A Rare Population of Intermediate-mass Helium Stars Between Hot Subdwarfs and Wolf-Rayet Stars

TL;DR

This study uses binary population synthesis to predict the formation, properties, and population estimates of intermediate-mass helium stars, bridging a gap in understanding between hot subdwarfs and Wolf-Rayet stars.

Contribution

It provides the first systematic theoretical investigation of intermediate-mass helium stars, highlighting the roles of metallicity and common-envelope ejection in their formation.

Findings

Several thousand IMHeS in the Milky Way and hundreds in the Magellanic Clouds.

Over 90% of IMHeS are in binary systems, mostly with main-sequence companions.

Majority formed through stable mass transfer or common envelope evolution.

Abstract

Helium stars stripped of their hydrogen envelopes represent pivotal phases in binary evolution, yet their origins, particularly within the intermediate-mass range of $2-8\, M_{\odot}$, still remain poorly understood. This population bridges the gap between low-mass hot subdwarfs and massive Wolf-Rayet stars, but has remained largely unobserved. In this study, we employ binary population synthesis to systematically investigate the formation and properties of intermediate-mass helium stars (IMHeS) across various galactic metallicities. Our results indicate that metallicity and common-envelope ejection efficiency are the dominant factors shaping the IMHeS population. We estimate that several thousand IMHeS exist in the Milky Way, with several hundred more in the Magellanic Clouds. The vast majority of IMHeS reside in binaries, with fewer than $10\%$ appearing as single stars. Among IMHeS binaries, $\gtrsim 50\%$ are expected to have main-sequence companions, and the remainder host compact companions (including helium stars, white dwarfs, neutron stars, or black holes). The former systems form mainly through stable mass transfer, whereas the latter arise predominantly from common envelope evolution. Our work provides quantitative predictions for the populations of these elusive stars formed through binary interactions and offers guidance for future observational searches.