782 research outputs found
Galactic Centre stellar winds and Sgr A* accretion
(ABRIDGED) We present in detail our new 3D numerical models for the accretion
of stellar winds on to Sgr A*. In our most sophisticated models, we put stars
on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account
for radiative cooling. We first model only one phase `fast' stellar winds (1000
km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5
Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models.
In contrast, Mdot decreases by an order of magnitude for stars following
circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to
produce slow winds. Much of these winds cool radiatively, forming cold clumps
immersed into the X-ray emitting gas. We test two orbital configurations for
the stars in this scenario, an isotropic distribution and two rotating discs
with perpendicular orientation. The morphology of cold gas is quite sensitive
to the orbits. In both cases, however, most of the accreted gas is hot, with an
almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The
cold gas accretes in intermittent, short but powerful episodes which may give
rise to large amplitude variability in the luminosity of Sgr A* on time scales
of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and
1e4 Rsch, for the one and two-phase wind simulations, respectively, never
forming the quasi-spherical accretion flows suggested in some previous work.
Our work suggests that, averaged over time scales of 100s to 1000s of years,
the radiative and mechanical luminosity of Sgr A* may be substantially higher
than it is in its current state. Further improvements of the wind accretion
modelling of Sgr A* will rely on improved observational constraints for the
wind properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper
and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2:
minor changes
Tidal disruptions in circumbinary disks. II: Observational signatures in the reverberation spectra
Supermassive Binary Black Holes (SMBBHs) with sub-pc separations form in the
course of galaxy mergers, if both galaxies harbour massive black holes. Clear
observational evidence for them however still eludes us. We propose a novel
method of identifying these systems by means of reverberation mapping their
circumbinary disk after a tidal disruption event has ionized it. The tidal
disruption of a star at the secondary leads to strong asymmetries in the disk
response. We model the shape of the velocity--delay maps for various toy disk
models and more realistic gas distributions obtained by SPH simulations. The
emissivity of the ionized disk is calculated with {\em Cloudy}. We find
peculiar asymmetries in the maps for off center ionizing sources that may help
us constrain geometrical parameters of a circumbinary disk such as semi-major
axis and orbital phase of the secondary, as well as help strengthen the
observational evidence for sub-parsec SMBBHs as such.Comment: 10 pages, 5 figures. Accepted for publication in Ap
The Milky Way's Fermi Bubbles: Echoes of the Last Quasar Outburst?
{\it Fermi}-LAT has recently detected two gamma ray bubbles disposed
symmetrically with respect to the Galactic plane. The bubbles have been
suggested to be in a quasi-steady state, inflated by ongoing star formation
over the age of the Galaxy. Here we propose an alternative picture where the
bubbles are the remnants of a large-scale wide-angle outflow from \sgra, the
SMBH of our Galaxy. Such an outflow would be a natural consequence of a short
but bright accretion event on to \sgra\ if it happened concurrently with the
well known star formation event in the inner 0.5 pc of the Milky Way
Myr ago. We find that the hypothesised near-spherical outflow is focussed into
a pair of symmetrical lobes by the greater gas pressure along the Galactic
plane. The outflow shocks against the interstellar gas in the Galaxy bulge.
Gamma--ray emission could be powered by cosmic rays created by either \sgra\
directly or accelerated in the shocks with the external medium. The Galaxy disc
remains unaffected, agreeing with recent observational evidence that
supermassive black holes do not correlate with galaxy disc properties. We
estimate that an accreted mass \sim 2 \times 10^3\msun is needed for the
accretion event to power the observed {\it Fermi}--LAT lobes. Within a factor
of a few this agrees with the mass of the young stars born during the star
formation event. This estimate suggests that roughly 50% of the gas was turned
into stars, while the rest accreted onto \sgra. One interpretation of this is a
reduced star formation efficiency inside the \sgra\ accretion disc due to
stellar feedback, and the other a peculiar mass deposition geometry that
resulted in a significant amount of gas falling directly inside the inner pc of the Galaxy.Comment: 6 pages, 0 figures; accepted for publication in MNRA
Self-gravitating fragmentation of eccentric accretion disks
We consider the effects of eccentricity on the fragmentation of
gravitationally unstable accretion disks, using numerical hydrodynamics. We
find that eccentricity does not affect the overall stability of the disk
against fragmentation, but significantly alters the manner in which such
fragments accrete gas. Variable tidal forces around an eccentric orbit slow the
accretion process, and suppress the formation of weakly-bound clumps. The
"stellar" mass function resulting from the fragmentation of an eccentric disk
is found to have a significantly higher characteristic mass than that from a
corresponding circular disk. We discuss our results in terms of the disk(s) of
massive stars at ~0.1pc from the Galactic Center, and find that the
fragmentation of an eccentric accretion disk, due to gravitational instability,
is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap
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