2,393 research outputs found
Common envelope ejection in massive binary stars - Implications for the progenitors of GW150914 and GW151226
The recently detected gravitational wave signals (GW150914 and GW151226) of
the merger event of a pair of relatively massive stellar-mass black holes (BHs)
calls for an investigation of the formation of such progenitor systems in
general. We analyse the common envelope (CE) stage of the "traditional"
formation channel in binaries where the first-formed compact object undergoes
an in-spiral inside the envelope of its evolved companion star and ejects the
envelope in that process. We calculate envelope binding energies of donor stars
with initial masses between 4 and 115 Msun for metallicities of Z=Zsun/2 and
Z=Zsun/50, and derive minimum masses of in-spiralling objects needed to eject
these envelopes. We find that CE evolution, besides from producing WD-WD and
NS-NS binaries, may, in principle, also produce massive BH-BH systems with
individual BH component masses up to ~50-60 Msun, in particular for donor stars
evolved to giants. However, the physics of envelope ejection of massive stars
remains uncertain. We discuss the applicability of the energy-budget formalism,
the location of the bifurcation point, the recombination energy and the
accretion energy during in-spiral as possible energy sources, and also comment
on the effect of inflated helium cores. Massive stars in a wide range of
metallicities and with initial masses up to at least 115 Msun may possibly shed
their envelopes and survive CE evolution, depending on their initial orbital
parameters, similarly to the situation for intermediate mass and low-mass stars
with degenerate cores. We conclude that based on stellar structure
calculations, and in the view of the usual simple energy budget analysis,
events like GW150914 and GW151226 could possibly be produced from the CE
channel. Calculations of post-CE orbital separations, however, and thus the
estimated LIGO detection rates, remain highly uncertain. [Abridged]Comment: 13 pages, 7 figures, A&A accepte
Simulations of stripped core-collapse supernovae in close binaries
We perform smoothed-particle hydrodynamical simulations of the explosion of a
helium star in a close binary system, and study the effects of the explosion on
the companion star as well as the effect of the presence of the companion on
the supernova remnant. By simulating the mechanism of the supernova from just
after core bounce until the remnant shell passes the stellar companion, we are
able to separate the various effects leading to the final system parameters. In
the final system, we measure the mass stripping and ablation from, and the
velocity kick imparted to, the companion star, as well as the structure of the
supernova shell. The presence of the companion star produces a conical cavity
in the expanding supernova remnant, and loss of material from the companion
causes the supernova remnant to be more metal-rich on one side and more
hydrogen-rich (from the companion material) around the cavity. Following the
removal of mass from the companion, we study its subsequent evolution and
compare it with a single star not subjected to a supernova impact.Comment: 20 pages, 14 figures, submitted to Computational Astrophysics and
Cosmolog
Possible confirmation of the existence of ergoregion by the Kerr quasinormal mode in gravitational waves from Pop III massive black hole binary
The existence of the ergoregion of the Kerr space-time has not been confirmed
observationally yet. We show that the confirmation would be possible by
observing the quasinormal mode in gravitational waves. As an example, using the
recent population synthesis results of Pop III binary black holes, we find that
the peak of the final merger mass () is about , while
the fraction of the final spin needed for the
confirmation of a part of ergoregion is . To confirm the frequency
of the quasinormal mode, is needed. The standard model of Pop
III population synthesis tells us that the event rate for the confirmation of
more than of the ergoregion by the second generation gravitational wave
detectors is where and
are the peak value of the Pop III star formation rate and the
fraction of binaries, respectively.Comment: Accepted for publication in PTEP. Comments welcom
Ultra-luminous X-ray sources and neutron-star-black-hole mergers from very massive close binaries at low metallicity
Gravitational waves from the binary black hole (BH) merger GW150914 may
enlighten our understanding of ultra-luminous X-ray sources (ULXs), as
BHs>30Msun can reach luminosities>4x10^39 erg s^-1 without exceeding their
Eddington limit. It is then important to study variations of evolutionary
channels for merging BHs, which might instead form accreting BHs and become
ULXs. It was recently shown that massive binaries with mass ratios close to
unity and tight orbits can undergo efficient rotational mixing and evolve
chemically homogeneously, resulting in a compact BH binary. We study similar
systems by computing ~120000 detailed binary models with the MESA code covering
a wide range of initial parameters. For initial mass ratios M2/M1~0.1-0.4,
primaries >40Msun can evolve chemically homogeneously, remaining compact and
forming a BH without undergoing Roche-lobe overflow. The secondary then expands
and transfers mass to the BH, initiating a ULX phase. We predict that ~1 out of
10^4 massive stars evolves this way, and that in the local universe 0.13 ULXs
per Msun yr^-1 of star-formation rate are observable, with a strong preference
for low-metallicities. At metallicities log Z>-3, BH masses in ULXs are limited
to 60Msun due to the occurrence of pair-instability supernovae which leave no
remnant, resulting in an X-ray luminosity cut-off. At lower metallicities, very
massive stars can avoid exploding as pair-instability supernovae and instead
form BHs with masses above 130Msun, producing a gap in the ULX luminosity
distribution. After the ULX phase, neutron-star-BH binaries that merge in less
than a Hubble time are produced with a formation rate <0.2 Gpc^-3 yr^-1. We
expect that upcoming X-ray observatories will test these predictions, which
together with additional gravitational wave detections will provide strict
constraints on the origin of the most massive BHs that can be produced by
stars.Comment: Accepted for publication in A&A. 19 Pages plus 16 pages of
appendices. Abstract abridge
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