Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate

TL;DR

This study uses Chandra X-ray observations of the massive star HD 93129A to analyze wind shocks and determine a lower mass-loss rate than previous estimates, supporting the embedded wind shock model.

Contribution

First consistent modeling of X-ray line profiles and broadband spectrum in a massive star, constraining wind properties and mass-loss rate with new observational evidence.

Findings

Continuum wind absorption causes the X-ray spectrum hardness.

Intrinsic colliding wind shocks contribute less than 10% of X-ray flux.

Measured mass-loss rate is significantly lower than previous H-alpha estimates.

Abstract

We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths Rstar. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star's closest visual companion at a distance of ~100 AU. The broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first consistent modeling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind.