Context. The outflow force of molecular bipolar outflows is a key parameter in theories of young stellar feedback on their surroundings. The focus of many outflow studies is the correlation between... Show moreContext. The outflow force of molecular bipolar outflows is a key parameter in theories of young stellar feedback on their surroundings. The focus of many outflow studies is the correlation between the outflow force, bolometric luminosity, and envelope mass. However, it is difficult to combine the results of different studies in large evolutionary plots over many orders of magnitude due to the range of data quality, analysis methods, and corrections for observational effects, such as opacity and inclination. Aims: We aim to determine the outflow force for a sample of low-luminosity embedded sources. We quantify the influence of the analysis method and the assumptions entering the calculation of the outflow force. Methods: We used the James Clerk Maxwell Telescope to map $^{12}$CO J = 3-2 over 2'{ imes} 2' regions around 16 Class I sources of a well-defined sample in Ophiuchus at 15{Prime} resolution. The outflow force was then calculated using seven different methods differing, e.g., in the use of intensity-weighted emission and correction factors for inclination. Two well studied outflows (HH 46 and NGC1 333 IRAS4A) are added to the sample and included in the comparison. Results: The results from the analysis methods differ from each other by up to a factor of 6, whereas observational properties and choices in the analysis procedure affect the outflow force by up to a factor of 4. Subtraction of cloud emission and integrating over the remaining profile increases the outflow force at most by a factor of 4 compared to line wing integration. For the sample of Class I objects, bipolar outflows are detected around 13 sources including 5 new detections, where the three nondetections are confused by nearby outflows from other sources. New outflow structures without a clear powering source are discovered at the corners of some of the maps. Conclusions: When combining outflow forces from different studies, a scatter by up to a factor of 5 can be expected. Although the true outflow force remains unknown, the separation method (separate calculation of dynamical time and momentum) is least affected by the uncertain observational parameters. The correlations between outflow force, bolometric luminosity, and envelope mass are further confirmed down to low-luminosity sources. Appendices are available in electronic form at http://www.aanda.org Show less
Marel, N. van der; Kristensen, L.; Visser, R.; Mottram, J.C.; Yildiz, U.; Dishoeck, E.F. van 2013
Context. The outflow force of molecular bipolar outflows is a key parameter in theories of young stellar feedback on their surroundings. The focus of many outflow studies is the correlation... Show moreContext. The outflow force of molecular bipolar outflows is a key parameter in theories of young stellar feedback on their surroundings. The focus of many outflow studies is the correlation between the outflow force, bolometric luminosity, and envelope mass. However, it is difficult to combine the results of different studies in large evolutionary plots over many orders of magnitude due to the range of data quality, analysis methods, and corrections for observational effects, such as opacity and inclination. Aims: We aim to determine the outflow force for a sample of low-luminosity embedded sources. We quantify the influence of the analysis method and the assumptions entering the calculation of the outflow force. Methods: We used the James Clerk Maxwell Telescope to map $^{12}$CO J = 3-2 over 2'{ imes} 2' regions around 16 Class I sources of a well-defined sample in Ophiuchus at 15{Prime} resolution. The outflow force was then calculated using seven different methods differing, e.g., in the use of intensity-weighted emission and correction factors for inclination. Two well studied outflows (HH 46 and NGC1 333 IRAS4A) are added to the sample and included in the comparison. Results: The results from the analysis methods differ from each other by up to a factor of 6, whereas observational properties and choices in the analysis procedure affect the outflow force by up to a factor of 4. Subtraction of cloud emission and integrating over the remaining profile increases the outflow force at most by a factor of 4 compared to line wing integration. For the sample of Class I objects, bipolar outflows are detected around 13 sources including 5 new detections, where the three nondetections are confused by nearby outflows from other sources. New outflow structures without a clear powering source are discovered at the corners of some of the maps. Conclusions: When combining outflow forces from different studies, a scatter by up to a factor of 5 can be expected. Although the true outflow force remains unknown, the separation method (separate calculation of dynamical time and momentum) is least affected by the uncertain observational parameters. The correlations between outflow force, bolometric luminosity, and envelope mass are further confirmed down to low-luminosity sources. Appendices are available in electronic form at http://www.aanda.orgShow less
Context. According to traditional gas-phase chemical models, O$_{2}$ should be abundant in molecular clouds, but until recently, attempts to detect interstellar O$_{2}$ line emission with ground-... Show moreContext. According to traditional gas-phase chemical models, O$_{2}$ should be abundant in molecular clouds, but until recently, attempts to detect interstellar O$_{2}$ line emission with ground- and space-based observatories have failed. Aims: Following the multi-line detections of O$_{2}$ with low abundances in the Orion and {$ρ$} Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O$_{2}$ abundance near a solar-mass protostar. Methods: Observations of molecular oxygen, O$_{2}$, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. Results: The deep HIFI spectrum fails to show O$_{2}$ at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O$_{2}$/H$_{2}$ at {le}6 { imes} 10$^{-9}$ (3{$σ$}). The O$_{2}$/CO abundance ratio is less than 0.005. However, a tentative (4.5{$σ$}) detection of O$_{2}$ is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km s$^{-1}$ relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~{}0.7-1 { imes} 10$^{6}$ years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H$_{2}$O, to avoid overproduction of O$_{2}$ in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule if hydrogenation of NO on the grains to more complex molecules such as NH$_{2}$OH, found in recent laboratory experiments, is included. The tentative detection of O$_{2}$ in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. Conclusions: The low O$_{2}$ abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O$_{2}$. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgReduced spectra (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/558/A58Show less
San Jose Garcia, I.; Mottram, J.C.; Kristensen, L.; Dishoeck, E.F. van; Yıldız, U.; Tak, F.; ... ; Johnstone, D. 2013
Context. Our understanding of the star formation process has traditionally been confined to certain mass or luminosity boundaries because most studies focus only on low-, intermediate-, or high... Show moreContext. Our understanding of the star formation process has traditionally been confined to certain mass or luminosity boundaries because most studies focus only on low-, intermediate-, or high-mass star-forming regions. Therefore, the processes that regulate the formation of these different objects have not been effectively linked. As part of the ''Water In Star-forming regions with Herschel'' (WISH) key programme, water and other important molecules, such as CO and OH, have been observed in 51 embedded young stellar objects (YSOs). The studied sample covers a range of luminosities from {lt}1 to {gt}10$^{5}$L$_{⊙}$. Aims: We analyse the CO line emission towards a large sample of embedded protostars in terms of both line intensities and profiles. This analysis covers a wide luminosity range in order to achieve better understanding of star formation without imposing luminosity boundaries. In particular, this paper aims to constrain the dynamics of the environment in which YSOs form. Methods: Herschel-HIFI spectra of the $^{12}$CO J = 10-9, $^{13}$CO J = 10-9 and C$^{18}$O J = 5-4, J = 9-8 and J = 10-9 lines were analysed for a sample of 51 embedded protostars. In addition, JCMT spectra of $^{12}$CO J = 3-2 and C$^{18}$O J = 3-2 extend this analysis to cooler gas components. We focussed on characterising the shape and intensity of the CO emission line profiles by fitting the lines with one or two Gaussian profiles. We compared the values and results of these fits across the entire luminosity range covered by WISH observations. The effects of different physical parameters as a function of luminosity and the dynamics of the envelope-outflow system were investigated. Results: All observed CO and isotopologue spectra show a strong linear correlation between the logarithms of the line and bolometric luminosities across six orders of magnitude on both axes. This suggests that the high-J CO lines primarily trace the amount of dense gas associated with YSOs and that this relation can be extended to larger (extragalactic) scales. The majority of the detected $^{12}$CO line profiles can be decomposed into a broad and a narrow Gaussian component, while the C$^{18}$O spectra are mainly fitted with a single Gaussian. For low- and intermediate-mass protostars, the width of the C$^{18}$O J = 9-8 line is roughly twice that of the C$^{18}$O J = 3-2 line, suggesting increased turbulence/infall in the warmer inner envelope. For high-mass protostars, the line widths are comparable for lower- and higher-J lines. A broadening of the line profile is also observed from pre-stellar cores to embedded protostars, which is due mostly to non-thermal motions (turbulence/infall). The widths of the broad $^{12}$CO J = 3-2 and J = 10-9 velocity components correlate with those of the narrow C$^{18}$O J = 9-8 profiles, suggesting that the entrained outflowing gas and envelope motions are related but independent of the mass of the protostar. These results indicate that physical processes in protostellar envelopes have similar characteristics across the studied luminosity range. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgShow less
Context. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation under interstellar conditions has never been... Show moreContext. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation under interstellar conditions has never been properly studied. The partitioning of nitrogen between N and N$_{2}$ controls the formation of more complex prebiotic nitrogen-containing species. Aims: The aim of this work is to gain a better understanding of the interstellar N$_{2}$ photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models. Methods: We used an approach similar to that adopted for CO in which we simulated the full high-resolution line-by-line absorption + dissociation spectrum of N$_{2}$ over the relevant 912-1000 å wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schrödinger equation. The simulated N$_{2}$ spectra were compared with the absorption spectra of H$_{2}$, H, CO, and dust to compute photodissociation rates in various radiation fields and shielding functions. The effects of the new rates in interstellar cloud models were illustrated for diffuse and translucent clouds, a dense photon dominated region and a protoplanetary disk. Results: The unattenuated photodissociation rate in the Draine (1978, ApJS, 36, 595) radiation field assuming an N$_{2}$ excitation temperature of 50 K is 1.65 { imes} 10$^{-10}$ s$^{-1}$, with an uncertainty of only 10%. Most of the photodissociation occurs through bands in the 957-980 å range. The N$_{2}$ rate depends slightly on the temperature through the variation of predissociation probabilities with rotational quantum number for some bands. Shielding functions are provided for a range of H$_{2}$ and H column densities, with H$_{2}$ being much more effective than H in reducing the N$_{2}$ rate inside a cloud. Shielding by CO is not effective. The new rates are 28% lower than the previously recommended values. Nevertheless, diffuse cloud models still fail to reproduce the possible detection of interstellar N$_{2}$ except for unusually high densities and/or low incident UV radiation fields. The transition of N { arr} N$_{2}$ occurs at nearly the same depth into a cloud as that of C$^{+}$ { arr} C { arr} CO. The orders-of-magnitude lower N$_{2}$ photodissociation rates in clouds exposed to black-body radiation fields of only 4000 K can qualitatively explain the lack of active nitrogen chemistry observed in the inner disks around cool stars. Conclusions: Accurate photodissociation rates for N$_{2}$ as a function of depth into a cloud are now available that can be applied to a wide variety of astrophysical environments. Appendices are available in electronic form at http://www.aanda.orgShow less
Mottram, J.C.; Dishoeck, E.F. van; Schmalzl, M.; Kristensen, L.; Visser, R.; Hogerheijde, M.R.; Bruderer, S. 2013
Context. For stars to form, material must fall inwards from core scales through the envelope towards the central protostar. While theories of how this takes place have been around for some time,... Show moreContext. For stars to form, material must fall inwards from core scales through the envelope towards the central protostar. While theories of how this takes place have been around for some time, the velocity profile around protostars is poorly constrained. The combination of observations in multiple transitions of a tracer which is sensitive to kinematics and radiative transfer modelling of those lines has the potential to break this deadlock. Aims: Seven protostars observed with the Heterodyne Instrument for the Far-Infrared (HIFI) on board the Herschel Space Observatory as part of the ''Water in star-forming regions with Herschel'' (WISH) survey show infall signatures in water line observations. We aim to constrain the infall velocity and the radii over which infall is taking place within the protostellar envelopes of these sources. We will also use these data to constrain the chemistry of cold water. Methods: We use 1-D non-LTE ratran radiative transfer models of the observed water lines to constrain the infall velocity and chemistry in the protostellar envelopes of six Class 0 protostars and one Class I source. We assume a free-fall velocity profile and, having found the best fit, vary the radii over which infall takes place. Results: In the well-studied Class 0 protostar NGC 1333-IRAS4A we find that our observations probe infall over the whole envelope to which our observations are sensitive (r {gsim} 1000 AU). For L1527, L1157, BHR71 and IRAS 15398 infall takes place on core to envelope scales (i.e. ~{}10 000-3000 AU). In Serpens-SMM4 and GSS30 the inverse P-Cygni profiles seen in the ground-state lines are more likely due to larger-scale motions or foreground clouds. Models including a simple consideration of the chemistry are consistent with the observations, while using step abundance profiles are not. The non-detection of excited water in the inner envelope in six out of seven protostars is further evidence that water must be heavily depleted from the gas-phase at these radii. Conclusions: Infall in four of the sources is supersonic and in all sources must take place at the outer edge of the envelope, which may be evidence that collapse is global or outside-in rather than inside-out. The mass infall rate in NGC 1333-IRAS4A is large ({gsim}10$^{-4}$M$_{⊙}$ yr$^{-1}$), higher than the mass outflow rate and expected mass accretion rates onto the star. This suggests that any flattened disk-like structure on small scales will be gravitationally unstable, potentially leading to rotational fragmentation and/or episodic accretion. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendix A is available in electronic form at http://www.aanda.orgShow less
Aims: Herschel-HIFI spectra of H$_{2}$O towards low-mass protostars show a distinct velocity component not seen in observations from the ground of CO or other species. The aim is to characterise... Show moreAims: Herschel-HIFI spectra of H$_{2}$O towards low-mass protostars show a distinct velocity component not seen in observations from the ground of CO or other species. The aim is to characterise this component in terms of excitation conditions and physical origin. Methods: A velocity component with an offset of ~{}10 km s$^{-1}$ detected in spectra of the H$_{2}$O 1$_{10}$-1$_{01}$ 557 GHz transition towards six low-mass protostars in the ''Water in star-forming regions with Herschel'' (WISH) programme is also seen in higher-excited H$_{2}$O lines. The emission from this component is quantified and local excitation conditions are inferred using 1D slab models. Data are compared to observations of hydrides (high-J CO, OH$^{+}$, CH$^{+}$, C$^{+}$, OH) where the same component is uniquely detected. Results: The velocity component is detected in all six targeted H$_{2}$O transitions (E$_{up}$ ~{} 50-250 K), as well as in CO 16-15 towards one source, Ser SMM1. Inferred excitation conditions imply that the emission arises in dense (n ~{} 5 { imes} 10$^{6}$-10$^{8}$ cm$^{-3}$) and hot (T ~{} 750 K) gas. The H$_{2}$O and CO column densities are {gsim}10$^{16}$ and 10$^{18}$ cm$^{-2}$, respectively, implying a low H$_{2}$O abundance of ~{}10$^{-2}$ with respect to CO. The high column densities of ions such as OH$^{+}$ and CH$^{+}$ (both {gsim}10$^{13}$ cm$^{-2}$) indicate an origin close to the protostar where the UV field is strong enough that these species are abundant. The estimated radius of the emitting region is 100 AU. This component likely arises in dissociative shocks close to the protostar, an interpretation corroborated by a comparison with models of such shocks. Furthermore, one of the sources, IRAS 4A, shows temporal variability in the offset component over a period of two years which is expected from shocks in dense media. High-J CO gas detected with Herschel-PACS with T$_{rot}$ ~{} 700 K is identified as arising in the same component and traces the part of the shock where H$_{2}$ reforms. Thus, H$_{2}$O reveals new dynamical components, even on small spatial scales in low-mass protostars. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Show less
Context. The measure of the water deuterium fractionation is a relevant tool for understanding mechanisms of water formation and evolution from the prestellar phase to the formation of planets and... Show moreContext. The measure of the water deuterium fractionation is a relevant tool for understanding mechanisms of water formation and evolution from the prestellar phase to the formation of planets and comets. Aims: The aim of this paper is to study deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B, to compare their HDO abundance distributions with other star-forming regions, and to constrain their HDO/H$_{2}$O abundance ratios. Methods: Using the Herschel/HIFI instrument as well as ground-based telescopes, we observed several HDO lines covering a large excitation range (E$_{up}$/k = 22-168 K) towards these protostars and an outflow position. Non-local thermal equilibrium radiative transfer codes were then used to determine the HDO abundance profiles in these sources. Results: The HDO fundamental line profiles show a very broad component, tracing the molecular outflows, in addition to a narrower emission component and a narrow absorbing component. In the protostellar envelope of NGC 1333 IRAS 4A, the HDO inner (T {ge} 100 K) and outer (T {lt} 100 K) abundances with respect to H$_{2}$ are estimated with a 3{$σ$} uncertainty at 7.5$_{-3.0}$$^{+3.5}$ { imes} 10$^{-9}$ and 1.2$_{-0.4}$$^{+0.4}$ { imes} 10$^{-11}$, respectively, whereas in NGC 1333 IRAS 4B they are 1$_{-0.9}$$^{+1.8}$ { imes} 10$^{-8}$ and 1.2$_{-0.4}$$^{+0.6}$ { imes} 10$^{-10}$, respectively. Similarly to the low-mass protostar IRAS 16293-2422, an absorbing outer layer with an enhanced abundance of deuterated water is required to reproduce the absorbing components seen in the fundamental lines at 465 and 894 GHz in both sources. This water-rich layer is probably extended enough to encompass the two sources, as well as parts of the outflows. In the outflows emanating from NGC 1333 IRAS 4A, the HDO column density is estimated at about (2-4) { imes} 10$^{13}$ cm$^{-2}$, leading to an abundance of about (0.7-1.9) { imes} 10$^{-9}$. An HDO/H$_{2}$O ratio between 7 { imes} 10$^{-4}$ and 9 { imes} 10$^{-2}$ is also derived in the outflows. In the warm inner regions of these two sources, we estimate the HDO/H$_{2}$O ratios at about 1 { imes} 10$^{-4}$-4 { imes} 10$^{-3}$. This ratio seems higher (a few %) in the cold envelope of IRAS 4A, whose possible origin is discussed in relation to formation processes of HDO and H$_{2}$O. Conclusions: In low-mass protostars, the HDO outer abundances range in a small interval, between ~{}10$^{-11}$ and a few 10$^{-10}$. No clear trends are found between the HDO abundance and various source parameters (L$_{bol}$, L$_{smm}$, L$_{smm}$/L$_{bol}$, T$_{bol}$, L$_{bol}$$^{0.6}$/M$_{env}$). A tentative correlation is observed, however, between the ratio of the inner and outer abundances with the submillimeter luminosity. Based on observations carried out with the Herschel/HIFI instrument, the Institut de Radioastronomie Millimétrique (IRAM) 30 m Telescope, the James Clerk Maxwell Telescope (JCMT), and one of the ESO telescopes at the La Silla Paranal, the Atacama Pathfinder Experiment (APEX, programme ID 090.C-0239). Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The JCMT is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the United Kingdom, the Netherlands Organization for Scientific Research, and the National Research Council of Canada. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the ESO, and the Onsala Space Observatory.Appendices are available in electronic form at http://www.aanda.orgShow less
Yıldız, U.; Kristensen, L.; Dishoeck, E.F. van; San Jose Garcia, I.; Karska, A.; Harsono, D.S.; ... ; Hogerheijde, M.R. 2013
Context. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through... Show moreContext. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through dust continuum spectral energy distributions (SEDs), but the use of CO excitation as an evolutionary probe has not yet been explored due to the lack of high-J CO observations. Aims: The aim is to constrain the physical characteristics (excitation, kinematics, column density) of the warm gas in low-mass protostellar envelopes using spectrally resolved Herschel data of CO and compare those with the colder gas traced by lower excitation lines. Methods: Herschel-HIFI observations of high-J lines of $^{12}$CO, $^{13}$CO, and C$^{18}$O (up to J$_u$ = 10, E$_u$ up to 300 K) are presented toward 26 deeply embedded low-mass Class 0 and Class I young stellar objects, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. This is the first large spectrally resolved high-J CO survey conducted for these types of sources. Complementary lower J CO maps were observed using ground-based telescopes, such as the JCMT and APEX and convolved to matching beam sizes. Results: The $^{12}$CO 10-9 line is detected for all objects and can generally be decomposed into a narrow and a broad component owing to the quiescent envelope and entrained outflow material, respectively. The $^{12}$CO excitation temperature increases with velocity from ~{}60 K up to ~{}130 K. The median excitation temperatures for $^{12}$CO, $^{13}$CO, and C$^{18}$O derived from single-temperature fits to the J$_u$ = 2-10 integrated intensities are ~{}70 K, 48 K and 37 K, respectively, with no significant difference between Class 0 and Class I sources and no trend with M$_{env}$ or L$_{bol}$. Thus, in contrast to the continuum SEDs, the spectral line energy distributions (SLEDs) do not show any evolution during the embedded stage. In contrast, the integrated line intensities of all CO isotopologs show a clear decrease with evolutionary stage as the envelope is dispersed. Models of the collapse and evolution of protostellar envelopes reproduce the C$^{18}$O results well, but underproduce the $^{13}$CO and $^{12}$CO excitation temperatures, due to lack of UV heating and outflow components in those models. The H$_{2}$O 1$_{10}$ - 1$_{01}$/CO 10-9 intensity ratio does not change significantly with velocity, in contrast to the H$_{2}$O/CO 3-2 ratio, indicating that CO 10-9 is the lowest transition for which the line wings probe the same warm shocked gas as H$_{2}$O. Modeling of the full suite of C$^{18}$O lines indicates an abundance profile for Class 0 sources that is consistent with a freeze-out zone below 25 K and evaporation at higher temperatures, but with some fraction of the CO transformed into other species in the cold phase. In contrast, the observations for two Class I sources in Ophiuchus are consistent with a constant high CO abundance profile. Conclusions: The velocity resolved line profiles trace the evolution from the Class 0 to the Class I phase through decreasing line intensities, less prominent outflow wings, and increasing average CO abundances. However, the CO excitation temperature stays nearly constant. The multiple components found here indicate that the analysis of spectrally unresolved data, such as provided by SPIRE and PACS, must be done with caution. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices C and D are available in electronic form at http://www.aanda.orgShow less
Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a... Show moreContext. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H$_{2}$O, OH, and atomic lines. For four targets the spectra span the entire 55-200 {$μ$}m region. The PACS field-of-view covers ~{}47'' with the resolution of 9.4''. Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H$_{2}$O 8$_{18}$-7$_{07}$ 63.3 {$μ$}m line (E$_u$/k$_B$ = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~{}350 K and ~{}700 K. H$_{2}$O has typical excitation temperatures of ~{}150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on {amp}{ge}10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H$_{2}$O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H$_{2}$O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H$_{2}$O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H$_{2}$O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. Appendices A-J are available in electronic form at http://www.aanda.orgShow less
