M. T. Schuster (U. Minn., CfA), R. M. Humphreys (U. Minn.), M. Marengo (CfA), R. D. Gehrz, C. E. Woodward, E. Polomski (U. Minn.)
We present HST and Spitzer images of several of the most luminous cool stars in the Galaxy. These highly unstable, very massive stars lie on or near the empirical upper luminosity boundary in the H-R diagram, and are characterized by high mass loss phenomena, sometimes violent, which may be responsible for the upper boundary. These observations are designed to search for circumstellar structures close to the star as well as more distant nebulosity. We discuss the presence, or lack of, ejecta around these hypergiants, and the evolutionary implications. Our high-resolution WFPC2 images show compact nebulosity around the cool M-type hypergiants NML Cyg, VX Sgr and S Per. The powerful OH/IR source NML Cyg exhibits a small, peculiar bean-shaped asymmetric nebula that closely matches the distribution of the surrounding H2O vapor masers. NML Cyg's concave outer envelope is likely shaped by photo-dissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. VX Sgr and S Per, also OH/IR sources, have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a NE/SW orientation similar to that for its surrounding OH and H2O masers, while VX Sgr is obscured by a spheroidal envelope. We find no evidence in our WFPC2 images for circumstellar nebulosity around the intermediate-type hypergiants \rho Cas, HR 8752, HR 5171a nor the normal M-type supergiant \mu Cep. We conclude that very likely, there has been no high mass loss event prior to 500-1000 yrs ago for these four stars. Our IRAC images (\rho Cas, HR 8752, and R 150) also show no evidence for extended structure.
This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work is in part provided by NASA through contracts 1256406 and 1215746 issued by JPL/Caltech to the University of Minnesota.
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.