57 research outputs found
The influence of the distribution of cosmic star formation at different metallicities on the properties of merging double compact objects
Binaries that merge within the local Universe originate from progenitor
systems that formed at different times and in various environments. The
efficiency of formation of double compact objects is highly sensitive to
metallicity of the star formation. Therefore, to confront the theoretical
predictions with observational limits resulting from gravitational waves
observations one has to account for the formation and evolution of progenitor
stars in the chemically evolving Universe. In particular, this requires
knowledge of the distribution of cosmic star formation rate at different
metallicities and times, probed by redshift (SFR(Z,z)). We investigate the
effect of the assumed SFR(Z,z) on the properties of merging double compact
objects, in particular on their merger rate densities. Using a set of binary
evolution models from Chruslinska et al. (2018) we demonstrate that the
reported tension between the merger rates of different types of double compact
objects and current observational limits in some cases can be resolved if a
SFR(Z,z) closer to that expected based on observations of local star-forming
galaxies is used, without the need for changing the assumptions about the
evolution of progenitor stars of different masses. This highlights the
importance of finding tighter constraints on SFR(Z,z) and understanding the
associated uncertainties.Comment: 6 pages, 4 figures, resubmitted to MNRAS after minor revisio
Double neutron stars: merger rates revisited
We revisit double neutron star (DNS) formation in the classical binary evolution scenario in light of the recent Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo DNS detection (GW170817). The observationally estimated Galactic DNS merger rate of RMW = 21+28 −14 Myr−1, based on three Galactic DNS systems, fully supports our standard input physics model with RMW = 24 Myr−1. This estimate for the Galaxy translates in a non-trivial way (due to cosmological evolution of progenitor stars in chemically evolving Universe) into a local (z ≈ 0) DNS merger rate density of Rlocal = 48 Gpc−3 yr−1, which is not consistent with the current LIGO/Virgo DNS merger rate estimate (1540+3200 −1220 Gpc−3 yr−1). Within our study of the parameter space, we find solutions that allow for DNS merger rates as high as Rlocal ≈ 600+600 −300 Gpc−3 yr−1 which are thus consistent with the LIGO/Virgo estimate. However, our corresponding BH–BH merger rates for the models with high DNS merger rates exceed the current LIGO/Virgo estimate of local BH–BH merger rate (12–213 Gpc−3 yr−1). Apart from being particularly sensitive to the common envelope treatment, DNS merger rates are rather robust against variations of several of the key factors probed in our study (e.g. mass transfer, angular momentum loss, and natal kicks). This might suggest that either common envelope development/survival works differently for DNS (∼10–20M stars) than for BH–BH (∼40–100M stars) progenitors, or high black hole (BH) natal kicks are needed to meet observational constraints for both types of binaries. Our conclusion is based on a limited number of (21) evolutionary models and is valid within this particular DNS and BH–BH isolated binary formation scenario
Impact of inter-correlated initial binary parameters on double black hole and neutron star mergers
The distributions of the initial main-sequence binary parameters are one of
the key ingredients in obtaining evolutionary predictions for compact binary
(BH-BH / BH-NS / NS-NS) merger rates. Until now, such calculations were done
under the assumption that initial binary parameter distributions were
independent. Here, we implement empirically derived inter-correlated
distributions of initial binary parameters primary mass (M1), mass ratio (q),
orbital period (P), and eccentricity (e). Unexpectedly, the introduction of
inter-correlated initial binary parameters leads to only a small decrease in
the predicted merger rates by a factor of 2 3 relative to the previously
used non-correlated initial distributions. The formation of compact object
mergers in the isolated classical binary evolution favors initial binaries with
stars of comparable masses (q = 0.5 1) at intermediate orbital periods (log
P (days) = 2 4). New distributions slightly shift the mass ratios towards
smaller values with respect to the previously used flat q distribution, which
is the dominant effect decreasing the rates. New orbital periods only
negligibly increase the number of progenitors. Additionally, we discuss the
uncertainty of merger rate predictions associated with possible variations of
the massive-star initial mass function (IMF). We argue that evolutionary
calculations should be normalized to a star formation rate (SFR) that is
obtained from the observed amount of UV light at wavelength 1500{\AA} (SFR
indicator). In this case, contrary to recent reports, the uncertainty of the
IMF does not affect the rates by more than a factor of 2. Any change to the IMF
slope for massive stars requires a change of SFR in a way that counteracts the
impact of IMF variations on the merger rates. In contrast, we suggest that the
uncertainty in cosmic SFR at low metallicity can be a significant factor at
play.Comment: accepted for publication in A&
The impact of the FMR and starburst galaxies on the (low metallicity) cosmic star formation history
The question how much star formation is occurring at low metallicity throughout the cosmic history appears crucial for the discussion of the origin of various energetic transients, and possibly double black hole mergers. We revisit the observation-based distribution of birth metallicities of stars (fSFR(Z,z)), focusing on several factors that strongly affect its low metallicity part: (i) the method used to describe the metallicity distribution of galaxies (redshift-dependent mass metallicity relation - MZR, or redshift-invariant fundamental metallicity relation - FMR), (ii) the contribution of starburst galaxies and (iii) the slope of the MZR. We empirically construct the FMR based on the low-redshift scaling relations, which allows us to capture the systematic differences in the relation caused by the choice of metallicity and star formation rate (SFR) determination techniques and discuss the related fSFR(Z,z) uncertainty. We indicate factors that dominate the fSFR(Z,z) uncertainty in different metallicity and redshift regimes. The low metallicity part of the distribution is poorly constrained even at low redshifts (even a factor of ∼200 difference between the model variations) The non-evolving FMR implies a much shallower metallicity evolution than the extrapolated MZR, however, its effect on the low metallicity part of the fSFR(Z,z) is counterbalanced by the contribution of starbursts (assuming that they follow the FMR). A non-negligible fraction of starbursts in our model may be necessary to satisfy the recent high-redshift SFR density constraints
The origin of the first neutron star -- neutron star merger
The first neutron star-neutron star (NS-NS) merger was discovered on August
17, 2017 through gravitational waves (GW170817) and followed with
electromagnetic observations. This merger was detected in an old elliptical
galaxy with no recent star formation. We perform a suite of numerical
calculations to understand the formation mechanism of this merger. We probe
three leading formation mechanisms of double compact objects: classical
isolated binary star evolution, dynamical evolution in globular clusters and
nuclear cluster formation to test whether they are likely to produce NS-NS
mergers in old host galaxies. Our simulations with optimistic assumptions show
current NS-NS merger rates at the level of 10^-2 yr^-1 from binary stars, 5 x
10^-5 yr^-1 from globular clusters and 10^-5 yr^-1 from nuclear clusters for
all local elliptical galaxies (within 100 Mpc^3). These models are thus in
tension with the detection of GW170817 with an observed rate 1.5 yr^-1 (per 100
Mpc^3; LIGO/Virgo estimate). Our results imply that either (i) the detection of
GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is
a statistical coincidence; or that (ii) physics in at least one of our three
models is incomplete in the context of the evolution of stars that can form
NS-NS mergers; or that (iii) another very efficient (unknown) formation channel
with a long delay time between star formation and merger is at play.Comment: A&A: accepte
It has to be cool: on supergiant progenitors of binary black hole mergers from common-envelope evolution
Common-envelope (CE) evolution in massive binary systems is thought to be one
of the most promising channels for the formation of compact binary mergers. In
the case of merging binary black holes (BBHs), the essential CE phase takes
place at a stage when the first BH is already formed and the companion star
expands as a supergiant. We study which BH binaries with supergiant companions
will evolve through and potentially survive a CE phase. To this end, we compute
envelope binding energies from detailed massive stellar models at different
evolutionary stages and metallicities. We make multiple physically extreme
choices of assumptions that favor easier CE ejection as well as account for
recent advancements in mass transfer stability criteria. We find that even with
the most optimistic assumptions, a successful CE ejection in BH (and also NS)
binaries is only possible if the donor is a massive convective-envelope giant,
a red supergiant (RSG). In other words, pre-CE progenitors of BBH mergers are
BH binaries with RSG companions. We find that due to its influence on the
radial expansion of massive giants, metallicity has an indirect but a very
strong effect on the envelope structure and binding energies of RSGs. Our
results suggest that merger rates from population synthesis models could be
severely overestimated, especially at low metallicity. Additionally, the lack
of observed RSGs with luminosities above log() = 5.6-5.8,
corresponding to stars with , puts into question the
viability of the CE channel for the formation of the most massive BBH mergers.
Either such RSGs elude detection due to very short lifetimes, or they do not
exist and the CE channel can only produce BBH systems with total mass . We discuss an alternative CE scenario, in which a partial envelope
ejection is followed by a phase of possibly long and stable mass transfer.Comment: 20 pages + App., accepted for publication in A&A. For fits, see: https://ftp.science.ru.nl/astro/jklencki
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