The first detection of thermal water emission from a Herbig-Haro object is presented. The observations were performed with the LWS (Long Wavelength Spectrograph) aboard ISO (Infrared Space Observatory). Besides H2O, rotational lines of CO are present in the spectrum of HH 54. These high-J CO lines are used to derive the physical model parameters of the FIR (far-infrared) molecular line emitting regions. This model fits simultaneously the observed OH and H2O spectra for an OH abundance X(OH)=10-6 and a water vapour abundance X(H2O)=10-5. 



At a distance of 250pc, the total CO, OH and H2O rotational line cooling rate is estimated to be 1.3x10-2 L⊙, which is comparable to the mechanical luminosity generated by the 10km s-1 shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone

Ade, P.

Armand, C.

Burgdorf, M.

Caux, E.

Ceccarelli, C.

Cerulli, R.

Church, S.

Clegg, P. E.

Digorgio, A.

Furniss, I.

Giannini, T.

Glencross, W.

Gry, C.

King, K.

Larsson, B.

Lim, T.

Liseau, R.

Lorenzetti, D.

Molinari, S.

Naylor, D.

Nisini, B.

Orfei, R.

Saraceno, P.

Sidher, S.

Smith, H.

Spinoglio, L.

Swinyard, B.

Texier, D.

Tommasi, E.

Trams, N.

Unger, S.

White, G. J.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThermal H2O emission from the Herbig-Haro flow HH54Journal ItemHow to cite:Liseau, R.; Ceccarelli, C.; Larsson, B.; Nisini, B.; White, G. J.; Ade, P.; Armand, C.; Burgdorf, M.; Caux, E.;Cerulli, R.; Church, S.; Clegg, P. E.; Digorgio, A.; Furniss, I.; Giannini, T.; Glencross, W.; Gry, C.; King, K.; Lim, T.;Lorenzetti, D.; Molinari, S.; Naylor, D.; Orfei, R.; Saraceno, P.; Sidher, S.; Smith, H.; Spinoglio, L.; Swinyard, B.;Texier, D.; Tommasi, E.; Trams, N. and Unger, S. (1996). Thermal H2O emission from the Herbig-Haro flow HH 54.Astronomy & Astrophysics, 315(2) L181-L184.For guidance on citations see FAQs.c© 1996 European Southern ObservatoryVersion: Version of RecordLink(s) to article on publisher’s website:http://cdsads.u-strasbg.fr/abs/1996A%26A...315L.181LCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukAstron. Astrophys. 315, L181–L184 (1996) ASTRONOMYANDASTROPHYSICSThermal H2O emission from the Herbig-Haro flow HH 54?R. Liseau1;2, C. Ceccarelli3;2, B. Larsson1, B. Nisini2, G.J. White4, P. Ade4, C. Armand5, M. Burgdorf5, E. Caux6,R. Cerulli2, S. Church7, P.E. Clegg4, A. Di Gorgio5;2, I. Furniss8, T. Giannini2, W. Glencross8, C. Gry5;9, K. King10, T. Lim5,D. Lorenzetti11, S. Molinari5;2, D. Naylor12, R. Orfei2, P. Saraceno2, S. Sidher5, H. Smith4, L. Spinoglio2, B. Swinyard10,D. Texier5, E. Tommasi2, N. Trams5, and S. Unger101 Stockholm Observatory, S-133 36 Saltsjo¨baden, Sweden, (rene@astro.su.se)2 CNR-Istituto di Fisica dello Spazio Interplanetario, Casella Postale 27, I-00044 Frascati (Rome), Italy3 Laboratoire d’Astrophysique de l’Observatoire de Grenoble, 414, rue de la Piscine, BP 53, F-38041 Grenoble, France4 Physics Department, Queen Mary & Westfield College, University of London, Mile End Road, London E1 4NS, UK5 The Lws Instrument-Dedicated Team, Iso Science Operations Centre, P.O. Box 50727, E-28080 Madrid, Spain6 Centre d’Etude Spatiale des Rayonnements, BP 4346, F-31029 Toulouse Cedex, France7 California Institute of Technology, Pasadena, CA 91125, USA8 Department of Physics and Astronomy, University College London, Gower Street, London, WC1 E 6BT, UK9 Laboratoire d’Astronomie Spatiale, BP8, F-13376 Marseille Cedex 12, France10 Space Science Department, Rutherford Appleton Laboratory, Chilton, Oxon OX11 OQX, UK11 Osservatorio Astronomico di Roma, I-00044 Monteporzio, Italy12 Department of Physics, University of Lethbridge, Lethbridge, Alberta T1K 3M4, CanadaReceived 2 July 1996 / Accepted 20 August 1996Abstract. The first detection of thermal water emission froma Herbig-Haro object is presented. The observations were per-formed with the Lws (Long Wavelength Spectrograph) aboardIso (Infrared Space Observatory). Besides H2O, rotational linesof CO are present in the spectrum of HH 54. These high-J COlines are used to derive the physical model parameters of theFir (far-infrared) molecular line emitting regions. This modelfits simultaneously the observed OH and H2O spectra for anOH abundance X(OH) = 10 6 and a water vapour abundanceX(H2O) = 10 5.At a distance of 250 pc, the total CO, OH and H2O rota-tional line cooling rate is estimated to be 1.3 10 2 L, whichis comparable to the mechanical luminosity generated by the10 km s 1 shocks, suggesting that practically all of the cool-ing of the weak-shock regions is done by these three molecularspecies alone.Key words: stars: formation – ISM: molecules – ISM: jets andoutflows – ISM: individual objects: HH 54 – physical processes:shock waves – physical processes: radiative transfer? Based on observations with Iso, an Esa project with instrumentsfunded by Esa Member States (especially the PI countries: France,Germany, the Netherlands and the United Kingdom) and with the par-ticipation of Isas and Nasa (see: Kessler et al. 1996).1. IntroductionThe Herbig-Haro object HH 54 is associated with the star form-ing dark cloud Cha ii, at a distance d = 250 pc, although this isuncertain by as much as >100 pc. The object consists of sev-eral arcsecond scale bright knots enveloped by diffuse emission( 4000; Graham & Hartigan 1988). The overall optical emissionline spectrum is indicative of the presence of relatively highlyexcited gas and can be explained with J-shocks of velocitiesof at least 60 km s 1 (Schwartz & Dopita 1980). Considerablylower velocities, of  10 km s 1, are observed in the CO (1–0)emission from a cold ( 15 K), extended molecular flow, whoseemission peaks>30 away from HH 54 (Knee 1992). RecentNir(near-infrared) observations by Gredel (1994) testify to the pres-ence of warm, purely collisionally excited, H2 gas. This ratherhot gas [ 2 000 K, N (H2) = 3.6 1017 cm 2] might be expectedto emit strongly in relatively highly excited rotational lines ofCO and H2O. Many of these transitions fall within the wavebandof the Lws and in this Letter we report on the first detection ofthermally excited H2O emission from an HH flow, as well asobservations of lines of CO and OH.2. ObservationsLws-grating scans (43–196.7m, R = 140   330), wereobtained towards HH 54B (= 12h52m10s6,  = –764000400,equinox 1950.0) during Iso-revolution no. 92 on February 17,1996. The instrument and its capabilities are described by Clegget al. (1996) and by Swinyard et al. (1996) respectively. The ab-L182 R. Liseau et al.: Thermal H2O emission from the Herbig-Haro flow HH 54Fig. 1. Lws spectrum towards HH 54 with emission lines identified.The oversampled data have been rebinned and smoothed with a run-ning mean filter to  = 0:42m. The filled dots refer to Iras data(Psc 2+Lrp+ zodiacal background)solute flux calibration is based on observations of the planetUranus (Swinyard et al.). Full grating scans were also obtainedtowards HH 52, HH 53 and the CO outflow, which was mapped(3 3, 10000 grid). These observations are described by Nisiniet al. (1996).3. ResultsThe Fir spectrum towards HH 54 consists of emission linesand continuous radiation (Fig. 1). The line spectrum of HH 54is dominated by [O i] 63m, as expected for shocked gas. The[O i] 63/145m emission is intrinsic to HH 54 and compatiblewith the predictions of existing J-shock models, whereas theobserved spatial distribution and intensity of the [C ii] 158mline is characteristic of a Pdr at the cloud surface (see: Nisiniet al.).In addition, several, weaker lines are present longward of 100m, coinciding in wavelength with rotational transitionsof CO, H2O and OH. No such emission could be discerned inany of the other spectra of this region. All the line measurementsare given by Nisini et al..The continuum level is consistent with the sum of the zo-diacal and galactic backgrounds and the dark cloud emissiondetermined by Iras. Situated within HH 54 is the Iras pointsource 12522–7640, which has significant flux density only at60m (0:59 0:09 Jy). This flux can probably be ascribed en-tirely to [O i] line emission, for which we find (correspondingto the Iras band) S60 = 0:50 0:03 Jy, i.e. any stellar sourcewould be extremely faint at these wavelengths. We conclude,therefore, that the observed diffuse radiation field is weak, acircumstance that largely simplifies the analysis of the molecu-lar line emission in the following sections.4. Discussion4.1. The CO spectrumCO emission lines from HH 54 were traced from the rotationalupper level J = 14 to J <19. The observed decrease in CO lineflux with J (Fig. 1) implies that the CO emitting gas is not veryhighly excited and, consequently, that the emission feature at113.5m is unlikely to be attributable to CO (23–22).The quite limited spatial ( 8000) andspectral ( 1 500 km s 1) resolution of the observations justi-fied initially only average values of the physical parametersbeing estimated. The observed CO line ratios are reasonablywell fit by a model of HH 54, where the average kinetic tem-perature of the gas is Tkin = (330  30) K and the logarithmicvolume density (in cm 3) is logn(H2) = 5:3 0:2 (Fig. 2). Thehydrogen column density of the model,N (H2) = 5.0 1020cm 2,is larger by three orders of magnitude than that of the hot H2gas. We assume that the relative abundance of carbon monoxidemolecules X(CO) = [N (CO)/N (H2)] = 8 10 5.Further, the absolute flux level of the observed CO linesthen implies that only about fb = (10 2) 1 of the Lws-beamis filled by the emission. Hence, we derive the CO source size 2500 to 3000 (0.03 pc), comparable to the extent of HH 54 atoptical and Nir wavelengths.The excitation and radiative transfer calculations were allperformed in the Sobolev approximation using a Large VelocityGradient code. The EinsteinA values and energies were adoptedfrom Chackerian & Tipping (1983), with collisional excitationrate coefficients, extrapolated beyond J = 20 to J = 25, be-ing taken from Schinke et al. (1985). An intrinsic line width of10 km s 1 was adopted, a choice which will be justified below(sect. 4.3), and all CO transitions were found to be opticallythin. Our model of the CO emission is not particularly sensitiveto the choice of line widths, as long as v > 0:5 km s 1.The total cooling rate in the CO lines from HH 54 is foundto be LCO = 1.0 10 2 L. The thickness of the emitting layeris dr =N (H2)/n(H2) = 2.5 1015 cm and, hence, the mass of themolecular material is 6.8 10 3 M.For the lower rotational transitions, the predictions of ourmodel can be compared to observations obtained from theground. Towards HH 54, spectra of CO (1–0) and (2–1) havebeen obtained with the 15 m Sest (L. Knee, private commu-nication). At 5.0 km s 1 from line centre, the intensities areTmb(1–0) = (0:360:06)K andTmb(2–1) = (0:710:08) K. Thecorresponding values of our model would be somewhat lessthan 0.075 K and 0.82 K for the (1–0) and (2–1) transitions re-spectively. It thus appears that the observed lowest transitionis dominated by the cold component ( 15 K) of the extendedoutflow, whereas the warm gas contributes significantly to the(2–1) line. This assertion could be further tested by (2–1) map-ping and Sest observations in the (3–2) line for which thepredicted on-source radiation temperature is 5.7 K.R. Liseau et al.: Thermal H2O emission from the Herbig-Haro flow HH 54 L183Fig. 2. Observed CO line fluxes towards HH 54 as function of the ro-tational quantum number of the upper state are compared to modelcalculations described in the text. Shown errors are statistical and are1 for unblended lines and 2  for blends and upper limits. The pre-dicted radiation temperatures of the lower transitions for 15 m and 1 mtelscopes are also indicated4.2. The H2O and OH spectraKeeping all parameters of the CO model of HH 54 fixed, wecomputed the ortho- and para-water spectra with the H2O abun-dance,X(H2O), as the only free parameter. For both, the numberof included energy levels was 45, involving 164 transitions, sev-eral of which are masing. The radiative transition rates are thoseof Chandra et al. (1984), whereas for the collision rates we usedthe H2O–He values (1:35) presented by Green et al. (1993).The distribution of the ortho and para forms was assumed to bein the ratio of the statistical weights of their nuclear spins (3 : 1).As for the CO computations, the radiation field included boththe cosmic microwave background and that of the diffuse dustemission (Sect. 3), the effects of which on the level populationsare relatively minor, though.The data are well represented by the adopted CO model and awater abundanceX(H2O) = 10 5, i.e. [H2O/CO] 10 1. Simi-larly, this model fits the OH spectrum as well, forX(OH)= 10 6(44 transitions from theHitrandatabase, Rothman etal. 1987,with collision rate coefficients from Offer et al. 1994). In Fig. 3,the modeled intensities of CO, OH and H2O, convolved withthe instrumental profile, are shown superposed onto the ob-served spectral line data. The water spectrum is very sensitiveto changes of the model parameters: the fact that the 179.5mline, connecting to the ground state of ortho-H2O, is observedto be the strongest, puts severe constraints on both the tempera-ture and density of the emitting gas. The overall success of themodel, viz. the capability to simultaneously fit the CO, OH andFig. 3. The computed H2O, OH and CO spectra, convolved withthe Lws instrumental profile for R(), are shown by the continuousline, superposed onto the Lws observations of HH 54, displayed inhistogram form. The abundances for the fit are X(OH) = 10 6 andX(H2O) = 10 5H2O observations, lends credence to the probable uniquenessof our model of HH 54.The H2O lines identified in Fig. 3 originate from levelswithinE=k 300 K above ground and, with the exception of the138.5m para-line, are all optically thick ( >1 to  70). Theopacity in the 557 GHz ortho ground state transition is   60and we predict a line temperature of about 0.3 K for future ob-servations with the 1 m-dish aboard the Odin satellite. The to-tal rotational line cooling, due to both ortho- and para-H2O,amounts to LH2O = 2:0 10 3 L, i.e. about 20% of that due toCO, being ten times more abundant. The OH cooling rate is halfof that of H2O. Hence, the radiative losses from HH 54 throughCO, OH and H2O line emission are  1.3 10 2 L. Anticipat-ing the results of the next section, the mechanical power inputis Lk = 12 mH n0 v3s  area = 1:8 10 2 L, so that more than70% is radiated away in the CO, OH and H2O lines. In view ofthe uncertainties involved it is not excluded that essentially all(with some dust cooling) of the shock luminosity is radiated bythese species alone.4.3. Excitation of the moleculesIn the absence of a strong radiation field the most likely phys-ical mechanism of excitation of HH 54 is collisional heatingby shock waves. The relatively low temperatures derived aboveL184 R. Liseau et al.: Thermal H2O emission from the Herbig-Haro flow HH 54( 300 K) are indicative of rather slow shocks, whose veloc-ities are of order 10 km s 1. The compression of the gas be-hind the shocks is given by nmax=n0 = C vs=p, where C = 77for vs in 100 km s 1, and where p = B0?=nq0 relates the nor-mal component of the pre-shock magnetic field, B0?, to thepre-shock density, n0 (see: Hollenbach et al. 1989). Crutcheret al. (1993) found B0? < 16G for dark clouds (averagedensity n(H) 2 103 cm 3) so that p<0.4 for q = 0.5. Tak-ing nmax to be the observed post-shock value of the vol-ume density, i.e. 2 105 cm 3, and using vs = 10 km s 1 leadsto n0(H2)< 104 cm 3. The value of p is uncertain by at leasta factor of two, affecting the estimate of the pre-shock densityaccordingly.Kaufman & Neufeld (1996) have recently presented detailedmodel calculations of molecular, non-dissociative C-shocks.The lowest pre-shock density (104 cm 3) considered in thesemodels is of the order of our estimate for the conditions inHH 54. Low-velocity models for logn(H2) = 4.0 reproduce ourCO, OH and H2O observations very well, with the observedline ratios falling in between the models for vs = 10 km s 1 and15 km s 1 respectively. On the basis of the previously estimatedsource solid angle, viz. source = 1:8 10 8 sr (Sect. 4.1), thelower shock velocity of 10 km s 1 would be favoured by themodel’s absolute fluxes. In this case, model fluxes would be invery good agreement with the observations, i.e. within the ob-servational error of the brighter lines, the exception being theOH 119m doublet which would appear underestimated by themodel by a factor of two to three.Although the identification of C-shocks as the exciting agentof the molecules seems basically correct in the case of HH 54,a few comments may be in order. (1) The C-shock models havebeen computed for planar, stationary shock waves. However, ourestimates of the shock parameters are close to those where C-shocks may become unstable, developing cooler clumps of neu-tral material (Wardle 1990). The clumpy morphology of HH 54would be qualitatively in accord with such a scenario. In addi-tion, the brightness variability of the H2 (1, 0) S(1) line reportedby Gredel (1994) is a clear indicator of non-stationary flows:the time scale of this variability <10 yr, significantly shorterthan the flow time scale, which is estimated to be  102 yr. (2)The models assume that p= 1.0, implying uncompressed fieldstrengths of at least 141G. This certainly appears excessivefor dark clouds by large factors. The effect of a significantlyreduced value of p would be to increase the post-shock temper-ature (Hollenbach et al. 1989). When comparing with observa-tions, this could mean that one is using the incorrect diagnostics.(3) Emission from higher excitation material, e.g. in the Nir,is outside the domain of C-shock excitation. A unifying shockmodel would thus be desirable. E.g., models of wide bow shockshave yet to be developed to the level of sophistication as thoseof Kaufman & Neufeld. Needless to say that further progresson the observational side would require improved spatial andspectral resolution.5. ConclusionsIn short, we conclude the following:1. The shocked flow HH 54 has been successfully observedwith the Lws. The spectrum of thermally excited H2O linesis the first obtained towards an HH-object.2. The high-J CO transitions present in the spectrum are usedto estimate the physical characteristics of the line emittingregions. This CO model fits simultaneously also the ob-served OH and H2O spectra for abundancesX(OH) = 10 6and X(H2O) = 10 5 respectively.3. Essentially all of the weak-shock energy of HH 54 is dissi-pated in rotational lines of CO, OH and H2O.Acknowledgements. RL enjoyed interesting discussions with M. Kauf-man and D. Neufeld and we thank them for making available to us theresults of their C-shock models. We thank P. Bergman for the Lvgcode and appreciate the valuable suggestions made by the referee,E. van Dishoeck.ReferencesChackerian C.Jr., Tipping R.H., 1983, J.Mol.Spec. 99, 431Chandra S., Varshalovich D.A., Kegel W.H., 1984, A&AS 55, 51Clegg P.E. et al., 1996, this volumeCrutcher R.M., Troland T.H., Goodman A.A., Heiles C., Kaze`s I., My-ers P.C., 1993, ApJ 407, 175Graham J.A., Hartigan P., 1988, AJ 95, 1197Gredel R., 1994, A&A 292, 580Green S., Maluendes S., McLean A.D., 1993, ApJS 85, 181Hollenbach D.J., Chernoff D.F., McKee C.F., 1989, in: Proc. 22ndEslab Symp. on Infrared Spectroscopy in Astronomy, ESA SP-290, p. 245Kaufman M.J., Neufeld D.A., 1996 ApJ 456, 611Kessler M. et al., 1996, this volumeKnee L.B.G., 1992, A&A 259, 283Nisini B. et al., 1996, this volumeOffer A.R., van Hemert M.C., van Dishoeck E.F., 1994,J. Chem. Phys. 100, 362Rothman L.S. et al., 1987, Appl. Opt. 26, 4078Schinke R., Engel V., Buck U., Meyer H., Diercksen G.H.F., 1985, ApJ299, 939Schwartz R.D., Dopita M.A., 1980, ApJ 236, 543Swinyard B.M. et al., 1996, this volumeWardle M., 1990, MNRAS 246, 98

Thermal H<sub>2</sub>O emission from the Herbig-Haro flow HH 54

Thermal H<sub>2</sub>O emission from the Herbig-Haro flow HH 54

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