Alladi Ramakrishnan Hall

Star clusters: a versatile laboratory for astrophysical phenomena

Sambaran Banerjee

Argelander-Institut fur Astronomie, University of Bonn

Star clusters serve as a factory of a wide range of astrophysical phenomena by virtue of their high spatial density of stars and stellar binaries, a deep gravitational potential well, and near-isotropic stellar motion resulting in frequent interactions and collisions among stars of various evolutionary types and stellar remnants. In this talk, I shall focus on one highly-contextual aspect that interests me greatly, namely, the dynamics of stellar-remnant black holes (BH) in clusters. Such BHs in dense stellar clusters comprise a natural setup to trigger general-relativistic (GR) inspiral and merger of binary black holes (BBH), detectable by the LISA and the LIGO-Virgo, through dynamical encounters inside such environments. In my latest studies, state-of-the-art schemes for stellar wind and remnant formation are adopted in realistic models of young massive and open clusters which are evolved ab initio via state-of-the-art, relativistic, direct N-body computations. The GR BBH mergers obtained in these computations show, for the first time, the prominence in in-cluster, dynamically-formed, triple-mediated mergers. This effect, which stems from the elevated tendency to form compact subsystems in such relatively lower-mass clusters and their self-consistent post-Newtonian treatment, remains to be true over a wide variety of cluster conditions. These dynamical BBH mergers from young massive and open clusters are well consistent with the to-date LIGO-observed BBH merger events and would add up to merger rates comparable to those from massive globular and nuclear clusters. I shall also touch upon some of my other star cluster-related research of current interest, namely, dynamically-ejected runaway massive stars from R136-like starburst clusters, massive stellar-collision products, and the effects of residual gas expulsion on the structure and kinematics of newborn and young clusters.