25,617 research outputs found
3-D SPH simulations of colliding winds in eta Carinae
We study colliding winds in the superluminous binary eta Carinae by
performing three-dimensional, Smoothed Particle Hydrodynamics (SPH)
simulations. For simplicity, we assume both winds to be isothermal. We also
assume that wind particles coast without any net external forces. We find that
the lower density, faster wind from the secondary carves out a spiral cavity in
the higher density, slower wind from the primary. Because of the
phase-dependent orbital motion, the cavity is very thin on the periastron side,
whereas it occupies a large volume on the apastron side. The model X-ray light
curve using the simulated density structure fits very well with the observed
light curve for a viewing angle of i=54 degrees and phi=36 degrees, where i is
the inclination angle and phi is the azimuth from apastron.Comment: 6 pages, 3 figures, To be published in Proceedings of IAU Symposium
250: Massive Stars as Cosmic Engines, held in Kauai, Hawaii, USA, Dec 2007,
edited by F. Bresolin, P.A. Crowther & J. Puls (Cambridge University Press
Pre-processing for approximate Bayesian computation in image analysis
Most of the existing algorithms for approximate Bayesian computation (ABC)
assume that it is feasible to simulate pseudo-data from the model at each
iteration. However, the computational cost of these simulations can be
prohibitive for high dimensional data. An important example is the Potts model,
which is commonly used in image analysis. Images encountered in real world
applications can have millions of pixels, therefore scalability is a major
concern. We apply ABC with a synthetic likelihood to the hidden Potts model
with additive Gaussian noise. Using a pre-processing step, we fit a binding
function to model the relationship between the model parameters and the
synthetic likelihood parameters. Our numerical experiments demonstrate that the
precomputed binding function dramatically improves the scalability of ABC,
reducing the average runtime required for model fitting from 71 hours to only 7
minutes. We also illustrate the method by estimating the smoothing parameter
for remotely sensed satellite imagery. Without precomputation, Bayesian
inference is impractical for datasets of that scale.Comment: 5th IMS-ISBA joint meeting (MCMSki IV
Will Carbon Find a Home on the Range? A Monte Carlo Simulation
Rangeland Carbon Sequestration, Agribusiness, Agricultural and Food Policy, Land Economics/Use,
Far Ultraviolet Spectroscopic Explorer Observations of a Supernova Remnant in the Line of Sight to HD 5980 in the Small Magellanic Cloud
We report a detection of far ultraviolet absorption from the supernova
remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is
seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the
LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a
velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s
with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2
absorption, but is present. These lines are not seen in the FUSE spectrum of
Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032 absorption
from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is
well described by a Gaussian with FWHM=75 km/s. We find log N(O
VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC
(excluding the SNR) and greater than the O VI column in the Milky Way halo
along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in
the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and
lower than the halo value, supporting models in which SNRs produce the highly
ionized gas close to the plane of the Galaxy, while other mechanisms occur in
the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this
sight line, suggesting that other mechanisms contribute to the creation of the
global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column
density profiles suggest the presence of a multi-phase shell followed by a
region of higher temperature gas.Comment: 7 pages, 3 figures, 2 tables, uses emulateapj5.sty. Accepted for
publication in ApJ Letter
X-ray Modeling of \eta\ Carinae and WR140 from SPH Simulations
The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions produce
hard X-rays that have been monitored extensively by several X-ray telescopes,
including RXTE. To interpret these RXTE X-ray light curves, we model the
wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations.
Adiabatic simulations that account for the absorption of X-rays from an assumed
point source at the apex of the wind-collision shock cone by the distorted
winds can closely match the observed 2-10keV RXTE light curves of both \eta\
Car and WR140. This point-source model can also explain the early recovery of
\eta\ Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4
reduction in the mass loss rate of \eta\ Car. Our more recent models relax the
point-source approximation and account for the spatially extended emission
along the wind-wind interaction shock front. For WR140, the computed X-ray
light curve again matches the RXTE observations quite well. But for \eta\ Car,
a hot, post-periastron bubble leads to an emission level that does not match
the extended X-ray minimum observed by RXTE. Initial results from incorporating
radiative cooling and radiatively-driven wind acceleration via a new
anti-gravity approach into the SPH code are also discussed.Comment: 5 pages, 3 figures, Proceedings of the 39th Li\'ege Astrophysical
Colloquium, held in Li\`ege 12-16 July 2010, edited by G. Rauw, M. De Becker,
Y. Naz\'e, J.-M. Vreux, P. William
Mean shear flows generated by nonlinear resonant Alfven waves
In the context of resonant absorption, nonlinearity has two different
manifestations. The first is the reduction in amplitude of perturbations around
the resonant point (wave energy absorption). The second is the generation of
mean shear flows outside the dissipative layer surrounding the resonant point.
Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the
slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494}, 317
(2009)] investigated nonlinearity at the Alfven resonance, however, they did
not include the generation of mean shear flow. In this present paper, we
investigate the mean shear flow, analytically, and study its properties. We
find that the flow generated is parallel to the magnetic surfaces and has a
characteristic velocity proportional to , where is
the dimensionless amplitude of perturbations far away from the resonance. This
is, qualitatively, similar to the flow generated at the slow resonance. The
jumps in the derivatives of the parallel and perpendicular components of mean
shear flow across the dissipative layer are derived. We estimate the generated
mean shear flow to be of the order of in both the solar
upper chromosphere and solar corona, however, this value strongly depends on
the choice of boundary conditions. It is proposed that the generated mean shear
flow can produce a Kelvin--Helmholtz instability at the dissipative layer which
can create turbulent motions. This instability would be an additional effect,
as a Kelvin--Helmholtz instability may already exist due to the velocity field
of the resonant Alfven waves. This flow can also be superimposed onto existing
large scale motions in the solar upper atmosphere.Comment: 11 page
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
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