HV 11423: The Coolest Supergiant in the SMC

We call attention to the fact that one of the brightest red supergiants inthe SMC has recently changed its spectral type from K0-1 I (December 2004) toM4 I (December 2005) and back to K0-1 I (September 2006). An archival spectrumfrom the Very Large Telescope reveals that the star was even cooler (M4.5-M5 I)in December 2001. By contrast, the star was observed to be an M0 I in bothOctober 1978 and October 1979. The M4-5 I spectral types is by far the latesttype seen for an SMC supergiant, and its temperature in that state places itwell beyond the Hayashi limit into a region of the H-R diagram where the starshould not be in hydrostatic equilibrium. The star is variable by nearly 2 magin V, but essentially constant in K. Our modeling of its spectral energydistribution shows that the visual extinction has varied during this time, butthat the star has remained essentially constant in bolometric luminosity. Wesuggest that the star is currently undergoing a period of intense instability,with its effective temperature changing from 4300 K to 3300 K on the time-scaleof months. It has one of the highest 12-micron fluxes of any RSG in the SMC,and we suggest that the variability at V is due primarily to changes ineffective temperature, and secondly, due to changes in the local extinction dueto creation and dissipation of circumstellar dust. We speculate that the starmay be nearing the end of its life.Comment: Accepted by the Astrophysical Journa