The Effects of Clumps in Explaining X‐Ray Emission Lines from Hot Stars

Abstract

It is now well established that stellar winds of hot stars are fragmentary and that the X-ray emission from stellar winds has a strong contribution from shocks in winds. Chandra high spectral resolution observations of line profiles of O and B stars have shown numerous properties that had not been expected. Here we suggest explanations by considering the X-rays as arising from bow shocks that occur where the stellar wind impacts on spherical clumps in the winds. We use an accurate and stable numerical hydrodynamical code to obtain steady-state physical conditions for the temperature and density structure in a bow shock. We use these solutions plus analytic approximations to interpret some major X-ray features: the simple power-law distribution of the observed emission measure derived from many hot star X-ray spectra and the wide range of ionization stages that appear to be present in X-ray sources throughout the winds. Also associated with the adiabatic cooling of the gas around a clump is a significant transverse velocity for the hot plasma flow around the clumps, and this can help to understand anomalies associated with observed line widths, and the differences in widths seen in stars with high and low mass-loss rates. The differences between bow shocks and the planar shocks that are often used for hot stars are discussed. We introduce an "on the shock" (OTSh) approximation that is useful for interpreting the X-rays and the consequences of clumps in hot star winds and elsewhere in astronomy.

High-Resolution X-Ray Spectroscopy Reveals the Special Nature of Wolf-Rayet Star Winds

“…Exceptionally large, pancake-shaped clumps which form close to the photosphere at low velocities would resist the radiative acceleration. Being an obstacle to the generally much faster wind, they create strong reverse shocks that may add more mass to the clump if the gas cools quickly, or generate bow shocks if it does not (Cassinelli et al 2008). Self-consistent kinematic models of Gayley (in prep.…”

Modeling X-ray emission line profiles from massive star winds – A review

“…By contrast the case of clump bow-shocks indicate that the spatial correlation of cool and hot components can in principle produce reasonable emission line profiles. However, the simulations of Cassinelli et al (2008) did not evolve a clump through the flow. Instead, Ignace et al (2012) made use of beta velocity laws to impose a velocity difference between the clumps and interclump gas.…”

The Importance of Xuv Radiation as a Solution to the P v Mass Loss Rate Discrepancy in O Stars

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