Group member Prof. Stephen Justham coauthored this new textbook on what is without question the least understood type of binary interaction: the common envelope phase. During this phase stars temporarily share the same outer layers. This phase is essential for a wide variety of astrophysical phenomena: including cataclysmic variables, X-ray binaries, progenitors for type Ia supernovae, and gravitational-wave mergers. Read the introductory chapter here for free: https://iopscience.iop.org/book/978-0-7503-1563-0/chapter/bk978-0-7503-1563-0ch1
Stars are cosmic factories that produce most elements heavier than helium in their deep interiors through nuclear fusion. The rate at which Carbon is fused into Oxygen is notoriously uncertain. It cannot be measured at any laboratory at earth, at least not under the conditions that occur in the interiors of stars.
Rob Farmer (postdoc at UvA) showed earlier that the location of the Pair Instability mass gap is a remarkably robust prediction, more solid than anything we know about the final stages of the most massive stars.
In this new paper, he shows that this prediction depends on the assumed rate for the rate at which Carbon is destroyed by alpha particles to make Oxygen. Why? High rates basically lead the star to consume all it’s carbon prematurely, already during the helium-burning phase. Being devoid of carbon, such a start skips the Carbon burning phase and has to resort to burning Oxygen immediately. In very massive stars this ignites explosively leading to a pair-instability supernova or pulsations.
Instead, stellar models in which a lower rate is assumed for this reaction, still have carbon that they can burn. When burning Carbon in a shell they prevent premature contraction of the star holding up the outer layers. This can prevent the explosive ignition of Oxygen. Instead, the more gentle burning calmly depletes the core of its fuel leading to an inescapable final implosion to a black hole.
Long story short, assuming that the reaction is less efficient allows more massive stars to form black holes. Reducing the rate by 3 sigma moves the lower mass gap up to 100 solar masses. The hope is that gravitational wave detections will probe the location of the black hole mass gap and will eventually allow is to start to learn about their very massive brilliant progenitor stars.
The theory of (single) stellar evolution predicts that stars can end their lives as Black holes, but masses between about 45 and 130 times the mass of the Sun are “forbidden”. Stellar cores that could have made Black Holes with such masses either lose a lot of mass before they can implode to become a black hole or they explode and leave nothing behind. An exciting prediction that can now be tested with the new heavy black holes found through Gravitational Wave Searches.
Lieke van Son (PhD student at Harvard University and the University of Amsterdam) investigated if this is still true for a star that has a binary companion. Black holes formed in binary systems can, in principle, steal mass from their stellar companion and grow to become heavier. How heavy? Can they fill the predicted mass gap?
Group member Tom Wagg has been awarded the Leo Goldberg prize by the Harvard Astronomy Department for the best astronomy senior thesis. He estimated the detectability of black hole–neutron star binaries with LISA, a planned future mission to detect gravitational waves from space.
Tom will continue as a research fellow in the group working on gravitational waves before starting his PhD studies in 2021.
Gravitational-wave (GW) detections are now starting to probe the mass distribution of stellar-mass black holes (BHs). We investigate the predicted gap in the BH mass distribution and find that the location of the lower edge of the gap, at 45 solar masses, is remarkably robust against model assumptions and composition variations, making it the most robust predictions for the final stages of massive star evolution we have. We do find a dependency on the reaction rates, which implies that GW detections will constrain nuclear astrophysics. The robustness implies that there is a universal maximum for the location of the lower edge of the gap 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. Farmer, Renzo, de Mink et al. (2019, ApJ in press) https://ui.adsabs.harvard.edu/abs/2019arXiv191012874F Rob Farmer, the lead other, is a postdoc in my group in Amsterdam and is a visiting scientist at Harvard University.
Former Ph.D. student Ylva Götberg (now Nashman theory fellow at Carnegie observatories) estimated the relative contribution of massive stars, stars stripped in binaries and active galactic nuclei to the epoch of reionization. We estimate that stripped stars contributed tens of percent of the photons that caused cosmic reionization of hydrogen, depending on the assumed escape fractions. More importantly, stripped stars harden the ionizing emission. At high redshift, stripped stars and massive single stars combined dominate the He II-ionizing emission, but we still expect active galactic nuclei drive cosmic helium reionization.
Ylva Götberg’s third thesis paper has accepted for publication A&A. In this paper, we present predictions for the integrated spectra of stellar populations including the hard ionizing photons expected from hot stars that have lost their envelope through interaction with a binary companion. The SED’s will be made available through the STARBURST99 portal. https://arxiv.org/abs/1908.06102