EDP Sciences
Herschel: the first science highlights
Free Access
Issue
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
Article Number L120
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201014603
Published online 16 July 2010
A&A 518, L120 (2010)

Herschel: the first science highlights

LETTER TO THE EDITOR

Water cooling of shocks in protostellar outflows

Herschel-PACS map of L1157[*]

B. Nisini1 - M. Benedettini2 - C. Codella3 - T. Giannini1 - R. Liseau4 - D. Neufeld5 - M. Tafalla6 - E. F. van Dishoeck7,8 - R. Bachiller6 - A. Baudry9 - A. O. Benz10 - E. Bergin11 - P. Bjerkeli4 - G. Blake12 - S. Bontemps9 - J. Braine9 - S. Bruderer10 - P. Caselli13,3 - J. Cernicharo14 - F. Daniel14 - P. Encrenaz15 - A. M. di Giorgio2 - C. Dominik16,17 - S. Doty18 - M. Fich19 - A. Fuente6 - J. R. Goicoechea14 - Th. de Graauw20 - F. Helmich20 - G. Herczeg8 - F. Herpin9 - M. Hogerheijde7 - T. Jacq9 - D. Johnstone21,22 - J. Jørgensen23 - M. Kaufman24 - L. Kristensen7 - B. Larsson25 - D. Lis12 - M. Marseille20 - C. McCoey19 - G. Melnick26 - M. Olberg4 - B. Parise25 - J. Pearson28 - R. Plume29 - C. Risacher20 - J. Santiago6 - P. Saraceno2 - R. Shipman20 - T. A. van Kempen26 - R. Visser7 - S. Viti30,2 - S. Wampfler10 - F. Wyrowski27 - F. van der Tak20,31 - U. A. Yildiz7 - B. Delforge32,17 - J. Desbat9,33 - W. A. Hatch29 - I. Péron34,32,17 - R. Schieder35 - J. A. Stern29 -
D. Teyssier36 - N. Whyborn37

1 - INAF - Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
2 - INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
3 - INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
4 - Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
5 - Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
6 - IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
7 - Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
8 - Max Planck Institut for Extraterestrische Physik, Garching, Germany
9 - Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
10 - Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
11 - Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
12 - California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
13 - School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
14 - Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain
15 - LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
16 - Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
17 - Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
18 - Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
19 - University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
20 - SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
21 - National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
22 - Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
23 - Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
24 - Department of Physics and Astronomy, San Jose State University, One Washington Square, San Jose, CA 95192, USA
25 - Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
26 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
27 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
28 - Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
29 - Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
30 - Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
31 - Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
32 - Institute Laboratoire d'Etudes du Rayonnement et de la Matire en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
33 - CNRS/INSU, UMR 5804, B.P. 89, 33271 Floirac cedex, France
34 - Institute Institut de Radioastronomie Millimetrique, IRAM, 300 rue de la Piscine, 38406 St Martin d'Heres, France
35 - KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
36 - European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
37 - ALMA

Received 31 March 2010 / Accepted 23 April 2010

Abstract
Context. The far-IR/sub-mm spectral mapping facility provided by the Herschel-PACS and HIFI instruments has made it possible to obtain, for the first time, images of H2O emission with a spatial resolution comparable to ground based mm/sub-mm observations.
Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H2O 179 $\mu $m  transition obtained toward the young outflow L1157.
Methods. The 179 $\mu $m  map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the H2O abundance and total cooling.
Results. Strong H2O  peaks are localized on both shocked emission knots and the central source position. The H2O  179 $\mu $m  emission is spatially correlated with emission from H2 rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH3. A strong H2O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 $\mu $m  intensity and its intensity ratio to the H2O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H2O  abundance of the order of 10-4. This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total H2O cooling is $\sim $10 $^{-1}~ L_\odot$, about 40% of the cooling due to H2 and 23% of the total energy released in shocks along the L1157 outflow.

Key words: stars: formation - ISM: jets and outflows - ISM: molecules

1 Introduction

Among the main coolants in molecular shocks, water is the tracer most sensitive to physical variations and the temporal evolution of protostellar outflows, thus representing a very powerful probe of their shock conditions and thermal history (e.g., Bergin et al. 1998). Water emission and excitation in shocks were studied extensively for the first time with ISO, the first space facility with spectroscopic capabilities in the mid- and far-IR. ISO surveyed the water emission in a large sample of outflows from young stellar objects (YSOs), providing a global statistical picture of the importance of water in the outflow cooling and of variations in its abundance with shock properties and ages (see e.g., Nisini 2003; van Dishoeck 2004). Following ISO, the SWAS and Odin facilities made it possible to observe the ortho-H2Ofundamental line at 557 GHz, providing important constraints on the water abundance and kinematics in the cold outflow gas components (e.g., Franklin et al. 2008; Bjerkeli et al. 2009). All these facilities, however, had poor spatial resolution (i.e. greater than 80 $\hbox{$^{\prime\prime}$ }$), which did not allow one to locate the origin of the water emission nor study variations in abundances and excitation within individual flows.

In this framework, a sample of YSO outflows will be surveyed in different water lines by the PACS and HIFI instruments onboard the Herschel satellite, as part of the key program WISH (Water In Star-forming-regions with Herschel[*]). This paper presents the first results obtained from this survey, consisting of a PACS map of the H2O 2 12-101 179 $\mu $m line covering the outflow of the protostar L1157-mm, obtained during the Herschel science demonstration phase. The 179 $\mu $m line is the transition connecting the lower two back-bone levels of ortho-H2O. It is therefore one of the brightest water lines expected in collisionally excited conditions, thus representing an ideal tracer of the water distribution in shocked regions. L1157 is a well known outflow driven by a low mass class 0 object (L1157-mm, $ L_{\rm bol} \sim
8.3~ L_\odot$, D=440 pc, Froebrich 2005). It is considered to be the prototype of chemically active flows, given the large number of different species detected in its shocked regions (e.g., Bachiller & Perez-Gutierrez 1997). This paper is presenting the first of several observations planned for this source by the WISH team.

2 Observations

Observations were performed on 26 October 2009 with the PACS instrument (Poglitsch et al. 2010) onboard the Herschel Space Observatory (Pilbratt et al. 2010) in line spectroscopic mode, with the grating centred on the H2O 212-101 line at 179.527 $\mu $m. The L1157 outflow region (of about $6\hbox{$^\prime$ }\times 2\hbox{$^\prime$ }$) was covered by 3 individual PACS raster maps, arranged along the outflow axis. Each map consists of $3\times3$ PACS frames acquired in steps of 40 $\hbox{$^{\prime\prime}$ }$. The instrument is a $5 \times 5$ pixel array providing a spatial sampling of 9.4 $\hbox{$^{\prime\prime}$ }$/pixel, while the spectral resolution at 179 $\mu $m is $R\sim1500$(i.e., $\sim $210 km s-1). The data were reduced with HIPE 2.0. Additional IDL routines were developed to construct a final integrated and continuum-subtracted line map. Flux calibrations used calibration files obtained by ground tests that remain very uncertain at the time of paper writing, especially for extended sources. To evaluate the flux uncertainty, we compared with the three measurements performed by the ISO satellite along the outflow (Giannini et al. 2001). To do that, we performed aperture photometry of the line emission in the PACS map within the 80 $\hbox{$^{\prime\prime}$ }$ ISO circular beam. The ratio of PACS to ISO fluxes ranges between 1.1 and 1.8 at the three positions: we adopt this as the uncertainty in our quantitative analysis. The typical rms noise across the map is of the order of $2\times10^{-6}$ erg s-1 cm-2 sr-1.

\begin{figure}
\par\includegraphics[angle=-90,width=16cm,clip]{14603fig1.eps}\vspace*{-3mm}\vspace{-2mm}
\end{figure} Figure 1:

Continuum subtracted PACS map of the integrated H2O 179 $\mu $m  emission along the L1157 outflow. Offsets are with respect to the L1157-mm source, at coordinates $\alpha (2000) = 20$:39:06.2, $\delta (2000) = +68$:02:16. The different emission peaks are labelled following the nomenclature adopted by Bachiller et al. (2001) for individual CO peaks. The same map is shown in the other panels with overlays of other tracers, namely H2 0-0 S(1) at 17 $\mu $m  (Neufeld et al. 2009), CO 2-1, and SiO 3-2 (Bachiller et al. 2001). The spatial resolution of these images are $\sim $11 $^{\prime \prime }$, for H2  and CO, and 18 $^{\prime \prime }$  for SiO. Note that the H2 observed region does not cover the B2 and R2 shocked peaks.

Open with DEXTER

3 Results and comparison with other tracers

Figure 1 presents the PACS map of the 179 $\mu $m line emission. In the same figure, the H2O  map is overlaid with contours of the emission from the H2 0-0 S(1) (Neufeld et al. 2009), CO 2-1 and SiO 3-2 (Bachiller et al. 2001) transitions. The water map exhibits several emission peaks corresponding to the positions of previously-known shocked knots, labelled as B0-B1-B2 for the south east blue-shifted lobe, and R0-R-R2 for the north west red-shifted lobe, following the nomenclature of Bachiller et al. (2001). These emission knots represent the actual working surfaces of a precessing and pulsed jet and are thus associated with the present location of the active shock regions. With respect to CO, H2O emission appears more localized, having a less prominent diffuse component. About 60% of the total 179 $\mu $m  flux is found within 30 $\hbox{$^{\prime\prime}$ }$ apertures centered on the knots. This could be partly related to the line excitation: the 179 $\mu $m line excitation temperature is $\sim $80 K above the o-H2O ground state (compared to the 17 K for CO 2-1), and the critical density of its upper level is above 108 cm-3  for $T \la 500$ K. It may however also be a consequence of the specific conditions needed to ensure a significant production of water. The H2O abundance is indeed significantly higher only in shocks strong enough to release the water ice located on grain mantles by sputtering and grain-grain collisions or to activate the gas-phase reactions that convert the gas-phase oxygen into water. Both these processes become efficient at shock velocities $v_{\rm s} \ga 15$ km s-1  (Caselli et al. 1997; Jiménez-Serra et al. 2008; Kaufman & Neufeld 1996). In this respect, we note that the H2O emission peaks correspond rather closely to both the position and the relative intensity of the H2 rotational emission (with the H2O 179 $\mu $m/H2 17 $\mu $m  ratio in the range ${\sim}(2{-}3)\times 10^{-2}$ for all the H2  peaks). Peaks of low-J H2 pure rotational lines are associated with warm gas (with $T \sim 300{-}500$ K) excited in low velocity non-dissociative shocks that are tracers of regions in which a high H2O abundance is expected. Other molecules are known to have strongly enhanced abundances in shocks. One of the most well studied of these molecules is SiO, for which Fig. 1 shows that, like water, its emission is very localized around the shocked knots. A similar behavior is found for other molecules, such as NH3 and CH3OH (Bachiller et al. 2001; Tafalla & Bachiller 1995).

The strongest water peak is located at the position of the B1 knot, which is known to be the most chemically active of the L1157 spots (e.g. Bachiller & Perez Guitierrez 1997; Benedettini et al. 2007; Codella et al. 2010). This knot at near-IR wavelengths appears as a bow shock with intense H2 2.12 $\mu $m emission (Davis & Eislöffel 1995) and has a significant H2 column density enhancement (Nisini et al. 2010). Although the spatial resolution of the present observations prevents us from completely resolving the bow shock structure, the observed morphology at the B1/B0 positions suggests that water emission is mainly localized at the bow apex and eastern wing. A similar morphology has been observed for molecules such as SiO, NH3, and CS (Benedettini et al. 2007; Tafalla & Bachiller 1997), while other shock produced molecules, such as CH3OH, noticeably have emission localized on the bow western wing (e.g. Codella et al. 2009). This behavior probably relates to an asymmetry in the excitation conditions along the bow structure, most likely induced by the jet precession or the propagation of shocks in an inhomogeneous medium.

Strong, spatially unresolved, water emission is also detected on-source. This localized emission can originate in different components, including shocks impacting on a dense medium at the jet base, the infalling protostellar envelope, or emission from a UV-heated outflow cavity, as discussed in van Kempen et al. (2010) for the HH46-IRS case. The precise origin of this emission will be investigated by dedicated Herschel observations, but we note here the interesting evidence that no other molecule exhibits significant emission at the central position. In particular, the non-detection of strong emission from molecules such as CH3OH indicates that grain ice mantle evaporation in the protostellar envelope is unlikely to be the origin of the on-source H2O  emission, since the two molecules should desorb at similar temperatures. The non-detection of the H2 0-0 S(1) line at the central position is also remarkable. This may be caused by the heavy extinction close to the central source. Assuming an intrinsic H2O 179 $\mu $m/H2 17 $\mu $m  ratio in the range of that observed along the outflow, we estimate that Av on-source should be $\ga$150 mag to be able to explain the H2  line non-detection. Alternatively, C-type shocks with very high pre-shock densities ($\ge$106 cm-3) and velocities between 20 and 40 km s-1  are expected to have a large H2O/H2  cooling ratio (Kaufman & Neufeld 1996).

4 Water abundance and total cooling

To constrain the range of water column densities that could produce the observed 179 $\mu $m emission, we consider the SWAS and Odin observations of the H2O 1 10-101 557 GHz (538 $\mu $m) line observed in this outflow (Franklin et al. 2008; Bjerkeli et al. 2009). Given the large size of the apertures of these two instruments relative to the PACS spatial resolution, we evaluate here only properties averaged over large outflow regions. In particular, we consider the Odin observations acquired towards the blue (B) and red (R) outflow lobes at offsets (+29 $\hbox{$^{\prime\prime}$ }$, $-52\hbox{$^{\prime\prime}$ }$) and ( $-21 \hbox{$^{\prime\prime}$ }$, +121 $\hbox{$^{\prime\prime}$ }$) (Bjerkeli et al. 2009). The 179 $\mu $m/557 GHz intensity ratios are obtained by diluting the PACS observations to the 126 $\hbox{$^{\prime\prime}$ }$ Odin resolution. The same procedure was adopted for the SWAS observation that encompasses almost the entire L1157 PACS mapped region with its 3.5 $^{\prime} \times 5.0^{\prime}$ elliptical aperture.
\begin{figure}
\par\includegraphics[width=8.5cm,clip]{14603fig2.eps}\vspace{-1mm}\vspace*{-3mm}
\end{figure} Figure 2:

LVG theoretical predictions of the 179 $\mu $m line brightness versus the 179 $\mu $m/557 GHz line ratio, compared with observed values. See text for the details.

Open with DEXTER
Figure 2 presents large velocity gradient (LVG) predictions, assuming a slab geometry, of the 179 $\mu $m line brightness versus the 179 $\mu $m/557 GHz line ratio, compared to the observations combined above. The absolute brightnesses are those averaged within an area enclosing 90% of the total PACS emission inside each considered Odin/SWAS aperture. These emitting areas are $5.9\times 10^{-8}$, $8.0\times 10^{-8}$, and $2.7\times 10^{-7}$ sr for the R, B, and the SWAS apertures, respectively. The line intensity derived in this way was considered to be a lower limit to the true 179 $\mu $m brightness if the PACS emission originates in a clumpy medium, of which the clump size is smaller than the Herschel diffraction limit at 179 $\mu $m.

In the figure, observations are indicated as boxes that take into consideration the uncertainty of a factor of about 1.5 in the 179 $\mu $m flux, estimated by comparing with the ISO observations (Sect. 2). Theoretical curves were derived as a function of the o-H2O  column density, using the RADEX code (Van der Tak et al. 2007) assuming temperature and density conditions measured from the H2  Spitzer observations or ground-based millimeter observations (Nisini et al. 2010, 2007; Mikami et al. 1992). The temperature is between 300 and 500 K and the density is in the range $1{-}5\times10^{5}$ cm-3, the blue lobe being on average colder and denser than the red lobe. Part of the 557 GHz emission can arise from a gas colder than these assumed values, given the lower excitation temperature of this line with respect to the 179 $\mu $m  line. To evaluate the effect of different temperature components along the line of sight on the ratio of the two considered transitions, Fig. 2 also plots the theoretical predictions assuming a temperature stratification where the column density in each layer at a given T varies as T-b (Neufeld & Yuan 2008). A minimum and maximum temperature of 100 K and 4000 K, respectively are assumed, and b values between 2 and 4, i.e., the range of values that consistently fit the H2 rotational lines (Neufeld et al. 2009). These curves give the same range of predicted values as the single T curves, indicating that contributions from high-temperature gas do not significantly affect the considered transitions.

Several general conclusions can be drawn from the inspection of Fig. 2. Firstly, the data are consistent with model predictions only if we assume that the real emitting areas are smaller than those estimated from the PACS map. In particular, agreement with the theoretical curves is found for covering factors ($f_{\rm c}$) $\sim $0.1-0.2, which suggests that the emission is concentrated on some unresolved emission knots that together do not fill an area larger than a few tens of arcsec. This is not unexpected, since interferometric mm observations illustrate the extreme clumpiness of the shocked gas, individual knots being of sizes of a few arcsec each (e.g., Benedettini et al. 2007; Lefloch et al. 2010). We note that the typical length scale for planar C-type shocks at the considered densities is of the order of 1016 cm, i.e., about 1/10 of the PACS spatial resolution at D=440 pc. The observed 179 $\mu $m/557 GHz ratios, ranging between 10 and 20, are consistent with N(H2O) $\sim $ 2- $9~\times~ 10^{16}$ cm-2  (assuming a $\Delta v = 15$ km s-1 from the 557 GHz line width). The H2  column densities, averaged within the PACS emitting areas, were measured from the H2 mid-IR rotational lines and results in ${\sim}5\times 10^{19}$ cm-2  in both regions covered by the B and R observations. The water abundance in the unresolved clumps is therefore estimated to be ${\sim} N$(H2O)/N(H2) $~\times f_{\rm c} \sim 0.6{-}3\times10^{-4}$(with a H2O o/p ratio of 3). Table 1 reports in more detail the range of values derived in each considered aperture. The total mass of the shocked gas involved in the 179 $\mu $m  emission is of the order of $5\times10^{-3}~ M_\odot$, which is only a small fraction ($\sim $1/100) of the total mass of the outflow estimated from CO observations (e.g. Bachiller et al. 2001). Lefloch et al. (2010) show that H2O  components with different velocities are discernible in the 557 GHz data acquired by HIFI in a 40 $^{\prime \prime }$ beam centred on the L1157-B1 knot. They separately analyse the different velocity components, confirming that small filling factors are required to explain their observations and finding that the component of higher velocity is the one exhibiting the water abundance of the order of 10-4. Lower H2O  abundance values, between 10-6 and 10-5, were estimated using only the SWAS and Odin 557 GHz emission, assuming that the 557 GHz emission originates in the same cool gas traced by the low-J CO emission, thus a gas with a larger covering factor and lower temperature than considered here (Neufeld et al. 2000; Franklin et al. 2008; Bjerkeli et al. 2009). Combining ISO-179 $\mu $m emission and SWAS observations, Benedettini et al. (2002) derived a water abundance for the warm shocked gas of ${\sim}5\times10^{-5}$, thus in the lower range of values estimated in the present analysis. However, the ISO observations did not cover the entire L1157 outflow 179 $\mu $m emission, and the inferred ISO 179 $\mu $m/SWAS 557 GHz ratio was underestimated by about a factor of 2.

Table 1:   Estimated water abundances.

Given the considered conditions, the 179 $\mu $m line contributes to about 30-40% of the water emission in the outflow: the total estimated H2O luminosity is ${\sim} 8{-}9\times 10^{-2}~L_\odot$, which is about 40% of the total H2 shock luminosity ( $0.2~ L_\odot$, Nisini et al. 2010) and about 23% of the total shock cooling in the L1157 outflow, if we also consider the contributions given by CO and [O I] derived from ISO observations by Giannini et al. (2001). The high water abundance estimatedin the present analysis is consistent with predictions of non-dissociative shock models, in which water is mainly produced by endothermic reactions, activated at $T \ga 300$ K, where all the available gas-phase oxygen is converted into H2O, or by the sputtering of icy grain mantles behind the shock. According to Bergin et al. (1998), the time needed to complete this process is of the order of 103 yr, for T = 400 K. This is comparable to the shock timescales estimated from H2 observations of individual emission knots of the L1157 outflow (Nisini et al. 2010), thus supporting the idea that the water in this outflow has had time to reach its maximum allowed abundance.

5 Conclusions

We have presented a PACS spectral map of the H2O 179 $\mu $m transition obtained toward the L1157 protostellar outflow. Strong water emission peaks have been found at the location of previously-known shocked spots and correlate well with H2 mid-IR rotational lines, as well as other important shock tracers, such as SiO and NH3. The absolute 179 $\mu $m  intensity and the intensity ratios with respect to the previously-observed 557 GHz line, indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, that have a H2O abundance of the order of 10-4. The total H2O cooling has been estimated to be of the order of $8{-}9\times10^{-2}~L_\odot$, representing about 40% of the cooling due to H2 and 23% of the total energy released in shocks along the L1157 outflow.

Additional Herschel PACS/HIFI observations of the L1157 outflow are planned by the WISH program. These will enable us to investigate variations in the water abundance within the outflow and correlate these with kinematical information.

Acknowledgements
This program is made possible thanks to the HIFI guaranteed time and the PACS instrument builders.

References

Footnotes

... L1157[*]
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important partecipation from NASA.
... Herschel[*]
http://www.strw.leidenuniv.nl/WISH/

All Tables

Table 1:   Estimated water abundances.

All Figures

  \begin{figure}
\par\includegraphics[angle=-90,width=16cm,clip]{14603fig1.eps}\vspace*{-3mm}\vspace{-2mm}
\end{figure} Figure 1:

Continuum subtracted PACS map of the integrated H2O 179 $\mu $m  emission along the L1157 outflow. Offsets are with respect to the L1157-mm source, at coordinates $\alpha (2000) = 20$:39:06.2, $\delta (2000) = +68$:02:16. The different emission peaks are labelled following the nomenclature adopted by Bachiller et al. (2001) for individual CO peaks. The same map is shown in the other panels with overlays of other tracers, namely H2 0-0 S(1) at 17 $\mu $m  (Neufeld et al. 2009), CO 2-1, and SiO 3-2 (Bachiller et al. 2001). The spatial resolution of these images are $\sim $11 $^{\prime \prime }$, for H2  and CO, and 18 $^{\prime \prime }$  for SiO. Note that the H2 observed region does not cover the B2 and R2 shocked peaks.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8.5cm,clip]{14603fig2.eps}\vspace{-1mm}\vspace*{-3mm}
\end{figure} Figure 2:

LVG theoretical predictions of the 179 $\mu $m line brightness versus the 179 $\mu $m/557 GHz line ratio, compared with observed values. See text for the details.

Open with DEXTER
In the text


Copyright ESO 2010

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.