The Cosmic Carbon Footprint of Massive Stars Stripped in Binary Systems

The cosmic origin of carbon, a fundamental building block of life, is still uncertain. Yield predictions for massive stars are almost exclusively based on single-star models, even though a large fraction interact with a binary companion. Using the MESA stellar evolution code, we predict the amount of carbon ejected in the winds and supernovae of single and binary-stripped stars at solar metallicity. We find that binary-stripped stars are twice as efficient at producing carbon (1.5–2.6 times, depending on choices regarding the slope of the initial mass function and black hole formation). We confirm that this is because the convective helium core recedes in stars that have lost their hydrogen envelope, as noted previously. The shrinking of the core disconnects the outermost carbon-rich layers created during the early phase of helium burning from the more central burning regions. The same effect prevents carbon destruction, even when the supernova shock wave passes. The yields are sensitive to the treatment of mixing at convective boundaries, specifically during carbon-shell burning (variations up to 40%), and improving upon this should be a central priority for more reliable yield predictions. The yields are robust (variations less than 0.5%) across our range of explosion assumptions. Black hole formation assumptions are also important, implying that the stellar graveyard now explored by gravitational-wave detections may yield clues to better understand the cosmic carbon production. Our findings also highlight the importance of accounting for binary-stripped stars in chemical yield predictions and motivates further studies of other products of binary interactions

The expansion of stripped-envelope stars:Consequences for supernovae and gravitational-wave progenitors

Massive binaries that merge as compact objects are the progenitors of gravitational-wave sources. Most of these binaries experience one or more phases of mass transfer, during which one of the stars loses part or all of its outer envelope and becomes a stripped-envelope star. The evolution of the size of these stripped stars is crucial in determining whether they experience further interactions and their final fate. We present new calculations of stripped-envelope stars based on binary evolution models computed with MESA. We use these to investigate their radius evolution as a function of mass and metallicity. We further discuss their pre-supernova observable characteristics and potential consequences of their evolution on the properties of supernovae from stripped stars. At high metallicity we find that practically all of the hydrogen-rich envelope is removed, in agreement with earlier findings. Only progenitors with initial masses below 10\Msun expand to large radii (up to 100\Rsun), while more massive progenitors stay compact. At low metallicity, a substantial amount of hydrogen remains and the progenitors can, in principle, expand to giant sizes (> 400\Rsun), for all masses we consider. This implies that they can fill their Roche lobe anew. We show that the prescriptions commonly used in population synthesis models underestimate the stellar radii by up to two orders of magnitude. We expect that this has consequences for the predictions for gravitational-wave sources from double neutron star mergers, in particular for their metallicity dependence.Comment: Main text 17 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Mind the Gap:The Location of the Lower Edge of the Pair-instability Supernova Black Hole Mass Gap

Gravitational-wave detections are now starting to probe the mass distribution of stellar-mass black holes (BHs). Robust predictions from stellar models are needed to interpret these. Theory predicts the existence of a gap in the BH mass distribution because of pair-instability supernova. The maximum BH mass below the gap is the result of pulsational mass loss. We evolve massive helium stars through their late hydrodynamical phases of evolution using the open-source MESA stellar evolution code. We find that the location of the lower edge of the mass gap at 45$M_\odot$ is remarkably robust against variations in the metallicity ($\approx 3M_\odot$), the treatment of internal mixing ($\approx 1M_\odot$), stellar wind mass loss ($\approx 4M_\odot$), making it the most robust predictions for the final stages of massive star evolution. The reason is that the onset of the instability is dictated by the near-final core mass, which in turn sets the resulting BH mass. However, varying $^{12}C\left(\alpha,\gamma\right)^{16}O$ reaction rate within its $1\sigma$ uncertainties shifts the location of the gap between $40M_\odot$ and $56M_\odot$. We provide updated analytic fits for population synthesis simulations. Our results imply that the detection of merging BHs can provide constraints on nuclear astrophysics. Furthermore, the robustness against metallicity suggests that there is a universal maximum for the location of the lower edge of the gap, which is insensitive to the formation environment and redshift for first-generation BHs. This is promising for the possibility to use the location of the gap as a "standard siren" across the Universe.Comment: 17 pages, 5 figures, 1 Table, Accepted Ap

Close encounters of star-black hole binaries with single stars

Multibody dynamical interactions of binaries with other objects are one of the main driving mechanisms for the evolution of star clusters. It is thus important to bring our understanding of three-body interactions beyond the commonly employed point-particle approximation. To this end, we here investigate the hydrodynamics of three-body encounters between star–black hole (BH) binaries and single stars, focusing on the identification of final outcomes and their long-term evolution and observational properties, using the moving-mesh hydrodynamics code AREPO. This type of encounter produces five types of outcomes: stellar disruption, stellar collision, weak perturbation of the original binary, binary member exchange, and triple formation. The two decisive parameters are the binary phase angle, which determines which two objects meet at the first closest approach, and the impact parameter, which sets the boundary between violent and non-violent interactions. When the impact parameter is smaller than the semimajor axis of the binary, tidal disruptions and star-BH collisions frequently occur when the BH and the incoming star first meet, while the two stars mostly merge when the two stars meet first instead. In both cases, the BHs accrete from an accretion disc at super-Eddington rates, possibly generating flares luminous enough to be observed. The stellar collision products either form a binary with the BH or remain unbound to the BH. Upon collision, the merged stars are hotter and larger than the main sequence stars of the same mass at similar age. Even after recovering their thermal equilibrium state, stellar collision products, if isolated, would remain hotter and brighter than main sequence stars until becoming giants