43 research outputs found
On the Bardeen-Petterson Effect in black hole accretion discs
We investigate the effect of black hole spin on warped or misaligned
accretion discs - in particular i) whether or not the inner disc edge aligns
with the black hole spin and ii) whether the disc can maintain a smooth
transition between an aligned inner disc and a misaligned outer disc, known as
the Bardeen-Petterson effect. We employ high resolution 3D smoothed particle
hydrodynamics simulations of -discs subject to Lense-Thirring
precession, focussing on the bending wave regime where the disc viscosity is
smaller than the aspect ratio . We first address the
controversy in the literature regarding possible steady-state oscillations of
the tilt close to the black hole. We successfully recover such oscillations in
3D at both small and moderate inclinations (), provided
both Lense-Thirring and Einstein precession are present, sufficient resolution
is employed, and provided the disc is not so thick so as to simply accrete
misaligned. Second, we find that discs inclined by more than a few degrees in
general steepen and break rather than maintain a smooth transition, again in
contrast to previous findings, but only once the disc scale height is
adequately resolved. Finally, we find that when the disc plane is misaligned to
the black hole spin by a large angle, the disc 'tears' into discrete rings
which precess effectively independently and cause rapid accretion, consistent
with previous findings in the diffusive regime (). Thus
misalignment between the disc and the spin axis of the black hole provides a
robust mechanism for growing black holes quickly, regardless of whether the
disc is thick or thin.Comment: 15 pages, 18 figures, movies available at
http://users.monash.edu.au/~rnealon/ or YouTub
On the Papaloizou-Pringle instability in tidal disruption events
We demonstrate that the compact, thick disc formed in a tidal disruption
event may be unstable to non-axisymmetric perturbations in the form of the
Papaloizou-Pringle instability. We show this can lead to rapid redistribution
of angular momentum that can be parameterised in terms of an effective
Shakura-Sunyaev parameter. For remnants that have initially weak
magnetic fields, this may be responsible for driving mass accretion prior to
the onset of the magneto-rotational instability. For tidal disruptions around a
M black hole, the measured accretion rate is super-Eddington
but is not sustainable over many orbits. We thus identify a method by which the
torus formed in tidal disruption event may be significantly accreted before the
magneto-rotational instability is established.Comment: 9 pages, 10 figures, accepted for publication in MNRAS. Movies of
simulations available at https://youtu.be/kBLAjY8n9vI and
https://youtu.be/F8F0tmLbX3
WInDI: a Warp-Induced Dust Instability in protoplanetary discs
We identify a new dust instability that occurs in warped discs. The
instability is caused by the oscillatory gas motions induced by the warp in the
bending wave regime. We first demonstrate the instability using a local 1D
(vertical) toy model based on the warped shearing box coordinates and
investigate the effects of the warp magnitude and dust Stokes number on the
growth of the instability. We then run 3D SPH simulations and show that the
instability is manifested globally by producing unique dust structures that do
not correspond to gas pressure maxima. The 1D and SPH analysis suggest that the
instability grows on dynamical timescales and hence is potentially significant
for planet formation.Comment: Accepted for publication in MNRAS, 13 pages, 10 figure
Generalized Warped Disk Equations
The manner in which warps in accretion disks evolve depends on the magnitude of the viscosity. ... See full text for complete abstract
The System and Freedom in the Work of Karl Marx
We consider black hole - galaxy coevolution using simple analytic arguments.
We focus on the fact that several supermassive black holes are known with
masses significantly larger than suggested by the relation,
sometimes also with rather small stellar masses. We show that these are likely
to have descended from extremely compact `blue nugget' galaxies born at high
redshift, whose very high velocity dispersions allowed the black holes to reach
unusually large masses. Subsequent interactions reduce the velocity dispersion,
so the black holes lie above the usual relation and expel a
large fraction of the bulge gas (as in WISE J104222.11+164115.3) that would
otherwise make stars, before ending at low redshift as very massive holes in
galaxies with relatively low stellar masses, such as NGC 4889 and NGC 1600. We
further suggest the possible existence of two new types of galaxy: low-mass
dwarfs whose central black holes lie below the relation at low
redshift, and galaxies consisting of very massive (M) black holes with extremely small stellar masses. This
second group would be very difficult to detect electromagnetically, but
potentially offer targets of considerable interest for LISA.Comment: Accepted for publication in MNRAS. 5 pages, 3 figure
Dwarf galaxies and the black hole scaling relations
The sample of dwarf galaxies with measured central black hole masses M and velocity dispersions Ï has recently doubled, and gives a close fit to the extrapolation of the M - Ï relation for more massive galaxies. We argue that this is difficult to reconcile with suggestions that the scaling relations between galaxies and their central black holes are simply a statistical consequence of assembly through repeated mergers. This predicts black hole masses significantly larger than those observed in dwarf galaxies unless the initial distribution of uncorrelated seed black hole and stellar masses is confined to much smaller masses than earlier assumed. It also predicts a noticeable flattening of the M - Ï relation for dwarfs, to M â Ï2 compared with the observed M â Ï4. In contrast black hole feedback predicts that black hole masses tend towards a universal M â Ï4 relation in all galaxies, and correctly gives the properties of powerful outflows recently observed in dwarf galaxies. These considerations emphasize once again that the fundamental physical black hole â galaxy scaling relation is between M and Ï. The relation of M to the bulge mass Mb is acausal, and depends on the quite independent connection between Mb and Ï set by stellar feedback
Warping away gravitational instabilities in protoplanetary discs
We perform three-dimensional smoothed-particle hydrodynamics simulations of warped, non-coplanar gravitationally unstable discs to show that as the warp propagates through the self-gravitating disk, it heats up the disk rendering it gravitationally stable, thus losing their spiral structure and appearing completely axisymmetric. In their youth, protoplanetary discs are expected to be massive and self-gravitating, which results in nonaxisymmetric spiral structures. However recent observations of young protoplanetary discs with the Atacama Large Millimeter/submillimeter Array have revealed that discs with large-scale spiral structure are rarely observed in the midplane. Instead, axisymmetric discs, with some also having ring and gap structures, are more commonly observed. Our work invloving warps, non-coplanar disk structures that are expected to commonly occur in young discs, potentially resolves this discrepancy between observations and theoretical predictions. We demonstrate that they are able to suppress the large-scale spiral structure of self-gravitating protoplanetary discs
Continuing to hide signatures of gravitational instability in protoplanetary discs with planets
We carry out three dimensional smoothed particle hydrodynamics simulations to study the impact of planet-disc interactions on a gravitationally unstable protoplanetary disc. We find that the impact of a planet on the discâs evolution can be described by three scenarios. If the planet is sufficiently massive, the spiral wakes generated by the planet dominate the evolution of the disc and gravitational instabilities are completely suppressed. If the planetâs mass is too small, then gravitational instabilities are unaffected. If the planetâs mass lies between these extremes, gravitational instabilities are weakened. We present mock Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations showing that the observability of large-scale spiral structures is diminished or completely suppressed when the planet is massive enough to influence the discâs evolution. Our results show that massive discs that would be expected to be gravitationally unstable can appear axisymmetric in the presence of a planet. Thus, the absence of observed large-scale spiral structures alone is not enough to place upper limits on the discâs mass, which could have implications on observations of young Class I discs with rings & gaps