The Anomalous A-Type Supergiants in the Magellanic Clouds—Evidence for Post-Red Supergiant Evolution

Abstract

A group of A-type supergiants in the Large and Small Magellanic Clouds have hydrogen lines and a Balmer jump that are too strong for their high luminosities. Their U-B colors are also too red by about 0.2-0.3 mag for their spectral types and luminosities, consistent with the strong Balmer jump. Using NLTE model atmospheres we show that the strength of the hydrogen lines and the Balmer jump can be best explained by enhanced helium (N He /N H ⇒0.3 to 0.5) in the atmospheres of these anomalous A-type supergiants