The Circumstellar Environments of NML Cygni and the Cool Hypergiants

Abstract

We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H_{2}Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H_{2}O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants {ρ} Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant {μ} Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.