68 research outputs found
Accretion disk warping by resonant relaxation: The case of maser disk NGC4258
The maser disk around the massive black hole (MBH) in active galaxy NGC 4258
exhibits an O(10 deg) warp on the O(0.1 pc) scale. The physics driving the warp
are still debated. Suggested mechanisms include torquing by relativistic frame
dragging or by radiation pressure. We propose here a new warping mechanism:
resonant torquing of the disk by stars in the dense cusp around the MBH. We
show that resonant torquing can induce such a warp over a wide range of
observed and deduced physical parameters of the maser disk.Comment: 4 pp, 2 figure
Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre
Observations of massive stars within the central parsec of the Galaxy show
that, while most stars orbit within a well-defined disc, a significant fraction
have large eccentricities and / or inclinations with respect to the disc plane.
Here, we investigate whether this dynamically hot component could have arisen
via scattering from an initially cold disc -- the expected initial condition if
the stars formed from the fragmentation of an accretion disc. Using N-body
methods, we evolve a variety of flat, cold, stellar systems, and study the
effects of initial disc eccentricity, primordial binaries, very massive stars
and intermediate mass black holes. We find, consistent with previous results,
that a circular disc does not become eccentric enough unless there is a
significant population of undetected 100--1000 Msun objects. However, since
fragmentation of an eccentric disc can readily yield eccentric stellar orbits,
the strongest constraints come from inclinations. We show that_none_ of our
initial conditions yield the observed large inclinations, regardless of the
initial disc eccentricity or the presence of massive objects. These results
imply that the orbits of the young massive stars in the Galactic Centre are
largely primordial, and that the stars are unlikely to have formed as a
dynamically cold disc.Comment: 5 pages, 6 colour figures. MNRAS Letters in press. (v2: very minor
changes
Simulations of the formation of stellar discs in the Galactic centre via cloud-cloud collisions
Young massive stars in the central parsec of our Galaxy are best explained by
star formation within at least one, and possibly two, massive self-gravitating
gaseous discs. With help of numerical simulations, we here consider whether the
observed population of young stars could have originated from a large angle
collision of two massive gaseous clouds at R approx. 1 parsec from Sgr A*. In
all the simulations performed, the post-collision gas flow forms an inner,
nearly circular gaseous disc and one or two eccentric outer filaments,
consistent with the observations. Furthermore, the radial stellar mass
distribution is always very steep, Sigma proportional to R^-2, again consistent
with the observations. All of our simulations produce discs that are warped by
between 30 to 60 degrees, in accordance with the most recent observations. The
3D velocity structure of the stellar distribution is sensitive to initial
conditions (e.g., the impact parameter of the clouds) and gas cooling details.
For example, the runs in which the inner disc is fed intermittently with
material possessing fluctuating angular momentum result in multiple stellar
discs with different orbital orientations, contradicting the observed data. In
all the cases the amount of gas accreted by our inner boundary condition is
large, enough to allow Sgr A* to radiate near its Eddington limit over approx.
10^5 years. This suggests that a refined model would have physically larger
clouds (or a cloud and a disc such as the circumnuclear disc) colliding at a
distance of a few parsecs rather than 1 parsec as in our simulations.Comment: 18 pages, 14 figures. Accepted for publication in MNRAS. Minor
additions at referee request. Movies of simulations available at
http://www.astro.le.ac.uk/~aph11/movies.htm
Simulations of Direct Collisions of Gas Clouds with the Central Black Hole
We perform numerical simulations of clouds in the Galactic Centre (GC)
engulfing the nuclear super-massive black hole and show that this mechanism
leads to the formation of gaseous accretion discs with properties that are
similar to the expected gaseous progenitor discs that fragmented into the
observed stellar disc in the GC. As soon as the cloud hits the black hole, gas
with opposite angular momentum relative to the black hole collides downstream.
This process leads to redistribution of angular momentum and dissipation of
kinetic energy, resulting in a compact gaseous accretion disc. A parameter
study using thirteen high resolution simulations of homogeneous clouds falling
onto the black hole and engulfing it in parts demonstrates that this mechanism
is able to produce gaseous accretion discs that could potentially be the
progenitor of the observed stellar disc in the GC. A comparison of simulations
with different equations of state (adiabatic, isothermal and full cooling)
demonstrates the importance of including a detailed thermodynamical
description. However the simple isothermal approach already yields good results
on the radial mass transfer and accretion rates, as well as disc eccentricities
and sizes. We find that the cloud impact parameter strongly influences the
accretion rate whereas the impact velocity has a small affect on the accretion
rate.Comment: 21 pages, 18 figures, Accepted for publication in MNRA
Feeding SMBHs through supersonic turbulence and ballistic accretion
It has long been recognised that the main obstacle to accretion of gas onto
supermassive black holes (SMBHs) is large specific angular momentum. It is
feared that the gas settles in a large scale disc, and that accretion would
then proceed too inefficiently to explain the masses of the observed SMBHs.
Here we point out that, while the mean angular momentum in the bulge is very
likely to be large, the deviations from the mean can also be significant.
Indeed, cosmological simulations show that velocity and angular momentum fields
of gas flows onto galaxies are very complex. Furthermore, inside bulges the gas
velocity distribution can be further randomised by the velocity kicks due to
feedback from star formation. We perform hydrodynamical simulations of gaseous
rotating shells infalling onto an SMBH, attempting to quantify the importance
of velocity dispersion in the gas at relatively large distances from the black
hole. We implement this dispersion by means of a supersonic turbulent velocity
spectrum. We find that, while in the purely rotating case the circularisation
process leads to efficient mixing of gas with different angular momentum,
resulting in a low accretion rate, the inclusion of turbulence increases this
accretion rate by up to several orders of magnitude. We show that this can be
understood based on the notion of "ballistic" accretion, whereby dense
filaments, created by convergent turbulent flows, travel through the ambient
gas largely unaffected by hydrodynamical drag. We derive a simple analytical
formula that captures the numerical results to within a factor of a few.
Rescaling our results to astrophysical bulges, we argue that this "ballistic"
mode of accretion could provide the SMBHs with a sufficient supply of fuel
without the need to channel the gas via large-scale discs or bars, and
therefore that star formation in bulges can be a strong catalyst for SMBH
accretion.Comment: 21 pages, 21 figures. Accepted and published by MNRA
In situ formation of SgrA* stars via disk fragmentation: parent cloud properties and thermodynamics
The formation of the massive young stars surrounding SgrA* is still an open
question. In this paper, we simulate the infall of a turbulent molecular cloud
towards the Galactic Center (GC). We adopt two different cloud masses (4.3x10^4
and 1.3x10^5 solar masses). We run five simulations: the gas is assumed to be
isothermal in four runs, whereas radiative cooling is included in the fifth
run. In all the simulations, the molecular cloud is tidally disrupted, spirals
towards the GC, and forms a small, dense and eccentric disk around SgrA*. With
high resolution simulations, we follow the fragmentation of the gaseous disk.
Star candidates form in a ring at ~0.1-0.4 pc from the super-massive black hole
(SMBH) and have moderately eccentric orbits (~0.2-0.4), in good agreement with
the observations. The mass function of star candidates is top-heavy only if the
local gas temperature is high (>~100 K) during the star formation and if the
parent cloud is sufficiently massive (>~10^5 solar masses). Thus, this study
indicates that the infall of a massive molecular cloud is a viable scenario for
the formation of massive stars around SgrA*, provided that the gas temperature
is kept sufficiently high (>~100 K).Comment: 16 pages, 16 figures, accepted for publication in Ap
A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits
We present new proper motions from the 10 m Keck telescopes for a puzzling
population of massive, young stars located within 3.5" (0.14 pc) of the
supermassive black hole at the Galactic Center. Our proper motion measurements
have uncertainties of only 0.07 mas/yr (3 km/s), which is ~7 times better than
previous proper motion measurements for these stars, and enables us to measure
accelerations as low as 0.2 mas/yr^2 (7 km/s/yr). Using these measurements,
line-of-sight velocities from the literature, and 3D velocities for additional
young stars in the central parsec, we constrain the true orbit of each
individual star and directly test the hypothesis that the massive stars reside
in two stellar disks as has been previously proposed. Analysis of the stellar
orbits reveals only one of the previously proposed disks of young stars using a
method that is capable of detecting disks containing at least 7 stars. The
detected disk contains 50% of the young stars, is inclined by ~115 deg from the
plane of the sky, and is oriented at a position angle of ~100 deg East of
North. Additionally, the on-disk and off-disk populations have similar K-band
luminosity functions and radial distributions that decrease at larger projected
radii as \propto r^-2. The disk has an out-of-the-disk velocity dispersion of
28 +/- 6 km/s, which corresponds to a half-opening angle of 7 +/- 2 deg, and
several candidate disk members have eccentricities greater than 0.2. Our
findings suggest that the young stars may have formed in situ but in a more
complex geometry than a simple, thin circular disk.Comment: accepted to ApJ, 64 pages, 20 figure
Are SMBHs shrouded by "super-Oort" clouds of comets and asteroids?
The last decade has seen a dramatic confirmation that an in situ star
formation is possible inside the inner parsec of the Milky Way. Here we suggest
that giant planets, solid terrestrial-like planets, comets and asteroids may
also form in these environments, and that this may have observational
implications for Active Galactic Nuclei (AGN). Like in debris discs around main
sequence stars, collisions of large solid objects should initiate strong
fragmentation cascades. The smallest particles in such a cascade - the
microscopic dust - may provide a significant opacity. We put a number of
observational and physical constraints on AGN obscuring torii resulting from
such fragmentation cascades. We find that torii fed by fragmenting asteroids
disappear at both low and high AGN luminosities. At high luminosities, , where is the Eddington limit, the AGN radiation
pressure blows out the microscopic dust too rapidly. At low luminosities, on
the other hand, the AGN discs may avoid gravitational fragmentation into stars
and solids. We also note that these fragmentation cascades may be responsible
for astrophysically "large" dust particles of approximately micrometer sizes
that were postulated by some authors to explain unusual absorption properties
of the AGN torii.Comment: a typo in the title correcte
Proposed update to the taxonomy of the genera Hepacivirus and Pegivirus within the Flaviviridae family
Proposals are described for the assignment of recently reported viruses, infecting rodents, bats and other mammalian species, to new species within the Hepacivirus and Pegivirus genera (Family Flaviviridae). Assignments into 14 Hepacivirus species (Hepacivirus A to N) and 11 Pegivirus species (Pegivirus A to K) are based on phylogenetic relationships and sequence distances between conserved regions extracted from complete coding sequences of each proposed taxon. We propose that the species hepatitis C virus is renamed Hepacivirus C in order to acknowledge its unique historical position and so as to minimise confusion. Despite the newly documented genetic diversity of hepaciviruses and pegiviruses, members of these genera remain phylogenetically distinct, and differ in hepatotropism and the possession of a basic core protein; pegiviruses in general lack these features. However, other characteristics that were originally used to support their division into separate genera are no longer definitive; there is overlap between the two genera in the type of internal ribosomal entry site (IRES) and the presence of miR-122 sites in the 5'untranslated region (UTR), the predicted number of N-linked glycosylation sites in the envelope E1 and E2 proteins, the presence of poly U tracts in the 3' UTR and the propensity of viruses to establish a persistent infection. While all classified hepaciviruses and pegiviruses have mammalian hosts, the recent description of a hepaci-/pegi-like virus from a shark and the likely existence of further homologues in other non-mammalian species indicates that further species or genera remain to be defined in the future
An orally bioavailable broad-spectrum antiviral inhibits SARS-CoV-2 in human airway epithelial cell cultures and multiple coronaviruses in mice
Coronaviruses (CoVs) traffic frequently between species resulting in novel disease outbreaks, most recently exemplified by the newly emerged SARS-CoV-2, the causative agent of COVID-19. Herein, we show that the ribonucleoside analog β-D-N4-hydroxycytidine (NHC, EIDD-1931) has broad spectrum antiviral activity against SARS-CoV-2, MERS-CoV, SARS-CoV, and related zoonotic group 2b or 2c Bat-CoVs, as well as increased potency against a coronavirus bearing resistance mutations to the nucleoside analog inhibitor remdesivir. In mice infected with SARS-CoV or MERS-CoV, both prophylactic and therapeutic administration of EIDD-2801, an orally bioavailable NHC-prodrug (β-D-N4-hydroxycytidine-5′-isopropyl ester), improved pulmonary function, and reduced virus titer and body weight loss. Decreased MERS-CoV yields in vitro and in vivo were associated with increased transition mutation frequency in viral but not host cell RNA, supporting a mechanism of lethal mutagenesis in CoV. The potency of NHC/EIDD-2801 against multiple coronaviruses and oral bioavailability highlight its potential utility as an effective antiviral against SARS-CoV-2 and other future zoonotic coronaviruses
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