Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

Abstract

We present Spitzer Space Telescope spectroscopic observations of 14 carbon-rich asymptotic giant branch (AGB) stars in the Small Magellanic Cloud (SMC). SiC dust is seen in most of the carbon-rich stars but it is weak compared to Large Magellanic Cloud (LMC) stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas in the Galaxy SiC and graphite condensate at more comparable temperatures. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-μm band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C2H2 bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour–colour diagram based on Spitzer InfraRed Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for ∼6 × 105 yr. For the LMC, this lifetime is ∼3 × 105 yr. The superwind phase traced with Spitzer lasts for ∼104 yr. Spitzer spectra of a K supergiant and a compact H ii region are also given.