Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

Abstract

Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various possible outcomes. In the case of nearly head-on collisions, the star is completely disrupted with roughly half of the stellar material becoming bound to the black hole. For more distant encounters near the classical tidal-disruption radius, the star is only partially disrupted on the first pericenter passage. Depending upon the interaction details, the partially disrupted stellar remnant may be tidally captured by the black hole or become unbound (in some cases, receiving a sufficiently large impulsive kick from asymmetric mass loss to be ejected from its host cluster). In the former case, the star will undergo additional pericenter passages before ultimately being disrupted fully. Based on the properties of the material bound to the black hole at the end of our simulations (in particular, the total bound mass and angular momentum), we comment upon the expected accretion process and associated electromagnetic signatures that are likely to result.

Tidal capture of stars by supermassive black holes: implications for periodic nuclear transients and quasi-periodic eruptions

“…Faber et al (2005) considered the tidal capture of a planet by a star in which the mass ratio was = 0.001, and found a minimum binding energy of ∼ 14% of the binding energy of the planet at = 10/19 ≃ 0.523 (see their Table 1). Kremer et al (2022) also recently considered black hole-star systems with mass ratios closer to unity, and found a similar effect to the one described here if the mass ratio was 0.02 or 0.05 if the star was modeled as a = 5/3 polytrope, but that the star was able to go from bound to completely disrupted -without being ejected -as the mass ratio increased beyond 0.05 and the star was modeled with the Eddington standard model (see their Figure 1). We defer an analysis of the minimum orbital energy -and the at which the minimum energy occurs -as a function of the mass ratio and the type of star to future work.…”

Stellar Escape from Globular Clusters. I. Escape Mechanisms and Properties at Ejection

“…Though much of the released gravitational energy goes into the internal energy of the collision remnant(s), asymmetric mass ejection during the collision can kick the remnant(s), and by momentum conservation, any other bodies in the interaction. Hydrodynamic simulations suggest such kicks can reach ∼10 km s −1 in starstar collisions (Gaburov et al 2010) or even up to ∼100 km s −1 in BH-star collisions (Kremer et al 2022a). In general, however, the kick speeds depend sensitively on the exact species and processes involved.…”

“…If the mass loss is asymmetric, the TDE also kicks the remnant (s). Hydrodynamic simulations suggest mass loss during the tidal capture of a star into a binary with a BH (or complete disruption of the star) can kick the new binary's center of mass (or, for full disruptions, the leftover lone BH) by ∼10-100 km s −1 (Kremer et al 2022a). The kick is typically highest for more-penetrating encounters, which cause more mass loss.…”

Light-curve Model for Luminous Red Novae and Inferences about the Ejecta of Stellar Mergers

“…Our model can also be applied to mergers of hydrogen-rich stars with compact objects, such as WDs (e.g., Shara & Shaviv 1977;Metzger et al 2021;Michaely & Shara 2021), neutron stars (NSs), or stellar-or intermediate-mass black holes (BHs; e.g., Perets et al 2016;Fragione et al 2020;Kremer et al 2021Kremer et al , 2022, which give rise to up to several solar masses of ejecta expanding at velocities of hundreds to thousands of kilometers per second. Due to the deeper potential well of the WDs/NSs/BHs, the kinetic energies of the ejecta from these events can exceed those of ordinary star mergers; however, the small ejection radii-less than the tidal disruption radius, itself typically comparable to the stellar radius-imply that most of the initial thermal energy will be lost to adiabatic expansion, as in the stellar mergers we have studied.…”