Mid‐Infrared Spectroscopy of Carbon Stars in the Small Magellanic Cloud

We have observed a sample of 36 objects in the Small Magellanic Cloud (SMC)with the Infrared Spectrometer on the Spitzer Space Telescope. Nineteen ofthese sources are carbon stars. An examination of the near- and mid-infraredphotometry shows that the carbon-rich and oxygen-rich dust sources follow twoeasily separated sequences. A comparison of the spectra of the 19 carbon starsin the SMC to spectra from the Infrared Space Observatory (ISO) of carbon starsin the Galaxy reveals significant differences. The absorption bands at 7.5 umand 13.7 um due to C2H2 are stronger in the SMC sample, and the SiC dustemission feature at 11.3 um is weaker. Our measurements of the MgS dustemission feature at 26-30 um are less conclusive, but this feature appears tobe weaker in the SMC sample as well. All of these results are consistent withthe lower metallicity in the SMC. The lower abundance of SiC grains in the SMCmay result in less efficient carbon-rich dust production, which could explainthe excess C2H2 gas seen in the spectra. The sources in the SMC with thestrongest SiC dust emission tend to have redder infrared colors than the othersources in the sample, which implies more amorphous carbon, and they also tendto show stronger MgS dust emission. The weakest SiC emission features tend tobe shifted to the blue; these spectra may arise from low-density shells withlarge SiC grains.Comment: Accepted by ApJ 21 March, 2006, 13 pages (emulateapJ), 12 figure