Authors: Todd A. Thompson
Date: 18 Nov 2010
Abstract: The mechanism of Type Ia supernovae and gamma-ray bursts (GRBs) is unknown, but a subset of both may be due to white dwarf-white dwarf (WD-WD) and neutron star-neutron star (NS-NS) mergers, respectively. A general problem with this picture is the production of binaries with semi-major axes small enough to merge via gravitational wave (GW) emission in significantly less than the Hubble time (t_H), and thus accommodate the observation that these events closely follow episodes of star formation in time. I explore the possibility that such systems are not binaries at all, but actually coeval, or dynamical formed, hierarchical triple systems. The tertiary induces Kozai oscillations in the inner binary, driving it to high eccentricity, and dramatically reducing its GW merger timescale. This effect significantly increases the allowed range of binary period P such that the merger time is t_merge < t_H. I find that Chandrasehkar mass binaries with P as large as ~300 days can in fact merge in < t_H if they contain a prograde solar-mass tertiary at high enough inclination. For systems with retrograde tertiaries, the allowed range of P such that t_merge < t_H is yet larger. In contrast, P < 0.3 days is required in the absence of a tertiary. I discuss implications of these findings for the production of Ia supernovae via WD-WD mergers, as well as GRBs formed via binary mergers composed of NSs, black holes, and WDs. Based on the statistics of solar-type binaries, I argue that nearly many tight WD-WD binaries should be in triple systems affected by the Kozai resonance. In analogy, the tightest NS-NS binaries may also have formed in triples. If true, expectations for the mHz GW signal from individual sources, the diffuse background, and the foreground for GW experiments like LISA are modified. This work motivates future studies of the triple fraction of intermediate mass A/B stars and massive O stars.
© M. Vallisneri 2012 — last modified on 2010/01/29
Tantum in modicis, quantum in maximis