9,097 research outputs found
CH observations toward the Orion Bar
CH is one of the first radicals to be detected in the interstellar
medium. Its higher rotational transitions have recently become available with
the Herschel Space Observatory. We aim to constrain the physical parameters of
the CH emitting gas toward the Orion Bar. We analyse the CH line
intensities measured toward the Orion Bar CO Peak and Herschel/HIFI maps of
CH, CH, and HCO, and a NANTEN map of [CI]. We interpret the observed
CH emission using radiative transfer and PDR models. Five rotational
transitions of CH have been detected in the HIFI frequency range toward the
CO peak. A single component rotational diagram gives a rotation temperature
of ~64 K and a beam-averaged CH column density of 410
cm. The measured transitions cannot be explained by any single parameter
model. According to a non-LTE model, most of the CH column density produces
the lower- CH transitions and traces a warm ( ~ 100-150 K)
and dense ((H)~10-10 cm) gas. A small fraction of the
CH column density is required to reproduce the intensity of the highest-
transitions (=9-8 and N=10-9) originating from a high density
((H)~510 cm) hot ( ~ 400 K) gas. The
total beam-averaged CH column density in the model is 10 cm.
Both the non-LTE radiative transfer model and a simple PDR model representing
the Orion Bar with a plane-parallel slab of gas and dust suggest, that CH
cannot be described by a single pressure component, unlike the reactive ion
CH, which was previously analysed toward the Orion Bar CO peak. The
physical parameters traced by the higher rotational transitions
(=6-5,...,10-9) of CH may be consistent with the edges of dense clumps
exposed to UV radiation near the ionization front of the Orion Bar.Comment: Proposed for acceptance in A&A, abstract abridge
Multi-jet cross sections in deep inelastic scattering at next-to-leading order
We present the perturbative prediction for three-jet production cross section
in DIS at the NLO accuracy. We study the dependence on the renormalization and
factorization scales of exclusive three-jet cross section. The perturbative
prediction for the three-jet differential distribution as a function of the
momentum transfer is compared to the corresponding data obtained by the H1
collaboration at HERA.Comment: 5 pages, 3 figure
Three-jet cross sections in hadron-hadron collisions at next-to-leading order
We present a new QCD event generator for hadron collider which can calculate
one-, two- and three-jet cross sections at next-to-leading order accuracy. In
this letter we study the transverse energy spectrum of three-jet hadronic
events using the kT algorithm. We show that the next-to-leading order
correction significantly reduces the renormalization and factorization scale
dependence of the three-jet cross section.Comment: 4 pages, 4 figures, REVTEX
Semi-numerical resummation of event shapes
For many event-shape observables, the most difficult part of a resummation in
the Born limit is the analytical treatment of the observable's dependence on
multiple emissions, which is required at single logarithmic accuracy. We
present a general numerical method, suitable for a large class of event shapes,
which allows the resummation specifically of these single logarithms. It is
applied to the case of the thrust major and the oblateness, which have so far
defied analytical resummation and to the two-jet rate in the Durham algorithm,
for which only a subset of the single logs had up to now been calculated.Comment: 29 pages, 7 figures. Version 2 adds some clarifications, a reference,
as well as corrections to the subleading fixed-order coefficients and to
figures 4 and
Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
The massive and luminous star-forming region W49A is a well known Galactic
candidate to probe the physical conditions and chemistry similar to those
expected in external starburst galaxies. We aim to probe the physical and
chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT
Spectral Legacy Survey, which covers the frequency range between 330 and 373
GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP
instrument on JCMT provides information on the spatial structure and kinematics
of the cloud. For species where multiple transitions are available, we estimate
excitation temperatures and column densities. We detected 255 transitions
corresponding to 60 species in the 330-373 GHz range at the center position of
W49A. Excitation conditions can be probed for 16 molecules. The chemical
composition suggests the importance of shock-, PDR-, and hot core chemistry.
Many molecular lines show a significant spatial extent across the maps
including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of
UV-irradiation (e.g. CN and C2H). Large variations are seen between the
sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly
red-shifted emission toward the Northern clump, and emission peaking around the
expected source velocity toward the South-west clump. A comparison of column
density ratios of characteristic species observed toward W49A to Galactic PDRs
suggests that while the chemistry toward the W49A center is driven by a
combination of UV-irradiation and shocks, UV-irradiation dominates for the
Northern Clump, Eastern tail, and South-west clump regions. A comparison to a
starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with
respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A
and the >1 kpc regions in external galaxies with global star-formation.Comment: Proposed for acceptance in A&A, abstract abridge
The chemical structure of the very young starless core L1521E
L1521E is a dense starless core in Taurus that was found to have relatively
low molecular depletion by earlier studies, thus suggesting a recent formation.
We aim to characterize the chemical structure of L1521E and compare it to the
more evolved L1544 pre-stellar core. We have obtained 2.52.5
arcminute maps toward L1521E using the IRAM-30m telescope in transitions of
various species. We derived abundances for the species and compared them to
those obtained toward L1544. We estimated CO depletion factors. Similarly to
L1544, -CH and CHOH peak at different positions. Most species
peak toward the -CH peak. The CO depletion factor derived toward the
dust peak is 4.31.6, which is about a factor of three lower
than that toward L1544. The abundances of sulfur-bearing molecules are higher
toward L1521E than toward L1544 by factors of 2-20. The abundance of
methanol is similar toward the two cores. The higher abundances of
sulfur-bearing species toward L1521E than toward L1544 suggest that significant
sulfur depletion takes place during the dynamical evolution of dense cores,
from the starless to pre-stellar stage. The CO depletion factor measured toward
L1521E suggests that CO is more depleted than previously found. Similar
CHOH abundances between L1521E and L1544 hint that methanol is forming at
specific physical conditions in Taurus, characterized by densities of a few
10 cm and (H)10 cm, when CO
starts to catastrophically freeze-out, while water can still be significantly
photodissociated, so that the surfaces of dust grains become rich in solid CO
and CHOH, as already found toward L1544. Methanol can thus provide
selective crucial information about the transition region between dense cores
and the surrounding parent cloud.Comment: Accepted for publication in A&A, abstract abridge
QCD Corrections to Four-jet Production and Three-jet Structure in e+ e- annihilation
We report on the general purpose numerical program MERCUTIO, which can be
used to calculate any infrared safe four-jet quantity in electron-positron
annihilation at next-to-leading order. The program is based on the dipole
formalism and uses a remapping of phase-space in order to improve the
efficiency of the Monte Carlo integration. Numerical results are given for the
four-jet fraction and the D-parameter. These results are compared with already
existing ones in the literature and serve as a cross-check. The program can
also be used to investigate the internal structure of three-jet events at NLO.
We give results for previously uncalculated observables: the jet broadening
variable and the softest-jet explanarity.Comment: 28 pages, Latex, final versio
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