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

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 (fOBe), and how it depends on absolute magnitude, mass, and metallicity (Z). We find fOBe = 0.22 in the Large Magellanic Cloud (0.5 Z⊙), increasing to fOBe = 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.