© 1998 RAS

Mon. Not. R. Astron. Soc. 297, 687–691 (1998)

ISO observations of molecular hydrogen in the DR 21 bipolar outflow

Michael D. Smith,1w Jochen Eisloffel2w and Christopher J. Davis3,4w

1Astronomisches Institut, Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany2Thuringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany3Dublin Institute for Advanced Studies, School of Cosmic Physics, 5 Merrion Square, Dublin 2, Ireland4Joint Astronomy Centre, 660 N. A‘ohoku Place, University Park, Hilo, Hawaii 96720, USA

Accepted 1997 December 15. Received 1997 September 19

A B STR ACTWe demonstrate that a wide range of molecular hydrogen excitation can beobserved in protostellar outflows at wavelengths in excess of 5 lm. Cold H2 in DR 21is detected through the pure rotational transitions in the ground vibrational level(0–0). Hot H2 is detected in pure rotational transitions within higher vibrationallevels (1–1, 1–2, etc.). Although this emission is relatively weak, we have detectedtwo 1–1 lines in the DR 21 outflow with the ISO SWS instrument. We thusinvestigate molecular excitation over energy levels corresponding to thetemperature range 1015–15 722 K, without the uncertainty introduced bydifferential extinction when employing near-infrared data.

This gas is thermally excited. We uncover a rather low H2 excitation in the DR 21West Peak. The line emission cannot be produced from single C-shocks or J-shocks;a range of shock strengths is required. This suggests that bow shocks and/or bow-generated supersonic turbulence is responsible. We are able to distinguish thisshock-excited gas from the fluoresced gas detected in the K band, providing supportfor the dual-excitation model of Fernandes, Brand & Burton.

Key words: shock waves – ISM: individual: DR 21 – ISM: jets and outflows – ISM:molecules – infrared: ISM: lines and bands.

1 INTRODUCTION

DR 21 is a giant bipolar outflow driven by a high-massprotostar (Garden et al. 1986). It is one of the most lumin-ous H2 emission-line sources in our Galaxy, containing com-plex spatial and velocity structure (Garden et al. 1991). Thissuggests that a ‘supersonic turbulent reactor’ (STR) is atwork (Davis & Smith 1996) in which turbulence, driven bynumerous dense clumps, is dissipated through shocks on allscales and velocities. This shock spectrum implies thatenergy is released in considerable amounts on all scales,akin to a Kolmogorov spectrum for subsonic turbulence.One test for this hypothesis requires observations of H2

lines covering a wide range of rotational and vibrationalexcitation levels. In the near-infrared, we cannot observethe cool H2 gas. Below 2.5 lm we are limited to detectingthe vibrationally excited gas involving upper energy levels

above 6000 K. Furthermore, the 2.12-lm extinction is high,13.2¹1 mag (at the western peak, DR 21W: Garden et al.1986), providing an extra source of uncertainty. We turn,therefore, to the Infrared Space Observatory (ISO) to obtainthe required data.

Both hot and cold H2 can be detected at long wave-lengths. Cold H2 gas has indeed been detected at wave-lengths above 5 lm in numerous bipolar outflows with theISO Short Wavelength Spectrometer (SWS). However, thehot H2 gas has been neglected even though it is well knownthat pure rotational transitions within vibrationally excitedgas are excited at these wavelengths. Smith (1996) has pre-sented on CD-ROM the predicted H2 line intensities above4.8 lm from two contrasting shock models. To test these, aprogramme of observations was established to observeseveral lines in several objects. The high vibrational linesare, however, quite weak and the SWS was not as sensitiveas originally forecast. We thus chose five lines in DR 21(Table 1), an extremely bright outflow despite its distance of3 kpc. The lines are widely spaced in energy, which permitsshock models to be tested. Only ortho-H2 is considered, so

wE-mail: smith@astro.uni-wuerzburg.de (MDS);eisloeffel@tls-tautenburg.de (JE);cdavis@jach.hawaii.edu (CJD)

that the derived excitation is not dependent on the ortho–para ratio. The long wavelengths, partly necessary becausewe were limited to lines not observable from the ground,have the advantage of minimizing the effects of differentialextinction. All the lines were detected: two 1–1 emissionlines as well as three 0–0 lines, covering energies between1015 and 15 722 K.

Recently, Fernandes, Brand & Burton (1997) havestudied the K-band spectrum of DR 21. Their results can becompared with those presented here since their slit inDR 21 West cuts directly through the ISO aperture (dis-played in Fig. 1), centred on the peak 1–0 (S1) emission,although one must bear in mind that the slit has an extent of90 arcsec. They concluded that the H2 K-band emission inDR 21 West contains a fluorecent component which contri-butes 10–20 per cent to the 1–0 lines, 25–58 per cent to the2–1 lines and 60–78 per cent to the 3–2 lines. The ISO dataprovide a test: the 1–1 lines observed here are probablydominated by thermal (shock) excitation, even though theirupper energy levels are in the range of those of the K-bandlines from the second vibrational level. Hence, on compar-ing H2 column densities from transitions with similar upperenergy levels, the existence of two components should beapparent.

In Section 2 we present the observations and convert thefluxes into column densities. In Section 3 we then comparethe molecular excitation with present models.

2 OB SERVATION A ND R EDUCTION TECH NIQUE

The pointing towards the DR 21 West Peak was madeduring revolution 347, on 1996 October 28/29. Fig. 1 showsthe positioning of the spectrograph entrance slit on animage of DR 21 in the H2 S(1) line at 2.12 lm. The spectraobtained are displayed in Fig. 2. The wavelength bands werescanned in the Astronomical Observation Template (AOT)

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© 1998 RAS, MNRAS 297, 687–691

Table 1. The ro-vibrational transitions with intensities from theplanar shock models. The wavelengths are the vacuum valuesfrom Black & van Dishoeck (1987). The upper energy level ofthe transition is given in the third column. The 1–0 and 2–1 S(1)surface brightnesses (for a shock moving directly in the line ofsight) are in units of erg sÐ1 cmÐ2. All others are expressedrelative to the 1–0 S(1) line strength. Models for planar shocksC35 and J15 and their JHK fluxes are described by Smith(1995).

Figure 1. The 1–0 S(1) map of DR 21 West (see Davis & Smith 1996), showing the positioning of the 14Å20 arcsec2 aperture at 29h38m54 s. 9,+42°10p10 P. 6 (J2000).

SWS02 mode with a total on-target time of 21 000 s. Alllines were unresolved. The integrated line fluxes were com-puted from the Gaussian fits using IDL, with a linear func-tion for any continuum subtracted (Table 2). Systematic fluxerrors were estimated at 30 per cent. Statistical errors werecomputed from the noise level near the line.

Note the difference in wavelengths between the observedline peaks and the documented vacuum values. Specifically,the differences for the three lines in the 2B band between5.3 and 7.0 lm are prominent and systematic. However, theresolution in this band is low (of the order of 1000, althoughthe uncertainty in the wavelength calibration is smaller),and the pointing accuracy may also lead to a wavelengthshift. It is significant that 0–0 S(7) and 0–0 S(5) are two ofthese three lines, since it is then clear that the 1–1 S(7)identification is consistent. The 1–1 lines, however, are

weak and one must consider other possible candidates. Theother candidate lines, orders of magnitude weaker in shockmodels, but which could be strong in a fluorescence model,are 3–2 O(10) (5.8085 lm) and 6–5 O(6) (4.955 lm). Theline strengths relative to 0–0 S(7) predicted by the Black &van Dischoeck (1987) fluoresced-gas model (calculated byemploying quoted line fluxes from the same upper levels)are 3–2 O(10)/0–0 S(7)\6.0Å10Ð3 and 6–5 O(6)/0–0S(7)\2.9Å10Ð2. The predicted 3–2 flux is thus a factor of 3lower than observed, even if we ascribe all the 0–0 S(7)emission to fluorescence. However, the fluorescent modelof Fernandes et al. (1997) excludes the latter possibility. Wethus proceed with what we believe are secure identifica-tions.

Column densities for the five upper energy levels aregiven in Table 2. They were derived, ignoring extinction,from the formula Nj\ljI/(hcAj) with the relevant wave-lengths lj and Einstein coefficients Aj with h being Planck’sconstant, c the speed of light and I the surface brightness.The columns are converted to column density ratios’(CDRs) by taking the ratio with the 0–0 S(7) column[usually we use 1–0 (S(1) at 6951 K, a similar energy]. Wethen normalize to the CDRs for a 2000-K slab of gas (toextract the exponential dependence). The result is shown inFig. 3.

An excitation temperature can be derived from each pairof columns (e.g. equation 3 of Smith, Davis & Lioure 1997).We find excitation temperatures of 668, 1065, 11530 and12200 K for the four successive pairs in Table 2. Here,however, we shall employ the CDR method to constrain themodels.

3 COLLISION A L OR FLUOR ESCENT EXCITATION?

To establish the excitation mechanism, we first compare theCDRs for the ISO and K-band data in Fig. 4. We normalizethe data so that the 1–0 S(1) K-band line strength and the0–0 S(7) SWS line strength both have a CDR of unity. Thejustification for this procedure is that these are relativelylow-excitation lines, and could well be in thermal equili-brium. Moreover, the difference in measured columns isconsistent with local thermodynamic equilibrium (LTE)plus the differential extinction derived from K-band data(see Section 5).

Pure shock models are immediately excluded. If collisionsdominated the excitation, the data should all lie along asingle line (if in LTE), or the CDRs of the higher vibrationallevels should lie below those of the lower vibrational levels

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Table 2. Parameters for the observed pure rotational transitions. The surface brightness I (in erg sÐ1

cmÐ2 srÐ1) and the column N are the averages over the corresponding aperture, which is somewhatlarger for the 0–0 S(1) line (14Å27 arcsec2) than for the others (14Å20 arcsec2).

Figure 2. The molecular hydrogen emission lines detected inDR 21. Wavelengths on the abscissa are in lm; fluxes on the ordi-nate are in Jy. The Gaussian fits are displayed as thinner lines.

[non-LTE case, as demonstrated for Cepheus A by Smith(1998)]. Fig. 5 demonstrates that the reverse is true.

We can now test the fluorescence-shock model in whichthe line emissions from high vibrational levels contain signi-ficant fluorescent contributions. The offsets between thetwo sets of data displayed in Fig. 4 imply that at least 70percent of the columns in the v\2 level (K-band data) arepopulated through fluorscence. This is a larger percentagethan predicted by Fernandes et al. (1997), reflecting theweaker shock component revealed here.

The ground and first vibrational levels are predominantlythermally populated. This conclusion is based on the follow-ing three pieces of evidence. The fluorescence model of

Black & van Dishoeck predicts flux ratios of 54: 10: 18 for0–0 S(7): 1–1 S(7): 1–1 S(9), whereas the observed ratiosare 460: 10: 12. This still allows up to 150 per cent of thefirst vibrational level to be directly populated by fluor-escence, as required by the fluorescence-shock model ofFernandes et al. Secondly, the extinction-corrected 1–0S(1)/1–1 S(7) flux ratio is 160, whereas the fluorescentmodel predicts a ratio of 32. Finally, the 1–0 emission-lineratios in the K band are typical of collisional excitation, notfluorescence (Fernandes et al. 1997).

4 SHOCK A N A LYSIS

What is the dynamical process behind the collisional excita-tion of the H2 gas observed by ISO? Molecular hydrogencools to temperatures below 1000 K on a time-scale of1/Aj1107 s. On the other hand, the dynamical time-scale ofthe outflow is (100 pc)/(100 km sÐ1)1104 yr. Hence werequire a heating mechanism that is local and effective. Todate, only shocks are viable. Below, we compare the datawith the various shock scenarios.

C-type planar shock models fail to reproduce the columndensity ratios. As shown in Fig. 3 (the broken lines), indivi-dual C-shocks are similar in excitation to constant-tempera-ture slabs of gas and so yield near-linear log (CDR)–Tj

relationships. The data, in contrast, possess a clear system-atic curvature.

J-type shocks are also ruled out. This is evident fromTable 1, where the 1–1 S(9)/0–0 S(7) and 1–1 S(7)/0–0S(7) ratios for J-shocks are predicted to be almost 10 timesstronger than observed. J-type shocks and their other prob-lems were discussed by Smith (1994) and Eisloffel et al.(1996). Not only water cooling but also gas–grain coolingmust be almost completely suppressed in order to produce alow-excitation spectrum. We do not consider J-shocks fur-ther here.

A mixture of planar C-shocks would clearly be plausible.A paraboloidal bow shock is one such mixture. First, wesuperimpose (solid line in Fig. 3) the C-type bow modelconsistent with the OMC-1 Peak 1 data (Burton & Haas1997). However, this model predicts large quantities ofwarm and hot gas, which are not detected.

A recent C-type model takes the bow shape as a freeparameter [numerical hydroydnamic simulations suggestthat molecular bow shocks possess strong flanks (Suttner etal. 1997)]. Applied to Cepheus E, Eisloffel et al. (1997)found that cylindrically symmetric bow shapes of the formz;Rs with the shape s in the range 1.39–1.76 fit the K-bandexcitation data. We find here also that with s\1.4¹0.1 areasonably close fit is obtained (solid line in Fig. 5). Lowers-values result in an excess in the low-Tj columns, whereashigher values predict excesses in the high-Tj columns.

Intensities for the planar C-shock models are typicallytwo orders of magnitude higher than those observed forDR 21. This is, however, not inconsistent, since a low fillingfactor is indicated from Fig. 1. Furthermore, some extinc-tion even at 5 lm [10.3–0.7 mag for the 0–0 S(7) wave-length], and a somewhat lower density than the 106 cmÐ3

taken in the above models, can be considered. Also, one hassome freedom in the models: decreasing shock powers byreducing the shock speeds, while increasing the ion level tomaintain the excitation, would also reduce line luminosities.

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Figure 3. The log(CDR) diagram for DR 21. The columns ofTable 2 have been normalized to the 0–0 S(7) column and then tothe equivalent columns from a slab of 2000-K gas in local thermo-dynamic equilibrium. Error bars owing to noise are extremely smallon such logarithmic plots. 25 per cent error bars are shown for thetwo weak 1–1 lines. Superimposed models are standard C-typeplanar shocks of speed 30 km sÐ1 (dashed) and 25 km sÐ1 (dot–dashed) with an oxygen abundance of 4Å10Ð4, Alfven speed of 2km sÐ1 and ion fraction of 2Å10Ð7. The solid line is the bestparaboloidal shock fit to OMC-1 Peak 1 with low oxygen and COabundances, 1.5Å10Ð5 and bow speed 200 km sÐ1.

Figure 4. The log(CDR) diagram for DR 21, including the K-banddata of Fernandes et al. (1997, their table 2), normalized to the 1–0S(1) line.

Note, however, that the line ratios do not change: the onlyimportant free parameters for fast bow shocks are the bowshape and the oxygen abundance (Smith, Brand & Moor-house 1991).

Turbulence cannot be satisfactorily simulated at present(Moin & Kim 1997). We have no chance of writing con-fidently about compressible, supersonic and magnetohydro-dynamic turbulence. Nevertheless, turbulence is responsiblefor many of the dynamical properties of interstellar gas.Furthermore, supersonic turbulence is unavoidable: it isproduced in the vorticity created downstream of bow shocksas well as by dynamical instabilities of the shocks them-selves. We suppose here that a fully developed turbulentspectrum of shocks arises from a Kolmogorov turbulentvelocity spectrum in the molecular gas (Davis & Smith1995). This brew of shocks generates a fairly low-excitationspectrum. As shown in Fig. 5, turbulent models are able toexplain the data, although they do overproduce emission inthe vibrationally excited gas. Even a spectrum with a highcontribution from slow shocks (dot–dashed line in Fig. 5)does not greatly improve the fit.

5 CONCLUSIONS

We have shown that DR 21 West Peak contains large quan-tities of cold H2 gas. Comparison with K-band data confirmsthat a high fraction of the molecules in the second vibra-tional level arrive there through a fluorescent cascade.

On the other hand, the ISO data reveal the shocked com-ponent. The excitation state depends strongly only on thespectrum of shocks and the type of shocks involved. Theexcitation cannot be modelled through planar shocks.Instead, C-type bow shocks with wide flanks (s-param-eter11.4) and/or supersonic turbulence with a high contentof weak C-shocks are necessary. Although we are thus able

to constrain the processes involved, it is not yet possibleusefully to constrain these models further: predicted shockintensities are sensitive to too many parameters – the ionfraction, field strength, shock velocity, density and fillingfactor.

The extinction at 2 lm can be estimated from the 0–0S(7) and the 1–0 S(1) fluxes. The 1–0 S(1) flux at 2.12 lm is19.5¹0.5Å10Ð13 erg sÐ1 cmÐ2 within a 20-arcsec beam onthe West Peak (Garden et al. 1986), a similar aperture sizeand the same location as for the ISO data. Thus the fluxratio of 0–0 S(7)/1–0 S(1) is 15.6. In contrast, the pre-dicted intrinsic flux ratio is 0.50–0.55 for all LTE modelsdiscussed here. The factor of 11 then corresponds to a dif-ferential extinction between 5.51 and 2.12 lm of 2.56 mag.This corresponds to a 2.12 lm extinction of 3.1 mag and a5.5-lm extinction of 0.5 mag (for an extinction law with awavelength index of Ð1.8). This is consistent with the3.2¹1 mag for the K band derived by Garden et al. (1986).The extinction-corrected columns are shown in Fig. 5.

The advantages of observing hot gas with relatively lowvibrational excitation have been demonstrated here. Welook forward to the next instruments aboard the satellites,SOFIA and SIRTF, that can explore this region of thespectrum.

ACKNOWLEDGMENTS

This work was supported by the Deutsche Forschungsge-meinschaft through the Schwerpunkt Programm ‘Physics ofStar Formation’ (grant Zi-242/10-1). This paper is based onobservations with ISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries:France, Germany, the Netherlands and the United King-dom) and with the participation of ISAS and NASA.

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Figure 5. The log(CDR) diagram for DR 21 West Peak with STRmodels superimposed (the dashed line corresponds to the standard1/3 Kolmogorov law; the dot–dashed line shows a 1/4 Kolmogorovspectrum). Also shown is a close-fitting bow model with shapeparameter s\1.4 and oxygen and CO abundances of 10Ð4. Thediamonds denote the uncorrected values and the + signs denotethe extinction-corrected values.

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