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

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: aschoot@astro.uni-bonn.de
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁴ Centro de Astrobiología, CSIC-INTA, Crtra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz (Madrid), Spain
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 10 August 2022
Accepted: 11 September 2022

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 among them tend to form decretion disks and manifest themselves as OBe stars. We use a combination of UB, Gaia, Spitzer, and Hubble Space Telescope photometry to identify the complete populations of massive OBe stars – from one hundred to thousands in number – in five nearby dwarf galaxies. This allows us to derive the galaxy-wide fraction 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_⊙), increasing to f_OBe = 0.31 in the Small Magellanic Cloud (0.2 Z_⊙). In the thus-far unexplored metallicity regime below 0.2 Z_⊙, 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 tendency for long-duration gamma-ray bursts and superluminous supernovae to occur in metal-poor galaxies.