Impact of companion and surrounding environment on stars and black holes

Abstract

Stars and their compact remnants rarely exist in isolation. Instead, most stars are part of binary or higher multiplicity systems, engaging in interactions throughout their lifetimes. This interaction may continue to the stage when the initially more massive star transforms into a compact remnant, subsequently interacting with its companion star. When both stars eventually die, they might leave behind a compact binary (e.g., a binary black hole system), which then continues to interact with its surrounding environment, a process that may persist for billions of years. This thesis explores the impact and implications of such interactions on stellar and compact binary evolution. We first consider the impact of interaction between binary stars on their evolution. Such an interaction - either in the form of tides or mass accretion - may spin up the star thus altering its subsequent evolution. We study the efficiency of such interactions in spinning up the stars and show that such stars may be important progenitors of energetic transients like superluminous supernovae, hypernovae, and γ-ray bursts. We then study the impact of this angular momentum reservoir on the star's eventual collapse dynamics with a focus on the ensuing black hole properties. Next, we explore the impact of mass accretion on neutron stars and black holes, quantifying the possibility of sustained and unimpeded super-Eddington accretion on such compact objects. Then we consider the impact of interstellar medium interaction on binary black hole dynamics. We quantify the properties of the interstellar medium properties under which it can lead to efficient hardening of binary black hole orbit, thus acting as an important pathway for driving LIGO-Virgo-KAGRA black hole mergers. Finally, as an extension of this formalism, we also consider the impact of the recent cosmological coupling hypothesis on binary black hole dynamics. We find that cosmologically coupled black holes that can account for the accelerated expansion of the present-day Universe would lead to severe inconsistency with pre-existing observations.