9 research outputs found
Hydrodynamical moving-mesh simulations of the tidal disruption of stars by supermassive black holes
When a star approaches a black hole closely, it may be pulled apart by
gravitational forces in a tidal disruption event (TDE). The flares produced by
TDEs are unique tracers of otherwise quiescent supermassive black holes (SMBHs)
located at the centre of most galaxies. In particular, the appearance of such
flares and the subsequent decay of the light curve are both sensitive to
whether the star is partially or totally destroyed by the tidal field. However,
the physics of the disruption and the fall-back of the debris are still poorly
understood. We are here modelling the hydrodynamical evolution of realistic
stars as they approach a SMBH on parabolic orbits, using for the first time the
moving-mesh code AREPO, which is particularly well adapted to the problem
through its combination of quasi-Lagrangian behaviour, low advection errors,
and high accuracy typical of mesh-based techniques. We examine a suite of
simulations with different impact parameters, allowing us to determine the
critical distance at which the star is totally disrupted, the energy
distribution and the fallback rate of the debris, as well as the hydrodynamical
evolution of the stellar remnant in the case of a partial disruption.
Interestingly, we find that the internal evolution of the remnant's core is
strongly influenced by persistent vortices excited in the tidal interaction.
These should be sites of strong magnetic field amplification, and the
associated mixing may profoundly alter the subsequent evolution of the tidally
pruned star.Comment: 13 pages, 9 figures. Accepted for publication by MNRA
Accretion of clumpy cold gas onto massive black holes binaries: the challenging formation of extended circumbinary structures
Massive black hole binaries (MBHBs) represent an unavoidable outcome of
hierarchical galaxy formation, but their dynamical evolution at sub-parsec
scales is poorly understood, due to a combination of uncertainties in
theoretical models and lack of firm observational evidence. In gas rich
environments, it has been shown that a putative extended, steady circumbinary
gaseous disc plays an important role in the MBHB evolution, facilitating its
coalescence. How gas on galactic scales is transported to the nuclear region to
form and maintain such a stable structure is, however, unclear. If, following a
galaxy merger, turbulent gas is condenses in cold clumps and filaments that are
randomly scattered, gas is naturally transported on parsec scales and interacts
with the MBHB in discrete incoherent pockets. The aim of this work is to
investigate the gaseous structures arising from this interaction. We employ a
suite of smoothed-particle-hydrodynamic simulations to study the formation and
evolution of gaseous structures around a MBHB constantly perturbed by the
incoherent infall of molecular clouds. We investigate the influence of the
infall rate and angular momentum distribution of the clouds on the geometry and
stability of the arising structures. We find that the continuous supply of
incoherent clouds is a double-edge sword, resulting in the intermittent
formation and disruption of circumbinary structures. Anisotropic cloud
distributions featuring an excess of co-rotating events generate more prominent
co-rotating circumbinary discs. Similar structures are seen when mostly
counter-rotating clouds are fed to the binary, even though they are more
compact and less stable. In general, our simulations do not show the formation
of extended smooth and stable circumbinary discs, typically assumed in
analytical and numerical investigations of the the long term evolution of
MBHBs. (Abridged)Comment: 22 Pages, 17 Figures. To be submitted to MNRA
Accretion of clumpy cold gas onto massive black hole binaries: a possible fast route to binary coalescence
In currently favoured hierarchical cosmologies, the formation of massive
black hole binaries (MBHBs) following galaxy mergers is unavoidable. Still, due
the complex physics governing the (hydro)dynamics of the post-merger dense
environment of stars and gas in galactic nuclei, the final fate of those MBHBs
is still unclear. In gas-rich environments, it is plausible that turbulence and
gravitational instabilities feed gas to the nucleus in the form of a series of
cold incoherent clumps, thus providing a way to exchange energy and angular
momentum between the MBHB and its surroundings. Within this context, we present
a suite of smoothed-particle-hydrodynamical models to study the evolution of a
sequence of near-radial turbulent gas clouds as they infall towards equal-mass,
circular MBHBs. We focus on the dynamical response of the binary orbit to
different levels of anisotropy of the incoherent accretion events. Compared to
a model extrapolated from a set of individual cloud-MBHB interactions, we find
that accretion increases considerably and the binary evolution is faster. This
occurs because the continuous infall of clouds drags inwards circumbinary gas
left behind by previous accretion events, thus promoting a more effective
exchange of angular momentum between the MBHB and the gas. These results
suggest that sub-parsec MBHBs efficiently evolve towards coalescence during the
interaction with a sequence of individual gas pockets.Comment: 18 pages, 17 figures. Accepted for publication by MNRAS. Find
companion paper at arXiv:1801.06179 Animations available at
http://multipleclouds.xyz/movies
Resolving the Binary Components of the Outbursting Protostar HBC 494 with ALMA
Episodic accretion is a low-mass pre-main sequence phenomenon characterized
by sudden outbursts of enhanced accretion. These objects are classified into
two: protostars with elevated levels of accretion that lasts for decades or
more, called FUors, and protostars with shorter and repetitive bursts, called
EXors. HBC 494 is a FUor object embedded in the Orion Molecular Cloud. Earlier
Atacama Large (sub-)Millimeter Array (ALMA) continuum observations showed an
asymmetry in the disk at 0.''2 resolution. Here, we present follow-up
observations at ~0.''03, resolving the system into two components: HBC 494 N
(primary) and HBC 494 S (secondary). No circumbinary disk was detected. Both
disks are resolved with a projected separation of ~0.''18 (75 au). Their
projected dimensions are 84+/-1.8 x 66.9+/-1.5 mas for HBC 494 N and 64.6+/-2.5
x 46.0+/-1.9 mas for HBC 494 S. The disks are almost aligned and with similar
inclinations. The observations show that the primary is ~5 times brighter/more
massive and ~2 times bigger than the secondary. We notice that the northern
component has a similar mass to the FUors, while the southern has to EXors. The
HBC 494 disks show individual sizes that are smaller than single eruptive YSOs.
In this work, we also report 12CO, 13CO, and C18O molecular line observations.
At large scale, the 12CO emission shows bipolar outflows, while the 13CO and
C18O maps show a rotating and infalling envelope around the system. At a
smaller scale, the 12CO and 13CO moment zero maps show cavities within the
continuum disks' area, which may indicate continuum over-subtraction or
slow-moving jets and chemical destruction along the line-of-sight.Comment: Published in MNRAS. Main text (11 pages, 8 figures). Appendix (11
pages, 9 figures). Total (22 pages, 17 figures