Yellow Supergiants in the Small Magellanic Cloud (SMC): Putting Current Evolutionary Theory to the Test

Kathryn F. Neugent, Philip Massey, Brian Skiff, Maria R. Drout, Georges Meynet, Knut A. G. Olsen

1,2,3 = Lowell Observatory; 4 = Department of Physics & Astronomy, University of Iowa; 5 = Geneva University, Geneva Observatory; 6 = National Optical Astronomy Observatory

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 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 12M⊙. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besancon model of the Milky Way suggest a foreground contamination of ≤4%. After placing the SMC supergiants on the H-R diagram 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 H-R diagram match the locations of evolutionary tracks well, the models over-predict the yellow supergiant lifetime by a factor of ten. Uncertainties about the mass-loss rates on the main-sequence thus cannot be the primary problem with the models.

Reference: ApJ
Status: Manuscript has been accepted