G. Pineau des Forêts

The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse

Context. We have studied the evolution of molecular gas during the early stages of protostellar collapse, when the freeze-out of “heavy” species on to grains occurs. Aims. In addition to studying the freeze-out of “heavy” species on to grains, we wished to compute the variation of the population densities of the different nuclear spin states of `tracer molecular ions, such asxa0H 2 D + and D 2 H + , which are currently observed only in their ortho and para forms, respectively. Methods. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly. Nuclear spin-changing reactions have received much less attention in the literature than those leading to deuteration; but, in fact, the former processes are as significant as the latter and often involve the same reactants. A “free-fall” model of gravitational collapse was adopted. Results. We found that the ortho:para ratios of some species, e.g.xa0H 2 D + , vary considerably as the density increases. Because the dynamical timescale is much shorter than some of the chemical timescales, there can be large departures of the predictions of the free-fall model from the steady-state solution at the same density and temperature. In the case ofxa0H 2 , it seems unlikely that the steady state value of the ortho:para ratio is attained before protostellar collapse from the progenitor molecular cloud commences. Values of the ortho:paraxa0H 2 ratio much higher than in steady state, which would prevail in “young” molecular clouds, are found to be inconsistent with high levels of deuteration of the gas. The internal energy of ortho-H 2 acts as a reservoir of chemical energy which inhibits the deuteration ofxa0H

Some empirical estimates of the H2 formation rate in photon-dominated regions

_3^+

H2 formation and excitation in the diffuse interstellar medium

and hence of other species, such as N 2 H + and NH 3 . Conclusions. The principal conclusion is that the degree of deuteration of molecular ions and molecules is sensitive to the ortho:paraxa0H 2 ratio and hence to the chemical and thermal history of the precursor molecular cloud.

H2 infrared line emission across the bright side of the ρ Ophiuchi main cloud

We combine recent ISO observations of the vibrational ground state lines of H 2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground–based telescopes in order to constrain the formation rate ofxa0H 2 on grain surfaces under the physical conditions in the layers responsible for H 2 xa0emission. We briefly review the data available for five nearby PDRs. We use steady state PDR models in order to examine the sensitivity of different H 2 xa0line ratios to the H 2 xa0formation rate R f . We show that the ratio of the 0–0xa0S(3) to the 1–0xa0S(1) line increases withxa0 R f but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer R f from the H 2 line data. We confirm earlier work by [CITE] on the Ophxa0Wxa0PDR which showed that an H 2 xa0formation rate higher than the standard value of

Shocks in dense clouds II. Dust destruction and SiO formation in J shocks

3 times 10^{-17}

SHOCK-ENHANCED C+ EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USING HERSCHEL

xa0cm 3 xa0s -1 inferred from UVxa0observations of diffuse clouds is needed to explain the observed H 2 xa0excitation. From comparison of the ISO and ground-based data, we find that moderately excited PDRs such as Ophxa0W, S140 and ICxa063 require an H 2 xa0formation rate of about five times the standard value whereas the data for PDRs with a higher incident radiation field such as NGCxa02023 and the Orion Bar can be explained with the standard value of R f . We compare also the H 2 1–0xa0S(1) line intensities with the emission in PAHxa0features and find a rough scaling of the ratio of these quantities with the ratio of local density to radiation field. This suggests but does not prove that formation of H 2 on PAHs is important in PDRs. We also consider some empirical models of the H 2 xa0formation process with the aim of explaining these results. Here we consider both formation on classical grains of size roughly 0.1xa0 μ m and on very small (~10xa0A) grains by either direct recombination from the gas phase (Eley–Rideal mechanism) or recombination of physisorbed Hxa0atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption where a physisorbed H-atom scans the grain surface before recombining with a chemisorbed H-atom is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low (below 30xa0K for χ ≤ 3000) temperatures may be the most promising surface for forming H 2 in PDRs.

Shocks in dense clouds - III. Dust processing and feedback effects in C-type shocks

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and ex- citation of H2 in the diuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon- illuminated gas. The data constrain well the nRproduct between gas density and H2 formation rate on dust grains: nR= 1t o 2:2 10 15 s 1 . For each line of sight the mean eective H2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J= 1)=N(J= 0) ratio, since the radiation field is known. Combining n with the nRvalues, we find similar H2 formation rates for the three stars of about R= 4 10 17 cm 3 s 1 . Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the J> 2 levels cannot be accounted for by the UV pumping of the cold H2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H2.

Excitation of H2 in photodissociation regions as seen by Spitzer

We present imaging and spectroscopic observations of dust and gas (H2) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ ∼ 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 ± 15 pc from the stellar parallax) and the layer of UV light penetration and of H2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H2 emitting layer, in which the ortho-to-para H2 ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ((C/H)PAH � 5 × 10 −5 ), we find that the H2 formation rate Rf must be high in warm gas (∼6 times the rate derived by Jura, 1975), in order to account for the observed H2 emission. This result and the spatial coincidence between the PAHs and H2 emission suggest that H2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in Rf may also result from an increased PAH abundance: assuming that Rf scales with the PAH abundance, the observed H2 excitation is well explained with Rf � 1 × 10 −16 cm 3 s −1 at Tgas = 330 K (∼3 times the rate derived by Jura 1975) and (C/H)PAH � 7.5 × 10 −5 .

Shocks in dense clouds - I. Dust dynamics

Context. Observations of SiO line emission in shocks in star-forming regions indicate that silicate dust destruction must be occurring in these dense regions. Current models rely on predictions for dust destruction by sputtering in C-type shock waves. However, J-type shocks may also be relevant for interpreting the widely-observed optical line emission from species such as Ou2009I and Feu2009II. Aims. In this work we explore, for the first time, dust destruction in J-type shocks slower than 50xa0kmu2009s -1 . Methods. We follow the dust trajectories throughout the shock using a model for the dust dynamics that allows us to solve the shock structure and at the same time calculate the degree of dust processing. We include the effects of sputtering in gas-grain collisions, and vaporisation and shattering in grain-grain collisions. Results. We find that the amount of silicon released into the gas phase is a few percent. The dominant destructive process is vaporisation, not sputtering. The degree of dust destruction increases with the shock velocity but decreases as the preshocku2000density increases. Conclusions. Our results compare well with that of C-type shock models. J-type shocks are therefore reasonable candidates for an interpretation of SiO line emission in molecular outflows and jets.

Photoelectric effect on dust grains across the L1721 cloud in the

We present the first Herschel spectroscopic detections of the [OI]63µm and [CII]158µm fine-structure transitions, and a single para-H_2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephans Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^(-1)) luminous [CII] line profiles, as well as fainter [OI]63µm emission. SPIRE FTS observations reveal water emission from the p-H_2O (1_(11)-0_(00)) transition at several positions in the filament, but no other molecular lines. The H_2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH_(tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII nregion excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H_2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. nLow-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H_2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.