Resolving the dynamical mass tension of the massive binary 9 Sagittarii

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.

14 pages, 5 figures, 4 tables (+6 figures and 3 tables in appendices). Accepted for publication in Astronomy and Astrophysics

References in corpus (11)

• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
• UBVJHK synthetic photometry of Galactic O stars
• BONNSAI: a Bayesian tool for comparing stars with stellar evolution models
• An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
• Radio, X-ray, and gamma-ray Emission Models of the Colliding Winds Binary WR 140
• Is HR 6819 a triple system containing a black hole? -- An alternative explanation
• The Tarantula Massive Binary Monitoring. IV. Double-lined photometric binaries
• A close encounter of the massive kind
• The Massive Binary System 9 Sgr Revisited: New Insights into Disentangling Methods
• The Three body system delta Circini

Cited by in corpus (6)

• Identifying quiescent compact objects in massive Galactic single-lined spectroscopic binaries
• A magnetic massive star has experienced a stellar merger
• HR 6819 is a binary system with no black hole -- revisiting the source with infrared interferometry and optical integral field spectroscopy
• Multiplicity of northern bright O-type stars with optical long baseline interferometry
• Are magnetic fields universal in O-type multiple systems?
• Quantitative spectroscopy of multiple OB stars: I. The quadruple system HD 37061 at the centre of Messier 43