Successive SiO shocks along the L 1448 jet axis

Transcription

1 Astron. Astrophys. 325, (1997) ASTRONOMY AND ASTROPHYSICS Successive SiO shocks along the L 1448 jet axis A. Dutrey 1, S. Guilloteau 1, and R. Bachiller 2 1 Institut de Radio Astronomie Millimétrique, 300 Rue de la Piscine, F Saint Martin d Hères, France 2 Centro Astronómico de Yebes (OAN, IGN). Apartado 148. E Guadalajara, Spain Received 2 October 1996 / Accepted 13 January 1997 Abstract. We present a complete SiO v = 0 J = 2 1 synthesis image of the red-shifted outflow lobe from the Class 0 protostar L 1448-mm. The image is a 5-field mosaic obtained with the IRAM interferometer at 3 angular resolution. The SiO emission arises in a highly-collimated jet extending over 0.2 pc. The jet consists of four main blobs, probably corresponding to successive episodes of mass-loss from the central object. The kinematical structure of the jet is studied by the means of velocity channel maps and position-velocity diagrams, and is compared to current models of jet-driven bipolar outflows. We have constructed a kinematical model of bow-shock to make such comparison as detailed as possible. We find that most of the SiO clumps delineate partial bow-shock structures, but important SiO emission is also seen along the jet axis itself. Shockprocessing of the dust grains, and perhaps chemical gas-phase reactions in the protostellar wind and in the mixing layer, could explain the enhancement of SiO in the different jet blobs. The CO outflow in L1448, and perhaps in all bipolar outflows, is thought to be driven by a primary jet ejected from the central star/disk system. We conclude that, if not the primary wind itself, the material traced by the SiO emission reported here is very closely linked to this primary jet. Key words: stars: formation ISM: individual objects L1448; jets and outflows; molecules radio lines: ISM 1. Introduction Rotational line emission from silicon monoxide (SiO) is one of the most interesting probes of the shocked molecular gas in the interstellar medium. The emission from the highly refractory SiO molecules is not generally detected in molecular clouds, but it marks very distinctively the regions in which shocks destroy the dust grains enhancing the gas-phase silicon abundance. High-velocity winds from young stellar objects (YSOs) are expected to generate strong shocks, and indeed broad emission Send offprint requests to: A.Dutrey lines of SiO have been detected in a number of star-forming regions (Downes et al. 1982). The jets from low-mass YSOs are of special interest, since these sources present much less confusion than higher-mass young objects. Moreover, molecular outflows seem to exhibit higher symmetry in low-mass objects than in high-mass sources, probably providing more clues about their origin. Finally, the outflows closer to the Sun are from low-mass YSOs, which provides a better spatial resolution in the observations. The highly collimated outflow in L1448 (Bachiller et al. 1990) was one of the first outflows from a low-mass YSO in which SiO was realized to be a good probe of the internal jet that could drive the molecular outflow (Bachiller et al. 1991). Since then, SiO emission is being detected in an increasing number of lowmass sources (Martín-Pintado et al. 1992; Mikami et al. 1992; Bachiller et al. 1993; McMullin et al. 1994; Blake et al. 1995). L1448 remains however one of the most interesting SiO cases, since the emission was found to consist in a narrow jet of dense high-velocity bullets in which the SiO abundance is enhanced up to 5 orders of magnitudes. The SiO emission around the driving YSO, L1448-mm, was mapped with the IRAM interferometer at 2.5 resolution (Guilloteau et al. 1992a, hereafter GBFL), revealing that the acceleration of the outflow is achieved in a region of 6 (2000 AU or 0.01 pc). A complete high-resolution map of the SiO jet has indeed a great potential interest, since it could provide important information on the structure of the jet/outflow system, but is difficult to obtain, because the L1448 outflow extends over (at least) 5 on the sky. In this paper, we present a complete map of the redshifted lobe of the L1448 outflow with a resolution of 3. The map is a high-sensitivity mosaic of five 50 fields and provides a detailed view of the jet kinematic structure. The data are compared with models of jet-driven molecular outflows. 2. Observations and data reduction The observations were carried out with the 3-antenna IRAM interferometer at Plateau de Bure (near Gap, France) between April and December A first description of the interferometer was done by Guilloteau et al. (1992b). The observing frequency was 86.8 GHz in the lower side-band (LSB) and 89.8

2 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis 759 Fig. 1). These fields were alternatively observed for 5 minutes each in order to get similar UV coverages. The data were reduced using the GILDAS software package. The maps produced by the interferometer in each field were combined and deconvolved simultaneously using a generalization of the CLEAN algorithm to mosaics (Gueth et al. 1995). The resulting images are corrected for primary beam attenuation, and thus show noise enhancement at the edge of the imaged field. For SiO, the synthesized beam has a HPBW of with P.A. 62. For H 13 CO +, an image was produced with a velocity resolution of 0.5 km.s 1, and a synthesized beam of with P.A Results Fig. 1. In grey scale, map of the CO J=2-1 emission observed at the 30-m radiotelescope (resolution 11 ). The 5 circles show the location of the fields observed with the PdBI. GHz in the upper side-band (USB) allowing the simultaneous observation of the SiO v =0 J=2 1 and the H 13 CO + J = 1 0 lines. SIS receivers provided system temperature between in LSB and in USB. We used 3 configurations, providing 18 baselines with length ranging from 24 to 176 m. The correlator was configured to offer (1) two overlapping 20 MHz bands on the SiO v =0 J=2 1 line at GHz with channels, (2) one 20 MHz band centered on the H 13 CO + J=1 0 line with 256 channels, and (3) 3 overlapping bands of 160 MHz width, with 64 channels for the continuum. The spectral resolution is 1.6 times the channel spacing (78 khz), or about 0.4 km.s 1. The flux density scale was derived from observations of 3C84 which was also used as phase calibrator. At this frequency, the flux density of 3C84 increased slowly with time from 6.5 to 7.5 Jy. We estimate that the flux density calibration is accurate to 10 %. The rms phase noise was between 10 and 25 on all baselines. Such a phase noise introduces position errors which are below 0.4. The continuum map was obtained by summing the upper and lower side-band of the 160 MHz subbands, corresponding to an effective total bandwidth of about 700 MHz. A mosaic of 5 fields (overlapping at half power of the primary beam) was observed in order to cover the whole redshifted lobe of the L1448 outflow (see Figs. 2 and 3 show maps of the SiO emission in velocity channels of 3.9 km.s 1 width. The ambient gas is at V lsr 4.7km.s 1 and it does not emit in the SiO line (Bachiller et al. 1991). All the observed SiO emission comes from the flow itself, and extends in velocity from 6.6 km.s 1 to 82.5 km.s 1. The emission is aligned on a well defined jet axis and presents a very clumpy structure. In Fig. 4 we present maps of the emission integrated in two velocity intervals which provide a good overall view of the jet. The four EHV blobs ( molecular bullets ) denoted R1, R2, R3, and R4 by Bachiller et al. (1990, 1991) are well identified in the maps. These bullets are elongated along the jet axis, and present a complex substructure; in particular R1 consists of two maxima referred to as RI, RII by GBFL. Near the position of R3 a sudden variation in the jet direction (from P.A 160 to 180 ) is observed. Moreover, a significant east-west velocity gradient is seen on R2 and R3. Previous H 13 CO + observations showed that there is a V- shaped structure near the base of the outflow (GBFL). In addition to this condensation around L1448-mm, another H 13 CO + emission peak has been revealed by the present observations close to the position of the R3 bullet, where the jet is observed to suffer an abrupt change of direction. The kinematical structure of the jet can be further investigated by the means of position-velocity diagrams. Fig. 4 presents three of such diagrams constructed along lines 1,2 and 3 which are close to the jet main directions. A similar diagram for the region around the driving source was presented by GBFL. The data reported here are noisier in this region which falls at the edge of the northern-field primary beam, but the structure observed is in excellent agreement with GBFL observations. The velocity structure of the different bullets is in good agreement with previous single-dish observations (Bachiller et al. 1991). Negative velocities are only observed near R3 in the form of a small isolated blob near 6 km.s 1 (see also Fig.2), and near the central source, where the blueshifted flow lobe starts to be seen. Following Bachiller et al. (1991) the range of velocities in which the emission is observed can be divided in three intervals denoted EHV, IHV, and SHV (for extreme, intermediate, and standard high-velocity, respectively). Little emission is observed in the IHV interval.

3 760 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis Fig. 2. Channel maps of the SiO v = 0 J=2-1 emission associated with the red lobe of L1448-mm. Contour step is 25 mjy/beam or 0.28 K. The typical noise level is 10 mjy/beam. Beam size is with P.A. 62. The 3 lines correspond to the position-velocity plots of Fig. 4.

4 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis 761 Fig. 3. Channel maps of the SiO v = 0 J=2-1 emission associated with the red lobe of L1448-mm (cont.). Contour step is 25 mjy/beam or 0.28 K. The typical noise level is 10 mjy/beam.

5 762 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis Fig. 4. Left: Maps of the SiO v = 0 J=2-1 emission associated with the red lobe of L1448-mm. The intensity is integrated from 7 to 34 km.s 1 (left panel) and 35 to 80 km.s 1 (center panel). The beam is at P.A. 62. Contour step is 0.5 Jy/beam. km.s 1 or 0.56 K. km.s 1 in both maps. Right: position-velocity diagram along the lines First contour and step are 20 mjy or 0.23 K 4. A bow-shock model Since the SiO enhancement is believed to be caused by shocks, one of the most likely alternatives for the origin of the SiO emission is that it arises from bow-shocks generated by a highvelocity jet impacting on the ambient medium. Such bowshocks have been modelled in the context of Herbig-Haro jets (e.g. Hartigan et al. 1987, Raga 1988, Blondin et al. 1990), and more recently for the specific case of molecular outflows (Raga & Cabrit 1993). A complete treatment of such bow-shock structures would at least need solving the 2-D hydrodynamics coupled to radiative transfer equations, something which is out of the scope of our work. In fact, simple kinematical models have shown to be very useful to asses the validity of the theoretical ideas, and to help identifying the basic physical mechanism at work (e.g. Gueth et al. 1996) Model description The geometry of the model is depicted in Fig. 5, the velocities are given in the frame of the exciting source. We assume axial symmetry around the jet axis z. The shape of the bow-shock is described by a function z(r), where r is the distance from the jet axis, as that used by Hartigan et al. (1987) in their radiative bow-shock models for HH objects i.e. z(r) =A r 2 +B r 4 (1) where A and B are constants. In the frame of the bow-shock, at a given point along its surface z(r), material impacting with velocity V = V amb V S is driven sideways along the bow-shock at velocity v. We derive v from V by neglecting the normal component of the velocity of the material V Sn after the shock while the tangential velocity V St remains continuous. This assumption is realistic when most of the energy is quickly radiated, which would imply important cooling effects. In the case of L1448, comparisons between two shock tracers like SiO v =0 J=2 1 and H 2 vibrationally excited (Bally et al. 1993) permit to conclude that the shocks are essentially radiative. Thus the assumption of neglecting the normal component of the velocity seems to be well fundamented. According to this assumption, the angle between the material velocity and shock axis θ =(z,v) (see Fig.5) is π 2 ξ where ξ =(V S,V Sn ) and the velocity v in the frame of the exciting source is v z =(V amb V S ) sin 2 (ξ)+v S (2) v r =(V amb V S ) sin(ξ)cos(ξ) (3) where V S and V amb are deduced from the observations as described below. The distribution of SiO along the bow shock is empirically adjusted to the observations. The density n is assumed to follow a gaussian profile of width dz as a function of the distance from the apex z o (since the model is not valid near the apex, we cut this gaussian distribution at z o ) and another gaussian profile of width dr as a function of distance to the bow shock z. Typical values of dz and dr are in the range AU and AU respectively. After computing n and v, we project the velocity along the line-of-sight and, assuming LTE conditions, we integrate the

6 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis 763 speed), the shock (or working surface, Raga et al., 1990) velocity V S can be derived from the approximate ram pressure balance between upstream and downstream flow. ρ jup (V jup V S ) 2 = ρ jdown (V S V jdown ) 2 (8) i.e. V S = βv jup + V jdown 1+β and V jup >V S >V jdown (9) Fig. 5. Geometry of the bow-shock in case of an upstream jet at V jup impacting a downstream medium moving at V amb in the frame of the exciting source. ρ jup and ρ amb are the upstream and downstream density. V S is the velocity of the bow-shock. i is the inclination angle between the jet and the observer. The bow-shock shape z(r) is given in black line. radiative transfer equation as explained in Dutrey et al. (1994). We assume that the kinetic temperature is uniform inside the SiO cooling region with values T o 50 K (Bachiller et al. 1993). Since turbulence can be very important in shocks, the local velocity dispersion v is taken to be 1km.s 1. Some purely geometrical considerations help to retrieve from the observations the fundamental parameters of the model which are V S, V amb and the inclination angle i along the lineof-sight, as demonstrated by Hartigan et al. (1987). Assuming that most of the emission originates from the bowshock itself and is not restricted to the apex, the total velocity dispersion V of the bow-shock feature is independant of the inclination angle i and is given by: V = V S V amb (4) In the frame of the observer, the extreme velocities V min and V max are: V min = V S /2 (1 sin(i)) + V amb /2 (1 + sin(i)) (5) V max = V S /2 (1 + sin(i)) V amb /2 (1 sin(i)) (6) Thus the median velocity V median =(V min + V max )/2 is equal to: V median =(V S +V amb )/2 sin(i) (7) If i is independantly known, from these simple formulae we can derive V S and V amb for each possible internal bow-shock. In the L1448 case, we assume an inclination angle i =22 as derived from the CO J = 1 0 interferometric map of Bachiller et al. (1995). Further physical considerations allow in some cases to derive the jet velocity itself, depending on whether the bow-shock is generated by an internal shock in the jet, or by the terminal shock propagating in the ambient cloud. For an internal shock with an upstream jet impacting at V jup the downstream jet at V jdown (V jup V jdown >> C s sound in which β = ρ jup /ρ jdown, ρ jdown and ρ jup are the downstream and upstream density respectively. For the terminal bow-shock, the downstream medium is identical to the ambient medium, and the equations above can be applied by replacing jdown by amb in Eqs In this case, a direct estimate of the upstream jet velocity is possible if β = ρ jup /ρ amb is known Model results We convolved the best model by the spatial resolution of the observations ( with P.A.=62 ). The simulation, which corresponds to a very broad bow shape, is shown in the frame of the observer (assuming V lsr =0km.s 1 ) where the jet is moving from the north to the south showing us its red counterpart. We assume V amb =50km.s 1 (for the observer: V amb sin(22 o )= 18.7km.s 1 ) and V S =92.5km.s 1 (for the observer: V S sin(22 o )=34.7km.s 1 ). Thus the total velocity dispersion is V =42.5km.s 1 and V median =26.7km.s 1. Figs. 6 and 7 present the results from the kinematical model. From blue to red velocities, the bow shape evolves as follows (see Fig. 6). Extreme blue velocities are first encountered from the front part of the apex of the bow-shock. Then the feature opens and adopts a bow shape (channel at 9 km.s 1 ) as soon as the intersection between the iso-velocity surface and the bowshock is an open curve. At higher velocities, this intersection closes again, and delineates a kind of ring located far away from the apex at intermediate velocities (around km.s 1 ). Depending on the SiO line opacity, this ring-like structure is dominated by the east-west edges (optically thin case) or by the back side (optically thick case τ = 1 5). At still higher velocities the iso-velocity surface becomes smaller ( km.s 1 ), the ring shrinks and moves back toward the apex. The most extreme red velocity is generated from the back side near the apex Comparison with observations We first compare the model results (Fig. 6a, 6c and 6d) with the images of the R4 structure (Fig. 6b) who reveal most of the expected features. Particularly, the back side feature dominates at 35 and 39 km.s 1, and the emission becomes smaller and moves south at higher velocities. The observed shift implies a large bow-shock. Furthermore, the lack of prominent bow shape at intermediate velocities imply that the SiO v =0 J=2 1 line is optically thick. In our simulation, we took a width dz =

7 764 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis Fig. 7. Comparison of bow-shock models with the R4 feature: Velocity-position diagrams. From top to bottom 1) optically thin model 2) observations, 3) optically thick models (τ >> 1) and 4) τ 1. A) Left side: strip along the jet axis B) Right side: the transverse dimension is integrated. Velocities of the bow-shock (V S sin(i)) and of the ambient medium in front of the shock (V amb sin(i)) are shown. Fig. 6. Comparison of bow-shock models with the R4 feature: channel maps. From left to right: 1) optically thin bow-shock, 2) observed R4 feature, 3) optically thick bow-shocks (τ >>1) and 4) τ 1. Note that 3 channels at intermediate velocities (between 15 and 25 km.s 1 ) have not been displayed, since the bow-like aspect hardly changes with velocity in between 12 and 28 km.s 1. Contours are regularly spaced AU or 13.5 at the L1448 distance of 300 parsecs. In our case, observations are in agreement with simulations in between 6c and 6d, i.e. an optically thick model suffering from slight dilution. However, the perfect bow shape profile obtained in the model is not really seen: the bow wings are weaker than expected. Several effects may explain such behaviour. In the model, we assume a uniform kinetic temperature of T k =50 K inside the whole bow shape. A more realistic model would assume at least a decreasing temperature with the z axis. In this case, the emission coming from the wings would be smaller. Moreover, we used a relatively large width dr = 500 AU or 1.7 ( half of the clean beam). Assuming a smaller width would

8 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis 765 Table 1. L1448-mm: Properties of possible the Bow-Shocks BS i ( ) V S V amb V jup R2 22 o (*) R3 22 o (*) R4 22 o (*) Notes to Table 1: Velocities are given in km.s 1. i =0 means the shock is in the plane of sky. See Sect. 4 for details about the model. (*) Velocities V jup in last column are equal to 2 V s V amb ; they are purely indicative since the above equation assumes V amb = V jdown and a density ratio β =1. imply a more important beam dilution leading to reduction of the wings. Clumpiness of the bow shock can also explain this disagreement. Fig. 7 presents two sets of velocity-position diagrams. In the right panels, the emission is integrated along the transverse dimension ( slit spectrum ), while the left panels display a strip along the jet axis. Again, reasonable agreement is obtained between the optically thick case and the general velocity-position pattern of the R4 feature. Nevertheless, two differences appear between the model results and the observations. First, the R4 feature lacks the highest velocities near the apex. Again, clumpiness can provide a possible explanation for this, since these velocities are generated by a very small part of the bow shock. Second, and most important, the R4 feature presents a downstream precursor to the main bow -shock, as if it consisted of two successive shocks indeed. This can be related to the high value of V amb required in the model, which implies that the R4 feature is not the terminal bow-shock of the outflow. This problem will be adressed in the Sect Discussion The data presented here have implications on different aspects of the outflow physics, and we discuss them in turn Role of the bow-shocks Since bow-shock models explain (at least partially) the structure named R4, we explore here wether the other prominent SiO features can be attributed to bow-shocks. The R3 bullet shows negative velocities which require transverse motions as obtained in bow-shocks: such velocities cannot be obtained in turbulent entrainment mechanisms. Also the R2 structure can be decomposed in two relatively well separated shocks, as can be seen by comparing the velocity-position diagram of Fig.4 with those of Fig.7, moreover R2 shows an incomplete bowshock pattern (channels 9 to 20 km.s 1, Fig.2-3). The lack of resolution precludes interpretation of the R1 bullets. Assuming that the bow-shocks are reasonably complete, we can use the total velocity dispersion determined from the observations and derive V S and V amb from Eqs.4-6. The inclination is taken from Bachiller et al. (1995) (precession can modify the respective inclinations of each shock by ±4, see Sect. 5.2). As mentioned above, this interpretation implies that R4 is propogating into a moving medium and that it cannot be the terminal bow-shock. It also requires the bow-shock to be optically thick. Since the average brightness (1-10 K) is low compared to the expected kinetic temperature, the beam filling factor must be very small. The derived thickness of the bow-shock is about a tenth of the beam size, i.e. about 150 AU. Also, complete bow-shock structures are obviously not seen, and a significant amount of clumpiness is needed Jet precession or deflection by an ambient obstacle? Precession is observed in a high number of Herbig-Haro optical jets, as well as in some molecular outflows, L1157 being one of the most convincing cases (Gueth et al. 1996). Precession has also been proposed as responsible for excavating the large opening-angle cavities observed at the base of bipolar outflows (Masson & Chernin 1993). Arguments for precession in L1448 come from the morphology of the SiO jet axis. GBFL already mentionned that the two sub-blobs RI and RII of the R1 structure were not perfectly aligned along the jet axis. In Figs. 2-3, the possible variations of the jet axis from R1 (RI+RII) to R4 have been displayed. 3 major directions are easily distinguished, corresponding to possible precession by about 8. One objection against the precession interpretation is the lack of corresponding features in the blueshifted lobe of L1448. This argument is nevertheless not decisive to rule out precession. Bachiller et al. (1995) argued that the blueshifted lobe of L1448-mm physically collides with the outflow from L1448-IRS3, thereby destructing any a-priori symmetry. An alternative explanation for the jet axis variation is given by a possible deflection of the jet by ambient material. H 13 CO + emission is observed at the position where the jet direction is changing. It is at this position that both blueshifted and redshifted SiO emission is well observed, and that relatively strong NH 3 (3,3) emission has been detected with the VLA (Tafalla & Bachiller 1995, unpublished observations). These observations indicate the presence of a strong shock. Moreover, the ambient velocity necessary to explain the SiO bow-shocks decreases precisely at this position from 90 to 35 km.s 1 (Table 1). The spatial coincidence of a H 13 CO + peak, the strong shock, and the change observed in the direction of the jet propagation seems to suggest that the jet is being deflected by the collision with a dense ambient clump found in its path. The collision of a jet with a dense obstacle has been recently modelled by Raga & Canto (1995). At an initial stage it is found that the jet beam is deflected along the cloud surface. However, after some time the jet digs a hole into the cloud, the deflected beam is pinched off, and the jet ends going through the cloud along a straight path. This model seems to explain the structure of some HH jets which are not aligned with any exciting source, such as HH110. Following Raga & Canto (1995), we can estimate the digging time as t = β 2R j (10) V j sin(θ)

9 766 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis Fig. 8. SiO contour maps overlaid on the H 2 emission (left), and CO J=1-0 integrated intensity (right). The arrow shows the global shift (6 ) of the H 2 emission from the nominal position given by Bally et al. (1993), who mentioned positional errors of 5. The area covered by the CO image (Bachiller et al. 1995) is indicated by the purple colour. where β is the density ratio (clump over jet), θ the angle of incidence of the jet and V j the jet velocity. With R j =20AU, V j = 180 km.s 1, θ 20, we obtain t β years. This should be compared to the dynamical time required for the shock fronts to propagate from R3 to R4, about 3000 years for V S 100 km.s 1 (see Table 1). Hence, unless the density of the jet is 10 6 times that of the dense clump, the digging time is too short. If precession exists, the digging time becomes much larger, since the jet never hits the same region of the dense clump. Moreover, in case of precession, the dense condensation also plays a role by blocking the development of the outflow cavity. Accordingly, based on the current observations, we conclude that precession exists, but deflection cannot be ruled out.

10 A. Dutrey et al.: Successive SiO shocks along the L 1448 jet axis Relative spatial distribution of SiO, CO, and H 2 emissions Fig. 8 shows a superposition of the SiO images of the L1448 jet with the H 2 v =1 0 S(1) image taken by Bally et al. (1993), and with the slow-moving CO J = 1 0 from Bachiller et al. (1995). The mean jet direction is along the axis of the conical cavity delineated by the slow-moving CO. Most of the high velocity SiO is concentrated around this axis while the CO emission traces the edges of the flow cavity. Some shocked CO and SiO moving at the same velocity ( km.s 1 ) are spatially associated only on R2 (Bachiller et al. 1995, figure 4). The H 2 emission has a filamentary appearance with several peaks which seem to be closely associated with the R3 and R4 SiO maxima. For the H 2 map, Bally et al. (1993) announced positional uncertainties of 5 while positional uncertainties of SiO emission are 0.4. In Fig.8, we shifted the H 2 emission by 6 from its nominal position. Even allowing for such high uncertainties, the coincidence between the SiO and the H 2 emitting regions is not perfect. H 2 filamentary structure similar to that seen in L1448 is also observed toward some other jets such as HH211 (McCaughrean et al. 1994) and IRAS03282 (Bachiller et al. 1994). In those cases, the H 2 emission does not coincide with the jet itself, but it is believed to arise from a narrow cocoon surrounding the true jet. The fact that the vibrational levels of H 2 are significantly populated at those interface regions indicates that the kinetic temperature of the gas is about K. SiO molecules might not survive in the hot medium traced by the H 2. In fact, ammonia observations (Bachiller et al. 1993) show that the kinetic temperature of SiO v = 0 J = 2 1 emitting region is at K, a temperature which is too low to excite the H 2 vibrational levels. Thus vibrationally excited H 2 and rotational transitions of SiO v = 0 trace different temperature regimes and it is not surprising to find spatial offsets between them. However, shocks at velocities of about 50 km.s 1 are necessary to excite the H 2 molecules as well as to enhance the SiO abundance from dust grain destruction. Since cooling times are known to be relatively short at the shocked regions ( yr), we should expect to find strong temperature gradients near the bow-shocks. Hence, if not strictly coincident, it is reasonable to find that the H 2 and the SiO emitting regions are closely associated. The lack of H 2 emission associated with the R1 and R2 bullets could be due to a higher extinction toward this part of the jet which is more deeply embedded within the molecular cloud Origin and abundance of the SiO molecules Although bow-shocks are definitely required, the above model fails to reproduce for example the precursor seen down-stream of the R4 bullet. In fact, the impression obtained from the images in Fig 2-3 is that the SiO emission is seen all along the jet, rather than being concentrated in a few well developed shocks, a situation highly reminiscent of the geometry observed in the knotty Herbig-Haro optical jets. This suggests that the creation of SiO is not exclusively linked to the bow-shock existence. The presence of SiO within the jet can be attributed 1) to the synthesis of molecules within the primary (originally atomic) protostellar wind (Glassgold et al. 1989, 1991), 2) partial destruction of dust grains in jet shocks generated for instance if the ejection velocity is time-variable, as modelled by Raga & Kofman (1992) in the context of HH jets, 3) dust destruction in the turbulent mixing layer between the jet and its surrounding and 4) all combinations of the 3 possibilities mentionned above. Unfortunately it is very difficult to distinguish observationally between these different processes. Nevertheless, our observations suggest that the SiO emission is the observational phenomenon most intimately related to the primary jet. 6. Conclusion We have shown here that the combination of images in different molecular lines (H 2, SiO, slow-moving CO, H 13 CO + )is necessary to obtain a detailed picture of the physical processes operating during the jet propagation, particularly the SiO v = 0 J=2 1 emission is a very useful tool to study the properties of the early stages of the outflow evolution. Concerning the L1448 case, we can conclude that: - Most of the redshifted SiO bullets, except R1, are in correct agreement with bow-shock models. R4 should not be the terminal bow-shock. A high degree of clumpiness and in some cases optically thick SiO lines are required to explain the observed SiO patterns. - The jet seems to precess, but we cannot exclude that it has also been deflected on a dense clump. - The precession alone cannot explain the large cavities. Bow-shocks seem to be the dominant process. - SiO abundance is highly enhanced (factor ), but the origin of SiO molecules is unclear. A non negligible amount of SiO molecules is linked to the jet itself, but our observations do not constrain its production mechanism. Finally, the processes by which a highly-collimated jet produces a CO outflow of large opening angle still remains unclear. The ambient material should be entrained by the jet via prompt entrainment (De Campli 1981) at the head of the bow-shocks, but turbulent entrainment (Stahler 1993) could also operate in the wake of the shock. Hydrodynamical models including both types of entrainment are needed to explain the structural details revealed by these observations. Although still very approximate, the recent models and simulations by Chernin et al. (1994), Raga et al. (1995) and Suttner et al. (1996) constitute very promising first steps in developing a satisfactory theoretical scenario. Acknowledgements. We thank Dr. J.Bally for providing the H 2 image of the outflow, F.Gueth for many fruitful discussions and F.Viallefond for the developments of the mosaicing software. R.B. acknowledges partial support from Spanish DGICYT grant PB References Bachiller R., Gómez-González J., 1992, A & AR 3, 257

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