Pushing it over the edge: How to make BHs with “forbidden masses” and what they can teach us
Abstract: Gravitational-wave detections are starting to reveal the properties of the population of merging binary black holes (BBH).  Stellar theory predicts a gap in the black hole mass function between 45 and 130 Msun, referred to as the pair-instability supernovae (PISN) mass gap. This prediction of a mass gap is remarkably robust against model variations to the extent that it could be considered among the most robust predictions from massive stellar theory available today. The recently revealed properties of GW190521 are challenging this PISN mass gap.It’s total mass of ~150 Msun make it into one of the most surprising BBH merger detections to date. The median inferred masses of both BH components are within the gap, which implies either a re-writing of our understanding of PISN, or the first confirmation of more exotic double hierarchical BBH mergers. In this talk, I will explore possibilities for creating black holes with masses in the gap, and I will discuss several formation channels, ranging from dynamical interactions to isolated binary evolution including super-Eddington mass accretion onto black holes.

Bio: I am a second year graduate student at the Harvard Smithsonian Center for Astrophysics, part of the BinCosmos group of Selma de Mink which is partially situated in the Anton Pannekoek Institute at the University of Amsterdam. My work mainly focuses on gravitational wave progenitors, specifically BBHs, and the evolution of the massive stars that precede them.

Email: lieke.van.son@cfa.harvard.edu
Host: Hui Li

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Bio: Qi is a PhD candidate at the Department of Astronomy, University of Florida. He works with Dr. Desika Narayanan on cosmic dust evolution using hydrodynamic simulations. He received B.Sc. in Astronomy at University of Science and Technology of China in 2015.

Email: pg3552@ufl.edu
Host: Vogelsberger Group

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Tidal Disruption Events: Multi-messenger probes of outflows and accretion in supermassive black holes
Abstract: Supermassive black holes (SMBHs) reside in the centers of most galaxies, but the processes by which they accrete material, launch jets, and shape the galaxies around them remain poorly understood. Occasionally, a star wandering too close to a SMBH will be torn apart by tidal forces, briefly shining light on these questions as the SMBH gobbles up stellar debris. These tidal disruption events (TDEs) provide a valuable opportunity to test theories of SMBH accretion and to study the formation and growth of relativistic jets and outflows. I will present recent radio and millimeter observations of TDEs that allow us to determine the properties of outflowing material (energy, size, expansion velocity) and to trace the ambient density profile around previously-dormant SMBHs on otherwise unresolvable scales of ~0.1 – 10 pc. The increased TDE sample size now being realized by wide-field optical and X-ray surveys, together with new information from multi-messenger facilities (including the first astrophysical neutrino possibly associated with a TDE), will shed further light on the physical conditions required for jet and outflow formation and particle acceleration. Within the next few years, I expect to increase the sample of radio-detected TDEs to a truly statistical sample, via a newly approved Large Program on the Very Large Array.

Email: kate.alexander@northwestern.edu
Host: Erin Kara