Tidal Disruptions of Stars by Black Hole Remnants in Dense Star Clusters

Abstract

In a dense stellar environment, such as the core of a globular cluster (GC), dynamical interactions with black holes (BHs) are expected to lead to a variety of astrophysical transients. Here we explore tidal disruption events (TDEs) of stars by stellar-mass BHs through collisions and close encounters. Using state-of-the-art cluster simulations, we show that these TDEs occur at significant rates throughout the evolution of typical GCs and we study how their relative rates relate to cluster parameters such as mass and size. By incorporating a realistic cosmological model of GC formation, we predict a BH -main-sequence-star TDE rate of approximately 3 Gpc −3 yr −1 in the local universe (z < 0.1) and a cosmological rate that peaks at roughly 25 Gpc −3 yr −1 for redshift 3. Furthermore, we show that the ejected mass associated with these TDEs could produce optical transients of luminosity ∼ 10 41 − 10 44 erg s −1 with timescales of about a day to a month. These should be readily detectable by optical transient surveys such as the Zwicky Transient Facility. Finally, we comment briefly on BH -giant encounters and discuss how these events may contribute to the formation of BH -white-dwarf binaries.

Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star Clusters

“…In the case of extremely efficient energy release in the form of a jet, a bright X-ray or γray flare may result with the overall phenomenology possibly resembling ultra-long gamma-ray bursts (Perets et al 2016). These TDEs are expected to occur in GCs at rates of roughly 3-10 Gpc −3 yr −1 (Perets et al 2016;Lopez et al 2020;Kremer et al 2019c) and at similar rates in both nuclear star clusters (Fragione et al 2020) and in stellar triples under the influence of Lidov-Kozai oscillations (Fragione et al 2019).…”

“…BH-star encounters are expected to play a crucial role in the formation of both accreting and detached BH binaries (e.g., Ivanova et al 2010Ivanova et al , 2017Giesler et al 2018;Kremer et al 2018a) with properties similar to the BH candidates detected to date in Milky Way GCs (e.g., Kremer et al 2019a). Additionally, such dynamical encounters may occasionally cause a star to cross a BH within its tidal disruption radius, leading to a tidal disruption of the star (Perets et al 2016;Lopez et al 2020;Kremer et al 2019bKremer et al , 2019cSamsing et al 2019;Fragione et al 2020). These stellar-mass BH tidal disruption events (TDEs) may occur during close encounters of pairs of single stars (i.e., single-single interactions) and also during small-N (typically three-or four-body) resonant encounters that occur through binary-mediated dynamical interactions (e.g., Fregeau & Rasio 2007).…”

“…In order to treat stellar-mass BH TDEs, we adopt the same prescriptions as in Kremer et al (2019c). In short, if a dynamical encounter involving at least one BH and one star 6 leads to a BH-star pericenter passage, r p , within the star's tidal disruption radius…”

“…We record TDEs/collisions that occur during both single-single encounters and binary-mediated dynamical encounters that are integrated directly using Fewbody. For further details, see Fregeau & Rasio (2007) and Kremer et al (2019cKremer et al ( , 2020a.…”

“…Regardless of the dynamical pathway, these TDEs are expected to lead to transients with fast rise times of roughly a day driven by viscous accretion onto the BH (e.g., Perets et al 2016). Depending on the assumed accretion efficiency of the subsequently formed accretion disk, outflows associated with disk wind mass loss are expected to reprocess the inner-disk radiation on a timescale of a few to 10 days, leading to peak bolometric luminosities up to roughly 10 44 erg s −1 that peak in the optical (Kremer et al 2019c). In the case of extremely efficient energy release in the form of a jet, a bright X-ray or γray flare may result with the overall phenomenology possibly resembling ultra-long gamma-ray bursts (Perets et al 2016).…”