Post-common envelope binaries from SDSS - XVI. Long orbital period systems and the energy budget of CE evolution

TL;DR

This study reports the discovery of long orbital period post-common envelope binaries with white dwarfs, analyzing their evolution and discussing the role of recombination energy in the common-envelope phase.

Contribution

It presents the discovery of the second and third longest orbital period PCEBs and evaluates the significance of recombination energy in their evolution.

Findings

Long orbital period PCEBs discovered with white dwarfs.

Evolution of these systems can be explained without recombination energy.

Possible existence of more long-period PCEBs with massive WDs if recombination energy is significant.

Abstract

Virtually all close compact binary stars are formed through common-envelope (CE) evolution. It is generally accepted that during this crucial evolutionary phase a fraction of the orbital energy is used to expel the envelope. However, it is unclear whether additional sources of energy, such as the recombination energy of the envelope, play an important role. Here we report the discovery of the second and third longest orbital period post-common envelope binaries (PCEBs) containing white dwarf (WD) primaries, i.e. SDSSJ121130.94-024954.4 (Porb = 7.818 +- 0.002 days) and SDSSJ222108.45+002927.7 (Porb = 9.588 +- 0.002 days), reconstruct their evolutionary history, and discuss the implications for the energy budget of CE evolution. We find that, despite their long orbital periods, the evolution of both systems can still be understood without incorporating recombination energy, although at least small contributions of this additional energy seem to be likely. If recombination energy significantly contributes to the ejection of the envelope, more PCEBs with relatively long orbital periods (Porb >~ 1-3 day) harboring massive WDs (Mwd >~ 0.8 Msun) should exist.