Yellow Supergiants in the Andromeda Galaxy (M31)

TL;DR

This study identifies and analyzes yellow supergiants in M31, comparing observations with stellar evolution models, revealing discrepancies in predicted versus observed numbers and durations of the yellow supergiant phase.

Contribution

First spectroscopic identification of yellow supergiants in M31 using velocity data to confirm membership, and comparison with evolutionary models to test their accuracy.

Findings

54 certain and 66 probable yellow supergiants identified.

Observed yellow supergiant numbers are much lower than model predictions for high masses.

Yellow supergiant phase duration is about 3000 years, shorter than some models predict.

Abstract

The yellow supergiant content of nearby galaxies can provide a critical test of stellar evolution theory, bridging the gap between the hot, massive stars and the cool red supergiants. But, this region of the color-magnitude diagram is dominated by foreground contamination, requiring membership to somehow be determined. Fortunately, the large negative systemic velocity of M31, coupled to its high rotation rate, provides the means for separating the contaminating foreground dwarfs from the bona fide yellow supergiants within M31. Using the MMT, we obtained spectra of about 2900 stars, selected using the color and magnitude range to be yellow supergiants. Comparing the velocities to that of M31's rotation curve, we identified 54 certain, and 66 probable yellow supergiants from among the sea of foreground dwarfs. We find excellent agreement between the location of yellow supergiants in the H-R diagram and that predicted by the latest Geneva evolutionary tracks which include rotation. However, the relative number of yellow supergiants seen as a function of mass varies from that predicted by the models by a factor of more than 10, in the sense that more high mass yellow supergiants are predicted than are actually observed. Comparing the total number of yellow supergiants with masses above 20Mo with the estimated number of unevolved O stars indicates that the duration of the yellow supergiant phase is about 3000 years. This is consistent with what the 12Mo and 15Mo evolutionary tracks predict, but disagrees with the 20,000-80,000 year time scales predicted by the models for higher masses.