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arXiv · 2105.09968

Resolving the dynamical mass tension of the massive binary 9 Sagittarii

M. Fabry·C. Hawcroft·A. J. Frost·L. Mahy·P. Marchant·J-B. Le Bouquin·H. Sana
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Abstract

Direct dynamical mass measurements of stars with masses above 30 M${}_\odot$ are rare. This is the result of the low yield of the upper initial mass function and the limited number of such systems in eclipsing binaries. Long-period, double-lined spectroscopic binaries that are also resolved astrometrically offer an alternative for obtaining absolute masses of stellar objects. 9 Sgr is one such long-period, high-mass binary. Unfortunately, a large amount of tension exists between its total dynamical mass inferred from radial velocity measurements and that from astrometric data. We obtained the astrometric orbit from VLTI/PIONIER and VLTI/GRAVITY interferometric measurements. Using archival and new spectroscopy, we performed a grid-based spectral disentangling search to constrain the semi-amplitudes of the radial velocity curves. We computed atmospheric parameters and surface abundances by adjusting \textsc{fastwind} atmosphere models and we compared our results with evolutionary tracks computed with the Bonn Evolutionary Code (BEC). Grid spectral disentangling of 9 Sgr supports the presence of a 53 M${}_\odot$ primary and a 39 M${}_\odot$ secondary. Comparison with BEC evolutionary tracks shows the components of 9 Sgr are most likely coeval with an age of roughly 1 Myr. Our analysis clears up the contradiction between mass and orbital inclination estimates reported in previous studies. We detect the presence of significant CNO-processed material at the surface of the primary, suggesting enhanced internal mixing compared to currently implemented in the BEC models. The present measurements provide a high-quality high-mass anchor to validate stellar evolution models and to test the efficiency of internal mixing processes.

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M. Fabry, C. Hawcroft, A. J. Frost, L. Mahy, P. Marchant, J-B. Le Bouquin, H. Sana. 2021-05-20. Resolving the dynamical mass tension of the massive binary 9 Sagittarii. https://doi.org/10.1051/0004-6361%2F202140452

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