Dormant black holes and neutron stars in stellar binaries

Black holes (BHs) are perhaps the most bizarre prediction of Einstein's theory of general relativity: regions of spacetime where gravity is so strong that nothing, not even light, can escape. Models predict that more than 100 million BHs exist in the Milky Way, yet only about 20 have been discovered to date. This proposal seeks to increase that number by using telescopes on the ground, data from the Gaia spacecraft, and novel data analysis methods to detect BHs via their gravitation effects on stars that orbit them. A research group at the California Institute of Technology will carry out the investigation using a variety of observational and theoretical tools developed by that group. Along the way, the researchers also expect to discover an unprecedented sample of neutron stars -- close cousins of BHs with lower masses -- orbited by stars like the Sun. The ultimate goal is a census of our Galaxy's BH and neutron star population, which will ultimately improve our understanding of the physics of stellar evolution, supernovae, and compact object formation. A majority of the research will be carried out by a PhD student and will form the central pillar of that student's thesis. The group will also develop open-source material for undergraduate labs centered around orbital dynamics. The PI will continue to give public talks and work with the Future Ignited program at Caltech to support students who are members of under-represented minorities in STEM (for example, Black, African-American, Native American and Pacific Islanders). The team will use a range of ground- and space-based facilities to discover and characterize the first statistical sample of dormant black holes (BHs) and neutron stars (NSs) in binaries with widely separated stellar companions. Candidates will be selected from the recent 3rd data release of the Gaia mission, which provides orbital solutions for an unprecedented 350,000 astrometric and spectroscopic binaries. The dataset represents a factor of 50 increase in sample size over all previous work and thus provides novel opportunities to search for rare objects. The group will use ground-based telescopes to follow-up the best candidates spectroscopically. The main goals of the program are to (1) obtain spectroscopy and multi-epoch radial velocities for all ~100 binaries in the Gaia sample with high probability of having BH or NS components; (2) measure the mass distribution of BHs and NSs from 1 to 15 solar masses, detecting or ruling out the mass gap; (3) constrain the frequency and magnitude of natal kicks for BHs and NSs from binary eccentricities and separations; and (4) detect or rule out the presence of unseen tertiaries within binaries containing BHs. 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.