Abstract. Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to E/k ∼ 1000 K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/∼moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

Aims. The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. Methods. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/∼vdtak/radex/index.shtml. The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. Results. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. Conclusions. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes.

Context. Water is a key tracer of dynamics and chemistry in low-mass star-forming regions, but spectrally resolved observations have so far been limited in sensitivity and angular resolution, and only data from the brightest low-mass protostars have been published. Aims. The first systematic survey of spectrally resolved water emission in 29 low-mass (L < 40 L ) protostellar objects is presented. The sources cover a range of luminosities and evolutionary states. The aim is to characterise the line profiles to distinguish physical components in the beam and examine how water emission changes with protostellar evolution. Methods. H 2 O was observed in the ground-state 1 10 -1 01 transition at 557 GHz (E up /k B ∼ 60 K) as single-point observations with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel in 29 deeply embedded Class 0 and I low-mass protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our programme) are used to constrain the spectral energy distribution (SED) of each source. H 2 O intensities are compared to inferred envelope properties, e.g., mass and density, outflow properties and CO 3-2 emission. Results. H 2 O emission is detected in all objects except one (TMC1A). The line profiles are complex and consist of several kinematic components tracing different physical regions in each system. In particular, the profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not in the quiescent envelope. Several sources show multiple shock components appearing in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in seven sources (six Class 0, one Class I) indicative of infalling envelopes, and regular P-Cygni profiles in four sources (three Class I, one Class 0) indicative of expanding envelopes. Molecular "bullets" moving at > ∼ 50 km s −1 with respect to the source are detected in four Class 0 sources; three of these sources were not known to harbour bullets previously. In the outflow, the H 2 O/CO abundance ratio as a function of velocity is nearly the same for all line wings, increasing from 10 −3 at low velocities (<5 km s −1 ) to > ∼ 10 −1 at high velocities (>10 km s −1 ). The water abundance in the outer cold envelope is low, > ∼ 10 −10 . The different H 2 O profile components show a clear evolutionary trend: in the younger Class 0 sources the emission is dominated by outflow components originating inside an infalling envelope. When large-scale infall diminishes during the Class I phase, the outflow weakens and H 2 O emission all but disappears.

The physical structure of the envelopes around a sample of fourteen massive young stars is investigated using maps and spectra in submillimeter continuum and lines of C 17 O, CS, C 34 S and H 2 CO. Nine of the sources are highly embedded luminous (10 3 − 10 5 L ⊙ ) young stellar objects which are bright near-infrared sources, but weak in radio continuum; the other objects are similar but not bright in the near-infrared, and contain "hot core"-type objects and/or ultracompact H II regions. The data are used to constrain the temperature and density structure of the circumstellar envelopes on 10 2 − 10 5 AU scales, to investigate the relation between the different objects and to search for evolutionary effects.The total column densities and the temperature profiles are obtained by fitting self-consistent dust models to submillimeter photometry. The calculated temperatures range from 300 to 1000 K at ∼ 10 2 AU and from 10 to 30 K at ∼ 10 5 AU from the star. Visual extinctions are a few hundred to a few thousand magnitudes, assuming a grain opacity at λ1300µm, of ≈ 1 cm −2 g −1 of dust, as derived earlier for one of our sources. The mid-infrared data are consistent with a 30% decrease of the opacity at higher temperatures, caused by the evaporation of the ice mantles.The CS, C 34 S and H 2 CO data as well as the submillimeter dust emission maps indicate density gradients n ∝ r −α . Assuming a constant CS abundance throughout the envelope, values of α = 1.0 − 1.5 are found, significantly flatter than the α = 2.0 ± 0.3 generally found for low-mass objects. This flattening may indicate that in massive