8,126 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
Do wavelets really detect non-Gaussianity in the 4-year COBE data?
We investigate the detection of non-Gaussianity in the 4-year COBE data
reported by Pando, Valls-Gabaud & Fang (1998), using a technique based on the
discrete wavelet transform. Their analysis was performed on the two DMR faces
centred on the North and South Galactic poles respectively, using the
Daubechies 4 wavelet basis. We show that these results depend critically on the
orientation of the data, and so should be treated with caution. For two
distinct orientations of the data, we calculate unbiased estimates of the
skewness, kurtosis and scale-scale correlation of the corresponding wavelet
coefficients in all of the available scale domains of the transform. We obtain
several detections of non-Gaussianity in the DMR-DSMB map at greater than the
99 per cent confidence level, but most of these occur on pixel-pixel scales and
are therefore not cosmological in origin. Indeed, after removing all multipoles
beyond from the COBE maps, only one robust detection remains.
Moreover, using Monte-Carlo simulations, we find that the probability of
obtaining such a detection by chance is 0.59. We repeat the analysis for the
53+90 GHz coadded COBE map. In this case, after removing
multipoles, two non-Gaussian detections at the 99 per cent level remain.
Nevertheless, again using Monte-Carlo simulations, we find that the probability
of obtaining two such detections by chance is 0.28. Thus, we conclude the
wavelet technique does {\em not} yield strong evidence for non-Gaussianity of
cosmological origin in the 4-year COBE data.Comment: 7 pages, 5 figures. Revised version including discussion of
orientation sensitivity of the wavelet decomposition. MNRAS submitte
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Illumination system for the MICE tracker station assembly QA
Copyright @ 2007 MICEThis document describes the design and preparation of the optical system used to illuminate the scintillating-fibre planes to be used in the MICE Tracker. This illumination test during the tracker station assembly is a part of the quality assurance (QA) scheme. The optical design uses a two-stage approach: first, cylindrical optics are used to focus the round beam from the LED into to a long, thin shape. A mechanical slit is placed here to select an evenly illuminated region, providing it with well-defined edges. The second stage is a set of relay optics which project an image of the slit aperture on to the scintillating-fibre plane. A useful consequence of using relay optics rather than a simple slit close to the fibre plane is that wear or accidental damage to the fibres are avoided when the illumination system is being scanned across
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