X-ray emission from O stars

TL;DR

Young O stars emit variable, hard X-rays due to magnetic wind shocks, while older stars produce softer X-rays from wind shocks; modeling suggests previous mass-loss estimates are too high.

Contribution

This study combines high-resolution X-ray spectroscopy and MHD modeling to explain X-ray emission mechanisms in young and old O stars, providing new insights into their magnetic fields and mass-loss rates.

Findings

Young O stars exhibit variable, hard X-ray emission explained by magnetic wind shocks.

Older O stars produce steady, soft X-rays from shock heating in stellar winds.

Mass-loss rates of O stars are likely overestimated based on X-ray spectral modeling.

Abstract

Young O stars are strong, hard, and variable X-ray sources, properties which strongly affect their circumstellar and galactic environments. After ~1 Myr, these stars settle down to become steady sources of soft X-rays. I use high-resolution X-ray spectroscopy and MHD modeling to show that young O stars like theta-1 Ori C are well explained by the magnetically channeled wind shock scenario. After their magnetic fields dissipate, older O stars produce X-rays via shock heating in their unstable stellar winds. Here too I use X-ray spectroscopy and numerical modeling to confirm this scenario. In addition to elucidating the nature and cause of the O star X-ray emission, modeling of the high-resolution X-ray spectra of O supergiants provides strong evidence that mass-loss rates of these O stars have been overestimated.