Astron. Astrophys. 343, 571-584 (1999)

3. Results

The CO emission clearly delineates two separate entities (see Fig. 4): the outflow cavities seen at low velocities (i.e. velocities close to the systemic velocity), and an extremely well collimated jet-like structure, which is essentially visible at high velocities. These two components can also be seen in the position-velocity diagram (Fig. 5), where they have different kinematical behaviours.

3.1. The outflow cavities

The images presented in Figs. 2 to 4 do not include short-spacing information. We used the CO  IRAM 30-m spectra obtained throughout HH 211 to estimate the total flux of the source. Fig. 6 compares the single-dish and interferometer integrated spectra: in the few channels in which the cavities are detected, the interferometer has clearly filtered out 60 to 80% of the flux. Correcting for this missing flux and assuming an optically thin emission and a CO abundance of , one can estimate the mass of the outflow to be for an excitation temperature of 50 K (i.e. a mass of for higher ). Note that if the missing fluxes of 80 and 200 Jy at velocities of 12.2 and 8.2 km  s ^-1, respectively, are uniformly spread across the extent of the outflow, they correspond to brightness temperatures of and 5 K, respectively, i.e. only to 2 contours in the corresponding panels of Fig. 3. At all other velocities, the missing flux is much less important.

Fig. 6. CO  spectrum, spatially integrated over the extent of the HH 211 outflow. The systemic velocity is 9.2 km  s -1. White histogram: IRAM 30-m observations (a minimal spectrum common to all observed positions, which is likely to correspond to the cloud emission, has been removed; it amounts  Jy at the systemic velocity, and is negligible in other channels). Shaded histogram: IRAM Plateau de Bure observations.

The eastern, blueshifted lobe of the HH 211 molecular outflow, and especially its northern (i.e. north of the flow axis) part, has an extremely regular, well-defined shape. The walls of the CO cavity appear exactly in the wake of the terminal H₂ shock, which has a clear bow shape. The southern flank of the lobe seems to be perturbed: the CO emission stops at roughly half the length of the cavity (this can be seen in the channel maps as well as in the integrated emission). Although instrumental artifacts (e.g. short-spacing filtering) and/or sensitivity limitations could explain this feature and cannot be ruled out, the coincidence between the point where the emission breaks off and a small H₂ shock (see Fig. 4) suggests this interruption is real.

The western, redshifted lobe has a slightly smaller opening angle than the eastern one, but the morphology of the two lobes is nevertheless extremely similar near the protostar. However, in contrast to the eastern cavity, the terminal part of the western lobe does not present a well-defined shape, but a more complex structure: there is no CO emission at the end of the H₂ main shock (which has a "bubble-like" morphology), but bright CO blobs are located just upstream. As discussed in the next section, the observations of the H¹³CO⁺  transition provide a clue to explain this perturbation.

3.2. A molecular filament

Bachiller et al. (1987) presented ammonia observations of the IC 348 region, revealing the large-scale structure of the molecular cloud. HH 211 lies at the center of one of the three main clumps which were identified (see McCaughrean et al. 1994). These globules have a mean density of  cm^-3 and their temperature seems to increase from  K at the center to  K at the edges (Bachiller et al. 1987). Interferometric observations of the H¹³CO⁺  line reveal a large elongated filament oriented roughly northeast-southwest (Fig. 7). The same structure is observed through NH₃(1,1) emission measured with the VLA (Bachiller 1996). It is likely that this filament corresponds to a denser region within the molecular clump surrounding HH 211. Assuming a mean temperature of 15 K, optically thin emission, and an abundance for H¹³CO⁺ of , we obtain a mass of for this filament. Incidentally, the H¹³CO⁺  spectra allow us to estimate the systemic velocity of the HH 211 system to be km  s ^-1.

Fig. 7. Overlay of the H13CO+  integrated emission (in greyscale; contour step is 50 mJy km  s -1/beam or 0.75 K km  s -1; clean beam is at PA ) and the low-velocity CO  emission (cf. Fig. 4, upper panel; contour step is 1.6 Jy km  s -1/beam; clean beam is at PA ; for clarity, the noise at the edges of the mosaic has been masked and no negative contours have been drawn). The cross denotes the position of the exciting source.

Interestingly, the protostar is located on the eastern side of this filament, and the western, redshifted lobe of the outflow appears thus to be propagating into a dense environment. Note that extinction could therefore weaken the H₂ emission, which is indeed fainter in this lobe than in the eastern one. Further downstream, the flow seems to have traversed the filament and propagate in a lower-density medium. Hence, we can speculate that this sudden change in the ambient properties affects the structure of the flow and causes the complex morphology of the CO and H₂ emission observed in the terminal part of the western lobe.

3.3. The CO jet

The high velocity CO  emission traces an extremely collimated (length-to-width ratio ) jet-like structure, which emerges from the protostellar condensation and is located close to the axis of the low-velocity CO cavities. Note however that there is a slight misalignment between the jet and cavities mean axes (see Sect. 5.2).

To investigate the internal structure of this "jet" (we further discuss below, in Sect. 5, its actual nature), we have performed gaussian fits perpendicular to its mean axis (defined with a position angle of ). Fig. 8 presents the resulting positions, widths and intensities. Several sub-structures are clearly present within the jet and are showing up as variations of the width and brightness. The closest part from the protostar has a regularly increasing width, up to a distance of from the central position (points BI and RI in Fig. 8). Note that these two points are not symmetrically placed about the central source, as the blueshifted one is farther away by a factor of . In addition, both lobes are showing a "step" in the width curve (points BII and RII), which actually correspond to the region of maximal intensities. It is noteworthy that, in the blueshifted lobe, the points BI and BII are located just upstream of the two H₂ features present along the jet axis (see Fig. 4). In the position-velocity plot (Fig. 5), BI corresponds to the extreme velocity region of the apparent acceleration zone. These properties strongly suggest that (at least) two internal shocks are propagating within the jet and are showing up as variation of the kinematical and morphological characteristics of the associated CO emission. Finally, the terminal portions of both the blue- and redshifted jets are weaker, not transversally resolved, and have a different position angle as the inner parts of the jet, as most clearly seen in the blueshifted lobe (Fig. 8, first panel; see Sect. 5.2).

Fig. 8. Results (position offset from the mean axis, width, and peak intensity) of gaussian fits performed in slices perpendicular to the jet axis on the high-velocity CO  integrated emission (Fig. 4, lower panel). The continuum emission has not been removed and shows up as a peak at the central position in the second and third panels. The horizontal dashed line in the second panel indicates the angular resolution of the observations ( in the direction perpendicular to the jet axis).

In addition to the striking morphological differences between the jet and the cavities, the position-velocity diagram (Fig. 5) also reveals strongly different kinematical behaviours. The CO jet of HH 211 shows an almost perfectly linear velocity increase with distance from the protostar ("Hubble-law"). The apparent acceleration is of the order of km  s ^-1 AU^-1, while the maximal detected velocity is km  s ^-1. For such a velocity, the kinematical age (i.e. crossing time) of the HH 211 CO jet is years, where i is the inclination on the plane of the sky. Assuming , we obtain a deprojected velocity of km  s ^-1 and an age of years.

The mass of the jet can be estimated from simple excitation considerations. The lack of short spacing information in the dataset is not crucial for that purpose, thanks to the small intrinsic width of this collimated structure (see Fig. 6). Assuming optically thin emission, a CO abundance of , and an excitation temperature of 50 K, we obtain for both CO lines a mass of (and thus a mass of for higher ). If the whole apparent volume of the CO jet is filled, the derived density is of the order of . Note that the interpretation of such values strongly depends on the actual nature of this high-velocity CO jet (see Sect. 5).

3.4. The protostellar condensation

The exciting protostar (labelled HH 211-mm) of the HH 211 outflow is associated with a small condensation located on one side of the molecular filament detected in the H¹³CO⁺ emission (Fig. 7). Fig. 9 presents the maps of the continuum emission (which traces the thermal dust emission of the protostellar condensation) that we obtained at 86 GHz (simultaneously with the observations of the H¹³CO⁺  line), 115 GHz (CO ) and 230 GHz (CO ). On the image with the highest angular resolution ( at 230 GHz), the peak is located at , (J2000.0), with a positional uncertainty of about . The condensation surrounding this peak is well resolved. The image at 230 GHz reveals a flattened structure elongated roughly perpendicular to the jet axis. Assuming a gaussian distribution, the fitted (in the uv plane, i.e. deconvolved) source FWHM is at PA , which corresponds to a linear size of  AU.

Fig. 9. Continuum emission of the HH 211-mm protostellar condensation, at three different frequencies. The axis of the jet is indicated as a dashed line. a.  86 GHz ( mm): resolution is at PA  and the contour step is 1.5 mJy/beam (). b.  115 GHz ( mm): resolution is at PA  and the contour step is 3 mJy/beam (). The 0.9, 2.7 and 4.5 Jy km  s -1/beam levels of the integrated CO  emission (see Fig. 2) are represented in greyscale. c.  230 GHz ( mm): resolution is at PA  and the contour step is 10 mJy/beam (). The 2, 6, 10 and 14 Jy km  s -1/beam levels of the integrated CO  emission (see Fig. 3) are represented in greyscale.

The integrated fluxes of HH 211-mm are 25 mJy at 86 GHz, 40 mJy at 115 GHz and 275 mJy at 230 GHz. The three available fluxes, as well as the non-detection of the source in the near-IR (McCaughrean et al. 1994), are typical of the spectral energy distribution of an extremely young low-mass protostar (a Class 0 object, André et al. 1993). There is unfortunately no IRAS source that can be associated with HH 211-mm but, as quoted by McCaughrean et al. (1994), the IRAS measurements in this region are rather confused due to the presence of a bright extended emission.

The spectral index derived from the three millimeter fluxes reported here is . For optically thin emission, which is a reasonable hypothesis at these wavelengths, and assuming that the dust absorption coefficient varies as (Beckwith et al. 1990), the spectral index should be . We thus derive . However, the filtering of the extended structures, on different scales in the three images, most probably affect (reduce) the measured spectral index. Hence, a higher value of cannot be ruled out from these observations.

Finally, we can estimate the mass of the protostellar condensation from the 230 GHz flux, assuming a dust temperature of  K (as typical for embedded protostellar cores) and  cm² g^-1 (Beckwith et al. 1990). We obtain a mass ranging from (, i.e.  cm² g^-1) to (, i.e.  cm² g^-1).

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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