X-ray-Induced Ionization in the Winds of \\ Near-Main-Sequence O and B Stars
Session 31 -- Massive Hot Stars
Display presentation, Tuesday, 31, 1994, 9:20-6:30

## [31.08] X-ray-Induced Ionization in the Winds of \\ Near-Main-Sequence O and B Stars

J. J. MacFarlane, D. H. Cohen, and P. Wang (U. Wisc.)

We investigate the effects of X-rays on the ionization state of stellar winds for O and early-B stars along the main-sequence. In our investigation, detailed statistical equilibrium, radiative transfer, and atomic physics models are used to compute ionization distributions for H, He, C, N, O, and Si. X-rays are modeled as a spatially-distributed source within the wind, with parameters constrained by ROSAT and Einstein observations. Our results indicate that the ionization balance in the winds of stars with spectral type B0~V and later is significantly altered by the X-ray radiation field. Unlike the case of denser O star winds, where the X-rays tend to perturb the level of ionization, the ionization state of the bulk wind of early-B stars can be significantly increased by soft X-rays. We examine in detail the case of $\tau$~Sco (B0~V), which has been well-studied at UV and X-ray wavelengths. Comparisons are made between calculated ionization fractions and those deduced from UV observations. In addition, we address the sensitivity of our results to the X-ray source characteristics, the wind temperature, and the photospheric EUV flux. Our results suggest the possibility that for early-B stars X-rays play a critical role in both influencing the radiation line driving force, as well as ionizing and heating the wind all the way down to the top of the photosphere.