Formation of Extremely Low-Mass White Dwarfs in Wide Orbits

TL;DR

This paper explores how wide-orbit extremely low-mass white dwarfs can form through hierarchical triple systems, especially via supernova explosions, explaining some observed systems.

Contribution

It introduces a new formation channel for wide-orbit ELM WDs involving hierarchical triples and supernovae, addressing previous limitations of binary evolution models.

Findings

Supernova explosions can produce single ELM WDs in hierarchical triples.

The model can explain the orbital parameters of HE 0430-2457.

It fails to reproduce the features of KIC 8145411, supporting its triple system nature.

Abstract

Helium white dwarfs (WDs) with masses less than 0.3 $\rm M_{\sun}$ are known as extremely low-mass WDs (ELM WDs), which cannot be produced by single stellar evolution in theory. Generally, these stars are believed to form through binary interactions. Recently, two ELM WDs in unusually wide orbits were reported, i.e., KIC 8145411 and HE 0430-2457. Their orbital separations are too wide to be produced by the binary evolution scenario. In this work, we study the formation of wide-orbit ELM WD binaries from hierarchical triple systems. In this scenario, an ELM WD is formed from the inner binary and subsequently forms a wide binary system with the third object. We find that the merger of an evolved star with a brown dwarf in the inner binary fails to produce single ELM WDs, but Type Ia supernovae (SNe Ia) explosions can successfully do so. Furthermore, we investigate the impact of the supernova explosion on the orbital distribution of the surviving binary and find that this channel may have a probability of reproducing the orbital parameters of HE 0430-2457, but fails to reproduce the observed features of KIC 8145411. This supports recent observational recalibrations suggesting that KIC 8145411 resides in a triple system rather than a binary.