An Overview of Compact Star Populations and Some of Its Open Problems

The study of compact object populations has come a long way since the determination of the mass of the Hulse-Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects from binary mergers detected from gravitational-waves by the Ligo-Virgo-KAGRA Collaboration. With a growing understanding of the variety of systems that host these objects, their formation, evolution and frequency, we are now in a position to evaluate the statistical nature of these populations, their properties, parameter correlations and long-standing problems, such as the maximum mass of neutron stars and the black hole lower mass gap, to a reasonable level of statistical significance. Here, we give an overview of the evolution and current state of the field and point to some of its standing issues. We focus on Galactic black holes, and offer an updated catalog of 35 black hole masses and orbital parameters, as well as a standardized procedure for dealing with uncertainties.
39 pages, 12 figures, 3 tables. Published in the Galaxies Special Issue "Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations. Quo Vadis?"