Mass from a third star: transformations of close compact-object binaries within hierarchical triples

Abstract

ABSTRACT Close-orbit binaries consisting of two compact objects are a centre of attention because of the detection of gravitational-radiation-induced mergers. The creation of close, compact-object binaries involves physical processes that are not yet well understood; there are open questions about the manner in which two compact objects come to be close enough to merge within a Hubble time. Here, we explore an important, and likely common physical process: mass transfer from a third star in a wider, hierarchical orbit. Mass added to the close binary’s components can reduce the time to merger and can even change the nature of an accretor, transforming a white dwarf to a neutron star and/or a neutron star to a black hole. Some accreting WDs in close binaries may even explode as Type Ia supernovae. Given the ubiquity of higher order multiples, the evolutionary channels we lay out may be important pathways to gravitational mergers, including Type Ia supernovae. Fortunately, these pathways also lead to testable predictions.