The infrared spectral properties of Magellanic carbon stars

TL;DR

This study analyzes infrared spectra of 184 Magellanic carbon stars, revealing how dust production, composition, and stellar properties vary with pulsation period, metallicity, and evolutionary stage, providing insights into dust formation and stellar evolution.

Contribution

It presents the first comprehensive analysis of dust production and composition in Magellanic carbon stars using Spitzer spectra, highlighting the effects of metallicity and stellar mass.

Findings

Dust production rate increases with pulsation period.

Condensation sequences depend on metallicity, with SiC and amorphous carbon order reversing.

Deeper embedded stars show signs of evolution off the AGB or non-spherical dust geometries.

Abstract

The Infrared Spectrograph on the Spitzer Space Telescope observed 184 carbon stars in the Magellanic Clouds. This sample reveals that the dust-production rate (DPR) from carbon stars generally increases with the pulsation period of the star. The composition of the dust grains follows two condensation sequences, with more SiC condensing before amorphous carbon in metal-rich stars, and the order reversed in metal-poor stars. MgS dust condenses in optically thicker dust shells, and its condensation is delayed in more metal-poor stars. Metal-poor carbon stars also tend to have stronger absorption from C2H2 at 7.5 um. The relation between DPR and pulsation period shows significant apparent scatter, which results from the initial mass of the star, with more massive stars occupying a sequence parallel to lower-mass stars, but shifted to longer periods. Accounting for differences in the mass distribution between the carbon stars observed in the Small and Large Magellanic Clouds reveals a hint of a subtle decrease in the DPR at lower metallicities, but it is not statistically significant. The most deeply embedded carbon stars have lower variability amplitudes and show SiC in absorption. In some cases they have bluer colors at shorter wavelengths, suggesting that the central star is becoming visible. These deeply embedded stars may be evolving off of the AGB and/or they may have non-spherical dust geometries.