Wide post-common envelope binaries containing ultramassive white dwarfs: evidence for efficient envelope ejection in massive AGB stars

TL;DR

This paper presents evidence that efficient envelope ejection in massive AGB stars can produce wide post-common-envelope binaries with ultramassive white dwarfs, challenging previous models that predicted only short-period systems.

Contribution

It demonstrates that recombination energy significantly contributes to envelope ejection, enabling the formation of wide PCEBs with ultramassive white dwarfs, supported by observations and stellar models.

Findings

Observed wide PCEBs with ultramassive WDs and periods of 18-49 days.

Recombination energy is crucial for ejecting envelopes in massive AGB stars.

Models show wide PCEBs can form via Roche lobe overflow of luminous AGB stars.

Abstract

Post-common-envelope binaries (PCEBs) containing a white dwarf (WD) and a main-sequence (MS) star can constrain the physics of common envelope evolution and calibrate binary evolution models. Most PCEBs studied to date have short orbital periods ($P_{\rm orb} \lesssim 1\,$d), implying relatively inefficient harnessing of binaries' orbital energy for envelope expulsion. Here, we present follow-up observations of five binaries from {\it Gaia} DR3 containing solar-type MS stars and probable ultramassive WDs ($M\gtrsim 1.2\,M_{\odot}$) with significantly wider orbits than previously known PCEBs, $P_{\rm orb} = 18-49\,$d. The WD masses are much higher than expected for systems formed via stable mass transfer at these periods, and their near-circular orbits suggest partial tidal circularization when the WD progenitors were giants. These properties strongly suggest that the binaries are PCEBs. Forming PCEBs at such wide separations requires highly efficient envelope ejection, and we find that the observed periods can only be explained if a significant fraction of the energy released when the envelope recombines goes into ejecting it. Our 1D stellar models including recombination energy confirm prior predictions that a wide range of PCEB orbital periods, extending up to months or years, can potentially result from Roche lobe overflow of a luminous AGB star. This evolutionary scenario may also explain the formation of several wide WD+MS binaries discovered via self-lensing, as well as a significant fraction of post-AGB binaries and barium stars.