A census of OBe stars in nearby metal-poor dwarf galaxies reveals a high fraction of extreme rotators

TL;DR

This study identifies and quantifies the high fraction of OBe stars in nearby metal-poor dwarf galaxies, revealing a strong dependence on metallicity and implications for stellar evolution and explosive transients.

Contribution

It provides the first comprehensive census of OBe stars in multiple low-metallicity dwarf galaxies, highlighting their high prevalence and potential impact on galaxy evolution and transient phenomena.

Findings

OBe star fraction increases as metallicity decreases.

High OBe fractions are observed below 0.2 Z_Sun.

Results suggest a link between OBe stars and explosive transients in metal-poor environments.

Abstract

The Early Universe, together with many nearby dwarf galaxies, is deficient in heavy elements. The evolution of massive stars in such environments is thought to be affected by rotation. Extreme rotators amongst them tend to form decretion disks and manifest themselves as OBe stars. We use a combination of U B, GAIA, Spitzer, and Hubble Space Telescope photometry to identify the complete populations of massive OBe stars - one hundred to thousands in number - in five nearby dwarf galaxies. This allows us to derive the galaxy-wide fractions of main sequence stars that are OBe stars (f_OBe), and how it depends on absolute magnitude, mass, and metallicity (Z). We find f_OBe = 0.22 in the Large Magellanic Cloud (0.5 Z_Sun), increasing to f_OBe = 0.31 in the Small Magellanic Cloud (0.2 Z_Sun). In the so far unexplored metallicity regime below 0.2 Z_Sun, in Holmberg I, Holmberg II, and Sextans A, we also obtain high OBe star fractions of 0.27, 0.27, and 0.27, respectively. These high OBe star fractions, and the strong contribution in the stellar mass range which dominates the production of supernovae, shed new light on the formation channel of OBe stars, as well as on the preference of long-duration gamma-ray bursts and superluminous supernovae to occur in metal-poor galaxies.