Yellow supergiants in the Small Magellanic Cloud: Putting current evolutionary theory to the test

Abstract

The yellow supergiant content of nearby galaxies provides a critical test of massive star evolutionary theory. While these stars are the brightest in a galaxy, they are difficult to identify because a large number of foreground Milky Way stars have similar colors and magnitudes. We previously conducted a census of yellow supergiants within M31 and found that the evolutionary tracks predict a yellow supergiant duration an order of magnitude longer than we observed. Here we turn our attention to the Small Magellanic Cloud (SMC), where the metallicity is 10 × lower than that of M31, which is important as metallicity strongly affects massive star evolution. The SMC's large radial velocity (∼160 km s-1) allows us to separate members from foreground stars. Observations of ∼500 candidates yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along with 306 foreground stars, and provide good relative numbers of yellow supergiants down to 12 M ⊙. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besançon model of the Milky Way suggest a foreground contamination of ≤4%. After placing the SMC supergiants on the Hertzsprung-Russell diagram (HRD) and comparing our results to the Geneva evolutionary tracks, we find results similar to those of the M31 study: while the locations of the stars on the HRD match the locations of evolutionary tracks well, the models overpredict the yellow supergiant lifetime by a factor of 10. Uncertainties about the mass-loss rates on the main sequence thus cannot be the primary problem with the models. © 2010. The American Astronomical Society.