Context. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with... Show moreContext. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play. Aims: Our goal is to investigate the spatial and velocity distribution of H$_{2}$O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H$_{2}$O emitters among the class 0 outflows. Methods: To this end, maps of the o-H$_{2}$O 1$_{10}$-1$_{01}$ and 2$_{12}$-1$_{01}$ transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H$_{2}$ S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H$_{2}$O column densities were inferred using large velocity gradient radiative transfer calculations. Results: The water distribution appears to be clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557 GHz line is confined to radial velocities in the range {plusmn}10-50 km s$^{-1}$, but extended emission at extreme velocities (up to v$_r$ ~{} 80 km s$^{-1}$) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H$_{2}$O 1$_{10}$-1$_{01}$/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H$_{2}$O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H$_{2}$O lines and the comparison with H$_{2}$ indicate averaged T$_{kin}$ and n(H$_{2}$) values of ~{}300-500 K and 5 { imes} 10$^{6}$ cm$^{-3}$, respectively, while a water abundance with respect to H$_{2}$ of about 0.5-1 { imes} 10$^{-6}$ along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H$_{2}$O chemistry. Conclusions: The results of our analysis show that the bulk of the observed H$_{2}$O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H$_{2}$O gas in the post-shock region. Among the possible scenarios, we favor H$_{2}$O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength. 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.org Show less
Nisini, B.; Santangelo, G.; Antoniucci, S.; Benedettini, M.; Codella, C.; Giannini, T.; ... ; Dishoeck, E.F. van 2013
Context. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with... Show moreContext. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play. Aims: Our goal is to investigate the spatial and velocity distribution of H$_{2}$O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H$_{2}$O emitters among the class 0 outflows. Methods: To this end, maps of the o-H$_{2}$O 1$_{10}$-1$_{01}$ and 2$_{12}$-1$_{01}$ transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H$_{2}$ S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H$_{2}$O column densities were inferred using large velocity gradient radiative transfer calculations. Results: The water distribution appears to be clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557 GHz line is confined to radial velocities in the range {plusmn}10-50 km s$^{-1}$, but extended emission at extreme velocities (up to v$_r$ ~{} 80 km s$^{-1}$) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H$_{2}$O 1$_{10}$-1$_{01}$/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H$_{2}$O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H$_{2}$O lines and the comparison with H$_{2}$ indicate averaged T$_{kin}$ and n(H$_{2}$) values of ~{}300-500 K and 5 { imes} 10$^{6}$ cm$^{-3}$, respectively, while a water abundance with respect to H$_{2}$ of about 0.5-1 { imes} 10$^{-6}$ along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H$_{2}$O chemistry. Conclusions: The results of our analysis show that the bulk of the observed H$_{2}$O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H$_{2}$O gas in the post-shock region. Among the possible scenarios, we favor H$_{2}$O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength. 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. 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
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
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. 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
Tak, F.; Chavarría, L.; Herpin, F.; Wyrowski, F.; Walmsley, C.; Dishoeck, E.F. van; ... ; Tafalla, M. 2013
Context. Water is a key constituent of star-forming matter, but the origin of its line emission and absorption during high-mass star formation is not well understood. Aims: We study the velocity... Show moreContext. Water is a key constituent of star-forming matter, but the origin of its line emission and absorption during high-mass star formation is not well understood. Aims: We study the velocity profiles of low-excitation H$_{2}$O lines toward 19 high-mass star-forming regions and search for trends with luminosity, mass, and evolutionary stage. Methods: We decompose high-resolution Herschel-HIFI line spectra near 990, 1110 and 1670 GHz into three distinct physical components. Dense cores (protostellar envelopes) are usually seen as narrow absorptions in the H$_{2}$O 1113 and 1669 GHz ground-state lines, the H$_{2}$O 987 GHz excited-state line, and the H$_{2}$$^{18}$O 1102 GHz ground-state line. In a few sources, the envelopes appear in emission in some or all studied lines, indicating higher temperatures or densities. Broader features due to outflows are usually seen in absorption in the H$_{2}$O 1113 and 1669 GHz lines, in 987 GHz emission, and not seen in H$_{2}$$^{18}$O, indicating a lower column density and a higher excitation temperature than the envelope component. A few outflows are detected in H$_{2}$$^{18}$O, indicating higher column densities of shocked gas. In addition, the H$_{2}$O 1113 and 1669 GHz spectra show narrow absorptions by foreground clouds along the line of sight. The lack of corresponding features in the 987 GHz and H$_{2}$$^{18}$O lines indicates a low column density and a low excitation temperature for these clouds, although their derived H$_{2}$O ortho/para ratios are close to 3. Results: The intensity of the ground state lines of H$_{2}$O at 1113 and 1669 GHz does not show significant trends with source luminosity, envelope mass, or evolutionary state. In contrast, the flux in the excited-state 987 GHz line appears correlated with luminosity and the H$_{2}$$^{18}$O line flux appears correlated with the envelope mass. Furthermore, appearance of the envelope in absorption in the 987 GHz and H$_{2}$$^{18}$O lines seems to be a sign of an early evolutionary stage, as probed by the mid-infrared brightness and the L$_{bol}$/M$_{env}$ ratio of the source. Conclusions: The ground state transitions of H$_{2}$O trace the outer parts of the envelopes, so that the effects of star formation are mostly noticeable in the outflow wings. These lines are heavily affected by absorption, so that line ratios of H$_{2}$O involving the ground states must be treated with caution, especially if multiple clouds are superposed as in the extragalactic case. The isotopic H$_{2}$$^{18}$O line appears to trace the mass of the protostellar envelope, indicating that the average H$_{2}$O abundance in high-mass protostellar envelopes does not change much with time. The excited state line at 987 GHz increases in flux with luminosity and appears to be a good tracer of the mean weighted dust temperature of the source, which may explain why it is readily seen in distant galaxies. 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
