Description:

 Elena Prieto, PhD Student, Rasio Group

"Formation of Intermediate-mass Black Holes in Star Clusters"

The existence of black holes (BHs) with masses in the range between stellar remnants and supermassive BHs has only recently become unambiguously established. GW190521, a gravitational wave signal detected by the LIGO/ Virgo Collaboration, provides the first direct evidence for the existence of such intermediate-mass BHs (IMBHs). This event sparked and continues to fuel discussion on the possible formation channels for such massive BHs. As the detection revealed, IMBHs can form via binary mergers of BHs in the “upper mass gap” (≈40–120 solar masses). Alternatively, IMBHs may form via the collapse of a very massive star formed through stellar collisions and mergers in dense star clusters. In this study, we explore the formation of IMBHs with masses between 120 and 500 solar masses in young, massive star clusters using state-of-the-art Cluster Monte Carlo models. To study the dependence of IMBH formation on initial cluster properties, we have created a grid of over a hundred simulations that explores the relationship between initial number of objects, cluster viral radius, high-mass binary fraction and the slope of the Initial Mass Function. The results from this study will give us insight about the regime in parameter space where we expect to form “small” IMBHs as opposed to forming a very massive stars in a runaway fashion. Furthermore, we will be able to understand better the interplay between the different primordial physical parameters and their effect on the initial evolutionary stages of the star cluster. 

Adrian Thompson, Postdoctoral Scholar, de Gouvea Group

"Insights into Dark Matter and Baryogenesis from Binary Pulsar Data"

We explore baryon number violating interactions (BNV) using a simple model involving a charged iso-singlet, color-triplet scalar and a Majorana fermion with interactions in the quark sector. This model has been useful for explaining baryogenesis and the DM-baryon coincidence puzzle. We revisit this model, with chiral perturbation theory as a guide, at the level of baryons and mesons in the dense environments of neutron stars and binary pulsar systems. BNV neutron decays become accessible in this environment where in vacuum they would be kinematically forbidden. By considering several equations of state in binary pulsar candidates, we establish strong constraints on the model parameter space from these decays, and the subsequent scattering of the Majorana fermions, in total amounting to a ΔB=2 loss in the star. These limits are highly complementary to a number of terrestrial probes, from neutron-anti-neutron oscillations and di-proton decay to collider searches. Lastly, we suggest a new DM direct detection channel.