Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The... Show moreContext. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 {$μ$}m emission band of the crystalline silicate forsterite. Aims: This work aims at providing an overview of the 69 {$μ$}m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods: The 69 {$μ$}m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 {$μ$}m band to the forsterite bands at shorter wavelengths. Results: A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 {$μ$}m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 {$μ$}m bands are found to be warm (~{}100-200 K) and iron-poor (less than ~{}2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions: Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 {$μ$}m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 {$μ$}m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result. Appendix A is available in electronic form at http://www.aanda.org Show less
We present far-infrared (50-200 {$μ$}m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe... Show moreWe present far-infrared (50-200 {$μ$}m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [O i], [C ii], CO, OH, H$_{2}$O, CH$^{+}$. The most common feature is the [O i] 63 {$μ$}m line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10$^{13}$ {lt} N$_{OH}$ {lt} 10$^{16}$ cm$^{-2}$, emitting radii 15 {lt} r {lt} 100 AU and excitation temperatures 100 {lt} T$_{ex}$ {lt} 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n {ge} 10$^{10}$ cm$^{-3}$) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H$_{2}$O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H$_{2}$O abundance ratios across the disk than do T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH$^{+}$ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T$_{ex}$ ~{} 100 K) and compact (r ~{} 60 AU) emission in the case of HD 100546. Off-source [O i] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [C ii] emission is spatially extended in all sources where the line is detected. This suggests that not all [C ii] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([O i] 63 {$μ$}m, [O i] 145 {$μ$}m, [C ii]) are analyzed with PDR models and require high gas density (n {gsim} 10$^{5}$ cm$^{-3}$) and high UV fluxes (G$_o$ ~{} 10$^{3}$ - 10$^{7}$), consistent with a disk origin for the oxygen lines for most of the sources. 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
We present a large, comprehensive survey of rovibrational CO line emission at 4.7 {$μ$}m from 69 protoplanetary disks, obtained with CRIRES on the ESO Very Large Telescope at the highest available... Show moreWe present a large, comprehensive survey of rovibrational CO line emission at 4.7 {$μ$}m from 69 protoplanetary disks, obtained with CRIRES on the ESO Very Large Telescope at the highest available spectral resolving power (R = 95,000, {$Delta$}v = 3.2 km s$^{-1}$). The CO fundamental band ({$Delta$}v = 1) is a well-known tracer of warm gas in the inner, planet-forming regions of gas-rich disks around young stars, with the lines formed in the super-heated surfaces of the disks at radii of 0.1-10 AU. Consistent with earlier studies, the presence of 100-1000 K CO is found to be ubiquitous around young stars which still retain disks. Our high spectral resolution data provide new insight into the kinematics of the inner disk gas. The observed line profiles are complex and reveal several different components. Pure double-peaked Keplerian profiles are surprisingly uncommon in our sample, beyond the frequency expected based on disk inclination. The majority of the profiles are consistent with emission from a disk plus a slow (few km s$^{-1}$) molecular disk wind. This is evidenced by analysis of different classes as well as an overall tendency for line profiles to have excess emission on their blue side. The data support the notion that thermal molecular winds are common for young disks. Thanks to the high spectral resolution, narrow absorption lines and weak emission lines from isotopologues and from vibrationally excited levels are readily detected. In general, $^{13}$CO lines trace cooler gas than the bulk $^{12}$CO emission and may arise from further out in the disk, as indicated by narrower line profiles. A high fraction of the sources show vibrationally excited emission (~{}50%) which is correlated with accretion luminosity, consistent with ultraviolet fluorescent excitation. Disks around early-type Herbig AeBe stars have narrower line profiles, on average, than their lower-mass late-type counterparts, due to their increased luminosity. Evolutionary changes in CO are also seen. Removal of the protostellar envelope between class I and II results in the disappearance of the strong absorption lines and CO ice feature characteristic of class I spectra. However, CO emission from class I and II objects are similar in detection frequency, excitation, and line shape, indicating that inner disk characteristics are established early. Show less
We present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in... Show moreWe present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in Time Key Program. Some sources exhibit up to 75 H$_{2}$O lines ranging in excitation energy from 100 to 2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 { arr} 13 up to J = 40 { arr} 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two very low luminosity objects. The mean 63/145 {$μ$}m [O I] flux ratio is 17.2 {plusmn} 9.2. The [O I] 63 {$μ$}m line correlates with L $_{bol}$, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample L $_{bol}$ increased by 1.25 (1.06) and T $_{bol}$ decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (${$$ackslash$langle ${$ ${$N$}$$}$$ackslash$rangle$}$ = (0.70 +/- 1.12)${$${$$}$ $ackslash$times 10^{}${$49$}$$}$ total particles). ${$ ${$N$}$$}$_CO correlates strongly with L $_{bol}$, but neither T $_{rot}$ nor ${$ ${$N$}$$}$_CO(warm)/${$ ${$N$}$$}$_CO(hot) correlates with L $_{bol}$, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H$_{2}$O (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ = 194 +/- 85 K) and OH (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ =183 +/- 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths among the detected lines. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Show less
Aims: In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H$_{2}$O (E$_u$ {gt} 190 K), high-J CO, [Oi], and OH transitions are mapped with Herschel... Show moreAims: In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H$_{2}$O (E$_u$ {gt} 190 K), high-J CO, [Oi], and OH transitions are mapped with Herschel-PACS in two shock positions along two prototypical outflows around the low-luminosity sources L1448 and L1157. Previous Herschel-HIFI H$_{2}$O observations (E$_u$ = 53-249 K) are also used. The aim is to derive a complete picture of the excitation conditions at the selected shock positions. Methods: We adopted a large velocity gradient analysis (LVG) to derive the physical parameters of the H$_{2}$O and CO emitting gas. Complementary Spitzer mid-IR H$_{2}$ data were used to derive the H$_{2}$O abundance. Results: Consistent with other studies, at all selected shock spots a close spatial association between H$_{2}$O, mid-IR H$_{2}$, and high-J CO emission is found, whereas the low-J CO emission traces either entrained ambient gas or a remnant of an older shock. The excitation analysis, conducted in detail at the L1448-B2 position, suggests that a two-component model is needed to reproduce the H$_{2}$O, CO, and mid-IR H$_{2}$ lines: an extended warm component (T ~{} 450 K) is traced by the H$_{2}$O emission with E$_u$ = 53-137 K and by the CO lines up to J = 22-21, and a compact hot component (T = 1100 K) is traced by the H$_{2}$O emission with E$_u$ {gt} 190 K and by the higher-J CO transitions. At L1448-B2 we obtain an H$_{2}$O abundance (3-4) { imes} 10$^{-6}$ for the warm component and (0.3-1.3) { imes} 10$^{-5}$ for the hot component and a CO abundance of a few 10$^{-5}$ in both components. In L1448-B2 we also detect OH and blue-shifted [Oi] emission, spatially coincident with the other molecular lines and with [Feii] emission. This suggests a dissociative shock for these species, related to the embedded atomic jet. On the other hand, a non-dissociative shock at the point of impact of the jet on the cloud is responsible for the H$_{2}$O and CO emission. The other examined shock positions show an H$_{2}$O excitation similar to L1448-B2, but a slightly higher H$_{2}$O abundance (a factor of ~{}4). Conclusions: The two gas components may represent a gas stratification in the post-shock region. The extended and low-abundance warm component traces the post-shocked gas that has already cooled down to a few hundred Kelvin, whereas the compact and possibly higher-abundance hot component is associated with the gas that is currently undergoing a shock episode. This hot gas component is more affected by evolutionary effects on the timescales of the outflow propagation, which explains the observed H$_{2}$O abundance variations. 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
In this work, we introduce the use of H I Pfund {$β$} (Pf{$β$} 4.6538 {$μ$}m) as a tracer of mass accretion from protoplanetary disks onto young stars. Pf{$β$} was serendipitously observed in... Show moreIn this work, we introduce the use of H I Pfund {$β$} (Pf{$β$} 4.6538 {$μ$}m) as a tracer of mass accretion from protoplanetary disks onto young stars. Pf{$β$} was serendipitously observed in NIRSPEC and CRIRES surveys of CO fundamental emission, amounting to a sample size of 120 young stars with detected Pf{$β$} emission. Using a subsample of disks with previously measured accretion luminosities, we show that Pf{$β$} line luminosity is well correlated with accretion luminosity over a range of at least three orders of magnitude. We use this correlation to derive accretion luminosities for all 120 targets, 65 of which are previously unreported in the literature. The conversion from accretion luminosity to accretion rate is limited by the availability of stellar mass and radius measurements; nevertheless, we also report accretion rates for 67 targets, 16 previously unmeasured. Our large sample size and our ability to probe high extinction values allow for relatively unbiased comparisons between different types of disks. We find that the transitional disks in our sample have lower than average Pf{$β$} line luminosities, and thus accretion luminosities, at a marginally significant level. We also show that high Pf{$β$} equivalent width is a signature of transitional disks with high inner disk gas/dust ratios. In contrast, we find that disks with signatures of slow disk winds have Pf{$β$} luminosities comparable to those of other disks in our sample. Finally, we investigate accretion rates for stage I disks, including significantly embedded targets. We find that stage I and stage II disks have statistically indistinguishable Pf{$β$} line luminosities, implying similar accretion rates, and that the accretion rates of stage I disks are too low to be consistent with quiescent accretion. Our results are instead consistent with both observational and theoretical evidence that stage I objects experience episodic, rather than quiescent, accretion. Show less
Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The... Show moreContext. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 {$μ$}m emission band of the crystalline silicate forsterite. Aims: This work aims at providing an overview of the 69 {$μ$}m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods: The 69 {$μ$}m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 {$μ$}m band to the forsterite bands at shorter wavelengths. Results: A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 {$μ$}m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 {$μ$}m bands are found to be warm (~{}100-200 K) and iron-poor (less than ~{}2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions: Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 {$μ$}m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 {$μ$}m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result. Appendix A is available in electronic form at http://www.aanda.orgShow less
Tafalla, M.; Liseau, R.; Nisini, B.; Bachiller, R.; Santiago-García, J.; Dishoeck, E.F. van; ... ; Yıldız, U. 2013
Context. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks. Aims: We present a survey of the water emission in... Show moreContext. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks. Aims: We present a survey of the water emission in a sample of more than 20 outflows from low-mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas. Methods: We used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the ''Water In Star-forming regions with Herschel'' (WISH) key program, and have been complemented with CO and H$_{2}$ data. Results: The emission of water has a different spatial and velocity distribution from that of the J = 1-0 and 2-1 transitions of CO. On the other hand, it has a similar spatial distribution to H$_{2}$, and its intensity follows the H$_{2}$ intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins that is responsible for the H$_{2}$ emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates that the emitting gas has a narrow range of excitations. A radiative transfer analysis shows that while there is some ambiguity in the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4 { imes} 10$^{9}$ cm$^{-3}$K, which represents an increase of 10$^{4}$ with respect to the ambient value. The data also constrain the water column density within a factor of 2 and indicate values in the sample between 2 { imes} 10$^{12}$ and 10$^{14}$ cm$^{-2}$. When these values are combined with estimates of the H$_{2}$ column density, the typical water abundance is only 3 { imes} 10$^{-7}$, with an uncertainty of a factor of 3. Conclusions: Our data challenge current C-shock models of water production through the combination of wing-line profiles, high gas compressions, and low abundances. 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. Mid- and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the... Show moreContext. Mid- and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation or heating by the embedded protostar of its immediate surroundings. Aims: By studying of the most important molecular and atomic coolants, we aim at constraining the physical conditions around the embedded protostars SMM3 and SMM4 in the Serpens molecular cloud core and measuring the CO/H$_{2}$ ratio in warm gas. Methods: Spectro-imaging observations from the Spitzer Infrared Spectrograph (IRS) and the Herschel Photodetector Array Camera and Spectrometer (PACS) provide an almost complete wavelength coverage between 5 and 200 {$μ$}m. Within this range, emission from all major molecular (H$_{2}$, CO, H$_{2}$O and OH) and many atomic ([OI], [CII], [FeII], [SiII] and [SI]) coolants of excited gas are detected. Emission line maps reveal the morphology of the observed emission and indicate associations between the different species. The excitation conditions for molecular species are assessed through rotational diagrams. Emission lines from major coolants are compared to the results of steady-state C- and J-type shock models. Results: Line emission tends to peak at distances of ~{}10-20{Prime} from the protostellar sources with all but [CII] peaking at the positions of outflow shocks seen in near-IR and sub-millimeter interferometric observations. The [CII] emission pattern suggests that it is most likely excited from energetic UV radiation originating from the nearby flat-spectrum source SMM6. Excitation analysis indicates that H$_{2}$ and CO originate in gas at two distinct rotational temperatures of ~{}300 K and 1000 K, while the excitation temperature for H$_{2}$O and OH is ~{}100-200 K. The morphological and physical association between CO and H$_{2}$ suggests a common excitation mechanism, which allows direct comparisons between the two molecules. The CO/H$_{2}$ abundance ratio varies from ~{}10$^{-5}$ in the warmer gas up to ~{}10$^{-4}$ in the hotter regions. Shock models indicate that C-shocks can account for the observed line intensities if a beam filling factor and a temperature stratification in the shock front are considered. C-type shocks can best explain the emission from H$_{2}$O. The existence of J-shocks is suggested by the strong atomic/ionic (except for [CII]) emission and a number of line ratio diagnostics. Dissociative shocks can account for the CO and H$_{2}$ emission in a single excitation temperature structure. Conclusions: The bulk of cooling from molecular and atomic lines is associated with gas excited in outflow shocks. The strong association between H$_{2}$ and CO constrain their abundance ratio in warm gas. Both C- and J-type shocks can account for the observed molecular emission; however, J-shocks are strongly suggested by the atomic emission and provide simpler and more homogeneous solutions for CO and H$_{2}$. The variations in the CO/H$_{2}$ abundance ratio for gas at different temperatures can be interpreted by their reformation rates in dissociative J-type shocks, or the influence of both C and J shocks. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices A-C are available in electronic form at http://www.aanda.orgShow less
CO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas,... Show moreCO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas, tracing conditions in both the inner and outer disc. We present Herschel/PACS spectroscopy of a sample of 22 Herbig Ae/Be (HAEBEs) and eight T Tauri stars (TTS), covering the pure rotational CO transitions from J = 14 { arr} 13 up to J = 49 { arr} 48. CO is detected in only five HAEBEs, namely AB Aur, HD 36112, HD 97048, HD 100546, and IRS 48, and in four TTS, namely AS 205, S CrA, RU Lup, and DG Tau. The highest transition detected is J = 36 { arr} 35 with E$_{up}$ of 3669 K, seen in HD 100546 and DG Tau. We construct rotational diagrams for the discs with at least three CO detections to derive T$_{rot}$ and find average temperatures of 270 K for the HAEBEs and 485 K for the TTS. The HD 100546 star requires an extra temperature component at T$_{rot}$ ~{} 900-1000 K, suggesting a range of temperatures in its disc atmosphere, which is consistent with thermo-chemical disc models. In HAEBEs, the objects with CO detections all have flared discs in which the gas and dust are thermally decoupled. We use a small model grid to analyse our observations and find that an increased amount of flaring means higher line flux, as it increases the mass in warm gas. CO is not detected in our flat discs as the emission is below the detection limit. We find that HAEBE sources with CO detections have high L$_{UV}$ and strong PAH emission, which is again connected to the heating of the gas. In TTS, the objects with CO detections are all sources with evidence of a disc wind or outflow. For both groups of objects, sources with CO detections generally have high UV luminosity (either stellar in HAEBEs or due to accretion in TTS), but this is not a sufficient condition for the detection of the far-IR CO lines. 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. 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
