Double Compact Objects. I. The Significance of the Common Envelope on Merger Rates

Abstract

The last decade of observational and theoretical developments in stellar and binary evolution provides an opportunity to incorporate major improvements to the predictions from populations synthesis models. We compute the Galactic merger rates for NS-NS, BH-NS, and BH-BH mergers with the StarTrack code. The most important revisions include: updated wind mass loss rates (allowing for stellar mass black holes up to 80 M ⊙ ), a realistic treatment of the common envelope phase (a process that can affect merger rates by 2-3 orders of magnitude), and a qualitatively new neutron star/black hole mass distribution (consistent with the observed "mass gap"). Our findings include: (i) The binding energy of the envelope plays a pivotal role in determining whether a binary merges within a Hubble time. (ii) Our description of natal kicks from supernovae plays an important role, especially for the formation of BH-BH systems. (iii) The masses of BH-BH systems can be substantially increased in the case of low metallicities or weak winds. (iv) Certain combinations of parameters underpredict the Galactic NS-NS merger rate, and can be ruled out. (v) Models incorporating delayed supernovae do not agree with the observed NS/BH "mass gap", in accordance with our previous work. This is the first in a series of three papers. The second paper will study the merger rates of double compact objects as a function of redshift, star formation rate, and metallicity. In the third paper we will present the detection rates for gravitational wave observatories, using up-to-date signal waveforms and sensitivity curves.

Accessibility of the gravitational-wave background due to binary coalescences to second and third generation gravitational-wave detectors

“…The factor (1 + z f ) in the denominator corrects for the time dilation due to the cosmic expansion and converts the rate from the source frame into the detector frame. Population synthesis [34][35][36][37][38][39][40][41] suggests that the probability distribution for the delay time is well described by…”

“…We will assume the following ranges for M c : 1-2.5 M ⊙ for BNS, 2.5-10 M ⊙ for the BHNS, and 2.5-20 M ⊙ for the BBH models. These mass ranges include the average chirp masses obtained in population synthesis models [41], and allow for uncertainties in possible neutron star and black hole masses. For the BBH case, however, we will use the more complex functional form derived by [14] and used by [28], which includes the inspiral, merger, and ringdown contributions to the gravitational-wave signal (see [28] for more detail).…”

Enrichment of r-Process Elements by Neutron Star Mergers through the Sub-Halo Clustering

“…Binary population synthesis models indicate an average t NSM is ∼ 1 Gyr, while a significant fraction of NSMs is expected to have t NSM ≲ 100 Myr [4]. In addition, a short-lived channel of t NSM ≲ 1 Myr is also predicted, though the fraction of a short-lived channel is estimated to be up to ∼ 7% [4]. Thus, we assume here a bimodal distribution of t NSM = 1 Myr and 100 Myr and with the corresponding fractions of 5% and 95%, respectively.…”

“…In fact, a number of recent nucleosynthesis studies based on hydrodynamical simulations support NSMs to be the promising sources of r-process elements in the Galaxy [3]. On the other hand, previous GCE models appear to disfavor the NSM scenario, because estimates of binary lifetimes, t NSM ∼ 0.1-1 Gyr [4], are in conflict with the presence of high [r/Fe] stars with [Fe/H]≲ −2.5 in those models [5].…”

Rapid Mergers in a Mixed System of Black Holes and Neutron Stars

“…To reproduce the distribution of r-process elements ([Eu/Fe]) of metal poor stars in the Galactic halo, a very short timescale (1-10 Myr) of NSMs is required [3]. On the other hand, the timescale of NSMs is evaluated to be 0.1-1 Gyr from the study of the population synthesis [4]. Besides, the timescale of NSMs is longer than 100 Myr from the observations of binary pulsars [5].…”