Herschel PACS observations of shocked gas associated with the jets of L1448 and L1157
G. Santangelo, B. Nisini, S. Antoniucci, C. Codella, S. Cabrit, T. Giannini, G. Herczeg, R. Liseau, M. Tafalla, E.F. van Dishoeck
aa r X i v : . [ a s t r o - ph . GA ] J u l Astronomy&Astrophysicsmanuscript no. PACS˙santangelo c (cid:13)
ESO 2018August 8, 2018
Herschel ⋆ PACS observations of shocked gas associated with thejets of L1448 and L1157
G. Santangelo , B. Nisini , S. Antoniucci , C. Codella , S. Cabrit , T. Giannini , G. Herczeg , R. Liseau , M. Tafalla ,and E.F. van Dishoeck , Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italye-mail: [email protected] Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Florence, Italy LERMA, Observatoire de Paris, UMR 8112 of the CNRS, 61 Av. de LObservatoire, 75014 Paris, France Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian Qu, 100871 Beijing, P.R. China Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden Observatorio Astron´omico Nacional (IGN), Alfonso XII 3, E-28014 Madrid, Spain Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands Max Planck Institut f¨ur Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, GermanyReceived ; accepted
ABSTRACT
Aims.
In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H O ( E u >
190 K), high- J CO, [O i ], and OH transitions are mapped with Herschel -PACS in two shock positions along two prototypical outflows around thelow-luminosity sources L1448 and L1157. Previous
Herschel -HIFI H O observations ( E u = −
249 K) are also used. The aim is toderive 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 O and CO emitting gas.Complementary Spitzer mid-IR H data were used to derive the H O abundance.
Results.
Consistent with other studies, at all selected shock spots a close spatial association between H O, mid-IR H , and high- J COemission is found, whereas the low- J CO emission traces either entrained ambient gas or a remnant of an older shock. The excitationanalysis, conducted in detail at the L1448-B2 position, suggests that a two-component model is needed to reproduce the H O, CO,and mid-IR H lines: an extended warm component ( T ∼
450 K) is traced by the H O emission with E u = −
137 K and by theCO lines up to J = −
21, and a compact hot component ( T = O emission with E u >
190 K and by thehigher- J CO transitions. At L1448-B2 we obtain an H O abundance (3 − × − for the warm component and (0 . − . × − for the hot component and a CO abundance of a few 10 − in both components. In L1448-B2 we also detect OH and blue-shifted [O i ]emission, spatially coincident with the other molecular lines and with [Fe ii ] emission. This suggests a dissociative shock for thesespecies, related to the embedded atomic jet. On the other hand, a non-dissociative shock at the point of impact of the jet on the cloudis responsible for the H O and CO emission. The other examined shock positions show an H O excitation similar to L1448-B2, but aslightly higher H O abundance (a factor of ∼ Conclusions.
The two gas components may represent a gas stratification in the post-shock region. The extended and low-abundancewarm component traces the post-shocked gas that has already cooled down to a few hundred Kelvin, whereas the compact and possiblyhigher-abundance hot component is associated with the gas that is currently undergoing a shock episode. This hot gas component ismore a ff ected by evolutionary e ff ects on the timescales of the outflow propagation, which explains the observed H O abundancevariations.
Key words.
Stars: formation – Stars: low-mass – ISM: jets and outflows – ISM: individual objects: L1448 and L1157 – ISM:molecules
1. Introduction
During the earliest stages of star formation, young stars producefast collimated jets, that collide with the dense parent cloud gen-erating strong interstellar shocks. These processes strongly mod-ify the chemical composition of the surrounding gas and areidentified by intense line emission. Among the di ff erent trac-ers of shocks, water is a key molecule and a unique diagnos-tic tool of local conditions and energetic processes occurringin star-forming regions (e.g. van Dishoeck et al. 2011), since itsabundance varies by many orders of magnitude during the shock ⋆ Herschel is an ESA space observatory with science instrumentsprovided by European-led Principal Investigator consortia and with im-portant participation from NASA. lifetime (e.g. Bergin et al. 1998). In particular, water abundancewith respect to H is expected to increase from < − in cold re-gions to about 10 − in the warm gas, due to the combined e ff ectsof evaporation of icy mantles and high-temperature chemical re-actions which drive all the atomic oxygen into H O.Space instruments, such as SWAS, Odin, and ISO, have al-lowed the study of water in outflows. It was possible to resolvethe water line profiles (e.g. Benedettini et al. 2002, Bjerkeli et al.2009), to derive the excitation conditions of the emitting gas(e.g. Liseau et al. 1996, Ceccarelli et al. 1998, Nisini et al. 1999,2000) and to measure the water abundance in shocks throughcomparison with CO emission. In particular, values within therange ∼ − − − have been derived for the H O abundanceshowing that it depends on the gas temperature and velocity (e.g.
1. Santangelo et al.:
Herschel
PACS observations of shocked gas associated with the jets of L1448 and L1157
Giannini et al. 2001, Franklin et al. 2008). However, the limitedspatial and spectral resolution of these instruments have pre-vented a clear association of the shocked gas with a specifickinematical component and with a specific region along the out-flow, thus preventing the origin of the shocked gas from beingderived. Thanks to the
Herschel Space Observatory , we are nowable to improve our view of the shock processes occurring dur-ing the very early stages of star formation and to test the modelpredictions for the water formation and abundance during theseprocesses.In this context, the low-luminosity Class 0 protostellar sys-tems L1448 (7.5 L ⊙ ) and L1157 (4 L ⊙ ) are excellent targets. Atthe distance of 232 pc (Hirota et al. 2011), L1448 is the proto-type of a source driving a molecular jet (Eisl¨o ff el 2000). It has apowerful and highly collimated outflow, which has been exten-sively studied (e.g. Guilloteau et al. 1992, Bachiller et al. 1995,Hirano et al. 2010). Gas excited in shocks has been detectedalong the outflow through near- and mid-IR emission of molec-ular hydrogen (e.g. Neufeld et al. 2009, Giannini et al. 2011).At a distance of 250 pc, L1157 is perhaps the most active out-flow from a chemical point of view (Bachiller & Perez Gutierrez1997, Bachiller et al. 2001), often quoted as being the proto-type of the so-called chemically rich outflows. Detailed Herschel observations of the L1157 outflow by the Chemical HErschelSurveys of Star forming regions (CHESS) program havebeen presented by Codella et al. (2010), Lefloch et al. (2010),Codella et al. (2012a,b), Benedettini et al. (2012), Lefloch et al.(2012).The
Herschel
Key Program Water In Star-forming regionswith Herschel (WISH, van Dishoeck et al. 2011) employed morethan 400 hours of telescope time to observe H O and relatedmolecules toward about 80 protostars at di ff erent evolutionarystages and masses to study the physical and chemical condi-tions of the gas in nearby star-forming regions. Within the WISHframework, several results concerning outflows have been pre-sented (e.g. Bjerkeli et al. 2011, Kristensen et al. 2011, 2012,Bjerkeli et al. 2012, Herczeg et al. 2012, Tafalla et al. 2013).Both the L1448 and L1157 outflows have been mapped tostudy the spatial distribution of water and the results havebeen presented by Nisini et al. (2010a) for the L1157 outflowand Nisini et al. (2013) for the L1448 outflow. They show aclumpy water distribution, with emission peaks corresponding toshock positions along the outflow. Multi-transition observations(with excitation energies ranging from 53 to 249 K), performedwith the Heterodyne Instrument for the Far Infrared (HIFI,de Graauw et al. 2010) toward two shock positions of each out-flow, have been presented by Vasta et al. (2012) for the L1157outflow and by Santangelo et al. (2012) for the L1448 outflowto constrain the water excitation conditions. These studies haveshown strong variations of the H O line profiles with excitation,which indicate that gas components with di ff erent physical andexcitation conditions coexist at the shock positions. Complexline profiles have also been observed at the position of the centraldriving source of the L1448 outflow by Kristensen et al. (2012),with a broad velocity component possibly associated with theinteraction of the outflow with the protostellar envelope and theextreme high-velocity gas (EHV) associated with the collimatedmolecular jet.In this context as part of the WISH key program, we reporthere on the results of new Herschel observations of the sameshock regions along the L1448 and L1157 outflows. A set of highexcitation H O lines and several transitions of CO, OH, and [O i ]have been mapped with the Photodetecting Array Camera andSpectrometer (PACS, Poglitsch et al. 2010) instrument. Unlike the previous HIFI observations, the PACS data will allow us todetect and characterize the higher excitation gas with a higherangular resolution, thus providing a complete and consistent pic-ture of the shocked gas along the two outflows. This in turn willallow us to settle the conditions for water formation and to ex-plore its ability to probe specific excitation regimes.The paper is organized as follows. The PACS observationsare described in Sect. 2. In Sect. 3 we present the PACS maps andthe main observational results. A detailed analysis of the PACSmaps is discussed in Sect. 4, starting from the study of the phys-ical and excitation conditions in the B2 shocked position alongthe L1448 outflow and subsequently discussing the implicationsfor the other selected shocked spots. The results are discussedin Sect. 5, in the context of current shock models. Finally, theconclusions are presented in Sect. 6.
2. Observations and data reduction
We performed a survey of key far-IR lines with the PACS in-strument on board
Herschel (Pilbratt et al. 2010, Poglitsch et al.2010) toward two shock spots along each interested outflow (seeFig. 1): the B2 and R4 spots along L1448 (hereafter L1448-B2and L1448-R4, respectively; Bachiller et al. 1990); and B2 andR along L1157 (hereafter L1157-B2 and L1157-R, respectively;Bachiller et al. 2001). The PACS instrument is an integral fieldunit (IFU), consisting of a 5 × . ′′ × . ′′
4, providing a totalfield of view of 47 ′′ × ′′ . The line spectroscopy mode wasused to cover short spectral regions and thus observe selectedlines at the interested shock positions. The line survey comprisesortho- and para-H O transitions with excitation energies rang-ing from 194 to 396 K. In addition, high- J CO, [O i ], and OHlines have been observed (see Table 1 for a summary of the tar-geted lines). These observations are complementary to observa-tions of lower excitation H O transitions (Santangelo et al. 2012,Vasta et al. 2012) conducted with the HIFI heterodyne instru-ment (de Graauw et al. 2010) with excitation energies rangingfrom 53 to 249 K, and to PACS maps of the H O 2 − line at179.5 µ m along the two outflows (Nisini et al. 2010a, 2013, seealso Fig. 1).The data were processed with the ESA-supported packageHIPE ( Herschel
Interactive Processing Environment, Ott 2010)version 4.2 (except for the data relative to the L1157-R positionthat were processed with HIPE version 5) . The observed fluxeswere normalized to the telescope background and then convertedinto absolute fluxes using Neptune as a calibrator. The flux cal-ibration uncertainty of the PACS observations is 30%, based onthe flux repeatability for multiple observations of the same tar-get in di ff erent programs and on cross-calibration with HIFI andISO. Further data reduction, to obtain continuum subtracted linemaps, and the analysis of the data were performed using IDLand the GILDAS software.The Herschel di ff raction limit at 179 µ m is 12 . ′′ µ m it is smaller than the PACS spaxelsize of 9 . ′′
4. To correct for the di ff erent beam sizes in the excita-tion analysis presented in Sect. 4, we convolved all maps to the HIPE is a joint development by the
Herschel
Science GroundSegment Consortium, consisting of ESA, the NASA
Herschel
ScienceCenter, and the HIFI, PACS, and SPIRE consortia. We have checked the data processed with the latest version of HIPE(version 10) and we find an agreement in the flux densities within 15 − http: // / IRAMFR / GILDAS /
2. Santangelo et al.:
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PACS observations of shocked gas associated with the jets of L1448 and L1157
Fig. 1: PACS H O 179 µ m images of L1448 and L1157 (Nisini et al. 2010a, 2013). The PACS line survey positions are indicatedfor L1448-B2 and R4 with crosses, for L1157-B2 and R with triangles. The field of view of the PACS observations is displayed as abox at the selected positions. CO(3 −
2) and SiO(3 −
2) emissions for L1448 and L1157, respectively, are superimposed on the H Omaps.resolution of the transition with the longest wavelength, that is12 . ′′ µ m, and then extracted the fluxes at each selectedshock spot.
3. Results
The PACS spectra of all the lines detected in the four examinedshocked positions are presented in Appendix A, whereas a sum-mary of the main line parameters is given in Table 1, along withthe fluxes of the detected lines. The source L1448-B2 representsthe position in which we detected the largest number of linesand it is the only position where we detected the OH fundamen-tal line at 119 µ m.The original PACS maps of selected lines, not convolved toa common angular resolution, are shown in Fig. 2 and 3, respec-tively, for L1448 and L1157. The figures present the overlay be-tween the H O 3 − (174 µ m), H O 2 − (108.1 µ m),CO(16 − i ] P − P (63.2 µ m), OH (119 µ m) emis-sion, and other tracers from complementary observations. Alongthe L1448 outflow several interesting features can be noticed. Inparticular, the peak of the H O emission at L1448-B2 is at theapex of the bow-shock, as traced by the H emission at 2.12 µ m,and at L1448-R4 it corresponds to the peak of the IRAC 8 µ memission. We found no shift at this angular resolution betweenthe H O emission at 174 µ m and CO(16 −
15) emission, bothin L1448-B2 and L1448-R4, which indicates that at this angularresolution high- J CO and H O are spatially coincident and traceshocked gas. The bottom-right panel relative to L1448-B2 shows the com-parison between H O and CO(3 − v & −
50 km s − , Bachiller et al. 1990) has been separatedfrom the standard outflow high-velocity (HV, v . −
40 km s − )gas emission. We see that the HV gas is totally uncorrelatedwith the water emission, a result already found in other studies(Santangelo et al. 2012, Nisini et al. 2013, Tafalla et al. 2013).The EHV gas, on the other hand, has a peak shifted north-westwith respect to the H O peak. Thus, the low- J CO emissiontraces entrained ambient gas and not the shocked gas, indepen-dent of the velocity components.Finally, we do not see any significant spatial shift at this an-gular resolution between the peaks of H O, [O i ], and [Fe ii ] emis-sion, although the H O emission appears to be more extendedthan [O i ] and [Fe ii ]. Nevertheless, at the L1448-B2 position, andpossibly at the adjacent spaxels along the outflow direction, ahint of a velocity shift in the [O i ] line at 63 µ m was detected: theline is blue-shifted by ∼
80 km s − (see Fig. A.1), which is com-parable with the resolution element of PACS at this wavelength( ∼
90 km s − ). Similar [O i ] velocity shifts have been found inHH46 by van Kempen et al. (2010a) and in Serpens SMM1 byGoicoechea et al. (2012), who suggested the presence of fast dis-sociative shocks close to the protostar, related with an embeddedatomic jet. Moreover, Karska et al. (2013) analysed PACS spec-tra of a large sample of Class 0 / I protostars and they found suchprofile shifts toward at least 1 / eV and thus we expected tofind [Fe ii ] co-spatial with [O i ] (ionization potential of 13.6 eV ).
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PACS observations of shocked gas associated with the jets of L1448 and L1157
Fig. 2: Overlay between PACS H O 3 − (174 µ m), H O 2 − (108.1 µ m), [O i ] P − P (63.2 µ m), CO(16 − µ m), and other tracers in the B2 ( upper panel ) and R4 ( lower panel ) shocked spots along the L1448 outflow. In particular,JCMT CO(3 −
2) emission (Half Power Beam Width, HPBW ∼ ′′ ) from Nisini et al. (2013), Spitzer [Fe ii ] emission at 26 µ mfrom Neufeld et al. (2009), IRAC 8 µ m emission from Tobin et al. (2007), and H emission at 2.12 µ m from Davis & Smith (1995)are shown. In the bottom-right panel relative to L1448-B2 the EHV CO(3 −
2) emission ( v & −
50 km s − ) and the HV CO(3 − v . −
40 km s − ) are shown in dashed and solid lines, respectively. The contours in each map are traced every 3 σ , startingfrom a 5 σ level, except for the CO(3 −
2) in L1448-B2, where the contours are traced in steps of 5 σ , starting from 5 σ . The crossesrepresent the pointing of the 25 spaxels.
4. Santangelo et al.:
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PACS observations of shocked gas associated with the jets of L1448 and L1157
Fig. 3: The same as in Fig. 2, but for the L1157 outflow. Contours are displayed for IRAM-30m CO(2 −
1) and SiO(3 −
2) emission(HPBW equal to 11 ′′ and 18 ′′ , respectively) from Bachiller et al. (2001) and are traced in steps of 5 σ , starting from 5 σ . Spitzer-IRAC 8 µ m emission and Spitzer-IRS H S(1) emission at 17 µ m from Neufeld et al. (2009) are shown.
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PACS observations of shocked gas associated with the jets of L1448 and L1157
Table 1: Fluxes of the lines observed with PACS and relative 1 σ errors in parentheses. L1448 - B2 L1448 - R4 L1157 - B2 L1157 - R a Transition Frequency Wavelength E u / k B Flux(GHz) ( µ m) (K) (10 − erg s − cm − )[O i ] P − P O 2 − <
16 9 (2)CO 24 −
23 2756.39 108.8 1656.5 14 (4) < < < −
21 2528.17 118.6 1397.4 13 (2) 7 (2) < < Π / J = / − − / + b (2) < < < Π / J = / + − / − O 4 − <
11 5 (1)p-H O 3 − −
17 2070.62 144.8 945.0 32 (2) 11 (2) < < i ] P − P < < −
15 1841.35 162.8 751.7 39 (2) 17 (2) 5 (2) 10 (2)o-H O 3 − O 2 − c Notes.
Fluxes are measured at the central spaxel of the maps after convolving to 12 . ′′
6, i.e. the PACS resolution of the transition with the longestwavelength (179 µ m). The relative rms error is measured at the same spaxel and does not include 30% calibration accuracy. ( a ) The fluxes andrelative rms errors are given at the peak of the H O emission, which is at a position o ff set of (2 ′′ , − ′′ ) from the central spaxel (see Fig. 3). ( b ) Thevalue represents the sum of the fluxes of the two listed OH lines (at 119.2 and 119.4 µ m). The OH 119.4 / ( c ) The values are measured from the PACS maps of the two outflows at 179 µ m (Nisini et al. 2013). A much smaller number of lines was detected along theL1157 outflow. In particular, only four lines were detected atL1157-B2. Here the emission is elongated in the outflow direc-tion, according to all tracers. Similarly to L1448, the H O emis-sion at 174 µ m is spatially associated with the [O i ] P − P emission at 63.2 µ m and the CO(16 −
15) emission. Two emis-sion peaks can be identified in the PACS maps: the brightest oneis found at the central spaxel and is spatially associated with theH emission, as seen from the overlay with the Spitzer-IRACimage at 8 µ m; the other emission peak is at a position o ff setof (12 ′′ , − ′′ ) from the central spaxel, close to the edge of thePACS map. The SiO(3 −
2) emission (Bachiller et al. 2001) alsoappears to be elongated along the outflow direction with a peakroughly corresponding to the central spaxel of the PACS maps.On the other hand, the CO(2 −
1) emission is not spatially as-sociated with any other molecular species. At L1157-R a brightemission peak is seen in H O and in all species observed withPACS. This H O peak is shifted with respect to the central spaxelof (2 ′′ , − ′′ ) and is spatially associated with the H emission. Asecond peak is found in the [O i ] P − P emission (63.2 µ m), ata position o ff set of ( − ′′ ,8 ′′ ) from the central spaxel, and is alsovisible in H O. On the other hand, the SiO(3 −
2) emission peaksat the central spaxel position, thus o ff set from the H O emission.Finally, the CO(2 −
1) emission is more di ff use than the othertracers and has an emission peak at the central spaxel position,thus shifted with respect to the H O and high- J CO emission.In conclusion, the inspection of the PACS maps highlightsthe following results: in both outflows the H O emission is spa-tially associated with mid-IR H emission and high- J CO emis-sion, whereas the low- J CO emission seems to be associatedwith a di ff erent gas component. Our findings are consistent withthe results obtained by Nisini et al. (2010a, 2013) from mappingthe H O 2 − emission along the L1448 and L1157 outflowsand by Tafalla et al. (2013) from the analysis of H O 1 − and 2 − emission in a large sample of shocked positions.Moreover, Karska et al. (2013) found a tight correlation betweenH O 2 − at 179 µ m and high- J CO line fluxes, conclud-ing that they likely arise in the same gas component. The SiOemission appears to be slightly shifted with respect to H O, con-sistent with the two gas components tracing shock regions with di ff erent excitation conditions, as discussed in Santangelo et al.(2012) and Vasta et al. (2012). Finally, no shift is observed atthe PACS angular resolution between the [O i ] and [Fe ii ] linesand the H O emission.
4. Analysis
In this section we discuss the excitation conditions of the ob-served lines, complemented with Spitzer H data, when avail-able. In particular, we will concentrate our analysis on theL1448-B2 shock, where we detected the largest number of lineswith high signal-to-noise ratio ( S / N ). Given the observed strictcorrelation between H O and H mid-IR emission, we will firstuse the Spitzer H lines to constrain the H O temperature. TheH O line ratios and absolute intensities will then be used to de-rive the density and column density of the gas and the size ofthe emitting region. The physical parameters derived for H Oare then used for the CO emission and both H O and CO abun-dances with respect to H are estimated. Finally, the emission atthe other shock spots with respect to the excitation conditions inL1448-B2 will be discussed. rotational diagram We used Spitzer H observations by Giannini et al. (2011) andconverted the H intensities, averaged in a 13 ′′ beam, into col-umn densities ( N u ) to construct the H rotational diagram, whichis presented in the upper panel of Fig. 4. The fluxes have not beencorrected for visual extinction, since it is only a marginal e ff ect( A V = J transitions that trace gas at T .
400 K,while the high temperature equilibrium value of 3 is reached bythe lines with J larger than 3.The fact that the observed transitions do not align on a singlestraight line on the rotational diagram indicates that gas com-ponents at di ff erent temperatures are present within the spa-
6. Santangelo et al.:
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PACS observations of shocked gas associated with the jets of L1448 and L1157
Fig. 4:
Upper:
Rotational diagram at L1448-B2 for the H emis-sion lines detected with Spitzer by Giannini et al. (2011). Thevalues have been derived from the H fluxes integrated over a13 ′′ area for comparison with the PACS data. The black dotsare the observed values, whereas the empty dot represents theH S(1) line corrected for an ortho-to-para ratio equal to 1 (seeGiannini et al. 2011). The green solid line represents the linearfit to the S(0)–S(3) H lines, while the green dotted line is the fitobtained using the S(0)–S(2) H lines. Finally, the blue line is thelinear fit to the S(3)–S(7) lines. The resulting parameters ( T , N )or range of parameters of the linear fits are reported in the dia-gram. Lower: H rotational diagram at L1157-R. The symbolsare the same as in the upper panel.tial resolution element or along the line of sight. In particu-lar, two temperature components can be identified in the dia-gram, in the assumption of LTE conditions: a warm componentat T in the range ∼ −
450 K, where the uncertainty de-pends on the lines considered for the fit, i.e. the S(0)–S(2) orthe S(0)–S(3) lines, and a hot component at T ∼ column density is Fig. 5: Top : Ratio between H O 2 − (179 µ m) and 3 − (174 µ m) emission, as a function of T for two values of H den-sity ( n H = × cm − and 5 × cm − ) and two values ofH O column density ( N H O = × and 4 × cm − ) ofL1448-B2. The shaded band highlights the H O ratio observedwith PACS and the arrows indicate that this ratio can be con-sidered an upper limit (see text for details).
Middle and
Bottom :Ratios between H O 3 − (174 µ m) and 3 − (138.5 µ m)emission and between H O 3 − (174 µ m) and 4 − (125.4 µ m) emission, respectively, as a function of T . The sym-bols are the same as in the top panel.
7. Santangelo et al.:
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PACS observations of shocked gas associated with the jets of L1448 and L1157 N (H ) ∼ . − − . × cm − for the warm component and N (H ) ∼ . × cm − for the hot component. In conclusion,the properties of the hot component are well defined from theH rotational diagram, while a slightly larger range of parame-ters can be associated with the warm component. O emission
Previous HIFI observations in L1448-B2 of H O lines with exci-tation energies E u ranging from 53 to 137 K (Santangelo et al.2012) are consistent with very dense gas with n (H ) ∼ cm − and T =
450 K and with moderate H O column densities of ∼ cm − . The bulk of the HIFI H O emission can be thusassociated with the warm component identified from the H ro-tational diagram.To analyse the excitation conditions of the H O emission ob-served with PACS, we used the radiative transfer code RADEX(van der Tak et al. 2007) in the plane-parallel geometry, with thecollisional rate coe ffi cients from Dubernet et al. (2006, 2009)and Daniel et al. (2010, 2011), to build a grid of models withdensity ranging between 10 and 10 cm − and H O column den-sity between 10 and 10 cm − . We adopted a typical line width ∆ v of 50 km s − (full-width at zero intensity, FWZI), from thespectrally resolved HIFI observations of H O (Santangelo et al.2012). An uncertainty on the assumed line width value translatesinto an uncertainty on the H O column density determination,since the H O line ratios depend on the ratio N (H O) /∆ v . Anortho-to-para ratio equal to 3 was assumed, as implied by theHIFI observations of the warm component.Figure 5 shows the ratios between H O lines observed withPACS (having higher excitation than those observed with HIFI)as a function of temperature, for two values of H density andH O column density. The warm component at T ∼
450 K doesnot reproduce the PACS H O line ratios. In particular, the toppanel shows the 179 µ m / µ m line ratio. The arrows in the plotindicate that the observed ratio (shaded band) is an upper limit,since the H O 2 − line ( E u ∼
114 K) is more contaminatedthan the H O 3 − line ( E u ∼
197 K) by the warm gas com-ponent traced by the lower excitation H O lines observed withHIFI. The low H O column density N (H O) ∼ × cm − , de-rived for the warm component from the HIFI H O observations,does not reproduce the 179 µ m / µ m line ratio. This indicatesthe presence of an additional gas component with respect to thewarm one traced by the bulk of the HIFI observations. In par-ticular, a hot component with temperature higher than 600 Kand H O column density larger than a few 10 cm − is requiredto reproduce the higher excitation H O emission observed withPACS.This evidence suggests that the bulk of the H O emission ob-served with PACS is associated with the hot component whichis seen in the H rotational diagram. Assuming a temperature T = rotational diagram(see Fig. 4) and H density n (H ) ≥ cm − , as obtained byGiannini et al. (2011), the density and column density of thiscomponent are derived by fitting the intensity of all the PACSlines with excitation energy level E u &
190 K and varying n (H ), N (H O) and the size of the emitting region θ . Table 2 summa-rizes the results of the fit: unlike the warm component, the emis-sion region of the hot component should be compact (a few arc-sec). In particular, assuming an emitting size of 1 arcsec, the hot The ortho-H O 3 − line with E u =
249 K was not detected inB2 (see Santangelo et al. 2012), therefore the H O line with the highestenergy used for the fit was the para-H O 2 − line with E u =
137 K. component requires a density n (H ) ∼ × − × cm − and column density N (H O) ∼ × − × cm − . The ob-tained column densities correspond to moderately optically thicklines (the optical depths of the H O lines are lower than ∼ µ m line).A visualization of the obtained results is presented in Fig. 6,which shows the two separate models for the H O emission, i.e.the warm and hot components, and the sum of the two modelsin red. The fluxes predicted by the models have been correctedfor the filling factors (using θ / ( θ + θ )) obtained byassuming the emitting size derived from the excitation analysis.Except for the H O 2 − line at 108.1 µ m, which is over-estimated by a factor of 2.5, and the 2 − line at 752 GHz,which is under-estimated by a factor of 2, all H O lines are wellreproduced by the two-component model.We note that this model predicts that the hot component seenin L1448-B2 should contribute very little to the emission of thelow-excitation H O lines observed with HIFI (Santangelo et al.2012) and indeed these lines show very similar profiles with noclear evidence of variations in shape with excitation. However,the HIFI observations of L1448-R4 and L1157-R show a di ff er-ent trend, with high velocity gas preferentially associated withthe low-excitation lines (see Santangelo et al. 2012, Vasta et al.2012). Nevertheless, in these cases geometrical e ff ects related tothe presence of bow shocks and self-absorption by cold H O gasin the lines at lower excitation may contribute to modifying theline profiles.Finally, Fig. 7 presents the H O rotational diagram, withthe flux predictions from the two separate models for H O andfrom their sum. The models identify two gas components in therotational diagram, with the blue one (associated with the hotcomponent) showing more scatter than the green one (associ-ated with the warm component), because of the larger associ-ated optical depths. However, when the sum of the two separatemodels is considered, the two temperature components are nolonger discernible. The total rotational ladder shows a single-temperature aspect, although the large scatter suggests that sub-thermal excitation and optical depth e ff ects are significant. Therotational temperature obtained from a single-temperature fit is ∼
50 K, which is within the range of rotational temperatures ob-tained for low-mass Class 0 protostars (e.g. Herczeg et al. 2012,Goicoechea et al. 2012, Karska et al. 2013).
Figure 8 shows the CO rotational diagram obtained by convert-ing the fluxes of the high- J CO lines observed with PACS andthe CO(3 −
2) line observed with the JCMT telescope (beamsize equal to 14 ′′ ) into column densities ( N u ). A global fit tothe PACS CO lines reveals a gas with rotational temperature T ∼
290 K and CO column density, averaged in the 12 . ′′ N (CO) ∼ cm − . This is consistent with the warmgas component identified from the H rotational diagram andassociated with the bulk of the H O emission observed withHIFI. Although only four CO lines have been detected withPACS, a hint of a possible curvature occurs at excitation ener-gies E u ≥ −
23) transition lies abovethe straight line followed by the other PACS lines. If we assumethat this line comes from a di ff erent gas component, a slightlylower temperature T ∼
240 K is found from the lower excitationPACS lines and correspondingly N (CO) ∼ × cm − . Thesame diagram shows that the CO(3 −
2) line lies well above theother CO lines, which is consistent with its origin in a colder gas.
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Table 2: Summary of the best-fit models derived for the two gas components at the L1448-B2 position.
Comp.
T n (H ) N (H O) Θ N (CO) [H O] / [H ] a [CO] / [H ] a (K) (cm − ) (cm − ) (arcsec) (cm − )Warm b
450 10 ×
17 3 × − × − − × − Hot 1100 (0 . − × (0 . − × ∼ . − × (0 . − . × − (1 − × − Notes. ( a ) The H O and CO abundances of each gas component are obtained from the H O and CO column densities after correcting for therelative beam filling factor. ( b ) See the B2-2 model from Santangelo et al. (2012), shown in their Table 2. The H O column density is, however,slightly di ff erent because it has been derived using the collisional rate coe ffi cients from Dubernet et al. (2006, 2009) and Daniel et al. (2010, 2011). Fig. 6: Comparison between the observed H O fluxes (black dots) and the two best-fit models for L1448-B2, which are given inTable 2: the green model is the fit to the HIFI H O lines and the two blue models are the extremes of the obtained density rangethat fits the PACS H O lines (squares represent n (H ) = × cm − and circles represent n (H ) = × cm − ). The red modelrepresents the sum of the fluxes predicted for each line by the green and the two blue models. The fluxes predicted by the modelshave been corrected for the relative predicted filling factors. Calibration uncertainties of 20% for the HIFI data and 30% for thePACS data have been assumed. The open triangle represents the upper limits of the HIFI H O 3 − line ( E u =
249 K).The presence of multiple excitation temperature compo-nents in the CO emission has been found by other stud-ies of CO ladders in low-mass Class 0 protostars and theiroutflows (see e.g. van Kempen et al. 2010b, Benedettini et al.2012, Goicoechea et al. 2012, Herczeg et al. 2012, Yıldız et al.2012, 2013, Karska et al. 2013, Manoj et al. 2013). In particular,Karska et al. (2013) present CO rotational diagrams for a largesample of protostars, showing two distinct components, a warmcomponent with T rot ∼
300 K and a hot component with T rot ∼
700 K, in addition to a cold component with T rot ∼
100 K, ob-served in the J .
14 lines (Goicoechea et al. 2012, Yıldız et al.2012, 2013). They found the break between warm and hot gasin the CO diagrams around E u ∼ ff erent components in our PACS CO data, a warm and a hot component, is probably valid and may reflect true di ff erences inthe excitation conditions of the gas traced by the di ff erent rangesof CO transitions in Class 0 sources.We can then use the physical conditions derived for the warmand hot H O components to verify whether they are able toreproduce our PACS CO observations. The comparison is pre-sented in Fig. 9. In particular, we used the temperature and den-sity derived from the H O analysis to fit the CO line ratios,normalizing the warm component to the CO(16 −
15) line andderiving the CO column density of the hot component so thatthe sum of the two components (warm plus hot) reproduced theobserved CO fluxes and upper limits. The CO column densi-ties derived in this fashion are reported in the last column ofTable 2. The two blue models in Fig. 9 for the hot gas com-
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Fig. 7: H O rotational diagram at L1448-B2. Calibration uncertainties of 20% for the HIFI data and 30% for the PACS data havebeen assumed. The predictions of the two best-fit models for L1448-B2, corrected for the relative predicted filling factors, are shown(see Table 2 and Fig. 6). Symbols are as in Fig. 6.ponent represent the extremes of the density range obtainedfrom the H O excitation analysis (see Table 2). We obtained N (CO) = (1 . − × cm − for the hot gas component and N (CO) = × cm − for the warm component, both averagedover the relative emitting size.To summarize, the CO and H O line ratios trace two gascomponents, a warm gas component at T ∼
450 K (with n (H ) = cm − ), which is visible in the H O emission with E u = −
137 K and the PACS CO data up to J = −
21, and a hot gascomponent at T ∼ n (H ) = (0 . − × cm − ),which is traced by the H O observations with E u >
190 K andthe PACS higher- J CO emission. These two gas components areassociated with a warm and hot component, respectively, in theSpitzer mid-IR H emission. O and CO abundance ratios
A direct estimate of the H O and CO abundances with respectto H for both gas components can be obtained by comparingthe column density of these species, averaged over a 13 ′′ beam.We find an [H O] / [H ] abundance ratio of (3 − × − forthe warm component and (0 . − . × − for the hot com-ponent. The inferred H O abundances are much higher than thetypical value of ∼ − − − , which is found in cold inter-stellar clouds (e.g. Caselli et al. 2010). However, even for thehot component, this is lower than ∼ − , which is the valueexpected in hot shocked gas (e.g. Kaufman & Neufeld 1996,Flower & Pineau Des Forˆets 2010). The derived H O abun-dances for the warm component are consistent with the val- ues obtained by Santangelo et al. (2012), Vasta et al. (2012),Nisini et al. (2013) from HIFI velocity-resolved observations. Inparticular, Nisini et al. (2013), from the analysis of H O 2 − and 1 − maps of L1448, derived a relatively constant waterabundance along the outflow of about (0 . − × − , with anincrease by roughly one order of magnitude at the protostar posi-tion. Similarly, low H O abundances in the warm gas have beenderived in other outflows by several authors (e.g. Bjerkeli et al.2012, Tafalla et al. 2013).On the other hand, we derive a [CO] / [H ] abundance of(3 − × − for the warm component and (1 − × − forthe hot component. The derived [CO] / [H ] abundances do notdepend on the emitting size, because both CO and H lines areoptically thin; therefore, their absolute intensity depends on thebeam diluted column density.Our data suggest that the CO abundance is lower by a factorfrom 3 to 10 with respect to the canonical value of 2 . × − measured for dense interstellar clouds (e.g. Lacy et al. 1994).Shocks that are non-dissociative, like those implied by ourmolecular observations (see Sect. 5), are not expected to al-ter the original CO / H abundance ratio in the cloud. We pointout however that a CO abundance less than the canonical valuehas been recently measured in di ff erent environments, includ-ing the inner envelopes of low- and intermediate-mass protostars(e.g. Yıldız et al. 2010, 2012, Fuente et al. 2012) and toward theOrion region (Wilson et al. 2011), which indicates that such lowvalues are indeed not peculiar to the considered shock regions.By comparing the H O and CO column densities, we find a[H O] / [CO] abundance ratio of 0.1 for the warm and 0 . − . O abundance,
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Fig. 8: Rotational diagram at L1448-B2 for the CO emission lines (both the detections and the non-detections) observed with PACS(in a 12 . ′′ −
2) line (empty symbol; beam size equal to 14 ′′ ). Calibration uncertainties of 30% have beenassumed. The solid line represents the linear fit to the four detected CO lines, the dotted line the linear fit only to the three lowerexcitation CO lines. As in Fig. 7, the predictions of the two best-fit models for L1448-B2 corrected for the relative predicted fillingfactors are shown.based on the assumption that the CO abundance with respectto H is equal to 10 − , would lead to higher values for the hotcomponent (about 1 −
13 10 − ). For this reason our obtainedH O abundance values are di ff erent from those obtained pre-viously from ISO observations (e.g. Nisini et al. 1999, 2000,Giannini et al. 2001): our analysis points to a CO abundancewith respect to H lower than the standard value of 10 − for thehot component and correspondingly to a lower H O abundance.
According to the excitation analysis, di ff erent sizes are associ-ated with the two H O gas components: the warm gas is foundto be rather extended (17 ′′ ), while the hot gas should be compact( < ′′ ). Based on our model, from Fig. 6 we expect the contribu-tion of the warm component to the total H O flux at 179 µ m tobe similar or stronger than that of the hot component, whereasat 174 µ m the hot component dominates the H O emission withlittle contribution from the warm component. One way of study-ing the spatial extent of the two components and verifying theresults obtained from our analysis is to use the maps of thesetwo H O lines (179 and 174 µ m), which are also the strongestH O lines we detected with PACS, and analyse the relative con-tribution of the two predicted components from their line ratio.In particular, we expect the ratio between the 179 µ m and the174 µ m H O lines to increase going from the central position outwards, thanks to the dominant contribution of the compactcentral component to the H O 174 µ m flux.Figure 10 presents the ratio between the PACS maps of thetwo H O lines (at 179 µ m and 174 µ m). As predicted by our exci-tation analysis, the H O line ratio increases going from the cen-tre of the map toward the edges in both directions along the out-flow. This result supports the scenario in which two gas compo-nents coexist: a compact component which dominates the H Oemission above E u ∼
190 K and an extended component thatdominates the H O emission at lower excitation energies.
At the other selected shock spots a detailed analysis like the onewe performed for L1448-B2 is precluded because of the smallernumber of lines, the lower S / N of the detections, and the lackof H data that would allow us to get a direct measure of thewater abundance. The only other position, among the selectedones, where Spitzer spectroscopic data are available is L1157-R(Nisini et al. 2010b). Therefore, at this position an estimate ofthe water abundance can be obtained in a similar fashion.The rotational diagram, constructed in L1157-R from theSpitzer mid-IR data (see lower panel of Fig. 4), shows once morethe presence of two gas components: a warm component at atemperature of about T ∼
420 K and a hot component with T ∼ −
850 K. The break is found at approximately 2000 K. The
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Fig. 9: The same comparison presented in Fig. 6, but for the CO fluxes measured with PACS toward L1448-B2 (both the detectionsand the non-detections). The two blue models represent the extremes of the density range derived from the H O excitation analysis.Fig. 10: Ratio between the H O 2 − map at 179 µ m and the3 − map at 174 µ m for L1448-B2. The ratio is shown onlyabove a 5 σ detection level in both maps. The crosses representthe pointing of the 25 spaxels.corresponding H column densities are N (H ) ∼ . × cm − for the warm component and N (H ) ∼ (0 . − . × cm − for the hot component, both averaged over 13 ′′ . As we did for L1448-B2, we assume that the bulk of theH O emission observed with PACS is associated with the hotcomponent identified from the H rotational diagram, and weadopt a RADEX analysis to derive the excitation conditions ofthis hot component. In particular, we derived the H density, theH O column density, and the emitting size by fitting the PACSH O lines with excitation energy level E u &
190 K, under theassumption of a temperature T ∼ −
850 K from the H dataand a line width of 25 km s − from the HIFI observations byVasta et al. (2012).A compact gas component is found to be associated with thebulk of the PACS emission, with n (H ) ∼ (0 . − × cm − and N (H O) ∼ (0 . − × cm − . The obtained excitation condi-tions appear to be similar to those derived for the hot componentat L1448-B2, but a larger uncertainty on the H O column densi-ties is associated with L1157-R. The corresponding abundance,obtained by comparing the H O column densities (corrected forthe relative filling factor) with the H column density, is in therange (0 . − × − (see Sect. 4.1.4 for comparison).For the remaining two shock spots, namely L1448-R4 andL1157-B2, the lack of mid-IR H data in both cases does notallow us to get constraints on the temperature and to estimatethe H O abundance. To investigate the physical and excitationconditions of the hot gas component at these positions, we usedthe L1448-B2 shock position as a template and compared theratios of the detected lines with the relative line ratios observedin L1448-B2. The comparison is presented for all the selectedshock spots in Fig. 11, where all the line ratios are normalizedwith respect to the H O 3 − line at 174 µ m. The observed
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Fig. 11: Line ratios between the H O, CO, [O i ], and OH lines andthe H O 3 − line at 174 µ m at all selected shock positions.1 σ errors are indicated with errorbars.H O line ratios are roughly comparable within the relative errorswith those observed in L1448-B2, within a factor of 2. We canthus conclude that the excitation conditions of the hot gas com-ponent are comparable in all selected shock positions, as alreadydeduced for L1157-R. We note that the bright H O 179 / µ mline ratio at the L1157-B2 shock position may provide evidencefor an older shock with respect to the other selected positions.Because this line ratio is indicative of the relative contribu-tion between the warm and the hot component, the high valueobserved at L1157-B2 may suggest a smaller contribution ofthe hot component relative to the warm component comparedto the other shock positions. This is consistent with this posi-tion being the signpost of an older shock, as already suggestedby previous studies (e.g. Bachiller & Perez Gutierrez 1997,Rodr´ıguez-Fern´andez et al. 2010, Vasta et al. 2012). Assuming aconstant shock propagation velocity, Gueth et al. (1998) deriveda dynamical age for the L1157-B2 shock spot of ∼ . − yr, respectively,Flower & Pineau Des Forˆets 2010). Therefore, in L1157-B2 thehot component has already had the time to cool down to a fewhundred Kelvin.Figure 11 shows that CO / H O line ratios lower by a factorof ∼ O excitation conditions, thiswould suggest a higher H O abundance with respect to L1448-B2, which is in line with the range estimated for L1157-R usingSpitzer mid-IR H data.The L1448-R4 and L1157-B2 shock positions thus appear tobe more similar to L1157-R than to L1448-B2 in terms of H Oabundance. This conclusion is supported by the PACS detectionat L1448-B2 of OH and brighter [O i ] emission (see Fig. 11),which suggests either that not all oxygen has been converted intoH O or that water is partially dissociated. Indeed, the L1448- Fig. 12: Optically thin [O i ]63 µ m / [O i ]145 µ m flux ratios as afunction of temperature are shown in dotted lines for collisionswith atomic hydrogen H and in solid lines for collisions withmolecular hydrogen H (as in Liseau et al. 2006). The loga-rithms of the density (in cm − ) are indicated for each curve. Thebroken line outlines the ratio of optically thick lines. Observedline ratios are depicted by the shaded areas for the L1448-B2 po-sition and the L1157-R position. The data have been smoothedto a common angular resolution of 12 . ′′ ff ected by the strong UVradiation field coming from the central protostar or from dis-sociative internal jet shocks (e.g. Hollenbach & McKee 1989,van Kempen et al. 2009), which can photodissociate the freshlyformed H O. i ] ratio It is useful to compare the observed ratio between the [O i ] P − P line at 63.2 µ m and the [O i ] P − P line at 145.5 µ mto infer additional information on the gas excitation conditions(see Liseau et al. 2006). We detected both [O i ] lines only at theL1448-B2 and L1157-R positions (at the latter position the [O i ]line at 145 µ m was detected only at ∼ σ level) and the mea-sured [O i ]63 / µ m ratios are ∼
20 and ∼
6, respectively. Theobserved line ratios are displayed in Fig. 12, along with the lineratios predicted from the RADEX code assuming optically thinlines, for collisions with atomic hydrogen or with molecular hy-drogen; the predicted line ratio for optically thick lines is alsoshown. We have neglected O excitation due to collisions withelectrons, since it becomes relevant (i.e. it contributes more than10%) only for n ( e ) / n (H) fractions larger than 0.6, clearly in con-trast with the mostly molecular / atomic gas observed in the con-sidered shocks. Thus, assuming optically thin lines excited bycollisions with H , the ratio observed at L1448-B2 is consistentwith a H volume density between 10 and a few 10 cm − and atemperature T &
100 K, which is within the range of parametersderived from our excitation analysis (see Sects. 4.1.2 and 4.1.3).On the other hand, assuming collisions with H, it corresponds to
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PACS observations of shocked gas associated with the jets of L1448 and L1157 n (H) ∼ − cm − and T &
100 K. We can thus distinguishtwo possibilities for the origin of the [O i ] emission at L1448-B2:either H O, CO, and [O i ] emission arise from the same molecu-lar gas with density n (H ) ∼ × cm − , or the [O i ] emissionoriginates in a low-density component of atomic gas. This willbe discussed further in Sect. 5. Instead, the lower line ratio mea-sured at L1157-R is consistent either with n (H ) ∼ − cm − and T &
200 K for optically thin lines excited by collisions withH , or with optically thick lines and temperatures lower than200 K.Finally, in both positions the [O i ] column density averagedover the PACS beam is of the order of 2 − cm − at L1448-B2 and 5 −
10 10 cm − at L1157-R, which is consistent withoptically thin lines.
5. Comparison with shock models
The observed fluxes are compared with the grid provided byFlower & Pineau Des Forˆets (2010) for stationary C- and J-typeshock models. The grid explores a range of shock velocitiesfrom 10 to 40 km s − and two pre-shock densities, 2 × and2 × cm − . In the upper panel of Fig. 13 we present the ob-served flux of the [O i ] P − P line at 63.2 µ m with the shockmodel predictions. Unsmoothed peak line fluxes have been usedto minimize beam dilution e ff ects. At the L1448-B2 positiononly a J-type shock, with velocity v s >
20 km s − for pre-shockdensity n = × cm − and v s >
10 km s − for n = × cm − ,can reproduce the observed flux; C-type shocks under-estimatethis line by at least one order of magnitude. A pre-shock den-sity n = × cm − , and a corresponding shock velocity v s >
10 km s − , are not consistent with the results of the H O andCO excitation analysis (Table 2). From the comparison betweenthis pre-shock density and the maximum post-shock density thatcan be evinced from the [O i ] line ratio (see Sect. 4.3 and Fig. 12),a very small compression factor would be derived. In addition, acomparison with shock models by Hollenbach & McKee (1989)shows that even a lower pre-shock density of 10 cm − and shockvelocity v s &
30 km s − can reproduce our [O i ] data. This sug-gests that at L1448-B2 the [O i ] emission originates in a fast dis-sociative shock with pre-shock density n . × cm − and v s >
20 km s − . The presence of a dissociative shock giving riseto ionizing photons is also supported by the detection at the [O i ]peak of OH at 119 µ m and [Fe ii ] at 26 µ m (see Fig. 2). On theother hand, at the other shock positions we are not able to dis-criminate between C- and J-type shocks. The observations areconsistent either with a low-velocity ( v s <
20 km s − ) C-typeshock or with a J-type shock with velocity v s >
20 km s − for n = × cm − and v s >
10 km s − for n = × cm − .A comparison of the observed CO and H O emission withshock models is presented in the lower panel of Fig. 13 for theL1448-B2 and L1448-R4 shock positions. At the L1448-B2 po-sition the CO and H O emissions are also consistent with a J-type shock having a pre-shock density n = × cm − , but at alower shock velocity ( .
20 km s − ) with respect to the [O i ] emis-sion. According to Flower & Pineau Des Forˆets (2010), a highcompression factor of about 100 is predicted for a low-velocity( v s .
20 km s − ) J-type shock, as observed at this shock position.This corresponds to a post-shock density of about 2 × cm − ,which is within the range of post-shock density derived from theH O and CO excitation analysis (Table 2) for the hot gas compo-nent. Once more, the plot highlights that the physical conditionsat this shock position are di ff erent with respect to the other se-lected shock spots. In particular, at the L1448-R4 position theobservations are consistent with a C-type shock with pre-shock Fig. 13: Upper panel : Comparison between the [O i ] P − P (63.2 µ m) flux observed at the investigated shock positions(shaded bands) and the corresponding theoretical values pre-dicted by the Flower & Pineau Des Forˆets (2010) shock modelsfor C-type shocks (magenta) and J-type shocks (blue), as a func-tion of the shock velocity in units of km s − . The fluxes mea-sured in the central spaxel of the PACS maps have been usedwithout smoothing to a common angular resolution. Calibrationuncertainties of 30% have been assumed. The arrows in the plotindicate that the absolute fluxes have to be considered as lowerlimits; they are beam diluted because we do not resolve the emit-ting size of the shock. Lower panel : Same comparison as in theupper panel, but for CO and H O line ratios. The observed valuesare depicted as black dots and the errorbars represent 1 σ errors.The data have been smoothed to a common angular resolution of12 . ′′ n = × cm − and velocity larger than 20 km s − , incontrast with the predictions from C-type shock models for the[O i ] emission at the same shock position. A lower compressionfactor with respect to L1448-B2, in the range 2 . −
25, is sug-gested at L1448-R4 from the comparison between the derived
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Table 3: Origin of the emission observed with PACS.
Position [O i ] & OH H O & high- J COL1448-B2 J-type shock ( v s >
20 km s − , n . × cm − ) J-type shock ( v s .
20 km s − , n = × cm − )L1448-R4 J-type shock ( v s >
10 km s − ) C-type shock ( v s >
20 km s − , n = × cm − ) pre-shock densities and the post-shock densities obtained fromthe excitation analysis. This is consistent with the proposed sce-nario in which CO and H O emissions are produced in a C-typeshock.In conclusion (see Table 3 for a summary), our analysis sug-gests that at the L1448-B2 shock position the H O and COemissions are produced in a low-velocity non-dissociative J-type shock along the outflow cavity walls, whereas the [O i ]and maybe the OH emission originate in a fast dissocia-tive shock. The bright and velocity-shifted [O i ] emission at63 µ m, along with the detection of the [Fe ii ] line and the high[O i ]63 / µ m line ratio ( ∼ O and high- J CO emissions are produced in a C-typeshock with velocity greater than 20 km s − , whereas a partiallydissociative J-type shock is needed to explain the [O i ] emission.As discussed in Sect. 4.2, the H O abundance of the hot gas atthese positions appears to be higher than at L1448-B2 by a factorof ∼
4. This is consistent with L1448-B2 being the signpost ofa J-type shock, in which the predicted H O abundance is of theorder of 2 × − (see Flower & Pineau Des Forˆets 2010).Finally, our results are also consistent with previous HIFIobservations by Santangelo et al. (2012), showing that the H Oline ratios at L1448-B2 are consistent with a non-dissociative J-type shock, with pre-shock density n = cm − . On the otherhand, the authors found that in L1448-R4 the shock conditionsof the low-velocity component, which dominates the emissionin the relatively higher excitation lines, are more degenerate anda C-type shock origin could not be ruled out. The same degen-eracy has been inferred for the two positions along the L1157outflow by Vasta et al. (2012), thus consistent with a possibleC-type shock origin for the H O emission.We point out, however, that any comparison with availableshock models can only be roughly indicative of the real physicalsituation occurring in the investigated shock events. In particular,geometrical complexity as well as chemical e ff ects induced bydi ff use UV fields (both from the star and from associated fastshocks) would need to be properly included in a more realisticmodel.
6. Conclusions
Herschel -PACS observations of H O, high- J CO, [O i ], and OHtoward two selected positions along the bright outflows L1448and L1157 have been presented, as part of the WISH key pro-gram. The main conclusions of this work are the following:1. Consistent with other studies, at all selected shock positionswe find a close spatial association, at the angular resolutionof our PACS observations, between H O emission and high- J CO emission, whereas the low- J CO emission seems totrace a di ff erent gas component, not directly associated withshocked gas. A spatial association is also found betweenH O emission and mid-IR H emission at all selected po-sitions. Moreover, no shift is found at this angular resolution between H O, [O i ], and [Fe ii ] emission, although the H Oemission appears to be more extended than [O i ] and [Fe ii ].2. The excitation conditions at the L1448-B2 shock positionclose to the driving outflow source indicate a two-componentmodel to reproduce the H O and CO emission. In partic-ular, an extended warm component with temperature T ∼
450 K and density n (H ) = cm − is traced by the bulkof the HIFI H O emission ( E u = −
137 K) and bythe PACS CO emission up to J = −
21; furthermore,a compact hot component with T = n (H ) = (0 . − × cm − is traced by the bulk of thePACS higher-excitation H O emission ( E u >
190 K) and bythe PACS higher- J CO emission. A similar stratification ofgas components at di ff erent temperatures has been found forthe Spitzer H gas.3. Among the selected positions L1448-B2 is found to be pecu-liar, possibly because of its proximity to the central drivingsource of the L1448 outflow. In particular, a non-dissociativeJ-type shock at the point of impact of the jet on the cloudseems to be responsible for the H O and CO hot gas com-ponent at this position, whereas a C-type shock is neededto explain the origin of the hot component at the other se-lected positions. On the other hand, the observations suggesta dissociative J-type shock at L1448-B2, related to the pres-ence of an embedded atomic jet, to explain the observed OHand [Fe ii ] emission and the bright and velocity-shifted [O i ]emission. A J-type shock that is at least partially dissociativeis needed to explain the [O i ] emission at the other selectedpositions as well.4. From the comparison between H O and H , at L1448-B2we obtain a H O abundance of (3 − × − for the warmcomponent and of (0 . − . × − for the hot component.At the other examined shock positions the H O abundanceof the hot component appears to be higher by a factor of ∼
4, reflecting evolutionary e ff ects on the timescales of theoutflow propagation. The indication that the H O abundancemay be higher in the hotter gas in some shock positions isin line with ISO data by other authors (e.g. Giannini et al.2001). This result is also consistent with L1448-B2 beingcloser to the driving outflow source than the other selectedpositions. This makes it more a ff ected by the strong FUVradiation field coming from the nearby protostar that mayphotodissociate H O in the post-shock gas and thus decreasethe H O abundance. An estimate of the CO abundance wasalso derived at L1448-B2 and is of the order of (3 − × − for the warm component, whereas it is (1 − × − for thehot component.5. These results, along with the spatial extent inferred for thedi ff erent gas components, lead us to the conclusion that thetwo gas components represent a gas stratification in the post-shock region. In particular, the extended and low-abundancewarm component traces the post-shocked gas that has al-ready cooled down to a few hundred Kelvin, whereas thecompact and possibly more abundant hot component is as-sociated with the gas that is currently undergoing a shockepisode, being compressed and heated to a thousand Kelvin.This hot gas component is thus possibly a ff ected by evo-
15. Santangelo et al.:
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PACS observations of shocked gas associated with the jets of L1448 and L1157 lutionary e ff ects on the timescales of the outflow propaga-tion, which explains the variations of H O abundance weobserved at the di ff erent positions along the outflows. Acknowledgements.
WISH activities at Osservatorio Astronomico di Roma aresupported by the ASI project 01 / / /
0. G.S. and B.N. also acknowledge fi-nancial contribution from the agreement ASI-INAF I / / /
0. Astrochemistryin Leiden is supported by NOVA, by a Spinoza grant and grant 614.001.008 fromNWO, and by EU FP7 grant 238258. HIFI has been designed and built by a con-sortium of institutes and university departments from across Europe, Canadaand the United States under the leadership of SRON Netherlands Institutefor Space Research, Groningen, The Netherlands and with major contribu-tions 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, OsservatorioAstrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK;Spain: Observatorio Astron´omico Nacional (IGN), Centro de Astrobiolog´ıa(CSIC-INTA). Sweden: Chalmers University of Technology - MC2, RSS &GARD; Onsala Space Observatory; Swedish National Space Board, StockholmUniversity - Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA:Caltech, JPL, NHSC.
References
Bachiller, R., Martin-Pintado, J., Tafalla, M., Cernicharo, J., & Lazare ff , B. 1990,A&A, 231, 174Bachiller, R., Guilloteau, S., Dutrey, A., Planesas, P., & Martin-Pintado, J. 1995,A&A, 299, 857Bachiller, R., & Perez Gutierrez, M. 1997, ApJ, 487, L93Bachiller, R., P´erez Guti´errez, M., Kumar, M. S. N., & Tafalla, M. 2001, A&A,372, 899Benedettini, M., Viti, S., Giannini, T., et al. 2002, A&A, 395, 657Benedettini, M., Busquet, G., Lefloch, B., et al. 2012, A&A, 539, L3Bergin, E. A., Neufeld, D. A., & Melnick, G. J. 1998, ApJ, 499, 777Bjerkeli, P., Liseau, R., Olberg, M., et al. 2009, A&A, 507, 1455Bjerkeli, P., Liseau, R., Nisini, B., et al. 2011, A&A, 533, A80Bjerkeli, P., Liseau, R., Larsson, B., et al. 2012, A&A, 546, A29Caselli, P., Keto, E., Pagani, L., et al. 2010, A&A, 521, L29Ceccarelli, C., Caux, E., White, G. J., et al. 1998, A&A, 331, 372Codella, C., Lefloch, B., Ceccarelli, C., et al. 2010, A&A, 518, L112Codella, C., Ceccarelli, C., Lefloch, B., et al. 2012a, ApJ, 757, L9Codella, C., Ceccarelli, C., Lefloch, B., et al. 2012b, ApJ, 759, L45Daniel, F., Dubernet, M.-L., Pacaud, F., & Grosjean, A. 2010, A&A, 517, A13Daniel, F., Dubernet, M.-L., & Grosjean, A. 2011, A&A, 536, A76Davis, C. J., & Smith, M. D. 1995, ApJ, 443, L41de Graauw, T., et al. 2010, A&A, 518, L6Dubernet, M.-L., Daniel, F., Grosjean, A., et al. 2006, A&A, 460, 323Dubernet, M.-L., Daniel, F., Grosjean, A., & Lin, C. Y. 2009, A&A, 497, 911Dutrey, A., Guilloteau, S., & Bachiller, R. 1997, A&A, 325, 758Eisl¨o ff el, J. 2000, A&A, 354, 236Flower, D. R., & Pineau Des Forˆets, G. 2010, MNRAS, 406, 1745Franklin, J., Snell, R. L., Kaufman, M. J., Melnick, G. J., Neufeld, D. A.,Hollenbach, D. J., & Bergin, E. A. 2008, ApJ, 674, 1015Fuente, A., Caselli, P., McCoey, C., et al. 2012, A&A, 540, A75Giannini, T., Nisini, B., & Lorenzetti, D. 2001, ApJ, 555, 40Giannini, T., Nisini, B., Neufeld, D., et al. 2011, ApJ, 738, 80Goicoechea, J. R., Cernicharo, J., Karska, A., et al. 2012, A&A, 548, A77Gueth, F., Guilloteau, S., & Bachiller, R. 1998, A&A, 333, 287Guilloteau, S., Bachiller, R., Fuente, A., & Lucas, R. 1992, A&A, 265, L49Herczeg, G. J., Karska, A., Bruderer, S., et al. 2012, A&A, 540, A84Hirano, N., Ho, P. P. T., Liu, S.-Y., et al. 2010, ApJ, 717, 58Hirota, T., Honma, M., Imai, H., et al. 2011, PASJ, 63, 1Hollenbach, D., & McKee, C. F. 1989, ApJ, 342, 306Karska, A., Herczeg, G. J., van Dishoeck, E. F., et al. 2013, A&A, 552, A141Kaufman, M. J., & Neufeld, D. A. 1996, ApJ, 456, 611Kristensen, L. E., van Dishoeck, E. F., Tafalla, M., et al. 2011, A&A, 531, L1Kristensen, L. E., van Dishoeck, E. F., Bergin, E. A., et al. 2012, A&A, 542, A8Lacy, J. H., Knacke, R., Geballe, T. R., & Tokunaga, A. T. 1994, ApJ, 428, L69Lefloch, B., Cabrit, S., Codella, C., et al. 2010, A&A, 518, L113Lefloch, B., Cabrit, S., Busquet, G., et al. 2012, ApJ, 757, L25Liseau, R., et al. 1996, A&A, 315, L181Liseau, R., Justtanont, K., & Tielens, A. G. G. M. 2006, A&A, 446, 561Manoj, P., Watson, D. M., Neufeld, D. A., et al. 2013, ApJ, 763, 83Neufeld, D. A., & Dalgarno, A. 1989, ApJ, 340, 869Neufeld, D. A., Nisini, B., Giannini, T., et al. 2009, ApJ, 706, 170Nisini, B., Benedettini, M., Giannini, T., et al. 1999, A&A, 350, 529Nisini, B., Benedettini, M., Giannini, T., et al. 2000, A&A, 360, 297 Nisini, B., Benedettini, M., Codella, C., et al. 2010a, A&A, 518, L120Nisini, B., Giannini, T., Neufeld, D. A., et al. 2010b, ApJ, 724, 69Nisini, B., Santangelo, G., Antoniucci, S., et al. 2013, A&A, 549, A16Ott, S. 2010, Astronomical Data Analysis Software and Systems XIX, 434, 139Pilbratt, G. L., et al. 2010, A&A, 518, L1Poglitsch, A., Waelkens, C., Geis, N., et al. 2010, A&A, 518, L2Rodr´ıguez-Fern´andez, N. J., Tafalla, M., Gueth, F., & Bachiller, R. 2010, A&A,516, A98Santangelo, G., Nisini, B., Giannini, T., et al. 2012, A&A, 538, A45Tafalla, M., Liseau, R., Nisini, B., et al. 2013, A&A, 551, A116Tobin, J. J., Looney, L. W., Mundy, L. G., Kwon, W., & Hamidouche, M. 2007,ApJ, 659, 1404van der Tak, F. F. S., Black, J. H., Sch¨oier, F. L., Jansen, D. J., & van Dishoeck,E. F. 2007, A&A, 468, 627van Dishoeck, E. F., Kristensen, L. E., Benz, A. O., et al. 2011, PASP, 123, 138van Kempen, T. A., van Dishoeck, E. F., G¨usten, R., et al. 2009, A&A, 507, 1425van Kempen, T. A., Kristensen, L. E., Herczeg, G. J., et al. 2010a, A&A, 518,L121van Kempen, T. A., Green, J. D., Evans, N. J., et al. 2010b, A&A, 518, L128Vasta, M., Codella, C., Lorenzani, A., et al. 2012, A&A, 537, A98Visser, R., Kristensen, L. E., Bruderer, S., et al. 2012, A&A, 537, A55Wilson, T. L., Muders, D., Dumke, M., Henkel, C., & Kawamura, J. H. 2011,ApJ, 728, 61Yıldız, U. A., van Dishoeck, E. F., Kristensen, L. E., et al. 2010, A&A, 521, L40Yıldız, U. A., Kristensen, L. E., van Dishoeck, E. F., et al. 2012, A&A, 542, A86Yıldız, U. A., Kristensen, L. E., van Dishoeck, E. F., et al. 2013, submitted
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Appendix A: PACS maps
The PACS maps of all the lines observed at the four selectedshock positions (B2 and R4 along L1448 and B2 and R alongL1157) are presented in this section (see also Table 1) . In par-ticular, Figs. A.1 and A.2 are relative to the L1448 outflow (B2and L1157 positions, respectively), whereas Figs. A.3 and A.4concern the L1157 outflow (B2 and R positions, respectively). . Santangelo et al.:
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Fig. A.1: PACS spectra of the detected transitions at the L1448-B2 position. The centre of each spaxel box corresponds to its o ff setposition with respect to the coordinates of the central shock position. The velocity (km s − ) and intensity scale (Kelvin) are indicatedfor one spaxel box and refer to all spectra of the relative transition. The labels in the top-right corner of every box indicate the relativetransition. . Santangelo et al.: Herschel
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Fig. A.1 – Continued.. Santangelo et al.:
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Fig. A.1 – Continued.. Santangelo et al.:
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Fig. A.2: Same as Fig. A.1 for the L1448-R4 position. . Santangelo et al.:
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Fig. A.2 – Continued.. Santangelo et al.:
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Fig. A.3: Same as Fig. A.1 for the L1157-B2 position. . Santangelo et al.:
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Fig. A.4: Same as Fig. A.1 for the L1157-R position. . Santangelo et al.:
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