The mass-radius relationship for very low mass stars: four new discoveries from the HATSouth Survey

Abstract

We report the discovery of four transiting F-M binary systems with companions between 0.1 and 0.2 M in mass by the HATSouth survey. These systems have been character- ized via a global analysis of the HATSouth discovery data, combined with high-resolution radial velocities and accurate transit photometry observations. We determined the masses and radii of the component stars using a combination of two methods: isochrone fitting of spectroscopic primary star parameters and equating spectroscopic primary star rotation veloc- ity with spin–orbit synchronization. These new very low mass companions are HATS550- 016B (0 . 110 + 0 . 005 − 0 . 006 M ,0 . 147 + 0 . 003 − 0 . 004 R ), HATS551-019B (0 . 17 + 0 . 01 − 0 . 01 M ,0 . 18 + 0 . 01 − 0 . 01 R ), HATS551-021B (0 . 132 + 0 . 014 − 0 . 005 M ,0 . 154 + 0 . 006 − 0 . 008 R ) and HATS553-001B (0 . 20 + 0 . 01 − 0 . 02 M , 0 . 22 + 0 . 01 − 0 . 01 R ). We examine our sample in the context of the radius anomaly for fully convec- tive low-mass stars. Combining our sample with the 13 other well-studied very low mass stars, we find a tentative 5 per cent systematic deviation between the measured radii and theoretical isochrone models.