41 research outputs found
The cosmic merger rate of neutron stars and black holes
Six gravitational wave detections have been reported so far, providing
crucial insights on the merger rate of double compact objects. We investigate
the cosmic merger rate of double neutron stars (DNSs), neutron star-black hole
binaries (NSBHs) and black hole binaries (BHBs) by means of
population-synthesis simulations coupled with the Illustris cosmological
simulation. We have performed six different simulations, considering different
assumptions for the efficiency of common envelope (CE) ejection and exploring
two distributions for the supernova (SN) kicks. The current BHB merger rate
derived from our simulations spans from to Gpc
yr and is only mildly dependent on CE efficiency. In contrast, the
current merger rates of DNSs (ranging from to Gpc
yr) and NSBHs (ranging from to Gpc
yr) strongly depend on the assumptions on CE and natal kicks. The merger
rate of DNSs is consistent with the one inferred from the detection of GW170817
only if a high efficiency of CE ejection and low SN kicks (drawn from a
Maxwellian distribution with one dimensional root mean square km
s) are assumed.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in MNRA
The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks
Six gravitational wave events have been reported by the LIGO-Virgo
collaboration (LVC), five of them associated with black hole binary (BHB)
mergers and one with a double neutron star (DNS) merger, while the coalescence
of a black hole-neutron star (BHNS) binary is still missing. We investigate the
progenitors of double compact object binaries with our population-synthesis
code MOBSE. MOBSE includes advanced prescriptions for mass loss by stellar
winds (depending on metallicity and on the Eddington ratio) and a formalism for
core-collapse, electron-capture and (pulsational) pair instability supernovae.
We investigate the impact of progenitor's metallicity, of the common-envelope
parameter and of the natal kicks on the properties of DNSs, BHNSs
and BHBs. We find that neutron-star (NS) masses in DNSs span from 1.1 to 2.0
M, with a preference for light NSs, while NSs in merging BHNSs have
mostly large masses ( M). BHs in merging BHNSs are
preferentially low mass ( M). BH masses in merging BHBs strongly
depend on the progenitor's metallicity and span from to
M. The local merger rate density of both BHNSs and BHBs derived from
our simulations is consistent with the values reported by the LVC in all our
simulations. In contrast, the local merger rate density of DNSs matches the
value inferred from the LVC only if low natal kicks are assumed. This result
adds another piece to the intricate puzzle of natal kicks and DNS formation.Comment: 22 pages, 15 figures, 2 tables, published in MNRAS, We corrected a
bug in the script for producing table 2 and figure 1
The impact of electron-capture supernovae on merging double neutron stars
Natal kicks are one of the most debated issues about double neutron star
(DNS) formation. Several observational and theoretical results suggest that
some DNSs have formed with low natal kicks ( km s), which
might be attributed to electron-capture supernovae (ECSNe). We investigate the
impact of ECSNe on the formation of DNSs by means of population synthesis
simulations. In particular, we assume a Maxwellian velocity distribution for
the natal kick induced by ECSNe with one dimensional root-mean-square
km s. The total number of DNSs
scales inversely with and the number of DNS mergers is
higher for relatively low kicks. This effect is particularly strong if we
assume low efficiency of common-envelope ejection (described by the parameter
), while it is only mild for high efficiency of common-envelope
ejection (). In most simulations, more than 50 per cent of the
progenitors of merging DNSs undergo at least one ECSN and the ECSN is almost
always the first SN occurring in the binary system. Finally, we have considered
the extreme case in which all neutron stars receive a low natal kick
(~km~s). In this case, the number of DNSs increases by a
factor of ten and the percentage of merging DNSs which went through an ECSN is
significantly suppressed ( per cent).Comment: 11 pages, 7 figures, 1 tables, to appear in MNRA
The High Mass X-ray Binaries in star-forming galaxies
The high mass X-ray binaries (HMXBs) provide an exciting framework to
investigate the evolution of massive stars and the processes behind binary
evolution. HMXBs have shown to be good tracers of recent star formation in
galaxies and might be important feedback sources at early stages of the
Universe. Furthermore, HMXBs are likely the progenitors of gravitational wave
sources (BH--BH or BH--NS binaries that may merge producing gravitational
waves). In this work, we investigate the nature and properties of HMXB
population in star-forming galaxies. We combine the results from the population
synthesis model MOBSE (Giacobbo et al. 2018) together with galaxy catalogs from
EAGLE simulation (Schaye et al. 2015). Therefore, this method describes the
HMXBs within their host galaxies in a self-consistent way. We compute the X-ray
luminosity function (XLF) of HMXBs in star-forming galaxies, showing that this
methodology matches the main features of the observed XLF.Comment: 4 pages, 2 figures. To appear in Proc. IAUS 346: High-mass X-ray
binaries: illuminating the passage from massive binaries to merging compact
object
Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data
Ten binary black-hole mergers have already been detected during the first two
observing runs of advanced LIGO and Virgo, and many more are expected to be
observed in the near future. This opens the possibility for gravitational-wave
astronomy to better constrain the properties of black hole binaries, not only
as single sources, but as a whole astrophysical population. In this paper, we
address the problem of using gravitational-wave measurements to estimate the
proportion of merging black holes produced either via isolated binaries or
binaries evolving in young star clusters. To this end, we use a Bayesian
hierarchical modeling approach applied to catalogs of merging binary black
holes generated using state-of-the-art population synthesis and N-body codes.
In particular, we show that, although current advanced LIGO/Virgo observations
only mildly constrain the mixing fraction between the two
formation channels, we expect to narrow down the fractional errors on to
after a few hundreds of detections.Comment: 17 pages, 4 figure
Fingerprints of Binary Black Hole Formation Channels Encoded in the Mass and Spin of Merger Remnants
Binary black holes (BBHs) are thought to form in different environments, including the galactic field and (globular, nuclear, young, and open) star clusters. Here, we propose a method to estimate the fingerprints of the main BBH formation channels associated with these different environments. We show that the metallicity distribution of galaxies in the local universe along with the relative amount of mergers forming in the field or in star clusters determine the main properties of the BBH population. Our fiducial model predicts that the heaviest merger to date, GW170729, originated from a progenitor that underwent 2–3 merger events in a dense star cluster, possibly a galactic nucleus. The model predicts that at least one merger remnant out of a hundred BBH mergers in the local universe has mass , and one in a thousand can reach a mass as large as . Such massive black holes would bridge the gap between stellar-mass and intermediate-mass black holes. The relative number of low- and high-mass BBHs can help us unravel the fingerprints of different formation channels. Based on the assumptions of our model, we expect that isolated binaries are the main channel of BBH merger formation if of the whole BBH population has remnants with masses , whereas % of remnants having masses points to a significant subpopulation of dynamically formed BBH binaries
The cosmic merger rate density of compact objects: impact of star formation, metallicity, initial mass function and binary evolution
We evaluate the redshift distribution of binary black hole (BBH), black hole
- neutron star binary (BHNS) and binary neutron star (BNS) mergers, exploring
the main sources of uncertainty: star formation rate (SFR) density, metallicity
evolution, common envelope, mass transfer via Roche lobe overflow, natal kicks,
core-collapse supernova model and initial mass function. Among binary evolution
processes, uncertainties on common envelope ejection have a major impact: the
local merger rate density of BNSs varies from to
Gpc yr if we change the common envelope efficiency parameter from
to 0.5, while the local merger rates of BBHs and BHNSs vary
by a factor of . The BBH merger rate changes by one order of
magnitude, when uncertainties on metallicity evolution are taken
into account. In contrast, the BNS merger rate is almost insensitive to
metallicity. Hence, BNSs are the ideal test bed to put constraints on uncertain
binary evolution processes, such as common envelope and natal kicks. Only
models assuming values of and moderately low natal
kicks (depending on the ejected mass and the SN mechanism), result in a local
BNS merger rate density within the 90% credible interval inferred from the
second gravitational-wave transient catalogue.Comment: 14 pages, 12 figures, 2 tables, accepted for publication in MNRA
Merging black hole binaries with the SEVN code
Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the
SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 x 10(8) binary systems, with metallicity in the range [10(-4); 4 x 10(-2)]. From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is similar to 30, 45, and 55 M-circle dot at metallicity Z = 2 x 10(-2), 6 x 10(-3), and 10(-4), respectively. A few massive single BHs may also form (less than or similar to 0.1 per cent of the total number of BHs), with mass up to similar to 65, 90, and 145 M-circle dot at Z = 2 x 10(-2), 6 x 10(-3),
and 10(-4), respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814, and GW170104) originate from metal-poor (Z less than or similar to 6 x 10(-3)) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10(-4). The BHB merger rate in the local Universe obtained from our simulations is similar to 90Gpc(-3)yr(-1), consistent with the rate inferred from LIGO-Virgo data