Yellow Supergiants In Open Clusters (superluminous Giants(slg)).

Abstract

Superluminous giant stars (SLGs) have been reported in young globular clusters in the Large Magellanic Cloud (LMC). These stars appear to be in the post-asymptotic-giant-branch phase of evolution. This program was an investigation of galactic SLG candidates in open clusters, which are more like the LMC young globular clusters. These were chosen because luminosity, mass, and age determinations can be made for members since cluster distances and interstellar reddenings are known. Color-magnitude diagrams were searched for candidates, using the same selection criteria as for SLGs in the LMC. Classification spectra were obtained of 115 program stars from McGraw-Hill Observatory and of 68 stars from Cerro Tololo Inter-American Observatory in Chile. These stars were visually classified on the MK system using spectral scans of standard stars taken at the respective observations. A total of 62 objects were found to be of high luminosity and to warrant further investigations. Seven stars were monitored over a 90-day period for luminosity variations at Dyer Observatory and at Kitt Peak National Observatory (KPNO). High-resolution H(,(alpha)) scans were acquired of twelve stars with the KPNO Coude Feed and of ten stars at the McMath Solar Observatory. These observations were analyzed to measure radial velocities and to detect emission features. The former item was used to test for cluster membership; the latter for possible mass loss. A search of the IRAS Point Source Catalog for circumstellar dust revealed that eight stars have infrared fluxes at least six times greater than predicted. Published information was combined with this program's data for 83 stars in 30 clusters. Membership probabilities were assigned to these stars, and the clusters were analyzed according to age. It was seen that the intrinsically brightest supergiants are found in the youngest clusters. With increasing cluster age, the absolute luminosities attained by the supergiants decline. Also, it appears that the evolutionary tracks of luminosity class II stars are more similar to those of class I than of class III. Only two SLG candidates were found in open clusters. This low number is due in part to these clusters being less rich and younger than their LMC counterparts.