The impact of tides and mass transfer on the evolution of metal-poor massive binary stars

Abstract

Stars more massive than 10Msun , although few in number compared to objects like our sun, play a very large role in the evolution of the universe. Through strong winds and supernovae they enrich the interstellar medium with heavy elements and provide mechanical feedback on galactic scales. Their large flux of ionizing photons may dominate the reionization of the universe. A thorough understanding of massive stellar evolution is then paramount to our understanding of the universe.<br /> In the last years, observations have established that binary interaction is a fundamental part of the evolution of massive stars, most of which will undergo mass exchange with a nearby companion. In addition, the recent detection of gravitational waves from the merger of two ∼ 30M black holes opens up exciting new possibilities to improve our understanding of the evolution of massive stars. To this purpose, we have extended the open-source stellar evolution code MESA to include the necessary physics to model binary systems. This new version of MESA allows us to explore in detail the parameter space of massive and very massive binary stars.<br /> The first problem we consider involves the evolution of very massive stars orbiting each other with periods of a few days. Owing to tidal locking, both components are fast rotators, which induces large scale mixing throughout their radiative envelopes. The stars then evolve chemically homogeneously, burning all their available nuclear fuel instead of developing a core envelope structure. This modifies their evolution dramatically: Instead of expanding significantly, these stars contract inside their Roche lobes to eventually form a pair of close black holes of similar mass that can merge in less than a Hubble time. For the design sensitivity of advanced LIGO, we expect ∼ 20 − 900 detections per year from this channel, with the large uncertainty arising from uncertainties in the chemical evolution of the universe. Owing to a range of very massive stars exploding as pair instability supernovae rather than forming a black hole, we expect to detect a gap in the distribution of black hole masses between 60M and 130M .<br /> Considering similar systems for which the secondary component is significantly less massive, we show that only the more massive star evolves homogeneously to become a black hole. On a longer timescale, the secondary expands and initiates mass transfer to the compact object, which makes it active as an X-ray source. Owing to their large black hole masses (in excess of 20M ), these X-ray binaries would have luminosities characteristic of those of observed ultra-luminous X-ray sources, which are difficult to explain through standard stellar evolution. The occurrence of pair-instability supernovae, just as was the case for binary black holes, produces a gap in black hole masses, which could be observable as a gap in the luminosity distribution of ultra-luminous sources. Our simulations show a strong metallicity dependence, and future X-ray surveys such as eROSITA could potentially test our claims.<br /> Finally, we modelled a large set of binary and single models of massive stars to study the population of objects in the LMC. Under extreme assumptions that minimize the contribution of binaries to our sample, we show that the large observed binary fraction inescapably implies a degeneracy between the predictions of rotational mixing in single stars and mass transfer in binaries. Binaries with low transfer efficiencies explain observed rapidly rotating stars with low nitrogen enrichment at their surfaces, but still undergo mixing and a slight enrichment in nitrogen. Accurate measurements of the abundances of rapid rotators could then provide valuable information on the efficiency of rotational mixing.<br /> Throughout this thesis we have extensively demonstrated the use of detailed stellar evolution models to cover the wide parameter space of single and binary stellar evolution in a statistically significant way. This new capability is certain to enable many new venues of research.