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 μm. 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.
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 μm. 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.
| Original language | English |
|---|---|
| Article number | 44 |
| Journal | Astrophysical Journal |
| Volume | 826 |
| Issue number | 1 |
| Early online date | 18 Jul 2016 |
| DOIs | |
| Publication status | Published - 20 Jul 2016 |