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.

Context. Herschel observations of water and highly excited CO (J >9) have allowed the physical and chemical conditions in the more active parts of protostellar outflows to be quantified in detail for the first time. However, to date, the studied samples of Class 0/I protostars in nearby star-forming regions have been selected from bright, well-known sources and have not been large enough for statistically significant trends to be firmly established. Aims. We aim to explore the relationships between the outflow, envelope and physical properties of a flux-limited sample of embedded low-mass Class 0/I protostars. Methods. We present spectroscopic observations in H 2 O, CO and related species with Herschel HIFI and PACS, as well as ground-based follow-up with the JCMT and APEX in CO, HCO + and isotopologues, of a sample of 49 nearby (d <500 pc) candidate protostars selected from Spitzer and Herschel photometric surveys of the Gould Belt. This more than doubles the sample of sources observed by the WISH and DIGIT surveys. These data are used to study the outflow and envelope properties of these sources. We also compile their continuum spectral energy distributions (SEDs) from the near-IR to mm wavelengths in order to constrain their physical properties (e.g. L bol , T bol and M env ). Results. Water emission is dominated by shocks associated with the outflow, rather than the cooler, slower entrained outflowing gas probed by ground-based CO observations. These shocks become less energetic as sources evolve from Class 0 to Class I. Outflow force, measured from low-J CO, also decreases with source evolutionary stage, while the fraction of mass in the outflow relative to the total envelope (i.e. M out /M env ) remains broadly constant between Class 0 and I. The median value of ∼1% is consistent with a core to star formation efficiency on the order of 50% and an outflow duty cycle on the order of 5%. Entrainment efficiency, as probed by F CO /Ṁ acc , is also invariant with source properties and evolutionary stage. The median value implies a velocity at the wind launching radius of 6.3 km s −1 , which in turn suggests an entrainment efficiency of between 30 and 60% if the wind is launched at ∼1AU, or close to 100% if launched further out. L[O i] is strongly correlated with L bol but not with M env , in contrast to low-J CO, which is more closely correlated with the latter than the former. This suggests that [O i] traces the present-day accretion activity of the source while CO traces time-averaged accretion over the dynamical timescale of the outflow. H 2 O is more strongly correlated with M env than L bol , but the difference is smaller than low-J CO, consistent with water emission primarily tracing actively shocked material between the wind, traced by [O i], and the entrained molecular outflow, traced by low-J CO. L[O i] does not vary from Class 0 to Class I, unlike CO and H 2 O. This is likely due to the ratio of atomic to molecular gas in the wind increasing as the source evolves, balancing out the decrease in ...

Aims. Our aim is to study the response of the gas-to-energetic processes associated with high-mass star formation and compare it with previously published studies on low-and intermediate-mass young stellar objects (YSOs) using the same methods. The quantified far-IR line emission and absorption of CO, H 2 O, OH, and [O i] reveals the excitation and the relative contribution of different atomic and molecular species to the gas cooling budget. Methods. Herschel/PACS spectra covering 55-190 μm are analyzed for ten high-mass star forming regions of luminosities L bol ∼ 10 4 −10 6 L and various evolutionary stages on spatial scales of ∼10 4 AU. Radiative transfer models are used to determine the contribution of the quiescent envelope to the far-IR CO emission. Results. The close environments of high-mass protostars show strong far-IR emission from molecules, atoms, and ions. Water is detected in all 10 objects even up to high excitation lines, often in absorption at the shorter wavelengths and in emission at the longer wavelengths. CO transitions from J = 14−13 up to typically 29−28 (E u /k B ∼ 580−2400 K) show a single temperature component with a rotational temperature of T rot ∼ 300 K. Typical H 2 O excitation temperatures are T rot ∼250 K, while OH has T rot ∼ 80 K.Far-IR line cooling is dominated by CO (∼75%) and, to a smaller extent, by [O i] (∼20%), which becomes more important for the most evolved sources. H 2 O is less important as a coolant for high-mass sources because many lines are in absorption. Conclusions. Emission from the quiescent envelope is responsible for ∼45-85% of the total CO luminosity in high-mass sources compared with only ∼10% for low-mass YSOs. The highest−J lines (J up ≥ 20) originate most likely in shocks, based on the strong correlation of CO and H 2 O with physical parameters (L bol , M env ) of the sources from low-to high-mass YSOs. The excitation of warm CO described by T rot ∼ 300 K is very similar for all mass regimes, whereas H 2 O temperatures are ∼100 K high for high-mass sources compared with low-mass YSOs. The total far-IR cooling in lines correlates strongly with bolometric luminosity, consistent with previous studies restricted to low-mass YSOs. Molecular cooling (CO, H 2 O, and OH) is ∼4 times greater than cooling by oxygen atoms for all mass regimes. The total far-IR line luminosity is about 10 −3 and 10 −5 times lower than the dust luminosity for the lowand high-mass star forming regions, respectively.