5,904 research outputs found
Fuelling Active Galactic Nuclei
We suggest that most nearby active galactic nuclei are fed by a series of
small--scale, randomly--oriented accretion events. Outside a certain radius
these events promote rapid star formation, while within it they fuel the
supermassive black hole. We show that the events have a characteristic time
evolution. This picture agrees with several observational facts. The expected
luminosity function is broadly in agreement with that observed for
moderate--mass black holes. The spin of the black hole is low, and aligns with
the inner disc in each individual feeding event. This implies radio jets
aligned with the axis of the obscuring torus, and uncorrelated with the
large--scale structure of the host galaxy. The ring of young stars observed
about the Galactic Centre are close to where our picture predicts that star
formation should occur.Comment: MNRAS, in pres
Radiation induced warping of protostellar accretion disks
We examine the consequences of radiatively driven warping of accretion disks
surrounding pre-main-sequence stars. These disks are stable against warping if
the luminosity arises from a steady accretion flow, but are unstable at late
times when the intrinsic luminosity of the star overwhelms that provided by the
disk. Warps can be excited for stars with luminosities of around 10 solar
luminosities or greater, with larger and more severe warps in the more luminous
systems. A twisted inner disk may lead to high extinction towards stars often
viewed through their disks. After the disk at all radii becomes optically thin,
the warp decays gradually on the local viscous timescale, which is likely to be
long. We suggest that radiation induced warping may account for the origin of
the warped dust disk seen in Beta Pictoris, if the star is only around 10-20
Myr old, and could lead to non-coplanar planetary systems around higher mass
stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres
An Investigation into the Geometry of Seyfert Galaxies
We present a new method for the statistical investigation into the
distributions of the angle beta between the radio axis and the normal to the
galactic disk for a sample of Seyfert galaxies. We discuss how further
observations of the sample galaxies can strengthen the conclusions. Our data
are consistent with the hypothesis that AGN jets are oriented randomly in
space, independent of the position of the plane of the galaxy. By making the
simple assumption that the Standard Model of AGN holds, with a universal
opening angle of the thick torus of phi_c, we demonstrate a statistical method
to obtain an estimate of phi_c. Our data are not consistent with the
simple-minded idea that Seyfert 1s and Seyfert 2s are differentiated solely by
whether or not our line of sight lies within some fixed angle of the jet axis.
Our result is significant on the 2 sigma level and can thus be considered only
suggestive, not conclusive. A complete sample of Seyfert galaxies selected on
an isotropic property is required to obtain a conclusive result.Comment: 13 pages, Tex, 5 Postscript figures. Accepted Ap
Investigating Binary Properties with Next-Generation Microlensing Surveys
We explore the usefulness of future gravitational microlensing surveys in the
study of binary properties such as the binary fraction and the distributions of
binary separation and mass ratio by using the binary sample detectable through
a channel of repeating events. For this, we estimate the rate of repeating
microlensing eventstoward the Galactic bulge field based on standard models of
dynamical and physical distributions of Galactic matter combined with models of
binary separation and mass function. From this, we find that the total number
of repeating events expected to be detected from -year space-based
surveys will be --400, that is --50 times higher than the
rate of current surveys. We find that the high detection rate is due to the
greatly improved sensitivity to events associated with faint source stars and
low-magnification events. We find that the separation range of the binaries to
be covered by the repeating events will extend up to 100 AU. Therefore, the
future lensing surveys will provide a homogeneous sample that will allow to
investigate the statistical properties of Galactic binaries unbiased by
brightness of the binary components.Comment: total 6 pages, including 4 figures, ApJ, in pres
Parallel Performance for a Real Time Lattice Boltzmann Code
The paper will present the details of a Lattice Boltzmann solver running in real time for unsteady
wake computations. In addition to algorithmic implementation, computational results, single
core and parallel optimization of the methods are also discussed
Increased C3 productivity in Midwestern lawns since 1982 revealed by carbon isotopes in Amanita thiersii
How climate and rising carbon dioxide concentrations (pCO2) have influenced competition between C3 and C4 plants over the last 50âyears is a critical uncertainty in climate change research. Here we used carbon isotope (ÎŽ13C) values of the saprotrophic lawn fungus Amanita thiersii to integrate the signal of C3 and C4 carbon in samples collected between 1982 and 2009 from the Midwestern USA. We then calculated 13C fractionation (Î) to assess the balance between C3 and C4 photosynthesis as influenced by mean annual temperature (MAT), mean annual precipitation over a 30âyear period (MAPâ30), and pCO2. Sporocarp Î correlated negatively with MAT (â1.74â°â°Câ1, 79% of variance) and positively with MAP (9.52â°âmâ1, 15% of variance), reflecting the relative productivity of C3 and C4 grasses in lawns. In addition, Î values correlated positively with pCO2 (0.072â°âppmâ1, 5% of variance). Reduced photorespiration with rising pCO2 accounted for 20% of this increased Î, but the remaining 80% is consistent with increased assimilation of C3âderived carbon by Amanita thiersii resulting from increased productivity of C3 grasses with rising pCO2. Between 1982 and 2009, pCO2 rose by 46âppm and the relative contribution of C3 photosynthesis to Amanita thiersii carbon increased 18.5%. The ÎŽ13C value of Amanita thiersii may integrate both lawn maintenance practices and the physiological responses of turf grasses to rising CO2 concentrations
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