Testing the Wind-Shock Paradigm for B-Type Star X-Ray Production with θ Carinae
We present Chandra X-ray grating spectroscopy of the B0.2V star, θ Carinae. θ Car is in a critical transition region between the latest O-type and earliest B-type stars, where some of these stars are observed to have UV-determined mass-loss rates much lower than theoretically expected. In general, X-ray emission in this low-luminosity regime should be less prominent than in O-star winds, but observations have shown a higher than expected production of X-ray emission from the winds of these stars (e.g., Cohen et al. 2008; Huenemoerder et al. 2012). A hot wind could explain weak UV wind signatures, but this severely challenges the predictions of radiatively-driven wind theory for low-luminosity stars. We measure the f /i (forbidden to intercombination intensity) ratio of several Helium-like lines and the widths of several Helium-like, Hydrogen-like and Fe lines in the X-ray spectrum of θ Car. The f /i ratio is a diagnostic of the radial location of the X-ray emitting plasma, which is modified by the distance to the UV-emitting stellar photosphere. We measure a low flow velocity from the widths of the X-ray lines of θ Car, which agrees with a slower and lower density wind as compared to what is theoretically predicted from the theory for strong winds in O-stars. The measured widths, less than 300 km s−1 , are also consistent with other weak-wind stars, for exmple, β Cru (Cohen et al., 2008). The location of the X-ray emitting plasma, measured to be relatively close to the star at less than 5R∗, is also consistent with a low density wind and with other weak-wind stars. We also estimate an X-ray mass-loss rate of ∼10−9 M⊙ yr−1 and place an upper limit on the mass-loss rate of ∼2 × 10−8 M⊙ yr−1 . We use θ Car to study how weak-wind stars agree or disagree with the theory for radiatively-driven winds using one-dimensional hydrodynamical simulations. We find that the perturbations produced by instabilities in the radiatively-driven wind are small and the shock temperatures are low overall. When radiative cooling is turned off, we find the shocked regions stay hot for longer and the wind is slower as a whole. We find that it is difficult to produce a hot wind with stronger shocks in the simulations for a B-type star, but hot winds are produced when radiative cooling is turned off.