149 research outputs found
Evolution of linear warps in accretion discs and applications to protoplanetary discs in binaries
Warped accretion discs are expected in many protostellar binary systems. In
this paper, we study the long-term evolution of disc warp and precession for
discs with dimensionless thickness larger than their viscosity parameter
, such that bending waves can propagate and dominate the warp
evolution. For small warps, these discs undergo approximately rigid-body
precession. We derive analytical expressions for the warp/twist profiles of the
disc and the alignment timescale for a variety of models. Applying our results
to circumbinary discs, we find that these discs align with the orbital plane of
the binary on a timescale comparable to the global precession time of the disc,
and typically much smaller than its viscous timescale. We discuss the
implications of our finding for the observations of misaligned circumbinary
discs (such as KH 15D) and circumbinary planetary systems (such as Kepler-413);
these observed misalignments provide useful constraints on the uncertain
aspects of the disc warp theory. On the other hand, we find that circumstellar
discs can maintain large misalignments with respect to the plane of the binary
companion over their entire lifetime. We estimate that inclination angles
larger than can be maintained for typical disc parameters.
Overall, our results suggest that while highly misaligned circumstellar discs
in binaries are expected to be common, such misalignments should be rare for
circumbinary discs. These expectations are consistent with current observations
of protoplanetary discs and exoplanets in binaries, and can be tested with
future observations.Comment: 15 pages, 10 figures, Accepted by MNRA
Remnant baryon mass outside of the black hole after a neutron star-black hole merger
Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a
Neutron Star (NS) and a Black Hole (BH) are a highly anticipated discovery in
extreme gravity, nuclear-, and astrophysics. We develop a simple formula that
distinguishes between merger outcomes and predicts the post-merger remnant
mass, validated with 75 simulations. Our formula improves on existing results
by describing critical unexplored regimes: comparable masses and higher BH
spins. These are important to differentiate NSNS from NSBH mergers, and to
infer source physics from EM signals.Comment: 9 pages, 5 figures, 2 table
Black Hole-Neutron Star Mergers: Disk Mass Predictions
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times
and its properties, has been one of the main motivations behind the numerical
simulation of these systems. Black hole-neutron star binaries are amongst the
most likely progenitors of short gamma-ray bursts --- as long as massive
(probably a few percents of a solar mass), hot accretion disks are formed
around the black hole. Whether this actually happens strongly depends on the
physical characteristics of the system, and in particular on the mass ratio,
the spin of the black hole, and the radius of the neutron star. We present here
a simple two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger (i.e. the combined mass of the accretion
disk, the tidal tail, and the potential ejecta). This model predicts the
remnant mass within a few percents of the mass of the neutron star, at least
for remnant masses up to 20% of the neutron star mass. Results across the range
of parameters deemed to be the most likely astrophysically are presented here.
We find that, for 10 solar mass black holes, massive disks are only possible
for large neutron stars (R>12km), or quasi-extremal black hole spins (a/M>0.9).
We also use our model to discuss how the equation of state of the neutron star
affects the final remnant, and the strong influence that this can have on the
rate of short gamma-ray bursts produced by black hole-neutron star mergers.Comment: 11 pages, 7 figure
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