PREPRINT
4AE211BA-7887-469E-80D4-C76ABA5F7F12

# Characterising the orbit and circumstellar environment of the high-mass binary MWC 166 A

Sebastian A. Zarrilli, Stefan Kraus, Alexander Kreplin, John D. Monnier, Tyler Gardner, Antoine Mérand, Sam Morrell, Claire L. Davies, Aaron Labdon, Jacob Ennis, Benjamin Setterholm, Jean-Baptiste Le Bouquin, Narsireddy Anugu, Cyprien Lanthermann, Gail Schaefer, Theo ten Brummelaar
arXiv:2207.02836

Submitted on 6 July 2022

## Abstract

Context: Stellar evolution models are highly dependent on accurate mass estimates, especially for high-mass stars in the early stages of evolution. The most direct method for obtaining model-independent masses is derivation from the orbit of close binaries. Aims: To derive the first astrometric+RV orbit solution for the single-lined spectroscopic binary MWC 166 A, based on CHARA and VLTI near-infrared interferometry over multiple epochs and ~100 archival radial velocity measurements, and to derive fundamental stellar parameters from this orbit. We also sought to model circumstellar activity in the system from K-band spectral lines. Methods: We geometrically modelled the dust continuum to derive astrometry at 13 epochs and constrain individual stellar parameters. We used the continuum models as a base to examine differential phases, visibilities and closure phases over the Br-$\gamma$ and He-I emission lines. Results: Our orbit solution suggests a period of $367.7±0.1$ d, twice as long as found with previous RV orbit fits, subsequently constraining the component masses to ${M}_{1}=12.2±2.2{M}_{\odot }$ and ${M}_{2}=4.9±0.5{M}_{\odot }$. The line-emitting gas was found to be localised around the primary and is spatially resolved on scales of ~11 stellar radii, with the spatial displacement between the line wings consistent with a rotating disc. Conclusions: The large radius and stable orientation of the line emission are inconsistent with magnetospheric or boundary-layer accretion, but indicate an ionised inner gas disk around MWC 166 Aa. We observe line variability that could be explained either with generic line variability in a Herbig star disc or V/R variations in a decretion disc. We also constrained the age of the system to ~$\left(7±2\right)×{10}^{5}$ yr, consistent with the system being comprised of a main-sequence primary and a secondary still contracting towards the main sequence.

## Preprint

Comment: 24 pages, 19 figures, 7 tables, 1 appendix. Accepted in A&A

Subject: Astrophysics - Solar and Stellar Astrophysics