Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility

TL;DR

This study analyzes 29 sdB+dM eclipsing binaries using ZTF and Gaia data, revealing how their properties relate to orbital period and identifying brown dwarf companions in very short-period systems.

Contribution

First comprehensive analysis of sdB+dM binaries combining ZTF light curves and Gaia parallaxes, providing new insights into their physical parameters and evolutionary implications.

Findings

Mass ratio and companion radius decrease with shorter orbital periods.

Brown dwarf companions found in systems with periods less than 0.075 days.

Companions follow the mass-radius relation for low-mass stars and brown dwarfs.

Abstract

The development of large-scale time-domain surveys provides an opportunity to study the physical properties as well as the evolutionary scenario of B-type subdwarfs (sdB) and M-type dwarfs (dM). Here, we obtained 33 sdB+dM eclipsing binaries based on the Zwicky Transient Facility (ZTF) light curves and $Gaia$ early data release 3 (EDR3) parallaxes. By using the PHOEBE code for light curve analysis, we obtain probability distributions for parameters of 29 sdB+dM. $R_1$, $R_2$, and $i$ are well determined, and the average uncertainty of mass ratio $q$ is 0.08. Our parameters are in good agreement with previous works if a typical mass of sdB is assumed. Based on parameters of 29 sdB+dM, we find that both the mass ratio $q$ and the companion's radius $R_2$ decrease with the shortening of the orbital period. For the three sdB+dMs with orbital periods less than 0.075 days, their companions are all brown dwarfs. The masses and radii of the companions satisfy the mass--radius relation for low-mass stars and brown dwarfs. Companions with radii between $0.12R_\odot$ and $0.15R_\odot$ seem to be missing in the observations. As more short-period sdB+dM eclipsing binaries are discovered and classified in the future with ZTF and $Gaia$, we will have more information to constrain the evolutionary ending of sdB+dM.