composed of either silicate or carbonaceous dust. We fit GRAMS models to ~30,000 asymptotic giant branch (AGB) and red supergiant (RSG) stars in the LMC, using 12 bands of photometry from the optical to the mid-infrared. Our published data set consists of thousands of evolved stars with individually determined evolutionary parameters such as luminosity and mass-loss rate. The GRAMS grid has a greater than 80% accuracy rate discriminating between oxygen- and carbon-rich chemistry. The global dust injection rate to the interstellar medium (ISM) of the LMC from RSGs and AGB stars is on the order of 2.1 × 10-5 M ⊙ yr-1, equivalent to a total mass injection rate (including the gas) into the ISM of ~6 × 10-3 M ⊙ yr-1. Carbon stars inject two and a half times as much dust into the ISM as do O-rich AGB stars, but the same amount of mass. We determine a bolometric correction factor for C-rich AGB stars in the K s band as a function of J - K s color, BC_{K_{s}} = -0.40(J-K_{s})^2 + 1.83(J-K_{s}) + 1.29. We determine several IR color proxies for the dust mass-loss rate (\\dot{M}_{d}) from C-rich AGB stars, such as log \\dot{M_{d}} = ({-18.90}/({(K_{s}-[8.0])+3.37}))-5.93. We find that a larger fraction of AGB stars exhibiting the "long-secondary period" phenomenon are more O-rich than stars dominated by radial pulsations, and AGB stars without detectable mass loss do not appear on either the first-overtone or fundamental-mode pulsation sequences.

Molecular and Dusty Layers of Asymptotic Giant Branch Stars Studied with the VLT Interferometer

DTIC Science & Technology

2011-09-01

the evolution of low to intermediate mass stars towards planetary nebulae . It is also one of the most important sources of chemical enrichment of...planetary nebula (PN) phases, and is thus the most important driver for the further stellar evolution (e.g., Habing & Olofsson, 2003). Mass loss from AGB...branch (AGB) stars is the most important driver for the evolution of low to intermediate mass stars towards planetary nebulae . It is also one of the

Monash Chemical Yields Project (Monχey) - Element production in low- and intermediate-mass stars of metallicities Z = 0 to 0.04

NASA Astrophysics Data System (ADS)

Doherty, Carolyn Louise; Lattanzio, John; Angelou, George; Wattana Campbell, Simon; Church, Ross; Constantino, Thomas; Cristallo, Sergio; Gil-Pons, Pilar; Karakas, Amanda; Lugaro, Maria; Stancliffe, Richard James

2015-08-01

The Monχey project provides a large and homogeneous set of stellar yields for the low- and intermediate- mass stars and has applications particularly to galactic chemical evolution modelling.We present a detailed grid of stellar evolutionary models and corresponding nucleosynthetic yields for stars of initial mass 0.8 M⊙ up to the limit for core collapse supernova ≈ 10 M⊙. Our study covers a broad range of metallicities, ranging from the first, primordial stars (Z=0) to those of super-solar metallicity (Z=0.04). The models are evolved from the zero-age main-sequence until the end of the asymptotic giant branch (AGB) and the nucleosynthesis calculations include all elements from H to Bi.A major innovation of our work is the first complete grid of heavy element nucleosynthetic predictions for primordial AGB stars as well as the inclusion of extra-mixing processes (in this case thermohaline) during the red giant branch. We provide a broad overview of our results with implications for galactic chemical evolution as well as highlight interesting results such as heavy element production in dredge-out events of super-AGB stars.We briefly introduce our easy to use web-based database which provides the evolutionary tracks, structural properties, internal/surface nucleosynthetic compositions and stellar yields. Our web interface includes user- driven plotting capabilities with output available in a range of formats. Our nucleosynthetic results are available for further use in post processing calculations for dust production yields.

HR 6094: A Young, Solar-Type, Solar-Metallicity Barium Dwarf Star

NASA Astrophysics Data System (ADS)

Porto de Mello, G. F.; da Silva, L.

1997-02-01

The young solar-type star HR 6094 is found to be a barium dwarf, overabundant in the s-process elements as well as deficient in C. It is a member of the solar-metallicity, 0.3 Gyr old Ursa Major kinematical group. Measurements of radial velocity and ultraviolet flux do not support the attribution of such abundance anomalies to an unseen degenerate companion. A common proper motion, V = 10, DA white dwarf (WD), located 5360 AU away, however, strongly supports the explanation of the origin of this barium star by the process of mass transfer in a binary system, in which the secondary component accreted matter from the primary one (now the WD) when it was an asymptotic giant branch (AGB) star self-enriched in the s-process elements. The membership in the UMa group of another s-process-rich and C-deficient star, HR 2047, suggests that these stars could have formed a multiple system in the past, which was disrupted by the mass-loss episode of the former AGB star. Their [C/Fe] deficiency could be explained by the action of the hot-bottomed envelope burning process in the late AGB, thereby reconverting it from a C-rich to an O-rich star, depleting C while enriching its envelope with Li and neutron capture elements. This is the first identification of the barium phenomenon in a near-zero-age star, besides being the first barium system in which the remnant of the late AGB star responsible for the heavy-element enrichment may have been directly spotted. Observations collected at the Cerro Tololo Inter-American Observatory (CTIO), Chile, and at the Observatório do Pico dos Dias, operated by the CNPq/Laboratório Nacional de Astrofísica, Brazil.

Constraints on Grain Formation around Carbon Stars from Laboratory Studies of Presolar Graphite

NASA Astrophysics Data System (ADS)

Bernatowicz, Thomas J.; Akande, Onaolapo Wali; Croat, Thomas K.; Cowsik, Ramanath

2005-10-01

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Msolar for stars capable of contributing grains to the solar nebula is derived. This mass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium thermodynamics, indicate that grain formation occurs at radii from 2.3 to 3.7 AU for AGB carbon stars in the 1.1-5 Msolar range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 μm) only under ideal growth conditions (perfect sticking efficiency, no evaporation, no depletion of gas species contributing to grain growth), and then only in outflows from carbon stars with masses <~2.5 Msolar. The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24+/-14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium thermodynamics, force us to conclude that presolar graphite

VLTI-Pionier Imaging of the Carbon AGB Star R Sculptoris and the Supergiant V766 Centauri

NASA Astrophysics Data System (ADS)

Wittkowski, Markus

2018-04-01

I will present reconstructed images of the carbon-rich AGB star R Scl and of the supergiant V766 Cen, both recently obtained from VLTI-PIONIER data. The images are compared to state-of-the art atmosphere and wind models. The images of R Scl exhibit a complex structure within the stellar disk. This structure is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere. Images of V 766 Cen were obtained at three epochs. The first epoch shows a complex elongated structure within the photospheric disk, consistent with a red supergiant harboring giant photospheric convection cells. The second and third epochs show a qualitatively and quantitatively different structure with significantly increased contrast, which is not compatible with current models of convection. Instead we interpret the 2016 and 2017 epochs as showing a previously suggested close eclipsing companion in front of the primary, which was located behind the primary at the 2014 epoch. Finally, I will show preliminary reconstructed images of a small sample of further red supergiants.

The ultraviolet-bright stars of Omega Centauri, M3, and M13

NASA Technical Reports Server (NTRS)

Landsman, Wayne B.; O'Connell, Robert W.; Whitney, Jonathan H.; Bohlin, Ralph C.; Hill, Robert S.; Maran, Stephen P.; Parise, Ronald A.; Roberts, Morton S.; Smith, Andrew A.; Stecher, Theodore P.

1992-01-01

Two new UV-bright stars detected within 2 arcmin of the center of Omega Cen are spectroscopically investigated with the short-wavelength spectrograph of the IUE. The IUE spectra of the UV-bright stars UIT-1 and UIT-2 in the core of Omega Cen superficially resemble those of Population I mid-B stars. The absorption lines of the core UV-bright stars are significantly weaker than in Population I stars, consistent with their membership in the cluster. Synthetic spectra calculated from low-metallicity Kurucz model stellar atmospheres are compared with the spectra. These objects are insufficiently luminous to be classical hydrogen-burning post-AGB stars. They may be evolved hot horizontal branch stars which have been brightened by more than 3 mag since leaving the zero-age horizontal branch. It is inferred from the spectra and luminosity of the core UV-bright stars that similar objects could provide the source of the UV light in elliptical galaxies.

Asymptotic giant branch and super-asymptotic giant branch stars: modelling dust production at solar metallicity

NASA Astrophysics Data System (ADS)

Dell'Agli, F.; García-Hernández, D. A.; Schneider, R.; Ventura, P.; La Franca, F.; Valiante, R.; Marini, E.; Di Criscienzo, M.

2017-06-01

We present dust yields for asymptotic giant branch (AGB) and super-asymptotic giant branch (SAGB) stars of solar metallicity. Stars with initial mass 1.5 M⊙ ≤ Mini ≤ 3 M⊙ reach the carbon star stage during the AGB phase and produce mainly solid carbon and SiC. The size and the amount of the carbon particles formed follows a positive trend with the mass of the star; the carbon grains with the largest size (aC ˜ 0.2 μm) are produced by AGB stars with Mini = 2.5-3 M⊙, as these stars are those achieving the greatest enrichment of carbon in the surface regions. The size of SiC grains, being sensitive to the surface silicon abundance, remains at about aSiC ˜ 0.1μm. The mass of carbonaceous dust formed is in the range 10-4-5 × 10-3 M⊙, whereas the mass of SiC produced is 2 × 10-4-10-3 M⊙. Massive AGB/SAGB stars with Mini > 3 M⊙ experience hot bottom burning, which inhibits the formation of carbon stars. The most relevant dust species formed in these stars are silicate and alumina dust, with grain sizes in the range 0.1 < aol < 0.15 μm and a_Al_2O_3 ˜ 0.07 μm, respectively. The mass of silicates produced spans the interval 3.4 × 10-3 M⊙ ≤ Mdust ≤ 1.1 × 10-2 M⊙ and increases with the initial mass of the star.

High Resolution Optical Spectroscopy of Hot Post-AGB Star Candidates LS IV-04 1 and LB3116

NASA Astrophysics Data System (ADS)

Şahin, T.

2018-04-01

We present LTE analysis of high resolution optical spectra for B-type hot PAGB stars LS IV-04 1 and LB3116 (LSE 237). The spectra of these high Galactic latitude stars were obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph. The standard 1D LTE analysis with line-blanketed LTE model atmospheres and spectral synthesis provided fundamental atmospheric parameters of T eff= 15 000±1000 K, log g= 2.5±0.2, ξ = 5.0±1.0 km s-1, [M/H] = -1.81 dex, and v sin i= 5 km s-1 for LSIV-04 1 and T eff= 16 000±1000 K, log g= 2.5±0.1, v sin i= 25 km s-1, and [Fe/H] = -0.93 dex for LB 3116. Chemical abundances of ten different elements were obtained. For LS IV-04 1, its derived model temperature contradicts with previous analysis results. The upper limits for its nitrogen and oxygen abundances were reported for the first time. The magnesium, silicon and calcium were overabundant (i.e. [Mg/Fe] = 0.8 dex, [Si/Fe] = 0.5 dex, [Ca/Fe] = 0.9 dex). With its metal-poor photosphere and VLSR ≈ 96 km s-1, LSIV-04 1 is likely a population II star and most probably a PAGB star. LTE abundances of LB 3116 were reported for the first time. The spectrum of this helium rich star shows 0.9 dex enhancement in the nitrogen. The photosphere of the star is slightly deficient in Mg, Si, and S. (i.e. [Mg/Fe] = -0.2 dex, [Si/Fe] = -0.4 dex, [S/Fe] = -0.2 dex). The Al is slightly enhanced. The phosphorus is overabundant, i.e. [P/Fe] ≈ 1.7 ± 0.47 dex, hence LB3116 may be the first example of a PAGB star which is rich in phosphorus. With its high radial velocity (i.e. V LSR = 73 km s-1), and the deficiencies observed in C, Mg, Si, and S indicate that LB 3116 is likely a hot PAGB star at high galactic latitude.

THE MASS-LOSS RETURN FROM EVOLVED STARS TO THE LARGE MAGELLANIC CLOUD. VI. LUMINOSITIES AND MASS-LOSS RATES ON POPULATION SCALES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Riebel, D.; Meixner, M.; Srinivasan, S.

We present results from the first application of the Grid of Red Supergiant and Asymptotic Giant Branch ModelS (GRAMS) model grid to the entire evolved stellar population of the Large Magellanic Cloud (LMC). GRAMS is a pre-computed grid of 80,843 radiative transfer models of evolved stars and circumstellar dust shells composed of either silicate or carbonaceous dust. We fit GRAMS models to {approx}30,000 asymptotic giant branch (AGB) and red supergiant (RSG) stars in the LMC, using 12 bands of photometry from the optical to the mid-infrared. Our published data set consists of thousands of evolved stars with individually determined evolutionarymore » parameters such as luminosity and mass-loss rate. The GRAMS grid has a greater than 80% accuracy rate discriminating between oxygen- and carbon-rich chemistry. The global dust injection rate to the interstellar medium (ISM) of the LMC from RSGs and AGB stars is on the order of 2.1 Multiplication-Sign 10{sup -5} M{sub Sun} yr{sup -1}, equivalent to a total mass injection rate (including the gas) into the ISM of {approx}6 Multiplication-Sign 10{sup -3} M{sub Sun} yr{sup -1}. Carbon stars inject two and a half times as much dust into the ISM as do O-rich AGB stars, but the same amount of mass. We determine a bolometric correction factor for C-rich AGB stars in the K{sub s} band as a function of J - K{sub s} color, BC{sub K{sub s}}= -0.40(J-K{sub s}){sup 2} + 1.83(J-K{sub s}) + 1.29. We determine several IR color proxies for the dust mass-loss rate (M-dot{sub d}) from C-rich AGB stars, such as log M-dot{sub d} = (-18.90/((K{sub s}-[8.0])+3.37) - 5.93. We find that a larger fraction of AGB stars exhibiting the 'long-secondary period' phenomenon are more O-rich than stars dominated by radial pulsations, and AGB stars without detectable mass loss do not appear on either the first-overtone or fundamental-mode pulsation sequences.« less

Distances of Dwarf Carbon Stars

NASA Astrophysics Data System (ADS)

Harris, Hugh C.; Dahn, Conard C.; Subasavage, John P.; Munn, Jeffrey A.; Canzian, Blaise J.; Levine, Stephen E.; Monet, Alice B.; Pier, Jeffrey R.; Stone, Ronald C.; Tilleman, Trudy M.; Hartkopf, William I.

2018-06-01

Parallaxes are presented for a sample of 20 nearby dwarf carbon stars. The inferred luminosities cover almost two orders of magnitude. Their absolute magnitudes and tangential velocities confirm prior expectations that some originate in the Galactic disk, although more than half of this sample are halo stars. Three stars are found to be astrometric binaries, and orbital elements are determined; their semimajor axes are 1–3 au, consistent with the size of an AGB mass-transfer donor star.

NuSTAR Observations of Heavily Obscured Quasars at z Is Approximately 0.5

NASA Technical Reports Server (NTRS)

Lansbury, G. B.; Alexander, D. M.; Del Moro, A.; Gandhi, P.; Assef, R. J.; Stern, D.; Aird, J.; Ballantyne, D. R.; Balokovic, M.; Bauer, F. E.;

Fluorine and Sodium in C-rich Low-metallicity Stars

NASA Astrophysics Data System (ADS)

Lucatello, Sara; Masseron, Thomas; Johnson, Jennifer A.; Pignatari, Marco; Herwig, Falk

2011-03-01

We present the N, O, F, and Na abundance and 12C/13C isotopic ratio measurements or upper limits for a sample of 10 C-rich, metal-poor giant stars: 8 enhanced in s-process (CEMP-s) elements and 2 poor in n-capture elements (CEMP-no). The abundances are derived from IR, K-band, high-resolution CRIRES@VLT obtained spectra. The metallicity of our sample ranges from [Fe/H] = -3.4 to -1.3. F abundance could be measured only in two CEMP-s stars. With [F/Fe] = 0.64, one is mildly F-overabundant, while the other is F-rich, at [F/Fe] = 1.44. For the remaining eight objects, including both CEMP-no stars in our sample, only upper limits on F abundance could be placed. Our measurements and upper limits show that there is a spread in the [F/C+N] ratio in CEMP-s stars as predicted by theory. Predictions from nucleosynthetic models for low-mass, low-metallicity asymptotic giant branch (AGB) stars account for the derived F abundances, while the upper limits on F content derived for most of the stars are lower than the predicted values. The measured Na content is accounted for by AGB models in the 1.25-1.75 M sun range, confirming that the stars responsible for the peculiar abundance pattern observed in CEMP-s stars are low-mass, low-metallicity AGB stars in agreement with the most accepted astrophysical scenario. We conclude that the mechanism of F production in current state-of-the-art low-metallicity low-mass AGB models needs further scrutiny and that F measurements in a larger number of metal-poor stars are needed to better constrain the models. Based on observations made with ESO Telescopes at Paranal Observatories under program ID 080.D-0606A. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation.

The mass-loss return from evolved stars to the Large Magellanic Cloud. V. The GRAMS carbon-star model grid

NASA Astrophysics Data System (ADS)

Srinivasan, S.; Sargent, B. A.; Meixner, M.

2011-08-01

Context. Outflows from asymptotic giant branch (AGB) and red supergiant (RSG) stars inject dust into the interstellar medium. The total rate of dust return provides an important constraint to galactic chemical evolution models. However, this requires detailed radiative transfer (RT) modeling of individual stars, which becomes impractical for large data sets. An alternative approach is to select the best-fit spectral energy distribution (SED) from a grid of dust shell models, allowing for a faster determination of the luminosities and mass-loss rates for entire samples. Aims: We have developed the Grid of RSG and AGB ModelS (GRAMS) to measure the mass-loss return from evolved stars. The models span the range of stellar, dust shell and grain properties relevant to evolved stars. The GRAMS model database will be made available to the scientific community. In this paper we present the carbon-rich AGB model grid and compare our results with photometry and spectra of Large Magellanic Cloud (LMC) carbon stars from the SAGE (Surveying the Agents of Galaxy Evolution) and SAGE-Spec programs. Methods: We generate models for spherically symmetric dust shells using the 2Dust code, with hydrostatic models for the central stars. The model photospheres have effective temperatures between 2600 and 4000 K and luminosities from ~2000 L⊙ to ~40 000 L⊙. Assuming a constant expansion velocity, we explore five values of the inner radius Rin of the dust shell (1.5, 3, 4.5, 7 and 12 Rstar). We fix the outer radius at 1000 Rin. Based on the results from our previous study, we use amorphous carbon dust mixed with 10% silicon carbide by mass. The grain size distribution follows a power-law and an exponential falloff at large sizes. The models span twenty-six values of 11.3 μm optical depth, ranging from 0.001 to 4. For each model, 2Dust calculates the output SED from 0.2 to 200 μm. Results: Over 12 000 models have dust temperatures below 1800 K. For these, we derive synthetic photometry

The Core Mass Growth and Stellar Lifetime of Thermally Pulsing Asymptotic Giant Branch Stars

NASA Astrophysics Data System (ADS)

Kalirai, Jason S.; Marigo, Paola; Tremblay, Pier-Emmanuel

2014-02-01

We establish new constraints on the intermediate-mass range of the initial-final mass relation, and apply the results to study the evolution of stars on the thermally pulsing asymptotic giant branch (TP-AGB). These constraints derive from newly discovered (bright) white dwarfs in the nearby Hyades and Praesepe star clusters, including a total of 18 high signal-to-noise ratio measurements with progenitor masses of M initial = 2.8-3.8 M ⊙. We also include a new analysis of existing white dwarfs in the older NGC 6819 and NGC 7789 star clusters, M initial = 1.6 and 2.0 M ⊙. Over this range of initial masses, stellar evolutionary models for metallicity Z initial = 0.02 predict the maximum growth of the core of TP-AGB stars. By comparing the newly measured remnant masses to the robust prediction of the core mass at the first thermal pulse on the AGB (i.e., from stellar interior models), we establish several findings. First, we show that the stellar core mass on the AGB grows rapidly from 10% to 30% for stars with M initial = 1.6 to 2.0 M ⊙. At larger masses, the core-mass growth decreases steadily to ~10% at M initial = 3.4 M ⊙, after which there is a small hint of a upturn out to M initial = 3.8 M ⊙. These observations are in excellent agreement with predictions from the latest TP-AGB evolutionary models in Marigo et al. We also compare to models with varying efficiencies of the third dredge-up and mass loss, and demonstrate that the process governing the growth of the core is largely the stellar wind, while the third dredge-up plays a secondary, but non-negligible role. Based on the new white dwarf measurements, we perform an exploratory calibration of the most popular mass-loss prescriptions in the literature, as well as of the third dredge-up efficiency as a function of the stellar mass. Finally, we estimate the lifetime and the integrated luminosity of stars on the TP-AGB to peak at t ~ 3 Myr and E = 1.2 × 1010 L ⊙ yr for M initial ~ 2 M ⊙ (t ~ 2 Myr

Intermediate-mass Asymptotic Giant Branch Stars and Sources of 26Al, 60Fe, 107Pd, and 182Hf in the Solar System

NASA Astrophysics Data System (ADS)

Wasserburg, G. J.; Karakas, Amanda I.; Lugaro, Maria

2017-02-01

We explore the possibility that the short-lived radionuclides {}26{{A}}l, {}60{{F}}e, {}107{{P}}d, and {}182{{H}}f inferred to be present in the proto-solar cloud originated from 3-8 {M}⊙ asymptotic giant branch (AGB) stars. Models of AGB stars with initial mass above 5 {M}⊙ are prolific producers of {}26{{A}}l owing to hot bottom burning (HBB). In contrast, {}60{{F}}e, {}107{{P}}d, and {}182{{H}}f are produced by neutron captures: {}107{{P}}d and {}182{{H}}f in models ≲ 5 {M}⊙ , and {}60{{F}}e in models with higher mass. We mix stellar yields from solar-metallicity AGB models into a cloud of solar mass and composition to investigate whether it is possible to explain the abundances of the four radioactive nuclides at the Sun’s birth using one single value of the mixing ratio between the AGB yields and the initial cloud material. We find that AGB stars that experience efficient HBB (≥slant 6 {M}⊙ ) cannot provide a solution because they produce too little {}182{{H}}f and {}107{{P}}d relative to {}26{{A}}l and {}60{{F}}e. Lower-mass AGB stars cannot provide a solution because they produce too little {}26{{A}}l relative to {}107{{P}}d and {}182{{H}}f. A self-consistent solution may be found for AGB stars with masses in between (4-5.5 {M}⊙ ), provided that HBB is stronger than in our models and the {}13{{C}}(α, n){}16{{O}} neutron source is mildly activated. If stars of {{M}}< 5.5 {M}⊙ are the source of the radioactive nuclides, then some basis for their existence in proto-solar clouds needs to be explored, given that the stellar lifetimes are longer than the molecular cloud lifetimes.

28SiO v = 0 J = 1-0 emission from evolved stars

NASA Astrophysics Data System (ADS)

de Vicente, P.; Bujarrabal, V.; Díaz-Pulido, A.; Albo, C.; Alcolea, J.; Barcia, A.; Barbas, L.; Bolaño, R.; Colomer, F.; Diez, M. C.; Gallego, J. D.; Gómez-González, J.; López-Fernández, I.; López-Fernández, J. A.; López-Pérez, J. A.; Malo, I.; Moreno, A.; Patino, M.; Serna, J. M.; Tercero, F.; Vaquero, B.

2016-05-01

Aims: Observations of 28SiO v = 0J = 1-0 line emission (7-mm wavelength) from asymptotic giant branch (AGB) stars show in some cases peculiar profiles, composed of a central intense component plus a wider plateau. Very similar profiles have been observed in CO lines from some AGB stars and most post-AGB nebulae and, in these cases, they are clearly associated with the presence of conspicuous axial symmetry and bipolar dynamics. We aim to systematically study the profile shape of 28SiO v = 0J = 1-0 lines in evolved stars and to discuss the origin of the composite profile structure. Methods: We present observations of 28SiO v = 0J = 1-0 emission in 28 evolved stars, including O-rich, C-rich, and S-type Mira-type variables, OH/IR stars, semiregular long-period variables, red supergiants and one yellow hypergiant. Most objects were observed in several epochs, over a total period of time of one and a half years. The observations were performed with the 40 m radio telescope of the Instituto Geográfico Nacional (IGN) in Yebes, Spain. Results: We find that the composite core plus plateau profiles are systematically present in O-rich Miras, OH/IR stars, and red supergiants. They are also found in one S-type Mira (χ Cyg) and in two semiregular variables (X Her and RS Cnc) that are known to show axial symmetry. In the other objects, the profiles are simpler and similar to those observed in other molecular lines. The composite structure appears in the objects in which SiO emission is thought to come from the very inner circumstellar layers, prior to dust formation. The central spectral feature is found to be systematically composed of a number of narrow spikes, except for X Her and RS Cnc, in which it shows a smooth shape that is very similar to that observed in CO emission. These spikes show a significant (and mostly chaotic) time variation, while in all cases the smooth components remain constant within the uncertainties. The profile shape could come from the superposition

Chemical Compositions of RV Tauri Stars and Related Objects

NASA Astrophysics Data System (ADS)

Rao, S. S.; Giridhar, S.

2014-04-01

We have undertaken a comprehensive abundance analysis for a sample of relatively unexplored RV Tauri and RV Tauri like stars to further our understanding of post-Asymptotic Giant Branch (post-AGB) evolution. From our study based on high resolution spectra and a grid of model atmospheres, we find indications of mild s-processing for V820 Cen and IRAS 06165+3158. On the other hand, SU Gem and BT Lac exhibit the effects of mild dust-gas winnowing. We have also compiled the existing abundance data on RV Tauri objects and find that a large fraction of them are afflicted by dust-gas winnowing and aided by the present work, we find a small group of two RV Tauris showing mild s-process enhancement in our Galaxy. With two out of three reported s-process enhanced objects belonging to RV Tauri spectroscopic class C, these intrinsically metal-poor objects appear to be promising candidates to analyse the possible s-processing in RV Tauri stars.

Iron Abundance in the Prototype PG 1159 Star, GW Vir Pulsator PG 1159-035, and Related Objects

NASA Technical Reports Server (NTRS)

Werner, K.; Rauch, T.; Kruk, J. W.; Kurucz, R. L.

2011-01-01

We performed an iron abundance determination of the hot, hydrogen deficient post-AGB star PG 1159-035. which is the prototype of the PG 1159 spectral class and the GW Vir pulsators, and of two related objects (PG 1520+525, PG 1144+005), based on the first detection of Fe VIII lines in stellar photospheres. In another PG 1159 star. PG 1424+535. we detect Fe VII lines. In all four stars, each within T(sub eff) = 110,000-150,000 K, we find a solar iron abundance. This result agrees with our recent abundance analysis of the hottest PG 1159 stars (T(sub eff) = 150,000-200,000 K) that exhibit Fe x lines. On the whole, we find that the PG 1159 stars are not significantly iron deficient, in contrast to previous notions.

Mass Loss at Higher Metallicity: Quantifying the Mass Return from Evolved Stars in the Galactic

NASA Astrophysics Data System (ADS)

Sargent, Benjamin

Bulge Mass-losing evolved stars, and in particular asymptotic giant branch (AGB) stars and red supergiant (RSG) stars, are expected to be the major producers of dust in galaxies. This dust will help form planetary systems around future generations of stars. Our ADAP program to measure the mass loss from the AGB and RSG stars in the Magellanic Clouds is nearing completion, and we wish to extend this successful study to the Galactic bulge of the Milky Way Galaxy. Metallicity should determine the amount of elements available to condense dust in the star's outflow, so evolved stars of differing metallicities should have differing mass-loss rates. Building upon our work on evolved stars in the Magellanic Clouds, we will compare the mass-loss rates from AGB and RSG stars in the older and potentially more metal-rich Bulge to the mass-loss rates of AGB and RSG stars in the Magellanic Clouds, which have lower metallicity, making for an interesting contrast. In addition, the Galactic bulge, like the Clouds, is located at a well-determined distance ( 8 kpc), thereby removing the distance ambiguities that present a major uncertainty in determining mass-loss rates and luminosities for evolved stars. To model photometric observations of outflowing dust shells around evolved stars, we have constructed the Grid of Red supergiant and Asymptotic giant branch ModelS (GRAMS; Sargent et al 2011; Srinivasan et al 2011) using the radiative transfer code 2Dust (Ueta and Meixner 2003). Our study will apply these models to the large photometric database of sources identified in the Spitzer Space Telescope GLIMPSE survey of the Milky Way and also to the various infrared spectra of Bulge AGB and RSG stars from Spitzer, ISO, etc. We have already modeled a few Galactic bulge evolved stars with GRAMS, and we will use these results as the foundation for modeling a large and representative sample of Galactic bulge evolved stars identified and measured photometrically by GLIMPSE. We will use our

Amorphous carbon in the disk around the post-AGB binary HR 4049. Discerning dust species with featureless opacity curves

NASA Astrophysics Data System (ADS)

Acke, B.; Degroote, P.; Lombaert, R.; de Vries, B. L.; Smolders, K.; Verhoelst, T.; Lagadec, E.; Gielen, C.; Van Winckel, H.; Waelkens, C.

2013-03-01

Context. Infrared spectroscopy has been extensively used to determine the mineralogy of circumstellar dust. The identification of dust species with featureless opacities, however, is still ambiguous. Here we present a method to lift the degeneracy using the combination of infrared spectroscopy and interferometry. Aims: The binary post-AGB star HR 4049 is surrounded by a circumbinary disk viewed at a high inclination angle. Apart from gaseous emission lines and molecular emission bands of polycyclic aromatic hydrocarbons (PAH), diamonds, and fullerenes, the 2-25 μm infrared spectrum is featureless. The goal of the paper is to identify the dust species responsible for the smooth spectrum. Methods: We gathered high-angular-resolution measurements in the near- and mid-infrared with the VLTI interferometric instruments AMBER and MIDI. The data set is expanded with archival Geneva optical photometry, ISO-SWS and Spitzer-IRS infrared spectroscopy, and VISIR N-band images and spectroscopy. We computed a grid of radiative-transfer models of the circumbinary disk of HR 4049 using the radiative-transfer code MCMax. We searched for models that provide good fits simultaneously to all available observations. Results: We find that the variable optical extinction towards the primary star is consistent with the presence of very small (0.01 μm) iron-bearing dust grains or amorphous carbon grains. The combination of the interferometric constraint on the disk extent and the shape of the infrared spectrum points to amorphous carbon as the dominant source of opacity in the circumbinary disk of HR 4049. The disk is optically thick to the stellar radiation in the radial direction. At infrared wavelengths it is optically thin. The PAH emission is spatially resolved in the VISIR data and emanates from a region with an extent of several hundreds of AU, with a projected photocenter displacement of several tens of AU from the disk center. The PAHs most likely reside in a bipolar outflow

The Evolution of Carbon Stars

NASA Astrophysics Data System (ADS)

Chan, S. Josephine

1993-04-01

This dissertation is concerned with the nature of the carbon stars, unusual late-type stars in which the abundance of carbon in the photosphere is greater than that of oxygen. Data from the Infrared Astronomical Satellite (IRAS) survey has shown that carbon stars which were identified from optical surveys and those identified from the SiC dust features in their IRAS Low Resolution Spectrometer LRS spectra have different IRAS colours. The former (which will be referred to as visual carbon stars) are visually bright and have large excesses at 6 microns, while the latter group (which will be referred to as infrared carbon stars) have blackbody energy distributions. The origin of visual carbon stars has been discussed by Chan and Kwok (1988) based on the hypothesis of Willems and de Jong (1988). A complete sample of visual carbon stars detected by IRAS with 12 microns flux densities greater than 5 Jy was selected, and 207 LRS spectra were extracted for those sources without previous \\lrs data. Of these, 152 sources had new LRS spectra with reasonably good signal-to-noise ratio and 575 sources had previously released LRS spectra. All these spectra have been classified with the scheme of Volk and Cohen (1989). When the LRS spectra of these 727 IRAS CCGCS sources were examined, 15 were found to show the 9.7 microns silicate emission feature which is expected to occur only in an oxygen-rich circumstellar shell. Eight of these are reported for the first time in this dissertation. This group of visual carbon stars (hereafter called silicate carbon stars) may represent transition objects between oxygen-rich and carbon stars on the asymptotic giant branch (AGB) because the photosphere is carbon-rich while the circumstellar material resembles that from a typical M-type star. A radiative transfer dust shell model for these silicate carbon stars is presented. The model spectra produce excellent fits to the observed energy distributions of these silicate carbon stars. The J

Dust clouds around red giant stars - Evidence of sublimating comet disks?

NASA Technical Reports Server (NTRS)

Matese, John J.; Whitmire, Daniel P.; Reynolds, Ray T.

1989-01-01

The dust production by disk comets around intermediate mass stars evolving into red giants is studied, focusing on AGB supergiants. The model of Iben and Renzini (1983) is used to study the observed dust mass loss for AGB stars. An expression is obtained for the comet disk net dust production rate and values of the radius and black body temperature corresponding to peak sublimation are calculated for a range of stellar masses. Also, the fractional amount of dust released from a cometesimal disk during a classical nova outburst is estimated.

The Far-Infrared Luminosity Function and Star Formation Rate Density for Dust Obscured Galaxies in the Bootes Field

NASA Astrophysics Data System (ADS)

Calanog, Jae Alyson; Wardlow, J. L.; Fu, H.; Cooray, A. R.; HerMES

2013-01-01

We present the far-Infrared (FIR) luminosity function (LF) and the star-formation rate density (SFRD) for dust-obscured galaxies (DOGs) in the Bootes field at redshift 2. These galaxies are selected by having a large rest frame mid-IR to UV flux density ratio ( > 1000) and are expected to be some of the most luminous and heavily obscured galaxies in the Universe at this epoch. Photometric redshifts for DOGs are estimated from optical and mid-IR data using empirically derived low resolution spectral templates for AGN and galaxies. We use HerMES Herschel-SPIRE data to fit a modified blackbody to calculate the FIR luminosity (LFIR) and dust temperature (Td) for all DOGs individually detected in SPIRE maps. A stacking analyses was implemented to measure a median sub-mm flux of undetected DOGs. We find that DOGs have LIR and Td that are similar with the sub-millimeter galaxy (SMG) population, suggesting these two populations are related. The DOG LF and SFRD at 2 are calculated and compared to SMGs.

THE STAR FORMATION HISTORIES OF z {approx} 2 DUST-OBSCURED GALAXIES AND SUBMILLIMETER-SELECTED GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bussmann, R. S.; Dey, Arjun; Jannuzi, B. T.

The Spitzer Space Telescope has identified a population of ultraluminous infrared galaxies (ULIRGs) at z {approx} 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M{sub *}) of two populations of Spitzer-selected ULIRGs that have extremely red R - [24] colors (dust-obscured galaxies, or DOGs) and compare our results with submillimeter-selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest frame 1.6 {mu}m associated with stellar emission ({sup b}ump DOGs{sup )}, while the other set of 51 DOGs havemore » power-law mid-IR SEDs that are typical of obscured active galactic nuclei ({sup p}ower-law DOGs{sup )}. We measure M{sub *} by applying Charlot and Bruzual stellar population synthesis models to broadband photometry in the rest-frame ultraviolet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function, we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 more massive than SMGs, respectively (median and inter-quartile M{sub *} values for SMGs, bump DOGs, and power-law DOGs are log(M{sub *}/M{sub Sun }) = 10.42{sup +0.42}{sub -0.36}, 10.62{sup +0.36}{sub -0.32}, and 10.71{sup +0.40}{sub -0.34}, respectively). More realistic star formation histories drawn from two competing theories for the nature of ULIRGs at z {approx} 2 (major merger versus smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star formation rate (SFR) in these z {approx} 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M{sub *}, a situation that arises more naturally in major mergers than in smooth accretion-powered systems.« less

Molecular processes from the AGB to the PN stage

NASA Astrophysics Data System (ADS)

García-Hernández, D. Anibal

2012-08-01

Many complex organic molecules and inorganic solid-state compounds have been observed in the circumstellar shell of stars (both C-rich and O-rich) in the transition phase between Asymptotic Giant Branch (AGB) stars and Planetary Nebulae (PNe). This short (~102-104 years) phase of stellar evolution represents a wonderful laboratory for astrochemistry and provides severe constraints on any model of gas-phase and solid-state chemistry. One of the major challenges of present day astrophysics and astrochemistry is to understand the formation pathways of these complex organic molecules and inorganic solid-state compounds (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene in the case of a C-rich chemistry and oxides and crystalline silicates in O-rich environments) in space. In this review, I present an observational review of the molecular processes in the late stages of stellar evolution with a special emphasis on the first detections of fullerenes and graphene in PNe.

Galaxy gas as obscurer - I. GRBs x-ray galaxies and find an NH3∝ M_{star} relation

NASA Astrophysics Data System (ADS)

Buchner, Johannes; Schulze, Steve; Bauer, Franz E.

2017-02-01

An important constraint for galaxy evolution models is how much gas resides in galaxies, in particular, at the peak of star formation z = 1-3. We attempt a novel approach by letting long-duration gamma ray bursts (LGRBs) x-ray their host galaxies and deliver column densities to us. This requires a good understanding of the obscurer and biases introduced by incomplete follow-up observations. We analyse the X-ray afterglow of all 844 Swift LGRBs to date for their column density NH. To derive the population properties, we propagate all uncertainties in a consistent Bayesian methodology. The NH distribution covers the 1020-23 cm-2 range and shows no evolutionary effect. Higher obscurations, e.g. Compton-thick columns, could have been detected but are not observed. The NH distribution is consistent with sources randomly populating a ellipsoidal gas cloud of major axis {N^{major}H }=10^{23}cm^{-2} with 0.22 dex intrinsic scatter between objects. The unbiased SHOALS survey of afterglows and hosts allows us to constrain the relation between Spitzer-derived stellar masses and X-ray derived column densities NH. We find a well-constrained power-law relation of NH = 1021.7 cm-2 × (M⋆/109.5 M⊙)1/3, with 0.5 dex intrinsic scatter between objects. The Milky Way and the Magellanic clouds also follow this relation. From the geometry of the obscurer, its stellar mass dependence and comparison with local galaxies, we conclude that LGRBs are primarily obscured by galaxy-scale gas. Ray tracing of simulated Illustris galaxies reveals a relation of the same normalization, but a steeper stellar-mass dependence and mild redshift evolution. Our new approach provides valuable insight into the gas residing in high-redshift galaxies.

Spitzer-IRS Spectroscopic Studies of the Properties of Dust from Oxygen-Rich Asymptotic Giant Branch and Red Supergiant Stars

NASA Astrophysics Data System (ADS)

Sargent, Benjamin A.; Speck, A.; Volk, K.; Kemper, C.; Reach, W. T.; Lagadec, E.; Bernard, J.; McDonald, I.; Meixner, M.; Srinivasan, S.

2014-01-01

We analyze the dust emission features seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of Oxygen-rich (O-rich) asymptotic giant branch (AGB) and red supergiant (RSG) stars. The spectra come from the Spitzer Legacy program SAGE-Spectroscopy (PI: F. Kemper) and other archival Spitzer-IRS programs. The broad 10 and 20 micron emission features attributed to amorphous dust of silicate composition seen in the spectra show evidence for systematic differences in the centroid of both emission features between O-rich AGB and RSG populations. Radiative transfer modeling using the GRAMS grid of models of AGB and RSG stars suggests that the centroid differences are due to differences in dust properties. We investigate differences in dust composition, size, shape, etc that might be responsible for these spectral differences. We explore how these differences may arise from the different circumstellar environments around RSG and O-rich AGB stars. BAS acknowledges funding from NASA ADAP grant NNX13AD54G.

Nonequilibrium iron oxide formation in some low-mass post-asymptotic giant branch stars

NASA Technical Reports Server (NTRS)

Rietmeijer, Frans J. M.

1992-01-01

Using experimental evidence that under highly oxidizing conditions gamma-Fe2O3 (maghemite) and Fe3O4 display refractory behavior, it is proposed that very low C/O ratios, that could be unique to evolving AGB stars, induce nonequilibrium formation of ferromagnetic iron oxide grains along with chondritic dust. The oxides are preferentially fractionated from chondritic dust in the stellar magnetic field which could account for the observed extreme iron underabundance in their photosphere. A search for the 1-2.5-micron IR absorption feature, or for diagnostic magnetite and maghemite IR absorption features, could show the validity of the model proposed.

AGB nucleosynthesis: The 19F(α, p)22Ne reaction at astrophysical energies

NASA Astrophysics Data System (ADS)

D'Agata, G.; Pizzone, R. G.; Spitaleri, C.; Blagus, S.; Figuera, P.; Grassi, L.; Guardo, G. L.; Gulino, M.; Hayakawa, S.; Indelicato, I.; Kshetri, R.; La Cognata, M.; Lamia, L.; Lattuada, M.; Mijatović, T.; Milin, M.; Miljanic, D.; Prepolec, L.; Sergi, M. L.; Skukan, N.; Soic, N.; Tokic, V.; Tumino, A.; Uroic, M.

2017-06-01

Learning how 19F is produced and destructed in AGB-stars is crucial. Fluorine abundance is in fact important, given that it is strongly tied to standard and extra-mixing processes taking place in AGB-stars. This kind of objects are considered to be the main sources of fluorine in galactic environment, in which experimental abundances are far overestimated. For this reason the reaction 19F(α, p)22Ne, that represents the main destruction channel in He-rich environment, was studied at energies corresponding to T˜2.108 K. Such reaction has been studied with direct method at Ebeam = 1100 keV for alpha particles impinging on a fluorine target, corresponding to EC.M. ˜ 900 keV, still far from the Gamow window, placed at 390÷800 keV, below the Coulomb barrier (3.81 MeV). An experiment was performed at Rujer Boskovic Institut (Zagreb), applying the Trojan Horse Method. With this experimental procedure we were able to select the quasi-free contribution coming from 6Li(19F,p 22Ne)2H at Ebeam = 6 MeV at kinematically useful angles. We measured the 19F(α, p)22Ne at 0 MeV ≤ EC.M ≤ 0.9 MeV, extracting the two body cross-section in absolute units at energies of astrophysical interest.

An unbiased view of X-ray obscuration amongst active galactic nuclei with NuLANDS

NASA Astrophysics Data System (ADS)

Boorman, Peter Gregory; Gandhi, Poshak; Stern, Daniel; Harrison, Fiona; NuSTAR Obscured AGN Team

2018-01-01

Nearly all active galactic nuclei (AGN) are obscured in X-rays behind column densities of NH ≥ 1022 cm-2. Hard X-ray studies have proven very effective to quanitfy the levels of obscuration amongst AGN, up to and just above the Compton-thick limit (NH ˜ 1.5 × 1024 cm-2). However, Compton-thick sources with NH values beyond this limit are typically missed in hard X-ray all-sky surveys such as Swift/BAT, requiring many studies to apply considerable bias corrections to account for the loss. Incorrectly quanitfying the heavily obscured AGN population can have a dramatic effect on synthesis models designed to fit the Cosmic X-ray Background spectrum, due to their significant contribution to the peak flux of the background at ~30 keV. This is what motivated the NuSTAR Local AGN NH Distribution Survey (NuLANDS) - a NuSTAR 1 Ms legacy survey of an obscuration-independent, infrared selected sample of AGN, undetected by BAT and unobserved by NuSTAR before - a considerable number of which are predicted to be heavily obscured. NuSTAR is the first true X-ray focusing instrument capable of spectral analysis > 10 keV, and as such can and will place robust constraints on the NH values of these elusive AGN. In this poster, I will present the first results from NuLANDS, including multiple newly identified Compton-thick AGN, previously undetected in the Swift/BAT 70-month catalog. I will further highlight the exciting prospects for the complete NuLANDS sample, with the ultimate goal of constructing a representative NH distribution of AGN in the local Universe, requiring minimal bias corrections.

Neutron-captures in Low Mass Stars and the Early Solar System Record of Short-lived Radioactivities

NASA Astrophysics Data System (ADS)

Busso, Maurizio; Vescovi, Diego; Trippella, Oscar; Palmerini, Sara; Cristallo, Sergio; Piersanti, Luciano

2018-01-01

Noticeable improvements were recently introduced in the modelling of n-capture nucleosynthesis in the advanced evolutionary stages of giant stars (Asymptotic Giant Branch, or AGB, stars). Two such improvements are closely linked together and concern the introduction of non-parameterized, physical models for extended mixing processes and the adoption of accurate reaction rates for H- and He-burning reactions, including the one for the main neutron source 13C(α,n)16O. These improvements profited of a longstanding collaboration between stellar physicists and C. Spitaleri's team and of his seminal work both as a leader in the Nuclear Astrophysics scenario and as a talent-scout in the recruitment of young researchers in the field. We present an example of the innovative results that can be obtained thanks to the novelties introduced, by estimating the contributions from a nearby AGB star to the synthesis of short-lived (t1/2 ≤ 10 Myr) radioactive nuclei which were alive in early Solar System condensates. We find that the scenario indicating an AGB star as the source of such radioactivities, discussed for many years by researchers in this field, appears now to be no longer viable, when the mentioned improvements of AGB models and nuclear parameters are considered.

Mining the Obscured OB Star Population in Carina

NASA Astrophysics Data System (ADS)

Smith, Michael

2016-04-01

Massive OB stars are very influential objects in the ecology of galaxies like our own. Current catalogues of Galactic OB stars are heavily biased towards bright (g < 13) objects, only typically including fainter objects when found in prominent star clusters (Garmany et al., 1982; Reed, 2003; Maíz-Apellaniz et al., 2004). Exploitation of the VST Photometric Hα Survey (VPHAS+) allows us to build a robust catalogue of photometrically-selected OB stars across the entire Southern Galactic plane, both within clusters and in the field, down to ∼20th magnitude in g. For the first time, a complete accounting of the OB star runaway phenomenon becomes possible. Along with making the primary selection using VPHAS+ colours, I have performed Markov-Chain Monte Carlo fitting of the spectral energy distributions of the selected stars by combining VPHAS+ u, g, r, i with published J, H, K photometry. This gives rough constraints on effective temperature and distance, whilst delivering much more precise reddening parameters A0 and RV - allowing us to build a much richer picture of how extinction and extinction laws vary across the Galactic Plane. My thesis begins with a description of the method of photometric selection of OB star candidates and its validation across a 2 square degree field including the well-known young massive star cluster Westerlund 2 (Mohr-Smith et al., 2015). Following on from this I present spectroscopy with AAOmega of 283 candidates identified by our method, which confirms that ∼94% of the sample are the expected O and early B stars. I then develop this method further and apply it to a Galactic Plane strip of 42 square-degrees that runs from the Carina Arm tangent region to the much studied massive cluster in NGC 3603. A new aspect I attend to in this expansion of method is tightening up the uniform photometric calibration of the data, paying particular attention to the always-challenging u band. This leads to a new and reliable catalogue of 5915 OB

Do meteoritic silicon carbide grains originate from asymptotic giant branch stars of super-solar metallicity?

NASA Astrophysics Data System (ADS)

Lugaro, Maria; Karakas, Amanda I.; Pető, Mária; Plachy, Emese

2018-01-01

We compare literature data for the isotopic ratios of Zr, Sr, and Ba from analysis of single meteoritic stardust silicon carbide (SiC) grains to new predictions for the slow neutron-capture process (the s process) in metal-rich asymptotic giant branch (AGB) stars. The models have initial metallicities Z = 0.014 (solar) and Z = 0.03 (twice-solar) and initial masses 2-4.5 M⊙ , selected such as the condition C/O > 1 for the formation of SiC is achieved. Because of the higher Fe abundance, the twice-solar metallicity models result in a lower number of total free neutrons released by the 13C(α ,n)16O neutron source. Furthermore, the highest-mass (4-4.5 M⊙) AGB stars of twice-solar metallicity present a milder activation of the 22Ne(α ,n)25Mg neutron source than their solar metallicity counterparts, due to cooler temperatures resulting from the effect of higher opacities. They also have a lower amount of the 13C neutron source than the lower-mass models, following their smaller He-rich region. The combination of these different effects allows our AGB models of twice-solar metallicity to provide a match to the SiC data without the need to consider large variations in the features of the 13C neutron source nor neutron-capture processes different from the s process. This raises the question if the AGB parent stars of meteoritic SiC grains were in fact on average of twice-solar metallicity. The heavier-than-solar Si and Ti isotopic ratios in the same grains are in qualitative agreement with an origin in stars of super-solar metallicity because of the chemical evolution of the Galaxy. Further, the SiC dust mass ejected from C-rich AGB stars is predicted to significantly increase with increasing the metallicity.

Heavy-element yields and abundances of asymptotic giant branch models with a Small Magellanic Cloud metallicity

NASA Astrophysics Data System (ADS)

Karakas, Amanda I.; Lugaro, Maria; Carlos, Marília; Cseh, Borbála; Kamath, Devika; García-Hernández, D. A.

2018-06-01

We present new theoretical stellar yields and surface abundances for asymptotic giant branch (AGB) models with a metallicity appropriate for stars in the Small Magellanic Cloud (SMC, Z = 0.0028, [Fe/H] ≈ -0.7). New evolutionary sequences and post-processing nucleosynthesis results are presented for initial masses between 1 and 7 M⊙, where the 7 M⊙ is a super-AGB star with an O-Ne core. Models above 1.15 M⊙ become carbon rich during the AGB, and hot bottom burning begins in models M ≥ 3.75 M⊙. We present stellar surface abundances as a function of thermal pulse number for elements between C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g. 12C/13C), which can be compared to observations. The integrated stellar yields are presented for each model in the grid for hydrogen, helium, and all stable elements from C to Bi. We present evolutionary sequences of intermediate-mass models between 4 and 7 M⊙ and nucleosynthesis results for three masses (M = 3.75, 5, and 7 M⊙) including s-process elements for two widely used AGB mass-loss prescriptions. We discuss our new models in the context of evolved AGB and post-AGB stars in the SMCs, barium stars in our Galaxy, the composition of Galactic globular clusters including Mg isotopes with a similar metallicity to our models, and to pre-solar grains which may have an origin in metal-poor AGB stars.

Radiation-pressure-driven sub-Keplerian rotation of the disc around the AGB star L2 Pup

NASA Astrophysics Data System (ADS)

Haworth, Thomas J.; Booth, Richard A.; Homan, Ward; Decin, Leen; Clarke, Cathie J.; Mohanty, Subhanjoy

2018-01-01

We study the sub-Keplerian rotation and dust content of the circumstellar material around the asymptotic giant branch (AGB) star L2 Puppis. We find that the thermal pressure gradient alone cannot explain the observed rotation profile. We find that there is a family of possible dust populations for which radiation pressure can drive the observed sub-Keplerian rotation. This set of solutions is further constrained by the spectral energy distribution (SED) of the system, and we find that a dust-to-gas mass ratio of ∼10-3 and a maximum grain size that decreases radially outwards can satisfy both the rotation curve and SED. These dust populations are dynamically tightly coupled to the gas azimuthally. However, grains larger than ∼ 0.5 μm are driven outwards radially by radiation pressure at velocities ∼5 km s-1, which implies a dust replenishment rate of ∼3 × 10-9 M⊙ yr-1. This replenishment rate is consistent with observational estimates to within uncertainties. Coupling between the radial motion of the dust and gas is weak and hence the gas does not share in this rapid outward motion. Overall, we conclude that radiation pressure is a capable and necessary mechanism to explain the observed rotation profile of L2 Pup, and offers other additional constraints on the dust properties.

Asymptotic Giant Branch stars as a source of short-lived radioactive nuclei in the solar nebula

NASA Astrophysics Data System (ADS)

Wasserburg, G. J.; Busso, M.; Gallino, R.; Raiteri, C. M.

1994-03-01

We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (106 less than or equal to Tau-bar less than or equal to 2 x 107 yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau0 were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau0. It is found that small masses MHe of He-shell material (10-4-10-7 solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau0 = 0.03 mbarn-1) which contaminated the cloud with a dilution factor of MHe/solar mass approximately 1.5 x 10-4. This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10-4 of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities

The "Water-Fountain Nebula" IRAS 16342-3814: Hubble Space Telescope/Very Large Array Study of a Bipolar Protoplanetary Nebula

NASA Technical Reports Server (NTRS)

Sahal, Raghvendra; teLintelHekkert, Peter; Morris, Mark; Zijlstra, Albert; Likkel, Lauren

1999-01-01

We present Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 images and VLA OH maser emission-line maps of the cold infrared object IRAS 16342-3814, believed to be a protoplanetary nebula. The HST images show an asymmetrical bipolar nebula, with the lobes separated by a dark equatorial waist. The two bright lobes and the dark waist are simply interpreted as bubble-like reflection nebulae illuminated by starlight escaping through polar holes in a dense, flattened, optically thick cocoon of dust, which completely obscures the central star. A faint halo can be seen surrounding each of the lobes. The bubbles are likely to have been created by a fast outflow (evidenced by H2O emission) plowing into a surrounding dense, more slowly expanding, circumstellar envelope of the progenitor asymptotic giant-branch (AGB) star (evidenced by the halo). The IRAS fluxes indicate a circumstellar mass of about 0.7 solar mass (D/2 kpc) and an AGB mass-loss rate of about 10(exp -4) solar mass/yr (V(sub exp)/15 km/s)(D/2 kpc)(sup 2) (assuming a gas-to-dust ratio of 200). OH features with the largest redshifted and blueshifted velocities are concentrated around the bright eastern and western polar lobes, respectively, whereas intermediate-velocity features generally occur at low latitudes, in the dark waist region. We critically examine evidence for the post-AGB classification of IRAS 16342-3814.

Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus. Non-LTE abundance analysis of CO, SiO, and HCN

NASA Astrophysics Data System (ADS)

Van de Sande, M.; Decin, L.; Lombaert, R.; Khouri, T.; de Koter, A.; Wyrowski, F.; De Nutte, R.; Homan, W.

2018-01-01

Context. The stellar outflows of low- to intermediate-mass stars are characterised by a rich chemistry. Condensation of molecular gas species into dust grains is a key component in a chain of physical processes that leads to the onset of a stellar wind. In order to improve our understanding of the coupling between the micro-scale chemistry and macro-scale dynamics, we need to retrieve the abundance of molecules throughout the outflow. Aims: Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. SiO is thought to play an essential role in the dust-formation process of oxygen-rich AGB stars. The presence of HCN in an oxygen-rich environment is thought to be due to non-equilibrium chemistry in the inner wind. Methods: We analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with data available in the literature. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. Results: We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the stellar outflow probed by our molecular data. For SiO, we find that the initial abundance lies between 5.5 × 10-5 and 6.0 × 10-5 with respect to H2. The abundance profile is constant up to 60 ± 10 R∗, after which it declines following a Gaussian profile with an e-folding radius of 3.5 ± 0.5 × 1013 cm or 1.4 ± 0.2 R∗. For HCN, we find an initial abundance of 5.0 × 10-7 with respect to H2. The Gaussian profile that describes the decline

A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph

DTIC Science & Technology

2006-10-23

spectral features, MSX SMC 029, in the Small Magellanic Cloud (SMC) usimg the low-resolution modules of the Infrared Spectrograph on the Spitzer Space ...029, in the Small Magellanic Cloud (SMC) using the low-resolution modules of the Infrared Spectrograph on the Spitzer Space Telescope. A cool dust... outer atmosphere expands and pulsates, pushing gas away from the star where it can cool and condense into dust grains. The resulting circumstellar dust

OXYGEN AND SODIUM ABUNDANCES IN M13 (NGC 6205) GIANTS: LINKING GLOBULAR CLUSTER FORMATION SCENARIOS, DEEP MIXING, AND POST-RGB EVOLUTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Christian I.; Pilachowski, Catherine A., E-mail: cijohnson@astro.ucla.edu, E-mail: catyp@astro.indiana.edu

We present O, Na, and Fe abundances, as well as radial velocities, for 113 red giant branch (RGB) and asymptotic giant branch (AGB) stars in the globular cluster M13. The abundances and velocities are based on spectra obtained with the WIYN-Hydra spectrograph, and the observations range in luminosity from the horizontal branch (HB) to RGB tip. The results are examined in the context of recent globular cluster formation scenarios. We find that M13 exhibits many key characteristics that suggest its formation and chemical enrichment are well described by current models. Some of these observations include the central concentration of O-poormore » stars, the notable decrease in [O/Fe] (but small increase in [Na/Fe]) with increasing luminosity that affects primarily the 'extreme' population, the small fraction of stars with halo-like composition, and the paucity of O-poor AGB stars. In agreement with recent work, we conclude that the most O-poor M13 giants are likely He-enriched and that most (all?) O-poor RGB stars evolve to become extreme HB and AGB-manque stars. In contrast, the 'primordial' and 'intermediate' population stars appear to experience standard HB and AGB evolution.« less

Rejuvenation of the Innocent Bystander: Testing Spin-Up in a Dwarf Carbon Star Sample

NASA Astrophysics Data System (ADS)

Green, Paul

2014-09-01

Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dC stars are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

Hot horizontal branch stars: Predictions for mass loss. Winds, rotation, and the low gravity problem

NASA Astrophysics Data System (ADS)

Vink, Jorick S.; Cassisi, Santi

2002-09-01

We predict mass-loss rates for the late evolutionary phases of low-mass stars, with special emphasis on the consequences for the morphology of the Horizontal Branch (HB). We show that the computed rates, as predicted by the most plausible mechanism of radiation pressure on spectral lines, are too low to produce EHB/sdB stars. This invalidates the scenario recently outlined by Yong et al. (2000) to create these objects by mass loss on the HB. We argue, however, that mass loss plays a role in the distribution of rotational velocities of hot HB stars, and may - together with the enhancement of heavy element abundances due to radiative levitation - provide an explanation for the so-called ``low gravity'' problem. The mass loss recipe derived for hot HB (and extreme HB, sdB, sdOB) stars may also be applied to post-HB (AGB-manqué, UV-bright) stars over a range in effective temperatures between 12 500-40 000 K.

An interferometric study of the post-AGB binary 89 Herculis. II. Radiative transfer models of the circumbinary disk

NASA Astrophysics Data System (ADS)

Hillen, M.; Menu, J.; Van Winckel, H.; Min, M.; Gielen, C.; Wevers, T.; Mulders, G. D.; Regibo, S.; Verhoelst, T.

2014-08-01

Context. The presence of stable disks around post-asymptotic giant branch (post-AGB) binaries is a widespread phenomenon. Also, the presence of (molecular) outflows is now commonly inferred in these systems. Aims: In the first paper of this series, a surprisingly large fraction of optical light was found to be resolved in the 89 Her post-AGB binary system. The data showed that this flux arises from close to the central binary. Scattering off the inner rim of the circumbinary disk, or scattering in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the circumbinary disk, we aim to discriminate between the two proposed configurations. Methods: By including Herschel/SPIRE photometry, we extend the spectral energy distribution (SED) such that it now fully covers UV to sub-mm wavelengths. The MCMax Monte Carlo radiative transfer code is used to create a large grid of disk models. Our models include a self-consistent treatment of dust settling as well as of scattering. A Si-rich composition with two additional opacity sources, metallic Fe or amorphous C, are tested. The SED is fit together with archival mid-IR (MIDI) visibilities, and the optical and near-IR visibilities of Paper I. In this way we constrain the structure of the disk, with a focus on its inner rim. Results: The near-IR visibility data require a smooth inner rim, here obtained with a double power-law parameterization of the radial surface density distribution. A model can be found that fits all of the IR photometric and interferometric data well, with either of the two continuum opacity sources. Our best-fit passive models are characterized by a significant amount of ~mm-sized grains, which are settled to the midplane of the disk. Not a single disk model fits our data at optical wavelengths because of the opposing constraints imposed by the optical and near-IR interferometric data. Conclusions: A geometry in which a passive, dusty, and puffed

Comparative Studies of the Dust around Red Supergiant and Oxygen-Rich Asymptotic Giant Branch Stars in the Local Universe

NASA Astrophysics Data System (ADS)

Sargent, Benjamin; Srinivasan, Sundar; Speck, Angela K.; Volk, Kevin; Kemper, Ciska; Reach, William; Lagadec, Eric; Bernard, Jean-Philippe; McDonald, Iain; Meixner, Margaret; Sloan, Greg; Jones, Olivia

2015-08-01

We analyze the dust emission features seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of red supergiant (RSG) and oxygen-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud galaxies and in various Milky Way globular clusters. The spectra come from the Spitzer Legacy program SAGE-Spectroscopy (PI: F. Kemper), the Spitzer program SMC-Spec (PI: G. Sloan), and other archival Spitzer-IRS programs. The broad 10 and 20 μm emission features attributed to amorphous dust of silicate composition seen in the spectra show evidence for systematic differences in the centroid of both emission features between O-rich AGB and RSG populations. Radiative transfer modeling using the GRAMS grid of models of AGB and RSG stars suggests that the centroid differences are due to differences in dust properties. We investigate differences in dust composition, size, shape, etc that might be responsible for these spectral differences. We explore how these differences may arise from the different circumstellar environments around RSG and O-rich AGB stars and assess effects of varying metallicity (LMC versus SMC versus Milky Way globular cluster) and other properties (mass-loss rate, luminosity, etc.) on the dust originating from these stars. BAS acknowledges funding from NASA ADAP grant NNX13AD54G.

Comparative Studies of the Dust around Red Supergiant and Oxygen-Rich Asymptotic Giant Branch Stars in the Local Universe

NASA Astrophysics Data System (ADS)

Sargent, B. A.; Srinivasan, S.; Speck, A.; Volk, K.; Kemper, F.; Reach, W.; Lagadec, E.; Bernard, J.-P.; McDonald, I.; Meixner, M.; Sloan, G. C.; Jones, O.

We analyze the dust emission features seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of red supergiant (RSG) and oxygen-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud galaxies and in various Milky Way globular clusters. The spectra come from the Spitzer Legacy program SAGE-Spectroscopy (PI: F. Kemper), the Spitzer program SMC-Spec (PI: G. Sloan), and other archival Spitzer-IRS programs. The broad 10 and 20 micron emission features attributed to amorphous dust of silicate composition seen in the spectra show evidence for systematic differences in the centroid of both emission features between O-rich AGB and RSG populations. Radiative transfer modeling using the GRAMS grid of models of AGB and RSG stars suggests that the centroid differences are due to differences in dust properties. We investigate differences in dust composition, size, shape, etc that might be responsible for these spectral differences. We explore how these differences may arise from the different circumstellar environments around RSG and O-rich AGB stars and assess effects of varying metallicity (LMC versus SMC versus Milky Way globular cluster) and other properties (mass-loss rate, luminosity, etc.) on the dust originating from these stars. BAS acknowledges funding from NASA ADAP grant NNX13AD54G.

Asymptotic Giant Branch stars as a source of short-lived radioactive nuclei in the solar nebula

NASA Technical Reports Server (NTRS)

Wasserburg, G. J.; Busso, M.; Gallino, R.; Raiteri, C. M.

1994-01-01

We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10(exp 6) less than or equal to Tau-bar less than or equal to 2 x 10(exp 7) yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau(sub 0) were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau(sub 0). It is found that small masses M(sub He) of He-shell material (10(exp -4)-10(exp -7) solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau(sub 0) = 0.03 mbarn(sup -1)) which contaminated the cloud with a dilution factor of M(sub He)/solar mass approximately 1.5 x 10(exp -4). This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10(exp -4) of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is

Mass-loss rates and luminosities of evolved stars in the Magellanic Clouds .

NASA Astrophysics Data System (ADS)

Groenewegen, M. A. T.; Sloan, G. C.

Stars on the asymptotic giant branch (AGB) stars play an important role in the chemical evolution of their host galaxies and the life cycle of dust in the interstellar medium. A detailed and quantitative understanding of they lose mass and eject their envelopes remains elusive, particularly how that process depends on metallicity. Groenewegen & Sloan (2017, hereafter GS17) recently presented dust radiative transfer models for 225 carbon stars and 171 oxygen-rich evolved stars in the Magellanic Clouds and four nearby dSphs which were observed with the Infrared spectrograph on the Spitzer Space Telescope. They applied a minimisation procedure to fit models to spectral energy distributions constructed from the infrared spectra and the available optical and infrared photometry for each star to determine its luminosity and dust mass-loss rate (MLR). In this contribution two items from that paper are highlighted: an update on MSX SMC 055, which Groenewegen et al. (2009) suggested could be a super-AGB star, and a discussion of synthetic colour-colour and colour-magnitude diagrams expected from the James Webb Space Telescope.

Radio Jet Feedback and Star Formation in Heavily Obscured, Hyperluminous Quasars at Redshifts ˜ 0.5-3. I. ALMA Observations

NASA Astrophysics Data System (ADS)

Lonsdale, Carol J.; Lacy, M.; Kimball, A. E.; Blain, A.; Whittle, M.; Wilkes, B.; Stern, D.; Condon, J.; Kim, M.; Assef, R. J.; Tsai, C.-W.; Efstathiou, A.; Jones, S.; Eisenhardt, P.; Bridge, C.; Wu, J.; Lonsdale, Colin J.; Jones, K.; Jarrett, T.; Smith, R.

2015-11-01

We present Atacama Large Millimeter/submillimeter Array (ALMA) 870 μm (345 GHz) data for 49 high-redshift (0.47 < z < 2.85), luminous (11.7\\lt {log}({L}{{bol}}/{L}⊙ )\\lt 14.2) radio-powerful active galactic nuclei (AGNs), obtained to constrain cool dust emission from starbursts concurrent with highly obscured radiative-mode black hole (BH) accretion in massive galaxies that possess a small radio jet. The sample was selected from the Wide-field Infrared Survey Explorer with extremely steep (red) mid-infrared colors and with compact radio emission from NVSS/FIRST. Twenty-six sources are detected at 870 μm, and we find that the sample has large mid- to far-infrared luminosity ratios, consistent with a dominant and highly obscured quasar. The rest-frame 3 GHz radio powers are 24.7\\lt {log}({P}\\text{3.0 GHz}/{{{W}} {Hz}}-1)\\lt 27.3, and all sources are radio-intermediate or radio-loud. BH mass estimates are 7.7 < log(MBH/M⊙) < 10.2. The rest-frame 1-5 μm spectral energy distributions are very similar to the “Hot DOGs” (hot dust-obscured galaxies), and steeper (redder) than almost any other known extragalactic sources. ISM masses estimated for the ALMA-detected sources are 9.9 < log (MISM/M⊙) < 11.75 assuming a dust temperature of 30 K. The cool dust emission is consistent with star formation rates reaching several thousand M⊙ yr-1, depending on the assumed dust temperature, but we cannot rule out the alternative that the AGN powers all the emission in some cases. Our best constrained source has radiative transfer solutions with approximately equal contributions from an obscured AGN and a young (10-15 Myr) compact starburst.

Rejuvenation of the Innocent Bystander: Testing Spin-Up in Dwarf Carbon Stars

NASA Astrophysics Data System (ADS)

Green, Paul

2013-09-01

Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dCs are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

Welcome to the Twilight Zone: The Mid-infrared Properties of Post-starburst Galaxies

NASA Astrophysics Data System (ADS)

Alatalo, Katherine; Bitsakis, Theodoros; Lanz, Lauranne; Lacy, Mark; Brown, Michael J. I.; French, K. Decker; Ciesla, Laure; Appleton, Philip N.; Beaton, Rachael L.; Cales, Sabrina L.; Crossett, Jacob; Falcón-Barroso, Jesús; Kelson, Daniel D.; Kewley, Lisa J.; Kriek, Mariska; Medling, Anne M.; Mulchaey, John S.; Nyland, Kristina; Rich, Jeffrey A.; Urry, C. Meg

2017-07-01

We investigate the optical and Wide-field Survey Explorer (WISE) colors of “E+A” identified post-starburst galaxies, including a deep analysis of 190 post-starbursts detected in the 2 μm All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone. Furthermore, we find that post-starbursts occupy a distinct region of [3.4]-[4.6] versus [4.6]-[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broadband photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that the mid-infrared (4-12 μm) properties of post-starbursts are consistent with either 11.3 μm polycyclic aromatic hydrocarbon emission, or thermally pulsating asymptotic giant branch (TP-AGB) and post-AGB stars. The composite SED of extended post-starburst galaxies with 22 μm emission detected with signal-to-noise ratio ≥slant 3 requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 μm. The composite SED of WISE 22 μm non-detections (S/N < 3), created by stacking 22 μm images, is also flat, requiring a hot dust component. The most likely source of the mid-infrared emission of these E+A galaxies is a buried active galactic nucleus (AGN). The inferred upper limits to the Eddington ratios of post-starbursts are 10-2-10-4, with an average of 10-3. This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections capable of identifying AGNs as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.

Complex dynamics in a simple model of pulsations for super-asymptotic giant branch stars.

PubMed

Munteanu, Andreea; Garcia-Berro, Enrique; Jose, Jordi; Petrisor, Emilia

2002-06-01

When intermediate mass stars reach their last stages of evolution they show pronounced oscillations. This phenomenon happens when these stars reach the so-called asymptotic giant branch (AGB), which is a region of the Hertzsprung-Russell diagram located at about the same region of effective temperatures but at larger luminosities than those of regular giant stars. The period of these oscillations depends on the mass of the star. There is growing evidence that these oscillations are highly correlated with mass loss and that, as the mass loss increases, the pulsations become more chaotic. In this paper we study a simple oscillator which accounts for the observed properties of this kind of stars. This oscillator was first proposed and studied in Icke et al. [Astron. Astrophys. 258, 341 (1992)] and we extend their study to the region of more massive and luminous stars -the region of super-AGB stars. The oscillator consists of a periodic nonlinear perturbation of a linear Hamiltonian system. The formalism of dynamical systems theory has been used to explore the associated Poincare map for the range of parameters typical of those stars. We have studied and characterized the dynamical behavior of the oscillator as the parameters of the model are varied, leading us to explore a sequence of local and global bifurcations. Among these, a tripling bifurcation is remarkable, which allows us to show that the Poincare map is a nontwist area preserving map. Meandering curves, hierarchical-islands traps and sticky orbits also show up. We discuss the implications of the stickiness phenomenon in the evolution and stability of the super-AGB stars. (c) 2002 American Institute of Physics.

Rotation of the asymptotic giant branch star R Doradus

NASA Astrophysics Data System (ADS)

Vlemmings, W. H. T.; Khouri, T.; Beck, E. De; Olofsson, H.; García-Segura, G.; Villaver, E.; Baudry, A.; Humphreys, E. M. L.; Maercker, M.; Ramstedt, S.

2018-05-01

High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 × 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of ωR sin i = (3.5 ± 0.3) × 10-9 rad s-1, which indicates a rotational velocity of |υrot sin i| = 1.0 ± 0.1 km s-1 at the stellar surface (R* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Φ = 7 ± 6°. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.

Collimated Outflow Formation via Binary Stars: Three-Dimensional Simulations of Asymptotic Giant Branch Wind and Disk Wind Interactions

NASA Astrophysics Data System (ADS)

García-Arredondo, F.; Frank, Adam

2004-01-01

We present three-dimensional hydrodynamic simulations of the interaction of a slow wind from an asymptotic giant branch (AGB) star and a jet blown by an orbiting companion. The jet or ``collimated fast wind'' is assumed to originate from an accretion disk that forms via Bondi accretion of the AGB wind or Roche lobe overflow. We present two distinct regimes in the wind-jet interaction determined by the ratio of the AGB wind to jet momentum flux. Our results show that when the wind momentum flux overwhelms the flux in the jet, a more disordered outflow results with the jet assuming a corkscrew pattern and multiple shock structures driven into the AGB wind. In the opposite regime, the jet dominates and will drive a highly collimated, narrow-waisted outflow. We compare our results with scenarios described by Soker & Rappaport and extrapolate to the structures observed in planetary nebulae (PNs) and symbiotic stars.

Dust-enshrouded asymptotic giant branch stars in the solar neighborhood

NASA Technical Reports Server (NTRS)

Jura, M.; Kleinmann, S. G.

1989-01-01

Using available infrared catalogs, an inventory is taken of the AGB star losing large amounts of mass within about 1 kpc of the sun. A surface density of these stars is estimated of about 25/sq kpc projected onto the plane of the Galaxy. Of these stars, about one-half are oxygen-rich while the other half are carbon-rich. The total mass-loss rate from AGB stars into the interstellar medium is probably between 3 and 6 x 10 to the -4th solar mass/sq kpc/yr. Within the uncertainties, this is in reasonable agreement with an estimated net loss rate of about 8 x 10 to the -4th solar mass/sq kpc/yr for main-sequence stars with initial masses between 1 and 5 solar masses as they evolve to white dwarfs. However, it is possible that there are important sources of mass loss which have not yet been identified. In the solar neighborhood, about one-half of all about 1.2 solar mass main-sequence stars spend greater than 30,000 yr in a carbon-star phase where they lose 1-2 x 10 to the -5th solar mass/yr and then become white dwarfs with about 0.7 solar mass.

(F)UV Spectral Analysis of 15 Hot, Hydrogen-Rich Central Stars of PNe

NASA Astrophysics Data System (ADS)

Ziegler, Marc

2013-07-01

the sample are still too small to start gravitational settling. For the elements C, N, O, Si, P, and S we find increasing abundances with increasing log(Teff^4/g), while the abundances for Ar and Fe decrease. The latter is unexpected as the higher the Teff^4/g ratio, the more the radiative force dominates the gravitational force and, thus, the elements should be kept in the atmosphere. The determined abundances were compared with previous literature values, with abundances predicted from diusion calculations, with abundances from Asymptotic Giant Branch (AGB) nucleosynthesis calculations, and, if available, with abundances found for the corresponding nebulae. The agreement was of mixed quality. The derived Teff and log g values confirmed some literature values while others had to be revised (e.g. for LSS 1362 and NGC1360). However, most of them agree with the previous literature values within the error limits. No difference in Teff can be found for DAO and O(H)-type stars, but O(H)-type stars have a lower log g (5.4 - 6.0) compared to the DAOs (6.5 - 7.4). The exception is the O(H)-type central star of the planetary nebula (CSPN) of Lo 1 with log g = 7.0. A comparison of the positions of each object with stellar evolutionary tracks for post-AGB stars in the log Teff - log g diagram lead to the respective stellar masses. The derived mean mass of the analyzed sample (M = 0.536 ± 0.023 Msol) agrees within the error limits with the expected mean mass for these objects. In the literature M = 0.638 - 0.145 Msol can be found for DA-type white dwarfs, the immediate successors of DAO-type white dwarfs. For two objects (A 35, Sh 2-174) extremely low masses were found. For A35 the derived mass (M_A35 = 0.523 ± 0.05Msol) lies at the lower end of possible masses predicted for post-AGB stars. The very low mass of Sh 2-174 (M_Sh 2-174 = 0.395 ± 0.05Msol) points at Sh 2-174 being a post-extended horizontal branch (EHB) star and not a CSPN. If a stellar mass is too low, it is

The 2014 AGB Survey of Higher Education Governance

ERIC Educational Resources Information Center

Hodge-Clark, Kristen

2014-01-01

"The 2014 AGB Survey of Higher Education Governance" is the fourth in AGB's studies of college and university governance. This report, based on survey responses from 592 public and independent boards, addresses a range of important governance topics that are receiving attention from boards and the news media, including presidential…

Abundance of SiC2 in carbon star envelopes

NASA Astrophysics Data System (ADS)

Massalkhi, S.; Agúndez, M.; Cernicharo, J.; Velilla Prieto, L.; Goicoechea, J. R.; Quintana-Lacaci, G.; Fonfría, J. P.; Alcolea, J.; Bujarrabal, V.

2018-03-01

Context. Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich asymptotic giant branch (AGB) stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si-C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216. Aim. We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars, and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. Methods: We carried out sensitive observations with the IRAM 30 m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes. Results: We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source except IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked; the SiC radical is probably the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend where the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as evidence of efficient incorporation of SiC2 onto dust grains, a process that is favored at high densities owing to the higher rate at which collisions between particles take place. Conclusions: The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an

a Study of the AGB in Local Group Bulge Populations

NASA Astrophysics Data System (ADS)

Rich, R.

1994-01-01

We propose to survey the bolometric luminosities, colors, and space distribution of the most luminous asymptotic giant branch (AGB) stars in the bulges of M31, M32, and M33. We seek to discover whether the bulges of these galaxies are relatively young, of order 10 Gyr rather than 15 Gyr. We will use WFPC2 and the R, I, and F1042M (1 micron) filters. Knowing that F1042M falls on the first continuum point of M giants, we have shown that we can use 1.04 micron fluxes to reliably calculate bolometric magnitudes for these very red stars. Color information from R and I will permit (1) comparison with Galactic bulge M giants, (2) an estimate of the spread of abundance and (3) increase the accuracy of the bolometric magnitudes. Frames with the damaged HST show signs of resolution to within 3" of the M31 nucleus; Red images with the aberrated HST show a red star cluster associated with the nucleus. Ground-based studies of M32 find an intermediate-age population from spectroscopy and infrared photometry. The repaired HST should resolve stars close to the nuclei of these galaxies. We will measure bolometric luminosity functions to determine if the populations are intermediate age, and attempt to measure the abundance range for stars near the nuclei of these galaxies. If metals have been lost due to winds, theory predicts that we should see a substantial spread of abundances even near the nucleus.

SALT reveals the barium central star of the planetary nebula Hen 2-39

NASA Astrophysics Data System (ADS)

Miszalski, B.; Boffin, H. M. J.; Jones, D.; Karakas, A. I.; Köppen, J.; Tyndall, A. A.; Mohamed, S. S.; Rodríguez-Gil, P.; Santander-García, M.

2013-12-01

Classical barium stars are binary systems which consist of a late-type giant enriched in carbon and slow neutron capture (s-process) elements and an evolved white dwarf (WD) that is invisible at optical wavelengths. The youngest observed barium stars are surrounded by planetary nebulae (PNe), ejected soon after the wind accretion of polluted material when the WD was in its preceding asymptotic giant branch (AGB) phase. Such systems are rare but powerful laboratories for studying AGB nucleosynthesis as we can measure the chemical abundances of both the polluted star and the nebula ejected by the polluter. Here, we present evidence for a barium star in the PN Hen 2-39 (PN G283.8-04.2) as one of only a few known systems. The polluted giant is very similar to that found in WeBo 1 (PN G135.6+01.0). It is a cool (Teff = 4250 ± 150 K) giant enhanced in carbon ([C/H] = 0.42 ± 0.02 dex) and barium ([Ba/Fe] = 1.50 ± 0.25 dex). A spectral type of C-R3 C24 nominally places Hen 2-39 amongst the peculiar early R-type carbon stars; however, the barium enhancement and likely binary status mean that it is more likely to be a barium star with similar properties, rather than a true member of this class. An AGB star model of initial mass 1.8 M⊙ and a relatively large carbon pocket size can reproduce the observed abundances well, provided mass is transferred in a highly conservative way from the AGB star to the polluted star (e.g. wind Roche lobe overflow). It also shows signs of chromospheric activity and photometric variability with a possible rotation period of ˜5.5 d likely induced by wind accretion. The nebula exhibits an apparent ring morphology in keeping with the other PNe around barium stars (WeBo 1 and A 70) and shows a high degree of ionization implying the presence of an invisible hot pre-WD companion that will require confirmation with UV observations. In contrast to A 70, the nebular chemical abundance pattern is consistent with non-Type I PNe, in keeping with the

NON-LOCAL THERMODYNAMICAL EQUILIBRIUM EFFECTS ON THE IRON ABUNDANCE OF ASYMPTOTIC GIANT BRANCH STARS IN 47 TUCANAE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lapenna, E.; Mucciarelli, A.; Lanzoni, B.

2014-12-20

We present the iron abundance of 24 asymptotic giant branch (AGB) stars, members of the globular cluster 47 Tucanae, obtained with high-resolution spectra collected with the FEROS spectrograph at the MPG/ESO 2.2 m Telescope. We find that the iron abundances derived from neutral lines (with a mean value [Fe I/H] =–0.94 ± 0.01, σ = 0.08 dex) are systematically lower than those derived from single ionized lines ([Fe II/H] =–0.83 ± 0.01, σ = 0.05 dex). Only the latter are in agreement with those obtained for a sample of red giant branch (RGB) cluster stars, for which the Fe I andmore » Fe II lines provide the same iron abundance. This finding suggests that non-local thermodynamical equilibrium (NLTE) effects driven by overionization mechanisms are present in the atmosphere of AGB stars and significantly affect the Fe I lines while leaving Fe II features unaltered. On the other hand, the very good ionization equilibrium found for RGB stars indicates that these NLTE effects may depend on the evolutionary stage. We discuss the impact of this finding on both the chemical analysis of AGB stars and on the search for evolved blue stragglers.« less

The Carbon Stars Adventure

NASA Astrophysics Data System (ADS)

Rau, Gioia; Paladini, C.; Hron, J.; Aringer, B.; Eriksson, K.; Groenewegen, M. A. T.; Nowotny, W.

We compare in a systematic way spectrometric, photometric and mid-infrared (VLTI/MIDI) interferometric measurements with different types of model atmospheres. Self-consistent dynamic model atmospheres in particular were used to interpret in a consistent way the dynamic behavior of gas and dust. The results underline how the joint use of different kind of observations, as photometry, spectroscopy and interferometry, is essential to understand the atmospheres of pulsating C-rich AGB stars. The sample of C-rich stars discussed in this work provides crucial constraints for the atmospheric structure.

Evidence for the Presence of Hn-PAHs in Post AGB Stars

NASA Technical Reports Server (NTRS)

Materese, Christopher K.; Bregman, Jesse D.; Sandford, Scott A.

2017-01-01

Polycyclic aromatic hydrocarbons (PAHs) are believed to be ubiquitous in space therefore represent an important class of molecules for the field of astrochemistry. PAHs are relatively stable under interstellar conditions, account for a significant fraction of the known Universe's molecular carbon inventory, and are believed responsible for numerous telltale interstellar infrared emission bands. PAHs can be subdivided into numerous classes, one of which is Hydrogenated PAHs (Hn-PAHs). Hn-PAHs are multi-ringed partially aromatic compounds with excess hydrogenation, leading to a partial disruption of the aromatic system. The infrared spectra of these compounds produce telltale signatures that make them distinct from ordinary aromatic or aliphatic molecules (or a mixture of both). Hn-PAHs may be an important subclass of PAHs that could explain the spectra of some astronomical objects with anomalously large 3.4 micron features. The 3.4 micron feature observed in these objects may be associated with the aliphatic C-H stretching vibrations of the excess hydrogen. If this presumption is correct, we also expect to observe methylene scissoring modes at 6.9 microns. We have recently conducted a series of follow-up observations to compliment our laboratory experiments into the properties of Hn-PAHs. Here we present our laboratory and observational results in support of the hypothesis that Hn-PAHs are a viable candidate molecule as the emission source for numerous post-asymptotic giant branch objects with abnormally large 3.4 micron features.

Heavy Metal Stars

NASA Astrophysics Data System (ADS)

2001-08-01

atomic mass and hence, increasing positive charge of the nuclei, the electric repulsion between the nuclei becomes stronger and stronger. In fact, the fusion process only works up to a certain mass limit, corresponding to the element Iron [2]. All elements that are heavier than Iron cannot be produced via this path. But then, how were those heavy elements we now find on the Earth produced in the first place? From where comes the Zirconium in artificial diamonds, the Barium that colours fireworks, the Tungsten in the filaments in electric bulbs? Which process made the Lead in your car battery? Beyond iron The production of elements heavier than Iron takes place by adding neutrons to the atomic nuclei . These neutral particles do not feel any electrical repulsion from the charged nuclei. They can therefore easily approach them and thereby create heavier nuclei. This is indeed the way the heaviest chemical elements are built up. There are actually two different stellar environments where this process of "neutron capture" can happen. One place where this process occurs is inside very massive stars when they explode as supernovae . In such a dramatic event, the build-up proceeds very rapidly, via the so-called "r-process" ( "r" for rapid ). The AGB stars But not all heavy elements are created in such an explosive way. A second possibility follows a more "peaceful" road. It takes place in rather normal stars, when they burn their Helium towards the end of their lives. In the so-called "s-process" ( "s" for slow ), heavier elements are then produced by a rather gentle addition of neutral neutrons to atomic nuclei. In fact, roughly half of all the elements heavier than Iron are believed to be synthesized by this process during the late evolutionary phases of stars. This process takes place during a specific stage of stellar evolution, known as the "AGB" phase [3]. It occurs just before an old star expels its gaseous envelope into the surrounding interstellar space and sometime

HUBBLE SPACE TELESCOPE MORPHOLOGIES OF z {approx} 2 DUST-OBSCURED GALAXIES. II. BUMP SOURCES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bussmann, R. S.; Dey, Arjun; Lotz, J.

We present Hubble Space Telescope imaging of 22 ultra-luminous infrared galaxies (ULIRGs) at z {approx} 2 with extremely red R - [24] colors (called dust-obscured galaxies, or DOGs) which have a local maximum in their spectral energy distribution (SED) at rest-frame 1.6 {mu}m associated with stellar emission. These sources, which we call 'bump DOGs', have star formation rates (SFRs) of 400-4000 M{sub sun} yr{sup -1} and have redshifts derived from mid-IR spectra which show strong polycyclic aromatic hydrocarbon emission-a sign of vigorous ongoing star formation. Using a uniform morphological analysis, we look for quantifiable differences between bump DOGs, power-law DOGsmore » (Spitzer-selected ULIRGs with mid-IR SEDs dominated by a power law and spectral features that are more typical of obscured active galactic nuclei than starbursts), submillimeter-selected galaxies, and other less-reddened ULIRGs from the Spitzer Extragalactic First Look Survey. Bump DOGs are larger than power-law DOGs (median Petrosian radius of 8.4 {+-} 2.7 kpc versus 5.5 {+-} 2.3 kpc) and exhibit more diffuse and irregular morphologies (median M{sub 20} of -1.08 {+-} 0.05 versus -1.48 {+-} 0.05). These trends are qualitatively consistent with expectations from simulations of major mergers in which merging systems during the peak SFR period evolve from M{sub 20} = -1.0 to M{sub 20} = -1.7. Less-obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally peaked, single-object morphologies rather than diffuse and irregular morphologies. This distinction in morphologies may imply that less-obscured ULIRGs sample the merger near the end of the peak SFR period. Alternatively, it may indicate that the intense star formation in these less-obscured ULIRGs is not the result of a recent major merger.« less

The evolving far-IR galaxy luminosity function and dust-obscured star formation rate density out to z≃5.

NASA Astrophysics Data System (ADS)

Koprowski, M. P.; Dunlop, J. S.; Michałowski, M. J.; Coppin, K. E. K.; Geach, J. E.; McLure, R. J.; Scott, D.; van der Werf, P. P.

2017-11-01

We present a new measurement of the evolving galaxy far-IR luminosity function (LF) extending out to redshifts z ≃ 5, with resulting implications for the level of dust-obscured star formation density in the young Universe. To achieve this, we have exploited recent advances in sub-mm/mm imaging with SCUBA-2 on the James Clerk Maxwell Telescope and the Atacama Large Millimeter/Submillimeter Array, which together provide unconfused imaging with sufficient dynamic range to provide meaningful coverage of the luminosity-redshift plane out to z > 4. Our results support previous indications that the faint-end slope of the far-IR LF is sufficiently flat that comoving luminosity density is dominated by bright objects (≃L*). However, we find that the number density/luminosity of such sources at high redshifts has been severely overestimated by studies that have attempted to push the highly confused Herschel SPIRE surveys beyond z ≃ 2. Consequently, we confirm recent reports that cosmic star formation density is dominated by UV-visible star formation at z > 4. Using both direct (1/Vmax) and maximum likelihood determinations of the LF, we find that its high-redshift evolution is well characterized by continued positive luminosity evolution coupled with negative density evolution (with increasing redshift). This explains why bright sub-mm sources continue to be found at z > 5, even though their integrated contribution to cosmic star formation density at such early times is very small. The evolution of the far-IR galaxy LF thus appears similar in form to that already established for active galactic nuclei, possibly reflecting a similar dependence on the growth of galaxy mass.

Establishing binarity amongst Galactic RV Tauri stars with a disc⋆

NASA Astrophysics Data System (ADS)

Manick, Rajeev; Van Winckel, Hans; Kamath, Devika; Hillen, Michel; Escorza, Ana

2017-01-01

Context. Over the last few decades it has become more evident that binarity is a prevalent phenomenon amongst RV Tauri stars with a disc. This study is a contribution to comprehend the role of binarity upon late stages of stellar evolution. Aims: In this paper we determine the binary status of six Galactic RV Tauri stars, namely DY Ori, EP Lyr, HP Lyr, IRAS 17038-4815, IRAS 09144-4933, and TW Cam, which are surrounded by a dusty disc. The radial velocities are contaminated by high-amplitude pulsations. We disentangle the pulsations from the orbital signal in order to determine accurate orbital parameters. We also place them on the HR diagram, thereby establishing their evolutionary nature. Methods: We used high-resolution spectroscopic time series obtained from the HERMES and CORALIE spectrographs mounted on the Flemish Mercator and Swiss Leonhard Euler Telescopes, respectively. An updated ASAS/AAVSO photometric time series is analysed to complement the spectroscopic pulsation search and to clean the radial velocities from the pulsations. The pulsation-cleaned orbits are fitted with a Keplerian model to determine the spectroscopic orbital parameters. We also calibrated a PLC relationship using type II cepheids in the LMC and apply the relation to our Galactic sample to obtain accurate distances and hence luminosities. Results: All six of the Galactic RV Tauri stars included in this study are binaries with orbital periods ranging between 650 and 1700 days and with eccentricities between 0.2 and 0.6. The mass functions range between 0.08 to 0.55 M⊙ which points to an unevolved low-mass companion. In the photometric time series we detect a long-term variation on the timescale of the orbital period for IRAS 17038-4815, IRAS 09144-4933, and TW Cam. Our derived stellar luminosities indicate that all except DY Ori and EP Lyr are post-AGB stars. DY Ori and EP Lyr are likely examples of the recently discovered dusty post-RGB stars. Conclusions: The orbital parameters

Molecular Astrophysics from Space: the Physical and Chemical Effects of Star Formation and the Destruction of Planetary Systems around Evolved Stars

NASA Technical Reports Server (NTRS)

Neufeld, David

2005-01-01

The research conducted during the reporting period is grouped into three sections: 1) Warm molecular gas in the interstellar medium (ISM); 2) Absorption line studies of "cold" molecular clouds; 3) Vaporization of comets around the AGB star IRC+10216.

Constraints on Grain Formation Around Carbon Stars from Laboratory Studies of Presolar Graphite

NASA Technical Reports Server (NTRS)

Bernatowicz, T. J.; Akande, O. W.; Croat, T. K.; Cowsik, R.

2005-01-01

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, either with or without internal crystals of titanium carbide (TiC). In addition to providing detailed information about stellar nucleosynthesis, the structure and composition of presolar grains give unique information about the conditions of grain formation. In the present work we use laboratory observations of presolar graphite to gain insight into the physical conditions in circumstellar outflows from carbon AGB stars. The periodic pulsation of AGB stars enhances the gas density through shocks in the stellar atmosphere above the photosphere, promoting the condensation of dust grains. Copious mass outflow occurs largely because grains are coupled to the radiation field of the star, which accelerates them by radiation pressure; momentum is in turn transferred to gas molecules by collisions with grains. The dust/gas mixture is effectively a two-component fluid whose motion depends on atmospheric structure and which, in turn, influences that structure. In particular, the radiation pressure on the grains determines the velocity field of the outflow and thus the density distribution, while the density distribution itself determines the conditions of radiative transfer within the outflow and thus the effective radiation pressure.

Nuclear reactions in AGB nucleosynthesis: the 19F(α, p)22Ne at energies of astrophysical relevance

NASA Astrophysics Data System (ADS)

D'Agata, G.; Pizzone, R. G.; La Cognata, M.; Indelicato, I.; Spitaleri, C.; Blagus, S.; Cherubini, S.; Figuera, P.; Grassi, L.; Guardo, G. L.; Gulino, M.; Hayakawa, S.; Kshetri, R.; Lamia, L.; Lattuada, M.; Mijatović, T.; Milin, M.; Miljanić, Ð.; Prepolec, L.; Rapisarda, G. G.; Romano, S.; Sergi, M. L.; Soić, N.; Tokić, V.; Tumino, A.; Uroić, M.

2018-01-01

The abundance of 19F in the universe is strictly related to standard and extra-mixing processes taking place inside AGB-stars, that are considered to be the most important sites for its production. Nevertheless the way in which it is destroyed is far from being well understood. For this reason we studied the 19F(α,p)22Ne reaction, that is supposed to be the main destruction channel in the Helium-rich part of the star. In this experiment, the reaction has been studied in the energy range of relevance for astrophysics (0÷1 MeV) via the Trojan Horse Method (THM), using the three-body reaction 6Li(19F,p22Ne)d.

Ages and Heavy Element Abundances from Very Metal-poor Stars in the Sagittarius Dwarf Galaxy

NASA Astrophysics Data System (ADS)

Hansen, Camilla Juul; El-Souri, Mariam; Monaco, Lorenzo; Villanova, Sandro; Bonifacio, Piercarlo; Caffau, Elisabetta; Sbordone, Luca

2018-03-01

Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky Way halo that has undergone several stripping events. Previous chemical studies were restricted mainly to a few, metal-rich ([Fe/H] \\gtrapprox -1) stars that suggested a top-light initial mass function (IMF). Here we present the first high-resolution, very metal-poor ([Fe/H] =‑1 to ‑3) sample of 13 giant stars in the main body of Sgr. We derive abundances of 13 elements, namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th, that challenge the interpretation based on previous studies. Our abundances from Sgr mimic those of the metal-poor halo, and our most metal-poor star ([Fe/H] ∼ -3) indicates a pure r-process pollution. Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing for age determination using nuclear cosmochronology. We calculate ages of 9+/- 2.5 {Gyr}. Most of the sample stars have been enriched by a range of asymptotic giant branch (AGB) stars with masses between 1.3 and 5 M ⊙. Sgr J190651.47–320147.23 shows a large overabundance of Pb (2.05 dex) and a peculiar abundance pattern best fit by a 3 M ⊙ AGB star. Based on star-to-star scatter and observed abundance patterns, a mixture of low- and high-mass AGB stars and supernovae (15–25 M ⊙) is necessary to explain these patterns. The high level (0.29 ± 0.05 dex) of Ca indicates that massive supernovae must have existed and polluted the early ISM of Sgr before it lost its gas. This result is in contrast with a top-light IMF with no massive stars polluting Sgr. Based on data obtained UVES/VLT ID: 083.B-0774, 075.B-0127.

Models of the Hydrodynamic Histories of Post-AGB Stars. I. Multiflow Shaping of OH 231.8+04.2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Balick, Bruce; Frank, Adam; Liu, Baowei

2017-07-10

We present a detailed hydrodynamic model that matches the present structure of the well-observed preplanetary nebula (“pPN”) OH 231.8+04.2 (“OH231”). The purpose of the model is to present a physically justified and coherent picture of its evolutionary history from about 100 years from the start of the formation of its complex outer structures to the present. We have adopted a set of initial conditions that are heavily constrained by high-quality observations of its present structure and kinematics. The shaping of the nebula occurs while the densities of the flows are “light,” i.e., less than the surrounding AGB-wind environment. The simulationsmore » show that pairs of essentially coeval clumps and sprays of the same extent and density, but different outflow speeds, sculpted both the pair of thin axial flow “or spine” and the bulbs. The total ejected mass and momentum in the best-fit model are surprisingly large—3 M {sub ⊙} and 2.2 × 10{sup 41} gm cm s{sup −1}, respectively—however, these values are reduced by up to a factor of 10 in other models that fit the data almost as well. Our ultimate goal is to combine the present model results of masses, momenta, flow speeds, and flow geometries for OH231 with those of other models to be published in the future in order to find common attributes of their ejection histories.« less

Determining Mass-Loss Rates of Evolved Stars in the Galactic Bulge from Infrared Surveys

NASA Astrophysics Data System (ADS)

Riley, Allyssa; Sargent, Benjamin A.; Srinivasan, Sundar; Meixner, Margaret; Kastner, Joel H.

2018-06-01

To investigate the relationship between mass loss from evolved stars and host galaxy metallicity, we are computing the dust mass loss budget due to red supergiant (RSG) and asymptotic giant branch (AGB) stars in the Galactic Bulge and comparing this result to that previously obtained for the Magellanic Clouds. We construct spectral energy distributions (SEDs) for our candidate RSG and AGB stars using observations from various infrared surveys, including the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). Because Robitaille et al (2008, AJ, 136, 2413) have already identified Intrinsically Red Objects from the GLIMPSE I and II surveys, we use their method as a starting point and expand the study by using the GLIMPSE 3D survey. Because AGB stars can be variable, we also match the GLIMPSE I, II, and 3D sources to other surveys, such as DEEP GLIMPSE, WISE, VVV, and DENIS, in order to characterize the variability across the spectral energy distribution (SED) of each source. This allows us to determine the source’s average SED over multiple epochs. We use extinction curves derived from Spitzer studies of extinction in the Galaxy to determine the extinction corrections for our sample. To establish mass-loss rates of evolved stars in the Bulge, we use the Grid of Red supergiant and Asymptotic giant branch ModelS (GRAMS) of dust-enshrouded evolved stars (2011, A&A, 532, A54; 2011, ApJ, 728, 93). This allows us to determine the total mass return to the Bulge from these stars. This work has been supported by NASA ADAP grant 80NSSC17K0057.

Galaxy Zoo: star formation versus spiral arm number

NASA Astrophysics Data System (ADS)

Hart, Ross E.; Bamford, Steven P.; Casteels, Kevin R. V.; Kruk, Sandor J.; Lintott, Chris J.; Masters, Karen L.

2017-06-01

Spiral arms are common features in low-redshift disc galaxies, and are prominent sites of star formation and dust obscuration. However, spiral structure can take many forms: from galaxies displaying two strong 'grand design' arms to those with many 'flocculent' arms. We investigate how these different arm types are related to a galaxy's star formation and gas properties by making use of visual spiral arm number measurements from Galaxy Zoo 2. We combine ultraviolet and mid-infrared (MIR) photometry from GALEX and WISE to measure the rates and relative fractions of obscured and unobscured star formation in a sample of low-redshift SDSS spirals. Total star formation rate has little dependence on spiral arm multiplicity, but two-armed spirals convert their gas to stars more efficiently. We find significant differences in the fraction of obscured star formation: an additional ˜10 per cent of star formation in two-armed galaxies is identified via MIR dust emission, compared to that in many-armed galaxies. The latter are also significantly offset below the IRX-β relation for low-redshift star-forming galaxies. We present several explanations for these differences versus arm number: variations in the spatial distribution, sizes or clearing time-scales of star-forming regions (I.e. molecular clouds), or contrasting recent star formation histories.

Evolved stars in the Local Group galaxies - II. AGB, RSG stars and dust production in IC10

NASA Astrophysics Data System (ADS)

Dell'Agli, F.; Di Criscienzo, M.; Ventura, P.; Limongi, M.; García-Hernández, D. A.; Marini, E.; Rossi, C.

2018-06-01

We study the evolved stellar population of the Local Group galaxy IC10, with the aim of characterizing the individual sources observed and to derive global information on the galaxy, primarily the star formation history and the dust production rate. To this aim, we use evolutionary sequences of low- and intermediate-mass (M < 8 M⊙) stars, evolved through the asymptotic giant branch phase, with the inclusion of the description of dust formation. We also use models of higher mass stars. From the analysis of the distribution of stars in the observational planes obtained with IR bands, we find that the reddening and distance of IC10 are E(B - V) = 1.85 mag and d = 0.77 Mpc, respectively. The evolved stellar population is dominated by carbon stars, that account for 40% of the sources brighter than the tip of the red giant branch. Most of these stars descend from ˜1.1 - 1.3 M⊙ progenitors, formed during the major epoch of star formation, which occurred ˜2.5 Gyr ago. The presence of a significant number of bright stars indicates that IC10 has been site of significant star formation in recent epochs and currently hosts a group of massive stars in the core helium-burning phase. Dust production in this galaxy is largely dominated by carbon stars; the overall dust production rate estimated is 7 × 10-6 M⊙/yr.

HEAVY ELEMENT NUCLEOSYNTHESIS IN THE BRIGHTEST GALACTIC ASYMPTOTIC GIANT BRANCH STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karakas, Amanda I.; Garcia-Hernandez, D. A.; Lugaro, Maria, E-mail: akarakas@mso.anu.edu.au, E-mail: agarcia@iac.es, E-mail: maria.lugaro@monash.edu.au

2012-05-20

We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5 M{sub Sun} and 9 M{sub Sun }, with an initial metallicity of Z = 0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis and Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis and Wood noted that for stars over 2.5 M{sub Sun} the superwind should be delayed until P Almost-Equal-To 750 days at 5 M{sub Sun }. Wemore » calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P Almost-Equal-To 700-800 days in models of M = 5, 6, and 7 M{sub Sun }. Post-processing nucleosynthesis calculations show that the 6 and 7 M{sub Sun} models produce the most Rb, with [Rb/Fe] Almost-Equal-To 1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] Almost-Equal-To 1.4 {+-} 0.8 dex). Changing the rate of the {sup 22}Ne +{alpha} reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the {sup 22}Ne({alpha}, n){sup 25}Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of Almost-Equal-To 1.4 dex although with C/O ratios >1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.« less

An Abundance Analysis of Two Carbon-Rich Proto-Planetary Nebulae: IRAS Z02229+6208 And IRAS 07430+1115

NASA Technical Reports Server (NTRS)

Reddy, Bacham E.; Bakker, Eric J.; Hrivnak, Bruce J.

1999-01-01

In this paper, we present an LTE abundance analysis of two new proto-planetary nebulae, IRAS Z02229 + 6208 and IRAS 07430 + 1115, based on high-resolution (R approximately equal 55,000) optical echelle spectra. Results show that both stars are metal-poor ([Fe/H] = -0.5) and overabundant in C, N, and s-process elements. The average elemental abundances are [C/Fe] = +0.8, [N/Fe] = +1.2, and [s-process/Fe] = +1.4 for IRAS Z02229 + 6208, and [C/Fe] = +0.6, [N/Fe] = +0.4, and [s-process/Fe] = +1.6 for IRAS 07430+ 1115. These abundances suggest that the stars have experienced nucleo-synthesis on the asymptotic giant branch (AGB), and the resultant products of CNO, 3alpha, and s-process reactions were brought to the photosphere during shell flashes and deep mixing episodes during the AGB phase of their evolution. Of major significance is the measurement of a high Li abundance in both stars, log epsilon(Li) approximately equal 2.3 and 2.4 for IRAS Z02229 + 6208 and IRAS 07430 + 1115, respectively. This may be the result of hot bottom burning, below the deep convective zone. We also present an analysis of the circumstellar molecular (C2 and CN) and atomic (Na I and K I) absorption spectra of both stars. We derive rotational temperatures, column densities, and envelope expansion velocities using molecular C2 Phillips and CN Red system bands. The values derived for expansion velocities, 8-14 km/s, are typical of the values found for post-AGB stars. IRAS 07430+ 1115 is unusual in that it shows P Cygni-shaped C2 emission profiles in the spectra of the circumstellar envelope. A minimum distance for IRAS Z02229+6208, determined from interstellar Na I lines, suggests that it is evolved from an intermediate-mass star. Including these two stars, the number of post-AGB stars for which clear C, N, and s-process elemental overabundances are found rises to eight. IRAS Z02229 + 6208 is known to possess the 21 micron emission feature in its mid-infrared spectrum; these results support the

Rejuvenation of the Innocent Bystander: Results from a Pilot X-ray Study of Dwarf Carbon Stars

NASA Astrophysics Data System (ADS)

Mazzoni, Fernando; Montez, Rodolfo; Green, Paul

2018-01-01

We present the results of a pilot study by the Chandra X-ray Observatory of X-ray emission from dwarf Carbon (dC) stars. Carbon stars were thought to be exclusively AGB stars but main sequence dwarfs showing carbon molecular bands appear to be the dominant variety. The existence of dC stars is surprising since dwarf stars cannot intrinsically produce carbon as an AGB star can. It is hypothesized that dC stars are polluted by an evolved companion star. Evidence of past pollution can appear in X-ray emission where increased coronal activity (“spin-up”) or mass accretion via a disk can be detected. Using the Chandra X-ray Observatory we detected X-ray photons in the vicinity of all the dC stars in our a pilot sample. For each detection we characterized the X-ray emission and compared to the emission expected from potential emission scenarios. Although the process that produces the X-ray emission from dC stars is presently unclear and our pilot sample is small, our results suggest that X-ray emission might be a universal characteristic of dC stars. Further examination of the X-ray emission plus future X-ray and multiwavelength observations will help us better understand the nature of these intriguing stars.

Age Dating Merger Events in Early Type Galaxies via the Detection of AGB Light

NASA Technical Reports Server (NTRS)

Bothun, G.

2005-01-01

A thorough statistical analysis of the J-H vs. H-K color plane of all detected early type galaxies in the 2MASS catalog with velocities less than 5000 km/s has been performed. This all sky survey is not sensitive to one particular galactic environment and therefore a representative range of early type galaxy environments have been sampled. Virtually all N-body simulation so major mergers produces a central starburst due to rapid collection of gas. This central starburst is of sufficient amplitude to change the stellar population in the central regions of the galaxy. Intermediate age populations are given away by the presence of AGB stars which will drive the central colors redder in H-K relative to the J- H baseline. This color anomaly has a lifetime of 2-5 billion years depending on the amplitude of the initial starburst Employing this technique on the entire 2MASS sample (several hundred galaxies) reveals that the AGB signature occurs less than 1% of the time. This is a straightforward indication that virtually all nearby early type galaxies have not had a major merger occur within the last few billion years.

Flamingos 2 Spectroscopy of Obscured and Unobscured Quasars

NASA Astrophysics Data System (ADS)

Ridgway, Susan; Lacy, Mark; Urrutia, Tanya; Petric, Andreea

2013-08-01

We will use Flamingos-2 to obtain spectra of luminous AGN and quasars selected in the mid-infrared. Mid-infrared selection is much less biased with respect to obscuration than optical and X-ray techniques, and hence allows for finding obscured (Type-2) quasars as well as Type-1 quasars. Our survey so far has been very successful and has provided an unique opportunity to construct luminosity functions for both Type-1 and Type-2 quasars selected in the same way and covering similar redshifts and luminosities. We have quantifed the change in the obscured fraction with luminosity and redshift for the first time, and find interesting indications that at high redshift the obscured fraction rises, consistent with models for the joint formation of the galaxy and black hole populations. Our samples are, however, still quite incomplete at low fluxes (and therefore lower luminosities at a given redshift), particularly in the southern hemisphere. Near-infrared spectroscopy, such as that we have previously obtained with NIRI at Gemini N, offers us the best possibility of bringing these southern samples to a reasonable completeness level, and will greatly increase the number of high z quasars in our sample. This will allow us to better judge our tantalizing initial results on the redshift evolution of the obscured fraction. In addition, these southern targets can be followed up with ALMA and GEMS/GSAOI to study the morphologies and star-formation properties of the hosts, allowing further exploration of the relationship between the formation of massive bulges and supermassive blackholes in the early universe.

Galaxy IC 3639 with Obscured Active Galactic Nucleus

NASA Image and Video Library

2017-01-07

IC 3639, a galaxy with an active galactic nucleus, is seen in this image combining data from the Hubble Space Telescope and the European Southern Observatory. This galaxy contains an example of a supermassive black hole hidden by gas and dust. Researchers analyzed NuSTAR data from this object and compared them with previous observations from NASA's Chandra X-Ray Observatory and the Japanese-led Suzaku satellite. The findings from NuSTAR, which is more sensitive to higher energy X-rays than these observatories, confirm the nature of IC 3639 as an active galactic nucleus that is heavily obscured, and intrinsically much brighter than observed. http://photojournal.jpl.nasa.gov/catalog/PIA21087

Do All O Stars Form in Star Clusters?

NASA Astrophysics Data System (ADS)

Weidner, C.; Gvaramadze, V. V.; Kroupa, P.; Pflamm-Altenburg, J.

The question whether or not massive stars can form in isolation or only in star clusters is of great importance for the theory of (massive) star formation as well as for the stellar initial mass function of whole galaxies (IGIMF-theory). While a seemingly easy question it is rather difficult to answer. Several physical processes (e.g. star-loss due to stellar dynamics or gas expulsion) and observational limitations (e.g. dust obscuration of young clusters, resolution) pose severe challenges to answer this question. In this contribution we will present the current arguments in favour and against the idea that all O stars form in clusters.

The First Stars: A Low-Mass Formation Mode

NASA Technical Reports Server (NTRS)

Stacy, Athena; Bromm, Volker

2014-01-01

We perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as approx. 1 AU, corresponding to gas densities of 10(exp 16)/cu cm. Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1Stellar Mass to approx. 5 Stellar Mass by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion.

The TGAS HR diagram of S-type stars

NASA Astrophysics Data System (ADS)

Shetye, Shreeya; van Eck, Sophie; Jorissen, Alain; van Winckel, Hans; Siess, Lionel

2018-04-01

S-type stars are late-type giants enhanced with s-process elements originating either from nucleosynthesis during the Asymptotic Giant Branch (AGB) or from a pollution by a binary companion. The former are called intrinsic S stars, and the latter extrinsic S stars. The atmospheric parameters of S stars are more numerous than those of M-type giants (C/O ratio and s-process abundances affect the thermal structure and spectral synthesis), and hence they are more difficult to derive. Nevertheless, high-resolution spectroscopic data of S stars combined with the TGAS (Tycho-Gaia Astrometric solution) parallaxes were used to derive effective temperatures, surface gravities, and luminosities. These parameters allow to locate the intrinsic and extrinsic S stars in the Hertzsprung-Russell diagram.

POST T-Tauri Stars in Galactic Clusters

NASA Astrophysics Data System (ADS)

Haro, G.

1983-08-01

There is a number of theoretical and observational reasons to support a view of star formation and evolution as a continuous process which covers a rather long period of time, On the other hand, it can be stressed that some particular evolutionary stages are confined to relatively short lengths of time. On a purely observational basis, it seems quite evident that the typical and most "advanced" T Tauri phenomenon in a given star -and consequently its extreme spectroscopic and photometric characteristics- manifest itself during an extremely short period of time in relation to the whole evolutionary process for intermediate and late type stars. Without doubt the extreme or advanced" features of a T Tauri object tend to diminish in periods of only -in most cases- a few million years. However, a considerably longer time is required for the process of weakening or apparent total disappearance of the most persistent T Tauri features. Nevertheless, among other problems, there emerges one of fundamental importance: can we arrive to an acceptable definition of a bon T Tauri star? In the present work we repeat our attempt to define what can characterize an "advanced" T Tauri-type star or the minimum spectroscopic and photometric features required to classify a young star within the family that unmistakably includes all typical T Tauri objects. At the same time, and following the trends of modern astronomy, we try to demonstrate that certain T Tauri-type stars evolve, during different periods of time and that, although they lose mass and their most conspicuous spectroscopic characteristics, they can still be described as what Herbig calls "post-T Tauri" stars, keeping some remnants of their primitive spectroscopic and photometric features. Several years ago, we stressed that in the great majority of T Tauri stars it seems that the time required for the diminishing or even apparent disappearance of the last typical T Tauri vestiges depends on the mass or on the observable

Asymmetric Planetary Nebulae VI: the conference summary

NASA Astrophysics Data System (ADS)

De Marco, O.

2014-04-01

The Asymmetric Planetary Nebulae conference series, now in its sixth edition, aims to resolve the shaping mechanism of PN. Eighty percent of PN have non spherical shapes and during this conference the last nails in the coffin of single stars models for non spherical PN have been put. Binary theories abound but observational tests are lagging. The highlight of APN6 has been the arrival of ALMA which allowed us to measure magnetic fields on AGB stars systematically. AGB star halos, with their spiral patterns are now connected to PPN and PN halos. New models give us hope that binary parameters may be decoded from these images. In the post-AGB and pre-PN evolutionary phase the naked post-AGB stars present us with an increasingly curious puzzle as complexity is added to the phenomenologies of objects in transition between the AGB and the central star regimes. Binary central stars continue to be detected, including the first detection of longer period binaries, however a binary fraction is still at large. Hydro models of binary interactions still fail to give us results, if we make an exception for the wider types of binary interactions. More promise is shown by analytical considerations and models driven by simpler, 1D simulations such as those carried out with the code MESA. Large community efforts have given us more homogeneous datasets which will yield results for years to come. Examples are the ChanPlaN and HerPlaNe collaborations that have been working with the Chandra and Herschel space telescopes, respectively. Finally, the new kid in town is the intermediate-luminosity optical transient, a new class of events that may have contributed to forming several peculiar PN and pre-PN.

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

NASA Astrophysics Data System (ADS)

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

2016-04-01

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

Arabidopsis G-protein β subunit AGB1 interacts with NPH3 and is involved in phototropism.

PubMed

Kansup, Jeeraporn; Tsugama, Daisuke; Liu, Shenkui; Takano, Tetsuo

2014-02-28

Heterotrimeric G proteins (Gα, Gβ and Gγ) have pleiotropic roles in plants, but molecular mechanisms underlying them remain to be elucidated. Here we show that Arabidopsis Gβ (AGB1) interacts with NPH3, a regulator of phototropism. Yeast two-hybrid assays, in vitro pull-down assays and bimolecular fluorescence complementation assays showed that AGB1 and NPH3 physically interact. NPH3-null mutation (nph3) is known to completely abolish hypocotyl phototropism. Loss-of-function mutants of AGB1 (agb1-1 and agb1-2) showed decreased hypocotyl phototropism, and agb1/nph3 double mutants showed no hypocotyl phototropism. These results suggest that AGB1 is involved in the NPH3-mediated regulation of phototropism. Copyright © 2014 Elsevier Inc. All rights reserved.

MASS OUTFLOW AND CHROMOSPHERIC ACTIVITY OF RED GIANT STARS IN GLOBULAR CLUSTERS. II. M13 AND M92

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meszaros, Sz.; Dupree, A. K.; Szalai, T.

High-resolution spectra of 123 red giant stars in the globular cluster M13 and 64 red giant stars in M92 were obtained with Hectochelle at the MMT telescope. Emission and line asymmetries in H{alpha} and Ca II K are identified, characterizing motions in the extended atmospheres and seeking differences attributable to metallicity in these clusters and M15. On the red giant branch, emission in H{alpha} generally appears in stars with T {sub eff} {approx}< 4500 K and log L/L {sub sun}{approx}> 2.75. Fainter stars showing emission are asymptotic giant branch (AGB) stars or perhaps binary stars. The line-bisector for H{alpha} revealsmore » the onset of chromospheric expansion in stars more luminous than log (L/L {sub sun}) {approx} 2.5 in all clusters, and this outflow velocity increases with stellar luminosity. However, the coolest giants in the metal-rich M13 show greatly reduced outflow in H{alpha} most probably due to decreased T {sub eff} and changing atmospheric structure. The Ca II K{sub 3} outflow velocities are larger than shown by H{alpha} at the same luminosity and signal accelerating outflows in the chromospheres. Stars clearly on the AGB show faster chromospheric outflows in H{alpha} than RGB objects. While the H{alpha} velocities on the RGB are similar for all metallicities, the AGB stars in the metal-poor M15 and M92 have higher outflow velocities than in the metal-rich M13. Comparison of these chromospheric line profiles in the paired metal-poor clusters, M15 and M92, shows remarkable similarities in the presence of emission and dynamical signatures, and does not reveal a source of the 'second-parameter' effect.« less

Determining the Stellar Initial Mass by Means of the 17O/18O Ratio on the AGB

NASA Astrophysics Data System (ADS)

De Nutte, Rutger; Decin, Leen; Olofsson, Hans; de Koter, Alex; Karakas, Amanda; Lombaert, Robin; Milam, Stefanie; Ramstedt, Sofia; Stancliffe, Richard; Homan, Ward; Van de Sande, Marie

2016-07-01

This poster presentsnewly obtainedcircumstellar 12C17O and 12C18O line observations, from which theline intensity are then related directly tothe 17O/18O surface abundance ratiofor a sample of nine AGB stars covering the three spectral types ().These ratios are evaluated in relation to a fundamental stellar evolution parameters: the stellar initial mass. The17O/18O ratio is shown to function as an effective method of determining the initial stellar mass. Through comparison with predictions bystellar evolution models, accurate initial mass estimates are calculated for all nine sources.

S stars in the Gaia era: stellar parameters and nucleosynthesis

NASA Astrophysics Data System (ADS)

van Eck, Sophie; Karinkuzhi, Drisya; Shetye, Shreeya; Jorissen, Alain; Goriely, Stéphane; Siess, Lionel; Merle, Thibault; Plez, Bertrand

2018-04-01

S stars are s-process and C-enriched (0.5stars (TP-AGB) and extrinsic S stars (binary masqueraders) can provide strong constraints on the s-process nucleosynthesis: in particular, the s-process temperature can be determined using zirconium and niobium abundances, independently of stellar evolution models. Synthetic spectra of dwarf S stars have been computed and will be sought for in spectroscopic survey data, constraining their luminosity thanks to Gaia parallaxes.

The Post-starburst Evolution of Tidal Disruption Event Host Galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

French, K. Decker; Zabludoff, Ann; Arcavi, Iair

We constrain the recent star formation histories of the host galaxies of eight optical/UV-detected tidal disruption events (TDEs). Six hosts had quick starbursts of <200 Myr duration that ended 10–1000 Myr ago, indicating that TDEs arise at different times in their hosts’ post-starburst evolution. If the disrupted star formed in the burst or before, the post-burst age constrains its mass, generally excluding O, most B, and highly massive A stars. If the starburst arose from a galaxy merger, the time since the starburst began limits the coalescence timescale and thus the merger mass ratio to more equal than 12:1 inmore » most hosts. This uncommon ratio, if also that of the central supermassive black hole (SMBH) binary, disfavors the scenario in which the TDE rate is boosted by the binary but is insensitive to its mass ratio. The stellar mass fraction created in the burst is 0.5%–10% for most hosts, not enough to explain the observed 30–200× boost in TDE rates, suggesting that the host’s core stellar concentration is more important. TDE hosts have stellar masses 10{sup 9.4}–10{sup 10.3} M {sub ☉}, consistent with the Sloan Digital Sky Survey volume-corrected, quiescent Balmer-strong comparison sample and implying SMBH masses of 10{sup 5.5}–10{sup 7.5} M {sub ☉}. Subtracting the host absorption line spectrum, we uncover emission lines; at least five hosts have ionization sources inconsistent with star formation that instead may be related to circumnuclear gas, merger shocks, or post-AGB stars.« less

Evolution, Nucleosynthesis, and Yields of Low-mass Asymptotic Giant Branch Stars at Different Metallicities. II. The FRUITY Database

NASA Astrophysics Data System (ADS)

Cristallo, S.; Piersanti, L.; Straniero, O.; Gallino, R.; Domínguez, I.; Abia, C.; Di Rico, G.; Quintini, M.; Bisterzo, S.

2011-12-01

By using updated stellar low-mass stars models, we systematically investigate the nucleosynthesis processes occurring in asymptotic giant branch (AGB) stars. In this paper, we present a database dedicated to the nucleosynthesis of AGB stars: FRANEC Repository of Updated Isotopic Tables & Yields (FRUITY). An interactive Web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up (TDU) episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 <=M/M ⊙ <= 3.0 and metallicities 1 × 10-3 <= Z <= 2 × 10-2, is discussed. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parameterization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the TDU efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find good agreement with observations.

Sensitive radio survey of obscured quasar candidates

NASA Astrophysics Data System (ADS)

Alexandroff, Rachael M.; Zakamska, Nadia L.; van Velzen, Sjoert; Greene, Jenny E.; Strauss, Michael A.

2016-12-01

We study the radio properties of moderately obscured quasars in samples at both low (z ˜ 0.5) and high (z ˜ 2.5) redshift to understand the role of radio activity in accretion, using the Karl G. Jansky Very Large Array (VLA) at 6.0 GHz and 1.4 GHz. Our z ˜ 2.5 sample consists of optically selected obscured quasar candidates, all of which are radio-quiet, with typical radio luminosities of νLν[1.4 GHz] ≲ 1040 erg s-1. Only a single source is individually detected in our deep (rms˜10 μJy) exposures. This population would not be identified by radio-based selection methods used for distinguishing dusty star-forming galaxies and obscured active nuclei. In our pilot A-array study of z ˜ 0.5 radio-quiet quasars, we spatially resolve four of five objects on scales ˜5 kpc and find they have steep spectral indices with an average value of α = -0.75. Therefore, radio emission in these sources could be due to jet-driven or radiatively driven bubbles interacting with interstellar material on the scale of the host galaxy. Finally, we also study the additional population of ˜200 faint ( ˜ 40 μJy-40 mJy) field radio sources observed over ˜120 arcmin2 of our data. 60 per cent of these detections (excluding our original targets) are matched in the Sloan Digital Sky Survey (SDSS) and/or Wide-Field Infrared Survey Explorer (WISE) and are, in roughly equal shares, active galactic nuclei (AGN) at a broad range of redshifts, passive galaxies with no other signs of nuclear activity and infrared-bright but optically faint sources. Spectroscopically or photometrically confirmed star-forming galaxies constitute only a small minority of the matches. Such sensitive radio surveys allow us to address important questions of AGN evolution and evaluate the AGN contribution to the radio-quiet sky.

ALMA observations of the host galaxy of GRB 090423 at z = 8.23: deep limits on obscured star formation 630 million years after the big bang

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berger, E.; Zauderer, B. A.; Chary, R.-R.

2014-12-01

We present rest-frame far-infrared (FIR) and optical observations of the host galaxy of GRB 090423 at z = 8.23 from the Atacama Large Millimeter Array (ALMA) and the Spitzer Space Telescope, respectively. The host remains undetected to 3σ limits of F {sub ν}(222 GHz) ≲ 33 μJy and F {sub ν}(3.6 μm) ≲ 81 nJy. The FIR limit is about 20 times fainter than the luminosity of the local ULIRG Arp 220 and comparable to the local starburst M 82. Comparing this with model spectral energy distributions, we place a limit on the infrared (IR) luminosity of L {sub IR}(8-1000more » μm) ≲ 3 × 10{sup 10} L {sub ☉}, corresponding to a limit on the obscured star formation rate of SFR{sub IR}≲5 M {sub ☉} yr{sup –1}. For comparison, the limit on the unobscured star formation rate from Hubble Space Telescope rest-frame ultraviolet (UV) observations is SFR{sub UV} ≲ 1 M {sub ☉} yr{sup –1}. We also place a limit on the host galaxy stellar mass of M {sub *} ≲ 5 × 10{sup 7} M {sub ☉} (for a stellar population age of 100 Myr and constant star formation rate). Finally, we compare our millimeter observations to those of field galaxies at z ≳ 4 (Lyman break galaxies, Lyα emitters, and submillimeter galaxies) and find that our limit on the FIR luminosity is the most constraining to date, although the field galaxies have much larger rest-frame UV/optical luminosities than the host of GRB 090423 by virtue of their selection techniques. We conclude that GRB host galaxies at z ≳ 4, especially those with measured interstellar medium metallicities from afterglow spectroscopy, are an attractive sample for future ALMA studies of high redshift obscured star formation.« less

An Infrared Study of the Circumstellar Material Associated with the Carbon Star R Sculptoris

NASA Astrophysics Data System (ADS)

Hankins, M. J.; Herter, T. L.; Maercker, M.; Lau, R. M.; Sloan, G. C.

2018-01-01

The asymptotic giant branch (AGB) star R Sculptoris (R Scl) is one of the most extensively studied stars on the AGB. R Scl is a carbon star with a massive circumstellar shell (M shell ∼ 7.3 × 10‑3 M ⊙) that is thought to have been produced during a thermal pulse event ∼2200 years ago. To study the thermal dust emission associated with its circumstellar material, observations were taken with the Faint Object InfraRed CAMera for the SOFIA Telescope (FORCAST) at 19.7, 25.2, 31.5, 34.8, and 37.1 μm. Maps of the infrared emission at these wavelengths were used to study the morphology and temperature structure of the spatially extended dust emission. Using the radiative-transfer code DUSTY, and fitting the spatial profile of the emission, we find that a geometrically thin dust shell cannot reproduce the observed spatially resolved emission. Instead, a second dust component in addition to the shell is needed to reproduce the observed emission. This component, which lies interior to the dust shell, traces the circumstellar envelope of R Scl. It is best fit by a density profile with n ∝ r α , where α ={0.75}-0.25+0.45 and a dust mass of {M}d={9.0}-4.1+2.3× {10}-6 {M}ȯ . The strong departure from an r ‑2 law indicates that the mass-loss rate of R Scl has not been constant. This result is consistent with a slow decline in the post-pulse mass loss that has been inferred from observations of the molecular gas.

The red/infrared evolution in galaxies - Effect of the stars on the asymptotic giant branch

NASA Technical Reports Server (NTRS)

Chokshi, Arati; Wright, Edward L.

1987-01-01

The effect of including the asymptotic giant branch (AGB) population in a spectral synthesis model of galaxy evolution is examined. Stars on the AGB are luminous enough and also evolve rapidly enough to affect the evolution of red and infrared colors in galaxies. The validity of using infrared colors as distance indicators to galaxies is then investigated in detail. It is found that for z of 1 or less infrared colors of model galaxies behave linearly with redshift.

Dust and molecular shells in asymptotic giant branch stars

NASA Astrophysics Data System (ADS)

Zhao-Geisler, R.; Quirrenbach, A.; Köhler, R.; Lopez, B.

2012-09-01

Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims: The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods: Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results: The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions: From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm

AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs

NASA Astrophysics Data System (ADS)

Ita, Y.; Matsuura, M.; Ishihara, D.; Oyabu, S.; Takita, S.; Kataza, H.; Yamamura, I.; Matsunaga, N.; Tanabé, T.; Nakada, Y.; Fujiwara, H.; Wada, T.; Onaka, T.; Matsuhara, H.

2010-05-01

Context. The AKARI, a Japanese infrared space mission, has performed an All-Sky Survey in six infrared-bands from 9 to 180 μm with higher spatial resolutions and better sensitivities than IRAS. Aims: We investigate the mid-infrared (9 and 18 μm) point source catalog (PSC) obtained with the infrared camera (IRC) onboard AKARI, in order to understand the infrared nature of the known objects and to identify previously unknown objects. Methods: Color-color diagrams and a color-magnitude diagram were plotted with the AKARI-IRC PSC and other available all-sky survey catalogs. We combined the Hipparcos astrometric catalog and the 2MASS all-sky survey catalog with the AKARI-IRC PSC. We furthermore searched literature and SIMBAD astronomical database for object types, spectral types, and luminosity classes. We identified the locations of representative stars and objects on the color-magnitude and color-color diagram schemes. The properties of unclassified sources can be inferred from their locations on these diagrams. Results: We found that the (B-V) vs. (V-S9W) color-color diagram is useful for identifying the stars with infrared excess emerged from circumstellar envelopes or disks. Be stars with infrared excess are separated well from other types of stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W) diagram is a powerful tool for classifying several object types. Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars form distinct sequences in this color-color diagram. Young stellar objects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, and planetary nebulae (PNe) have the largest mid-infrared color excess and can be identified in the infrared catalog. Finally, we plot the L18W vs. (S9W-L18W) color-magnitude diagram, using the AKARI data together with Hipparcos parallaxes. This diagram can be used to identify low-mass YSOs and AGB stars. We found that this diagram is comparable to the [24] vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources

Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

NASA Astrophysics Data System (ADS)

Bekki, Kenji

2017-05-01

Most old globular clusters (GCs) in the Galaxy are observed to have internal chemical abundance spreads in light elements. We discuss a new GC formation scenario based on hierarchical star formation within fractal molecular clouds. In the new scenario, a cluster of bound and unbound star clusters ('star cluster complex', SCC) that have a power-law cluster mass function with a slope (β) of 2 is first formed from a massive gas clump developed in a dwarf galaxy. Such cluster complexes and β = 2 are observed and expected from hierarchical star formation. The most massive star cluster ('main cluster'), which is the progenitor of a GC, can accrete gas ejected from asymptotic giant branch (AGB) stars initially in the cluster and other low-mass clusters before the clusters are tidally stripped or destroyed to become field stars in the dwarf. The SCC is initially embedded in a giant gas hole created by numerous supernovae of the SCC so that cold gas outside the hole can be accreted on to the main cluster later. New stars formed from the accreted gas have chemical abundances that are different from those of the original SCC. Using hydrodynamical simulations of GC formation based on this scenario, we show that the main cluster with the initial mass as large as [2-5] × 105 M⊙ can accrete more than 105 M⊙ gas from AGB stars of the SCC. We suggest that merging of hierarchical SSCs can play key roles in stellar halo formation around GCs and self-enrichment processes in the early phase of GC formation.

PULSATION-TRIGGERED MASS LOSS FROM AGB STARS: THE 60 DAY CRITICAL PERIOD

DOE Office of Scientific and Technical Information (OSTI.GOV)

McDonald, I.; Zijlstra, A. A., E-mail: iain.mcdonald-2@jb.man.ac.uk, E-mail: albert.zijlstra@manchester.ac.uk

2016-06-01

Low- and intermediate-mass stars eject much of their mass during the late, red giant branch (RGB) phase of evolution. The physics of their strong stellar winds is still poorly understood. In the standard model, stellar pulsations extend the atmosphere, allowing a wind to be driven through radiation pressure on condensing dust particles. Here, we investigate the onset of the wind, using nearby RGB stars drawn from the Hipparcos catalog. We find a sharp onset of dust production when the star first reaches a pulsation period of 60 days. This approximately coincides with the point where the star transitions to themore » first overtone pulsation mode. Models of the spectral energy distributions show stellar mass-loss rate suddenly increasing at this point, by a factor of ∼10 over the existing (chromospherically driven) wind. The dust emission is strongly correlated with both pulsation period and amplitude, indicating stellar pulsation is the main trigger for the strong mass loss, and determines the mass-loss rate. Dust emission does not strongly correlate with stellar luminosity, indicating radiation pressure on dust has little effect on the mass-loss rate. RGB stars do not normally appear to produce dust, whereas dust production by asymptotic giant branch stars appears commonplace, and is probably ubiquitous above the RGB-tip luminosity. We conclude that the strong wind begins with a step change in mass-loss rate and is triggered by stellar pulsations. A second rapid mass-loss-rate enhancement is suggested when the star transitions to the fundamental pulsation mode at a period of ∼300 days.« less

An Expanded Very Large Array and CARMA Study of Dusty Disks and Torii with Large Grains in Dying Stars

NASA Astrophysics Data System (ADS)

Sahai, R.; Claussen, M. J.; Schnee, S.; Morris, M. R.; Sánchez Contreras, C.

2011-09-01

We report the results of a pilot multiwavelength survey in the radio continuum (X, Ka, and Q bands, i.e., from 3.6 cm to 7 mm) carried out with the Expanded Very Large Array (EVLA) in order to confirm the presence of very large dust grains in dusty disks and torii around the central stars in a small sample of post-asymptotic giant branch (pAGB) objects, as inferred from millimeter (mm) and submillimeter (submm) observations. Supporting mm-wave observations were also obtained with the Combined Array for Research in Millimeter-wave Astronomy toward three of our sources. Our EVLA survey has resulted in a robust detection of our most prominent submm emission source, the pre-planetary nebula (PPN) IRAS 22036+5306, in all three bands, and the disk-prominent pAGB object, RV Tau, in one band. The observed fluxes are consistent with optically thin free-free emission, and since they are insignificant compared to their submm/mm fluxes, we conclude that the latter must come from substantial masses of cool, large (mm-sized) grains. We find that the power-law emissivity in the cm-to-submm range for the large grains in IRAS22036 is νβ, with β = 1-1.3. Furthermore, the value of β in the 3-0.85 mm range for the three disk-prominent pAGB sources (β <= 0.4) is significantly lower than that of IRAS22036, suggesting that the grains in pAGB objects with circumbinary disks are likely larger than those in the dusty waists of pre-planetary nebulae.

Is the interaction between Ti atoms and fullerenes the origin of the 21-μ m feature?

NASA Astrophysics Data System (ADS)

Kimura, Y.; Nuth, J. A., III; Ferguson, F. T.

2005-12-01

A 21-μ m-emission feature has been observed in the shells of carbon-rich post-asymptotic giant branch (AGB) stars. The carrier of the 21-μ m feature remains unidentified, although many candidate materials have been proposed, including nanodiamond, SiS2, a derivative of SiC and nanometer-sized TiC. In particular, TiC grains were extensively discussed after the report by von Helden (2000). Gas-phase TiC clusters less than 1 nm in diameter have been suggested as the source of the 21-μ m dust feature. The spectrum of TiC clusters recorded in the laboratory provides a good fit with the observational data. However, only negative results have been reported for both theoretical and laboratory experimental studies concerning TiC since the discovery by von Helden. Recent measurements of fullerenes and Ti atoms recorded in our laboratory have demonstrated the presence of an infrared feature near 21 μ m. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-AGB stars. In our experimental system, large-cage carbon particles, such as large fullerenes, were produced from CO gas by the Boudouard reaction. Large-cage carbon particles intermixed with Ti atoms were produced by the evaporation of a Ti-metal-wrapped carbon electrode in CO gas. The infrared spectra of large fullerenes interacting with Ti atoms show a characteristic feature at 20.3 μ m that closely corresponds to the 20.1-μ m feature observed in post-AGB stars. Both the laboratory and stellar spectra also show a small but significant peak at 19.0 μ m, which is attributed to fullerenes. We propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21-μ m feature seen in some post-AGB stars.

J-type Carbon Stars: A Dominant Source of 14 N-rich Presolar SiC Grains of Type AB

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Nan; Stephan, Thomas; Boehnke, Patrick

We report Mo isotopic data of 27 new presolar SiC grains, including 12 N-14-rich AB (N-14/N-15 > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as theirmore » stellar source. On the other hand, low-mass CO novae and early R-and J-type carbon stars show C-13 and N-14 excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%-15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.« less

(F)UV Spectroscopy of K648: Abundance Determination of Trace Elements

NASA Astrophysics Data System (ADS)

Mohamad-Yob, S. J.; Ziegler, M.; Rauch, T.; Werner, K.

2010-11-01

We present preliminary results of an ongoing spectral analysis of K 648, the central star of the planetary nebula Ps 1, based on high resolution FUV spectra. K 648, in M 15 is one of only four known PNe in globular clusters. The formation of this post-AGB object in a globular cluster is still unclear. Our aim is to determine Teff, log g, and the abundances of trace elements, in order to improve our understanding of post-AGB evolution of extremely metal-poor stars, especially PN formation in globular clusters. We analyzed FUSE, HST/STIS, and HST/FOS observations. A grid of stellar model atmospheres was calculated using the Tübingen NLTE Model Atmosphere Package (TMAP).

Olivier Chesneau's Work on Low Mass Stars

NASA Astrophysics Data System (ADS)

Lagadec, E.

2015-12-01

During his too short career, Olivier Chesneau pioneered the study of the circumstellar environments of low mass evolved stars using very high angular resolution techniques. He applied state of the art high angular resolution techniques, such as optical interferometry and adaptive optics imaging, to the the study of a variety of objects, from AGB stars to Planetary Nebulae, via e.g. Born Again stars, RCB stars and Novae. I present here an overview of this work and most important results by focusing on the paths he followed and key encounters he made to reach these results. Olivier liked to work in teams and was very strong at linking people with complementary expertises to whom he would communicate his enthusiasm and sharp ideas. His legacy will live on through the many people he inspired.

The 19F(α, p)22Ne Reaction at Energies of Astrophysical Relevance by Means of the Trojan Horse Method and Its Implications in AGB Stars

NASA Astrophysics Data System (ADS)

D’Agata, G.; Pizzone, R. G.; La Cognata, M.; Indelicato, I.; Spitaleri, C.; Palmerini, S.; Trippella, O.; Vescovi, D.; Blagus, S.; Cherubini, S.; Figuera, P.; Grassi, L.; Guardo, G. L.; Gulino, M.; Hayakawa, S.; Kshetri, R.; Lamia, L.; Lattuada, M.; Mijatovic`, T.; Milin, M.; Miljanic`, Đ.; Prepolec, L.; Rapisarda, G. G.; Romano, S.; Sergi, M. L.; Skukan, N.; Soic`, N.; Tokic`, V.; Tumino, A.; Uroic`, M.

2018-06-01

The main source of 19F in the universe has not yet been clearly identified and this issue represents one of the unanswered questions of stellar modeling. This lack of knowledge can be due to the 19F(α, p)22Ne reaction cross-section that has proven to be difficult at low energies: direct measurements stop only at about ∼660 keV, leaving roughly half of the astrophysical relevant energy region (from 200 keV to 1.1 MeV) explored only by R-matrix calculations. In this work, we applied the Trojan Horse Method to the quasi-free three-body 6Li(19F, p22Ne)d reaction performed at E beam = 6 MeV in order to indirectly study the 19F(α, p)22Ne reaction in the sub-Coulomb energy region. In this way, we obtained the cross-section and the reaction rate in the temperature region of interest for astrophysics and free from electron screening effects. A brief analysis of the impact of the new measured reaction rate in AGB star nucleosynthesis is also presented.

EVIDENCE FOR A WIDE RANGE OF ULTRAVIOLET OBSCURATION IN z {approx} 2 DUSTY GALAXIES FROM THE GOODS-HERSCHEL SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Penner, Kyle; Dickinson, Mark; Dey, Arjun

Dusty galaxies at z {approx} 2 span a wide range of relative brightness between rest-frame mid-infrared (8 {mu}m) and ultraviolet wavelengths. We attempt to determine the physical mechanism responsible for this diversity. Dust-obscured galaxies (DOGs), which have rest-frame mid-IR to UV flux density ratios {approx}> 1000, might be abnormally bright in the mid-IR, perhaps due to prominent emission from active galactic nuclei and/or polycyclic aromatic hydrocarbons, or abnormally faint in the UV. We use far-infrared data from the GOODS-Herschel survey to show that most DOGs with 10{sup 12} L {sub Sun} {approx}< L {sub IR} {approx}< 10{sup 13} L {submore » Sun} are not abnormally bright in the mid-IR when compared to other dusty galaxies with similar IR (8-1000 {mu}m) luminosities. We observe a relation between the median IR to UV luminosity ratios and the median UV continuum power-law indices for these galaxies, and we find that only 24% have specific star formation rates that indicate the dominance of compact star-forming regions. This circumstantial evidence supports the idea that the UV- and IR-emitting regions in these galaxies are spatially coincident, which implies a connection between the abnormal UV faintness of DOGs and dust obscuration. We conclude that the range in rest-frame mid-IR to UV flux density ratios spanned by dusty galaxies at z {approx} 2 is due to differing amounts of UV obscuration. Of galaxies with these IR luminosities, DOGs are the most obscured. We attribute differences in UV obscuration to either (1) differences in the degree of alignment between the spatial distributions of dust and massive stars or (2) differences in the total dust content.« less

The UK Infrared Telescope M33 monitoring project - IV. Variable red giant stars across the galactic disc

NASA Astrophysics Data System (ADS)

Javadi, Atefeh; Saberi, Maryam; van Loon, Jacco Th.; Khosroshahi, Habib; Golabatooni, Najmeh; Mirtorabi, Mohammad Taghi

2015-03-01

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope, of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this fourth paper of the series, we present a search for variable red giant stars in an almost square degree region comprising most of the galaxy's disc, carried out with the WFCAM (Wide Field CAMera) instrument in the K band. These data, taken during the period 2005-2007, were complemented by J- and H-band images. Photometry was obtained for 403 734 stars in this region; of these, 4643 stars were found to be variable, most of which are asymptotic giant branch (AGB) stars. The variable stars are concentrated towards the centre of M33, more so than low-mass, less-evolved red giants. Our data were matched to optical catalogues of variable stars and carbon stars and to mid-infrared photometry from the Spitzer Space Telescope. Most dusty AGB stars had not been previously identified in optical variability surveys, and our survey is also more complete for these types of stars than the Spitzer survey. The photometric catalogue is made publicly available at the Centre de Données astronomiques de Strasbourg.

The Dearth of UV-bright Stars in M32: Implications for Stellar Evolution Theory

NASA Technical Reports Server (NTRS)

Sweigart, Allen V.; Kimble, Randy A.; Bowers, Charles W.

2008-01-01

Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained deep far ultraviolet images of the compact elliptical galaxy M32. When combined with earlier near-ultraviolet images of the same field, these data enable the construction of an ultraviolet color-magnitude diagram of the hot horizontal branch (HB) population and other hot stars in late phases of stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the galaxy, implying that these stars either cross the HR diagram more rapidly than expected, and/or that they spend a significant fraction of their time enshrouded in circumstellar material. The predicted luminosity gap between the hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected, especially when compared to evolutionary tracks with enhanced helium abundances, implying that the presence of hot HB stars in this metal-rich population is not due to (Delta)Y/(Delta)Z greater than or approx. 4. Only a small fraction (approx. 2%) of the HB population is hot enough to produce significant UV emission, yet most of the W emission in this galaxy comes from the hot HB and AGBM stars, implying that PAGB stars are not a significant source of W emission even in those elliptical galaxies with a weak W excess. Subject headings: galaxies: evolution - galaxies: stellar content - galaxies: individual (M32) - stars: evolution - stars: horizontal branch

OBSCURED STAR FORMATION AND ENVIRONMENT IN THE COSMOS FIELD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feruglio, C.; Aussel, H.; Le Floc'h, E.

2010-09-20

We investigate the effects of the environment on star formation in a sample of massive luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) with S(24 {mu}m) >80 {mu}Jy and i {sup +} < 24 in the COSMOS field. We exploit the accurate photometric redshifts in COSMOS to characterize the galaxy environment and study the evolution of the fraction of LIRGs and ULIRGs in different environments in the redshift range z = 0.3-1.2 and in bins of stellar mass. We find that the environment plays a role in the star formation processes and evolution of LIRGs and ULIRGs. We find anmore » overall increase of the ULIRG+LIRG fraction in an optically selected sample with increasing redshift, as expected from the evolution of the star formation rate (SFR) density. We find that the ULIRG+LIRG fraction decreases with increasing density up to z {approx} 1, and that the dependence on density flattens with increasing redshift. We do not observe the reversal of the SFR density relation up to z = 1 in massive LIRGs and ULIRGs, suggesting that such reversal might occur at higher redshift in this infrared luminosity range.« less

Mass loss of stars on the asymptotic giant branch. Mechanisms, models and measurements

NASA Astrophysics Data System (ADS)

Höfner, Susanne; Olofsson, Hans

2018-01-01

As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made

Phosphorus Chemistry in Oxygen Rich Stars

NASA Astrophysics Data System (ADS)

Bernal, Jacob; Schmidt, Deborah; Anderson, Julie; Ziurys, Lucy M.

2017-06-01

Observations of PO and PN have been carried out at the Arizona Radio Observatory at 1, 2, and 3 mm. Multiple transitions of PO and PN have been detected towards the O-rich AGB stars TX Cam and RCas. Data obtained toward supergiant stars VY Canis Majoris and NML Cyg have also been analyzed. Abundances were obtained for these molecules in all four objects using the radiative transfer code ESCAPADE, which is suitable for symmetric and asymmetric stellar outflows. The abundances of PN and PO were found to be in the range 10^{-8} - 10^{-7} relative to H_{2}. While PN appears to be a parent molecule formed by LTE chemistry near the stellar photosphere, PO appears to be created further out from the star at r > 400 R_*.

The Problem of Spectral Mimicry of Supergiants

NASA Astrophysics Data System (ADS)

Klochkova, V. G.; Chentsov, E. L.

2018-01-01

The phenomenon of spectral mimicry refers to the fact that hypergiants and post-AGB supergiants—stars of different masses in fundamentally different stages of their evolution—have similar optical spectra, and also share certain other characteristics (unstable extended atmospheres, expanding dust-gas envelopes, high IR excesses). As a consequence, it is not always possible to distinguish post-AGB stars from hypergiants based on individual spectral observations in the optical. Examples of spectral mimicry are analyzed using uniform, high-quality spectral material obtained on the 6-m telescope of the Special Astrophysical Observatory in the course of long-term monitoring of high-luminosity stars. It is shown that unambiguously resolving the mimicry problem for individual stars requires the determination of a whole set of parameters: luminosity, wind parameters, spectral energy distribution, spectral features, velocity field in the atmosphere and circumstellar medium, behavior of the parameters with time, and the chemical composition of the atmosphere.

The UK Infrared Telescope M33 monitoring project - I. Variable red giant stars in the central square kiloparsec

NASA Astrophysics Data System (ADS)

Javadi, Atefeh; van Loon, Jacco Th.; Mirtorabi, Mohammad Taghi

2011-02-01

We have conducted a near-infrared monitoring campaign at the UK Infrared Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. The most extensive data set was obtained in the K band with the UIST instrument for the central 4 × 4 arcmin2 (1 kpc2) - this contains the nuclear star cluster and inner disc. These data, taken during the period 2003-2007, were complemented by J- and H-band images. Photometry was obtained for 18 398 stars in this region; of these, 812 stars were found to be variable, most of which are asymptotic giant branch (AGB) stars. Our data were matched to optical catalogues of variable stars and carbon stars and to mid-infrared photometry from the Spitzer Space Telescope. In this first of a series of papers, we present the methodology of the variability survey and the photometric catalogue - which is made publicly available at the Centre de Données astronomiques de Strasbourg - and discuss the properties of the variable stars. The most dusty AGB stars had not been previously identified in optical variability surveys, and our survey is also more complete for these types of stars than the Spitzer survey.

The effects of host obscuration on searches for tidal disruption events

NASA Astrophysics Data System (ADS)

Roth, Nathaniel; Mushotzky, Richard; Gezari, Suvi; van Velzen, Sjoert

2018-01-01

Tidal disruptions of stars by super-massive black holes (TDEs) offer opportunities to learn about black hole demographics and stellar dynamics. However, matching the observed TDE rate to that predicted by theory has remained a challenge, as most surveys to-date have found fewer flares than expected. Some of this discrepancy may relate to nuclear obscuration in host galaxies. This includes the effects of dust at optical and ultraviolet wavelengths, and the effects of neutral gas at x-ray wavelengths. I will discuss procedures to correct the observed TDE rate within existing and upcoming surveys to the intrinsic per-galaxy rate by accounting for host obscuration. I will also discuss how reddening might affect TDE selection criteria, and I will make predictions for the population of infrared TDE light echoes.

A Precessing Jet in a Dying Star: Adaptive Optics Imaging of the ``Water-Fountain" Nebula IRAS16342-3814

NASA Astrophysics Data System (ADS)

Sahai, R.; Le Mignant, D.; Sánchez Contreras, C.; Campbell, R. D.; Chaffee, F. H.

2004-12-01

Collimated jets are one of the most intriguing, yet poorly understood phenomena in astrophysics. Jets have been found in a wide variety of object classes which include active galactic nuclei, young stellar objects, massive X-ray binaries, black hole X-ray transients, symbiotic stars, supersoft X-ray sources, and finally, planetary and preplanetary nebulae (PNe & PPNe). In the case of PNe and PPNe, it has been proposed that wobbling collimated jets may be the universal mechanism which can explain a wide variety of bipolar and multipolar morphologies seen in these objects (Sahai 2000, ASP Conf.Ser. 199, 209). The ``Water-Fountain Nebula'', IRAS16342-3814 (IRAS1634) belongs to a class of very young PPNe with high-velocity molecular outflows traced in either or both of radio H2O and OH maser line emission, and are believed to result from the interaction of fast jets with ambient circumstellar material shed by the AGB progenitors of these objects. Hubble Space Telescope (HST) imaging of IRAS1634 showed a small bipolar nebula, with the lobes separated by a dark equatorial waist (Sahai et al. 1999, ApJ, 514, L115) -- the morphology was interpreted as bubble-like reflection nebulae illuminated by starlight escaping through polar holes in a dense, dusty waist obscuring the central star, with the bubbles created by a fast jet-like outflow plowing into the AGB mass-loss envelope. Here we report Adaptive Optics (AO) observations with the W. M. Keck Observatory at near-infrared wavelengths (in the H, K', L', Ms bands) which probe much deeper into the lobes and reveal a remarkable corkscrew-shaped structure apparently etched into the lobe walls. The corkscrew structure represents the proverbial ``writing on the wall" signature of an underlying precessing jet, and we compare our results with predictions from published numerical simulations of such jets. The results shown provide a dramatic example of the power of ground-based AO imaging with large telescopes to uncover phenomena

Study of the Outflow and Disk surrounding a Post-Outburst FU-Orionis Star

NASA Astrophysics Data System (ADS)

Mellon, Samuel N.; Perez, L. M.

2014-01-01

PP 13 is a fan-shaped cometary nebula located in the constellation of Perseus and embedded in the L1473 dark cloud. At optical wavelengths this region is obscured by the surrounding dark cloud, while at infrared and longer wavelengths two northern objects (PP13Na & PP13Nb) and one southern object (PP13S) are revealed. In the past, the young stellar object inside PP13S, called PP13S*, experienced an FU-Orionis type outburst due to a massive accretion episode and is currently returning to its quiescent state. Studying the FU-Orionis phase is crucial to our understanding of how low mass stars form; it is theorized that all low-mass stars go through this outburst phase while they are forming. I used CARMA 3mm interferometric observations of the PP13 region to study the continuum and molecular line emissions from PP13. With these observations, I determined the source of the previously detected outflow and learned new information about the double star system PP13Na and PP13Nb. Although I was not able to detect the accretion disk in the gas emissions, I plan to use computer modeling to help provide constraints on the properties of PP13S* and its outflow.

J-type Carbon Stars: A Dominant Source of 14 N-rich Presolar SiC Grains of Type AB

DOE PAGES

Liu, Nan; Stephan, Thomas; Boehnke, Patrick; ...

2017-07-21

Here, we report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB ( 14N/ 15N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takesmore » place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. And because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.« less

J-type Carbon Stars: A Dominant Source of 14 N-rich Presolar SiC Grains of Type AB

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Nan; Stephan, Thomas; Boehnke, Patrick

Here, we report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB ( 14N/ 15N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takesmore » place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. And because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.« less

Compton thick AGN in the NuSTAR era

NASA Astrophysics Data System (ADS)

Marchesi, Stefano; Ajello, Marco; Marcotulli, Lea; Comastri, Andrea

2017-08-01

The recent launch of the Nuclear Spectroscopic Telescope Array (NuSTAR), the first telescope with focusing optics at >10 keV, represented a major breakthrough in the study of obscured active galactic nuclei (AGN). In this talk, I present the results of the 0.3-100 keV spectral analysis of the 30 Compton thick (CT-; i.e., those sources having column density NH>1E24 cm-2) AGN detected within z~0.1 in the BAT 100-month survey with available NuSTAR data. Particularly, I will focus on how adding NuSTAR data to the 0.3-10 keV information helps to characterize the CT-AGN population, significantly improving the measurements of important X-ray spectral parameters such as the photon index, the intrinsic absorption, the intensity of the Iron K alpha line at 6.4 keV and the obscuring torus opening angle. Finally, I will discuss the role of these objects in the context of obscured AGN accretion physics, and their contribution to the cosmic X-ray background.

The planetary nebula IC 4776 and its post-common-envelope binary central star

NASA Astrophysics Data System (ADS)

Sowicka, Paulina; Jones, David; Corradi, Romano L. M.; Wesson, Roger; García-Rojas, Jorge; Santander-García, Miguel; Boffin, Henri M. J.; Rodríguez-Gil, Pablo

2017-11-01

We present a detailed analysis of IC 4776, a planetary nebula displaying a morphology believed to be typical of central star binarity. The nebula is shown to comprise a compact hourglass-shaped central region and a pair of precessing jet-like structures. Time-resolved spectroscopy of its central star reveals a periodic radial velocity variability consistent with a binary system. Whilst the data are insufficient to accurately determine the parameters of the binary, the most likely solutions indicate that the secondary is probably a low-mass main-sequence star. An empirical analysis of the chemical abundances in IC 4776 indicates that the common-envelope phase may have cut short the asymptotic giant branch evolution of the progenitor. Abundances calculated from recombination lines are found to be discrepant by a factor of approximately 2 relative to those calculated using collisionally excited lines, suggesting a possible correlation between low-abundance discrepancy factors and intermediate-period post-common-envelope central stars and/or Wolf-Rayet central stars. The detection of a radial velocity variability associated with the binarity of the central star of IC 4776 may be indicative of a significant population of (intermediate-period) post-common-envelope binary central stars that would be undetected by classic photometric monitoring techniques.

Stellar Evolutionary Effects on the Abundance of PAHS and SN-Condensed Dust in Galaxies

NASA Technical Reports Server (NTRS)

Dwek, Eli

2007-01-01

Spectral aid photometric observations of nearby galaxies show a correlation between the strength of their mid-IR aromatic features and their metal abundance, and a deficiency of these features in low-metallicity galaxies. The aromatic features are most commonly attributed to emission from PAH molecules. In this paper, we suggest that the observed correlation represents a trend of PAH abundance with galactic age, reflecting the delayed injection of PAHs and carbon dust into the ISM, by AGB stars in their final, post-AGB phase of their evolution. These AGB stars are the primary sources of PAHs and carbon dust in galaxies, and recycle their ejecta back to the interstellar medium only after a few hundred million years of evolution on the main sequence. In contrast, more massive stars that explode as Type II supernovae inject their metals and dust almost instantaneously after their formation. After determining the PAH abundances in 35 nearby galaxies, we use a chemical evolution model to show that the delayed injection of carbon dust by AGB stars provides a natural explanation to the dependence of the PAH content, in galaxies with metallicity. We also show that larger dust particles giving rise to the far-IR emission follow a distinct evolutionary trend closely related to the injection of dust by massive stars into the ISM.

A RE-EVALUATION OF THE EVOLVED STARS IN THE GLOBULAR CLUSTER M13

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sandquist, Eric L.; Gordon, Mark; Levine, Daniel

suggested. These stars are brighter than other stars of similar color (either redder or bluer), and may be examples of 'early hot flashers' that ignite core helium fusion shortly after leaving the RGB. We used ultraviolet photometry to identify hot post-HB stars, and based on their numbers (relative to canonical AGB stars) we estimate the position on the HB where the morphology of the post-HB tracks change to I {approx} 17.3, between the two peaks in the HB distribution. Concerning the possibility of helium enrichment in M13, we revisited the helium-sensitive R ratio, applying a new method for correcting star counts for larger lifetimes of hot HB stars. We find that M13's R ratio is in agreement with theoretical values for primordial helium abundance Y{sub P} = 0.245 and inconsistent with a helium enhancement {Delta}Y = 0.04. The brightness of the HB (both in comparison to the end of the canonical HB and to the tip of the RGB) also appears to rule out the idea that the envelopes of the reddest HB stars have been significantly enriched in helium. The absolute colors of the turnoffs of M3 and M13 potentially may be used to look for differences in their mean helium abundances, but there are inconsistencies in current data sets between colors using different filters that prevent a solid conclusion. The numbers of stars on the lower RGB and in the red giant bump agree very well with recent theoretical models, although there are slight indications of a deficit of red giant stars above the bump. There is not convincing evidence that a large fraction of stars leave the RGB before undergoing a core helium flash.« less

ROTATING STARS AND THE FORMATION OF BIPOLAR PLANETARY NEBULAE. II. TIDAL SPIN-UP

DOE Office of Scientific and Technical Information (OSTI.GOV)

García-Segura, G.; Villaver, E.; Manchado, A.

We present new binary stellar evolution models that include the effects of tidal forces, rotation, and magnetic torques with the goal of testing planetary nebulae (PNs) shaping via binary interaction. We explore whether tidal interaction with a companion can spin-up the asymptotic giant brach (AGB) envelope. To do so, we have selected binary systems with main-sequence masses of 2.5 M {sub ⊙} and 0.8 M {sub ⊙} and evolve them allowing initial separations of 5, 6, 7, and 8 au. The binary stellar evolution models have been computed all the way to the PNs formation phase or until Roche lobemore » overflow (RLOF) is reached, whatever happens first. We show that with initial separations of 7 and 8 au, the binary avoids entering into RLOF, and the AGB star reaches moderate rotational velocities at the surface (∼3.5 and ∼2 km s{sup −1}, respectively) during the inter-pulse phases, but after the thermal pulses it drops to a final rotational velocity of only ∼0.03 km s{sup −1}. For the closest binary separations explored, 5 and 6 au, the AGB star reaches rotational velocities of ∼6 and ∼4 km s{sup −1}, respectively, when the RLOF is initiated. We conclude that the detached binary models that avoid entering the RLOF phase during the AGB will not shape bipolar PNs, since the acquired angular momentum is lost via the wind during the last two thermal pulses. This study rules out tidal spin-up in non-contact binaries as a sufficient condition to form bipolar PNs.« less

Rotational broadening and conservation of angular momentum in post-extreme horizontal branch stars

NASA Astrophysics Data System (ADS)

Fontaine, G.; Latour, M.

2018-06-01

We show that the recent realization that isolated post-extreme horizontal branch (post-EHB) stars are generally characterized by rotational broadening with values of V rot sini between 25 and 30 km s-1 can be explained as a natural consequence of the conservation of angular momentum from the previous He-core burning phase on the EHB. The progenitors of these evolved objects, the EHB stars, are known to be slow rotators with an average value of V rot sini of 7.7 km s-1. This implies significant spin-up between the EHB and post-EHB phases. Using representative evolutionary models of hot subdwarf stars, we demonstrate that angular momentum conservation in uniformly rotating structures (rigid-body rotation) boosts that value of the projected equatorial rotation speed by a factor 3.6 by the time the model has reached the region of the surface gravity-effective temperature plane where the newly-studied post-EHB objects are found. This is exactly what is needed to account for their observed atmospheric broadening. We note that the decrease of the moment of inertia causing the spin-up is mostly due to the redistribution of matter that produces more centrally-condensed structures in the post-EHB phase of evolution, not to the decrease of the radius per se.

Mass Loss during Late Stellar Evolution

NASA Astrophysics Data System (ADS)

Olofsson, Hans

1999-10-01

Extensive post-main sequence mass loss occurs for low- and intermediate-mass (up to ~8MSun) stars on the asymptotic giant branch (AGB), and for the higher-mass stars during their red supergiant evolution. These winds have a profound effect on the evolution of the stars, as well as for the enrichment of the interstellar medium with heavy elements and grain particles. The mass loss on the AGB is the by far most well studied, but a good deal of the basic processes are still not understood or cannot be described in a proper quantitative way, e.g., the mass loss mechanism itself. Furthermore, these objects provide us with fascinating systems, where intricate interplays between various physical and chemical processes take place, and their relative simplicity in terms of geometry, density distribution, and kinematics makes them excellent astrophysical laboratories. In this review we will concentrate on those aspects of AGB mass loss that are particularly well studied using a large millimetre array.

Keck Observations of the UV-Bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)

NASA Astrophysics Data System (ADS)

Dixon, William Van Dyke; Chayer, Pierre; Reid, Iain N.

2016-06-01

In color-magnitude diagrams of globular clusters, stars brighter than the horizontal branch and bluer than the red-giant branch are known as UV-bright stars. Most are evolving from the asymptotic giant branch (AGB) to the tip of the white-dwarf cooling curve. To better understand this important phase of stellar evolution, we have analyzed a Keck HIRES echelle spectrum of the UV-bright star Barnard 29 in M13. We begin by fitting the star's H I (Hα, Hβ, and Hγ) and He I lines with a grid of synthetic spectra generated from non-LTE H-He models computed using the TLUSTY code. We find that the shape of the star's Hα profile is not well reproduced with these models. Upgrading from version 200 to version 204M of TLUSTY solves this problem: the Hα profile is now well reproduced. TLUSTY version 204 includes improved calculations for the Stark broadening of hydrogen line profiles. Using these models, we derive stellar parameters of Teff = 21,100 K, log g = 3.05, and log (He/H) = -0.87, values consistent with those of previous authors. The star's Keck spectrum shows photospheric absorption from N II, O II, Mg II, Al III, Si II, Si III, S II, Ar II, and Fe III. The abundances of these species are consistent with published values for the red-giant stars in M13, suggesting that the star's chemistry has changed little since it left the AGB.

J-type Carbon Stars: A Dominant Source of {sup 14}N-rich Presolar SiC Grains of Type AB

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Nan; Nittler, Larry R.; Alexander, Conel M. O’D.

We report Mo isotopic data of 27 new presolar SiC grains, including 12 {sup 14}N-rich AB ({sup 14}N/{sup 15}N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s -process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s -process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture processmore » ( i -process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show {sup 13}C and {sup 14}N excesses but no s -process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.« less

Observations of suspected low-mass post-T Tauri stars and their evolutionary status

NASA Technical Reports Server (NTRS)

Mundt, R.; Walter, F. M.; Feigelson, E. D.; Finkenzeller, U.; Herbig, G. H.; Odell, A. P.

1983-01-01

The results of a study of five X-ray discovered weak emission pre-main-sequence stars in the Taurus-Auriga star formation complex are presented. All are of spectral type K7-M0, and about 1-2 mag above the main sequence. One is a double-lined spectroscopic binary, the first spectroscopic binary PMS star to be confirmed. The ages, masses, and radii of these stars as determined by photometry and spectroscopy are discussed. The difference in emission strength between these and the T Tauri stars is investigated, and it is concluded that these 'post-T Tauri' stars do indeed appear more evolved than the T Tauri stars, although there is no evidence of any significant difference in ages.

Half of the Most Luminous Quasars May Be Obscured: Investigating the Nature of WISE-Selected Hot Dust-Obscured Galaxies

NASA Astrophysics Data System (ADS)

Assef, R. J.; Eisenhardt, P. R. M.; Stern, D.; Tsai, C.-W.; Wu, J.; Wylezalek, D.; Blain, A. W.; Bridge, C. R.; Donoso, E.; Gonzales, A.; Griffith, R. L.; Jarrett, T. H.

2015-05-01

The Wide-field Infrared Survey Explorer mission has unveiled a rare population of high-redshift (z = 1-4.6), dusty, hyper-luminous galaxies, with infrared luminosities {{L}IR}\\gt {{10}13} {{L}⊙ }, and sometimes exceeding {{10}14} {{L}⊙ }. Previous work has shown that their dust temperatures and overall far-infrared spectral energy distributions (SEDs) are significantly hotter than expected to be powered by star formation. We present here an analysis of the rest-frame optical through mid-infrared SEDs for a large sample of these so-called “hot, dust-obscured galaxies” (Hot DOGs). We find that the SEDs of Hot DOGs are generally well modeled by the combination of a luminous, yet obscured active galactic nuclei (AGNs) that dominates the rest-frame emission at λ \\gt 1 μ m and the bolometric luminosity output, and a less luminous host galaxy that is responsible for the bulk of the rest optical/UV emission. Even though the stellar mass of the host galaxies may be as large as 1011-1012 M⊙, the AGN emission, with a range of luminosities comparable to those of the most luminous QSOs known, require that either Hot DOGs have black hole masses significantly in excess of the local relations, or that they radiate significantly above the Eddington limit, at a level at least 10 times more efficiently than z ˜ 2 QSOs. We show that, while rare, the number density of Hot DOGs is comparable to that of equally luminous but unobscured (i.e., Type 1) QSOs. This may be at odds with the trend suggested at lower luminosities for the fraction of obscured AGNs to decrease with increasing luminosity. That trend may, instead, reverse at higher luminosities. Alternatively, Hot DOGs may not be the torus-obscured counterparts of the known optically selected, largely unobscured, hyper-luminous QSOs, and may represent a new component of the galaxy evolution paradigm. Finally, we discuss the environments of Hot DOGs and statistically show that these objects are in regions as dense as

Circumstellar Dust in Symbiotic Novae

NASA Astrophysics Data System (ADS)

Jurkic, T.; Kotnik-Karuza, D.

2015-12-01

We present a model of inner dust regions around the cool Mira component of the two symbiotic novae, RR Tel and HM Sge, based on the near-IR photometry, ISO spectra and mid-IR interferometry. The dust properties were determined using the DUSTY code. A compact circumstellar silicate dust shell with inner dust shell temperatures between 900 K and 1300 K and of moderate optical depth can explain all the observations. RR Tel shows the presence of an equatorially enhanced dust density during minimum obscuration. Obscuration events are explained by an increase in optical depth caused by the newly condensed dust. The mass loss rates are significantly higher than in intermediate-period single Miras but in agreement with longer-period O-rich AGB stars.

A Submillimeter Survey of Dust Continuum Emission in Local Dust-Obscured Galaxies

NASA Astrophysics Data System (ADS)

Lee, Jong Chul; Hwang, Ho Seong; Lee, Gwang-Ho

2015-08-01

Dusty star-forming galaxies are responsible for the bulk of cosmic star formation at 1star formation activity at high redshift. The submillimeter data on the 'Rayleigh-Jeans' side of the infrared spectral energy distributions (SEDs) of these galaxies are crucial for deriving the physical parameters of the dust content. We therefore conduct a submillimeter survey of local dust-obscured galaxies (DOGs) with the Caltech Submillimeter Observatory and the Submillimeter Array to study their dust properties. We determine the dust masses and temperatures for 16 local DOGs from the SED fit, and compare them with other dusty galaxies to understand a possible evolutionary link among them.

Chemical abundances of 1111 FGK stars from the HARPS GTO planet search program. II. Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd, and Eu

NASA Astrophysics Data System (ADS)

Delgado Mena, E.; Tsantaki, M.; Adibekyan, V. Zh.; Sousa, S. G.; Santos, N. C.; González Hernández, J. I.; Israelian, G.

2017-10-01

Aims: To understand the formation and evolution of the different stellar populations within our Galaxy it is essential to combine detailed kinematical and chemical information for large samples of stars. The aim of this work is to explore the chemical abundances of neutron capture elements which are a product of different nucleosynthesis processes taking place in diverse objects in the Galaxy, such as massive stars, asymptotic giant branch (AGB) stars and supernovae (SNe) explosions. Methods: We derive chemical abundances of Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd, and Eu for a large sample of more than 1000 FGK dwarf stars with high-resolution (R 115 000) and high-quality spectra from the HARPS-GTO program. The abundances are derived by a standard local thermodynamic equilibrium (LTE) analysis using measured equivalent widths (EWs) injected to the code MOOG and a grid of Kurucz ATLAS9 atmospheres. Results: We find that thick disc stars are chemically disjunct for Zn and Eu and also show on average higher Zr but lower Ba and Y than the thin disc stars. We also discovered that the previously identified high-α metal-rich population is also enhanced in Cu, Zn, Nd, and Eu with respect to the thin disc but presents lower Ba and Y abundances on average, following the trend of thick disc stars towards higher metallities and further supporting the different chemical composition of this population. By making a qualitative comparison of O (pure α), Mg, Eu (pure r-process), and s-process elements we can distinguish between the contribution of the more massive stars (SNe II for α and r-process elements) and the lower mass stars (AGBs) whose contribution to the enrichment of the Galaxy is delayed, due to their longer lifetimes. The ratio of heavy-s to light-s elements of thin disc stars presents the expected behaviour (increasing towards lower metallicities) and can be explained by a major contribution of low-mass AGB stars for s-process production at disc metallicities. However, the

Unveiling the Composite Nature of Dust-Obscured Galaxies (DOGs) with Herschel

NASA Astrophysics Data System (ADS)

Riguccini, Laurie A.; Le Floc'h, Emeric; Mullaney, James

2015-08-01

DOGs are bright 24um-selected sources with extreme obscuration at optical wavelengths. Some of them are characterized by a rising power-law continuum of hot dust (T_D ~ 200-1000 K) in the near-IR emission indicating that their mid-IR luminosity is dominated by an AGN. Whereas DOGs with a fainter 24um flux display a stellar bump and their mid-IR luminosity is believed to be mainly powered by dusty star-formation. Another explanation is that the mid-IR emission still comes from AGN activity but the torus emission is so obscured that it becomes negligible with respect to the emission from the host component.In an effort to characterize the nature of the physical processes underlying their IR emission, we focus on DOGs (F24/FR>982) within the COSMOS field with Herschel data and derive their far-IR properties (e.g., total IR luminosities; mid-to-far IR colors; dust temperatures and masses and AGN contribution) based on SED fitting.Of particular interest are the 24um-bright DOGs (F24>1mJy). They present bluer far-IR/mid-IR colors than the rest of the sample, unveiling the potential presence of an AGN. The AGN contribution to the total 8-1000um flux increases as a function of the rest-frame 8um-luminosity irrespective of the redshift, with a stronger contribution at lower redshift. This confirms that faint DOGs (F24<1mJy) are dominated by star-formation while brighter DOGs show a larger contribution from an AGN.Is this FIR-selection technique allowing us to probe a new population of obscured AGN? Or does it corresponds to already known AGN in the X-rays, NIR or radio? The wealth of multi wavelength data in COSMOS will allow us to describe our results here.

LITHIUM-RICH GIANTS IN GLOBULAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kirby, Evan N.; Cohen, Judith G.; Guhathakurta, Puragra

Although red giants deplete lithium on their surfaces, some giants are Li-rich. Intermediate-mass asymptotic giant branch (AGB) stars can generate Li through the Cameron–Fowler conveyor, but the existence of Li-rich, low-mass red giant branch (RGB) stars is puzzling. Globular clusters are the best sites to examine this phenomenon because it is straightforward to determine membership in the cluster and to identify the evolutionary state of each star. In 72 hours of Keck/DEIMOS exposures in 25 clusters, we found four Li-rich RGB and two Li-rich AGB stars. There were 1696 RGB and 125 AGB stars with measurements or upper limits consistentmore » with normal abundances of Li. Hence, the frequency of Li-richness in globular clusters is (0.2 ± 0.1)% for the RGB, (1.6 ± 1.1)% for the AGB, and (0.3 ± 0.1)% for all giants. Because the Li-rich RGB stars are on the lower RGB, Li self-generation mechanisms proposed to occur at the luminosity function bump or He core flash cannot explain these four lower RGB stars. We propose the following origin for Li enrichment: (1) All luminous giants experience a brief phase of Li enrichment at the He core flash. (2) All post-RGB stars with binary companions on the lower RGB will engage in mass transfer. This scenario predicts that 0.1% of lower RGB stars will appear Li-rich due to mass transfer from a recently Li-enhanced companion. This frequency is at the lower end of our confidence interval.« less

The dynamics of post-main sequence planetary systems

NASA Astrophysics Data System (ADS)

Mustill, Alexander James

2017-06-01

The study of planetary systems after their host stars have left the main sequence is of fundamental importance for exoplanet science, as the most direct determination of the compositions of extra-Solar planets, asteroids and comets is in fact made by an analysis of the elemental abundances of the remnants of these bodies accreted into the atmospheres of white dwarfs.To understand how the accreted bodies relate to the source populations in the planetary system, and to model their dynamical delivery to the white dwarf, it is necessary to understand the effects of stellar evolution on bodies' orbits. On the red giant branch (RGB) and asymptotic giant branch (AGB) prior to becoming a white dwarf, stars expand to a large size (>1 au) and are easily deformed by orbiting planets, leading to tidal energy dissipation and orbital decay. They also lose half or more of their mass, causing the expansion of bodies' orbits. This mass loss increases the planet:star mass ratio, so planetary systems orbiting white dwarfs can be much less stable than those orbiting their main-sequence progenitors. Finally, small bodies in the system experience strong non-gravitational forces during the RGB and AGB: aerodynamic drag from the mass shed by the star, and strong radiation forces as the stellar luminosity reaches several thousand Solar luminosities.I will review these effects, focusing on planet--star tidal interactions and planet--asteroid interactions, and I will discuss some of the numerical challenges in modelling systems over their entire lifetimes of multiple Gyr.

Mid-infrared luminous quasars in the GOODS-Herschel fields: a large population of heavily obscured, Compton-thick quasars at z ≈ 2

NASA Astrophysics Data System (ADS)

Del Moro, A.; Alexander, D. M.; Bauer, F. E.; Daddi, E.; Kocevski, D. D.; McIntosh, D. H.; Stanley, F.; Brandt, W. N.; Elbaz, D.; Harrison, C. M.; Luo, B.; Mullaney, J. R.; Xue, Y. Q.

2016-02-01

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL6 μm ≥ 6 × 1044 erg s-1) at redshift z ≈ 1-3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS-Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, NH > 1.5 × 1024 cm-2) quasars at the peak era of nuclear and star formation activities. Despite being very bright in the mid-IR band, ≈30 per cent of these quasars are not detected in the extremely deep 2 and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (≈67 per cent) are obscured by column densities NH > 1022 cm-2; this fraction reaches ≈80 per cent when including the X-ray-undetected sources (9 out of 33), which are likely to be the most heavily obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be ≈24-48 per cent, and their space density to be Φ = (6.7 ± 2.2) × 10-6 Mpc-3. From the investigation of the quasar host galaxies in terms of star formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily obscured quasars have, on average, more disturbed morphologies than the unobscured/moderately obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is ≈40 per cent, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z ≈ 2.

The AGB 2017 Trustee Index. Trustees and Higher Education's Value Proposition

ERIC Educational Resources Information Center

Association of Governing Boards of Universities and Colleges, 2017

2017-01-01

"The AGB 2017 Trustee Index" is the first in a series of three annual surveys, conducted by Gallup under the direction of AGB, to assess trustee perspectives on the state of higher education. This year's report reveals a number of interesting findings: the centrality of the liberal arts, the lack of trustee involvement in influencing…

Europium s-process Signature at Close-to-solar Metallicity in Stardust SiC Grains from Asymptotic Giant Branch Stars

NASA Astrophysics Data System (ADS)

Ávila, Janaína N.; Ireland, Trevor R.; Lugaro, Maria; Gyngard, Frank; Zinner, Ernst; Cristallo, Sergio; Holden, Peter; Rauscher, Thomas

2013-05-01

Individual mainstream stardust silicon carbide (SiC) grains and a SiC-enriched bulk sample from the Murchison carbonaceous meteorite have been analyzed by the Sensitive High Resolution Ion Microprobe-Reverse Geometry for Eu isotopes. The mainstream grains are believed to have condensed in the outflows of ~1.5-3 M ⊙ carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived from our measurements are compared with previous astronomical observations of carbon-enhanced metal-poor stars enriched in elements made by slow neutron captures (the s-process). Despite the difference in metallicity between the parent stars of the grains and the metal-poor stars, the fr(151Eu) values derived from our measurements agree well with fr(151Eu) values derived from astronomical observations. We have also compared the SiC data with theoretical predictions of the evolution of Eu isotopic ratios in the envelope of AGB stars. Because of the low Eu abundances in the SiC grains, the fr(151Eu) values derived from our measurements show large uncertainties, in most cases being larger than the difference between solar and predicted fr(151Eu) values. The SiC aggregate yields a fr(151Eu) value within the range observed in the single grains and provides a more precise result (fr(151Eu) = 0.54 ± 0.03, 95% conf.), but is approximately 12% higher than current s-process predictions. The AGB models can match the SiC data if we use an improved formalism to evaluate the contribution of excited nuclear states in the calculation of the 151Sm(n, γ) stellar reaction rate.

Characterizing the Population of Bright Infrared Sources in the Small Magellanic Cloud

NASA Astrophysics Data System (ADS)

Kraemer, K. E.; Sloan, G. C.; Wood, P. R.; Jones, O. C.; Egan, M. P.

2017-01-01

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to observe stars in the Small Magellanic Cloud (SMC) selected from the Point Source Catalog of the Midcourse Space Experiment (MSX). We concentrate on the dust properties of the oxygen-rich evolved stars. The dust composition has smaller contributions from alumina compared to the Galaxy. This difference may arise from the lower metallicity in the SMC, but it could be a selection effect, as the SMC sample includes more stars that are brighter and thus more massive. The distribution of the SMC stars along the silicate sequence looks more like the Galactic sample of red supergiants than asymptotic giant branch stars (AGBs). While many of the SMC stars are definitively on the AGB, several also show evidence of hot bottom burning. Three of the supergiants show PAH emission at 11.3 μm. Two other sources show mixed chemistry, with both carbon-rich and oxygen-rich spectral features. One, MSX SMC 134, may be the first confirmed silicate/carbon star in the SMC. The other, MSX SMC 049, is a candidate post-AGB star. MSX SMC 145, previously considered a candidate OH/IR star, is actually an AGB star with a background galaxy at z = 0.16 along the same line of sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of the higher sensitivity and resolution of Spitzer, and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the upcoming James Webb Space Telescope (JWST). Color-color diagrams generated from the IRS spectra and the mid-infrared filters on JWST show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different classes of YSOs.

The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust.

PubMed

Massalkhi, Sarah; Agúndez, M; Cernicharo, J; Velilla Prieto, L; Goicoechea, J R; Quintana-Lacaci, G; Fonfría, J P; Alcolea, J; Bujarrabal, V

2018-03-01

Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si-C bond detected in C-rich AGB stars are SiC 2 , SiC, and Si 2 C. To date, the ring molecule SiC 2 has been observed in a handful of evolved stars, while SiC and Si 2 C have only been detected in the C-star envelope IRC +10216. We aim to study how widespread and abundant SiC 2 , SiC, and Si 2 C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC 2 , SiC, and Si 2 C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC 2 and to derive SiC 2 fractional abundances in the observed envelopes. We detect SiC 2 in most of the sources, SiC in about half of them, and do not detect Si 2 C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC 2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC 2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC 2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC 2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. The observed behavior of a decline in the SiC 2 abundance with increasing density strongly suggests that SiC 2 is an important gas

Identifying Bright Stars in Crowded Environments Using Velocity Dispersion Measurements, and an Application to the Center of M32

NASA Astrophysics Data System (ADS)

Davidge, T. J.; Beck, Tracy L.; McGregor, Peter J.

2010-02-01

The identification of individual stars in crowded environments using photometric information alone is confounded by source confusion. However, with the addition of spectroscopic information it is possible to distinguish between blends and areas where the light is dominated by a single star, using the widths of absorption features. We describe a procedure for identifying locations in kinematically hot environments where the light is dominated by a single star, and apply this method to spectra with 0.1″ angular resolution covering the 2.1-2.3 μm interval in the central regions of M32. Targets for detailed investigation are selected as areas of localized brightness enhancement. Three locations where at least 60% of the K-band light comes from a single bright star, and another with light that is dominated by two stars with very different velocities, are identified. The dominant stars are evolving near the tip of the asymptotic giant branch (AGB), and have M5 III spectral type. The lack of a dispersion in spectral type suggests that the upper AGB within the central arcsecond of M32 has a dispersion in J - K of only a few hundredths of a magnitude, in agreement with what is seen at larger radii. One star has weaker atomic absorption lines than the others, such that [M/H] is 0.2 dex lower. Such a difference in metallicity is consistent with the metallicity dispersion inferred from the photometric width of the AGB in M32. The use of line width to distinguish between blends involving many relatively faint stars, none of which dominate the light output, and areas that are dominated by a single intrinsically bright star could be extended to crowded environments in other nearby galaxies. 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 (US), the Science and Technology

From starburst to quiescence: testing active galactic nucleus feedback in rapidly quenching post-starburst galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yesuf, Hassen M.; Faber, S. M.; Trump, Jonathan R.

Post-starbursts are galaxies in transition from the blue cloud to the red sequence. Although they are rare today, integrated over time they may be an important pathway to the red sequence. This work uses Sloan Digital Sky Survey, the Galaxy Evolution Explorer, and Wide-field Infrared Survey Explorer observations to identify the evolutionary sequence from starbursts to fully quenched post-starbursts (QPSBs) in the narrow mass range log M(M {sub ☉}) = 10.3-10.7, and identifies 'transiting' post-starbursts (TPSBs) which are intermediate between these two populations. In this mass range, ∼0.3% of galaxies are starbursts, ∼0.1% are QPSBs, and ∼0.5% are the transitingmore » types in between. The TPSBs have stellar properties that are predicted for fast-quenching starbursts and morphological characteristics that are already typical of early-type galaxies. The active galactic nucleus (AGN) fraction, as estimated from optical line ratios, of these post-starbursts is about three times higher (≳ 36% ± 8%) than that of normal star forming galaxies of the same mass, but there is a significant delay between the starburst phase and the peak of nuclear optical AGN activity (median age difference of ≳ 200 ± 100 Myr), in agreement with previous studies. The time delay is inferred by comparing the broadband near-NUV-to-optical photometry with stellar population synthesis models. We also find that starbursts and post-starbursts are significantly more dust obscured than normal star forming galaxies in the same mass range. About 20% of the starbursts and 15% of the TPSBs can be classified as 'dust-obscured galaxies' (DOGs), with a near-UV-to-mid-IR flux ratio of ≳ 900, while only 0.8% of normal galaxies are DOGs. The time delay between the starburst phase and AGN activity suggests that AGNs do not play a primary role in the original quenching of starbursts but may be responsible for quenching later low-level star formation by removing gas and dust during the post

Evolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey

NASA Astrophysics Data System (ADS)

Bourne, N.; Dunlop, J. S.; Merlin, E.; Parsa, S.; Schreiber, C.; Castellano, M.; Conselice, C. J.; Coppin, K. E. K.; Farrah, D.; Fontana, A.; Geach, J. E.; Halpern, M.; Knudsen, K. K.; Michałowski, M. J.; Mortlock, A.; Santini, P.; Scott, D.; Shu, X. W.; Simpson, C.; Simpson, J. M.; Smith, D. J. B.; van der Werf, P. P.

2017-05-01

We present a new exploration of the cosmic star formation history and dust obscuration in massive galaxies at redshifts 0.5 < z < 6. We utilize the deepest 450- and 850-μm imaging from SCUBA-2 CLS, covering 230 arcmin2 in the AEGIS, COSMOS and UDS fields, together with 100-250 μm imaging from Herschel. We demonstrate the capability of the t-phot deconfusion code to reach below the confusion limit, using multiwavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star formation rate (SFR) and stellar mass up to z ˜ 5, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-β relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in M_{\\ast }>10^{10} M_{⊙} galaxies at 0.5 < z < 6. This is dominated by obscured star formation by a factor of >10. One third of this is accounted for by 450-μm-detected sources, while one-fifth is attributed to UV-luminous sources (brighter than L_UV^\\ast), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at z ≲ 3, and from UV-only data at z ˜ 5. The cosmic star formation history undergoes a transition at z ˜ 3-4, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.

Stellar Clusters in the NGC 6334 Star-Forming Complex

NASA Astrophysics Data System (ADS)

Feigelson, Eric D.; Martin, Amanda L.; McNeill, Collin J.; Broos, Patrick S.; Garmire, Gordon P.

2009-07-01

The full stellar population of NGC 6334, one of the most spectacular regions of massive star formation in the nearby Galaxy, has not been well sampled in past studies. We analyze here a mosaic of two Chandra X-ray Observatory images of the region using sensitive data analysis methods, giving a list of 1607 faint X-ray sources with arcsecond positions and approximate line-of-sight absorption. About 95% of these are expected to be cluster members, most lower mass pre-main-sequence stars. Extrapolating to low X-ray levels, the total stellar population is estimated to be 20,000-30,000 pre-main-sequence stars. The X-ray sources show a complicated spatial pattern with ~10 distinct star clusters. The heavily obscured clusters are mostly associated with previously known far-infrared sources and radio H II regions. The lightly obscured clusters are mostly newly identified in the X-ray images. Dozens of likely OB stars are found, both in clusters and dispersed throughout the region, suggesting that star formation in the complex has proceeded over millions of years. A number of extraordinarily heavily absorbed X-ray sources are associated with the active regions of star formation.

Search for Carbon-Rich Asymptotic Giant Branch Stars in Milky Way Globular Clusters

NASA Astrophysics Data System (ADS)

Indahl, Briana; Pessev, P.

2014-01-01

From our current understanding of stellar evolution, it would not be expected to find carbon rich asymptotic giant branch (AGB) stars in Milky Way globular clusters. Due to the low metallicity of the population II stars making up the globular clusters and their age, stars large enough to fuse carbon should have already evolved off of the asymptotic giant branch. Recently, however, there have been serendipitous discoveries of these types of stars. Matsunaga et al. (2006) discovered a Mira variable in the globular cluster Lynga 7. It was later confirmed by Feast et al. (2012) that the star is a member of the cluster and must be a product of a stellar merger. In the same year, Sharina et al. (2012) discovered a carbon star in the low metallicity globular cluster NGC6426 and reports it to be a CH star. Five more of these types of stars have been made as serendipitous discoveries and have been reported by Harding (1962), Dickens (1972), Cote et al. (1997), and Van Loon (2007). The abundance of these types of carbon stars in Milky Way globular clusters has been unknown because the discovery of these types of objects has only ever been a serendipitous discovery. These stars could have been easily overlooked in the past as they are outside the typical parameter space of galactic globular clusters. Also advances in near-infrared instruments and observing techniques have made it possible to detect the fainter carbon stars in binary systems. Having an understanding of the abundances of carbon stars in galactic globular clusters will aid in the modeling of globular cluster and galaxy formation leading to a better understanding of these processes. To get an understanding of the abundances of these stars we conducted the first comprehensive search for AGB carbon stars into all Milky Way globular clusters listed in the Harris Catalog (expect for Pyxis). I have found 128 carbon star candidates using methods of comparing color magnitude diagrams of the clusters with the carbon

High-resolution speckle masking interferometry and radiative transfer modeling of the oxygen-rich AGB star AFGL 2290

NASA Astrophysics Data System (ADS)

Gauger, A.; Balega, Y. Y.; Irrgang, P.; Osterbart, R.; Weigelt, G.

1999-06-01

We present the first diffraction-limited speckle masking observations of the oxygen-rich AGB star AFGL 2290. The speckle interferograms were recorded with the Russian 6 m SAO telescope. At the wavelength 2.11 microns a resolution of 75 milli-arcsec (mas) was obtained. The reconstructed diffraction-limited image reveals that the circumstellar dust shell (CDS) of AFGL 2290 is at least slightly non-spherical. The visibility function shows that the stellar contribution to the total 2.11 microns flux is less than ~ 40%, indicating a rather large optical depth of the circumstellar dust shell. The 2-dimensional Gaussian visibility fit yields a diameter of AFGL 2290 at 2.11 microns of 43 masx51 mas, which corresponds to a diameter of 42 AUx50 AU for an adopted distance of 0.98 kpc. Our new observational results provide additional constraints on the CDS of AFGL 2290, which supplement the information from the spectral energy distribution (SED). To determine the structure and the properties of the CDS we have performed radiative transfer calculations for spherically symmetric dust shell models. The observed SED approximately at phase 0.2 can be well reproduced at all wavelengths by a model with T_eff=2000 K, a dust temperature of 800 K at the inner boundary r1, an optical depth tau_ {V}=100 and a radius for the single-sized grains of a_gr=0.1 microns . However, the 2.11 microns visibility of the model does not match the observation. Exploring the parameter space, we found that grain size is the key parameter in achieving a fit of the observed visibility while retaining the match of the SED, at least partially. Both the slope and the curvature of the visibility strongly constrain the possible grain radii. On the other hand, the SED at longer wavelengths, the silicate feature in particular, determines the dust mass loss rate and, thereby, restricts the possible optical depths of the model. With a larger grain size of 0.16 microns and a higher tau_ {V}=150, the observed

STELLAR POPULATIONS AND THE STAR FORMATION HISTORIES OF LSB GALAXIES. V. WFC3 COLOR–MAGNITUDE DIAGRAMS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schombert, James; McGaugh, Stacy, E-mail: jschombe@uoregon.edu, E-mail: stacy.mcgaugh@case.edu

2015-09-15

We present WFC3 observations of three low surface brightness (LSB) galaxies from the Schombert et al. LSB catalog that are within 11 Mpc of the Milky Way. Deep imaging at F336W, F555W, and F814W allow the construction of the V − I color–magnitude diagrams (CMD) to M{sub I} = −2. Overall 1869, 465, and 501 stellar sources are identified in the three LSB galaxies F415-3, F608-1, and F750-V1, respectively. The spatial distribution of young blue stars matches the Hα maps from ground-based imaging, indicating that star formation in LSB galaxies follows the same style as in other irregular galaxies. Severalmore » star complexes are identified, matching regions of higher surface brightness as seen from ground-based imaging. The CMD for each LSB galaxy has a similar morphology to Local Volume (LV) dwarf galaxies (i.e., a blue main sequence, blue and red He burning branches, and asymptotic giant branch (AGB) stars). The LSB CMD’s distinguish themselves from nearby dwarf CMD’s by having a higher proportion of blue main sequence stars and fewer AGB stars than expected from their mean metallicities. Current [Fe/H] values below −0.6 are deduced from the position of the red helium-burning branch (rHeB) stars in the V − I diagram. The distribution of stars on the blue helium-burning branch (bHeB) and rHeB from the U − V and V − I CMD indicate a history of constant star formation for the last 100 Myr.« less

Obscuring and Feeding Supermassive Black Holes with Evolving Nuclear Star Clusters

NASA Astrophysics Data System (ADS)

Schartmann, M.; Burkert, A.; Krause, M.; Camenzind, M.; Meisenheimer, K.; Davies, R. I.

2010-05-01

Recently, high-resolution observations made with the help of the near-infrared adaptive optics integral field spectrograph SINFONI at the VLT proved the existence of massive and young nuclear star clusters in the centers of a sample of Seyfert galaxies. With the help of high-resolution hydrodynamical simulations with the pluto code, we follow the evolution of such clusters, especially focusing on mass and energy feedback from young stars. This leads to a filamentary inflow of gas on large scales (tens of parsecs), whereas a turbulent and very dense disk builds up on the parsec scale. Here we concentrate on the long-term evolution of the nuclear disk in NGC 1068 with the help of an effective viscous disk model, using the mass input from the large-scale simulations and accounting for star formation in the disk. This two-stage modeling enables us to connect the tens-of-parsecs scale region (observable with SINFONI) with the parsec-scale environment (MIDI observations). At the current age of the nuclear star cluster, our simulations predict disk sizes of the order 0.8 to 0.9 pc, gas masses of order 106 M⊙, and mass transfer rates through the inner boundary of order 0.025 M⊙ yr-1, in good agreement with values derived from observations.

The Executive Committee. AGB Effective Committee Series

ERIC Educational Resources Information Center

Legon, Richard D.

2012-01-01

This publication is part of an Association of Governing Boards of Universities and Colleges (AGB) series devoted to strengthening the role of key standing committees of governing boards. While there is no optimum committee system for institutions of higher education, certain principles, practices, and procedures prevail. The best practices…

The Facilities Committee. AGB Effective Committee Series

ERIC Educational Resources Information Center

Kaiser, Harvey H.

2012-01-01

This publication is part of an Association of Governing Boards of Universities and Colleges (AGB) series devoted to strengthening the role of key standing committees of governing boards. While there is no optimum committee system for institutions of higher education, certain principles, practices, and procedures prevail. The best practices…

The Audit Committee. AGB Effective Committee Series

ERIC Educational Resources Information Center

Staisloff, Richard L.

2011-01-01

This publication is part of an Association of Governing Boards of Universities and Colleges (AGB) series devoted to strengthening the role of key standing committees of governing boards. While there is no optimum committee system for institutions of higher education, certain principles, practices, and procedures prevail. The best practices…

The Compensation Committee. AGB Effective Committee Series

ERIC Educational Resources Information Center

Hyatt, Thomas K.

2013-01-01

This publication is part of an Association of Governing Boards of Universities and Colleges (AGB) series devoted to strengthening the role of key standing committees of governing boards. While there is no optimum committee system for institutions of higher education, certain principles, practices, and procedures prevail. The best practices…

Gamma-ray line emission from Al-26 produced by Wolf-Rayet stars

NASA Technical Reports Server (NTRS)

Prantzos, N.; Casse, M.; Gros, M.; Doom, C.; Arnould, M.

1985-01-01

The recent satellite observations of the 1.8 MeV line from the decay of Al-26 has given a new impetus to the study of the nucleosynthesis of Al-26. The production and ejection of Al-26 by massive mass-losing stars (Of and WR stars) is discussed in the light of recent stellar models. The longitude distribution of the Al-26 gamma ray line emission produced by the galactic collection of WR stars is derived based on various estimates of their radial distribution. This longitude profile provides: (1) a specific signature of massive stars on the background of other potential Al-26 sources, as novae, supernovae, certain red giants and possibly AGB stars; and (2) a possible tool to improve the data analysis of the HEAO 3 and SMM experiments.

Leo P: A very low-mass, extremely metal-poor, star-forming galaxy

NASA Astrophysics Data System (ADS)

McQuinn, Kristen B.; Leo P Team

2017-01-01

Leo P is a low-luminosity dwarf galaxy just outside the Local Group with properties that make it an ideal probe of galaxy evolution at the faint-end of the luminosity function. Using combined data from 2 Hubble Space Telescope (HST) observing campaigns, the Very Large Array, the Spitzer Space telescope, as well as ground based data, we have constructed a robust evolutionary picture of Leo P. Leo P is one the most metal-poor, gas-rich galaxies ever discovered, has a stellar mass of a 5x105 Msun, comparable gas mass, and a single HII region. The star formation history reconstructed from the resolved stellar populations in Leo P shows it is unquenched, despite its very low mass. Based on the star formation history and metallicity measurements, the galaxy has lost 95% of its oxygen produced via nucleosynthesis, presumably to outflows. The neutral gas in the galaxy shows signs of rotation, although the velocity dispersion is comparable to the rotation velocity. Thus, Leo P bridges the gap between more massive dwarf irregular and less massive dwarf spheroidals on the baryonic Tully-Fisher relation. Furthermore, the galaxy hosts several, extremely dusty AGB candidates which will be probed with new HST and Spitzer observations. If confirmed as AGB stars, these may be our best local proxies for studying chemically unevolved star formation and subsequent dust production in metallicity environments comparable to the early universe.

Stellar Evolutionary Effects on the Abundances of PAH and SN-Condensed Dust in Galaxies

NASA Technical Reports Server (NTRS)

Dwek, Eliahu

2007-01-01

Spectral and photometric observations of nearby galaxies show a correlation between the strength of their mid-IR aromatic features, attributed to PAH molecules, and their metal abundance, leading to a deficiency of these features in low-metallicity galaxies. We suggest that the observed correlation represents a trend of PAH abundance with galactic age, reflecting the delayed injection of carbon dust into the ISM by AGB stars in the final post-AGB phase of their evolution. We also show that larger dust particles giving rise to the far-IR emission follow a distinct evolutionary trend closely related to the injection of dust by massive stars into the ISM.

2011 AGB Survey of Higher Education Governance

ERIC Educational Resources Information Center

Association of Governing Boards of Universities and Colleges, 2011

2011-01-01

This report, the second of AGB's studies of higher education governance, documents the extent to which college and university boards are following good-governance practices. In addition, it takes a focused look at board engagement to determine the degree to which governing boards are actively, intellectually, and strategically involved with their…

A Spectroscopic Study of the High-Latitude Far Evolved Star V534 Lyr

NASA Astrophysics Data System (ADS)

Sendzikas, E. G.; Chentsov, E. L.

2017-06-01

We study a pulsating variable post-AGB star V534 Lyr = HD172324 based on five high resolution spectra (R=60000) obtained with the NES echelle spectrograph of the 6-meter Russian telescope (BTA) in 2010 and 2013. Using the atmosphere modeling method and the Kurucz model set, we obtained the effective temperature Teff=10500 K, surface gravity log g=2.5, and microturbulent velocity ξt=4.0 km/s. The underabundance of the iron group elements [Met/H]⊙ = -0.50 was detected. This fact in combination with high spatial velocity indicates that V534 Lyr does not belong to the disk population. The radial velocity gradient in the V534 Lyr atmosphere is minimum: differential shifts of lines are close to measurement errors. The spectral class A0 Iab corresponds to the distance to V534 Lyr, d≍6 kpc.

FORMALDEHYDE MASERS: EXCLUSIVE TRACERS OF HIGH-MASS STAR FORMATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Araya, E. D.; Brown, J. E.; Olmi, L.

2015-11-15

The detection of four formaldehyde (H{sub 2}CO) maser regions toward young high-mass stellar objects in the last decade, in addition to the three previously known regions, calls for an investigation of whether H{sub 2}CO masers are an exclusive tracer of young high-mass stellar objects. We report the first survey specifically focused on the search for 6 cm H{sub 2}CO masers toward non high-mass star-forming regions (non HMSFRs). The observations were conducted with the 305 m Arecibo Telescope toward 25 low-mass star-forming regions, 15 planetary nebulae and post-AGB stars, and 31 late-type stars. We detected no H{sub 2}CO emission in ourmore » sample of non HMSFRs. To check for the association between high-mass star formation and H{sub 2}CO masers, we also conducted a survey toward 22 high-mass star-forming regions from a Hi-GAL (Herschel infrared Galactic Plane Survey) sample known to harbor 6.7 GHz CH{sub 3}OH masers. We detected a new 6 cm H{sub 2}CO emission line in G32.74−0.07. This work provides further evidence that supports an exclusive association between H{sub 2}CO masers and young regions of high-mass star formation. Furthermore, we detected H{sub 2}CO absorption toward all Hi-GAL sources, and toward 24 low-mass star-forming regions. We also conducted a simultaneous survey for OH (4660, 4750, 4765 MHz), H110α (4874 MHz), HCOOH (4916 MHz), CH{sub 3}OH (5005 MHz), and CH{sub 2}NH (5289 MHz) toward 68 of the sources in our sample of non HMSFRs. With the exception of the detection of a 4765 MHz OH line toward a pre-planetary nebula (IRAS 04395+3601), we detected no other spectral line to an upper limit of 15 mJy for most sources.« less

Non-Equilibrium Chemistry of O-Rich AGB Stars as Revealed by ALMA

NASA Astrophysics Data System (ADS)

Wong, Ka Tat

2018-04-01

Chemical models suggest that pulsation driven shocks propagating from the stellar surfaces of oxygen-rich evolved stars to the dust formation zone trigger non-equilibrium chemistry in the shocked gas near the star, including the formation of carbon-bearing molecules in the stellar winds dominated by oxygen-rich chemistry. Recent long-baseline ALMA observations are able to give us a detailed view of the molecular line emission and absorption at an angular resolution of a few stellar radii. I am going to present the latest results from the ALMA observations of IK Tau and o Cet in late 2017, with a particular focus on HCN.

The Investment Committee. AGB Effective Committee Series

ERIC Educational Resources Information Center

Yoder, Jay A.

2011-01-01

This publication is part of an AGB series devoted to strengthening the role of key standing committees of governing boards. While there is no optimum committee system for institutions of higher education, certain principles, practices, and procedures prevail. The best practices included in this text support the objectives of board committees:…

Lithium-rich Giants in Globular Clusters

NASA Astrophysics Data System (ADS)

Kirby, Evan N.; Guhathakurta, Puragra; Zhang, Andrew J.; Hong, Jerry; Guo, Michelle; Guo, Rachel; Cohen, Judith G.; Cunha, Katia

2016-03-01

Although red giants deplete lithium on their surfaces, some giants are Li-rich. Intermediate-mass asymptotic giant branch (AGB) stars can generate Li through the Cameron-Fowler conveyor, but the existence of Li-rich, low-mass red giant branch (RGB) stars is puzzling. Globular clusters are the best sites to examine this phenomenon because it is straightforward to determine membership in the cluster and to identify the evolutionary state of each star. In 72 hours of Keck/DEIMOS exposures in 25 clusters, we found four Li-rich RGB and two Li-rich AGB stars. There were 1696 RGB and 125 AGB stars with measurements or upper limits consistent with normal abundances of Li. Hence, the frequency of Li-richness in globular clusters is (0.2 ± 0.1)% for the RGB, (1.6 ± 1.1)% for the AGB, and (0.3 ± 0.1)% for all giants. Because the Li-rich RGB stars are on the lower RGB, Li self-generation mechanisms proposed to occur at the luminosity function bump or He core flash cannot explain these four lower RGB stars. We propose the following origin for Li enrichment: (1) All luminous giants experience a brief phase of Li enrichment at the He core flash. (2) All post-RGB stars with binary companions on the lower RGB will engage in mass transfer. This scenario predicts that 0.1% of lower RGB stars will appear Li-rich due to mass transfer from a recently Li-enhanced companion. This frequency is at the lower end of our confidence interval. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Massive Stars in the SDSS-IV/APOGEE SURVEY. I. OB Stars

NASA Astrophysics Data System (ADS)

Roman-Lopes, A.; Román-Zúñiga, C.; Tapia, Mauricio; Chojnowski, Drew; Gómez Maqueo Chew, Y.; García-Hernández, D. A.; Borissova, Jura; Minniti, Dante; Covey, Kevin R.; Longa-Peña, Penélope; Fernandez-Trincado, J. G.; Zamora, Olga; Nitschelm, Christian

2018-03-01

In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB stars. We developed a near-infrared semi-empirical spectral classification method that was successfully used in case of four new exemplars, previously classified as later B-type stars. Our results agree well with those determined independently from ECHELLE optical spectra, being in line with the spectral types derived from the “canonical” MK blue optical system. This confirms that the APOGEE spectrograph can also be used as a powerful tool in surveys aiming to unveil and study a large number of moderately and highly obscured OB stars still hidden in the Galaxy.

Planetary nebulae: 20 years of Hubble inquiry

NASA Astrophysics Data System (ADS)

Balick, Bruce

2012-08-01

The Hubble Space Telescope has served the critical roles of microscope and movie camera in the past 20 years of research on planetary nebulae (``PNe''). We have glimpsed the details of the evolving structures of neutral and ionized post-AGB objects, built ingenious heuristic models that mimic these structures, and constrained most of the relevant physical processes with careful observations and interpretation. We have searched for close physical binary stars with spatial resolution ~50 AU at 1 AU, located jets emerging from the nucleus at speeds up to 2000 km s-1 and matched newly discovered molecular and X-ray emission regions to physical substructures in order to better understand how stellar winds and ionizing radiation interact to form the lovely symmetries that are observed. Ultraviolet spectra of CNO in PNe help to uncover how stars process deep inside AGB stars with unstable nuclear burning zones. HST broadband imaging has been at the forefront of uncovering surprisingly complex wind morphologies produced at the tip of the AGB, and has led to an increasing realization of the potentially vital roles of close binary stars and emerging magnetic fields in shaping stellar winds.

Inter-relationships between the heterotrimeric Gβ subunit AGB1, the RLK FERONIA and RALF1 in salinity response.

PubMed

Yu, Yunqing; Assmann, Sarah M

2018-06-15

Plant heterotrimeric G proteins modulate numerous developmental stress responses. Recently, receptor-like kinases (RLKs) have been implicated as functioning with G proteins, and may serve as plant G-protein-coupled-receptors (GPCRs). The RLK FERONIA (FER), in the Catharantus roseus RLK1-like subfamily, is activated by a family of polypeptides called Rapid Alkalinization Factors (RALFs). We previously showed that the Arabidopsis G protein β subunit, AGB1, physically interacts with FER, and that RALF1 regulation of stomatal movement through FER requires AGB1. Here, we investigated genetic interactions of AGB1 and FER in plant salinity response by comparing salt responses in the single and double mutants of agb1 and fer. We show that AGB1 and FER act additively or synergistically depending on the conditions of the NaCl treatments. We further show that the synergism likely occurs through salt-induced ROS production. In addition, we show that RALF1 enhances salt toxicity through increasing Na + accumulation and decreasing K + accumulation rather than by inducing ROS production, and that the RALF1 effect on salt response occurs in an AGB1-independent manner. Our results indicate that RLK epistatic relationships are not fixed, as AGB1 and FER display different genetic relationships to RALF1 in stomatal vs. salinity responses. This article is protected by copyright. All rights reserved.

The circumstellar environment of evolved stars as traced by molecules and dust. The diagnostic power of Herschel

NASA Astrophysics Data System (ADS)

Lombaert, Robin

2013-12-01

Low-to-intermediate mass stars end their life on the asymptotic giant branch (AGB), an evolutionary phase in which the star sheds most of its mantle into the circumstellar environment through a stellar wind. This stellar wind expands at relatively low velocities and enriches the interstellar medium with elements newly made in the stellar interior. The physical processes controlling the gas and dust chemistry in the outflow, as well as the driving mechanism of the wind itself, are poorly understood and constitute the broader context of this thesis work. In a first chapter, we consider the thermodynamics of the high-density wind of the oxygen-rich star oh, using observations obtained with the PACS instrument onboard the Herschel Space Telescope. Being one of the most abundant molecules, water vapor can be dominant in the energy balance of the inner wind of these types of stars, but to date, its cooling contribution is poorly understood. We aim to improve the constraints on water properties by careful combination of both dust and gas radiative-transfer models. This unified treatment is needed due to the high sensitivity of water excitation to dust properties. A combination of three types of diagnostics reveals a positive radial gradient of the dust-to-gas ratio in oh. The second chapter deals with the dust chemistry of carbon-rich winds. The 30-mic dust emission feature is commonly identified as due to magnesium sulfide (MgS). However, the lack of short-wavelength measurements of the optical properties of this dust species prohibits the determination of the temperature profile of MgS, and hence its feature strength and shape, questioning whether this species is responsible for the 30-mic feature. By considering the very optically thick wind of the extreme carbon star LL Peg, this problem can be circumvented because in this case the short-wavelength optical properties are not important for the radial temperature distribution. We attribute the 30-mic feature to MgS, but

Imaging Variable Stars with HST

NASA Astrophysics Data System (ADS)

Karovska, Margarita

2011-05-01

The Hubble Space Telescope (HST) observations of astronomical sources, ranging from objects in our solar system to objects in the early Universe, have revolutionized our knowledge of the Universe its origins and contents.I will highlight results from HST observations of variable stars obtained during the past twenty or so years. Multiwavelength observations of numerous variable stars and stellar systems were obtained using the superb HST imaging capabilities and its unprecedented angular resolution, especially in the UV and optical. The HST provided the first detailed images probing the structure of variable stars including their atmospheres and circumstellar environments. AAVSO observations and light curves have been critical for scheduling of many of these observations and provided important information and context for understanding of the imaging results of many variable sources. I will describe the scientific results from the imaging observations of variable stars including AGBs, Miras, Cepheids, semi-regular variables (including supergiants and giants), YSOs and interacting stellar systems with a variable stellar components. These results have led to an unprecedented understanding of the spatial and temporal characteristics of these objects and their place in the stellar evolutionary chains, and in the larger context of the dynamic evolving Universe.

Imaging Variable Stars with HST

NASA Astrophysics Data System (ADS)

Karovska, M.

2012-06-01

(Abstract only) The Hubble Space Telescope (HST) observations of astronomical sources, ranging from objects in our solar system to objects in the early Universe, have revolutionized our knowledge of the Universe its origins and contents. I highlight results from HST observations of variable stars obtained during the past twenty or so years. Multiwavelength observations of numerous variable stars and stellar systems were obtained using the superb HST imaging capabilities and its unprecedented angular resolution, especially in the UV and optical. The HST provided the first detailed images probing the structure of variable stars including their atmospheres and circumstellar environments. AAVSO observations and light curves have been critical for scheduling of many of these observations and provided important information and context for understanding of the imaging results of many variable sources. I describe the scientific results from the imaging observations of variable stars including AGBs, Miras, Cepheids, semiregular variables (including supergiants and giants), YSOs and interacting stellar systems with a variable stellar components. These results have led to an unprecedented understanding of the spatial and temporal characteristics of these objects and their place in the stellar evolutionary chains, and in the larger context of the dynamic evolving Universe.

Inferring the post-merger gravitational wave emission from binary neutron star coalescences

NASA Astrophysics Data System (ADS)

Chatziioannou, Katerina; Clark, James Alexander; Bauswein, Andreas; Millhouse, Margaret; Littenberg, Tyson B.; Cornish, Neil

2017-12-01

We present a robust method to characterize the gravitational wave emission from the remnant of a neutron star coalescence. Our approach makes only minimal assumptions about the morphology of the signal and provides a full posterior probability distribution of the underlying waveform. We apply our method on simulated data from a network of advanced ground-based detectors and demonstrate the gravitational wave signal reconstruction. We study the reconstruction quality for different binary configurations and equations of state for the colliding neutron stars. We show how our method can be used to constrain the yet-uncertain equation of state of neutron star matter. The constraints on the equation of state we derive are complementary to measurements of the tidal deformation of the colliding neutron stars during the late inspiral phase. In the case of nondetection of a post-merger signal following a binary neutron star inspiral, we show that we can place upper limits on the energy emitted.

The great escape - III. Placing post-main-sequence evolution of planetary and binary systems in a Galactic context

NASA Astrophysics Data System (ADS)

Veras, Dimitri; Evans, N. Wyn; Wyatt, Mark C.; Tout, Christopher A.

2014-01-01

Our improving understanding of the life cycle of planetary systems prompts investigations of the role of the Galenvironment before, during and after asymptotic giant branch (AGB) stellar evolution. Here, we investigate the interplay between stellar mass-loss, Galactic tidal perturbations and stellar flybys for evolving stars which host one planet, smaller body or stellar binary companion and reside in the Milky Way's bulge or disc. We find that the potential evolutionary pathways from a main sequence (MS) to a white dwarf (WD) planetary system are a strong function of Galactocentric distance only with respect to the prevalence of stellar flybys. Planetary ejection and collision with the parent star should be more common towards the bulge. At a given location anywhere in the Galaxy, if the mass-loss is adiabatic, then the secondary is likely to avoid close flybys during AGB evolution, and cannot eventually escape the resulting WD because of Galactic tides alone. Partly because AGB mass-loss will shrink a planetary system's Hill ellipsoid axes by about 20 to 40 per cent, Oort clouds orbiting WDs are likely to be more depleted and dynamically excited than on the MS.

The Abundance of SiC2 in Carbon Star Envelopes⋆: Evidence that SiC2 is a gas-phase precursor of SiC dust

PubMed Central

Massalkhi, Sarah; Agúndez, M.; Cernicharo, J.; Velilla Prieto, L.; Goicoechea, J. R.; Quintana-Lacaci, G.; Fonfría, J. P.; Alcolea, J.; Bujarrabal, V.

2017-01-01

Context Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si–C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216. Aims We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. Methods We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes. Results We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. Conclusions The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an important

The Nature of Faint Spitzer-selected Dust-obscured Galaxies

NASA Astrophysics Data System (ADS)

Pope, Alexandra; Bussmann, R. Shane; Dey, Arjun; Meger, Nicole; Alexander, David M.; Brodwin, Mark; Chary, Ranga-Ram; Dickinson, Mark E.; Frayer, David T.; Greve, Thomas R.; Huynh, Minh; Lin, Lihwai; Morrison, Glenn; Scott, Douglas; Yan, Chi-Hung

2008-12-01

We use deep far-IR, submillimeter, radio, and X-ray imaging and mid-IR spectroscopy to explore the nature of a sample of Spitzer-selected dust-obscured galaxies (DOGs) in GOODS-N. A sample of 79 galaxies satisfy the criteria R - [ 24] > 14 (Vega) down to S24 > 100 μJy (median flux density S24 = 180 μJy). Twelve of these galaxies have IRS spectra available, which we use to measure redshifts and classify these objects as being dominated by star formation or active galactic nucleus (AGN) activity in the mid-IR. The IRS spectra and Spitzer photometric redshifts confirm that the DOGs lie in a tight redshift distribution around z ~ 2. Based on mid-IR colors, 80% of DOGs are likely dominated by star formation; the stacked X-ray emission from this subsample of DOGs is also consistent with star formation. Since only a small number of DOGs are individually detected at far-IR and submillimeter wavelengths, we use a stacking analysis to determine the average flux from these objects and plot a composite IR (8-1000 μm) spectral energy distribution (SED). The average luminosity of these star-forming DOGs is LIR ~ 1 × 1012 L⊙. We compare the average star-forming DOG to the average bright (S850 > 5 mJy) submillimeter galaxy (SMG); the S24 > 100 μJy DOGs are 3 times more numerous but 8 times less luminous in the IR. The far-IR SED shape of DOGs is similar to that of SMGs (average dust temperature of around 30 K), but DOGs have a higher mid-IR-to-far-IR flux ratio. The average star formation-dominated DOG has a star formation rate of 200 M⊙ yr -1, which, given their space density, amounts to a contribution of 0.01 M⊙ yr-1 Mpc-3 (or 5%-10%) to the star formation rate density at z ~ 2.

Weak metal lines in optical high-resolution Very Large Telescope and Keck spectra of "cool" PG 1159 stars

NASA Astrophysics Data System (ADS)

Werner, K.; Rauch, T.

2014-09-01

PG 1159 stars are very hot (effective temperatures Teff = 75 000-200 000 K), hydrogen-deficient (pre-) white dwarfs. They probably are the result of a late helium-shell flash that laid bare the He, C, and O rich intershell matter of the progenitor Asymptotic Giant Branch (AGB) star. Their chemical surface composition thus allows to conclude on details of AGB-star nucleosynthesis. Due to their very high effective temperatures, detailed spectral analyses are usually completely reliant on ultraviolet observations, except for some species in the hottest PG 1159 stars (Teff ≳ 130 000 K), which do exhibit highly excited lines from the CNO elements and neon (C iv, N v, O vi, Ne vii-viii) in optical spectra. Particularly problematic are, however, the coolest members of the PG 1159 class that exclusively show C iv lines in the optical. Access to the nitrogen abundance is important to decide which of the late-thermal pulse evolutionary scenarios was experienced by a particular star, while a high oxygen abundance is an important marker that the star could pulsate. In the present paper, we investigate high-resolution high signal-to-noise optical spectra of three "cool" PG 1159 stars (PG 0122+200, PG 2131+066, MCT 0130-1937, Teff = 80 000-95 000 K). With the help of non-LTE model atmospheres and synthetic spectra, we are able to identify a large number of weak CNO lines (C iii, N iv, O iii-v) that were not detected before in these stars. They allow abundance determinations and enable us to constrain the effective temperature to high precision through ionization equilibria without the requirement to access the ultraviolet spectral range.

Star formation history from the cosmic infrared background anisotropies

NASA Astrophysics Data System (ADS)

Maniyar, A. S.; Béthermin, M.; Lagache, G.

2018-06-01

We present a linear clustering model of cosmic infrared background (CIB) anisotropies at large scales that is used to measure the cosmic star formation rate density up to redshift 6, the effective bias of the CIB, and the mass of dark matter halos hosting dusty star-forming galaxies. This is achieved using the Planck CIB auto- and cross-power spectra (between different frequencies) and CIB × CMB (cosmic microwave background) lensing cross-spectra measurements, as well as external constraints (e.g. on the CIB mean brightness). We recovered an obscured star formation history which agrees well with the values derived from infrared deep surveys and we confirm that the obscured star formation dominates the unobscured formation up to at least z = 4. The obscured and unobscured star formation rate densities are compatible at 1σ at z = 5. We also determined the evolution of the effective bias of the galaxies emitting the CIB and found a rapid increase from 0.8 at z = 0 to 8 at z = 4. At 2 < z < 4, this effective bias is similar to that of galaxies at the knee of the mass functions and submillimetre galaxies. This effective bias is the weighted average of the true bias with the corresponding emissivity of the galaxies. The halo mass corresponding to this bias is thus not exactly the mass contributing the most to the star formation density. Correcting for this, we obtained a value of log(Mh/M⊙) = 12.77-0.125+0.128 for the mass of the typical dark matter halo contributing to the CIB at z = 2. Finally, using a Fisher matrix analysis we also computed how the uncertainties on the cosmological parameters affect the recovered CIB model parameters, and find that the effect is negligible.

The Environments of Obscured Quasars

NASA Astrophysics Data System (ADS)

Jones, Kristen M.; Lacy, Mark; Nielsen, Danielle

2016-01-01

Supermassive Black Hole (SMBH) feedback is prescribed for driving the high-end shape of the galaxy luminosity function, clearing the circumnuclear environment during the end stages of mergers, and eventually turning off its own accretion. Yet the dominant processes and characteristics of active galactic nuclei are indistinct. Chief among this confusion is how significant the role of dust is in each galaxy. Orientation of the dusty torus is attributed to causing the differences between Sy1 and Sy2, but whether obscured quasars are found in particularly dusty host galaxies, if they exist at a different stage in the merger process (early on, before the dust is blown out), or if they are merely oriented differently than optical quasars is not yet so well distinguished. With obscured quasars now observed to make up 50% or greater of the population of quasars, the question of what causes obscuration becomes vital to address. With this in mind, I study matched samples of obscured and unobscured quasars to characterize their environments, with the intent of addressing what contribution environment has to obscuration levels. I investigate the megaparsec-scale environments of SIRTF Wide-field Infra-Red Extragalactic Survey (SWIRE) quasars at z ˜ 1-3 by cross-correlating the sample with 3.8 million galaxies from the Spitzer Extragalactic Representative Volume Survey (SERVS). Optically obscured quasars are compared to a control sample of optically-bright quasars via selection in the mid-infrared. Environments were observed at 3.6 and 4.5 μm to a depth of ≈ 2 μJy (AB = 23.1). Recent work has found diverse results in such studies, with dependence of environmental richness on both redshift and level of obscuration. I find that, within reasonable error, on average there is no distinct difference between the level of clustering for obscured and normal quasars, and that there is no dependence on redshift of this result within the range of 1.3 < z < 2.5. I compare our results

CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kraemer, K. E.; Sloan, G. C.; Wood, P. R.

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to observe stars in the Small Magellanic Cloud (SMC) selected from the Point Source Catalog of the Midcourse Space Experiment (MSX). We concentrate on the dust properties of the oxygen-rich evolved stars. The dust composition has smaller contributions from alumina compared to the Galaxy. This difference may arise from the lower metallicity in the SMC, but it could be a selection effect, as the SMC sample includes more stars that are brighter and thus more massive. The distribution of the SMC stars along the silicate sequence looks moremore » like the Galactic sample of red supergiants than asymptotic giant branch stars (AGBs). While many of the SMC stars are definitively on the AGB, several also show evidence of hot bottom burning. Three of the supergiants show PAH emission at 11.3 μ m. Two other sources show mixed chemistry, with both carbon-rich and oxygen-rich spectral features. One, MSX SMC 134, may be the first confirmed silicate/carbon star in the SMC. The other, MSX SMC 049, is a candidate post-AGB star. MSX SMC 145, previously considered a candidate OH/IR star, is actually an AGB star with a background galaxy at z  = 0.16 along the same line of sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of the higher sensitivity and resolution of Spitzer , and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the upcoming James Webb Space Telescope ( JWST ). Color–color diagrams generated from the IRS spectra and the mid-infrared filters on JWST show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different classes of YSOs.« less

The evolution of high-metallicity horizontal-branch stars and the origin of the ultraviolet light in elliptical galaxies

NASA Technical Reports Server (NTRS)

Horch, E.; Demarque, P.; Pinsonneault, M.

1992-01-01

Evolutionary calculations of high-metallicity horizontal-branch stars show that for the relevant masses and helium abundances, post-HB evolution in the HR diagram does not proceed toward and along the AGB, but rather toward a 'slow blue phase' in the vicinity of the helium-burning main sequence, following the extinction of the hydrogen shell energy source. For solar and twice solar metallicity, the blue phase begins during the helium shell-burning phase (in agreement with the work of Brocato and Castellani and Tornambe); for 3 times solar metallicity, it begins earlier, during the helium core-burning phase. This behavior differs from what takes place at lower metallicities. The implications for high-metallicity old stellar populations in the Galactic bulge and for the integrated colors of elliptical galaxies are discussed.

The AGN-Star Formation Connection: Future Prospects with JWST

NASA Astrophysics Data System (ADS)

Kirkpatrick, Allison; Alberts, Stacey; Pope, Alexandra; Barro, Guillermo; Bonato, Matteo; Kocevski, Dale D.; Pérez-González, Pablo; Rieke, George H.; Rodríguez-Muñoz, Lucia; Sajina, Anna; Grogin, Norman A.; Mantha, Kameswara Bharadwaj; Pandya, Viraj; Pforr, Janine; Salvato, Mara; Santini, Paola

2017-11-01

The bulk of the stellar growth over cosmic time is dominated by IR-luminous galaxies at cosmic noon (z=1{--}2), many of which harbor a hidden active galactic nucleus (AGN). We use state-of-the-art infrared color diagnostics, combining Spitzer and Herschel observations, to separate dust-obscured AGNs from dusty star-forming galaxies (SFGs) in the CANDELS and COSMOS surveys. We calculate 24 μm counts of SFGs, AGN/star-forming “Composites,” and AGNs. AGNs and Composites dominate the counts above 0.8 mJy at 24 μm, and Composites form at least 25% of an IR sample even to faint detection limits. We develop methods to use the Mid-Infrared Instrument (MIRI) on JWST to identify dust-obscured AGNs and Composite galaxies from z˜ 1{--}2. With the sensitivity and spacing of MIRI filters, we will detect >4 times as many AGN hosts as with Spitzer/IRAC criteria. Any star formation rates based on the 7.7 μm PAH feature (likely to be applied to MIRI photometry) must be corrected for the contribution of the AGN, or the star formation rate will be overestimated by ˜35% for cases where the AGN provides half the IR luminosity and ˜50% when the AGN accounts for 90% of the luminosity. Finally, we demonstrate that our MIRI color technique can select AGNs with an Eddington ratio of {λ }{Edd}˜ 0.01 and will identify AGN hosts with a higher specific star formation rate than X-ray techniques alone. JWST/MIRI will enable critical steps forward in identifying and understanding dust-obscured AGNs and the link to their host galaxies.