109 research outputs found
Radiative Transfer Model of Dust Attenuation Curves in Clumpy, Galactic Environments
The attenuation of starlight by dust in galactic environments is investigated
through models of radiative transfer in a spherical, clumpy ISM. Extinction
properties for MW, LMC, and SMC dust types are considered. It is illustrated
that the attenuation curves are primarily determined by the wavelength
dependence of absorption rather than by the underlying extinction
(absorption+scattering) curve. Attenuation curves consistent with the "Calzetti
attenuation curve" are found by assuming the silicate-carbonaceous dust model
for the MW, but with the 2175A absorption bump suppressed or absent. The
discrepancy between our results and previous work that claimed the SMC-type
dust to be the most likely origin of the Calzetti curve is ascribed to the
difference in adopted albedos; this study uses the theoretically calculated
albedos whereas the previous ones adopted empirically derived albedos from
observations of reflection nebulae. It is also found that the model attenuation
curves calculated with the MW dust are well represented by a modified Calzetti
curve with a varying slope and UV bump strength. The strong correlation between
the slope and UV bump strength, with steeper curves having stronger bumps, as
found in star-forming galaxies at 0.5<z<2.0, is well reproduced by our models
if the abundance of the UV bump carriers or PAHs is assumed to be 30% or 40% of
that of the MW-dust. The trend is explained by radiative transfer effects which
lead to shallower attenuation curves with weaker UV bumps as the ISM is more
clumpy and dustier. We also argue that at least some of the IUE local starburst
galaxies may have a UV bump feature in their attenuation curves, albeit much
weaker than that of the MW extinction curve.Comment: 28 pages, 30 figures, submited to ApJ
Polarized Infrared Emission by Polycyclic Aromatic Hydrocarbons resulting from Anisotropic Illumination
We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons
(PAHs), when anisotropically illuminated by UV photons. PAH molecules are
modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As
first pointed out by Leger (1988), infrared emission features resulting from
in-plane and out-of-plane modes should have orthogonal polarization directions.
We show analytically how the degree of polarization depends on the viewing
geometry and the molecule's internal alignment between principal axis of
inertia and angular momentum, which gets worse after photon absorption. Longer
wavelength features, emitted after better internal alignment is recovered,
should be more strongly polarized. The degree of polarization for
uni-directional illumination (e.g., by a star) is larger than for diffuse
illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the
Cold Neutral Medium, the predicted polarization is probably too small to
distinguish from the contribution of linear dichroism by aligned foreground
dust. The level of polarization predicted for PAH emission from the Orion Bar
is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger
value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately
suprathermal rotation rates. Future observations of (or upper limits on) the
degree of polarization for the Orion Bar or for dust above edge-on galaxies
(e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs,
thus providing clues to their rotational dynamics.Comment: 9 pages, 4 figures, 1 table, submitted to Ap
Radiative properties of visible and subvisible Cirrus: Scattering on hexagonal ice crystals
One of the main objectives of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) is to provide a better understanding of the physics of upper level clouds. The focus is on just one specific aspect of cirrus physics, namely on characterizing the radiative properties of single, nonspherical ice particles. The basis for further more extensive studies of the radiative transfer through upper level clouds is provided. Radiation provides a potential mechanism for strong feedback between the divergence of in-cloud radiative flux and the cloud microphysics and ultimately on the dynamics of the cloud. Some aspects of ice cloud microphysics that are relevant to the radiation calculations are described. Next, the Discrete Dipole Approximation (DDA) is introduced and some new results of scattering by irregular crystals are presented. The Anomalous Diffraction Theory (ADT) was adopted to investigate the scattering properties of even larger crystals. In this way the scattering properties of nonspherical particles were determined over a range of particle sizes
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