Circumstellar Gas in the Wide Binary HD 188037

We report radio, optical, and ultraviolet studies of the wide binary HD 188037 (=IRC +20439). We detect the CO (2-1) and (1-0) rotational lines from the circumstellar envelope in this system, which consists of a luminous (~7000 L ☉ ) mass-losing M-type star and, at a separation of 0 75, a main-sequence A-type star. The strong (2-1) line has a very unusual skewed shape with a width at zero intensity of about 30 km s -1 , which we explain with a model wherein the gas ejected by the M-type star is preferentially photodissociated on one side of the outflow by ultraviolet light from the A-type companion. We infer that the wind from the M giant expands at 15 km s -1 and, for an estimated distance of 300 pc, carries ~3 × 10 -7 M ☉ yr -1 . Our simplified model for CO in a wind being photodissociated by ultraviolet from a companion explains why CO can be detected around Mira yet not around α Sco.