The White Dwarf Binary Pathways Survey -- IX. Three long period white dwarf plus subgiant binaries

TL;DR

This study reports the discovery of three long-period white dwarf plus subgiant binaries, providing insights into binary evolution pathways and the role of stable mass transfer versus common envelope phases.

Contribution

It presents the first detailed characterization of three intermediate-period white dwarf binaries, highlighting their formation via stable mass transfer and future evolutionary outcomes.

Findings

All systems contain low-mass white dwarfs (≤0.4 M☉).

Two shorter period systems are predicted to merge during common envelope evolution.

The longest period system may survive to form a close double white dwarf binary.

Abstract

Virtually all binaries consisting of a white dwarf with a non-degenerate companion can be classified as either close post-interaction systems (with orbital periods of a few days or less), or wide systems (with periods longer than decades), in which both components have effectively evolved as single stars. Binaries with periods between these two extremes can help constrain common envelope efficiency, or highlight alternative pathways towards the creation of compact binaries. To date such binaries have remained mostly elusive. Here we present three white dwarfs in binaries with evolved subgiant stars with orbital periods of 41, 52 and 461 d. Using Hubble Space Telescope spectroscopy we find that all three systems contain low mass white dwarfs ($\leq$0.4 M$_{\odot}$). One system, TYC 8394$-$1331$-$1, is the inner binary of a hierarchical triple, where the white dwarf plus subgiant binary is orbited by a more distant companion star. These binaries were likely formed from a phase of stable but non-conservative mass transfer, as opposed to common envelope evolution. All three systems will undergo a common envelope phase in the future, but the two shorter period systems are expected to merge during this event, while the longest period system is likely to survive and create a close binary with two low mass white dwarfs.