Magnetohydrodynamic equilibria in barotropic stars

Although barotropic matter does not constitute a realistic model for magnetic stars, it would be interesting to confirm a recent conjecture that states that magnetized stars with a barotropic equation of state would be dynamically unstable (Reisenegger 2009). In this work we construct a set of barotropic equilibria, which can eventually be tested using a stability criterion. A general description of the ideal MHD equations governing these equilibria is summarized, allowing for both poloidal and toroidal magnetic field components. A new finite-difference numerical code is developed in order to solve the so-called Grad-Shafranov equation describing the equilibrium of these configurations, and some properties of the equilibria obtained are briefly discussed.Comment: Conference Proceedings, Magnetic Fields in the Universe IV (2013

Testing eccentricity pumping mechanisms to model eccentric long period sdB binaries with MESA

Hot subdwarf-B stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts those objects to be circularised before the onset of Roche-lobe overflow (RLOF). We aim to find binary-evolution mechanisms that can explain these eccentric long-period orbits, and reproduce the currently observed period-eccentricity diagram. Three different processes are considered; tidally-enhanced wind mass-loss, phase-dependent RLOF on eccentric orbits and the interaction between a circumbinary disk and the binary. The binary module of the stellar-evolution code MESA (Modules for Experiments in Stellar Astrophysics) is extended to include the eccentricity-pumping processes. The effects of different input parameters on the final period and eccentricity of a binary-evolution model are tested with MESA. The end products of models with only tidally-enhanced wind mass-loss can indeed be eccentric, but these models need to lose too much mass, and invariably end up with a helium white dwarf that is too light to ignite helium. Within the tested parameter space, no sdBs in eccentric systems are formed. Phase-dependent RLOF can reintroduce eccentricity during RLOF, and could help to populate the short-period part of the period-eccentricity diagram. When phase-dependent RLOF is combined with eccentricity pumping via a circumbinary disk, the higher eccentricities can be reached as well. A remaining problem is that these models favour a distribution of higher eccentricities at lower periods, while the observed systems show the opposite. The models presented here are potentially capable of explaining the period-eccentricity distribution of long-period sdB binaries, but further theoretical work on the physical mechanisms is necessary.Comment: 18 pages, 9 figures, accepted for publication in A&

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

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. 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. 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 . 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. 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. 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

Tidally-driven Roche-Lobe Overflow of Hot Jupiters with MESA

Many exoplanets have now been detected in orbits with ultra-short periods, very close to the Roche limit. Building upon our previous work, we study the possibility that mass loss through Roche lobe overflow (RLO) may affect the evolution of these planets, and could possibly transform a hot Jupiter into a lower-mass planet (hot Neptune or super-Earth). We focus here on systems in which the mass loss occurs slowly ("stable mass transfer" in the language of binary star evolution) and we compute their evolution in detail with the binary evolution code MESA. We include the effects of tides, RLO, irradiation and photo-evaporation of the planet, as well as the stellar wind and magnetic braking. Our calculations all start with a hot Jupiter close to its Roche limit, in orbit around a sun-like star. The initial orbital decay and onset of RLO are driven by tidal dissipation in the star. We confirm that such a system can indeed evolve to produce lower-mass planets in orbits of a few days. The RLO phase eventually ends and, depending on the details of the mass transfer and on the planetary core mass, the orbital period can remain around a few days for several Gyr. The remnant planets have a rocky core and some amount of envelope material, which is slowly removed via photo-evaporation at nearly constant orbital period; these have properties resembling many of the observed super-Earths and sub-Neptunes. For these remnant planets we also predict an anti-correlation between mass and orbital period; very low-mass planets ($M_{\rm pl}\,\lesssim\,5\,M_{\oplus}$) in ultra-short periods ($P_{\rm orb}$<1d) cannot be produced through this type of evolution.Comment: 14 pages, 7 figures, 2 tables. Accepted by ApJ. The manuscript has been revised significantly to address the referee's comments. A link to MESA inlist files is now provided on page

Luminous supernovae associated with ultra-long gamma-ray bursts from hydrogen-free progenitors extended by pulsational pair-instability

We show that the luminous supernovae (SNe) associated with ultra-long gamma-ray bursts (GRBs) can be related to the slow cooling from the explosions of hydrogen-free progenitors extended by pulsational pair-instability. In the accompanying paper (Marchant & Moriya 2020), we have shown that some rapidly-rotating hydrogen-free GRB progenitors that experience pulsational pair-instability can keep an extended structure caused by pulsational pair-instability until the core collapse. Such progenitors have large radii exceeding 10 Rsun and they sometimes reach beyond 1000 Rsun at the time of the core collapse. They are, therefore, promising progenitors of ultra-long GRBs. We here perform the light-curve modeling of the explosions of one extended hydrogen-free progenitor with a radius of 1962 Rsun. Thanks to the large progenitor radius, the ejecta experience slow cooling after the shock breakout and they become rapidly evolving (~ 1e43 erg/s) SNe in optical even without the energy input from the 56Ni nuclear decay when the explosion energy is more than 1e52 erg. The 56Ni decay energy input can affect the light curves after the optical light-curve peak and make the light-curve decay slow when the 56Ni mass is around 1 Msun. They also have fast photospheric velocity above 10,000 km/s and hot photospheric temperature above 10,000 K at around the peak luminosity. We find that the rapid rise and luminous peak found in the optical light curve of SN 2011kl, which is associated with the ultra-long GRB 111209A, can be explained as the cooling phase of the extended progenitor. The ultra-long GRB progenitors proposed in Marchant & Moriya (2020) can explain both the ultra-long GRB duration and the accompanying SN properties. When the GRB jet is off-axis or choked, the luminous SNe could be observed as fast blue optical transients without accompanying GRBs. (abridged)Comment: 5 pages, 5 figures, accepted by Astronomy & Astrophysics Letter