Constraining Compact Object Formation with 2M0521

We show that the recently discovered binary 2M05215658+4359220 (2M0521), comprising a giant star (GS) orbiting a suspected black hole (BH) in an ∼80 day orbit, may be instrumental in shedding light on uncertain BH-formation physics and can be a test case for studying wind accretion models. Using binary population synthesis with a realistic prescription for the star formation history and metallicity evolution of the Milky Way, we analyze the evolution of binaries containing compact objects (COs) in orbit around GSs with properties that are similar to 2M0521. We find ∼102–103 CO–GS binaries in the Milky Way observable by Gaia and 0–12 BH–GS and 0–1 neutron star–GS binaries in the Milky Way with properties that are similar to 2M0521. We find that all CO–GSs with Porb < 5 yr, including 2M0521, go through a common envelope (CE), and hence form a class of higher-mass analogs to white dwarf post-CE binaries. We further show how the component masses of 2M0521-like binaries depend strongly on the supernova-engine model that we adopt. Thus, an improved measurement of the orbit of 2M0521, imminent with Gaia's third data release, will strongly constrain its component masses and as a result inform supernova-engine models that are widely used in binary population synthesis studies. These results have widespread implications for the origins and properties of CO binaries, especially those detectable by the Laser Interferometer Gravitational-Wave Observatory and the Laser Interferometer Space Antenna. Finally, we show that the reported X-ray nondetection of 2M0521 is a challenge for wind accretion theory, making 2M0521-like CO–GS binaries a prime target for further study with accretion models.