Hot subdwarfs in close binaries observed from space II: Analysis of the light curves

TL;DR

This study analyzes high-quality space-based light curves of hot subdwarf binaries to precisely determine the properties of their low-mass companions, revealing new insights into white dwarf and M dwarf companions and their orbital dynamics.

Contribution

It provides the first detailed constraints on the masses of companions in a large sample of hot subdwarf binaries using space-based photometry.

Findings

Determined masses of 19 companions, including white dwarfs and M dwarfs.

Identified most white dwarf companions as helium-core white dwarfs.

Measured rotation periods of sdBs, finding some are tidally locked, others not.

Abstract

Hot subdwarfs in close binaries with either M dwarf, brown dwarf or white dwarf companions show unique light variations. In hot subdwarf binaries with M dwarf or brown dwarf companions we can observe the so-called reflection effect and in hot subdwarfs with close white dwarf companions ellipsoidal modulation and/or Doppler beaming. Aims. The analysis of these light variations can be used to derive the mass and radius of the companion and hence determine its nature. Thereby we assume the most probable sdB mass and the radius of the sdB derived by the fit of the spectral energy distribution and the Gaia parallax. In the high signal-to-noise space-based light curves from the Transiting Exoplanet Survey Satellite and the K2 space mission, several reflection effect binaries and ellipsoidal modulation binaries have been observed with much better quality than possible for ground-based observations. The high quality of the light curves allowed us to analyse a large sample of sdB binaries with M dwarf or white dwarf companions using lcurve. For the first time we can constrain the absolute parameters of 19 companions of reflection effect systems covering periods from 2.5 to 19 hours and companion masses from the hydrogen burning limit to early M dwarfs. Moreover, we could determine the mass of eight white dwarf companion to hot subdwarf binaries showing ellipsoidal modulations, covering a so far unexplored period range from 7 to 19 hours. The derived masses of the white dwarf companions show that all but two of the white dwarf companions are most likely helium-core white dwarfs. Combining our results with previously measured rotation velocities allowed us to derive the rotation period of seven sdBs in short-period binaries. In four of those systems the rotation period of the sdB agrees with a tidally locked orbit, in the other three systems the sdB rotates significantly slower.