M. Cantiello

Binary star progenitors of long gamma-ray bursts

Context. The collapsar model for long gamma-ray bursts requires a rapidly rotating Wolf-Rayet star as progenitor. Aims. We test the idea of producing rapidly rotating Wolf-Rayet stars in massive close binaries through mass accretion and consecutive quasi-chemically homogeneous evolution — the latter had previously been shown to provide collapsars below a certain metallicity threshold. Methods. We use a 1-D hydrodynamic binary evolution code to simulate the evolution of a 16+15 M⊙ binary model with an initial orbital period of 5 days and SMC metallicity (Z=0.004). Internal differential rotation, rotationally induced mixing and magnetic fields are included in both components, as well as non-conservative mass and angular momentum transfer, and tidal spin-orbit coupling. Results. The considered binary system undergoes early Case B mass transfer. The mass donor becomes a helium star and dies as a Type Ib/c supernova. The mass gainer is spun-up, and internal magnetic fields e ffi ciently transport accreted angular momentum into the stellar core. The orbital widening prevents subseq uent tidal synchronization, and the mass gainer rejuvenates and evolves quasi-chemically homogeneously thereafter. The mass donor explodes 7 Myr before the collapse of the mass gainer. Assuming the binary to be broken-up by the supernova kick, the potential gamma-ray burst progenitor would become a runaway star with a space velocity of 27 km s −1 , traveling about 200 pc during its remaining lifetime. Conclusions. The binary channel presented here does not, as such, provide a new physical model for collapsar production, as the resulting stellar models are almost identical to quasi-che mically homogeneously evolving rapidly rotating single stars. However, it may provide a means for massive stars to obtain the required high rotation rates. Moreover, it suggests that a possibly la rge fraction of long gamma-ray bursts occurs in runaway stars.

Rotational mixing in massive binaries Detached short-period systems

Models of rotating single stars can successfully account fo r a wide variety of observed stellar phenomena, such as the surface enhancements of N and He observed in massive main-sequence stars. However, recent observations have questioned the idea that ro tational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test for rotational mixing. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In the se systems the tides are thought to spin up the stars to rapid rotation, sync hronous with their orbital revolution. We use a state-of-th e-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We adop t a rotational mixing effi ciency that has been calibrated against observations of rotating stars und er the assumption that rotational mixing is the main process responsible for the observed surface abundances. We find that the primaries of massive close binaries ( M1≈ 20 M⊙, Porb. 3 days) are expected to show significant enhancements in nitrogen (up to 0.6 dex in the Small Magellanic Cloud) for a significant fraction of their core hydrogen-burning lifetime . We propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their p arameters can be determined with high accuracy. For the primary stars of more massive and very close systems ( M1≈ 50 M⊙, Porb. 2 days) we find that centrally produced helium is effi ciently mixed throughout the envelope. The star remains blue and compact during the main sequence evolution and stays within its Roche lobe. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills it s Roche lobe first, contrary to what standard binary evolution theory pre dicts. The primaries will appear as “Wolf-Rayet stars in dis guise”: core hydrogen-burning stars with strongly enhanced He and N at the surface. We propose that this evolution path provides an al ternative channel for the formation of tight Wolf-Rayet binaries with a main-sequence companion and might explain massive black hole binaries such as the intriguing system M33 X-7.

Rotation and Massive Close Binary Evolution

We review the role of rotation in massive close binary systems. Rotation has been advocated as an essential ingredient in massive single star models. However, rotation clearly is most important in massive binaries where one star accretes matter from a close companion, as the resulting spin-up drives the accretor towards critical rotation. Here, we explore our understanding of this process, and its observable consequences. When accounting for these consequences, the question remains whether rotational effects in massive single stars are still needed to explain the observations.

The VLT-FLAMES Tarantula Survey

The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. Here we introduce the scientific motivations of the survey and give an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution.

Thermohaline mixing in low-mass giants

Thermohaline mixing has recently been proposed to occur in low mass red giants, with large consequences for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1M⊙ and 3M⊙, in comparison to other mixing processes acting in these stars. We confirm that thermohaline mixing has the potential to destroy most of the 3 He which is produced earlier on the main sequence during the red giant stage. In our models we find that this process is working only in stars with initial mass M � 1.5M⊙. Moreover, we report that thermohaline mixing can be present during core helium burning and beyond in stars which still have a 3 He reservoir. While rotational and magnetic mixing is negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with differential rotation and magnetic fields may be essential to establish the time scale of thermohaline mixing in red giants.

Evolution of Massive Stars at Very Low Metallicity, Including Rotation and Binary Interactions

We discuss recent models of the evolution of massive stars at very low metallicity, including the effects of rotation, magnetic fields and binarity. Very metal poor stars lose very little mass and angular momentum during their main sequence evolution, and rotation plays a dominant role in their evolution. In rapidly rotating massive stars, the rotationally induced mixing time scale can be even shorter than the nuclear time scale throughout the main sequence. The consequent quasi‐chemically homogeneous evolution greatly differs from the standard massive star evolution that leads to formation of red giants with strong chemical stratification. Interesting outcomes of such a new mode of evolution include the formation of rapidly rotating massive Wolf‐Rayet stars that emit large amounts of ionizing photons, the formation of long gamma‐ray bursts and hypernovae, and the production of large amounts of primary nitrogen. We show that binary interactions can further enhance the effects of rotation, as mass accretio...

Rotational mixing in close binaries

Rotational mixing is a very important but uncertain process in the evolution of massive stars. We propose to use close binaries to test its efficiency. Based on rotating single stellar models we predict nitrogen surface enhancements for tidally locked binaries. Furthermore we demonstrate the possibility of a new evolutionary scenario for very massive (M > 40 solar mass) close (P < 3 days) binaries: Case M, in which mixing is so efficient that the stars evolve quasi-chemically homogeneously, stay compact and avoid any Roche-lobe overflow, leading to very close (double) WR binaries.

Evolution of Progenitor Stars of Type Ibc Supernovae and Long Gamma-Ray Bursts

We discuss how rotation and binary interactions may be related to the diversity of type Ibc supernovae and long gamma-ray bursts. After presenting recent evolutionary models of massive single and binary stars including rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the nature of SNe Ibc progenitors from binary systems may not significantly differ from that of single star progenitors in terms of rotation, and that most long GRB progenitors may be produced via the quasi-chemically homogeneous evolution at sub-solar metallicity. We also briefly discuss the possible role of magnetic fields generated in the convective core of a massive star for the transport of angular momentum, which is potentially important for future stellar evolution models of supernova and GRB progenitors.

Thermohaline Mixing in Low‐mass Giants: RGB and Beyond

Thermohaline mixing has recently been proposed to occur in low mass red giants, with large consequence for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1u2009M⊙ and 3u2009M⊙, in comparison to other mixing processes acting in these stars. We use a stellar evolution code which includes rotational mixing and internal magnetic fields. We confirm that thermohaline mixing has the potential to destroy most of the 3He which is produced earlier on the main sequence during the red giant stage, in stars below 1.5u2009M⊙. We find this process to continue during core helium burning and beyond. We find rotational and magnetic mixing to be negligible compared to the thermohaline mixing in the relevant layers, even if the interaction of thermohaline motions with the differential rotation may be essential to establish the timescale of thermohaline mixing in red giants.

Light elements in massive single and binary stars

We highlight the role of the light elements (Li, Be, B) in the evolution of massive single and binary stars, which is largely restricted to a diagnostic value, and foremost so for the element boron. However, we show that the boron surface abundance in massive early type stars contains key information about their foregoing evolution which is not obtainable otherwise. In particular, it allows to constrain internal mixing processes and potential previous mass transfer event for binary stars (even if the companion has disappeared). It may also help solving the mystery of the slowly rotating nitrogen-rich massive main sequence stars.