Weighing Single-lined Spectroscopic Binaries Using Tidal Effects on Radial Velocities: The Case of V723 Monocerotis

Abstract

In single-lined spectroscopic binaries (SB1s) where flux variations due to tidal deformation of the primary star (ellipsoidal variations, EVs) are detected, the binary mass can be determined by combining EVs with the primary's radial velocity (RV) variations from orbital motion and information about the primary’s radius. This method has been used for mass estimation in close binaries including X-ray systems, but it has been pointed out that contaminating light from sources other than the primary star could introduce systematic errors in the mass and inclination estimates. Here, we focus on the apparent RV variations caused by asymmetric distortion of the absorption lines of the tidally deformed primary star (tidal RV). Because this signal contains information equivalent to that from photometric EVs, it enables mass estimation of the binary system using only the primary star’s absorption lines from high-resolution spectroscopic data, providing a potentially more robust approach against contaminating light. We apply the method to the binary system V723 Monocerotis, where both photometric EV and tidal RV signals are detected and successfully determine the component masses using only the primary star’s RVs and projected rotational velocity, without relying on absolute flux measurements or stellar evolutionary models. The masses derived from the tidal RV model shows a reasonable agreement with those obtained from EVs after carefully modeling the flux contamination from the secondary. This result demonstrates that tidal RVs provide a useful alternative means for mass estimation in SB1s.

Keywords

• Radial velocity
• Red giant stars
• Stellar mass black holes
• Tidal distortion