1,569 research outputs found
Stellar disruption by a supermassive black hole: is the light curve really proportional to ?
In this paper we revisit the arguments for the basis of the time evolution of
the flares expected to arise when a star is disrupted by a supermassive black
hole. We present a simple analytic model relating the lightcurve to the
internal density structure of the star. We thus show that the standard
lightcurve proportional to only holds at late times. Close to the
peak luminosity the lightcurve is shallower, deviating more strongly from
for more centrally concentrated (e.g. solar--type) stars. We test
our model numerically by simulating the tidal disruption of several stellar
models, described by simple polytropic spheres with index . The
simulations agree with the analytical model given two considerations. First,
the stars are somewhat inflated on reaching pericentre because of the effective
reduction of gravity in the tidal field of the black hole. This is well
described by a homologous expansion by a factor which becomes smaller as the
polytropic index becomes larger. Second, for large polytropic indices wings
appear in the tails of the energy distribution, indicating that some material
is pushed further away from parabolic orbits by shocks in the tidal tails. In
all our simulations, the lightcurve is achieved only at late stages.
In particular we predict that for solar type stars, this happens only after the
luminosity has dropped by at least two magnitudes from the peak. We discuss our
results in the light of recent observations of flares in otherwise quiescent
galaxies and note the dependence of these results on further parameters, such
as the star/hole mass ratio and the stellar orbit.Comment: 10 pages, 10 figures, MNRAS accepte
Black hole mergers: the first light
The coalescence of supermassive black hole binaries occurs via the emission
of gravitational waves, that can impart a substantial recoil to the merged
black hole. We consider the energy dissipation, that results if the recoiling
black hole is surrounded by a thin circumbinary disc. Our results differ
significantly from those of previous investigations. We show analytically that
the dominant source of energy is often potential energy, released as gas in the
outer disc attempts to circularize at smaller radii. Thus, dimensional
estimates, that include only the kinetic energy gained by the disc gas,
underestimate the real energy loss. This underestimate can exceed an order of
magnitude, if the recoil is directed close to the disc plane. We use three
dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional
finite difference simulations to verify our analytic estimates. We also compute
the bolometric light curve, which is found to vary strongly depending upon the
kick angle. A prompt emission signature due to this mechanism may be observable
for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about
1000 km/s. Emission at earlier times can mainly result from the response of the
disc to the loss of mass, as the black holes merge. We derive analytically the
condition for this to happen.Comment: 16 pages, accepted by MNRAS. Animations of the simulations are
available at http://jilawww.colorado.edu/~pja/recoil.htm
Planetesimal formation via fragmentation in self-gravitating protoplanetary discs
An unsolved issue in the standard core accretion model for gaseous planet formation is how kilometre-sized planetesimals form from, initially, micron-sized dust grains. Solid growth beyond metre sizes can be difficult both because the sticking efficiency becomes very small, and because these particles should rapidly migrate into the central star. We consider here how metre-sized particles evolve in self-gravitating accretion discs using simulations in which the gravitational influence of the solid particles is also included. Metre-sized particles become strongly concentrated in the spiral structures present in the disc and, if the solid to gas density ratio is sufficiently high, can fragment due to their own self-gravity to form planetesimals directly. This result suggests that planetesimal formation may occur very early in the star formation process while discs are still massive enough to be self-gravitating. The dependence of this process on the surface density of the solids is also consistent with the observation that extrasolar planets are preferentially found around high metallicity stars
The 2007 Stromboli eruption: event chronology and effusion rates using thermal infrared data
Using thermal infrared images recorded by a permanent thermal camera network maintained on Stromboli volcano (Italy), together with satellite and helicopter-based thermal image surveys, we have compiled a chronology of the events and processes occurring before and during Stromboli’s 2007 effusive eruption. These digital data also allow us to calculate the effusion rates and lava volumes erupted during the effusive episode. At the onset of the 2007 eruption, two parallel eruptive fissures developed within the North East crater, eventually breaching the NE flank of the summit cone and extending along the eastern margin of the Sciara del Fuoco. These fed a main effusive vent at 400 m a.s.l. to feed lava flows that extended to the sea. The effusive eruption was punctuated, on 15 March, by a paroxysm with features similar to the 5 April paroxysm that occurred during the 2002-03 effusive eruption. A total of between 3.2 x 106 m3 and 11 x 106 m3 of lava was erupted during the 2007 eruption over 34 days of effusive activity. More than half of this volume was emplaced during the first 5.5 days of the eruption. Although the 2007 effusive eruption had a comparable erupted volume to the previous (2002-03) effusive eruption, it had a shorter duration and thus a mean output rate (= total volume divided by eruption duration) that was one order of magnitude greater than the 2002-03 event (~2.4 m3 s-1 compared with 0.32±0.28 m3 s-1). In this paper, we discuss similarities and differences between these two effusive events, and interpret the processes occurring in 2007 in terms of the recent dynamics witnessed at Stromboli
The 2007 Stromboli eruption: Event chronology and effusion rates using thermal infrared data
Using thermal infrared images recorded by a permanent thermal camera network maintained on Stromboli volcano (Italy), together with satellite and helicopter‐based thermal image surveys, we have compiled a chronology of the events and processes occurring before and during Stromboli’s 2007 effusive eruption. These digital data also allow us to calculate the effusion rates and lava volumes erupted during the effusive episode.
At the onset of the 2007 eruption, two parallel eruptive fissures developed within the northeast crater, eventually breaching the NE flank of the summit cone and extending along the easternmargin of the Sciara del Fuoco. These fed amain effusive vent at 400m above sea level to feed lava flows that extended to the sea. The effusive eruption was punctuated,on 15 March, by a paroxysm with features similar to those of the 5 April paroxysm that occurred during the 2002–2003 effusive eruption. A total of between 3.2 × 106 and 11 × 106 m3
of lava was erupted during the 2007 eruption, over 34 days of effusive activity. More than half of this volume was emplaced during the first 5.5 days of the eruption. Although the 2007 effusive eruption had an erupted volume comparable to that of the previous (2002–2003) effusive eruption, it had a shorter duration and thus a mean output rate (=total volume divided by eruption duration) that was 1 order of magnitude higher than that of the 2002–
2003 event (∼2.4 versus 0.32 ± 0.28 m3
s−1). In this paper, we discuss similarities and differences between these two effusive events and interpret the processes occurring in 2007 in terms of the recent dynamics witnessed at Stromboli
Black-Hole Spin Dependence in the Light Curves of Tidal Disruption Events
A star orbiting a supermassive black hole can be tidally disrupted if the
black hole's gravitational tidal field exceeds the star's self gravity at
pericenter. Some of this stellar tidal debris can become gravitationally bound
to the black hole, leading to a bright electromagnetic flare with bolometric
luminosity proportional to the rate at which material falls back to pericenter.
In the Newtonian limit, this flare will have a light curve that scales as
t^-5/3 if the tidal debris has a flat distribution in binding energy. We
investigate the time dependence of the black-hole mass accretion rate when
tidal disruption occurs close enough the black hole that relativistic effects
are significant. We find that for orbits with pericenters comparable to the
radius of the marginally bound circular orbit, relativistic effects can double
the peak accretion rate and halve the time it takes to reach this peak
accretion rate. The accretion rate depends on both the magnitude of the
black-hole spin and its orientation with respect to the stellar orbit; for
orbits with a given pericenter radius in Boyer-Lindquist coordinates, a maximal
black-hole spin anti-aligned with the orbital angular momentum leads to the
largest peak accretion rate.Comment: 16 pages, 15 figures, 1 table, PRD published versio
Demographics of young stars and their protoplanetary disks: lessons learned on disk evolution and its connection to planet formation
Since Protostars and Planets VI (PPVI), our knowledge of the global
properties of protoplanetary and debris disks, as well as of young stars, has
dramatically improved. At the time of PPVI, mm-observations and optical to
near-infrared spectroscopic surveys were largely limited to the Taurus
star-forming region, especially of its most massive disk and stellar
population. Now, near-complete surveys of multiple star-forming regions cover
both spectroscopy of young stars and mm interferometry of their protoplanetary
disks. This provides an unprecedented statistical sample of stellar masses and
mass accretion rates, as well as disk masses and radii, for almost 1000 young
stellar objects within 300 pc from us, while also sampling different
evolutionary stages, ages, and environments. At the same time, surveys of
debris disks are revealing the bulk properties of this class of more evolved
objects. This chapter reviews the statistics of these measured global star and
disk properties and discusses their constraints on theoretical models
describing global disk evolution. Our comparisons of observations to
theoretical model predictions extends beyond the traditional viscous evolution
framework to include analytical descriptions of magnetic wind effects. Finally,
we discuss how recent observational results can provide a framework for models
of planet population synthesis and planet formation.Comment: Review Chapter for Protostars and Planets VII, Editors: Shu-ichiro
Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamura.
Accepted version after interaction with the referees and before community
feedback. 21 pages (24 with references), 8 figures. Data table available at
http://ppvii.org/chapter/15
The Shape of an Accretion Disc in a Misaligned Black Hole Binary
We model the overall shape of an accretion disc in a semi-detached binary
system in which mass is transfered on to a spinning black hole the spin axis of
which is misaligned with the orbital rotation axis. We assume the disc is in a
steady state. Its outer regions are subject to differential precession caused
by tidal torques of the companion star. These tend to align the outer parts of
the disc with the orbital plane. Its inner regions are subject to differential
precession caused by the Lense-Thirring effect. These tend to align the inner
parts of the disc with the spin of the black hole. We give full numerical
solutions for the shape of the disc for some particular disc parameters. We
then show how an analytic approximation to these solutions can be obtained for
the case when the disc surface density varies as a power law with radius. These
analytic solutions for the shape of the disc are reasonably accurate even for
large misalignments and can be simply applied for general disc parameters. They
are particularly useful when the numerical solutions would be slow.Comment: Accepted for publication in MNRA
Temporally divergent regulatory mechanisms govern neuronal diversification and maturation in the mouse and marmoset neocortex
Mammalian neocortical neurons span one of the most diverse cell type spectra of any tissue. Cortical neurons are born during embryonic development, and their maturation extends into postnatal life. The regulatory strategies underlying progressive neuronal development and maturation remain unclear. Here we present an integrated single-cell epigenomic and transcriptional analysis of individual mouse and marmoset cortical neuron classes, spanning both early postmitotic stages of identity acquisition and later stages of neuronal plasticity and circuit integration. We found that, in both species, the regulatory strategies controlling early and late stages of pan-neuronal development diverge. Early postmitotic neurons use more widely shared and evolutionarily conserved molecular regulatory programs. In contrast, programs active during later neuronal maturation are more brain- and neuron-specific and more evolutionarily divergent. Our work uncovers a temporal shift in regulatory choices during neuronal diversification and maturation in both mice and marmosets, which likely reflects unique evolutionary constraints on distinct events of neuronal development in the neocortex. The mechanisms underlying neuron specification and maturation are unclear. Here the authors provide an integrated epigenomic and transcriptomic analysis of mouse and marmoset neocortical neuronal classes. Pan-neuronal programs active during early development are more evolutionary conserved but not neuron-specific, whereas pan-neuronal programs active during later stages of maturation are more neuron- and species-specific
Warp propagation in astrophysical discs
Astrophysical discs are often warped, that is, their orbital planes change
with radius. This occurs whenever there is a non-axisymmetric force acting on
the disc, for example the Lense-Thirring precession induced by a misaligned
spinning black hole, or the gravitational pull of a misaligned companion. Such
misalignments appear to be generic in astrophysics. The wide range of systems
that can harbour warped discs - protostars, X-ray binaries, tidal disruption
events, quasars and others - allows for a rich variety in the disc's response.
Here we review the basic physics of warped discs and its implications.Comment: To be published in Astrophysical Black Holes by Haardt et al.,
Lecture Notes in Physics, Springer 2015. 19 pages, 2 figure
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