[Paper Review] Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

Kyle Kremer, James C. Lombardi+3 more|arXiv (Cornell University)|Jan 28, 2022

Astrophysical Phenomena and Observations117 references43 citations

TL;DR

This study uses smoothed particle hydrodynamics (SPH) simulations to model close encounters between stellar-mass black holes and main-sequence stars across a broad range of encounter parameters. It identifies three primary outcomes—complete disruption, partial disruption with unbound debris, and tidal capture with repeated pericenter passages—and shows that these occur at comparable rates in dense clusters, with implications for electromagnetic transients and black hole mass growth.

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.

Motivation & Objective

• To systematically map the hydrodynamic outcomes of close encounters between stellar black holes and main-sequence stars across the relevant parameter space.
• To quantify the fraction of stellar material bound to the black hole and its angular momentum distribution for different encounter types.
• To assess the electromagnetic transient signatures expected from these interactions, particularly in the context of optical and X-ray observatories like the Vera Rubin Observatory.
• To evaluate the potential for black hole mass growth via accretion of disrupted stellar material, especially in dense stellar environments.
• To provide a foundation for incorporating TDE and collision physics into N-body simulations of star clusters, including future feedback and accretion effects.

Proposed method

• Conducting a comprehensive suite of smoothed particle hydrodynamics (SPH) simulations using the StarSmasher code to model black hole–main-sequence star interactions.
• Varying key parameters: black hole mass (10–30 M⊙), stellar mass (0.5–20 M⊙), polytropic index (to represent stellar structure), and pericenter distance (from head-on collisions to distant encounters).
• Simulating encounters across the full range of impact parameters, from direct collisions to tidal disruption near the classical tidal radius.
• Analyzing the final state of the system to determine bound mass, angular momentum, and ejection fraction of debris.
• Using the results to estimate accretion timescales and luminosities under standard viscous disk models and Eddington-limited accretion physics.
• Projecting event rates for different outcomes based on simulation statistics and comparing them to predictions from N-body simulations of star clusters.

Experimental results

Research questions

• RQ1 What are the dominant hydrodynamic outcomes (complete disruption, partial disruption, tidal capture, collision) in close encounters between stellar black holes and main-sequence stars?
• RQ2 How much stellar material becomes bound to the black hole, and what are the angular momentum and mass distribution of the bound debris?
• RQ3 What are the expected electromagnetic transient signatures—particularly in X-ray and optical/UV bands—associated with these events?
• RQ4 How do the rates of full disruption, partial disruption with unbound remnants, and tidal capture compare in realistic star cluster environments?
• RQ5 To what extent can accretion feedback unbind the debris, and how might this affect the observed luminosity and duration of transients?

Key findings

• Nearly head-on collisions result in complete disruption of the star, with approximately half of the stellar material becoming bound to the black hole.
• For encounters near the classical tidal disruption radius, the star is only partially disrupted on the first pericenter passage, with the remnant either becoming unbound or tidally captured.
• Tidally captured remnants undergo multiple pericenter passages before full disruption, potentially leading to repeated accretion flares on timescales of days to years.
• The total bound mass ranges from 0.05 to 0.5 M⊙ for low-mass stars, with peak luminosity estimates of ~10^44 erg/s in the X-ray band under standard accretion assumptions.
• A significant fraction of the accretion energy may be reprocessed into bright optical/UV transients via disk winds, depending on accretion physics.
• The relative rates of full disruption, partial disruption with unbound debris, and tidal capture are predicted to be comparable in the local universe, within a small factor.