15 research outputs found
Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments
We present analytical approximations for calculating the scattering,
absorption and escape of nonionizing photons from a spherically symmetric
two-phase clumpy medium, with either a central point source of isotropic
radiation, a uniform distribution of isotropic emitters, or uniformly
illuminated by external sources. The analytical approximations are based on the
mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman,
combined with escape and absorption probability formulae for homogeneous media.
The accuracy of the approximations is examined by comparison with 3D Monte
Carlo simulations of radiative transfer, including multiple scattering. Our
studies show that the combined mega-grains and escape/absorption probability
formulae provide a good approximation of the escaping and absorbed radiation
fractions for a wide range of parameters characterizing the medium. A realistic
test is performed by modeling the absorption of a starlike source of radiation
by interstellar dust in a clumpy medium, and by calculating the resulting
equilibrium dust temperatures and infrared emission spectrum of both the clumps
and the interclump medium. In particular, we find that the temperature of dust
in clumps is lower than in the interclump medium if clumps are optically thick.
Comparison with Monte Carlo simulations of radiative transfer in the same
environment shows that the analytic model yields a good approximation of dust
temperatures and the emerging UV to FIR spectrum of radiation for all three
types of source distributions mentioned above. Our analytical model provides a
numerically expedient way to estimate radiative transfer in a variety of
interstellar conditions and can be applied to a wide range of astrophysical
environments, from star forming regions to starburst galaxies.Comment: 55 pages, 27 figures. ApJ 523 (1999), in press. Corrected equations
and text so as to be same as ApJ versio
Effects of clumping on temperature I: externally heated clouds
We present a study of radiative transfer in dusty, clumpy star-forming
regions. A series of self-consistent, 3-D, continuum radiative transfer models
are constructed for a grid of models parameterized by central luminosity,
filling factor, clump radius, and face-averaged optical depth. The temperature
distribution within the clouds is studied as a function of this
parameterization. Among our results, we find that: (a) the effective optical
depth is smaller in clumpy regions than in equivalent homogeneous regions; (b)
penetration of radiation is drive by the fraction of open sky (FOS) -- which
measures the fraction of solid angle which is devoid of clumps; (c) FOS
increases as clump radius increases and filling factor decreases; (d) for FOS >
0.6-0.8 the sky is sufficiently open that the temperature is relatively
insensitive to FOS; (e) the physical process by which radiation penetrates is
streaming between clumps; (f) filling factor dominates the temperature
distribution for large optical depths, and at small clump radii for small
optical depths; (g) at lower optical depths, the temperature distribution is
most sensitive to filling factors of 1-10 per cent, in accordance with many
observations; (h) direct shadowing can be important approximately one clump
radius behind a clump.Comment: 12 pages, 17 figures, accepted by MNRA
Deconvolution of Images from BLAST 2005: Insight into the K3-50 and IC 5146 Star-Forming Regions
We present an implementation of the iterative flux-conserving Lucy-Richardson
(L-R) deconvolution method of image restoration for maps produced by the
Balloon-borne Large Aperture Submillimeter Telescope (BLAST). We have analyzed
its performance and convergence extensively through simulations and
cross-correlations of the deconvolved images with available highresolution
maps. We present new science results from two BLAST surveys, in the Galactic
regions K3-50 and IC 5146, further demonstrating the benefits of performing
this deconvolution.
We have resolved three clumps within a radius of 4.'5 inside the star-forming
molecular cloud containing K3-50. Combining the well-resolved dust emission map
with available multi-wavelength data, we have constrained the Spectral Energy
Distributions (SEDs) of five clumps to obtain masses (M), bolometric
luminosities (L), and dust temperatures (T). The L-M diagram has been used as a
diagnostic tool to estimate the evolutionary stages of the clumps. There are
close relationships between dust continuum emission and both 21-cm radio
continuum and 12CO molecular line emission.
The restored extended large scale structures in the Northern Streamer of IC
5146 have a strong spatial correlation with both SCUBA and high resolution
extinction images. A dust temperature of 12 K has been obtained for the central
filament. We report physical properties of ten compact sources, including six
associated protostars, by fitting SEDs to multi-wavelength data. All of these
compact sources are still quite cold (typical temperature below ~ 16 K) and are
above the critical Bonner-Ebert mass. They have associated low-power Young
Stellar Objects (YSOs). Further evidence for starless clumps has also been
found in the IC 5146 region.Comment: 13 pages, 12 Figures, 3 Table
A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni
Observations of binary stars containing an accreting black hole or neutron star often show x-ray emission extending to high energies (>10 kilo–electron volts), which is ascribed to an accretion disk corona of energetic particles akin to those seen in the solar corona. Despite their ubiquity, the physical conditions in accretion disk coronae remain poorly constrained. Using simultaneous infrared, optical, x-ray, and radio observations of the Galactic black hole system V404 Cygni, showing a rapid synchrotron cooling event in its 2015 outburst, we present a precise 461 ± 12 gauss magnetic field measurement in the corona. This measurement is substantially lower than previous estimates for such systems, providing constraints on physical models of accretion physics in black hole and neutron star binary systems.
This article has a correction. Please see: http://science.sciencemag.org/content/360/6386/eaat927
Erratum for the Report “A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni”
In the Report “A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni,” a calculation error led to values of the magnetic field that were about 14 times too high. The mathematical expressions given in the Report were correct, but the code used to calculate the numerical values included an extraneous factor, which led to incorrect results. The magnetic fields calculated from the observations at different wavelengths were all scaled by the same factor, so after this is removed they remain consistent with each other. The corrected value of the magnetic field is lower than previously calculated, making the field in V404 Cygni even more unlike those estimated for other systems. However, the lower magnetic field is no longer consistent with the value predicted from the equipartition model. The text, materials and methods, Table S1, and Figure S3 have been updated to reflect the corrected magnetic field values and to state that the system was not in equipartition. No other results or conclusions of the study were affected. The authors thank J. Malzac (Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse) for alerting them to this error
A novel method for aircraft attitude estimation using magnetic field sensors and dynamic modeling
Orlando, Florida - USA
Attitude compensated electronic compass for aircraft navigation
Orlando, Florida - US