Free Access
Issue
A&A
Volume 568, August 2014
Article Number A125
Number of page(s) 12
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201424034
Published online 05 September 2014

© ESO, 2014

1. Introduction

Molecular outflows represent direct evidence of the earliest phases of star formation when collimated jets are driven (e.g., Arce et al. 2007; Ray et al. 2007). Shock fronts are generated at the point of impact of the ejected material with the surrounding cloud, inducing changes in the chemical composition and enhancing the abundance of several species. Along with H2 and CO, H2O is one of the main cooling agents in shocks (Kaufman & Neufeld 1996; Flower & Pineau des Forêts 2010; Karska et al. 2013). It is also very sensitive to physical conditions and chemical processes of the shocked gas (van Dishoeck et al. 2011). An increase of the water gas-phase abundance from <10-7 up to 3 × 10-4 is expected in warm shocked gas (100 K) because of the combined effects of sputtering of ice mantles and high-temperature reactions that drive atomic oxygen into H2O (Hollenbach & McKee 1989; Kaufman & Neufeld 1996; Flower & Pineau des Forêts 2010; Suutarinen et al. 2014).

Systematic observations of multiple H2O transitions in prototypical star-forming regions have been conducted with Herschel (Pilbratt et al. 2010). Thanks to its much higher sensitivity and spectral resolution and its smaller beam size with respect to previous space missions, such as ISO, Odin, and SWAS, Herschel has provided strong constraints on the water abundance and physical conditions in water-emitting gas. In particular, the Water In Star-forming regions with Herschel (WISH, van Dishoeck et al. 2011) key program has been dedicated to the study of the physical and dynamical properties of water and its role in shock chemistry. The H2O line profiles observed with the Heterodyne Instrument for the Far Infrared (HIFI, de Graauw et al. 2010) at outflow shocks show several kinematic components along with variations with excitation energy (Kristensen et al. 2012; Vasta et al. 2012; Santangelo et al. 2012). The observed H2O emission probes warm (>200 K) and very dense gas (nH2 ≳ 106 cm-3), which is associated with high-J CO emission (e.g Karska et al. 2013; Santangelo et al. 2013) and is not traced by other molecules seen from the ground, such as low-J CO and SiO (e.g., Vasta et al. 2012; Nisini et al. 2013; Tafalla et al. 2013; Santangelo et al. 2013). The differences in line profiles of the various tracers also confirm the uniqueness of H2O as a probe of shocked gas.

A two-temperature-components model has been suggested by Santangelo et al. (2013) to reproduce the H2O, CO, and mid-IR H2 lines, observed along the L1448 and L1157 protostellar outflows. This model consists of 1) an extended warm component (T~500 K) traced by lower excitation H2O emission (Eu ≲ 140 K) and by CO lines up to J = 22−21; and 2) a compact hot component (T ~ 1000 K) traced by higher excitation H2O emission and higher-J CO transitions. Two gas components with different excitation conditions have also been proposed by Busquet et al. (2014) to account for the H2O and CO emission observed at the bright shock region B1 in the L1157 outflow. Finally, multiple-temperature components have been suggested to explain the spectrally unresolved CO ladder at the position of several Class 0 sources (e.g., Goicoechea et al. 2012; Herczeg et al. 2012; Dionatos et al. 2013; Karska et al. 2013; Manoj et al. 2013; Green et al. 2013). The nature of these two components and, notably, their spatial extent has not yet been clarified, however. To understand this problem better, maps of velocity-resolved H2O lines that are sensitive to different excitation conditions are needed.

NGC 1333 (d = 235 pc, Hirota et al. 2008) is a well-studied star-forming region and contains many young stellar objects (YSOs) and outflows (e.g., Liseau et al. 1988; Bally et al. 1996; Knee & Sandell 2000). Within the region, IRAS 4A and IRAS 4B are two low-mass protostars with a projected separation of about 30′′; both have been identified as binary systems using mm interferometry (e.g., Lay et al. 1995; Looney et al. 2000; Choi 2005; Jørgensen et al. 2007). The companion to IRAS 4B is detected at a separation of 11′′, whereas IRAS 4A is resolved into two components with a separation of only 2′′. IRAS 4A is one of the youngest protobinary systems found so far, as inferred by its strong dust continuum emission with cold blackbody-like spectral energy distribution and its well-collimated outflow (e.g., Sandell et al. 1991; Blake et al. 1995), extending over arcminute scales. The IRAS 4A low-mass protostar has been the subject of extensive observations with ground-based submillimeter telescopes and interferometers (e.g., Blake et al. 1995; Di Francesco et al. 2001; Maret et al. 2005; Choi 2005; Jørgensen et al. 2007; Yıldız et al. 2012). As part of WISH, Kristensen et al. (2010, 2012) observed several H2O transitions towards the IRAS 4A source with Herschel-HIFI, showing the complex line profiles with multiple components within the HIFI beam. In the line profiles, these authors identified a broad Gaussian component (FWHM ≳ 20 km s-1) that was also detected in the CO (109) emission (Yıldız et al. 2013) and is associated with the molecular outflow. In addition, they identified a so-called medium-broad component, offset with respect to the source velocity and with smaller line widths (FWHM ~ 5 − 10 km s-1), which they associated with currently shocked gas close to the protostar (Kristensen et al. 2013).

In this paper, we present new Herschel-PACS and HIFI observations of several key H2O lines that are sensitive to different excitation conditions, and HIFI CO (1615) spectra at two shocked positions along the IRAS 4A outflow. The data are complemented by ground-based CO (32) and (65) maps by Yıldız et al. (2012). The goal is to study the spatial distribution of the water emission to spatially separate the multiple kinematic components that were previously detected towards the source within the HIFI beam. The observations and data reduction are described in Sect. 2. In Sect. 3 we present the observational results. The analysis and interpretation of the H2O excitation conditions and its physical origin are discussed in Sect. 4. Finally, in Sect. 5, we present the main conclusions.

2. Observations

2.1. PACS observations

thumbnail Fig. 1

PACS map of the H2O (212 − 101) emission at 1670 GHz of the IRAS 4 region compared with the Spitzer-IRAC emission at 4.5 μm and the CO (1413) emission in the left panel and with the JCMT CO (32) and APEX CO (65) emission (Yıldız et al. 2012) in the right panel. The CO (32) and CO (65) maps are integrated in the velocity ranges between 20 km s-1 and 3 km s-1 for the blue-shifted emission and 12 km s-1 and 50 km s-1 for the red-shifted emission. The contour levels start at the 5σ level and increase in steps of 10σ for the PACS H2O and CO (1413) maps, from the 5σ level emission in steps of 5σ for the CO (32) and from the 5σ level emission in steps of 3σ for the CO (65). Offsets are with respect to the central source IRAS 4A, at coordinates αJ2000 = 03h29m10.s50, δJ2000 = + 31°13′309. The positions of the IRAS 4A and IRAS 4B binary sources are marked with yellow symbols (Looney et al. 2000). The regions mapped with HIFI in the water lines around the selected shock positions (R1 and R2) are indicated.

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PACS maps of H2O (212101) at 1670 GHz and CO (1413) emission were used for the analysis. The maps are presented in Fig. 1, and a summary of the observations is given in Table 1. The observations are part of the OT1 program “Probing the physics and dynamics of the hidden warm gas in the youngest protostellar outflows” (OT1_bnisini_1). The PACS instrument is an Integral Field Unit (IFU), consisting of a 5 × 5 array of spatial pixels, each covering 4, for a total field of view of 47′′ × 47′′. PACS was used in line-spectroscopy mode to obtain a spectral Nyquist-sampled raster map of IRAS 4A. The reference coordinates are at the position of IRAS 4A, αJ2000 = 03h29m10.s50, δJ2000 = + 31°13′309. The diffraction-limited FWHM beam size at 179 μm is about 13′′. The data were reduced with HIPE1 (Herschel Interactive Processing Environment, Ott 2010) version 9.0. Within HIPE, they were flat-fielded and flux-calibrated by comparison with observations of Neptune. The calibration uncertainty is estimated to be around 20%, based on the flux repeatability for multiple observations of the same target in different programs and on cross-calibration with HIFI and ISO. Finally, continuum subtraction was performed in IDL, and integrated line maps were obtained.

Table 1

Parameters of the lines mapped with PACS.

2.2. HIFI observations

We selected two active shock positions along the IRAS 4A outflow: R1 (αJ2000 = 03h29m10.s82, 9), at the origin of the jet from the driving source, and R2 (αJ2000 = 03h29m14.s59, 8), which is the head of the red lobe of the outflow. They appear as very bright peaks in the PACS H2O (212101) and CO (1413) maps shown in Fig. 1.

Single-pointing observations of the o-H2O (110101) line at 557 GHz and the CO (1615) transition at the two selected positions were conducted with Herschel-HIFI in dual beam-switch and fast-chop mode. In addition, an area of size equal to the HIFI beam width at 557 GHz (38′′) was mapped in on-the-fly mode in three other H2O lines, spanning excitation energies Eu from 50 K to 250 K (see Table 2). The observations were carried out between July 2012 and August 2012 as part of the OT2 program “Solving the puzzle of water excitation in shocks” (OT2_gsantang_1). Contextually, the spectral set-up allowed us to observe transitions from other molecules: N2H+ (65), SO (13141213), CH3OH (3-22-1), NH3 (1000), and 13CO (109). A summary of the performed observations is given in Tables 2 and A.1.

thumbnail Fig. 2

HIFI spectra of the H2O and CO transitions observed at the R1 (left) and R2 (right) shock positions along IRAS 4A. The spectra are all convolved to the same angular resolution of the H2O (110 − 101) line at 557 GHz (~38′′), with the exception of the CO (1615) line, which is a single pointing HIFI observation at ~12′′ resolution. The spectra shown in red have intensities provided in the right-hand axes. The vertical dashed line marks the systemic velocity (vLSR = + 7.2 km s-1) and the H2O secondary emission peaks at +25 and +35 km s-1 for R1 and R2.

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thumbnail Fig. 3

Ratio between the o-H2O (110 − 101) (at 557 GHz) and o-H2O (312 − 303) (at 1097 GHz) lines as a function of velocity at the two observed shock positions R1 (upper panel) and R2 (lower panel). The 1097 GHz water spectra are convolved to the same angular resolution as the 557 GHz spectra, i.e., 38′′. The line ratios are plotted only where the S/N ratio is higher than three for the two lines, at a spectral resolution of 2 km s-1. The vertical dashed line indicates the source velocity, whereas the solid vertical lines mark the velocity range of the absorption dip (410 km s-1).

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The data were processed with the ESA-supported package HIPE version 11 for calibration. The calibration uncertainty is taken to be 20%. Further reduction of all the spectra, including baseline subtraction, and the analysis of the data were performed using the GILDAS2 software. H- and V-polarizations were co-added after inspection to increase sensitivity, since no significant differences were found between the two data sets. The calibrated scale from the telescope was converted into the Tmb scale using the main-beam efficiency factors provided by Roelfsema et al. (2012)3 and reported in Table 2. At the velocity resolution of 1 km s-1, the rms noise ranges between 10 mK and 20 mK (Tmb scale).

Table 2

Parameters of the lines observed with HIFI.

3. Results

Figure 1 shows the PACS maps of H2O (212101) at 1670 GHz and CO (1413), in comparison with the Spitzer-IRAC emission at 4.5 μm and the ground-based CO (32) and CO (65) (Yıldız et al. 2012) towards the IRAS 4 region. The figure highlights the spatial correlation between H2O, high-J CO, and Spitzer 4.5 μm emissions, in agreement with previous studies of outflows from low-mass protostars (e.g., Nisini et al. 2010; Santangelo et al. 2013; Tafalla et al. 2013). In particular, PACS CO (1413) and APEX CO (65) emissions are spatially associated with the PACS H2O emission, whereas the low-J CO (32) emission is more extended and offset. A sharp change of propagation direction of ~30° in the north-eastern outflow red lobe, occurring close to the R1 shock position, is visible both in H2O and CO emission. This directional variability was previously shown by both single-dish and interferometric observations, and several mechanisms were proposed to explain it, including magnetic deflection, a precessing jet, and collisions with a dense core in the ambient cloud (e.g., Blake et al. 1995; Girart et al. 1999; Choi 2001, 2005; Baek et al. 2009; Choi et al. 2011; Yıldız et al. 2012). Strong H2O emission peaks are found at the location of active shocked regions and at the position of IRAS 4A and its neighbour IRAS 4B (see also Nisini et al. 2010, 2013). In particular, the R1 and R2 shock positions appear as bright peaks in the H2O and high-J CO emission, as revealed by PACS and ground-based observations.

An overview of the HIFI observations is given in Fig. 2 and Fig. A.1, where H2O and CO spectra observed at the two selected shock positions and the spectra of additional lines detected with HIFI are shown. All H2O spectra observed in mapping mode are convolved to the same angular resolution of 38′′ for comparison with the observations of the ground-state o-H2O transition at 557 GHz. Several kinematic components can be distinguished in the observed H2O line profiles at both the R1 and R2 positions. First, an absorption dip around the systemic velocity of IRAS 4A (vLSR = +7.2 km s-1, Kristensen et al. 2012) is detected in the ground-state transitions of o- and p-H2O (at 557 GHz and 1113 GHz), associated with cold gas in the outer envelope. Second, a triangularly shaped outflow wing is present up to about 50 km s-1 at R1 and about 60 km s-1 at R2. Third, an excess of emission at high velocity is observed in the ground-state transitions of o- and p-H2O. This secondary high-velocity (HV) emission peak appears at a velocity of about +25 km s-1 and +35 km s-1 in R1 and R2. Similar variations of water line profiles with excitation were observed at the bow-shock positions along the red lobes of the L1448 (R4) and L1157 (R) outflows by Santangelo et al. (2012) and Vasta et al. (2012).

Figure 3 presents the H2O 557 GHz/1097 GHz line ratio as a function of velocity for R1 and R2. Water transitions with different upper level energies were chosen and convolved to the same angular resolution of 38′′, and the ratios are plotted only for velocities where both transitions have S/N> 3. Neglecting the velocity range 410 km s-1, where the absorption dip in the 557 GHz H2O contaminates the analysis, the ratio between the o-H2O (110101) and o-H2O (312 − 303) lines increases significantly with velocity, reflecting the fact that the secondary HV peak at both R1 and R2 appears in the lower excitation energy transitions (see Fig. 2). Finally, we note that the increasing trend is stronger at R2, the shock position farthest from the central driving source. We point out that these findings for the shock positions are in contrast to observations of H2O emission lines at the central protostellar positions, which show constant line ratios (Mottram et al., in prep.).

Recent studies have shown that high-J CO emission is associated with H2O emission, corresponding to a warm (300 K) and dense (nH2 ≳ 106 cm-3) gas component (e.g., Karska et al. 2013; Santangelo et al. 2013). This finding is consistent with our CO (1615) observations at R2, showing a similar line profile as the ground-state water transitions, that is, strong, broad emission around the systemic velocity of the source and an HV emission peak. We note that the detection of a secondary HV emission component at R2 in the H2O 557 GHz and in the CO (1615) lines, with angular resolutions of about 38′′ and 12′′, suggests that this emission is associated with a compact gas component centred on the shock. Therefore, the non-detection of this HV emission peak in the higher-excitation, smaller-beam size H2O transitions is probably due to an excitation effect, with the HV component being less excited than the low-velocity (LV) component, as previously found in the L1448 and L1157 bow shock positions (e.g., Vasta et al. 2012; Santangelo et al. 2012).

On the other hand, at the R1 position no clear secondary peak around +25 km s-1 is detected in CO (1615). This difference possibly arises because the HV peak is spatially shifted towards IRAS 4A by ~10′′ with respect to R1, while the CO (1615) was observed with a beam size of 12′′ (see also Sect. 3.1 and Figs. 7 and 8).

3.1. Water spatial distribution

Figure 4 shows the HIFI spectra at 1113 GHz observed around the R1 and R2 shock positions. The figure highlights the variation of the water line profiles around the shocked regions.

From examining the R1 position, we note that the H2O line profiles close to IRAS 4A resemble those along the outflow, which testifies that the outflow dominates the water profiles. This similarity is even clearer when we compare the H2O profiles centred at R1 and at IRAS 4A (Fig. 5). The p-H2O 1113 GHz spectra show the same profile in the red-shifted emission, although the on-source position is brighter than R1. On the other hand, the H2O profiles of the higher excitation energy H2O 1097 GHz line appear to be different, with the on-source position being fainter than R1. This difference suggests that at R1 the excitation conditions of the H2O emitting gas are different from the conditions at the position of the central source. In addition, in the on-source spectra, significant blue-shifted emission is detected that is not seen at the R1 position. The blue wing presents an excess of emission at ~0 km s-1 (−7 km s-1 with respect to the source velocity), which is quite symmetric in the 1113 GHz line with respect to the red-shifted secondary peak. Kristensen et al. (2013) interpreted this blue-shifted H2O component as originating from a compact dissociative shock close to the protostar. We remark that our profiles present some differences with those presented in Kristensen et al. (2013); in the latter, for example, the secondary red-shifted peak is not as bright and the relative intensity of the low- and high-velocity blue-shifted components is significantly different. These differences are probably due to the different observation pointings. However, Kristensen et al. (2013) suggested that time variability might also change the H2O line profiles.

thumbnail Fig. 4

HIFI spectra of the 1113 GHz H2O line, mapped at R1 (left) and R2 (right). The spectra are overlaid on the respective PACS H2O map at 1670 GHz (grey scale and white contours). Offsets are with respect to the R1 and R2 shock positions. The IRAS 4A binary source position is marked with green crosses.

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The R2 position, which is a pure shock position far from the driving source of the outflow, is a very interesting laboratory to study the water distribution around shocks. Our HIFI observations clearly show that the different water kinematic components mentioned above (see Sect. 3 and Fig. 2) are not uniformly distributed across the mapped region (Fig. 4). In particular, the HV peak is observed only at the bright shock peak, but is not detected in water line profiles offset from the shock position, which show a triangular wing shape. These two types of line profiles probably indicate two different gas components in the water emission: a compact gas component, located at the shock peak and associated with the HV peak, and a more diffuse component. The difference is shown in the left panel of Fig. 6, where line emission from the R2 position and a position (10′′, 10′′) offset from the R2 shock are chosen as representative. An emission residual between the two spectra is also displayed. The spectrum associated with the compact component shows an excess of emission at the systemic velocity and at the HV peak with respect to the extended component. The two detected components are therefore not kinematically distinct, since the velocity range of the emission is the same in both cases. Thus, it would not have been possible to distinguish them within the 38′′ beam size of single-pointing observations.

In Fig. 7, the maps of the H2O 1113 GHz emission, integrated in the three velocity ranges reported in the caption and corresponding to blue-shifted emission, LV red-wing emission, and HV emission peak, are presented for the R1 (upper panel) and the R2 (lower panel) positions, in comparison with the H2O 1670 GHz PACS map. Blue-shifted emission is only detected close to the central source IRAS 4A. At R1, the HV-peak emission appears to have a compact distribution, spatially associated with IRAS 4A, whereas the LV-wing emission is elongated in the outflow red-lobe direction. In contrast, no significant difference between the LV-wing and the HV-peak emission can be observed at R2 at this angular resolution (19′′, corresponding to about 4500 AU) with both distributions appearing unresolved. This difference is discussed in more detail in Sect. 4.

thumbnail Fig. 5

Comparison between the H2O (111 − 000) (1113 GHz) and (312 − 303) (1097 GHz) line profiles at R1 and on the IRAS 4A source. The vertical dashed line indicates the source velocity.

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thumbnail Fig. 6

Left: comparison between the H2O 111 − 000 emission at 1113 GHz (19′′ beam size) at the R2 shock position (black), corresponding to the compact gas component, and that at the offset position (10′′, 10′′) with respect to R2 (red), corresponding to the extended component. The residual spectrum, given by the difference between the two displayed spectra, is shown in blue. Offsets are with respect to R2. The vertical dashed line indicates the source velocity. Right: same as the left panel for the CO (32) emission.

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thumbnail Fig. 7

Velocity-integrated maps of the HIFI H2O 1113 GHz emission at the R1 (upper panel) and R2 (lower panel) positions compared with the PACS H2O 179 μm map. At R1, the H2O 1113 GHz emission is integrated in three velocity ranges: the blue-shifted emission (between −20 km s-1 and 3 km s-1), the low-velocity (LV) wing emission (between 11 km s-1 and 20 km s-1), and the high-velocity (HV) emission peak (between 20 km s-1 and 30 km s-1). At R2, two velocity ranges are considered: the LV wing emission (between 11 km s-1 and 30 km s-1) and the HV emission peak (between 30 km s-1 and 45 km s-1). The contour levels start from the 5σ level and increase in steps of 10σ for the PACS 179 μm emission and in steps of 3σ for the HIFI 1113 GHz emission. Offsets are with respect to the R1 and R2 shock positions. The magenta symbols represent the position of the IRAS 4A binary source.

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thumbnail Fig. 8

Same as Fig. 7 for the CO (32) emission. Offsets here are with respect to the central source IRAS 4A.

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thumbnail Fig. 9

H2O line ratios with respect to the o-H2O (312 − 303) line at 1097 GHz measured at the R2 shock position, corresponding to the compact component, and at the offset position (10′′, 10′′) with respect to R2, corresponding to the extended component. The HIFI water line intensities are integrated in the line wings (see Fig. 10) at the original angular resolution of the spectra (see Table 2), while the intensity of the H2O line at 1670 GHz in the two gas components is measured from the PACS map by averaging over 19′′. The intensity of the H2O (211 − 202) line at 752 GHz associated with the compact component is corrected for the beam size ratio with respect to the 1097 GHz line (28′′/19′′), assuming the source to be point-like.

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Similar conclusions can be drawn at R1 from Fig. 8, where the CO (32) emission is integrated in the same velocity ranges as adopted for the H2O maps of Fig. 7. The HV-peak emission appears to be more compact than the LV-wing emission and shifted with respect to R1 towards IRAS 4A. We note that a second north-eastern emission peak can be identified in the HV-peak emission, which is not associated with bright PACS H2O 179 μm emission. A different situation with respect to the H2O 1113 GHz emission is observed at R2 in the CO (32) emission, however. Here, the HV-peak emission shows a compact distribution that is not resolved on an angular scale of 14′′ and spatially associated with the H2O emission at 179 μm. In contrast, the LV-wing emission is associated with a diffuse gas component, following the outflow direction, and is spatially more extended than the H2O emission. The CO (32) observations thus seem to support the scenario of two distinct gas components in the R2 shock position.

Finally, a comparison between the CO (32) spectra observed at R2 and at a position (10′′, 10′′) offset from the shock is presented in the right panel of Fig. 6. Although the CO (32) spectra show different line profiles in the LV emission with respect to H2O, once more, they indicate that the HV peak is associated with a compact gas component not detected in the more diffuse LV gas.

We note that, because of the low sensitivity in the line wings, the APEX CO (65) data do not detect HV line emission and thus cannot be used to analyse the two spatial components observed in the H2O and CO (32) lines.

4. Two gas components in shocked H2O emission

To investigate the presence of two distinct gas components at R2, we compared their associated H2O line ratios with respect to the higher excitation energy H2O 1097 GHz line (Fig. 9). We assumed that the compact gas component dominates the H2O emission at the R2 shock position while the extended component dominates the H2O emission at the position (10′′, 10′′) offset from R2, as supported by the difference in the observed line profiles (see Figs. 4 and 6). The line intensities are integrated only in the line wings, between 10 km s-1 and 60 km s-1, because of the absorption dip at the source systemic velocity. The two distinct gas components with different excitation conditions at R2 are confirmed by the significantly different associated water line ratios. In particular, the three measured line ratios are higher in the compact component, which is consistent with this component being less excited than the extended one and detected only in the lower excitation water lines.

To characterize the two gas components at R2 in terms of excitation conditions, we ran the radex non-LTE molecular LVG radiative transfer code (van der Tak et al. 2007) in plane-parallel geometry, with collisional rate coefficients from Dubernet et al. (2006, 2009) and Daniel et al. (2010, 2011) and molecular data from the Leiden Atomic and Molecular Database (LAMDA4, Schöier et al. 2005). A grid of models with density ranging between 104 cm-3 and 108 cm-3 and temperatures ranging between 100 K and 1600 K was built. Two values of o-H2O column density were considered, corresponding to optically thin and moderately thick water emission. We adopted a typical line width of 20 km s-1 for both components from the HIFI spectra and an H2O ortho-to-para ratio equal to 3 (Emprechtinger et al. 2013), corresponding to the high-temperature equilibrium value. The ratio between the p-H2O 1113 GHz and the o-H2O 1097 GHz lines with similar beam sizes (19′′) was considered to avoid beam-filling problems. The line-wing intensities (between 10 km s-1 and 60 km s-1) were measured at the same chosen positions corresponding to the two gas components. A comparison between the observed and predicted water line ratios as a function of the H2 density for four values of temperature (100 K, 300 K, 500 K, and 1000 K) is presented in Fig. 10. In both cases, the low-density regime can be excluded, meaning that a gas density n ≳ 105 cm-3 is required. Such high densities are consistent with HIFI CS (1211) observations5 at a similar position about (9′′, 4′′) offset from R1 (Gómez-Ruiz et al., in prep.), suggesting that gas densities in excess of 105 cm-3 are needed to reproduce the CS (1211) intensity in the line wing. They are also consistent with JCMT observations of broad CS (109) emission at the position of the IRAS 4A central source, probing warm and dense gas (Jørgensen et al. 2005).

thumbnail Fig. 10

Radex predictions for the ratio between the p-H2O (111 − 000) (at 1113 GHz) and the o-H2O (312 − 303) (at 1097 GHz) lines as a function of the H2 density for four values of kinetic temperature (100, 300, 500, and 1000 K) and two values of o-H2O column density (1013 and 1016 cm-2). The shaded bands represent the water ratios observed for the compact and extended components at R2, i.e., the R2 position and a position (10′′, 10′′) offset from R2. The HIFI water line intensities are integrated in the line wings, between 10 and 60 km s-1, at the original angular resolution of the spectra (19′′).

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To investigate the excitation conditions of the two H2O components at R2 in detail, we assumed that these components correspond to the warm and hot components found in previous works, that is, the warm component at about 300500 K and the hot component at about 1000 K (see Santangelo et al. 2013; Busquet et al. 2014). By assuming these temperatures and modelling the observed H2O 1113/1097 GHz line ratio and the intensity of the H2O 1097 GHz line using the radex LVG code, we find that the hot component is associated with small emitting sizes (about 3′′, corresponding to about 700 AU), gas densities nH2 ~ (1−4) × 105 cm-3, and o-H2O column densities ~(0.5−1) × 1016 cm-2 (corresponding to τ557 GHz ~ 15−40 at the peak of the H2O emission). The warm component is instead more extended (sizes of about 10′′−17′′, corresponding to 24004000 AU) and associated with higher gas densities nH2 ~ (3−5) × 107 cm-3 and lower o-H2O column densities ~(1−2) × 1013 cm-2 (corresponding to optically thin H2O emission, with τ557 GHz ~ 0.04). The derived sizes for the extended and compact components are consistent with the fact that we cannot spatially resolve and separate them in the H2O emission with angular resolutions higher than 19′′ (see Fig. 7), while they can be spatially distinguished in the CO (32) emission, where the angular resolution is higher (14′′, see Fig. 8).

Next, we used the CO (1615) emission observed at R2 to estimate the H2O abundance of the two spatial components. Since similar line profiles are observed for CO (1615) and H2O at this position (see Sect. 3 and Fig. 2), we can assume that they share a common origin, meaning that the excitation conditions of CO (1615) are the same as derived for H2O in both components. Since the CO (1615) spectrum at R2 is a single-pointing observation, however, we cannot spatially separate the emission from the two components as we did with H2O. Although the two components are not kinematically distinct (see Sect. 3.1), a crude way of separating their relative contribution to the intensity of the single observed CO (1615) spectrum and thus deriving rough estimates of the H2O abundance, is to separate them in velocity; hence, we attribute the velocity range from +11 km s-1 to +30 km s-1 to the extended component and from 30 km s-1 to 45 km s-1 to the compact one, as suggested by the velocity-integrated maps of CO (32) (Fig. 8) and the CO (1615) line profile. Assuming for each component the same excitation conditions and emission sizes as derived from H2O, we obtain CO column densities of about 2 × 1015 cm-2 and 2 × 1016 cm-2 for the extended and compact components, respectively, which correspond to an H2O/H2 abundance of about (7−10) × 10-7 for the warm extended component and (3−7) × 10-5 for the hot compact component (assuming a typical CO/H2 abundance of 10-4). The low fractional H2O abundance associated with the warm gas component agrees with other studies of molecular outflows (e.g., Bjerkeli et al. 2012; Vasta et al. 2012; Nisini et al. 2013; Santangelo et al. 2013; Tafalla et al. 2013; Busquet et al. 2014). Moreover, our finding of higher fractional H2O abundance in the hot gas is consistent with ISO data (e.g., Giannini et al. 2001) and previous Herschel observations of shocked gas along the L1448 and L1157 outflows (e.g., Santangelo et al. 2013; Busquet et al. 2014).

We speculate that the compact hot component, detected in the H2O emission at R2, may be associated with the jet that impacts the surrounding material. Conversely, the warm, dense, and extended component originates from the compression of the ambient gas by the propagating flow. This picture was recently proposed by Busquet et al. (2014) for the L1157 outflow. Our data, however, allow for the first time to spatially resolve these two gas components through emission maps and confirm this scenario. We point out that high-angular resolution observations are crucial to probe the structure of the investigated shock region in depth.

5. Conclusions

We performed Herschel-HIFI observations of two shock positions (R1 and R2) along the IRAS 4A outflow. An area corresponding to the size of the largest HIFI beam of 38′′ at 557 GHz was mapped in several key water lines with different upper level energies to study the water spatial distribution and to separate spatially different gas components associated with the shock. The main results of the work can be summarized as follows:

  • 1.

    At both selected shock positions, we detect four H2O lines with upper energy levels in the range 50250 K and CO (1615). In addition, transitions from related outflow and envelope tracers are detected.

  • 2.

    At the R2 shock position, the head of the red-lobe of the outflow, two gas components with different excitation conditions are detected from the HIFI maps: a compact component, detected in the ground-state water lines, and a more extended component. They are not kinematically distinct, since the velocity range of the emission is similar in both cases, thus they cannot be distinguished within the large Herschel beam sizes of 19′′ and 38′′ at the frequencies of the ground-state H2O transitions.

  • 3.

    The LVG analysis of the H2O emission suggests that the compact (about 3′′) component is associated with a hot (T ~1000 K) gas with densities nH2 ~ (1−4) × 105 cm-3, while the extended (10′′−17′′) component traces a warm (T ~ 300−500 K) and dense (nH2 ~ (3−5) × 107 cm-3) gas.

  • 4.

    From a crude comparison between H2O and CO (1615) emission observed at R2, we estimate the H2O/H2 abundance of the warm and hot components to be (7−10) × 10-7 and (3−7) × 10-5.

Our H2O emission maps allow us, for the first time, to spatially resolve these two temperature components that were previously observed with HIFI and PACS. We suggest that the compact hot component is associated with the jet that impacts the surrounding material, while the warm, dense, and extended one originates from the compression of ambient gas by the propagating flow.


1

HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia.

3

See also http://herschel.esac.esa.int/twiki/bin/view/Public/ HifiCalibrationWeb?template=viewprint.

5

The data are part of the OT1 program “Peering into the protostellar shocks: NH3 emission at high-velocities”.

Acknowledgments

Herschel activities at INAF are financially supported by the ASI project 01/005/11/0. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri- INAF; The Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA); Sweden: Chalmers University of Technology – MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC.

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Appendix A: Additional lines observed with HIFI

The spectra of additional lines detected within the HIFI bands are presented in Fig. A.1 and summarized in Table A.1. We note that the R1 shock position seems to be more chemically rich than R2; more molecules are detected at R1 than at R2. The profiles of the lines detected at R1 show a red-wing component associated with the outflow and a low-velocity emission component possibly associated with the chemically enriched envelope material. The latter is clearly associated with the detection of N2H+ and the absorption dip, centred at the source velocity, present in the NH3 spectrum. This detection is consistent with the beam size of the observations being large enough (about 38′′) to encompass part of the emission associated with the central driving source of the outflow, along with the emission coming from the outflow itself. We finally point out that the absorption features seen in the NH3 spectrum at R2 are due to contamination from emission in the off-source reference position.

Table A.1

Parameters of additional lines observed with HIFI.

thumbnail Fig. A.1

Additional lines observed with HIFI at the selected shock positions. All observations were taken in single-pointing mode, with the exception of the 13CO (109) line. The absorption features seen in the NH3 spectrum at R2 are due to contamination from emission in the off-source reference position.

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All Tables

Table 1

Parameters of the lines mapped with PACS.

Table 2

Parameters of the lines observed with HIFI.

Table A.1

Parameters of additional lines observed with HIFI.

All Figures

thumbnail Fig. 1

PACS map of the H2O (212 − 101) emission at 1670 GHz of the IRAS 4 region compared with the Spitzer-IRAC emission at 4.5 μm and the CO (1413) emission in the left panel and with the JCMT CO (32) and APEX CO (65) emission (Yıldız et al. 2012) in the right panel. The CO (32) and CO (65) maps are integrated in the velocity ranges between 20 km s-1 and 3 km s-1 for the blue-shifted emission and 12 km s-1 and 50 km s-1 for the red-shifted emission. The contour levels start at the 5σ level and increase in steps of 10σ for the PACS H2O and CO (1413) maps, from the 5σ level emission in steps of 5σ for the CO (32) and from the 5σ level emission in steps of 3σ for the CO (65). Offsets are with respect to the central source IRAS 4A, at coordinates αJ2000 = 03h29m10.s50, δJ2000 = + 31°13′309. The positions of the IRAS 4A and IRAS 4B binary sources are marked with yellow symbols (Looney et al. 2000). The regions mapped with HIFI in the water lines around the selected shock positions (R1 and R2) are indicated.

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In the text
thumbnail Fig. 2

HIFI spectra of the H2O and CO transitions observed at the R1 (left) and R2 (right) shock positions along IRAS 4A. The spectra are all convolved to the same angular resolution of the H2O (110 − 101) line at 557 GHz (~38′′), with the exception of the CO (1615) line, which is a single pointing HIFI observation at ~12′′ resolution. The spectra shown in red have intensities provided in the right-hand axes. The vertical dashed line marks the systemic velocity (vLSR = + 7.2 km s-1) and the H2O secondary emission peaks at +25 and +35 km s-1 for R1 and R2.

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In the text
thumbnail Fig. 3

Ratio between the o-H2O (110 − 101) (at 557 GHz) and o-H2O (312 − 303) (at 1097 GHz) lines as a function of velocity at the two observed shock positions R1 (upper panel) and R2 (lower panel). The 1097 GHz water spectra are convolved to the same angular resolution as the 557 GHz spectra, i.e., 38′′. The line ratios are plotted only where the S/N ratio is higher than three for the two lines, at a spectral resolution of 2 km s-1. The vertical dashed line indicates the source velocity, whereas the solid vertical lines mark the velocity range of the absorption dip (410 km s-1).

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In the text
thumbnail Fig. 4

HIFI spectra of the 1113 GHz H2O line, mapped at R1 (left) and R2 (right). The spectra are overlaid on the respective PACS H2O map at 1670 GHz (grey scale and white contours). Offsets are with respect to the R1 and R2 shock positions. The IRAS 4A binary source position is marked with green crosses.

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In the text
thumbnail Fig. 5

Comparison between the H2O (111 − 000) (1113 GHz) and (312 − 303) (1097 GHz) line profiles at R1 and on the IRAS 4A source. The vertical dashed line indicates the source velocity.

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In the text
thumbnail Fig. 6

Left: comparison between the H2O 111 − 000 emission at 1113 GHz (19′′ beam size) at the R2 shock position (black), corresponding to the compact gas component, and that at the offset position (10′′, 10′′) with respect to R2 (red), corresponding to the extended component. The residual spectrum, given by the difference between the two displayed spectra, is shown in blue. Offsets are with respect to R2. The vertical dashed line indicates the source velocity. Right: same as the left panel for the CO (32) emission.

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In the text
thumbnail Fig. 7

Velocity-integrated maps of the HIFI H2O 1113 GHz emission at the R1 (upper panel) and R2 (lower panel) positions compared with the PACS H2O 179 μm map. At R1, the H2O 1113 GHz emission is integrated in three velocity ranges: the blue-shifted emission (between −20 km s-1 and 3 km s-1), the low-velocity (LV) wing emission (between 11 km s-1 and 20 km s-1), and the high-velocity (HV) emission peak (between 20 km s-1 and 30 km s-1). At R2, two velocity ranges are considered: the LV wing emission (between 11 km s-1 and 30 km s-1) and the HV emission peak (between 30 km s-1 and 45 km s-1). The contour levels start from the 5σ level and increase in steps of 10σ for the PACS 179 μm emission and in steps of 3σ for the HIFI 1113 GHz emission. Offsets are with respect to the R1 and R2 shock positions. The magenta symbols represent the position of the IRAS 4A binary source.

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In the text
thumbnail Fig. 8

Same as Fig. 7 for the CO (32) emission. Offsets here are with respect to the central source IRAS 4A.

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In the text
thumbnail Fig. 9

H2O line ratios with respect to the o-H2O (312 − 303) line at 1097 GHz measured at the R2 shock position, corresponding to the compact component, and at the offset position (10′′, 10′′) with respect to R2, corresponding to the extended component. The HIFI water line intensities are integrated in the line wings (see Fig. 10) at the original angular resolution of the spectra (see Table 2), while the intensity of the H2O line at 1670 GHz in the two gas components is measured from the PACS map by averaging over 19′′. The intensity of the H2O (211 − 202) line at 752 GHz associated with the compact component is corrected for the beam size ratio with respect to the 1097 GHz line (28′′/19′′), assuming the source to be point-like.

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In the text
thumbnail Fig. 10

Radex predictions for the ratio between the p-H2O (111 − 000) (at 1113 GHz) and the o-H2O (312 − 303) (at 1097 GHz) lines as a function of the H2 density for four values of kinetic temperature (100, 300, 500, and 1000 K) and two values of o-H2O column density (1013 and 1016 cm-2). The shaded bands represent the water ratios observed for the compact and extended components at R2, i.e., the R2 position and a position (10′′, 10′′) offset from R2. The HIFI water line intensities are integrated in the line wings, between 10 and 60 km s-1, at the original angular resolution of the spectra (19′′).

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In the text
thumbnail Fig. A.1

Additional lines observed with HIFI at the selected shock positions. All observations were taken in single-pointing mode, with the exception of the 13CO (109) line. The absorption features seen in the NH3 spectrum at R2 are due to contamination from emission in the off-source reference position.

Open with DEXTER
In the text

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