Effects of Metallicity on the Chemical Composition of Carbon Stars

Abstract

We present \emph{Spitzer} IRS data on 19 asymptotic giant branch (AGB) stars in the Large Magellanic Cloud, complementing existing published data sets of carbon stars in both Magellanic Clouds and the Milky Way, to investigate the effects of metallicity on dust and molecular spectral features arising from the circumstellar envelope. We find that the C${}_2$H${}_2$ P and R branches at 7.5 micron are affected by dust dilution at higher mass-loss rates -- albeit to a lesser extent for sources in the Magellanic Clouds, compared to the Milky Way -- while the narrow 13.7 micron C${}_2$H${}_2$ Q branch only shows the effect of dust dilution at low mass-loss rates. A strong metallicity dependence is not observed for the Q branch. Independent of metallicity, we also provide an explanation for the observed shifts in the central wavelength of the SiC emission feature, as we show that these are largely caused by molecular band absorption on either side of the dust emission feature, dominating over shifts in the central wavelength caused by self-absorption. We have devised a method to study the dust condensation history in carbon-rich AGB stars in different metallicity environments, by measuring the strength of the 11.3 \um SiC and 30 \um MgS features with respect to the continuum, as a function of mass-loss rate. With this method, it is possible to distinguish in what order SiC and graphite condense, which is believed to be sensitive to the metallicity, prior to the eventual deposit of the MgS layer.

18 pages, 13 figures, Accepted to ApJ

References in corpus (4)

• A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud
• 2D Models for Dust-driven AGB Star Winds
• Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud
• Spitzer observations of acetylene bands in carbon-rich AGB stars in the Large Magellanic Cloud

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• Luminosities and mass-loss rates of Local Group AGB stars and Red Supergiants
• Discovery of Extreme Carbon Stars in the Large Magellanic Cloud
• The mass-loss, expansion velocities and dust production rates of carbon stars in the Magellanic Clouds
• Wind morphology around cool evolved stars in binaries: the case of slowly accelerating oxygen-rich outflows
• Observations of the Planetary Nebula SMP LMC 058 with the JWST MIRI Medium Resolution Spectrometer
• AGB winds in interacting binary stars
• Chemical Abundances and Dust in the Halo Planetary Nebula K648 in M15: Its Origin and Evolution based on an Analysis of Multiwavelength Data
• Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy
• Characterizing AGB stars in Wide-field Infrared Survey Explorer (WISE) bands
• The Isaac Newton Telescope monitoring survey of Local Group dwarf galaxies. II. The star formation history of Andromeda I derived from long period variables
• Stellar Pulsation and the Production of Dust and Molecules in Galactic Carbon Stars
• The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. VI. The Star Formation History and Dust Production in Andromeda IX
• Effects of Metallicity on Graphite, TiC, and SiC Condensation in Carbon Stars