Population synthesis of hot-subdwarf B stars with COMPAS: parameter variations and a prescription for hydrogen-rich shells

Abstract: Subdwarf B stars are a well-known class of hot, low-mass stars thought to be formed through interactions in stellar binary systems. While different formation channels for subdwarf B stars have been studied through a binary population synthesis approach, it has also become evident that the characteristics of the found populations depend on the initial set of assumptions that describe the sometimes poorly constrained physical processes, such as common envelope episodes or angular momentum loss during mass transfer events. In this work we present a parameter study of subdwarf B populations, including a novel analytic prescription that approximates the evolution of subdwarf B stars with hydrogen-rich outer shells, an element previously overlooked in rapid binary population synthesis. We find that all studied parameters strongly impact the properties of the population, with the possibility of igniting helium below the expected core-mass value near the tip of the red giant branch strongly affecting the total number of subdwarf B candidates. Critically, our newly proposed prescription for the evolution of subdwarf B stars with hydrogen-shells helps to reconcile theoretical predictions of surface gravity and effective temperature with observational results. Our prescription is useful in the context of rapid binary population synthesis studies and can be applied to other rapid binary population synthesis codes' output.