The EREBOS project: Investigating the effect of substellar and low-mass stellar companions on late stellar evolution

Abstract

Eclipsing post-common envelope binaries are highly important for resolving the poorly understood, very short-lived common envelope phase of stellar evolution. Most hot subdwarfs (sdO/Bs) are the bare helium-burning cores of red giants which have lost almost all of their hydrogen envelopes. This mass loss is often triggered by common envelope interactions with close stellar or even sub-stellar companions. Cool companions to hot subdwarf stars such as late-type stars and brown dwarfs are detectable from characteristic light curve variations -reflection effects and often eclipses. In the recently published catalog of eclipsing binaries in the Galactic Bulge and in the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey, we discovered 125 new eclipsing systems showing a reflection effect by visual inspection of the light curves and using a machine-learning algorithm, in addition to the 36 systems discovered before by the OGLE (Optical Gravitational Lesing Experiment) team. The EREBOS (Eclipsing Reflection Effect Binaries from Optical Surveys) project aims at analyzing all newly discovered eclipsing binaries of the HW Vir type (hot subdwarf + close, cool companion) based on a spectroscopic and photometric follow up to derive the mass distribution of the companions, constrain the fraction of sub-stellar companions and determine the minimum mass needed to strip off the red-giant envelope. To constrain the nature of the primary we derived the absolute magnitude and the reduced proper motion of all our targets with the help of the parallaxes and proper motions measured by the Gaia mission and compared those to the Gaia white dwarf candidate catalogue. For a sub-set of our targets with observed spectra the nature could be derived by measuring the atmospheric parameter of the primary confirming that less than 10% of our systems are not sdO/Bs with cool companions but white dwarfs or central stars of planetary nebula. This large sample of eclipsing hot subdwarfs with cool companions allowed us to derive a significant period distribution for hot subdwarfs with cool companions for the first time showing that the period distribution is much broader than previously thought and ideally suited to find the lowest mass companions to hot subdwarf stars. The comparison with related binary populations shows that the period distribution of HW Vir systems is very similar to WD+dM systems and central stars of planetary nebula with cool companions. In the future several new photometric surveys will be carried out, which will increase the sample of this project even more giving the potential to test many aspects of common envelope theory and binary evolution.

Formation of sdB-stars via common envelope ejection by substellar companions

“…Our simulations show successful envelope ejection for systems with BD companions as indicated by the observations. Also, the lower mass limit determined here is supported by observations: Despite considerable effort no giant planet has so far been identified in close orbit around a hot subdwarf yet (Schaffenroth et al 2019;Casewell et al 2018). However, there may be a bias because the lowest-mass companions are expected at longer periods of about 0.3 d. Otherwise, the companion would exceed its Roche lobe leading to mass transfer onto the primary (see Fig.…”

“…However, there may be a bias because the lowest-mass companions are expected at longer periods of about 0.3 d. Otherwise, the companion would exceed its Roche lobe leading to mass transfer onto the primary (see Fig. 14 of Schaffenroth et al 2019). Such long-period systems are harder to follow up and only few of them have been investigated until now.…”

ZTFJ0038+2030: A Long-period Eclipsing White Dwarf and a Substellar Companion

“…Several sdB + brown dwarf systems are known, although typically seen with shorter orbital periods (e.g. Geier et al 2011;Schaffenroth et al 2015Schaffenroth et al , 2018Schaffenroth et al , 2019.…”

A search for transiting planets around hot subdwarfs

“…Several ground-based surveys with both photometric and RV techniques target the red dwarf or brown dwarf close companions to hot subdwarfs (Schaffenroth et al 2018(Schaffenroth et al , 2019. These companions are frequent (Schaffenroth et al 2018), but no Jupiter-like planets have been found to date.…”

“…all known sdB+red dwarf or brown dwarf post-CE binaries have orbital periods below 1 d (Schaffenroth et al 2018(Schaffenroth et al , 2019(Schaffenroth et al , 2021.…”