R CrB Candidates in the Small Magellanic Cloud: Observations of Cold, Featureless Dust with the Spitzer Infrared Spectrograph

摘要

We observed 36 evolved stars in the Small Magellanic Cloud (SMC) using thelow-resolution mode of the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope. Two of these stars, MSX SMC 014 and 155, have nearly featurelessspectral energy distributions over the IRS wavelength range (5.2-35 um) andF_nu peaking at ~8-9 um. The data can be fit by sets of amorphous carbon shellsor by single 600-700 K blackbodies. The most similar spectra found in extantspectral databases are of R CrB, although the spectral structure seen in R CrBand similar stars is much weaker or absent in the SMC sources. Both SMC starsshow variability in the near-infrared. Ground-based visual spectra confirm thatMSX SMC 155 is carbon-rich, as expected for R CrB (RCB) stars, and coincideswith an object previously identified as an RCB candidate. The temperature ofthe underlying star is lower for MSX SMC 155 than for typical RCB stars. Thestrength of the C_2 Swan bands and the low temperature suggest that it may be arare DY Per-type star, only the fifth such identified. MSX SMC 014 represents anew RCB candidate in the SMC, bringing the number of RCB candidates in the SMCto six. It is the first RCB candidate discovered with Spitzer and the firstidentified by its infrared spectral characteristics rather than its visualvariability.