Close detached white dwarf + brown dwarf binaries: further evidence for low values of the common envelope efficiency

TL;DR

This study investigates the evolution of white dwarf + brown dwarf binary systems, providing evidence that a low common envelope efficiency parameter explains their formation without needing extra energy sources, aligning with previous findings for systems with main sequence companions.

Contribution

It demonstrates that observed white dwarf + brown dwarf binaries can be explained with a low common envelope efficiency, challenging the necessity of additional energy sources in their formation.

Findings

Low common envelope efficiency ($_{ m CE}$) explains the evolution of these binaries.

Recombination energy is not required to account for envelope ejection.

Results are consistent with previous studies on systems with main sequence companions.

Abstract

Common envelope evolution is a fundamental ingredient in our understanding of the formation of close binary stars containing compact objects which includes the progenitors of type Ia supernovae, short gamma ray bursts and most stellar gravitational wave sources. To predict the outcome of common envelope evolution we still rely to a large degree on a simplified energy conservation equation. Unfortunately, this equation contains a theoretically rather poorly constrained efficiency parameter ($\alpha_{\mathrm{CE}}$) and, even worse, it is unclear if energy sources in addition to orbital energy (such as recombination energy) contribute to the envelope ejection process. In previous works we reconstructed the evolution of observed populations of post common envelope binaries (PCEBs) consisting of white dwarfs with main sequence star companions and found indications that the efficiency is rather small ($\alpha_{\mathrm{CE}}\simeq0.2-0.3$) and that extra energy sources are only required in very few cases. Here we used the same reconstruction tool to investigate the evolutionary history of a sample of observed PCEBs with brown dwarf companions. In contrast to previous works, we found that the evolution of observationally well characterized PCEBs with brown dwarf companions can be understood assuming a low common envelope efficiency ($\alpha_{\mathrm{CE}}=0.24-0.41$), similar to that required to understand PCEBs with main sequence star companions, and that contributions from recombination energy are not required. We conclude that the vast majority of PCEBs form from common envelope evolution that can be parameterized with a small efficiency and without taking into account additional energy sources.