Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

Abstract

We present a new method for using measured X-ray emission line fluxes from O stars to determine the shock-heating rate due to instabilities in their radiation-driven winds. The high densities of these winds means that their embedded shocks quickly cool by local radiative emission, while cooling by expansion should be negligible. Ignoring for simplicity any non-radiative mixing or conductive cooling, the method presented here exploits the idea that the cooling post-shock plasma systematically passes through the temperature characteristic of distinct emission lines in the X-ray spectrum. In this way, the observed flux distribution among these X-ray lines can be used to construct the cumulative probability distribution of shock strengths that a typical wind parcel encounters as it advects through the wind. We apply this new method (Gayley 2014) to Chandra grating spectra from five O stars with X-ray emission indicative of embedded wind shocks in effectively single massive stars. Correcting for wind absorption of the X-ray line emission is a crucial component of our analysis, and we use wind optical depth values derived from X-ray line-profile fitting (Cohen et al. 2014) in order to make that correction. The shock-heating rate results we derive for all the stars are quite similar: the average wind mass element passes through roughly one shock that heats it to at least $10^6$ K as it advects through the wind, and the cumulative distribution of shock strengths is a strongly decreasing function of temperature, consistent with a negative power-law of index $n\approx 3$, implying a marginal distribution of shock strengths that scales as $T^{-4}$, and with hints of an even steeper decline or cut-off above $10^7$ K.

11 pages, 6 figures (5 color); MNRAS in press

References in corpus (5)

• Quantitative studies of the optical and UV spectra of Galactic early B supergiants I. Fundamental parameters
• X-rays from Magnetically Confined Wind Shocks: Effect of Cooling-Regulated Shock Retreat
• X-Rays From Massive OB Stars: Thermal Emission From Radiative Shocks
• Evidence for the importance of resonance scattering in X-ray emission line profiles of the O star $\zeta$ Puppis
• Coronal properties of the EQ Peg binary system

Cited by in corpus (7)

• An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars
• Characterizing the turbulent porosity of stellar-wind structure generated by the line-deshadowing instability
• A JVLA survey of the high frequency radio emission of the massive magnetic B- and O-type stars
• Chandra X-ray observation of the young stellar cluster NGC 3293 in the Carina Nebula Complex
• Helium-like X-ray line complexes show that the hottest plasma on the O supergiant zeta Puppis is in its wind
• Wind accretion onto compact objects
• Fe XVII $2p$-$3s$ line ratio diagnostic of shock formation radius in O stars