Mass Loss History of the AGB star, R Cas

This research is based on observations with AKARI, a JAXA project with the participation of ESA, and Spitzer, which is operated by the JPL/Caltech under a contract with NASA.We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3′ and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled “Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)”

The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging

The circumstellar dust shells of intermediate initial-mass (about 1 to 8 solar masses) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. We used the AKARI Infrared Astronomy Satellite and the Spitzer Space Telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star's known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. Our data show that (1) the bow shock cone of 1 - 5 x 10^-5 solar masses (dust mass) is inclined at 68 degrees with respect to the plane of the sky, and (2) the dust temperature in the bow shock cone is raised to more than 20 K by collisional shock interaction in addition to the ambient interstellar radiation field. By comparison between the apex vector of the bow shock and space motion vector of the star we infer that there is a flow of interstellar medium local to R Cas whose flow velocity is at least 55.6 km/s, consistent with an environment conducive to dust heating by shock interactions.We are grateful for financial support from the Institute of Space and Astronautical Science under the auspices of the Japan Aerospace Exploration Agency as well as the Jet Propulsion Laboratory/California Institute of Technology

Massive-Star Supernovae as Major Dust Factories

DOI: 10.1126/science.1128131We present late-time optical and mid-infrared observations of the Type-II supernova 2003gd in NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499-678 days after outburst, and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae can be major dust producers throughout the history of the Universe

The mass-loss return from evolved stars to the Large Magellanic Cloud. III. Dust properties for carbon-rich asymptotic giant branch stars

DOI: 10.1051/0004-6361/201014991We present a radiative transfer model for the circumstellar dust shell around a Large Magellanic Cloud (LMC) long-period variable (LPV) previously studied as part of the Optical Gravitational Lensing Experiment (OGLE) survey of the LMC. OGLE LMC LPV 28579 (SAGE J051306.40-690946.3) is a carbon-rich asymptotic giant branch (AGB) star for which we have Spitzer broadband photometry and spectra from the SAGE and SAGE-Spec programs along with broadband UBVIJHKs photometry. By modeling this source, we obtain a baseline set of dust properties to be used in the construction of a grid of models for carbon stars. We reproduce the spectral energy distribution of the source using a mixture of amorphous carbon and silicon carbide with 15% SiC by mass. The grain sizes are distributed according to the KMH model, with γ = 3.5, amin = 0.01 μm and a0 = 1.0 μm. The best-fit model produces an optical depth of 0.28 for the dust shell at the peak of the SiC feature (11.3 μm), with an inner radius of about 1430 or 4.4 times the stellar radius. The temperature at this inner radius is 1310 K. Assuming an expansion velocity of 10 km s-1, we obtain a dust mass-loss rate of 2.5 × 10-9  yr-1. We calculate a 15% variation in this mass-loss rate by testing the sensitivity of the fit to variation in the input parameters. We also present a simple model for the molecular gas in the extended atmosphere that could give rise to the 13.7 μm feature seen in the spectrum. We find that a combination of CO and C2H2 gas at an excitation temperature of about 1000 K and column densities of 3 × 1021 cm-2 and 1019 cm-2 respectively are able to reproduce the observations. Given that the excitation temperature is close to the temperature of the dust at the inner radius, most of the molecular contribution probably arises from this region. The luminosity corresponding to the first epoch of SAGE observations is 6580 . For an effective temperature of about 3000 K, this implies a stellar mass of 1.5-2  and an age of 1-2.5 Gyr for OGLE LMC LPV 28579. We calculate a gas mass-loss rate of 5.0 × 10-7  yr-1 assuming a gas:dust ratio of 200. This number is comparable to the gas mass-loss rates estimated from the period, color and 8 μm flux of the source.This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. The research in this paper has been funded by Spitzer grant 1310534 and NASA grant NAG5-12595. The authors have made use of the SIMBAD astronomical database and would like to thank those responsible for its upkeep. The authors also thank Bernie Shiao at STScI for his hard work on the SAGE database and his kind assistance

Detection of a Far-Infrared Bow-Shock Nebula Around R Hya: the First MIRIAD Results

We present the first results of the MIRIAD (MIPS InfraRed Imaging of AGB Dust shells) project using the Spitzer Space Telescope. The primary aim of the project is to probe the material distribution in the extended circumstellar envelopes (CSEs) of evolved stars and recover the fossil record of their mass-loss history. Hence, we must map the whole of the CSEs plus the surrounding sky for background subtraction while avoiding the central star that is brighter than the detector saturation limit. With our unique mapping strategy, we have achieved better than 1 MJy sr 1 sensitivity in 3 hr of integration and successfully detected a faint (!5 MJy sr 1), extended (∼400 ) far-infrared nebula around the asymptotic giant branch (AGB) star R Hya. Based on the parabolic structure of the nebula, the direction of the space motion of the star with respect to the nebula shape, and the presence of extended Ha emission cospatial to the nebula, we suggest that the detected far-IR nebula is due to a bow shock at the interface of the interstellar medium and the AGB wind of this moving star. This is the first detection of the stellar-wind bow shock interaction for an AGB star and exemplifies the potential of Spitzer as a tool to examine the detailed structure of extended far-IR nebulae around bright central sources.This work is based on observations made with the Spitzer Space Telescope, which is operated by the JPL/Caltech under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. We also acknowledge additional support for the following individuals: an NPP Research Fellowship Award to T. Ueta, NASA ADP grant NAG 5-12675 to A. K. Speck, the LDRD program (20060357ER) at LANL for F. Herwig, NASA contract 1215746 issued by JPL/Caltech to R. D. Gehrz, grant 2.P03D.017.25 to R. Szczerba, a Grant-in-Aid (C) from JSPS (17540221) to H. Izumiura, JSPS for M. Matsuura, and NSF grant AST 05-07421 to M. Elitzur. We thank C. J. Wareing for sharing his insights on the stellar-wind bow shocks with us

The Mass-loss Return from Evolved Stars to the Large Magellanic Cloud. II. Dust Properties for Oxygen-rich Asymptotic Giant Branch Stars

doi: 10.1088/0004-637X/716/1/878We model multi-wavelength broadband UBVIJHKs and Spitzer IRAC and MIPS photometry and Infrared Spectrograph spectra from the SAGE and SAGE-Spectroscopy observing programs of two oxygen-rich asymptotic giant branch (O-rich AGB) stars in the LargeMagellanic Cloud (LMC) using radiative transfer (RT) models of dust shells around stars.We chose a star from each of the bright and faintO-richAGB populations found by earlier studies of the SAGE sample in order to derive a baseline set of dust properties to be used in the construction of an extensive grid of RT models of the O-rich AGB stars found in the SAGE surveys. From the bright O-rich AGB population, we chose HV 5715, and from the faint O-rich AGB population we chose SSTISAGE1C J052206.92−715017.6(SSTSAGE052206). We found the complex indices of refraction of oxygen-deficient silicates from Ossenkopf et al. and a power law with exponential decay grain size distribution like what Kim et al. used but with γ of −3.5, amin of 0.01μm, and a0 of 0.1μm to be reasonable dust properties for these models. There is a slight indication that the dust around the faint O-rich AGB may be more silica-rich than that around the bright O-rich AGB. Simple models of gas emission suggest a relatively extended gas envelope for the faint O-rich AGB star modeled, consistent with the relatively large dust shell inner radius for the same model. Our models of the data require the luminosity of SSTSAGE052206 and HV 5715 to be ∼5100L and ∼36,000L , respectively. This, combined with the stellar effective temperatures of 3700 K and 3500 K, respectively, that we find best fit the optical and near-infrared data, suggests stellar masses of ∼3M and ∼7M . This, in turn, suggests that HV 5715 is undergoing hot-bottom burning and that SSTSAGE052206 is not. Our models of SSTSAGE052206 and HV 5715 require dust shells of inner radius ∼17 and ∼52 times the stellar radius, respectively, with dust temperatures there of 900 K and 430 K, respectively, and with optical depths at 10μm through the shells of 0.095 and 0.012, respectively. The models compute the dust mass-loss rates for the two stars to be 2.0 × 10−9M yr−1 and 2.3 × 10−9M yr−1, respectively. When a dust-to-gas mass ratio of 0.002 is assumed for SSTSAGE052206 and HV 5715, the dust mass-loss rates imply total mass-loss rates of 1.0 × 10−6M yr−1 and 1.2 × 10−6M yr−1, respectively. These properties of the dust shells and stars, as inferred from our models of the two stars, are found to be consistent with properties observed or assumed by detailed studies of other O-rich AGB stars in the LMC and elsewhere.This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. We acknowledge funding from the NAG5-12595 grant, SAGE-LMC Spitzer grant 1275598, SAGE-SEEDS Spitzer grant 1310534, and Herschel/HERITAGE grant 1381522. This publication makes use of the Jena-St. Petersburg Database of Optical Constants (Henning et al. 1999). The authors wish to thank the anonymous referee for comments that greatly improved this manuscript. The authors also thank Kevin Volk, Sacha Hony, Albert Zijlstra, Jacco van Loon, and Martha Boyer for helpful comments and discussion. We wish to thank Peter Hauschildt for his assistance with the PHOENIX stellar photosphere models. The authors have made use of the SIMBAD astronomical database and thank those responsible for its upkeep. The authors also thank Bernie Shiao at STScI for his hard work on the SAGE database and his kind assistance

3-D Dynamics of Interactions between Stellar Winds and the Interstellar Medium as Seen by AKARI and Spitzer

http://arxiv.org/abs/0905.0756 Invited Talk, to be published in the proceedings of the conference "AKARI, a light to illuminate the misty Universe" held at University of Tokyo, Japan, 16-19 February 2009.Recent far-infrared mapping of mass-losing stars by the AKARI Infrared Astronomy Satellite and Spitzer Space Telescope have suggested that far-infrared bow shock structures are probably ubiquitous around these masslosing stars, especially when these stars have high proper motion. Higher spatial resolution data of such far-infrared bow shocks now allow detailed fitting to yield the orientation of the bow shock cone with respect to the heliocentric space motion vector of the central star, using the analytical solution for these bow shocks under the assumption of momentum conservation across a physically thin interface between the stellar winds and interstellar medium (ISM). This fitting analysis of the observed bow shock structure would enable determination of the ambient ISM flow vector, founding a new technique to probe the 3-D ISM dynamics that are local to these interacting systems. In this review, we will demonstrate this new technique for three particular cases, Betelgeuse, R Hydrae, and R Cassiopeiae.Support for this work was provided by University of Denver, ISAS/JAXA, and NASA via JPL/Caltech

The mineralogy of dust around evolved stars

Infrared (IR) observations of evolved red giant stars (AGB stars) have shown that many are surrounded by dust envelopes, which are ejected into the interstellar medium and seed the next generation of stars and planets. By studying these one can understand the origins of interstellar and solar system materials. AGB stars fall into two main categories: oxygen-rich and carbon-rich. The prominent features of the IR spectra of AGB stars are: the 11.3μm feature of C-stars, attributed to silicon carbide (SiC); and the 9.7μm feature of O-rich stars, attributed to silicates. There are also various minor features with less secure identifications. Identifying dust around stars requires the use of laboratory spectra of dust species analogous to those one expects to observe. I have compiled a database of such spectra, and thereby constrained the identifications of circumstellar dust, which I have also tried to ensure are compatible with data from meteoritic presolar grains. Some laboratory spectra need to be modified before they are relevant to the problem in hand, i.e. stardust. The techniques used for such modifications are outlined in the thesis. In order to fully comprehend the problems that can arise from using laboratory spectra, the way in which light interacts with matter must be understood. To this end the optical properties of matter are discussed. While the mineral constituents of the Earth have been reprocessed so extensively that they no longer contain any evidence of their stellar origins, the same is not true of primitive meteorites which contain "presolar" dust grains with isotopic fingerprints identifying their stellar sources. By comparing these presolar grains with nucleosynthesis models, grains expected to form around various stars and observational evidence of dust, we can gain a better picture of the formation mechanisms and sites of the various dust grains. I have investigated the mineralogy of SiC of 32 C-stars and its relationship to meteoritic dust by using a x2-minimisation routine to fit the observed SiC features, using laboratory optical constants that have been published for a variety of SiC samples. In addition to the extreme carbon star AFGL 3068, the only C-star previously known to show the 11.3μm SiC feature in absorption, I have discovered three further examples of sources that show SiC in net absorption. Previous attempts to identify the type of SiC present around carbon stars have all identified it with a-SiC. However, 1 have found that the previous work is based on flawed laboratory data and that a better fit is achieved using β-SiC, which agrees with meteoritic data. I have also used the same techniques to investigate the mineralogy of 80 O-rich stars. Dust mineralogy around O-rich stars is considerably more complicated. The 9.7μm feature attributed to silicates varies greatly in shape and intensity as well as exact peak position from star to star, and the number of possible laboratory analogues is much greater than for SiC. Most of these spectra have been fitted using some form of forsterite (Mg2SiO4) and/or enstatite (MgSiO3), although constraining the mineralogy further was not possible. There is little evidence of Al2O3 around these stars, contrary to theoretical predictions and previous radiative transfer models. Relating the O-rich dust species to meteoritic data is also more complicated, as most of the silicate material was reprocessed in the early solar system, although data on a small number of O-rich presolar grains have been used for this purpose. I have also discovered a previously unrecognized feature in the spectra of O-rich stars at 9.25μm. This feature, and the 12.5-13.0μm feature previously attributed to Al2O3, have been attributed to SiO2- Implications of the new attributions for both C- and O-rich stars are discussed

The SAGE-Spec Spitzer Legacy Program: The Life Cycle of Dust and Gas in the Large Magellanic Cloud

http://arxiv.org/abs/1004.1142The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 um" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z~0.14, which appears to be host-less.M. Cohen thanks NASA for supporting his participation in SAGE-Spec through JPL grant 1320707 with UC Berkeley. B. Sargent, M. Meixner, and B. Shiao were supported for SAGE-Spec through JPL/SSC grant 1310534 with STScI. M. Meixner was additionally supported by NASA NAG5-12595. R. Szczerba acknowledges support from grant N203 393334 (MNiSW)

The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud: Point source classification I.

http://arxiv.org/abs/1009.5929We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy program on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified.R. Sz. acknowledges support from grant N203 511838 (MNiSW). This paper utilizes public domain data obtained by the MACHO Project, jointly funded by the US Department of Energy through the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, by the National Science Foundation through the Center for Particle Astrophysics of the University of California under cooperative agreement AST8809616, and by the Mount Stromlo and Siding Spring Observatory, part of the Australian National University. 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/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Optical Gravitational Lensing Experiment OGLE-III online catalog of variable stars. This research has made use of the VizieR catalog access tool, CDS, Strasbourg, France. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of NASA's Astrophysics Data System Bibliographic Services