THE LOW-VELOCITY WIND FROM THE CIRCUMSTELLAR MATTER AROUND THE B9 V STAR σ HERCULIS

摘要

We have obtained Far Ultraviolet Spectroscopic Explorer spectra of σ Her, a nearby binary system with a main-sequence primary that has a Vega-like infrared excess. We observe absorption in the excited fine structure lines C Ⅱ~* at 1037 A, N Ⅱ~* at 1085 A, and N Ⅱ~(**) at 1086 A that are blueshifted by as much as ～30 km s~(-1) with respect to the star. Since these features are considerably narrower than the stellar lines and broader than interstellar features, the C Ⅱ and N Ⅱ are circumstellar. We suggest that there is a radiatively driven wind arising from the circumstellar matter rather than accretion as occurs around β Pic because of σ Her's high luminosity. Assuming that the gas is liberated by collisions between parent bodies at 20 AU, the approximate distance at which blackbody grains are in radiative equilibrium with the star and at which three-body orbits become unstable, we infer dM/dt ～ 6 x 10~(-12) solar mass yr~(-1). This wind depletes the minimum mass of parent bodies in less than the estimated age of the system.