The SiO lines exhibit a variety of profiles, ranging from narrow lines (1-3 km s width) at ambient velocities to broad profiles (10-20 km s ), with complex profiles consisting of a blend of low and high velocity components as intermediate stages. In the regions where SiO was mapped, the low velocity SiO emission comes from regions definitely offset from the position where the high velocity emission is present, indicating that the low and high velocity SiO emissions trace two distinct regimes. The SiO abundances are different in those two regimes: we estimate that typical SiO abundances are 10 -10 in the high velocity components, but they decrease by two orders of magnitude (10 -10 ) when SiO is detected at low velocities.
The hydrogen volume densities estimated from the multiline SiO
observations are in the range 10 to fews 10 cm , in both the
low and the high velocity regimes, indicating that all the SiO
emission arises in shock-compressed regions. We argue that the
different observed SiO profiles could be caused by an evolutionary
effect: the SiO molecules produced at high velocities could be slowed
down because of their interaction with the surrounding gas before they
stick onto the dust grains. However, the possibility that the low
velocity SiO emission is due to slow shocks cannot be ruled out,
but this would require the presence of a small amount of silicon
compounds on the dust grain mantles.Astronomy & Astrophysics, in press
Figure 13: Observed (filled symbols) and predicted (open symbols) column densities of the carbon-chain radicals in IRC+10216 (after Guélin et al. 1997&98). The predicted column densities are taken from Millar & Herbst (1994) and correspond to the peak abundances of the different species.
The current chemical models predict that C H forms from C H and C H from C H. C H itself results from the photodissociation of acetylene. A lower limit to the formation times of C H and C H can be derived by using the largest possible reaction rates, gas density and acetylene abundance. It takes yr to form C H from C H and C H from C H, which means, considering the envelope expansion velocity (14 kms ) and its distance (simeq 200 pc), that the abundances of these species should peak respectively and further out than H. In contrast, Figure 14 shows that the 3 species coexist spatially within and should form quasi-simultaneously (i.e. within 120 yr).
The very good agreement of Figure 13 is thus fortuitous and a new formation mechanism must be sought. This mechanism could be the desorption from dust grains of weakly attached carbon-chains, under the influence of interstellar UV and/or shocks.
Figure 14: Brightness temperature distribution of the 3-mm lines of C H, C H and C H, observed in IRC+10216 with the IRAM interferometer. The intensities (multiplied by 1,1.3, and 7, respectively) have been integrated over a narrow band centred on the star velocity and represent roughly the species' abundance distribution in a meridian plane. Except for the larger noise in C H, the 3 maps look very similar; their brightest spots lie all along a circle of radius 16''. Note that the emission from the central star has been removed.
Proc. 3rd Cologne-Zermatt Symposium The Physics and Chemistry of the Interstellar Medium, ed V. Ossenkopf.
Figure: CO emission (thin contours) integrated in two different velocity intervals and superimposed on the H v=1-0 S(1) emission (greyscale; from McCaughrean et al. 1994) and the 230 GHz continuum emission (thick contours; contours are 10, 30, 50 and 70 mJy/beam). The angular resolutions are for the H , at PA for the CO, and at PA for the continuum observations. Upper panel: CO emission integrated between LSR velocities 2.2 and 18.2kms (the systemic velocity is 9.2kms ); contours are 1.6 Jykms /beam. Lower panel: CO emission integrated for velocities lower than 2.2kms and larger than 18.2kms ; first contour is 1 Jykms /beam and contour step is 1.5 Jykms /beam.
We present high angular resolution (down to )
interferometric maps of the CO
and emission in the
molecular outflow associated with the extremely young HH211
jet, which is located in the IC348 molecular complex. At
velocities close to the systemic velocity, the CO emission
traces the outflow cavities, while an extremely collimated,
continuous jet-like structure is observed at high CO velocities.
The continuum emission reveals a dust condensation
surrounding the central exciting (Class 0) protostar, clearly
resolved and elongated perpendicular to the jet axis. The strong
(bow-)shocks observed in vibrationally excited H emission are
located at the terminal ends of the jet and the low-velocity CO
cavities are precisely situated in their wake. Hence, the overall
structure of HH211 perfectly fits into the picture of a
jet-driven flow and strongly supports shock-entrainment models as
the formation mechanisms of young, embedded molecular outflows.
The shape of the cavities traced by the low-velocity CO emission
can actually be (surprisingly well) reproduced by a simple,
semi-analytical toy-model of a jet-driven flow, in which prompt
entrainment occurs at the head of a travelling bow-shock. The
estimated jet mass and mass loss rate yield a timescale of order
one thousand years, in agreement with the kinematical age.
Finally, we discuss the physical properties of the different
parts of the outflow, and especially the actual nature of the
high-velocity CO jet.
Astronomy & Astrophysics, in press
e-mail: email@example.com Preprints:http://iram.fr/PP/papers.html
Figure 16: Zeeman observations of the H transition in MWC349 obtained with an autocorrelator backend and smoothed to a resolution of 0.7 kms . All data obtained during 23-28 June 1996 have been averaged, representing 245 min of polarization-switched spectra. The three subframes show: (a) the total power spectrum (Stokes I), (b) the V-spectrum (RHC-LHC), corrected for instrumental polarization, (c) same as (b), but observed with the filter bank at a spectral resolution 1.3 kms . Vertical dotted lines mark the maxima of the total power maser spikes.
Using Zeeman observations of the H recombination line maser transition at 1.3 mm we have detected a magnetic field which is associated with the corona of the circumstellar disk of MWC349A. At a radial distance of 40 a.u.,where the H maser is located, the line-of-sight component of the field is approximately parallel to the plane of this edge-on disk, and its average strength is 22 mG. The corresponding magnetic energy density is % of the thermal energy density of the plasma where the maser emission originates, very likely making the detected field dynamically important. Spectral fine structure of the detected Zeeman pattern suggests that the field may have a strong radial component, although other models for the field configurations are possible. The strength of the field at such a large distance from the star makes it unlikely that the field is of stellar origin. We suggest that it is generated by a local disk dynamo.%
Astron. & Astrophys., in press