Nathan W. C. Leigh

A binary origin for 'blue stragglers' in globular clusters.

Blue stragglers in globular clusters are abnormally massive stars that should have evolved off the stellar main sequence long ago. There are two known processes that can create these objects: direct stellar collisions and binary evolution. However, the relative importance of these processes has remained unclear. In particular, the total number of blue stragglers found in a given cluster does not seem to correlate with the predicted collision rate, providing indirect support for the binary-evolution model. Yet the radial distributions of blue stragglers in many clusters are bimodal, with a dominant central peak: this has been interpreted as an indication that collisions do dominate blue straggler production, at least in the high-density cluster cores. Here we report that there is a clear, but sublinear, correlation between the number of blue stragglers found in a cluster core and the total stellar mass contained within it. From this we conclude that most blue stragglers, even those found in cluster cores, come from binary systems. The parent binaries, however, may themselves have been affected by dynamical encounters. This may be the key to reconciling all of the seemingly conflicting results found to date.

An Analytic Technique for Constraining the Dynamical Origins of Multiple Star Systems Containing Merger Products

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 could be a significant contributor to BS 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.

MOCCA code for star cluster simulations – IV. A new scenario for intermediate mass black hole formation in globular clusters

We discuss a new scenario for the formation of intermediate mass black holes (IMBHs) in dense star clusters. In this scenario, IMBHs are formed as a result of dynamical interactions of hard binaries containing a stellar-mass black hole (BH), with other stars and binaries. We discuss the necessary conditions to initiate the process of intermediate mass BH formation and the influence of an IMBH on the host global globular cluster (GC) properties. We discuss two scenarios for IMBH formation. The SLOW and FAST scenarios. They occur later or earlier in the cluster evolution and require smaller or extremely large central densities, respectively. In our simulations, the formation of IMBHs is highly stochastic. In general, higher formation probabilities follow from larger cluster concentrations (i.e. central densities). We further discuss possible observational signatures of the presence of IMBHs in GCs that follow from our simulations. These include the spatial and kinematic structure of the host cluster, possible radio, X-ray and gravitational wave emissions due to dynamical collisions or mass transfer and the creation of hypervelocity main-sequence escapers during strong dynamical interactions between binaries and an IMBH. All simulations discussed in this paper were performed with the MOCCA (MOnte Carlo Cluster simulAtor) Monte Carlo code. MOCCA accurately follows most of the important physical processes that occur during the dynamical evolution of star clusters but, as with other dynamical codes, it approximates the dissipative processes connected with stellar collisions and binary mergers.

Masses and scaling relations for nuclear star clusters, and their co-existence with central black holes

Galactic nuclei typically host either a Nuclear Star Cluster (NSC, prevalent in galaxies with masses

The state of globular clusters at birth II: primordial binaries

\lesssim 10^{10}M_\odot

The dynamical significance of triple star systems in star clusters

) or a Massive Black Hole (MBH, common in galaxies with masses

The state of globular clusters at birth: emergence from the gas-embedded phase

\gtrsim 10^{12}M_\odot

The effect of orbital eccentricity on the dynamical evolution of star clusters

). In the intermediate mass range, some nuclei host both a NSC and a MBH. In this paper, we explore scaling relations between NSC mass (

An analytic model for blue straggler formation in globular clusters

{\cal M}_{\rm NSC}

Back to the Future: Estimating Initial Globular Cluster Masses from their Present Day Stellar Mass Functions

) and host galaxy total stellar mass (