We present the revised "Meudon" model of Photon Dominated Region (PDR code), presently available on the web under the Gnu Public Licence at: http://aristote.obspm.fr/MIS. General organisation of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelisation highlighted.

Abstract. We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and excitation of H 2 in the diffuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photonilluminated gas. The data constrain well the n R product between gas density and H 2 formation rate on dust grains: n R = 1 to 2.2 × 10 −15 s −1 . For each line of sight the mean effective H 2 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 n R values, we find similar H 2 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 H 2 excitation in the J > 2 levels cannot be accounted for by the UV pumping of the cold H 2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H 2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H 2 .

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H 2 , C i, CO, CH, and CH + column densities and excitation. These data are complemented by observations of C + , CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH + and H 2 in its excited (J ≥ 2) levels, are consistent with a cloud model implying a Galactic radiation field (G ∼ 0.4 in Draine's unit), a density of 80 cm −3 and a temperature (60−80 K) set by the equilibrium between heating and cooling processes. To account for excited (J ≥ 2) H 2 levels column densities, an additional component of warm (∼250 K) and dense (n H ≥ 10 4 cm −3 ) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H 2 ratio at formation is ∼1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H 2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H 2 is required to account for the CH + column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.

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We present JWST-MIRI Medium Resolution Spectrometer (MRS) spectra of the protoplanetary disk around the low-mass T Tauri star GW Lup from the MIRI mid-INfrared Disk Survey Guaranteed Time Observations program. Emission from 12CO2, 13CO2, H2O, HCN, C2H2, and OH is identified with 13CO2 being detected for the first time in a protoplanetary disk. We characterize the chemical and physical conditions in the inner few astronomical units of the GW Lup disk using these molecules as probes. The spectral resolution of JWST-MIRI MRS paired with high signal-to-noise data is essential to identify these species and determine their column densities and temperatures. The Q branches of these molecules, including those of hot bands, are particularly sensitive to temperature and column density. We find that the 12CO2 emission in the GW Lup disk is coming from optically thick emission at a temperature of ∼400 K. 13CO2 is optically thinner and based on a lower temperature of ∼325 K, and thus may be tracing deeper into the disk and/or a larger emitting radius than 12CO2. The derived N CO 2 / N H 2 O ratio is orders of magnitude higher than previously derived for GW Lup and other targets based on Spitzer-InfraRed-Spectrograph data. This high column density ratio may be due to an inner cavity with a radius in between the H2O and CO2 snowlines and/or an overall lower disk temperature. This paper demonstrates the unique ability of JWST to probe inner disk structures and chemistry through weak, previously unseen molecular features.

Aims. High-resolution spectroscopic observations (UV HST/STIS and optical) are used to characterize the physical state and velocity structure of the multiphase interstellar medium seen towards the nearby (170 pc) star HD 102065. The star is located behind the tail of a cometary-shaped, infrared cirrus-cloud, in the area of interaction between the Sco-Cen OB association and the Local Bubble. Methods. We analyze interstellar components present along the line of sight by fitting multiple transitions from a group of species all at once. We identify four groups of species: (1) molecules (CO, CH, CH + ), (2) atoms (C i, S i, Fe i) with ionization potential lower than H i, (3) neutral and low-ionized states of atoms (Mg i, Mg ii, Mn ii, P ii, Ni ii, C ii, N i and O i) with ionization potential larger than H i and (4) highly-ionized atoms (Si iii, C iv, Si iv). The absorption spectra are complemented by H i, CO and C ii emission-line spectra, H 2 column-densities derived from FUSE spectra, and IRAS images. Results. Gas components of a wide range of temperatures and ionization states are detected along the line of sight. Most of the hydrogen column-density is in cold, diffuse, molecular gas at low LSR velocity. This gas is mixed with traces of warmer molecular gas traced by H 2 in the J > 2 levels, in which the observed CH + must be formed. We also identify three distinct components of warm gas at negative velocities down to -20 km s −1 . The temperature and gas excitation are shown to increase with increasing velocity shift from the bulk of the gas. Hot gas at temperatures of several 10 5 K is detected in the most negative velocity component in the highly-ionized specie. This hot gas is also detected in very strong lines of less-ionized species (Mg ii, Si ii * and C ii * ) for which the bulk of the gas is cooler. Conclusions. We relate the observational results to evidence for dynamical impact of the Sco-Cen stellar association on the nearby interstellar medium. We propose a scenario where the infrared cirrus cloud has been hit a few 10 5 yr ago by a supernova blast wave originating from the Lower Centaurus Crux group of the Sco-Cen association. The observations provide detailed information on the interplay between ISM phases in relation with the origin of the Local and Loop I bubbles.

Terrestrial and sub-Neptune planets are expected to form in the inner (less than 10 au) regions of protoplanetary disks1. Water plays a key role in their formation2–4, although it is yet unclear whether water molecules are formed in situ or transported from the outer disk5,6. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks7, similar to PDS 70, the first system with direct confirmation of protoplanet presence8,9. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large (approximately 54 au) planet-carved gap separating an inner and outer disk10,11. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H2 and/or OH, and survival through water self-shielding5. This is also supported by the presence of CO2 emission, another molecule sensitive to ultraviolet photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir12. Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.

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