17 research outputs found
Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments
An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cmâ1, although resonances near threshold, below 5 cmâ1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cmâ1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool
Physical structure of the envelopes of intermediate-mass protostars
Context: Intermediate mass protostars provide a bridge between low- and
high-mass protostars. Furthermore, they are an important component of the UV
interstellar radiation field. Despite their relevance, little is known about
their formation process. Aims: We present a systematic study of the physical
structure of five intermediate mass, candidate Class 0 protostars. Our two
goals are to shed light on the first phase of intermediate mass star formation
and to compare these protostars with low- and high-mass sources. Methods: We
derived the dust and gas temperature and density profiles of the sample. We
analysed all existing continuum data on each source and modelled the resulting
SED with the 1D radiative transfer code DUSTY. The gas temperature was then
predicted by means of a modified version of the code CHT96. Results: We found
that the density profiles of five out of six studied intermediate mass
envelopes are consistent with the predictions of the "inside-out" collapse
theory.We compared several physical parameters, like the power law index of the
density profile, the size, the mass, the average density, the density at 1000
AU and the density at 10 K of the envelopes of low-, intermediate, and
high-mass protostars. When considering these various physical parameters, the
transition between the three groups appears smooth, suggesting that the
formation processes and triggers do not substantially differ
Ortho-to-para ratio of interstellar heavy water
Despite the low elemental deuterium abundance in the Galaxy, enhanced
molecular D/H ratios have been found in the environments of low-mass star
forming regions, and in particular the Class 0 protostar IRAS 16293-2422. The
CHESS (Chemical HErschel Surveys of Star forming regions) Key Program aims at
studying the molecular complexity of the interstellar medium. The high
sensitivity and spectral resolution of the HIFI instrument provide a unique
opportunity to observe the fundamental 1,1,1 - 0,0,0 transition of the
ortho-D2O molecule, inaccessible from the ground, and to determine the
ortho-to-para D2O ratio. We have detected the fundamental transition of the
ortho-D2O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in
absorption with a line opacity of 0.62 +/- 0.11 (1 sigma). From the previous
ground-based observations of the fundamental 1,1,0 - 1,0,1 transition of
para-D2O seen in absorption at 316.80 GHz we estimate a line opacity of 0.26
+/- 0.05 (1 sigma). We show that the observed absorption is caused by the cold
gas in the envelope of the protostar. Using these new observations, we estimate
for the first time the ortho to para D2O ratio to be lower than 2.6 at a 3
sigma level of uncertainty, to be compared with the thermal equilibrium value
of 2:1.Comment: 5 pages, 5 figures, accepted the A&A HIFI Special Issue as a lette
Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes
We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re
Résolution du problÚme inverse en métrologie optique par méthode statistique : Le Krigeage
National audienc
Assessment of the scatterometry capability to detect an etch process deviation
International audienc
Constraining the ortho-to-para ratio of H
Context. The ortho-to-para ratio (OPR) of molecular hydrogen is a
fundamental parameter in understanding the physics and chemistry of
molecular clouds. In dark and cold regions, however, H2 is not
directly observable and the OPR of H2 in these sources has so far
remained elusive.
Aims. We show that the 6Â cm absorption line of
ortho-formaldehyde (H2CO) can be employed to constrain both
the density and the OPR of H2 in dark clouds.
Methods. Green Bank
Telescope (GBT) observations of ortho-H2CO toward the molecular
cloud Barnard 68 (B68) are reported. Non-LTE radiative transfer
calculations combined with the well-constrained structure of B68 are
then employed to derive the physical conditions in the absorption
region.
Results. We provide the first firm confirmation of the Townes &
Cheung mechanism: propensity rules for the collisions of H2CO with
H2Â molecules are responsible for the sub-2.7Â K cooling of the
6Â cm doublet. Non-LTE calculations show that in the absorption
region of B68, the kinetic temperature is âŒ10 K, the
ortho-H2CO column density amounts to âŒÂ cm-2, the H2 density is in the range
 cm-3, and the OPR of H2 is close to
zero. Our observations thus provide fresh evidence that H2 is mostly in its para form in the cold gas, as expected from
theoretical considerations. Our results also suggest that
formaldehyde absorption originates in the edge of B68, at visual
extinctions A_{\rm V}\la 0.5Â mag
Rotational excitation of SO 2
The SO2 molecule is detected in a large variety of objects, notably cold dark clouds and star-forming regions. An accurate modeling of the observations requires a very good knowledge of the collisional excitation rates with H2 due to competition between collisional and radiative processes that excite and quench the different rotational levels of the molecule. The results of our recent collisional calculations are summarized. Pierre was associated to all steps of this collaborative work that was a key project in the Molecular Universe European FP6 network
Analyzing observations of molecules in the ISM: Theoretical and experimental studies of energy transfer
Our laboratories have recently calculated a set of collision coefficients characterizing
the efficiency of energy transfer between helium and/or hydrogen and a large variety of
interstellar molecules. We have considered with molecules ranging from light hydrides,
observed by the Herschel Space Observatory, to medium size molecules, observed by mm
antennas, to heavy complex organic molecules, observed also in the cm range. We present a
review of recent theoretical results obtained in our laboratories, for various kinds of
commonly observed molecules