290 research outputs found
Collisional excitation of water by hydrogen atoms
We present quantum dynamical calculations that describe the rotational
excitation of HO due to collisions with H atoms. We used a recent, high
accuracy potential energy surface, and solved the collisional dynamics with the
close-coupling formalism, for total energies up to 12 000 cm. From these
calculations, we obtained collisional rate coefficients for the first 45 energy
levels of both ortho- and para-HO and for temperatures in the range T =
5-1500 K. These rate coefficients are subsequently compared to the values
previously published for the HO / He and HO / H collisional
systems. It is shown that no simple relation exists between the three systems
and that specific calculations are thus mandatory
The Excitation of NH in Interstellar Molecular Clouds. I - Models
We present LVG and non-local radiative transfer calculations involving the
rotational and hyperfine structure of the spectrum of NH with
collisional rate coefficients recently derived by us. The goal of this study is
to check the validity of the assumptions made to treat the hyperfine structure
and to study the physical mechanisms leading to the observed hyperfine
anomalies.
We find that the usual hypothesis of identical excitation temperatures for
all hyperfine components of the =1-0 transition is not correct within the
range of densities existing in cold dense cores, i.e., a few 10 \textless
n(H) \textless a few 10 cm. This is due to different radiative
trapping effects in the hyperfine components. Moreover, within this range of
densities and considering the typical abundance of NH, the total
opacity of rotational lines has to be derived taking into account the hyperfine
structure. The error made when only considering the rotational energy structure
can be as large as 100%. Using non-local models we find that, due to
saturation, hyperfine anomalies appear as soon as the total opacity of the
=1-0 transition becomes larger than 20. Radiative scattering in
less dense regions enhance these anomalies, and particularly, induce a
differential increase of the excitation temperatures of the hyperfine
components. This process is more effective for the transitions with the highest
opacities for which emerging intensities are also reduced by self-absorption
effects. These effects are not as critical as in HCO or HCN, but should be
taken into account when interpreting the spatial extent of the NH
emission in dark clouds.Comment: 13 pages, 12 figure
Collisional excitation rate coefficients of N2H+ by He
Using a recoupling technique with close-coupling spin-free calculations de-excitation rate coefficients are obtained among hyperfine transitions for He colliding with N2H+. A recently determined potential energy surface suitable for scattering calculations is used to investigate rate coefficients for temperatures between 5 and 50 K, and for the seven lowest rotational levels of N2H+. Fitting functions are provided for the Maxwellian averaged opacity tensors and for the rotational de-excitation collisional rate coefficients. The fitting functions for the opacity tensors can be used to calculate hyperfine (de)-excitation rate coefficients among elastic and inelastic rotational levels, and among the corresponding magnetic sublevels of the hyperfine structure. Certain dynamical approximations are investigated and found to be invali
N2H+ and N2D+ in interstellar molecular clouds. II- Observations
We present observations of the =1--0, 2--1, and 3--2 rotational
transitions of NH and ND towards a sample of prototypical dark
clouds. The data have been interpreted using non--local radiative transfer
models.Comment: 12 pages, 18 figure
On the influence of collisional rate coefficients on the water vapour excitation
Water is a key molecule in many astrophysical studies. Its high dipole moment
makes this molecule to be subthermally populated under the typical conditions
of most astrophysical objects. This motivated the calculation of various sets
of collisional rate coefficients (CRC) for HO (with He or H) which are
necessary to model its rotational excitation and line emission. We performed
accurate non--local non--LTE radiative transfer calculations using different
sets of CRC in order to predict the line intensities from transitions that
involve the lowest energy levels of HO (E 900 K). The results obtained
from the different CRC sets are then compared using line intensity ratio
statistics. For the whole range of physical conditions considered in this work,
we obtain that the intensities based on the quantum and QCT CRC are in good
agreement. However, at relatively low H volume density ((H)
10 cm) and low water abundance ((HO) 10), these
physical conditions being relevant to describe most molecular clouds, we find
differences in the predicted line intensities of up to a factor of 3 for
the bulk of the lines. Most of the recent studies interpreting early Herschel
Space Observatory spectra used the QCT CRC. Our results show that although the
global conclusions from those studies will not be drastically changed, each
case has to be considered individually, since depending on the physical
conditions, the use of the QCT CRC may lead to a mis--estimate of the water
vapour abundance of up to a factor of 3
Causes et conséquences de la consommation de l'oxygène par les moûts de raisin
La consommation de l'oxygène par les moûts est mise en évidence et sa vitesse mesurée à l'aide d'une électrode à oxygène. Cette consommation est essentiellement d'origine enzymatique et fait intervenir la tyrosinase (catéchol oxydase) du raisin et la laccase de Botrytis cinerea, polyphénoloxydases catalysant l'oxydation des composés phénoliques du moût à partir de l'oxygène moléculaire. L'évolution de la consommation de l'oxygène par les moûts est suivie au cours de leur oxydation, au cours de traitements préfermentaires du moût tels que le sulfitage et le débourbage et en fonction de la température. Les conséquences de ces résultats sur la technologie de la production des vins sont discutées.Causes and consequences of oxygen consumption by grape mustsThe occurence of oxygen consumption by grape musts is shown. Its rate is measured by the use of a Clark oxygen electrode. This oxygen consumption is mainly due to enzymes which are a tyrosinase (catechol oxidase) from grape and a laccase from Botrytis cinerea. Bath polyphenoloxydases catalyze the oxidation of must phenolic compounds by atmospheric oxygen. The evolution of the oxygen consumption by musts is studied in the course of the treatments they receive before their fermentation. The importance of the oxygen consumption by musts with regard to the technology of winemaking is discussed
Urban forest usage and perception of ecosystem services – A comparison between teenagers and adults
We thank Christoph Düggelin and Marc Baume for the interpretation of the photographs and two anonymous reviewers for comments on the manuscript. The project was funded by the Swiss State Secretariat for Education, Research and Innovation SERI, Switzerland (Grant No. C13.0135) as a contribution to the COST Action PF1204 and by the Swiss Federal Office for the Environment, Switzerland (Grant No. 16.0074.PJ / S062-1129).Peer reviewedPublisher PD
Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
The interpretation of water line emission from existing observations and
future HIFI/Herschel data requires a detailed knowledge of collisional rate
coefficients. Among all relevant collisional mechanisms, the rotational
(de)excitation of H2O by H2 molecules is the process of most interest in
interstellar space. To determine rate coefficients for rotational de-excitation
among the lowest 45 para and 45 ortho rotational levels of H2O colliding with
both para and ortho-H2 in the temperature range 20-2000 K. Rate coefficients
are calculated on a recent high-accuracy H2O-H2 potential energy surface using
quasi-classical trajectory calculations. Trajectories are sampled by a
canonical Monte-Carlo procedure. H2 molecules are assumed to be rotationally
thermalized at the kinetic temperature. By comparison with quantum calculations
available for low lying levels, classical rates are found to be accurate within
a factor of 1-3 for the dominant transitions, that is those with rates larger
than a few 10^{-12}cm^{3}s^{-1}. Large velocity gradient modelling shows that
the new rates have a significant impact on emission line fluxes and that they
should be adopted in any detailed population model of water in warm and hot
environments.Comment: 8 pages, 2 figures, 1 table (the online material (4 tables) can be
obtained upon request to [email protected]
Physical conditions in the ISM towards HD185418
We have developed a complete model of the hydrogen molecule as part of the
spectral simulation code Cloudy. Our goal is to apply this to spectra of
high-redshift star-forming regions where H2 absorption is seen, but where few
other details are known, to understand its implication for star formation. The
microphysics of H2 is intricate, and it is important to validate these
numerical simulations in better-understood environments. This paper studies a
well-defined line-of-sight through the Galactic interstellar medium (ISM) as a
test of the microphysics and methods we use. We present a self-consistent
calculation of the observed absorption-line spectrum to derive the physical
conditions in the ISM towards HD185418, a line-of-sight with many observables.
We deduce density, temperature, local radiation field, cosmic ray ionization
rate, chemical composition and compare these conclusions with conditions
deduced from analytical calculations. We find a higher density, similar
abundances, and require a cosmic ray flux enhanced over the Galactic background
value, consistent with enhancements predicted by MHD simulations.Comment: 31 pages, accepted for publication in Ap
Rotational Excitation of HC_3N by H_2 and He at low temperatures
Rates for rotational excitation of HC3N by collisions with He atoms and H2
molecules are computed for kinetic temperatures in the range 5-20K and 5-100K,
respectively. These rates are obtained from extensive quantum and
quasi-classical calculations using new accurate potential energy surfaces
(PES)
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