160,127 research outputs found
Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium
We compare the performance of several dust models in reproducing the dust
spectral energy distribution (SED) per unit extinction in the diffuse
interstellar medium (ISM). We use our results to constrain the variability of
the optical properties of big grains in the diffuse ISM, as published by the
Planck collaboration.
We use two different techniques to compare the predictions of dust models to
data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared
emission spectrum to recover the dust extinction and the intensity of the
interstellar radiation field (ISRF). Second, we infer the ISRF intensity from
the total power emitted by dust per unit extinction, and then predict the
emission spectrum. In both cases, we test the ability of the models to
reproduce dust emission and extinction at the same time.
We identify two issues. Not all models can reproduce the average dust
emission per unit extinction: there are differences of up to a factor
between models, and the best accord between model and observation is obtained
with the more emissive grains derived from recent laboratory data on silicates
and amorphous carbons. All models fail to reproduce the variations in the
emission per unit extinction if the only variable parameter is the ISRF
intensity: this confirms that the optical properties of dust are indeed
variable in the diffuse ISM.
Diffuse ISM observations are consistent with a scenario where both ISRF
intensity and dust optical properties vary. The ratio of the far-infrared
opacity to the band extinction cross-section presents variations of the
order of ( in extreme cases), while ISRF intensity varies
by ( in extreme cases). This must be accounted for in
future modelling.Comment: A&A, in pres
The fine structure line deficit in S 140
We try to understand the gas heating and cooling in the S 140 star forming
region by spatially and spectrally resolving the distribution of the main
cooling lines with GREAT/SOFIA. We mapped the fine structure lines of [OI] (63
{\mu}m) and [CII] (158 {\mu}m) and the rotational transitions of CO 13-12 and
16-15 with GREAT/SOFIA and analyzed the spatial and velocity structure to
assign the emission to individual heating sources. We measure the optical depth
of the [CII] line and perform radiative transfer computations for all observed
transitions. By comparing the line intensities with the far-infrared continuum
we can assess the total cooling budget and measure the gas heating efficiency.
The main emission of fine structure lines in S 140 stems from a 8.3'' region
close to the infrared source IRS 2 that is not prominent at any other
wavelength. It can be explained by a photon-dominated region (PDR) structure
around the embedded cluster if we assume that the [OI] line intensity is
reduced by a factor seven due to self-absorption. The external cloud interface
forms a second PDR at an inclination of 80-85 degrees illuminated by an UV
field of 60 times the standard interstellar radiation field. The main radiation
source in the cloud, IRS 1, is not prominent at all in the fine structure
lines. We measure line-to-continuum cooling ratios below 10^(-4), i.e. values
lower than in any other Galactic source, rather matching the far-IR line
deficit seen in ULIRGs. In particular the low intensity of the [CII] line can
only be modeled by an extreme excitation gradient in the gas around IRS 1. We
found no explanation why IRS 1 shows no associated fine-structure line peak,
while IRS 2 does. The inner part of S 140 mimics the far-IR line deficit in
ULIRGs thereby providing a template that may lead to a future model.Comment: Accepted for publication in Astronomy & Astrophysic
Effect of Scatterering on Coherent Anti-Stokes Raman Scattering (CARS) signals
We develop a computational framework to examine the factors responsible for
scattering-induced distortions of coherent anti-Stokes Raman scattering (CARS)
signals in turbid samples. We apply the Huygens-Fresnel Wave-based Electric
Field Superposition (HF-WEFS) method combined with the radiating dipole
approximation to compute the effects of scattering-induced distortions of focal
excitation fields on the far-field CARS signal. We analyze the effect of
spherical scatterers, placed in the vicinity of the focal volume, on the CARS
signal emitted by different objects (2{\mu}m diameter solid sphere, 2{\mu}m
diameter myelin cylinder and 2{\mu}m diameter myelin tube). We find that
distortions in the CARS signals arise not only from attenuation of the focal
field but also from scattering-induced changes in the spatial phase that
modifies the angular distribution of the CARS emission. Our simulations further
show that CARS signal attenuation can be minimized by using a high numerical
aperture condenser. Moreover, unlike the CARS intensity image, CARS images
formed by taking the ratio of CARS signals obtained using x- and y-polarized
input fields is relatively insensitive to the effects of spherical scatterers.
Our computational framework provide a mechanistic approach to characterizing
scattering-induced distortions in coherent imaging of turbid media and may
inspire bottom-up approaches for adaptive optical methods for image correction.Comment: 15 pages, 7 figure
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