The Circumstellar Environments of NML Cygni and the Cool Hypergiants

We present high-resolution HST WFPC2 images of compact nebulosity surroundingthe cool M-type hypergiants NML Cyg, VX Sgr and S Per. The powerful OH/IRsource NML Cyg exhibits a peculiar bean-shaped asymmetric nebula that iscoincident with the distribution of its H2O vapor masers. We show that NMLCyg's circumstellar envelope is likely shaped by photo-dissociation from thepowerful, nearby association Cyg OB2 inside the Cygnus X superbubble. The OH/IRsources VX Sgr and S Per have marginally resolved envelopes. S Per'scircumstellar nebula appears elongated in a NE/SW orientation similar to thatfor its OH and H2O masers, while VX Sgr is embedded in a spheroidal envelope.We find no evidence for circumstellar nebulosity around the intermediate-typehypergiants rho Cas, HR 8752, HR 5171a, nor the normal M-type supergiant muCep. We conclude that there is no evidence for high mass loss events prior to500-1000 yrs ago for these four stars.Comment: 28 pages, 8 figures, accepted for publication in The Astronomical Journa