NSF-BSF: Stellar Collisions in Extreme Environments

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In the center of the Milky Way, stellar populations are orders of magnitude denser than in the Solar neighborhood, leading to high-velocity stellar collisions. The same violent events occur in the centers of other galaxies. Metzger and Stone will study these collisions using computer simulations. They will study: (1) tidal disruption event (TDE) impostors, where a star-star collision mimics a true TDE event (where a star passes close to a supermassive black hole (SMBH) and is pulled apart by tidal forces); (2) collisions in an accretion disk around a SMBH, and resulting flares in the disk, and (3) how to use the rate of collisions to predict how many gravitational wave sources are likely to be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO). This project is supported by both the National Science Foundation and the US-Israel Binational Science Foundation to further collaborations between the astrophysical communities in the two countries. Metzger and Stone will recruit graduate students and postdoctoral scholars to attend a week-long international summer school on time-domain gravitational wave astrophysics.The simulations will be used to precisely quantify the hydrodynamic outcomes and electromagnetic signatures of 1) grazing, hypersonic star-star collisions: a new type of transient first proposed by Metzger and Stone; 2) hypersonic star – AGN disk collisions, which holds significant promise for explaining the extreme tail of variability, i.e., changing-look AGN. The predictions made from this work will directly inform cutting-edge time-domain surveys, e.g., ASASSN, ZTF, and eROSITA. Related analytic work will simulate and quantify a wide range of “collision” scenarios to further our understanding of the formation of gravitational wave sources in dense star clusters. Modifications to continuum-limit, “n-sigma-v” collision rate prescriptions could strongly affect the plausibility of globular and nuclear star clusters for LIGO/Virgo source production.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.