A coordinated X-ray and Optical Campaign of the Nearest Massive Eclipsing Binary, $\delta$ Orionis Aa: IV. A multiwavelength, non-LTE spectroscopic analysis

TL;DR

This study conducts a comprehensive multiwavelength, non-LTE spectroscopic analysis of the massive eclipsing binary $\delta$ Orionis Aa, revealing stellar properties, wind characteristics, and X-ray emission origins, with implications for distance and mass estimates.

Contribution

It provides the first detailed multiwavelength non-LTE analysis of $\delta$ Ori Aa, clarifying component types, wind inhomogeneities, and X-ray emission sources, and suggests a revised distance estimate.

Findings

Primary star is an O9.5 II type with wind inhomogeneities.

X-ray emission mainly from the primary's wind, near the stellar surface.

Mass-loss rate consistent with hydrodynamic predictions.

Abstract

Eclipsing systems of massive stars allow one to explore the properties of their components in great detail. We perform a multi-wavelength, non-LTE analysis of the three components of the massive multiple system $\delta$ Ori A, focusing on the fundamental stellar properties, stellar winds, and X-ray characteristics of the system. The primary's distance-independent parameters turn out to be characteristic for its spectral type (O9.5 II), but usage of the ${\rm \it Hipparcos}$ parallax yields surprisingly low values for the mass, radius, and luminosity. Consistent values follow only if $\delta$ Ori lies at about twice the ${\rm \it Hipparcos}$ distance, in the vicinity of the $\sigma$-Orionis cluster. The primary and tertiary dominate the spectrum and leave the secondary only marginally detectable. We estimate the V-band magnitude difference between primary and secondary to be $\Delta V \approx 2.\!\!^{\rm m}8$. The inferred parameters suggest the secondary is an early B-type dwarf ($\approx$ B1 V), while the tertiary is an early B-type subgiant ($\approx$ B0 IV). We find evidence for rapid turbulent velocities ($\sim 200$ km ${\rm s}^{-1}$) and wind inhomogeneities, partially optically thick, in the primary's wind. The bulk of the X-ray emission likely emerges from the primary's stellar wind ($\log L_{\text{X}} / L_{\text{Bol}} \approx -6.85$), initiating close to the stellar surface at $R_0 \sim 1.1\,R_*$. Accounting for clumping, the mass-loss rate of the primary is found to be $\log \dot{M} \approx -6.4\,[M_\odot\,{\rm yr}^{-1}]$, which agrees with hydrodynamic predictions, and provides a consistent picture along the X-ray, UV, optical and radio spectral domains.