Abstract
We present high-resolution HST WFPC2 images of compact nebulosity surrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. The powerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers. We show that NML Cyg's circumstellar envelope is likely shaped by photodissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Per have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a northeast/southwest orientation similar to that for its OH and H2O masers, while VX Sgr is embedded in a spheroidal envelope. We find no evidence for circumstellar nebulosity around the intermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, nor around the normal M-type supergiant μ Cep. We conclude that there is no evidence for high mass loss events prior to 500–1000 years ago for these four stars.
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