Nuclear Star Clusters and the Stellar Spheroids of their Host Galaxies

(Abridged) We combine published photometry for the nuclear star clusters (NSCs) and stellar spheroids of 51 low-mass, early-type galaxies in the Virgo cluster with empirical mass-to-light ratios, in order to complement previous studies that explore the dependence of NSC masses (M_{NSC}) on various properties of their host galaxies. We confirm a roughly linear relationship between M_{NSC} and luminous host spheroid mass (M_{Sph}), albeit with considerable scatter (0.57 dex). We estimate velocity dispersions from the virial theorem, assuming all galaxies in our sample share a common DM fraction and are dynamically relaxed. We then find that M_{NSC} \sim \sigma^{2.73\pm 0.29}, with a slightly reduced scatter of 0.54 dex. This confirms recent results that the shape of the M_{CMO} - \sigma relation is different for NSCs and super-massive black holes (SMBHs). We discuss this result in the context of the generalized idea of "central massive objects" (CMOs). In order to assess which physical parameters drive the observed NSC masses, we also carry out a joint multi-variate power-law fit to the data. In this, we allow M_{NSC} to depend on M_{Sph} and R_{Sph} (and hence implicitly on \sigma), as well as on the size of the globular cluster reservoir. When considered together, the dependences on M_{Sph} and R_{Sph} are roughly consistent with the virial theorem, and hence M_{NSC} \propto \sigma^2. However, the only statistically significant correlation is a linear scaling between M_{NSC} and M_{Sph}. We compare M_{NSC} with predictions for two popular models for NSC formation, namely i) globular cluster infall due to dynamical friction, and ii) in-situ formation during the early phases of galaxy formation that is regulated via momentum feedback from stellar winds and/or supernovae. Neither model can directly predict the observations, and we discuss possible interpretations of our findings.Comment: 10 pages, 2 tables, 6 figures; accepted for publication in MNRAS; edited to match published versio

Stellar Encounters with Multiple Star Systems and the Blue Straggler Problem

We present a technique to identify the most probable dynamical formation scenario for an observed binary or triple system containing one or more merger products or, alternatively, to rule out the possibility of a dynamical origin. Our method relies on an analytic prescription for energy conservation during stellar encounters. With this, observations of the multiple star system containing the merger product(s) can be used to work backwards in order to constrain the initial orbital energies of any single, binary or triple systems that went into the encounter. The initial semi-major axes of the orbits provide an estimate for the collisional cross section and therefore the time-scale for the encounter to occur in its host cluster. We have applied our analytic prescription to observed binary and triple systems containing blue stragglers, in particular the triple system S1082 in M67 and the period distribution of the blue straggler binaries in NGC 188. We have shown that both S1082 and most of the blue straggler binaries in NGC 188 could have a dynamical origin, and that encounters involving triples are a significant contributor to blue straggler populations in old open clusters. In general, our results suggest that encounters involving triples could make up a significant fraction of those dynamical interactions that result in stellar mergers, in particular encounters that produce multiple star systems containing one or more blue stragglers.Comment: 8 pages, 2 figures; to be published in the proceedings of the Binary Star Evolution Conference held in Mykonos, Greece from June 22-25, 201

Tidal disruption events onto stellar black holes in triples

Stars passing too close to a black hole can produce tidal disruption events (TDEs), when the tidal force across the star exceeds the gravitational force that binds it. TDEs have usually been discussed in relation to massive black holes that reside in the centres of galaxies or lurk in star clusters. In this paper, we investigate the possibility that triple stars hosting a stellar black hole (SBH) may be sources of TDEs. We start from a triple system made up of three main sequence (MS) stars and model the supernova (SN) kick event that led to the production of an inner binary comprised of a SBH. We evolve these triples in isolation with a high precision $N$-body code and study their TDEs as a result of Kozai-Lidov oscillations. We explore a variety of distributions of natal kicks imparted during the SN event, various maximum initial separations for the triples, and different distributions of eccentricities. We show that the main parameter that governs the properties of the SBH-MS binaries which produce a TDE in triples is the mean velocity of the natal kick distribution. Smaller $\sigma$'s lead to larger inner and outer semi-major axes of the systems that undergo a TDE, smaller SBH masses, and longer timescales. We find that the fraction of systems that produce a TDE is roughly independent of the initial conditions, while estimate a TDE rate of $7.3\times 10^{-5}-4.1 \ \mathrm{yr}^{-1}$, depending on the prescriptions adopted for the SBH natal kicks. This rate is almost comparable to the expected TDE rate for massive black holes.Comment: 12 pages, 7 figures, 1 table, accepted by MNRAS. arXiv admin note: text overlap with arXiv:1903.1051