Probing the wind–wind collision in γ2 Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization |
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| Authors: | David B Henley Ian R Stevens Julian M Pittard |
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| Institution: | School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT;School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT |
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| Abstract: | We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary γ2 Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray-emitting plasma (∼4–40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of 1240 ± 30 km s−1 . Calculations of line profiles based on hydrodynamical simulations of the wind–wind collision predict lines that are blueshifted by a few hundred km s−1 . The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (>85°), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg xi emission originates from hotter gas closer to the O star than the Si xiii emission, which suggests that non-equilibrium ionization may be present. |
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| Keywords: | stars: individual: γ2 Velorum stars: winds outflows stars: Wolf–Rayet X-rays: stars |
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