The circumstellar environments of NML Cygni and the cool hypergiants

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 H ₂O 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 H ₂O 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 5171 a, 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.