Hot Subdwarf Formation: Confronting Theory with Observation

TL;DR

This paper reviews the current understanding of hot subdwarf star formation, comparing observational data with theoretical models, and discusses challenges to existing binary evolution theories based on recent findings.

Contribution

It critically evaluates observational evidence against standard binary formation scenarios for hot subdwarfs and highlights the need for revised models.

Findings

High binary fractions support binary formation scenarios

Recent observations challenge standard binary evolution models

Merger and envelope ejection are key processes in formation

Abstract

The formation of hot subdwarf stars is still unclear. Both single-star and binary scenarios have been proposed to explain the properties of these evolved stars situated at the extreme blue end of the horizontal branch. The observational evidence gathered in the last decade, which revealed high fractions of binaries, shifted the focus from the single-star to the binary formation scenarios. Common envelope ejection, stable Roche lobe overflow and the merger of helium white dwarfs seemed to be sufficient to explain the formation of both the binary as well as the remaining single hot subdwarfs. However, most recent and rather unexpected observations challenge the standard binary evolution scenarios.