The massive binary HD 152218 revisited: A new colliding wind system in NGC 6231

TL;DR

This study refines the orbital parameters of the massive binary HD 152218, confirms its spectral classification, and reports X-ray flux variations linked to wind-wind interactions, revealing a new colliding wind system.

Contribution

It provides a revised orbital solution, evidence of rapid apsidal motion, and detailed X-ray analysis of the first colliding wind system in NGC 6231.

Findings

Orbital period refined to approximately 5.604 days.

Detected about 30% increase in X-ray flux near apastron.

Confirmed wind-wind interaction causing X-ray flux variation.

Abstract

We present the results of an optical and X-ray monitoring campaign on the short-period massive SB2 binary HD 152218. Combining our HiRes spectroscopic data with previous observations, we unveil the contradictions between the published orbital solutions. In particular, we solve the aliasing on the period and derive a value close to 5.604 d. Our eccentricity e = 0.259 +/- 0.006 is slightly lower than previously admitted. We show that HD 152218 is probably undergoing a relatively rapid apsidal motion of about 3deg/yr and we confirm the O9IV + O9.7V classification. We derive minimal masses of 15.82 +/- 0.26 Msol operator and 12.00 +/- 0.19 Msol operator and constrain the radius of the components to R1 = 10.3 +/- 1.3 Rsol and R2 = 7.8 +/- 1.7 Rsol. We also report the results of an XMM-Newton monitoring of the HD 152218 X-ray emission throughout its orbital motion. The averaged X-ray spectrum is relatively soft and it is well reproduced by a 2-T optically thin thermal plasma model with component temperatures about 0.3 and 0.7 keV. The system presents an increase of its X-ray flux by about 30% near apastron compared to periastron, which is interpreted as the signature of an ongoing wind-wind interaction process occurring within the wind acceleration region.