Low-metallicity massive single stars with rotation

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.

An extreme [O III] emitter atz= 3.2: a low metallicity Lyman continuum source

“…The origin of He iiλ1640 emission in galaxies has been a matter of investigation, possibly produced by massive stars both in the local Universe (e.g., Szécsi et al 2015;Gräfener & Vink 2015) and at high-redshift (e.g., Cassata et al 2013;Sobral et al 2015;Pallottini et al 2015). He iiλ1640 detection is generally A51, page 10 of 13 associated with AGN activity, but it has also been reported that He iiλ1640 associated with no C ivλ1550 is a possible feature in star-forming galaxies at z ∼ 3 (Cassata et al 2013).…”

Merger Rates of Double Neutron Stars and Stellar Origin Black Holes: The Impact of Initial Conditions on Binary Evolution Predictions

“…For example, let us consider the fraction of massive binary systems in which mass transfer starts already during the main sequence evolution of the primary star. This depends on the maximum radius that a star reaches on the main sequence, which depends on a number of rather uncertain processes such as convection and overshooting, (e.g., calibrations by Pols et al 1997;Ribas et al 2000;Brott et al 2011a), further mixing processes such as those induced by rotation (e.g., Yoon & Langer 2005;Brott et al 2011b;Ekström et al 2012;Szécsi et al 2015), mass loss by stellar winds and eruptions (review by Smith 2014), and the possible density inversions in the outer layers of massive stars that can result in inflated envelopes (e.g., Yusof et al 2013;Köhler et al 2015, andJiang et al 2015, submitted). Varying the maximum radius of main sequence stars by 30% up and down and allowing for uncertainties in the initial distributions, we find a large variation for this fraction, 20%-50%.…”

Evolution and Nucleosynthesis of Massive Stars

“…For instance a homogeneous evolution during the Main-Sequence is obtained at a smaller initial rotation in meridional current models than in shear models. As an illustration of this, the models of [51] (that belong to the family of the meridional current models) show that for a metallicity corresponding to that I Zw 18, i.e. Z= 0.0004, a homogeneous evolution is obtained for a 39 M ⊙ already for an initial rotation equal to 350 km s −1 .…”

“…Also the minimum mass of single star entering a Wolf-Rayet stage is decreased by rotation. Many grids of rotating stellar models have appeared in the last decade [4,11,16,20,51]. Although these grids are all presented under the generic name of rotating stellar models, they are computed sometimes with very different physics.…”

“…The source of energy for such currents is the rotational energy of the star that deforms the stars. The main characteristics of stellar models accounting for this theory are the following ones [4,30,51]:…”