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 H$_2$O (with He or H$_2$) 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 H$_2$O (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$_2$ volume density ($n$(H$_2$) $<$ 10$^7$ cm$^{-3}$) and low water abundance ($\chi$(H$_2$O) $<$ 10$^{-6}$), these physical conditions being relevant to describe most molecular clouds, we find differences in the predicted line intensities of up to a factor of $\sim$ 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 $\sim$ 3

Observational studies of pre-stellar cores and infrared dark clouds

Stars like our Sun and planets like our Earth form in dense regions within interstellar molecular clouds, called pre-stellar cores (PSCs). PSCs provide the initial conditions in the process of star and planet formation. In the past 15 years, detailed observations of (low-mass) PSCs in nearby molecular cloud complexes have allowed us to find that they are cold (T < 10 K) and quiescent (molecular line widths are close to thermal), with a chemistry profoundly affected by molecular freeze-out onto dust grains. In these conditions, deuterated molecules flourish, becoming the best tools to unveil the PSC physical and chemical structure. Despite their apparent simplicity, PSCs still offer puzzles to solve and they are far from being completely understood. For example, what is happening to the gas and dust in their nuclei (the future stellar cradles) is still a mystery that awaits for ALMA. Other important questions are: how do different environments and external conditions affect the PSC physical/chemical structure? Are PSCs in high-mass star forming regions similar to the well-known low-mass PSCs? Here I review observational and theoretical work on PSCs in nearby molecular cloud complexes and the ongoing search and study of massive PSCs embedded in infrared dark clouds (IRDCs), which host the initial conditions for stellar cluster and high-mass star formation.Comment: To appear in the proceedings of IAU Symposium 280: "The Molecular Universe

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 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), 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

Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?

The star formation rates in starburst galaxies are orders of magnitude higher than in local star-forming regions, and the origin of this difference is not well understood. We use sub-mm spectral line maps to characterize the physical conditions of the molecular gas in the luminous Galactic star-forming region W49A and compare them with the conditions in starburst galaxies. We probe the temperature and density structure of W49A using H_2CO and HCN line ratios over a 2'x2' (6.6x6.6 pc) field with an angular resolution of 15" (~0.8 pc) provided by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines with multiple transitions with corrections for optical depth and beam dilution, and estimate excitation temperatures and column densities. Comparing the observed line intensity ratios with non-LTE radiative transfer models, our results reveal an extended region (about 1'x1', equivalent to ~3x3 pc at the distance of W49A) of warm (> 100 K) and dense (>10^5 cm^-3) molecular gas, with a mass of 2x10^4 - 2x10^5 M_Sun (by applying abundances derived for other regions of massive star-formation). These temperatures and densities in W49A are comparable to those found in clouds near the center of the Milky Way and in starburst galaxies. The highly excited gas is likely to be heated via shocks from the stellar winds of embedded, O-type stars or alternatively due to UV irradiation, or possibly a combination of these two processes. Cosmic rays, X-ray irradiation and gas-grain collisional heating are less likely to be the source of the heating in the case of W49A.Comment: Accepted for publication in A&A; 11 pages, 9 figure

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

On the robustness of the ammonia thermometer

Ammonia inversion lines are often used as probes of the physical conditions in the dense ISM. The excitation temperature between the first two para metastable (rotational) levels is an excellent probe of the gas kinetic temperature. However, the calibration of this ammonia thermometer depends on the accuracy of the collisional rates with H2. Here we present new collisional rates for ortho-NH3 and para-NH3 colliding with para-H2 (J=0) and we investigate the effects of these new rates on the excitation of ammonia. Scattering calculations employ a new, high accuracy, potential energy surface computed at the coupled-cluster CCSD(T) level with a basis set extrapolation procedure. Rates are obtained for all transitions involving ammonia levels with J <= 3 and for kinetic temperatures in the range 5-100 K. We find that the calibration curve of the ammonia thermometer -- which relates the observed excitation temperature between the first two para metastable levels to the gas kinetic temperature -- does not change significantly when these new rates are used. Thus, the calibration of ammonia thermometer appears to be robust. Effects of the new rates on the excitation temperature of inversion and rotation-inversion transitions are also found to be small.Comment: Accepted for publication in the MNRA

Radiative transfer and molecular data for astrochemistry (Review)

The estimation of molecular abundances in interstellar clouds from spectroscopic observations requires radiative transfer calculations, which depend on basic molecular input data. This paper reviews recent developments in the fields of molecular data and radiative transfer. The first part is an overview of radiative transfer techniques, along with a "road map" showing which technique should be used in which situation. The second part is a review of measurements and calculations of molecular spectroscopic and collisional data, with a summary of recent collisional calculations and suggested modeling strategies if collision data are unavailable. The paper concludes with an overview of future developments and needs in the areas of radiative transfer and molecular data.Comment: Contribution to proceedings of IAU Symposium 280, "The Molecular Universe", Toledo, June 2011; 12 page

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

Excitation Collisionnelle du formaldéhyde interstellaire : Théorie et Observations

Molecular hydrogen is the simplest and most abundant molecule in the Universe. Owing to the possible different nuclear spin alignments, it presents two forms, ortho- and para-H$_2$. The ortho-to-para ratio of H$_2$ is a fundamental parameter to understand the (inelastic and reactive) collisional processes in molecular astrophysical media. In this thesis, we focus on the determination of the ortho-to-para ratio of H$_2$ in dark clouds, where H$_2$ cannot be directly observed. To this aim, we use the formaldehyde molecule (H2CO) whose rotational excitation in these sources is dominated by H2 collisions. A peculiar transition has been selected in our study: the 6-cm transition (4.8 GHz) of ortho-H$_2$CO observed in absorption against the cosmic microwave background. Previous studies have shown that collisional effects can explain this (antimaser) absorption but the impact of the ortho- and para- form of H$_2$ on this absorption has been so far ignored. We first present high-precision computations of the (de)excitation rates of (ortho-, para-) H$_2$CO by (ortho-, para-)H$_2$. Significant differences are observed between ortho- and para-H$_2$ rates. We then use these collisional rates in radiative transfer calculations in order to model 6-cm observations carried out with the Green Bank Telescope towards 3 different dark clouds (B68, L134N and TMC-1). We show that the differences in para and ortho-H2 collisional rates have a significant impact on the modeling, allowing us to put interesting constraints on the ortho-to-para ratio of H$_2$ in dark clouds. We consider in detail the example of B68, prototype of pre-stellar cores.L'hydrogène moléculaire est la molécule la plus simple et la plus répandue dans l'Univers. Elle se présente sous deux formes, ortho- et para- H$_2$, correspondant aux différents alignements des spins de ses deux noyaux. Le rapport ortho/para de H$_2$ est un paramètre essentiel pour comprendre les processus collisionnels inélastiques et réactifs dans les milieux astrophysiques moléculaires. Dans cette thèse, nous nous sommes intéressés à la détermination de ce rapport dans les nuages sombres, régions où H$_2$ ne peut pas être observée directement. Dans ce but, nous avons utilisé la molécule de formaldéhyde (H$_2$CO) dont l'excitation rotationnelle dans ces sources est dominée par les collisions avec H$_2$. Une transition particulière a été choisie pour cette étude : la raie à 6 cm (4,8 GHz) de ortho-H$_2$CO observée en absorption devant le fond diffus cosmologique. Si des études précédentes ont montré que cette absorption (antimaser) peut être expliquée par des effets collisionnels, aucune n'a étudié jusqu'à présent l'impact des formes ortho- et para-H$_2$ sur l'absorption. Nous présentons, dans un premier temps, nos calculs haute-précision des taux d'excitation rotationnelle entre (ortho-,para-)H$_2$CO et (ortho-,para-)H$_2$. Nous montrons qu'il existe des différences significatives sur les taux de collisions selon le type de projectiles, et en particulier qu'il existe des différences entre ortho- et para-H$_2$. Nous présentons ensuite l'utilisation de ces taux dans un modèle de transfert radiatif afin de reproduire les observations que nous avons menées au Green Bank Telescope sur la transition a 6 cm de ortho-H$_2$CO en direction de 3 nuages sombres (B68, L134N et TMC-1). Nous montrons que les différences dans les taux de collisions calculés avec ortho- et para-H$_2$ ont un impact significatif sur les simulations, nous permettant ainsi d'apporter des contraintes sur la valeur du rapport ortho/para de H$_2$ dans ces milieux. Nous traitons en particulier l'exemple de B68, prototype de c\oe ur pre-stellaire