HE 0430-2457: a post merger extremely low-mass pre-white dwarf in a wide binary posing as an extreme horizontal branch star

TL;DR

The paper reports the discovery of HE 0430-2457, a unique binary system with an extremely low-mass pre-white dwarf, suggesting a higher prevalence of such objects in the EHB population and proposing a formation via hierarchical triple merger.

Contribution

This is the first identification of an ELM pre-white dwarf in a long-period binary, challenging previous assumptions about their rarity and origin.

Findings

HE 0430-2457 contains a 0.23 Msun pre-WD primary.

The primary is not an EHB star but a pre-WD of the ELM type.

The system likely formed through a hierarchical triple merger.

Abstract

We report the discovery of HE 0430-2457, the first extremely low-mass pre-white dwarf (ELM pre-WD) in a long period binary (P = 771 +- 3 d). The spectroscopic parameters of the primary are determined to be Teff = 26200 +- 1500 K and logg = 5.40 +- 0.35, placing it in the region occupied by core He-burning hot subdwarf B stars. By comparing the spectroscopic parameters of the K-type companion to stellar models, and using the mass ratio, the mass of the hot primary is determined to be 0.23 Msun. Given that this is too low for core He-burning, the primary in HE 0430-2457 is not an EHB star but a pre WD of the ELM type. As the lifetime of ELM pre-WDs in this region of the HR diagram populated by EHBs is thought to be very short, they are not considered to be part of the observed EHBs. However, the discovery of this system indicates that the percentage of ELM pre-WDs in the observed EHB population might be higher than previously thought. Binary evolution models indicate that HE 0430-2457 is likely formed by a merger of the inner binary in a hierarchical triple system.