Low-metallicity massive single stars with rotation

Evolutionary models applicable to I Zwicky 18^⋆,⋆⋆

¹ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: dorottya@astro.uni-bonn.de
² Department of Physics & Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, 151-742, Seoul, South Korea
³ Astronomical Institute Anton Pannekoek, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
⁴ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

Received: 27 May 2015
Accepted: 29 June 2015

Abstract

Context. Low-metallicity environments such as the early Universe and compact star-forming dwarf galaxies contain many massive stars. These stars influence their surroundings through intense UV radiation, strong winds and explosive deaths. A good understanding of low-metallicity environments requires a detailed theoretical comprehension of the evolution of their massive stars.

Aims. We aim to investigate the role of metallicity and rotation in shaping the evolutionary paths of massive stars and to provide theoretical predictions that can be tested by observations of metal-poor environments.

Methods. Massive rotating single stars with an initial metal composition appropriate for the dwarf galaxy I Zw 18 ([Fe/H] = −1.7) are modelled during hydrogen burning for initial masses of 9−300 M_⊙ and rotational velocities of 0−900 km s^-1. Internal mixing processes in these models were calibrated based on an observed sample of OB-type stars in the Magellanic Clouds.

Results. Even moderately fast rotators, which may be abundant at this metallicity, are found to undergo efficient mixing induced by rotation resulting in quasi chemically-homogeneous evolution. These homogeneously-evolving models reach effective temperatures of up to 90 kK during core hydrogen burning. This, together with their moderate mass-loss rates, make them transparent wind ultraviolet intense stars (TWUIN star), and their expected numbers might explain the observed He II ionising photon flux in I Zw 18 and other low-metallicity He II galaxies. Our slowly rotating stars above ~80 M_⊙ evolve into late B- to M-type supergiants during core hydrogen burning, with visual magnitudes up to 19^m at the distance of I Zw 18. Both types of stars, TWUIN stars and luminous late-type supergiants, are only predicted at low metallicity.

Conclusions. Massive star evolution at low metallicity is shown to differ qualitatively from that in metal-rich environments. Our grid can be used to interpret observations of local star-forming dwarf galaxies and high-redshift galaxies, as well as the metal-poor components of our Milky Way and its globular clusters.