1,784,345 research outputs found
Aggregate dust model to study the polarization properties of comet C/1996 B2 Hyakutake
In our present study, the observed linear polarization data of comet
Hyakutake are studied at wavelengths ,
and 0.684 through simulations using Ballistic Particle-Cluster Aggregate
and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical monomers.
We first investigated that the size parameter of the monomer, 1.56
-- 1.70, turned out to be most suitable which provides the best fits to the
observed dust scattering properties at three wavelengths , 0.485 and 0.684. Thus the effective radius of the aggregate
(r) lies in the range at ; at
and at . Now using
superposition \textsc{t-matrix} code and the power-law size distribution, , the best-fitting values of complex refractive indices are
calculated which can best fit the observed polarization data at the above three
wavelengths. The best-fitting complex refractive indices are found to
be (1.745, 0.095) at , (1.743, 0.100) at and (1.695, 0.100) at . The refractive
indices coming out from the present analysis correspond to mixture of both
silicates and organics, which are in good agreement with the \textit{in situ}
measurement of comets by different spacecraft.Comment: 9 pages, 3 figure
Grain size limits derived from 3.6 {\mu}m and 4.5 {\mu}m coreshine
Recently discovered scattered light from molecular cloud cores in the
wavelength range 3-5 {\mu}m (called "coreshine") seems to indicate the presence
of grains with sizes above 0.5 {\mu}m. We aim to analyze 3.6 and 4.5 {\mu}m
coreshine from molecular cloud cores to probe the largest grains in the size
distribution. We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in
the 3.6 and 4.5 {\mu}m bands for a sample of 10 low-mass cores. We used a new
modeling approach based on a combination of ratios of the two background- and
foreground-subtracted surface brightnesses and observed limits of the optical
depth. The dust grains were modeled as ice-coated silicate and carbonaceous
spheres. We discuss the impact of local radiation fields with a spectral slope
differing from what is seen in the DIRBE allsky maps. For the cores L260,
ecc806, L1262, L1517A, L1512, and L1544, the model reproduces the data with
maximum grain sizes around 0.9, 0.5, 0.65, 1.5, 0.6, and > 1.5 {\mu}m,
respectively. The maximum coreshine intensities of L1506C, L1439, and L1498 in
the individual bands require smaller maximum grain sizes than derived from the
observed distribution of band ratios. Additional isotropic local radiation
fields with a spectral shape differing from the DIRBE map shape do not remove
this discrepancy. In the case of Rho Oph 9, we were unable to reliably
disentangle the coreshine emission from background variations and the strong
local PAH emission. Considering surface brightness ratios in the 3.6 and 4.5
{\mu}m bands across a molecular cloud core is an effective method of
disentangling the complex interplay of structure and opacities when used in
combination with observed limits of the optical depth.Comment: 23 pages, 18 figures, accepted for publication in A&
Good measures on locally compact Cantor sets
We study the set M(X) of full non-atomic Borel (finite or infinite) measures
on a non-compact locally compact Cantor set X. For an infinite measure in
M(X), the set is called defective. We call
non-defective if . The class
consists of probability measures and infinite non-defective measures. We
classify measures from with respect to a homeomorphism. The
notions of goodness and compact open values set are defined. A
criterion when two good measures from are homeomorphic is given. For
any group-like we find a good probability measure on X
such that . For any group-like and any
locally compact, zero-dimensional, metric space A we find a good non-defective
measure on X such that and is
homeomorphic to A. We consider compactifications cX of X and give a criterion
when a good measure can be extended to a good measure on cX.Comment: 21 page
Far-Infrared Line Imaging of the Starburst Ring in NGC 1097 with the Herschel/PACS Spectrometer
NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst
ring. We study the physical properties of the interstellar medium (ISM) in the
ring using spatially resolved far-infrared spectral maps of the circumnuclear
starburst ring of NGC 1097, obtained with the PACS spectrometer on board the
Herschel Space Telescope. In particular, we map the important ISM cooling and
diagnostic emission lines of [OI] 63 m, [OIII] 88 m, [NII] 122
m, [CII] 158 m and [NII] 205 m. We observe that in the [OI] 63
m, [OIII] 88 m, and [NII] 122 m line maps, the emission is
enhanced in clumps along the NE part of the ring. We observe evidence of rapid
rotation in the circumnuclear ring, with a rotation velocity of ~220^{-1}\mu\mu\mu\mu\mu\mu^{-3}$.Comment: Accepted for publication on the A&A Herschel Special Issu
Anatomy of the Soft-Photon Approximation in Hadron-Hadron Bremsstrahlung
A modified Low procedure for constructing soft-photon amplitudes has been
used to derive two general soft-photon amplitudes, a two-s-two-t special
amplitude and a two-u-two-t special amplitude
, where s, t and u are the Mandelstam variables.
depends only on the elastic T-matrix evaluated at four sets
of (s,t) fixed by the requirement that the amplitude be free of derivatives
(T/s and /or T/). Likewise
depends only on the elastic T-matrix evaluated at four sets
of (u,t). In deriving these amplitudes, we impose the condition that
and reduce to and
, respectively, their tree level approximations. The
amplitude represents photon emission from a sum of
one-particle t-channel exchange diagrams and one-particle s-channel exchange
diagrams, while the amplitude represents photon
emission from a sum of one-particle t-channel exchange diagrams and
one-particle u-channel exchange diagrams. The precise expressions for
and are determined by using the
radiation decomposition identities of Brodsky and Brown. We point out that it
is theoretically impossible to describe all bremsstrahlung processes by using
only a single class of soft-photon amplitudes. At least two different classes
are required: the amplitudes which depend on s and t or the amplitudes which
depend on u and t. When resonance effects are important, the amplitude
, not , should be used. For processes with
strong u-channel exchange effects, the amplitude should be
the first choice.Comment: 49 pages report # LA-UR-92-270
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