Context. 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 H 2 O by H 2 molecules is the process of most interest in interstellar space. Aims. To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H 2 O colliding with both para and ortho-H 2 in the temperature range 20−2000 K. Methods. Rate coefficients are calculated on a recent high-accuracy H 2 O−H 2 potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H 2 molecules are assumed to be rotationally thermalized at the kinetic temperature. Results. 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.

Context. The low mass protostar IRAS16293-2422 is a prototype Class 0 source in studies of the chemical structure during the initial phases of life of solar type stars. Aims. To derive the chemical structure accurately, a precise determination of the source physical structure is required. The goal of this study is to derive the structure of IRAS16293-2422. Methods. We reanalyzed all available continuum data (single dish and interferometric, from millimeter to MIR) to derive accurate density and dust temperature profiles. Using ISO observations of water, we also reconstructed the gas temperature profile. Results. Our analysis shows that the envelope surrounding IRAS16293-2422 is described well by the Shu "inside-out" collapsing envelope model or a single power-law density profile with index equal to 1.8. In contrast to some previous studies, our analysis does not show evidence of a large (≥800 AU in diameter) cavity. Conclusions. Although IRAS16293-2422 is a multiple system composed of two or three objects, our reconstruction will be useful in deriving the chemical structure of the large cold envelope surrounding these objects and the warm component, treated here as a single source, from single-dish observations of molecular emission.

High resolution line spectra of star-forming regions are mines of information: they provide unique clues to reconstruct the chemical, dynamical, and physical structure of the observed source. We present the first results from the Herschel key project "Chemical HErschel Surveys of Star forming regions", CHESS. We report and discuss observations towards five CHESS targets, one outflow shock spot and four protostars with luminosities bewteen 20 and 2 × 10 5 L : L1157-B1, IRAS 16293-2422, OMC2-FIR4, AFGL 2591, and NGC 6334I. The observations were obtained with the heterodyne spectrometer HIFI on board Herschel, with a spectral resolution of 1 MHz. They cover the frequency range 555−636 GHz, a range largely unexplored before the launch of the Herschel satellite. A comparison of the five spectra highlights spectacular differences in the five sources, for example in the density of methanol lines, or the presence/absence of lines from S-bearing molecules or deuterated species. We discuss how these differences can be attributed to the different star-forming mass or evolutionary status.

We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5−636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-tonoise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC 2 ). We identified 55 SiC 2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC 2 is produced in the inner dust formation zone, with an abundance of ∼2 × 10 −7 relative to molecular hydrogen. These SiC 2 lines have been observed for the first time in space and have been used to derive an SiC 2 rotational temperature of ∼204 K and a source-averaged column density of ∼6.4 × 10 15 cm −2 . Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC 2 .