Detached cold dust shells around yellow variable supergiants TW Aql, AI CMi, V 925 SCO and related stars

Abstract

TW Azl, AI CMi, V 924 Sco, ST Pup, AX Sgr, AD Aql, V1027 Cyg are yellow variable supergiants. We find these stars to be far-infrared (IRAS) sources with flux maximum around 25 micrometers. Analysis of the IRAS fluxes shows the presence of detached cold circumstellar dust shells. Some of these stars are yellow semiregular variables and also show under abundance of metals. TW Aql, AI Cmi, V 925 Sco have far-infrared IRAS colors similar to those of planetary nebulae. V 925 Sco is near the cepheid instability strip. The detached cold dust shells around these stars suggest that they experienced severe mass loss in the recent past. Until now these stars were considered to be massive supergiants. V 925 Sco was considered as a very long period cepheid. We conclude that TW Aql and AI CMi are low mass stars in the post-AGB stage of evolution. The evolutionary status of V 925 Sco is not clear. However the presence of detached cold dust shell with far-IR colors similar to PN suggest that it may also be a low mass star in post asymptotic giant branch (AGB) stage of evolution