The evolution of rotating very massive stars with LMC composition^⋆,⋆⋆

¹ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: karenkoehler86@gmail.com
² Astronomical Institute “Anton Pannekoek”, Amsterdam University, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3011 Leuven, Belgium
⁴ Observatories of the Carnegie Institution for Science, 813 Santa Barbara St, Pasadena, CA 91101, USA
⁵ Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁶ Department of Physics & Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
⁷ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁸ Armagh Observatory, College Hill, Armagh BT61 9DG, UK
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 8 June 2014
Accepted: 14 October 2014

Abstract

Context. With growing evidence for the existence of very massive stars at subsolar metallicity, there is an increased need for corresponding stellar evolution models.

Aims. We present a dense model grid with a tailored input chemical composition appropriate for the Large Magellanic Cloud (LMC).

Methods. We use a one-dimensional hydrodynamic stellar evolution code, which accounts for rotation, transport of angular momentum by magnetic fields, and stellar wind mass loss to compute our detailed models. We calculate stellar evolution models with initial masses from 70 to 500  M_⊙ and with initial surface rotational velocities from 0 to 550 km s^-1, covering the core-hydrogen burning phase of evolution.

Results. We find our rapid rotators to be strongly influenced by rotationally induced mixing of helium, with quasi-chemically homogeneous evolution occurring for the fastest rotating models. Above 160 M_⊙, homogeneous evolution is also established through mass loss, producing pure helium stars at core hydrogen exhaustion independent of the initial rotation rate. Surface nitrogen enrichment is also found for slower rotators, even for stars that lose only a small fraction of their initial mass. For models above ~150 M_⊙ at zero age, and for models in the whole considered mass range later on, we find a considerable envelope inflation due to the proximity of these models to their Eddington limit. This leads to a maximum ZAMS surface temperature of ~56 000 K, at ~180 M_⊙, and to an evolution of stars in the mass range 50 M_⊙...100 M_⊙ to the regime of luminous blue variables in the Hertzsprung-Russell diagram with high internal Eddington factors. Inflation also leads to decreasing surface temperatures during the chemically homogeneous evolution of stars above ~180  M_⊙.

Conclusions. The cool surface temperatures due to the envelope inflation in our models lead to an enhanced mass loss, which prevents stars at LMC metallicity from evolving into pair-instability supernovae. The corresponding spin-down will also prevent very massive LMC stars to produce long-duration gamma-ray bursts, which might, however, originate from lower masses.