60 research outputs found
Wrapping an adhesive sphere with a sheet
We study the adhesion of an elastic sheet on a rigid spherical substrate.
Gauss'Theorema Egregium shows that this operation necessarily generates metric
distortions (i.e. stretching) as well as bending. As a result, a large variety
of contact patterns ranging from simple disks to complex branched shapes are
observed as a function of both geometrical and material properties. We describe
these different morphologies as a function of two non-dimensional parameters
comparing respectively bending and stretching energies to adhesion. A complete
configuration diagram is finally proposed
On the turbulent -disks and the intermittent activity in AGN
We consider effects of the MHD turbulence on the viscosity during the
evolution of the thermal-viscous ionization instability in the standard
-accretion disks. We consider the possibility that the accretion onto a
supermassive black hole proceeds through an outer standard accretion disk and
inner, radiatively inefficient and advection dominated flow. In this scenario
we follow the time evolution of the accretion disk in which the viscosity
parameter is constant throughout the whole instability cycle, as
implied by the strength of MHD turbulence. We conclude that the hydrogen
ionization instability is a promising mechanism to explain the intermittent
activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte
Interpreting the Variability of Double-Peaked Emission Lines in Active Galactic Nuclei with Stochastically Perturbed Accretion Disk Models
In an effort to explain the short-timescale variability of the broad,
double-peaked profiles of some active galactic nuclei, we constructed
stochastically perturbed accretion disk models and calculated H alpha line
profile series as the bright spots rotate, shear and decay. We determined the
dependence of the properties of the line profile variability on the spot
properties. We compared the variability of the line profile from the models to
the observed variability of the H alpha line of Arp 102B and 3C 390.3. We find
that spots need to be concentrated in the outer parts of the line emitting
region to reproduce the observed variability properties for Arp 102B. This
rules out spot production by star/disk collisions and favors a scenario where
the radius of marginal self-gravity is within the line emitting region,
creating a sharp increase in the radial spot distribution in the outer parts.
In the case of 3C 390.3, all the families of models that we tested can
reproduce the observed variability for a suitable choice of model parameters.Comment: 27 pages, 8 figures, accepted for publication in Ap
Mira's wind explored in scattering infrared CO lines
We have observed the intermediate regions of the circumstellar envelope of
Mira (o Ceti) in photospheric light scattered by three vibration-rotation
transitions of the fundamental band of CO, from low-excited rotational levels
of the ground vibrational state, at an angular distance of beta = 2"-7" away
from the star. The data were obtained with the Phoenix spectrometer mounted on
the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately
0.5" and seeing limited. Our observations provide absolute fluxes, leading to
an independent new estimate of the mass-loss rate of approximately 3e-7
Msun/yr, as derived from a simple analytic wind model. We find that the
scattered intensity from the wind of Mira for 2" < beta < 7" decreases as
beta^-3, which suggests a time constant mass-loss rate, when averaged over 100
years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa
Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and their Isotopes
A solar photospheric "thermal profiling" analysis is presented, exploiting
the infrared rovibrational bands of carbon monoxide (CO) as observed with the
McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above
the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum
intensities and center-limb behavior constrained the temperature profile of the
deep photosphere, while CO center-limb behavior defined the thermal structure
at higher altitudes. The oxygen abundance was self consistently determined from
weak CO absorptions. Our analysis was meant to complement recent studies based
on 3-D convection models which, among other things, have revised the historical
solar oxygen (and carbon) abundance downward by a factor of nearly two;
although in fact our conclusions do not support such a revision. Based on
various considerations, an oxygen abundance of 700+/-100 ppm (parts per million
relative to hydrogen) is recommended; the large uncertainty reflects the model
sensitivity of CO. New solar isotopic ratios also are reported for 13C, 17O,
and 18O.Comment: 90 pages, 19 figures (some with parts "a", "b", etc.); to be
published in the Astrophysical Journal Supplement
Observations of H3+ in the Diffuse Interstellar Medium
Surprisingly large column densities of H3+ have been detected using infrared
absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12,
Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating
that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard
model of diffuse cloud chemistry, our H3+ column densities imply unreasonably
long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary
millimeter-wave, infrared, and visible observations of related species suggest
that the chemical model is incorrect and that the number density of H3+ must be
increased by one to two orders of magnitude. Possible solutions include a
reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller
value of the H3+ dissociative recombination rate constant than implied by
laboratory experiments.Comment: To be published in Astrophysical Journal, March 200
Radiation pressure instability driven variability in the accreting black holes
The time dependent evolution of the accretion disk around black hole is
computed. The classical description of the -viscosity is adopted so the
evolution is driven by the instability operating in the innermost
radiation-pressure dominated part of the accretion disk. We assume that the
optically thick disk always extends down to the marginally stable orbit so it
is never evacuated completely. We include the effect of the advection, coronal
dissipation and vertical outflow. We show that the presence of the corona
and/or the outflow reduce the amplitude of the outburst. If only about half of
the energy is dissipated in the disk (with the other half dissipated in the
corona and carried away by the outflow) the outburst amplitude and duration are
consistent with observations of the microquasar GRS 1915+105. Viscous evolution
explains in a natural way the lack of direct transitions from the state C to
the state B in color-color diagram of this source. Further reduction of the
fraction of energy dissipated in the optically thick disk switches off the
outbursts which may explain why they are not seen in all high accretion rate
sources being in the Very High State.Comment: 31 pages, 14 figures; accepted to Ap
The Evolution of Black Hole Mass and Spin in Active Galactic Nuclei
We argue that supermassive black hole growth in AGN occurs via sequences of
randomly--oriented accretion discs with angular momentum limited by
self--gravity. These stably co-- or counter--align with the black hole spin
with almost equal frequency. Accretion from these discs very rapidly adjusts
the hole's spin parameter to average values (the precise
range depending slightly on the disc vertical viscosity coefficient )
from any initial conditions, but with significant fluctuations () about these. We conclude (a) AGN black holes should on average spin
moderately, with the mean value decreasing slowly as the mass
increases; (b) SMBH coalescences leave little long--term effect on ;
(c) SMBH coalescence products in general have modest recoil velocities, so that
there is little likelihood of their being ejected from the host galaxy; (d)
black holes can grow even from stellar masses to \sim 5\times 10^9 \msun at
high redshift ; (e) jets produced in successive accretion episodes can
have similar directions, but after several episodes the jet direction deviates
significantly. Rare examples of massive holes with larger spin parameters could
result from prograde coalescences with SMBH of similar mass, and are most
likely to be found in giant ellipticals. We compare these results with
observation.
(abridged)Comment: MNRAS, in pres
Symbiotic starburst-black hole AGN -- I. Isothermal hydrodynamics of the mass-loaded ISM
Compelling evidence associates the nuclei of active galaxies and massive
starbursts. The symbiosis between a compact nuclear starburst stellar cluster
and a massive black hole can self-consistently explain the properties of active
nuclei. The young stellar cluster has a profound effect on the most important
observable properties of active galaxies through its gravity, and by mass
injection through stellar winds, supernovae and stellar collisions. Mass
injection generates a nuclear ISM which flows under gravitational and radiative
forces until it leaves the nucleus or is accreted onto the black hole or
accretion disc.
The radiative force exerted by the black hole--accretion disc radiation field
is not spherically symmetric. This results in complex flows in which regions of
inflow can coexist with high Mach number outflowing winds and hydrodynamic
jets. We present two-dimensional hydrodynamic models of such nISM flows, which
are highly complex and time variable. Shocked shells, jets and explosive
bubbles are produced, with bipolar winds driving out from the nucleus. Our
results graphically illustrate why broad emission line studies have
consistently failed to identify any simple, global flow geometry. The real
structure of the flows is _inevitably_ yet more complex.Comment: 51 pages, 85 postscript figures, Latex, using MNRAS macros, to be
published in MNRAS. Postscript will full resolution pictures and mpeg
simulations available via http://ast.leeds.ac.uk/~rjrw/agn.htm
Phasic Phosphorylation of Caldesmon and ERK 1/2 during Contractions in Human Myometrium
Human myometrium develops phasic contractions during labor. Phosphorylation of caldesmon (h-CaD) and extracellular signal-regulated kinase 1/2 (ERK 1/2) has been implicated in development of these contractions, however the phospho-regulation of these proteins is yet to be examined during periods of both contraction and relaxation. We hypothesized that protein phosphorylation events are implicated in the phasic nature of myometrial contractions, and aimed to examine h-CaD and ERK 1/2 phosphorylation in myometrium snap frozen at specific stages, including; (1) prior to onset of contractions, (2) at peak contraction and (3) during relaxation. We aimed to compare h-CaD and ERK 1/2 phosphorylation in vitro against results from in vivo studies that compared not-in-labor (NIL) and laboring (L) myometrium. Comparison of NIL (nâ=â8) and L (nâ=â8) myometrium revealed a 2-fold increase in h-CaD phosphorylation (ser-789; Pâ=â0.012) during onset of labor in vivo, and was associated with significantly up-regulated ERK2 expression (Pâ=â0.022), however no change in ERK2 phosphorylation was observed (Pâ=â0.475). During in vitro studies (nâ=â5), transition from non-contracting tissue to tissue at peak contraction was associated with increased phosphorylation of both h-CaD and ERK 1/2. Furthermore, tissue preserved at relaxation phase exhibited diminished levels of h-CaD and ERK 1/2 phosphorylation compared to tissue preserved at peak contraction, thereby producing a phasic phosphorylation profile for h-CaD and ERK 1/2. h-CaD and ERK 1/2 are phosphorylated during myometrial contractions, however their phospho-regulation is dynamic, in that h-CaD and ERK 1/2 are phosphorylated and dephosphorylated in phase with contraction and relaxation respectively. Comparisons of NIL and L tissue are at risk of failing to detect these changes, as L samples are not necessarily preserved in the midst of an active contraction
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