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Astronomy & Astrophysics manuscript no. hh211 (DOI: will be inserted by hand later)

October 27, 2004

The near-infrared excitation of the HH 211 protostellar outflow
Barry O'Connell1,2 , Michael D. Smith1 , Dirk Froebrich3 , Christopher J. Davis4 , and Jochen Eisloffel ¨
1 2 3 4 5

5

Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK Physics Department, Trinity College Dublin, College Green, Dublin 2, Ireland School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland Joint Astronomy Centre, 660 N.A'ohoku Place, University Park, Hilo, Hawaii 96720, USA Thuringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenberg, Germany ¨

Received ...... / Accepted ........
Abstract. The protostellar outflow HH 211 is of considerable interest since it is extremely young and highly collimated. Here,

we explore the outflow through imaging and spectroscopy in the near-infrared to determine if there are further diagnostic signatures of youth. We confirm the detection of a near-infrared continuum of unknown origin. We propose that it is emitted by the driving millimeter source, escapes the core through tunnels, and illuminates features aligning the outflow. Narrow-band flux measurements of these features contain an unusually large amount of continuum emission. [FeII] emission at 1.644 µm has been detected and is restricted to isolated condensations. However, the characteristics of vibrational excitation of molecular hydrogen resemble those of older molecular outflows. We attempt to model the ordered structure of the western outflow as a series of shocks, finding that bow shocks with J-type dissociative apices and C-type flanks are consistent. Moreover, essentially the same conditions are predicted for all three bows except for a systematic reduction in speed and density with distance from the driving source. We find increased K-band extinctions in the bright regions as high as 2.9 magnitudes and suggest that the bow shocks become visible where the outflow impacts on dense clumps of cloud material. We propose that the outflow is carved out by episodes of pulsating jets. The jets, driven by central explosive events, are responsible for excavating a central tunnel through which radiation temporarily penetrates.
Key words. ISM: jets and outflows, stars: circumstellar matter, infrared: ISM, ISM: Herbig-Haro objects

1. Introduction
The processes controlling the birth of individual stars evade direct analysis in the optical because they operate within highly obscuring cores within dense molecular clouds. Nevertheless, the end of the period of gestation is heralded by often spectacular ejections of material (Bachiller 1996). We may thus deduce information concerning the accretion process by indirect means. Bipolar jets from protostars appear to be ubiquitous and their characteristics suggest a close relationship to the accretion process from a disc into a forming star (Froebrich et al. 2003b). In addition, they have drastic dynamical and chemical effects on their environment. They excavate cavities by sweeping up the molecular material which then forms an outflow visible in emission from CO molecules. As well as a leading bow wave which ploughs through the ambient medium, irregularities in
Send offprint requests to: B.O'Connell, e-mail: boc@arm.ac.uk Based in part on observations collected at the German-Spanish Astronomical Center, Calar Alto, operated jointly by Max-Planck Institut fur Astronomie and Instituto de Astrof´ ¨ isica de Andaluc´ ia (CSIC). Observations were also obtained with The United Kingdom Infrared Telescope, which is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council.

the outflow give rise to shocks which propagate away from the protostar into the ambient molecular gas (Froebrich et al. 2003a). These bow shocks excite atoms and molecules, inducing line emission which is often visible at optical wavelengths as Herbig-Haro (HH) objects (Reipurth & Bally 2001; Bally & Reipurth 2002). However, the near-infrared emission is particularly important for the youngest protostars since even the protruding outflows may still be obscured in the optical regime (Eisloffel et al. 2000). In fact, near-infrared observations reveal remarkable molecular hydrogen flows which hold valuable information concerning the environment through which they propagate as well as the mechanism which launches the driving jets (Davis 2002). HH 211 is a bipolar molecular outflow which was discovered by McCaughrean et al. (1994). It lies near the young stellar cluster IC 348 IR in the Perseus dark cloud complex at an estimated distance of 315 pc (Herbig 1998). The outflow is bilaterally symmetric and highly collimated with an aspect ratio of 15:1. The total extent of the outflow is 106 which is 0.16 pc at the adopted distance. A H2 (1,0) S(1) wide-field survey of the IC 348 cluster was carried out by Eisl ¨ ffel et al. (2003) o covering a 6 .8 â 6 .8 region; no HH 211 outflow remnants were detected beyond the outer knots. This makes it one of the


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O'Connell et al.: The HH 211 protostellar outflow

smaller outflows, which suggests that it may also be one of the youngest since the average length within an unbiased sample of Class O/I jets was found to be 0.6­0.8 pc (Stanke 2003). The conclusion that HH 211 is a jet driven outflow with a timescale of order 1000 years was derived from interferometric CO observations (Gueth & Guilloteau 1999), which confirms it as one of the youngest infrared outflows to be discovered. Although the outflow lobes are visible in both blue-shifted and red-shifted CO emission, a small inclination angle to the plane of the sky is suggested by (i) the lack of strong differential extinction in the H2 brightness distribution, (ii) the high degree of separation of the blue and red CO lobes and (iii) the relatively small radial components of the SiO and H2 flow speeds (Chandler & Richer 2001; Salas et al. 2003). The central engine driving the outflow is HH 211-mm, a low-mass protostar with a bolometric luminosity of 3.6 L and bolometric temperature of 33 K. It is surrounded by a 0.8 M dust condensation (Froebrich 2005)1. Since 4.6 per cent of the bolometric luminosity is attributed to the submillimeter luminosity, L smm , HH 211-mm is classified as a Class O type protostar. A compact and collimated SiO jet extends in both flow directions out to a projected distance of 20 from the central source (Chandler & Richer 1997, 2001). This is also another indication of the Class O nature of the source (Gibb et al. 2004). The clumpy nature of the observed SiO emission suggests a shock origin resulting from a time dependent jet velocity. Nisini et al. (2002) observed HH 211 in SiO lines originating from high rotational energy levels and deduced a high jet density of nH 2 2­5 â 106 cm-3 and gas temperature 250 K. However, no SiO emission is detected beyond 20 where the H2 (1,0) S(1) emission is found suggesting that the conditions, such as shock velocity and pre-shock density, vary considerably along the flow direction. Imaging of [FeII] emission in the H-band at 1.644µm has recently played a major role in our understanding of shocked outflows (see Reipurth et al. (2000) for a summary). Where [FeII] emission is observed it traces the fast (> 50 km s-1 ) and dissociative shocks. Combined with K-band imaging of H2 rotational-vibrational lines, which trace less extreme shock conditions, important information about outflow excitation may be gathered (Khanzadyan et al. 2004). Previous studies employing bow-shock models have focused on larger, more evolved systems (Eisl ¨ ffel et al. 2000; o Smith et al. 2003; O'Connell et al. 2004). HH 211 provides us with the unique opportunity of studying what has been deemed as an exceptionally young outflow covering a small spacial extent. We present here new high resolution images of H2 and [FeII] lines and K-band spectroscopy of the outflow (§ 3). We analyse the impact regions visible in the near-infrared. We then present in § 4 C-type bow-shock models in order to interpret the remarkable set of bows propagating through a changing environment along the western outflow. In § 5 we discuss the issues which have arisen from this set of data before pooling our findings together in conjunction with previous studies to suggest a global outflow mechanism for HH 211 in § 6.
1

2. Observations and data reduction

2.1. KSPEC obser vations
Our near-infrared (NIR) spectra cover the 1-2.5 µm region in medium resolution. They were obtained in the period 26-29 August 1996 with the KSPEC spectrograph on the University of Hawaii 2.2 meter telescope. This cross dispersed Echelle spectrograph is equipped with a HAWAII 1024 â 1024 detector array and optimised for 2.2 µm. Observations were performed at two bright H2 emission locations. The 0. 96 width slits ran in an east-west direction passing through knots f and d in the west and through knots i and j in the east (positions are indicated in Fig. 1). Data reduction (flatfielding, sky-subtraction, extraction of the spectra) was performed using our own MIDAS routines. An absolute calibration of the fluxes was not possible due to non-photometric weather conditions. Wavelength calibration was performed using OH-night-sky emission lines and tables of Rousselot et al. (2000). The results are presented in Table 1. Note that fainter emission lines are detected in knots i and j due to the stronger emission here compared to knot d (see Table 2.)
Table 1. KSPEC relative fluxes. For both slit positions, the measured fluxes lie above the continuum and are presented relative to the H2 (1,0) S(1) line flux. Three observations were carried out at each slit location. The relative fluxes have been averaged and the spread in values is quoted in brackets as an error estimate. Line (1,0) (1,0) (1,0) (1,0) (1,0) (1,0) (3,2) (2,1) (1,0) (3,2) (2,1) (3,2) (1,0) (2,1) (2,1) (3,2) (1,0) (1,0) (1,0) (1,0) (1,0) (1,0) S(9) S(7) S(6) S(5) S(4) S(2) S(5) S(3) S(1) S(4) S(2) S(3) S(0) S(1) S(0) S(1) Q(1) Q(2) Q(3) Q(4) Q(6) Q(7) (µm) 1.687 1.748 1.788 1.835 1.891 2.033 2.065 2.073 2.121 2.127 2.154 2.201 2.223 2.247 2.355 2.386 2.406 2.413 2.423 2.437 2.475 2.499 east 0.02 0.12 0.07 0.62 0.20 0.36 0.03 0.10 1.00 0.02 0.05 0.03 0.25 0.13 0.03 0.03 0.96 0.38 0.95 0.33 0.19 0.43 (0.01) (0.01) (0.01) (0.04) (0.01) (0.01) (0.01) (0.01) (0.05) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.05) (0.05) (0.05) (0.04) (0.06) (0.03) west ­ ­ ­ ­ ­ (0.05) ­ (0.07) (0.05) ­ ­ ­ (0.02) (0.06) ­ ­ (0.17) (0.11) (0.05) (0.15) ­ ­

0.30 0.18 1.00

0.31 0.17

1.25 0.54 1.13 0.41

2.2. MAGIC obser vations
The NIR images were taken in November 1995 at the 3.5 meter telescope on Calar Alto using the MAGIC infrared camera

http://www.dias.ie/protostars/


O'Connell et al.: The HH 211 protostellar outflow

3

Fig. 1. Broad-band K image of HH 211 which covers wavelengths between 2.03µm and 2.37µm. Features are labeled according to the nomenclature of McCaughrean et al. (1994). The exciting source HH 211-mm at position R.A.(2000) = 03h 43m 56.7s , Dec(2000) = +32 00 50.3 (Avila et al. 2001) is indicated by the cross. KSPEC spectroscopic slit positions are indicated by the dotted lines. Note that the star in the south-west of this image (and following images) has been masked for display purposes.

(Herbst et al. 1993) in its high resolution mode (0.32 per pixel). Images were obtained using narrow band filters centered on the H2 (1,0) S(1) emission line at 2.122 µm, the (2,1) S(1) line at 2.248 µm, the (3,2) S(3) line at 2.201 µm and on the nearby continuum at 2.140 µm. The per pixel integration time was 1740s. Seeing throughout the observations was 0.9 except for the 2.14 µm image where it is 1 .8. The narrow-band image containing the (1,0) S(1) line has been previously published in Eisl ¨ ffel et al. (2003). o The data could not be accurately flux calibrated due to non-photometric conditions. The total H2 (1,0) S(1) flux from the entire outflow was previously measured to be 1.0 â 10 -15 W m-2 by McCaughrean et al. (1994) which agrees with our flux calibration. We have deduced a calibration for all our narrow band images according to this measurement assuming that the average integrated flux from several bright unsaturated field of view stars should be similar for each filter. This should be the case because each filter FWHM is equal to 0.02 µm and the spectral energy distribution (SED) is likely to be relatively flat on average between 2.122 µm and 2.248 µm. We assume the accuracy of this method to be 15%.

(Roche et al. 2003). The camera is equipped with a Rockwell Hawaii 1024 â 1024 HgCdTe array which has a plate scale of 0 .091 per pixel and provides a total field of view of 92 .9 â 92 .9. Images in the [FeII] 4 D7/2 ­ 4 F9/2 transition were obtained using a narrow-band filter centered on = 1.644 µm with (FWHM) = 0.016 µm). The outflow was also imaged using the broad-band K[98] filter centered on = 2.20 µm with (FWHM) = 0.34µm. Seeing throughout the observations was 0 .8. Nine-point `jittered' mosaics were obtained in each filter. Standard reduction techniques were employed (using the Starlink packages CCDPACK and KAPPA) including badpixel masking, sky subtraction and flat-field creation (from the jittered source frames themselves). The images were registered using common stars in overlapping regions and mosaicked. The observations were conducted under photometric conditions, so the faint standard FS11 (spectral type A3; H = 11.267 mag) (Hawarden et al. 2001) was also observed and used to flux calibrate the [FeII] image. The final images were smoothed using a circular Gaussian filter of FWHM = 0 .25. in order to increase the signal to noise without compromising the resolution.

2.3. UFTI obser vations
Further NIR observations of HH 211 were carried out on December 12, 2000 (UT) at the U.K. Infrared Telescope (UKIRT) using the near-infrared Fast Track Imager UFTI

3. Results
Fig. 1 displays the HH 211 outflow in the K-band between 2.03µm and 2.37µm which contains all the K-band line emis-


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O'Connell et al.: The HH 211 protostellar outflow

Fig. 2. HH 211 at 2.248 µm. The grey-scale is the continuum subtracted image showing only the (2,1) S(1) line emission. Overlaid are contours representing the non continuum subtracted image. Thus the contours nearer the source trace predominantly scattered light while the contours further out mostly trace compact, line-emission features. The contour levels, which are scaled logarithmically, are at 0.22, 0.44, 0.87, 1.74, 3.47 â 10-18 W m-2 arcsec-2 .

Fig. 3. HH 211 at 1.644 µm. The greyscale and contours both trace the [FeII] line plus continuum emission. Consequently, most of the distributed flux can be attributed to continuum emission, although some concentrated condensations of [FeII] line emission are observed. These are labelled 1 ­3. The logarithmic contours measure the flux at 0.63, 1.0, 1.58, 2.51, 3.98, 6.31 â 10-19 W m-2 arcsec-2 .


O'Connell et al.: The HH 211 protostellar outflow

5

Table 2. Photometric results for HH 211. The flux in units of 10-18 W m-2 is measured over the indicated circular apertures. Note that the measurements were made from images which are not continuum subtracted because of the low S/N of the continuum image. To infer the line emission fluxes the continuum values need to be subtracted. The (2,1) S(1)/(1,0) S(1) (labeled 2/1) ratios have been determined after subtracting the 2.14 µm continuum emission. In the case of knot g the (2,1) S(1) flux is equal to the continuum flux so no ratio was derived. Knot HH 211-west HH 211-east bow-a bow-bc bow-de b c d e f g h ij


aperture 57 42 5.3 7.7 10.1 4.9 4.9 6.0 6.0 10.1 10.1 10.1 17.9

(1,0) S(1) 413 575 7.2 32.0 155.5 19.3 11.2 89.6 57.5 157.2 41.9 60.3 458.3



(2,1) S(1) 61 98 0.6 2.3 18.1 1.5 0.9 10.2 7.9 19.3 20.6 9.8 54.3



(3,2) S(3) 34 63 no det. no det. 7.3 no det. no det. 4.0 3.3 9.1 20.6 5.1 27.9



2.14µm Cont. 42 55 no det. no det. 3.1 no det. no det. 1.9 1.3 8.9 19.6 4.7 24.6



Broad-band K 1285 2197 9 65 335 41 21 192 131 395 380 190 1364



[FeII] 1.644µm 28 41 no det. no det. 11.8 no det. no det. 6.1 4.8 9.0 14.3 4.6 25.0



2/1 ratio 0.05 0.08 0.08 0.07 0.09 0.08 0.08 0.09 0.12 0.07 (0.04) (0.03) (0.04) (0.02) (0.02) (0.02) (0.03) (0.03) (0.03) (0.04) ­ 0.09 (0.08) 0.07 (0.03)

The background 1 noise estimates over an 8 aperture are: 2.8 for H2 (1,0) S(1); 0.2 for H2 (2,1) S(1); 0.7 for H2 (3,2) S(3); 1.1 for the 2.14µm continuum; and 3.4 for the [FeII] at 1.644µm, also in units of 10-18 W m-2 . As a standard star was not observed in the K[98] filter the broad-band fluxes are rough estimates. The flux calibration factor which we used was derived from the (1,0) S(1) calibration and a comparison of the broad-band and narrow-band filters.

trum of the outflow source. The band of continuum is prominent in the K-band as well as at 1.644µm. The photometric results for knots a ­ j are listed in Table 2. Knots d,e and b,c are interpreted as bow-shock components and The H2 (2,1) S(1) image is displayed in Fig. 2. The continuum at 2.14 µm has been subtracted in order to indicate loca- are labeled as bow-de and bow-bc. We will briefly discuss the tions of pure (2,1) S(1) emission (greyscale). Contours of the implications of these fluxes. The eastern outflow is 1.5 times non continuum subtracted image are also displayed in order brighter than the western outflow. However, the cause of this to indicate the extent of the continuum emission at 2.248 µm. difference is not necessarily due to an unequal jet power output The line emission is produced from an excitation level which is as Giannini et al. (2001) detected similar levels of OI 63 µm in 12,553 K above the ground state whereas the (1,0) S(1) arises both lobes (1.02 L in east; 0.94 L in west). This line is relfrom 6,953 K. Therefore, we expect it to highlight the hotter atively unaffected by extinction and represents the main cooling channel in the post-shocked gas. The non photometric conparts of molecular shocks. ditions during the observations are reflected in the relatively The western outflow shows particularly interesting struc- large 1 noise levels which have been estimated from the backtures which can be described as a series of bow shocks propa- ground flux variation using an 8 aperture. Most of the emisgating along the outflow away from the source. Bows de and bc sion in the (3,2) S(3) image is actually continuum emission exdisplay a common asymmetric structure: the lower bow wing cept possibly for bow-de. Interestingly, the fluxes at 1.644 µm is approximately 1.5 times brighter than the upper wing. We are also comparable to the continuum fluxes. This implies that will interpret this asymmetry in § 4 as due to a misalignment of the dereddened continuum fluxes are 2 ­ 5 times brighter at the magnetic field with the flow through which the bow shock 1.644 µm than at 2.14µm taking into consideration the higher configurations with C-type flanks are propagating. extinction (AH = 1.6 AK ), the range of extinctions explored (see below and § 5), and that the 1.644µm filter width is 25% Emission detected at 1.644 µm is displayed in Fig. 3. Most of the emission detected here is attributed to the high level of narrower than the 2.14µm filter. The (2,1) S(1)/(1,0) S(1) racontinuum flux, as is seen in the K-band (Fig. 1). Steeply ris- tios are calculated with the continuum subtracted. The error ing above this continuum level are several concentrated [FeII] propagation results in very large errors at locations where the emission condensations, labeled 1 ­ 3, one of which forms part continuum forms a large fraction of the H2 emission. From the KSPEC data, we can extract several pieces of inof a well defined bow-shock, bow-de. formation. We calculate the differential extinction between two We confirm the existence of a band of continuum emission transition lines (in magnitudes) using which extends along the western outflow. It becomes visible F 1 1 g 2 A2 5 from the driving source and maintains a relatively constant (1) = 2.512 log F 2 2 g 1 A1 flux out to 17 from the source. Eisl ¨ ffel et al. (2003) sugo gest that this continuum is scattered radiation from HH 211- where 1 and 2 are the transition wavelengths, F1 and F2 are mm which opens the possibility of indirectly obtaining a spec- the relative fluxes, g 1 and g2 are the upper level degeneracies,

sion as well as a large proportion of continuum emission. The principle knots have been labeled as in McCaughrean et al. (1994).


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O'Connell et al.: The HH 211 protostellar outflow

and A1 and A2 are the spontaneous electric quadrupole transition probabilities taken from Wolniewicz et al. (1998). When both lines originate from the same upper level and a K-band spectral index of 1.7 is adopted, the absolute extinction is given by . (2) AK = 1.7 2.2µm 1.7 - 2.2µm 2 1 Three pairs of H2 v=(1,0) lines from the KSPEC data were used in order to determine AK : Q(2)/S(0), Q(3)/S(1) and Q(4)/S(2). The results are presented in Table 3.
Table 3. AK values (in magnitudes) which have been determined from the KSPEC relative fluxes in Table 1. The flux errors have been propagated and yield a realistic AK error estimate. Q(2)/S(0) AK east AK west 2.7 ± 1.5 Q(3)/S(1) 1.5 ± 0.5 Q(4)/S(2) 1.8 ± 0.5

3.9 ± 2.3

2.4 ± 0.5

3.2 ± 2.0

Immediately evident is that the extinction is higher in the western knots although the extinction determination in the west is rather tentative due to the observing conditions. A higher extinction than in the east is plausible since the western lobe is redshifted and the embedded cloud (observed in H13 CO+ by Gueth & Guilloteau (1999)) extends predominantly in this direction. In order to determine the average value of A K each line pair was assigned with the relative line strength as a statistical weight. The statistically weighted average A K values for the eastern and western components are 1.8 and 2.9 magnitudes respectively. These values fall within the error limits and are used to deredden luminosities in § 4. In order to interpret the data, we employ the Column Density Ratio (CDR) method. We determine the column of gas, N j , in the upper energy level, k T j , necessary to produce each line. We then divide these values by the columns predicted from a gas at 2,000 K in local thermodynamic equilibrium with an ortho to para ratio of 3. Normalising to the (1,0) S(1) line, we then plot this as the CDR, as displayed in Fig. 4. We immediately see that the CDR is not constant but a function of excitation temperature, and thus is not consistent with emission from a uniform temperature region or from a single planar shock. Furthermore, no significant deviations from a single curve are identified (apart from that derived from the (1,0) S(5) which is not confirmed by the (1,0) Q(7) from the same Tj ). Hence, the ortho to para ratio is consistent with the value of three, usually associated with H2 shocks. The extinction determined in the east location also constrains the H-band lines. A significantly lower extinction would raise the CDRs for these (1,0) H2 above that of the (2,1) K-band lines which would have implied non-LTE low density conditions. As it stands, the fact that a single curve is predicted suggests a density sufficiently high to ensure LTE. Given a high fraction of hydrogen atoms, the lower vibrational levels of hydrogen molecules reach LTE at densities above 104 cm-3 .

Fig. 4. Column density ratio diagrams for HH 211, produced from vertical slits running through locations of peak emission. The extinction has been adjusted to minimise the differences in 1­0 S-branch and Q-branch lines originating from the same upper energy level. H2 (1,0) transitions are represented by squares, (2,1) transitions by crosses, and (3,2), (3,1) and (3,0) transitions by triangles. The faint squares represent (1,0) Q branch measurements.

4. Analysis

4.1. Modeling the bow shocks
The structure of the western outflow, shown in detail in Fig. 5, raises an ideal interpretation scenario. The excited H2 can be found in several distinct knots along an outflow axis. McCaughrean et al. (1994) suggested that this well organised appearance might prove particularly amenable to modeling. A series of bow-shocks is propagating along the outflow. They gradually exhaust their momentum and slow down as they plough through either ambient gas or the material in the wakes of upstream bow shocks. They encounter less dense gas towards the edge of the cloud. The submillimeter observations of Chandler & Richer (2000) show that the HH 211-mm envelope density decreases with distance from the source. The azimuthally averaged density structure is well fitted by a single power-law, r -1.5 , out to 0.1 pc (the projected distance of the outer knots of the outflow from the source). However, the density profile is extended in a direction perpendicular to


O'Connell et al.: The HH 211 protostellar outflow

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Table 4. Model parameters derived to fit the bow images with C-type shocks. Parameter Size, Lbow (cm) H Density, n (cm-3 ) Molecular Fraction Alfven Speed, vA (km s-1 ) ´ Magnetic Field (µG) Ion Fraction, Bow Velocity, vbow (km s-1 ) Angle to l.o.s. s Parameter Field angle, µ bow-de 1.0 â 10 8.0 â 10 0.2 4 193 1.0 â 10 55 100 2.10 60
16 3

bow-bc 1.0 â 10 4.0 â 10 0.2 4 137 3.0 â 10 40 100 1.90 60
16 3

bow-a 1.0 â 10 3.0 â 10 0.2 4 118 5.5 â 10 29 100 1.75 60
16 3

-5

-5

-5

can explain the high infrared fluxes which are observed in bow shocks. However, their observed velocities often exceed the H 2 dissociation speed of 40­50 km s-1 (Smith et al. 1991) giving rise to a double-zone bow shock composed of (1) a curved J-type dissociative cap (responsible for atomic emission) and (2) C-type wings (where H2 emission is radiated). Our C-type bow shock model consists of a three dimensional curved surface described by Z/L
bow

= (1/ s)(R/L

bow

)s,

(3)

where Z and R are cylindrical coordinates, Lbow characterises the bow size and s defines the shape or sharpness of the bow. The curved shock surface is then divided into a very large number of steady-state planar shocks for which the detailed physics and chemistry are computed (Draine 1980; Smith et al. 1991). Each mini-shock propagates at a different velocity depending Fig. 5. The HH 211 western outflow is shown here in H2 (1,0) S(1), on the angle between the shock surface normal and the direcH2 (2,1) S(1) and [FeII] emission lines. The images are not contintion of motion of the bow. The temperature reached, and hence uum subtracted as the bow shocks are relatively pure. Greyscale bars the excitation conditions, depends on the shock velocity, den-2 -18 -2 represent flux levels per arcsec in units of 10 W m . sity, magnetic field strength, etc., as well as the magnetic field direction (see Table 4). the outflow and the outer western knots are located outside this The systematic method of exploration of parameter space is given in O'Connell et al. (2004). In summary, the bow ludense core region. The bow-shock models described by Smith et al. (2003) minosities provide constraints on the density and bow speed. have been successfully employed to interpret the sequence of The location of the emission in the flanks or apex also conbow shaped features along the HH 240 outflow (O'Connell strains the bow speed. In addition, the ion fraction constrains et al. 2004). Here we apply the same model which serves as the transverse bow thickness as well as the bow speed. The a useful interpretive device. We wish to determine if a system- magnetic field strength strongly influences the extent of the atic change of one or more parameters results in a close match wing emission and the atomic fraction affects the line ratios. with the observed bow structures and thus to analyse the out- As demonstrated by previous modelling, the bow speed is limflow's changing environment. We have applied the bow-shock ited to within 15%, the orientation to within 10 and the density, magnetic field, atomic fraction and ion fraction to within model to the three leading structures: de, bc and a. Jump-type (J-type) bow shocks can explain many of the a factor of two. features associated with high excitation emission regions. The observed H2 (1,0) S(1) luminosity for each bow proThey cause rapid heating and dissociate H2 molecules for vides us with the strongest density and velocity constraint. For shock speeds greater than 24 km s-1 (Kwan 1977). However, a given bow size, Lbow , the line emission is directly proporContinuous-type (C-type) bow shocks have proved extremely tional to the mass density (= 2.32 â 10-24 nc ) and (vbow )3 . The successful at interpreting most of the observed structures. The observed bow luminosities have been dereddened using a Kmeasured low fraction of ions in molecular clouds is con- band extinction of AK = 2.9 mag (see § 3). Table 5 lists the sistent with their application. The magnetic field cushioning observed dereddened luminosities for each knot along with the means that less energy goes into molecule dissociation; they predicted model luminosities.


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O'Connell et al.: The HH 211 protostellar outflow

Table 5. Observed and predicted bow shock luminosities and (2,1) S(1) / (1,0) S(1) flux ratios. Luminosities are expressed in units of L and a distance of 315 pc is adopted. Luminosities have been dereddened using K-band and H-band extinctions of 2.9 and 4.5 magnitudes (using AH = 1.56 â AK from Rieke & Lebofsky (1985)) although the errors in these values are relatively large, see table 3. respectively. Note that the [FeII] luminosities have been predicted using a J-type shock model. Line bow-de H2 (1,0) S(1) H2 (2,1) S(1) H2 (3,2) S(3) [F eI I ] 4 D7/2 ­ 4 F 2/1 ratio bow-bc H2 (1,0) S(1) H2 (2,1) S(1) H2 (3,2) S(3) [F eI I ] 4 D7/2 ­ 4 F 2/1 ratio bow-a H2 (1,0) S(1) H2 (2,1) S(1) H2 (3,2) S(3) [F eI I ] 4 D7/2 ­ 4 F 2/1 ratio


Observed



Dereddened

C-type Model

9/2

4.6 â 10-4 4.3 â 10-5 9.8 â 10-6 3.6 â 10-5 0.09 (0.02)

6.5 â 10-3 6.1 â 10-4 1.4 â 10-4 2.2 â 10-3 0.09 (0.02)

6.8 7.3 1.6 2.3

â 10 â 10 â 10 â 10 0.11

-3 -4 -4 -3

9/2

9.7 â 10-5 6.85 â 10-6 no det. no det. 0.07 (0.02)

1.4 â 10-3 9.7 â 10-5 ­ ­ 0.07 (0.02)

1.4 1.5 2.7 1.2

â 10-3 â 10-4 â 10-5 â 10-4 0.12

9/2

2.2 â 10-5 1.7 â 10-6 no det. no det. 0.08 (0.04)

3.1 â 10-4 2.4 â 10-5 ­ ­ 0.08 (0.02)

3.2 2.5 2.6 1.2

â 10-4 â 10-5 â 10-6 â 10-6 0.08

The 3 detection limits for the observed knots over an 8 circular aperture are: 2.6 â 10-5 for (1,0) S(1); 1.8 â 10-6 for (2,1) S(1); 6.4 â 10-6 for (3,2) S(3); and 3.1 â 10-5 for the [FeII] image.

The parameters selected in order to model each bow are given in Table 4. A constant Alfv ´ speed is maintained so that en the magnetic field varies with density (B=vA (4)0.5). From the H2 radial velocity structure described in Salas et al. (2003) and that the outflow H 2 knots have an average velocity projected onto the plane of the sky of 50 km s-1 (such a velocity is consistent with our modelling results) we have estimated an inclination angle to the plane of the sky of between 5 and 10 . The bows have been modelled propagating at this angle, i.e. 100 to the line of sight as the western outflow is redshifted. At this angle the model images closely resemble the observations suggesting that the bow speed is similar to the shock speed, i.e the bows are propagationg in a medium which is relatively at rest. The field angle, µ, is the angle between the bow direction of motion and the magnetic field (see O'Connell et al. (2004) for a detailed description of the geometry). A misalignment of these directions results in asymmetric bow wings such as are observed for HH 211. In our case the best results were found for µ 60 ±10. Fig. 6 displays the simulated bow image to compare to bow-de in (1,0) S(1) and (2,1) S(1) ro-vibrational transition lines of H2 , as well as the [FeII] 4 D7/2 ­ 4 F9/2 transition line.

Fig. 6. A C-type bow shock model for bow-de shown in H2 (1,0) S(1) and (2,1) S(1) excitation lines. Adopting a source distance of 315 pc gives a pixel scale along the x and y axes of 1 pixel = 0 .18. The bow direction of motion is inclined to the plane of the sky by an angle of 10 , away from the observer, i.e. 100 to the angle of sight. The bow is moving at 55 km s-1 relative to the ambient medium of H density,n, = 8 â 103 cm-3 . All flux levels are indicated normalised to the maximum H2 (1,0) S(1) line flux. Hot dissociative shocks are necessary to induce emission from [FeII]. Here we have employed a J-shock model (same parameters) to simulate the flux distribution at 1.644 µm.


O'Connell et al.: The HH 211 protostellar outflow

9

Fig. 8. Faint (1,0) S(1) emission is detected at bow-a. A bow propagating at 29 km s-1 generates the correct (1,0) S(1) luminosity with luminosities from the other lines below the level of detectability, see table 5. Adopting a source distance of 315 pc gives a pixel scale of 1 pixel = 0 .18. The grey scale flux indications are normalised to the maximum (1,0) S(1) level for bow-de in Fig. 6.

Fig. 7. C-type bow shock model for bow-bc which reproduces the observed H2 (1,0) S(1) and (2,1) S(1) flux distribution. Adopting a source distance of 315 pc gives a pixel scale of 1 pixel = 0 .18. Here we take a lower bow speed of 40 km s-1 and a pre-shock H density of 4 â 103 cm-3 to match the observed luminosity. Other parameters are given in table 4. The grey scale flux indications are normalised to the maximum (1,0) S(1) level for bow-de in Fig. 6.

The shock speed is 55 km s-1 and the pre-shock density is 8 â 103 cm-3 . The bow size, Lbow , has been chosen in order to match the distance between the upper and lower wings, in this case 5 . The [FeII] emission is generated by a J-type dissociative bow and is restricted to a compact but elongated zone towards the bow apex where the highest temperatures are reached. However, the observed [FeII] emission is restricted to a single compact condensation, unlike the model distribution. According to this model the total cooling in the (1,0) S(1) line is 6.8â10-3 L , 1.4â10-1 L in all H2 rotational and vibrational lines and 2.4â10-1 L in all atomic and molecular lines. Bow-bc is observed in both the (1,0) S(1) and (2,1) S(1) lines. Fig. 7 displays the model generated images which closely resemble the observations. The distance between the upper and lower wings is 3 . This knot is modeled with a reduced bow speed of 40 km s-1 which is propagating into a lower density medium (n = 4.0 â 103 cm-3 . No [FeII] emission is observed,

consistent with the fact that its predicted luminosity lies below the detection threshold. According to this model the total cooling in the (1,0) S(1) line is 1.4â10-3 L , 3.6â10-2 L in all H2 rotational and vibrational lines and 5.4â10-2 L in all atomic and molecular lines. Bow-a appears in the (1,0) S(1) line as a compact knot of emission with a (2,1) S(1) / (1,0) S(1) ratio of 0.08 ± 0.02. A bow speed of 29 km s-1 results in emission restricted to the bow apex. The calculated line cooling is 3.2 â 10-4 L in the (1,0) S(1) line, 8.8 â 10-3 L in all H2 lines and 1.3 â 10-2 L total line cooling. We conclude that the observed structures are recreated in the model by solely altering the density, bow speed and ion fraction while all other parameters remain fixed. The extent of the bow is influenced principally by the bow speed as it determines the location of the H2 emission. A slower bow is characterised by emission concentrated closer to the bow front. For this reason we have maintained a constant Lbow for all the models. The bow speed systematically decreases as an otherwise similar bow ploughs into a material of decreasing density. An increase in the ion fraction is expected in less dense regions where cosmic rays and UV photons can more easily penetrate the gas. The driving power of a bow is converted into heat at a theoretical rate P = v3 L2 /2 or bow bow
2 Lbow L, -1 1.0 â 1016 cm (4) where is a non-dimensional factor related to the aerodynamical drag and is of order unity. is the mass density equal to 2.32 â 10-24 n (n is the hydrogen nucleon density). The derived numbers are thus consistent with expectations.

P = 0.13

n 8.0 â 103 cm-3

vbow 55 km s

3


10

O'Connell et al.: The HH 211 protostellar outflow

Fig. 9. A map showing the ratio of our 2.122µm and 2.248µm images. The two images have not been continuum-subtracted (due to the very low S/N of the continuum image), so the map only reveals the H2 (2,1) S(1) / (1,0) S(1) line ratio towards the ends of the outflow, where the continuum is weak; the darker regions near the center of the image are where the emission is dominated by the continuum. Logarithmically increasing contour levels are at 0.08, 0.11, 0.16, 0.22, 0.32, 0.45 (black) and 0.63, 0.89 (white).

the submillimeter dust emission maps of (Chandler & Richer 2000) which do not reveal a strong asymmetrical density disWe have also divided the fluxes in the narrow-band images con- tribution between the eastern and western outflows. Continuum taining the (1,0) S(1) flux and the (2,1) S(1) flux to produce the light encounters less hindrance along the eastern outflow where ratio image displayed in Fig. 9. In order to produce this map it terminates alongside shock excited H2 emission at knots i and both images were first smoothed with a Gaussian FWHM = j, 45 (2.1 â 1017 cm) from its protostellar origin. Here contin0 .6 and values lying below the noise level were excluded be- uum emission is seen at 2.14µm (Eisl ¨ ffel et al. 2003) but not o fore dividing. Note that the images were not continuum sub- in our ratio image as the H2 emission is relatively strong here. tracted, due to the poor quality of the continuum image at The origin and implications of the continuum emission will be 2.14µm, so Fig. 9 reveals two distinct outflow regions: (1) areas discussed in § 5. where the continuum emission is relatively strong possess a ratio above 0.3 and approach 1.0 where only continuum emission The excitation ratio along the western outflow, 0.1, is typis present and (2) parts of the outflow not effected by continuum ical of outflows seen in collisionally excited emission (Black emission where the H2 excitation ( 0.1) can be studied. & Dalgarno 1976; Shull & Beckwith 1982). There is an inThe outflow regions dominated by continuum emission are crease in the excitation ratio towards the leading edge of bowlocated along the edges of the SiO jet imaged by Chandler & de where higher shock velocities and temperatures are reached Richer (2001). This supports the idea that the continuum arises and the H2 line emission reaches its maximum value. Similar through scattered light from the protostar. The light escapes conditions were found for the HH 240 bow shocks (O'Connell along a jet excavated cavity but not along the high density jet et al. 2004) and they are well explained through the bow shock itself. It is cut off at 17 (8.0 â 1016 cm) along the western out- interpretation. However, a strong deviation from this picture is flow where it possibly encounters a high density 1 M filament found in the [Fe II] image: The localised [FeII] emission is co(Gueth & Guilloteau 1999). It is also possible that the filament incident with a region of higher ratio, (labelled [FeII]­3) and, lies in front of the flow. This projection effect would explain the puzzlingly, not at the expected H2 dissociated bow apex as seen higher extinction measured in the western outflow, contrary to in the model generated image, Fig. 6.

4.2. The outflow continuum emission and excitation


O'Connell et al.: The HH 211 protostellar outflow

11

nosity. These criteria are indeed met for HH 211 as Lkin 0.24 L and the CO and H2 are coincident suggesting that Which mechanisms give rise to the series of near-infrared bow the shock is essentially radiative (Gueth & Guilloteau 1999; shocks? Our analysis suggests a combination of two processes: Giannini et al. 2001). Note that the average extinction is re(1) The bow shocks are generated by a series of similar out- stricted to about 1 magnitude in the K-band in order to meet flow accretion/ejection events. Using the model velocities the these criteria. time lapses between the bows (and therefore between outflow The driving source HH 211­mm has a bolometric temperaevents) are 425 and 290 years. These numbers are consis- ture of 33K which implies a youthful outflow system. However, tent with the detection of three bows given the dynamical age the HH 211 outflow itself does not reveal any characteristic of 1000 years. Fluctuations in jet activity probably manifest signs of its assumed youthfulness besides the small spacial exthemselves as bow shocks at the jet cloud impact region which tent which implies a dynamical timescale of order 1000 years. is Knot-f where the high speed CO jet terminates and H2 shock This outflow age is limited by the density structure of the enheating is initialised. At Knot-f two H2 (1,0) S(1) velocity com- vironment, as older bow shocks may simply have disappeared ponents have been observed by Salas et al. (2003). This re- into sparse material. For this reason the outflow extent itself verse/transmitted shock pair seems to indicate the critical zone cannot be used to infer the full duration of outflow activity. of jet impact where H2 bow shocks are born. After formation, The high density jet (2­5 â 106 cm-3 ) observable in SiO J the bows propagate away from the protostar towards the cloud = 54 emission implies a pre-shock density of about 2 â 105 edge and through a changing environment; they become less cm-3 (Gibb et al. 2004) and thus a maximum jet-to-ambient luminous as the density decreases and they loose their momen- density ratio of 20. The CO J = 21 maximum radial vetum. Bow-a represents the final stages in the detectable life of locity is 40 km s-1 (Gueth & Guilloteau 1999) implying a one of these bow shocks. In support of this process, we note that jet velocity of 230 ­ 460 km s-1 given an inclination angle to the molecular jet demonstrates an apparent acceleration along the plane of the sky of between 5 and 10 . Hydrodynamic nuits length of value 5 â 10-3 km s-1 AU-1 (Chandler & Richer merical simulations find that a low density envelope generally 2001). Given ballistic motions, this implies that the entire jet develops around jets of density 105 cm-3 and a jet-to-ambient now observed was ejected within a relatively short period of density ratio of 10 (Suttner et al. 1997; V ¨ er et al. 1999; olk time just 100­150 years ago. Rosen & Smith 2004). Could (pulsed) C-type jets which are (2) The bow shocks become illuminated within regions heavier, denser and more ballistic than those simulated excawhere the outflow impacts on denser clumps of gas. This idea vate low density cocoons through which continuum emission is supported by the implied high K-band extinctions of 2.9 and might escape along the outflow? To date, such high density and 1.8 magnitudes for knots de and i. These values do not repre- high velocity jets involving C-type physics have not been simusent the entire outflow and cannot be used to infer the dered- lated. The new challenge for theorists is to simulate such MHD dened luminosity for the whole outflow. Additionally, strong jets. continuum emission is detected in the eastern outflow at knots HH 211 is of particular interest due to its unusually strong i and j where the extinction is high (AK 1.8). If this is indeed continuum emission. The original radiation may have escaped scattered light from the protostar then it has channeled through from the source along a cavity of low optical depth which must to where the enhanced density has resulted in scattering. The have been excavated by powerful jet events. This radiation is passage of a C-shock will increase the post-shock density with then scattered when it encounters dense walls or clumps where a compression ratio of 2 times the magnetic Mach num- the NIR optical depth along the direction facing the source is of ber (shock speed divided by the Alfv ´ speed) (Spitzer 1978). order unity. Some of the scattered radiation then exits along the en Therefore, the continuum emission is likely to be seen along- line of sight through a foreground of moderate NIR extinction. side the shock excited H2 emission as is the case for HH 211. This interpretation thus requires the original radiation to penMcCaughrean et al. (1994) suggested an average A K of 1.2 etrate a distance of order 1017 cm with a mean density of less magnitudes which is lower than the values that we derived in than 105 cm-3 before encountering walls of thickness of order § 3 but is consistent to within our error limits. Adopting this as 1016 cm and density 106 cm-3 . A moderate fraction of the scatthe average extinction over the entire outflow we find that the tered radiation then escapes without encountering further dense total H2 (1,0) S(1) luminosity of the entire outflow is 0.009 L . features along the line of sight. The high density and clumpiAccording to the predictions of our bow shock models, 4% of ness of the jet as revealed through SiO observations (Chandler the total H2 line emission (2.7% of total line cooling) is emit- & Richer 2001; Nisini et al. 2002) are important factors to conted in the (1,0) S(1) which gives an intrinsic H2 luminosity for sider in this interpretation. HH 211 of 0.23 L and a total luminosity, Lrad , resulting The [FeII] emission at 1.644 µm originates from an upper from all line emission of 0.34 L . If all the mechanical en- energy level of 11,300 K (compare to 6,953 K for H2 (1,0) S(1)) ergy is converted into radiation then Lmech is equivalent to Lrad therefore we expect it to highlight the hotter, high excitation reand Lmech / Lbol 10%. gions of an outflow. As shown in § 4, the [FeII] emission from Provided that the velocity of the swept up gas, which coin- a typical bow shock should be located towards the front of the cides with the CO outflow, roughly equals the shock velocity bow, where H2 is dissociated. Clearly, this is not the case for (i.e. a radiative shock), Davis & Eisloeffel (1996) have shown HH 211 (bow­de). The most likely explanation is that the front that Lrad should be roughly equal to the kinetic luminosity Lkin of the bow shock is traversing a low density region. Material of the outflowing material as measured through the CO lumi- has been swept out by outflow activity and the bow shocks be-

5. Discussion


12

O'Connell et al.: The HH 211 protostellar outflow

come luminous only where they interact with the wall of this hollowed out cavity. In this way it is possible see [Fe II] emission coincident with bright H2 emission in the `shoulders' of the bow shock. Such an explanation is also supported by the distinct possibility that continuum emission from the source is escaping along an outflow excavated cavity.

6. Conclusions
We have studied the HH 211 protostellar outflow in the nearinfrared regime through high resolution imaging and spectroscopy. Images in the (1,0) S(1) and (2,1) S(1) ro-vibrational transitions of H2 have been analysed in order to study the excitation throughout the outflow. A narrow-band image at 1.644µm was presented where the outflow is clearly seen and several confined condensations of emission are identified. In addition, we have presented K-band spectroscopic fluxes for two separate prominent locations from which the extinction and excitation conditions have been investigated. We have successfully modeled the series of bow-shocks in the western outflow as curved 3­dimensional shock fronts with steady state C­type physics. Our findings have lead to several conclusions about the nature of the outflow: ­ C-type bow shocks propagate along the western flow. Model fitting has constrained several parameters including the density, bow velocity, ion fraction, intrinsic bow shape and magnetic field strength and direction. ­ The bow shocks are passing through dense clumps where their luminosity is accentuated. High values of extinction are measured in these regions. ­ Bows de, bc and a are plausibly modeled as a series of initially identical bow shocks propagating through a medium of decreasing density. The bows slow down and disappear as they approach the cloud edge. ­ A misalignment of the magnetic field and outflow directions can account for the observed bow shock asymmetries. ­ The most likely source of the protracted continuum emission is light from the protostar which evades dense core obscuration by escaping through a low density cavity excavated by the jet, as discussed in § 5. The continuum light is scattered when it encounters the denser material aligning the jet tunnel and the dense clumps along the outflow. ­ The excitation along the outflow is typical of outflows in general. The ortho to para ratio of 3 for molecular hydrogen implies shock heating as the source of the near-infrared line emission. ­ The [FeII] emission is predicted and detected in isolated condensations. These condensations are coincident with strong H2 emission. However, the location of the [FeII] emission is puzzling; it is not found in the expected bow apex region as predicted. This may also be due to the low density tunnel through which the bow apex is propagating.

hundred years) give rise to a variable jet velocity. The resulting shocks manifest themselves as C-type bow shocks at the principle jet/ambient medium impact region where they are detected outside the dense core where the extinction is lower. The bows propagate towards the cloud edge through a changing environment. The model suggests that the mean density decreases with distance from the core but that the bow shocks brighten where they encounter dense clumps. It is feasible that these clumps consist of gas swept up by passage of previous bows driven by the alternating outflow power. It is clear that in-depth studies of a wide range of protostellar outflows will yield valuable insight into how the cloud environment sculpts the outflow and how much the environment itself has been influenced by the star forming process.
Acknowledgements. This research is supported by a grant to the Armagh Observatory from the Northern Ireland Department of Culture, Arts and Leisure and by the UK Particle and Astronomy Research Council (PARC). We would like to acknowledge the data analysis facilities provided by the Starlink Project which is run by CCLRC / Rutherford Appleton laboratory on behalf of PPARC. This publication makes use of the Protostars Webpage hosted by the Dublin Institute for Advanced Studies.

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