Heavy binary black holes in the making: constraining the physics of chemically homogeneous evolution using gravitational waves and electro-magnetic surveys of local analogues

Harvard University, Cambridge MA

Investigators

Abstract

Gravitational wave searches are now detecting the mergers of binary black holes and neutron stars (very compact stellar remnants) at a rate of almost one per week. Arguably, the biggest surprise is the detection of very heavy binary black holes with masses more than thirty times that of the Sun, well in excess of the known black hole binaries in our galaxy. A research group at Harvard University will carry out a theoretical investigation to help understand how these heavy binary black holes form. The scientists expect to learn more about the lives and deaths of massive stars, which are important in the history of the Milky Way and other galaxies, since they produce many of the chemical elements and influence their surroundings by emitting ionizing radiation and outflows. The lead researchers also plan to modernize STEM education through the development of a hands-on computer lab that will make direct use of the data and software that result from the project, and work to increase in participation of women and minorities in theoretical and computational astrophysics, through efforts that are supported by, and complementary to, the project. The two most popular ideas to explain the how heavy black holes are made are (i) the classic common envelope scenario and (ii) the dynamical formation scenario. Both are promising in some ways, but also face serious challenges and open questions. This research project will investigate a third possibility called the chemically homogeneous scenario, in which the stars in the system are very close at birth. Such stars are tidally deformed and believed to be prone to instabilities that cause chemical mixing. This supplies the nuclear burning regions with extra fuel and leads to very compact massive helium stars, which eventually collapse into black holes. The principal investigator and a graduate student will conduct an extensive theoretical investigation of this channel. The primary aim is to provide predictions that can be used to constrain the scenario by comparison with observations of the properties of local populations of massive near-contact binaries (periods, mass ratios, and surface abundances). The secondary aim is to compare with existing and future gravitational wave observations (merger rates, distributions of masses and mass ratios, effective spins, eccentricities, and orbital variations). The team hopes that these efforts will lead to the first solid constraints or upper limits on how much this channel contributes to the massive black hole population and on the physics that drives this channel. This project advances the goals of the NSF Windows on the Universe Big Idea. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.