The molecular jet driving the HH212 protostellar outflow

1
The molecular jet driving the HH212 protostellar
outflow
C. Codella1, S. Cabrit2, F. Gueth3, F.
Bacciotti4, R. Cesaroni4, B. Lefloch5, D.
Panoglou6, P. Garcia6 , M.J. McCaughrean7
1. INAF, Istituto di Radioastronomia, Firenze,
Italy 2. LERMA, Observatoire de Paris, France
3. IRAM, Saint Martin d'Heres, France 4. INAF,
Osservatorio Astrofisico di Arcetri, Firenze,
Italy 5. Observatoire de Grenoble, France 6.
University of Porto, Portugal 7. University of
Exeter, UK

We performed SiO(2-1) and (5-4), and continuum
observations of the bipolar HH212 Class 0 outflow
using the PdB interferometer with unprecedented
angular resolutions (up to 0.34?). The SiO
emission is confined in a highly collimated
bipolar jet, located along the outflow main axis.
It can be traced down to 500 AU of the driving
source, in a region that is heavily obscured in
H2 images. SiO shows the same kinematics as H2
supporting that both molecules are tracing the
same jet.
2
(a)
(b)
(c)
FIG. 1
3
(a)
(b)
H2
H2O
FIG. 2
4
Blue 1
Red 1
Blue 2
Red 2
FIG. 3
5
(a)
(b)
FIG. 4
Wiseman et al. (2001)
Davis et al. (2000)
-4.9 km/s
NK 1
-3.8 km/s
-5.5 km/s
(c)
H2
BLUE
RED
SE
BLUE
RED
BLUE
NW
RED
6.3 km/s
4.6 km/s
SK 1
4.0 km/s
6
Introduction jets from protostars
The launching of jets from protostars (Class 0
objects) is one of the most intriguing phenomena
in astrophysics. One fundamental problem to which
jets are believed to provide a solution is the
necessary removal of angular momentum from the
central accreting system, required to permit low
angular momentum material to fall onto the
protostar. Recent optical observations (e.g.
Bacciotti et al. 2002) suggest that atomic jets
from evolved T-Tauri stars may indeed transport
angular momentum away from the inner regions of
the accretion disk. Is the same mechanism at the
origin of the molecular jet counterparts observed
when the protostar is still deeply embedded in
its natal high-density core? To address this
question, we investigate HH212 which is a highly
symmetric bipolar H2 jet (Zinnecker et al. 1998)
driven by a low-luminosity Class 0 protostar in
Orion. HH212 is optimal to study the jet
kinematics, since it lies close to the plane of
sky (4º Claussen et 1998), shows hints of
rotation in one H2 knot (Davis et al. 2000), and
is surrounded by a compact ammonia core rotating
about an axis aligned with the jet (Wiseman et
al. 2001). As a tracer, we used SiO, which
generally suffers minimal contamination from
infalling envelopes or swept-up cavities (e.g.
Hirano et al. 2006).
7
HH212 a microjet from a Class 0 object
FIGURE 1ab. Contour PdBI maps of the integrated
J2-1 and 5-4 SiO emissions (contours). The
unprecedented angular resolution (up to 0.34? at
1.4mm) allows accurate comparison with a new,
deep H2 image obtained at the VLT (colour scale).
The white triangle marks the driving protostar
MM1 (FIG. 1c). The SiO emission is confined in a
highly collimated bipolar outflow. Two pairs of
bullets are present an outer pair (Blue 1, Red
1) that follows the H2 intensity distribution
beyond 10? from the protostar, and a new inner
pair within 2? (920 AU) of the source (Blue 2,
Red 2) tracing an unresolved transversally jet.
The inner jet has no H2 counterpart., as the
whole structure lies in the high extintion region
around the protostar. No SiO emission has been
detected towards the brightest H2 knots (SK1,
NK1). A possible explanation is that SK1 and NK1
trace powerful shocks where SiO does not
form/survive. The detection of FeII emission in
SK1 and NK1 (Zinnecker et al. 1998, Caratti o
Garatti 2006), and their wide bow-shock geometry
(FIG. 1ab) support this hypothesis. FIGURE 1c.
Zoom of the right panel of FIG. 1ab stressing
the central SiO(5-4) lobes. The colour image is
for the 1.4mm continuum emission, which clealry
shows the driving source (MM1). A second source
(MM2) is tentatively detected (S/N 6) towards
East.
8
Jet kinematics SiO, H2 and H2O
FIGURE 2a. Position-Velocity (PV) cut of SiO(2-1)
along the jet main axis. The ambient LSR velocity
is 1.6 km/s. Offset is computed with respect
to the driving source MM1. Blue-shifted emission
comes from the North-East bullets, while
red-shifted lines are detected towards the
South-West. Each bullet si associated with a
quite large velocity range (FWZI 20 km/s).
Filled triangles with error bars show the
velocity centroid and intrinsic FWHM of H2 knots
(Takami et al. 2006). SiO shows the same overall
kinematics as H2, indicating that both molecules
are tracing the same jet. In other words, SiO is
probably tracing the base of the large-scale
molecular jet. FIGURE 2b. Zoomed-in PV cut of
the inner jet in SiO(5-4). Black arrows show the
velocity range of H2O masers 0.1? from MM1
(Claussen et al. 1998). The little blue/red
overlap in each lobe despite the jet lies close
to the plane of the sky (4º) rules out that SiO
traces a wide-angle high-velocity wind. The fact
that SiO emission extends to ambient velocities
further argues for the presence of intrinsically
slow material. A possible origin for this slow
material would be unresolved bow-shock wings.
Indeed, H2O spots observed in VLBI reveal curved
bow-shocks covering a substantial range in radial
velocity. Given the restrictive excitation and
coherence conditions for maser amplification, the
observed range is only a lower limit to the full
velocity range in the bow. Hence, internal
bow-shocks probably contribute significantly to
the broad SiO line widths.
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SiO emission along the jet
FIGURE 3. (Upper panel) Comparison between the
integrated (K km/s) SiO(2-1) (solid line) and
SiO(5-4) (dot-dashed line) emission, corrected by
the primary beam effect, along the main outflow
axis. The SiO(5-4) values refer to the emission
convolved to the HPBW of the SiO(2--1) map. The
dotted vertical lines delimitate the SiO(5-4)
primary beam. (Lower panel) Integrated intensity
SiO(5-4)/SiO(2-1) ratio. The SiO excitation
conditions seem to vary along the molecular jet
embedded in the high density gas, being (i) the
inner jet more excited than the outer bullets,
and, in particular, (ii) North 2 more excited
than South 2.
10
Search for jet rotation
FIGURE 4a. PV plots of the SiO(5-4) intensity
perpendicular to the HH212 jet axis. The offset
is computed with respect to the driving source
MM1. The inner jet (Blue 2, Red 2) does not show
signs of velocity gradient. If rotation is
present, it has to occur in a region 0.3? (138
AU) and/or with velocities 1 km/s. This finding
does not agree with the velocity gradient
observed by Davis et al. (2000) towards the H2
knot SK1 on scales 2? (FIGURE 4c). As their
gradient goes in the same sense as that of the
NH3 protostellar core rotating about the jet
(FIGURE 4b), they interpreted it as jet rotation.
However, the present observations suggest that
the H2 map traces bow-shocks (see the SK1/NK1
morphology in FIG. 1ab) and thus the
correspondent gradient could just trace
asymmetries between bow-shocks wings. On the
other hand, a clear gradient is observed across
the Red 1 bullet (FIG. 4a, bottom panel, green
line) blue_at_South-East and red_at_North-West .
However, the gradient goes in opposite sense to
the rotation pattern of the NH3 core, ruling out
we are observing rotation. The fact that the
centroid of Red 1 is slightly displaced from the
jet axis also suggests that this effect is due to
a kinematical asymmetry in the shocked material.