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Session 25 - Stars in the Ultraviolet.
Display session, Tuesday, June 10
South Main Hall,
Hot stars are losing mass at large rates through their high-speed winds. Such stars are also know to emit X-rays with L_X/L_bol \simeq 10^-7 for O stars, falling to \simeq 10^-9 for B3 stars. It is particularly difficult to observationally determine the wind properties of early-B stars because their UV wind absorption lines are typically very weak. Extreme Ultraviolet Explorer (EUVE) observations of the B2 II star \epsilon CMa (\lambda \sim 100 - 700 Åshow strong line emission from He II, O III, and Fe IX - Fe XVI. The He II and O III emission lines originate in the ``cool'' portion of the wind, while the Fe IX - Fe XVI lines are produced in the hot X-ray-emitting plasma. Our analysis indicates that the He II \lambda304 line results from the recombination of He III, which is produced by X-ray photoionization of the cool wind. The He II Ly\alpha line produces the observed O III line emission at 374 Åvia the ``Bowen mechanism''; i.e., the photoexcitation of the coincident O III \lambda303-304 muliplet by the He II \lambda304 line. This interplay of the line emission from the hot plasma, the photoionization of the cooler wind, the He recombination line emission and the subsequent Bowen-produced O III line emission can be used to constrain the wind properties of \epsilon CMa. We shall present results from wind ionization dynamics calculations which model these processes, and discuss the ramifications for discerning the properties of the wind of \epsilon CMa.
