1,125 research outputs found
Chemistry of dense clumps near moving Herbig-Haro objects
Localised regions of enhanced emission from HCO+, NH3 and other species near
Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry
stimulated by the HHO radiation on high density quiescent clumps in molecular
clouds. Static models of this process have been successful in accounting for
the variety of molecular species arising ahead of the jet; however recent
observations show that the enhanced molecular emission is widespread along the
jet as well as ahead. Hence, a realistic model must take into account the
movement of the radiation field past the clump. It was previously unclear as to
whether the short interaction time between the clump and the HHO in a moving
source model would allow molecules such as HCO+ to reach high enough levels,
and to survive for long enough to be observed. In this work we model a moving
radiation source that approaches and passes a clump. The chemical picture is
qualitatively unchanged by the addition of the moving source, strengthening the
idea that enhancements are due to evaporation of molecules from dust grains. In
addition, in the case of several molecules, the enhanced emission regions are
longer-lived. Some photochemically-induced species, including methanol, are
expected to maintain high abundances for ~10,000 years.Comment: 7 pages, 3 figure
Molecular Clouds as Ensembles of Transient Cores
We construct models of molecular clouds that are considered as ensembles of
transient cores. Each core is assumed to develop in the background gas of the
cloud, grow to high density and decay into the background. The chemistry in
each core responds to the dynamical state of the gas and to the gas-dust
interaction. Ices are deposited on the dust grains in the core's dense phase,
and this material is returned to the gas as the core expands to low density.
The cores of the ensemble number typically one thousand and are placed randomly
in position within the cloud, and are assigned a random evolutionary phase.
The models are used to generate molecular line contour maps of a typical dark
cloud. These maps are found to represent extremely well the characteristic
features of observed maps of the dark cloud L673, which has been observed at
both low and high resolutions. The computed maps are found to exhibit the
general morphology of the observed maps, and to generate similar sizes of
emitting regions, molecular column densities, and the separations between peaks
of emissions of various molecular species. The models give insight into the
nature of molecular clouds and the dynamical processes occurring within them,
and significantly constrain dynamical and chemical processes in the
interstellar medium.Comment: 29 pages, 8 figures. Accepted for publication in Ap
Young starless cores embedded in the magnetically dominated Pipe Nebula
The Pipe Nebula is a massive, nearby dark molecular cloud with a low
star-formation efficiency which makes it a good laboratory to study the very
early stages of the star formation process. The Pipe Nebula is largely
filamentary, and appears to be threaded by a uniform magnetic field at scales
of few parsecs, perpendicular to its main axis. The field is only locally
perturbed in a few regions, such as the only active cluster forming core B59.
The aim of this study is to investigate primordial conditions in low-mass
pre-stellar cores and how they relate to the local magnetic field in the cloud.
We used the IRAM 30-m telescope to carry out a continuum and molecular survey
at 3 and 1 mm of early- and late-time molecules toward four selected starless
cores inside the Pipe Nebula. We found that the dust continuum emission maps
trace better the densest regions than previous 2MASS extinction maps, while
2MASS extinction maps trace better the diffuse gas. The properties of the cores
derived from dust emission show average radii of ~0.09 pc, densities of
~1.3x10^5 cm^-3, and core masses of ~2.5 M_sun. Our results confirm that the
Pipe Nebula starless cores studied are in a very early evolutionary stage, and
present a very young chemistry with different properties that allow us to
propose an evolutionary sequence. All of the cores present early-time molecular
emission, with CS detections toward all the sample. Two of them, Cores 40 and
109, present strong late-time molecular emission. There seems to be a
correlation between the chemical evolutionary stage of the cores and the local
magnetic properties that suggests that the evolution of the cores is ruled by a
local competition between the magnetic energy and other mechanisms, such as
turbulence.Comment: Accepted for publication in ApJ. 15 pages, 5 figures, 9 table
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
Performance of the Muon Identification at LHCb
The performance of the muon identification in LHCb is extracted from data
using muons and hadrons produced in J/\psi->\mu\mu, \Lambda->p\pi and
D^{\star}->\pi D0(K\pi) decays. The muon identification procedure is based on
the pattern of hits in the muon chambers. A momentum dependent binary
requirement is used to reduce the probability of hadrons to be misidentified as
muons to the level of 1%, keeping the muon efficiency in the range of 95-98%.
As further refinement, a likelihood is built for the muon and non-muon
hypotheses. Adding a requirement on this likelihood that provides a total muon
efficiency at the level of 93%, the hadron misidentification rates are below
0.6%.Comment: 17 pages, 10 figure
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