Binary Compact Objects and their Progenitors

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Heat from nuclear reactions inside stars creates pressure that prevents the star from collapsing under its own gravity. However, all stars will ultimately run out of nuclear fuel. When this happens, the star collapses under its own weight and the core will form a very dense, compact remnant. Depending on the mass of the star, the remnant will either be a black hole, neutron star or white dwarf. Many stars are formed in binary systems – two stars that are gravitationally bound to each other. If both stars in a binary evolve to form compact objects, the resulting system is known as a “compact object binary”. Compact object binaries that in-spiral and merge have been observed in recent sky surveys as bright transients and detected as sources of gravitational waves. A team from the California Institute of Technology (Caltech) will study the formation pathways of these compact binary systems. Their results will constrain the physical processes affecting the evolution of these systems, which determine their gravitational wave and electromagnetic signatures. The team will work on a variety of projects designed to retain under-represented minority (URM) undergraduates in STEM fields. In particular, the Principal Investigator (PI) will mentor URM undergraduate and graduate students from Caltech and neighboring institutions. The PI will also participate in Caltech's new FUTURE Ignited program designed to help students of color to prepare for graduate school, including help with the application process and research opportunities at Caltech. Finally, the PI will participate in several public outreach events. The team will use advanced computational techniques to model the formation pathways of binary compact objects. They will focus on understanding how orbital angular momentum (AM) is lost to enable the two remnants to in-spiral and merge. In particular, they will: (1) Perform evolutionary calculations for binary White Dwarf (WD) models with a range of WD masses, initial orbital periods, and hydrogen shell masses to determine the post common envelope conditions of observed WD binaries. (2) Implement extra AM loss via the Lagrangian-2 (L2) point into binary stellar evolution models for a range of progenitor mass, companion mass, and initial orbital period to account for the fact that material can escape through the higher Lagrangian points if the mass transfer rate is high enough. (3) Run binary models that include helium (Case BB) transfer, including extra AM loss due to L2 mass loss. (4) Model the light curves of heavily stripped type Ib/c supernovae produced by stars undergoing Case BB helium mass transfer.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.