A&A 372, 899-912 (2001)
DOI: 10.1051/0004-6361:20010519
R. Bachiller -
M. Pérez Gutiérrez -
M. S. N. Kumar - M. Tafalla
IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
Received 15 January 2001 / Accepted 6 April 2001
Abstract
We present millimeter-wave maps of the L1157 bipolar
outflow in several molecular emission lines. The CO
emission traces the bulk of the outflowing gas in the red
and blue shifted lobes displaying a remarkable S-shaped
symmetry indicating the presence of a precessing jet. We
determine the physical characteristics of the CO flow and
show evidence for 3 or 4 independent episodes of mass
ejection from the source. Molecules such as C3H2,
N2H+ and DCO+ are seen to be abundant only in the
quiescent medium, and result to be the best tracers of the
high-density core surrounding the driving source of the
outflow. Other molecules (SiO, CH3OH, H2CO, HCN, CN,
SO, SO2) are abundant in the outflow lobes, but exhibit
strong emission gradients. Multiline observations of some
species indicate that these gradients are not simply due to
excitation effects, but are caused by an actual
stratification in the chemical composition of the shocked
molecular gas. Shock tracers such as SiO, CH3OH, and
sulphur-bearing molecules result to be the most promising
candidates as potential chemical clocks to study the
evolution of outflows. The characteristics of the L1157
outflow, when compared to those of other outflows from
Class0 sources, indicate that L1157 is the prototype
of a category of bipolar outflows around Class0
protostars which we denominate "chemically active
outflows''.
Key words: stars: formation - interstellar medium: individual objects: L1157 - interstellar medium: jets and outflows - interstellar medium: molecules
The L1157 dark cloud harbors one of the most
illustrative cases of a bipolar outflow driven by a Class 0
protostar. Situated at a distance of only 440 pc from the
Sun, this flow is highly collimated, and is highly
inclined with respect to the line of sight. Thus the
overall geometry is very favorable to study the effects of
the propagation of the outflow in the surrounding medium.
The driving source, identified as L1157-mm (IRAS
20386+6751) is known to be a low luminosity (11 )
very young object. The blue lobe of this outflow has been
mapped in CO and SiO lines using both single dish and
interferometric observations (Umemoto et al.
1992; Mikami et al. 1992; Zhang et al.
1995; Gueth et al.
1996; Gueth et al. 1998). The
morphology of the shocked regions in a bipolar flow has
important implications on the theoretical models. For instance,
models for the propagation of a jet into the
dense surrounding medium predict the formation of large
molecular bow shocks (e.g. Raga & Cabrit 1993; Stahler
1994). These model predictions are indeed
supported by the large bow shocks seen in the SiO and CO
maps mentioned above and also in the near-IR images at
2.122
m of the H2 emission (Davis & Eislöffel
1995). We present complete CO maps of the L1157 outflow
and characterize its physical properties.
L1157 is also well-known for its rich mm-wave spectrum (e.g.: Avery & Chiao 1996). Bachiller & Pérez Gutiérrez (1997, hereafter BP97) conducted a deep survey of molecular lines at selected positions in this outflow and found strong enhancement of several molecules in the shocked regions. They noticed that the enhancement factor varied in space both as a function of the species type and of the species abundance. This was the motivation for us to conduct a detailed mapping of this outflow in several molecular lines. The shock heating and rich chemistry in the blue lobe of L1157 is comparable to that of the OriKL region. Unlike OriKL, L1157 is an isolated source without the confusion of flows from other sources in the surrounding region. It is therefore very advantageous and important to study the shock chemistry in this outflow. We present maps of the L1157 outflow in selected molecular lines and discuss the evidence for chemical stratification observed in these maps.
The observations were carried out with the IRAM 30-m
radiotelescope at Pico de Veleta (near Granada, Spain) in
1996 October. We used 9 different configurations of the
three SIS receivers operating at the bands around
3 mm, 2 mm and 1.3 mm to observe at least two
molecular lines simultaneously (Table 1). The CO
and
observations were taken with a
single configuration. We observed 320 points covering an
area of 2
6
with a spacing of
10
and an integration time of 20 seconds per
position. For the other molecules, we sampled about 80
points in the blue (southern) lobe covering an area of
1
3
with a spacing of
10
-16
.
We also observed the northern
lobe in lines of CH3OH and SiO. The SSB receiver
temperatures were about 100 K providing system temperatures
in the range
-800 K. The antenna
half-power beamwidths and main beam efficiencies ranged
from
and 0.75 at 90 GHz, to
and
0.37 at 240 GHz. This angular resolution corresponds to
4500-12000 AU at the distance of L1157 (
440 pc).
Pointing was found to be accurate to within 4
.
The
spectrometers used were filterbanks and autocorrelators
providing a spectral resolution up to 0.1 kms-1. The
observations were made in wobbler switching mode, with the
OFF position
from the position of L1157-mm
(
39
06
19;
,
J2000), except for the CO
lines, which were obtained in position switching. The
calibration of the data was achieved by the chopper wheel
method. Linear baselines were subtracted from the spectra,
and the intensities are reported here in units of main beam
brightness temperature.
![]() |
Figure 1:
Integrated emission of CO(
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Molecule | Transition | ![]() |
HPBW (
![]() |
Blue lobe: | |||
N2H+ | 1
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93.1733505 | 26 |
DCO+ | 2
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144.077321 | 16 |
C18O | 2
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219.560319 | 11 |
HCN | 1
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88.631847 | 28 |
SO | 4
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138.178648 | 17 |
SO | 6
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219.949391 | 11 |
HCO+ | 1
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89.188523 | 27 |
H2CO | 2
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140.839518 | 17 |
H2CO | 3
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225.697772 | 11 |
SO2 | 313
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104.029416 | 23 |
SO2 | 515
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135.696011 | 17 |
SO2 | 11![]() ![]() ![]() |
221.965210 | 11 |
CS | 2
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97.980968 | 25 |
CS | 3
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146.969049 | 16 |
CS | 5
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244.935606 | 22 |
CN | 1,
![]() ![]() ![]() |
113.490982 | 22 |
CN | 2,
![]() ![]() ![]() |
226.874780 | 11 |
CN | 2,
![]() ![]() ![]() |
226.659543 | 11 |
Both lobes: | |||
SiO | 2
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86.846891 | 28 |
SiO | 3
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130.268702 | 18 |
SiO | 5
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217.104935 | 11 |
CH3OH | 2k
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96.741420 | 25 |
C3H2 | 312
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145.089630 | 16 |
CH3OH | 3k
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145.103230 | 16 |
CH3OH | 5k
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241.791431 | 10 |
CO | 1
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115.271204 | 21 |
CO | 2
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230.537990 | 11 |
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Figure 2:
CO emission in several selected velocity
intervals. Each panel represents a velocity increase of 2.5 kms-1 for the blue lobe (South) and 4 kms-1 for the red one
(North) with respect to the central velocity of the quiescent
emission (
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Figure 1 shows the integrated blue and red shifted
12CO2
1 emission from L1157-mm. In agreement
with previous work (Umemoto et al. 1992; Zhang
et al. 2000), the CO emission is strongly bipolar
with respect to L1157-mm.
The western edge of the northern lobe corresponds
to a region of strong H2 emission, as seen in the images of Davis
& Eislöffel (1995) and Cabrit et al. (1998).
Both outflow lobes are very well aligned on an axis
at PA
161
.
The terminal projected velocity of the
northern lobe is about 65% higher than that of the southern
lobe. Since both lobes are well aligned on the plane of the sky at the
same PA, it seems likely that the inclination angles of both lobes
to the line of sight are also similar. Therefore, it is very likely that
both lobes are well aligned in space, and the observed difference in
projected velocity reflects an actual difference in the space
velocities of both lobes. This means that the northern lobe would be about
65% faster than the southern one. This is also consistent with the
fact that the northern lobe is more extended than the southern lobe in
a way that makes the mean kinematical ages of the two lobes very
similar (about 15000 yr), as expected in the most plausible physical
circumstance that both outflow lobes have been created
simultaneously. This common factor of space and velocity between the
lobes suggests that the outflow is better studied by comparing
velocity intervals spaced accordingly, and such a comparison
is presented in Fig. 2.
As seen in Fig. 2, each outflow lobe contains several clumps, and there is a correspondence between clumps in the sense that for each blue clump there is a corresponding red one symmetrically located with respect to the central source (a fact already noted by Zhang et al. 2000 in their SiO data). This symmetry occurs despite the curved shape of the northern lobe, which seems to be reflected in the southern lobe. The L1157 outflow, therefore, has a strong point reflection symmetry with respect to L1157-mm, and this symmetry is three-fold as it involves the (curved) lobe shape, the velocity field, and the clump structure. Such a degree of symmetry suggests an intrinsic cause, related more to the central source than to the surrounding environment, although the environment is probably responsible for slowing down the southern lobe and making it smaller. The symmetry in the individual clumps suggests that the outflow has undergone periods of enhanced ejection, and the curved shape suggests that there have been variations in the direction of the driving wind. In Sect. 5 we will explore these ideas with the help of a simplified model.
To further illustrate the symmetry of the outflow lobes (once the space/velocity factor is taken into account), we present in Fig. 3 a position-velocity diagram along the outflow axis. The clumps B0, B1, and B2, seem related, respectively, to R0, R1, and R2. Kinematical ages for the R0-B0 pair are 4500 yr, for the B1-R1 pair are 7000 yr, and for the B2-R2 pair are 15000 yr. Figure 3 shows that the events B1 and R1 have larger velocities (almost twice) compared to the other two events B0, B2 and R0, R2. This fact can also be noticed in the SiO data of the blue lobe from Gueth et al. (1996, hereafter referred to as GGB) and of the red lobe by Zhang et al. (2000). This result can be due to two possible reasons. The second events (B1, R1) follows the first events (B2, R2) in phase (by morphology) and therefore moves into a cavity that is already excavated by the first events, resulting in lesser obstruction and therefore higher velocity. Alternatively, this could be due to varying velocity of the jet in time (see, e.g., Reipurth & Raga 1999).
We first estimate the 12CO(2
1) optical depth
measuring the 12CO/13CO ratio toward selected
positions. To compute the value of this ratio (as well of that of
the CO 2
1/1
0 ratio, see below) we have first smeared
the 2
1 data to the angular resolution of the 1
0 data
(
21
).
In Fig. 4 we present the result for B0, one of
the brightest blue outflow positions (see Fig. 1), showing
that the ratio is close to 1 at ambient velocities (high
optical depth), but increases rapidly in the outflow
regime and stabilizes near 40 for velocities bluer than
km s-1.
This value is close to the isotopic ratio
(which is uncertain given isotopic fractionation, see
Wilson & Rood 1994), so we consider it
representative of the optically thin regime. For the red
lobe (not shown in Fig. 4), a similar situation is reached
for velocities larger
than
km s-1. In the following analysis, we will
concentrate on this optically thin outflow regime, ignoring
the highly uncertain optically thick part.
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Figure 3: Position-velocity diagrams of the CO emission along the L1157 outflow. First contour and step are 5% of the maximum, and the spectral resolution is 0.5 kms-1. The labels indicate the positions shown in Fig. 1. |
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For optically thin CO emission, the 2
1/1
0 ratio
(R21) is a good indicator of the CO excitation
temperature, which is expected to be close to the gas
kinetic temperature due to the low dipole moment of the
molecule. In Fig. 5 we present R21 towards
the positions marked in Fig. 1, showing that it varies
with position. Near the protostar and in most of the
northern lobe
,
indicating an excitation
temperature of
15 K, while at the lobe end the ratio
increases to
2.7, indicating higher temperatures, of
the order of 25-30 K. In the southern lobe, on the other
hand, ratios as high as 3.5 (indicating temperatures of 80
K) are seen toward B1, while at the tip of the lobe
temperatures are of the order of 25 K. These values are in
good agreement with the NH3 measurements of Tafalla &
Bachiller (1995), and seem not to depend on the
velocity of the outflowing gas. The largest temperatures
in the CO outflow are most likely due to gas heating by
outflow shocks, as previously suggested by Umemoto et al.
(1992) and Bachiller et al. (1993).
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Figure 4:
Profiles of the 12CO and 13CO lines and their
ratio towards the central position and the southern lobe (B0) of
L1157. The spectra are convolved to the same spectral and
spatial resolution. For B0, the line ratio raises to values
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Given the above temperatures and the fact that the outflow regime is
optically thin in CO for velocities smaller than 0.5 kms-1 and
larger than 5 kms-1, we calculate the outflow mass, momentum,
and energy. We assume a CO abundance of 10-4 and apply standard
techniques (see e.g. Margulis & Lada 1985). The results are
presented in Table 2. It is appropriate to remind here
that estimates of outflow physical parameters from CO observations
are subject to important sources of uncertainty. As concerns the
mass, the main uncertainties arise (i) in the placement of the
velocity boundary between the high velocity wing and the ambient line
and (ii) in the assumed kinetic temperature (note that the mass
estimate is nearly proportional to the assumed temperature). We
believe that our estimate of the L1157 outflow mass (0.6
)
is accurate within a factor of 2. With regard to the flow
momentum and energy, the estimates depend critically on the
knowledge of the inclination of the flow axis to the line of
sight. The values given in Table 2 are not corrected for
projection effects, and the corresponding correction factors are quite
important in the case of the L1157 outflow owing to its high
inclination with respect to the line of sight (as indicated by the
small spatial overlap of both outflow lobes around the exciting
source). We have estimated that if this inclination angle is 81
,
as suggested by GGB, the momentum and the kinetic energy reported in
Table 2 should be multiplied by factors of 6.4 and 41,
respectively. We finally point out that model simulations by Cabrit
& Bertout (1990) (see also Margulis & Lada 1985)
that the most accurate methods to
derive outflow parameters from CO observations estimate the mass
within a factor of
2, the momentum within a factor of
5,
and the kinetic energy within a factor of
10 (providing that
the momentum and energy have been corrected for projection effects).
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Figure 5:
12CO
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In Fig. 6 we present maps of the southern lobe obtained in the emission of molecules other than CO. Though we have observed more than one line per molecule, in this figure we present only those with the best S/N. The squares mark the positions from BP97 where the survey of the molecular lines have been presented. It can be immediately seen that there is a strong gradient in the distribution of emission among different species along the outflow lobe. Molecules such as C3H2, DCO+ and N2H+ trace only the central condensation (along the axis of a hypothetical disk) and are not observed in the outflow or the quiescent medium. In contrast, other molecules such as SiO, HCN, H2CO, CS, CH3OH, SO and SO2 follow a striking segregation tendency. The emission from HCO+and CN is present only between the protostar and the first peak B1. The structure of this region can be seen more clearly in H2CO, CS, CH3OH, and SO molecules. The structure around the B2 peak is best seen in the maps of SiO, CH3OH, and especially in SO and SO2 molecules.
These gradients in the molecular line distributions may be
due to a real segregation in the abundances of the chemical
species or to mere excitation effects. In order to
distinguish between these two possibilities, we have
obtained maps of the emission of several rotational lines
of the same species, which can trace regions of different
temperature due to their different excitation energies. In
Fig. 7 we show the maps of the
,
,
and
emission lines of SiO, CH3OH, and
CS. We can see that, after taking into account the
differences in the angular resolution of the maps, the
distribution of the emission is very similar for the
transitions of low and high excitation energies. Though the
effect of different spatial resolution is very obvious,
the positions of the emission peaks are almost the same.
This is clear evidence that the the distribution
gradients of the emission are not due to simple excitation
effects but mainly due to an actual segregation of the
molecular species.
Blue lobe | M | P | ![]() |
(![]() |
(![]() |
(1044 erg) | |
low v (-8.5 to -0.5) | 0.32 | 0.91 | 0.42 |
high v (-17.5 to -8.5) | 0.03 | 0.50 | 0.73 |
extreme v (-26.5 to -17.5) | 0.01 | 0.25 | 0.62 |
total blue wing | 0.36 | 1.66 | 1.77 |
Red lobe | M | P | ![]() |
(![]() |
(![]() |
(1044 erg) | |
low v (5 to 14) | 0.18 | 0.76 | 0.02 |
high v (14 to 23) | 0.06 | 0.91 | 0.53 |
extreme v (23 to 32) | 0.02 | 1.38 | 1.31 |
total red wing | 0.26 | 3.05 | 1.86 |
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Figure 6: Maps of the molecular emission towards the southern lobe of L1157. First contour and step are 0.07 K kms-1 for C3H2, 0.3 for DCO+, 0.8 for N2H+, 1.3 for HCO+ and SO, 0.7 for CN, 2 for H2CO, CS and SiO, 6 for CH3OH, and 0.5 K kms-1 for SO2. The star symbol marks the position of the Class 0 protostar L1157-mm, and the squares mark the positions where an initial molecular survey was carried out (BP97). Note the striking differences in the line emission distribution which are discussed in the text. |
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In order to confirm that the differences in line emission reflect actual differences in chemical abundances, we have carried out radiative transfer calculations using a Large-Velocity-Gradient (LVG) code to interpret the observations of molecular species for which we have observed more than a rotational line, namely SiO, CS, and CN. The case of CN is particularly complex because of its fine and hyperfine molecular structure, and because the collisional rates are unknown. In our calculations, we considered that the hyperfine components of each rotational transitions are completely coincident, and that the fine components present no line overlap. The CN collisional rates were estimated from those of CS (Turner et al. 1992) corrected for the different molecular sizes, and we assumed the infinite sudden (IOS) approximation to account for the fine structure. Although such rates are clearly inadequate to study details of the CN collisional excitation, they should be approximate enough for the estimates of the rotational temperature. The values of rotational temperatures of CH3OH were computed by using the "rotation diagram method'' (e.g. Cummings et al. 1986) at some selected positions of the L1157 southern lobe.
Although LVG calculations provide estimates of the excitation temperature and the line opacity, it is not possible to separate the effects on the excitation of the gas density and temperature. Therefore, some additional independent estimates of these parameters are necessary. Estimates of the kinetic temperature have been previously obtained at different outflow positions using multiline NH3 observations (Bachiller et al. 1993; Tafalla & Bachiller 1995) and CO observations (see Sect. 3.2). Estimates of the densities were obtained from the methanol rotational temperatures by comparing to detailed radiative transfer models as proposed by Bachiller et al. (1998, their Fig. 9).
As a result of these calculations we find that only moderate changes in the physical conditions (density and temperature) take place across the shocked regions sampled by the molecular species mentioned above (SiO, CS, CN, CH3OH; see also Sect. 5.2). The kinetic temperatures in the outflow range from 40 to 100 K. These values are in general agreement with very recent sub-mm observations (Hirano & Taniguchi 2001) showing that the region sampled by the CO J=6-5, 4-3, 3-2 lines has temperatures ranging from 50 to 170 K, and with observations of the very highly excited (5, 5) and (6, 6) metastable inversion lines of NH3 (Umemoto et al. 1999), which provide values of the kinetic temperatures in the range 40 to 140 K. Densities in this blueshifted lobe are in the range of a few 104 to a few 105 cm-3. Indeed the uncertainties in these LVG estimates are quite large. We believe that temperatures are accurate within 30%, and densities within a factor of 10. In any event, the moderate differences found in physical conditions cannot explain alone the large changes observed in the emission from the different molecules, which confirms that important variations in the chemical abundances take place along the outflow.
In Fig. 6, notice that H2CO, CS, CH3OH, and SO
molecules (maps of higher frequencies) display a prominent
peak of emission that is marked as B0 in Fig. 1.
This peak, which was not clearly seen in some previous maps of CO and
SiO, is unambiguously identified here at a position offset of
(23
,
-40
)
from the central protostar.
Its position, just north of
B1, is consistent with the idea of a underlying precessing
jet, and also compatible with the concept of a cavity wall
excavated by the propagation of the shock B1 (GGB, Gueth et al.
1998). This peak is indeed the
first peak seen in the position-velocity diagram of SiO
emission by GGB. The kinetic temperature we found for this
position is
40 K, significantly lower than the 80 K at B1.
This trend is in good agreement with estimates by Hirano & Taniguchi
(2001).
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Figure 7:
Maps of the integrated emission for several transitions
of SiO, CH3OH, and CS. First contour and step are 2, 2
and 1.5 K kms-1 for SiO
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In Fig. 8 we present the velocity channel maps of
the emission from some molecular lines with best S/N ratio.
Conforming to the CO maps of Fig. 2, we see that
the bulk of the emission is blue shifted with respect to the
systemic velocity (2.7 km s-1). Peak B1 dominates the
high velocities, whereas the emission from B2 is
concentrated at the lowest velocities. B0 presents an
intermediate velocity, and it is particularly prominent in the
CS(
)
maps. This line also shows appreciable
emission at the highest velocities, whereas HCO+ is seen
only at moderately low velocities. Such differences in the
terminal velocities of the different species is further
illustrated in the position-velocity diagrams of the blue
lobe along the outflow axis (
)
presented in
Fig. 9. We note that most of the emission takes
place at velocities close to that of the ambient gas
indicating that most of the material moves at low
velocities. However, CO and SiO reach terminal velocities
which are significantly higher than those of HCO+,
H2CO, SO and CS. This effect could be related to
differences in the mechanisms of formation of the different
species.
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Figure 8:
Channel maps of the molecular emission in the southern
lobe of the L1157 outflow. The emission has been integrated
over 3 kms-1 intervals centered around the LSR velocity
marked in the top row (lower-right corner of each panel).
The second panel from the right is centered at
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In our previous molecular survey (BP97) we saw that the
emission from the northern lobe of L1157 is much weaker
than that from the southern lobe. However, we have explored
the red lobe of L1157 and detected emission from SiO and
CH3OH at several regions along the outflow. In
Fig. 10 we present the 2mm maps of SiO
and CH3OH. The squares are the same positions as shown
in the integrated CO map of Fig. 1.
The most evident feature in these maps is the large asymmetry between the emission of the northern and southern lobe, even more pronounced than in the CO maps. The emission in the north is concentrated in some discrete and separated locations. These peaks are common to the emission from CH3OH, SiO and also CO, but the relative intensities are different. SiO peaks far away from the protostar whereas the emission from CH3OH is present in nearer points, probably related to more recent ejection events. The emission of CH3OH is too weak, but that of SiO can be separated into velocity channels, as shown in Fig. 11. We can see that the emission farther from the central source have lower velocities than those close to the source. This is consistent with the results of the CO maps in Fig. 5. We can also match the different episodes of ejection in the northern and southern lobes. As in the case of Fig. 3 we have used a scale factor between the two lobes to enable a direct comparison, owing to the asymmetry in size and velocity of the lobes. The velocity intervals are the same as in Fig. 3. This map reconfirms the change in the direction of the ejection from one episode to the other.
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Figure 9:
Position-velocity diagrams of the southern lobe of the
L1157 outflow in several strong emission lines. First contour and step are
2 K for CO, 0.3 for CS, 0.4 for H2CO, and 0.25 for SiO
and HCO+, in the
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As mentioned in Sect. 3.1, the L1157 outflow presents a
clear point reflection symmetry in both position and velocity,
with a one to one correlation between the
blue spots B0, B1, and B2 and the red peaks
R0, R1, and R2 (see CO and SiO maps in Figs. 1, 2,
10, and 11). This point reflection symmetry suggests that
the outflow has precessed over its lifetime, and that the
bright peaks correspond to events of enhanced ejection.
These ideas have been previously explored by
GGB, who modeled the southern CO lobe with two broad bow shocks
resulting from a precessing jet. The shocks in the GGB model were located
at the B1 and B2 positions, while the B0 peak was considered
to be part of the B1 bow shock. As B1 and B2
are almost aligned with L1157-mm, the precession
angle these authors estimate is
very small (). More recently, Zhang et al. (2000)
have presented CO and SiO maps of both lobes and suggested that
the B0 peak represents an independent ejection (due to its
pairing with R0), implying a larger precession angle (
15
)
and a narrower bow shock structure. Our observations
support this second interpretation, and to further explore this scenario,
we attempt to reproduce the outflow geometry with a simple,
geometrical model of a narrow, precessing jet.
Our model assumes that
the jet moves in a straight line with constant velocity,
and that the ejection direction rotates with constant
angular speed. We fit the asymmetry between the
blue and red lobes (Sect. 3.1) assuming that the blue
outflow velocity is 0.65 times the velocity in the
red lobe, and vary the outflow age and rotation period
to fit the positions of the bright outflow SiO spots.
We assume a precession angle of ,
in agreement
with Zhang et al. (2000), and present the result of the
modeling in Fig. 12.
As it can be seen in the left panel of Fig. 12, the simple model fits rather well the position of the SiO emission, both the bright peaks and the extended part. This good fit shows that the location of the most recently shocked material is consistent with the expected position of the working surface of a precessing jet. If this model is correct, the outflow time scale should be approximately equal to one precession period, which can also be seen from the fact that the northern lobe forms a single sinusoidal wave. This characteristic sets L1157 apart from other flows where precession has been proposed, like HH333 (Bally et al. 1996), HH34, RNO43 (Eislöffel & Mundt 1997) and PV Cep (Gómez et al. 1997). In all these cases, the jet/flow bends only once, giving them a global S-shape, irrespective of their sizes and dynamical ages. The repetitive S-shape of L1157 thus makes it an excellent case for further study of outflow precession.
Although the precession model fits well the location of the bright outflow shocks, it does not fully explain the distribution of all accelerated gas. This can be seen from the right panel of Fig. 12, where the thickest sinusoidal line indicates the actual ejecta position (as in the left panel) and the thinner sinusoidal lines indicate past ejecta locations. If we assume a narrow jet in the isothermal limit (see, e.g., Masson & Chernin 1993), the region swept up by the outflow will only extend over the region covered by thin sinusoidal waves. This makes it difficult to explain the weaker NE side of the red lobe and SW side of the blue lobe. We note, however, that this "anomalous'' emission arises from material moving at the lowest outflow velocities (Fig. 2), so it is possible that represents material accelerated not directly by the driving jet, but by ambient gas already set into motion. Alternatively, a less collimated outflow component may coexist with the precessing jet. The smaller fraction of this "anomalous'' gas, however, suggests that if this component exists, it represents a small perturbation to the jet driven flow.
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Figure 10:
Maps of the CH3OH and SiO
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Several mechanisms can explain the precession of an outflow jet.
Eislöffel & Mundt (1997) and Fendt & Zinnecker
(1998) have investigated the effects of the
forces or of a binary companion to the driving source,
and either of these processes can be at work in L1157. The
presence of a binary companion, in particular, is not completely
ruled out by present data, as mm observations from
Gueth et al. (1997) have a beam of about 3.5
,
which translates to more than 1000 AU at the distance of 440 pc.
Higher angular resolution observations are therefore needed
to settle this point.
An important result from our observations is the prominent
chemical segregation from the protostellar vicinity to the
southern lobe of L1157 and along the extent of this lobe
(see Sect. 4.1). The LVG
calculations with the molecules for which we have multiline
observations show that, although the physical conditions
only change moderately along the outflow, there
are important differences in the physical conditions
between the protostellar condensation and the shocked
regions. The intensities of the narrow lines around the
protostar are well explained as arising in a medium at a
kinetic temperature of 13 K (as indicated by NH3observations, see Bachiller et al. 1993) and
a density of 106 cm-3. Interferometric
observations of a narrow CH3OH line component by
Goldsmith et al. (1999), suggest higher
excitation conditions, densities of
107 cm-3,
and temperatures of
50 K, in the innermost region of
the protostar, probably a disk of <100 AU radius.
The physical conditions in the shocked regions can be
deduced from the broad emission lines observed toward the
outflow lobes (see Sect. 4.1). In the blue lobe, when one moves from B0 to
B2, the density changes from n3 to 6
105cm-3 and the kinetic temperature from 40 to 80 K. The
sharp increase of the CO brightness temperature observed
toward the blueshifted lobe (see Fig. 1) is clearly an
effect of the gas heating produced by the shock (see also
Umemoto et al. 1992; Bachiller et al.
1993; Hirano & Taniguchi 2001).
As previously noted, the changes in physical conditions
along the outflow lobes are insufficient to explain the
large differences observed in the emission of the different
molecular lines, so it is necessary to
invoke the presence of strong gradients in the chemical
composition of the shocked medium. BP97 already provided
estimates of the abundance enhancement factors at the positions of
the peaks. A extreme case is CN which appears to be
enhanced by a factor of 100 around B0,
30
around B1, and only
10 around B2. HCO+ exhibits a
similar behavior: the enhancement factor decreases from
>100 around B0 to values of
40 near B1 and
20
near B2. On the contrary, SO2 is not observed around B0,
but appears to be enhanced by factors of
8-20 around
B1 and around B2. No significant changes are perceived in
the abundances of SiO, CH3OH, and H2CO (which are
enhanced by factors of >105,
400, and
80
toward B0, B1 and B2). We refer to BP97 for a complete list
of the abundances estimated at the emission peaks.
It is very likely that the observed chemical gradients are related to the strong time dependence of the shock chemistry. As discussed in Sects. 3.1 and 5.1, the successive emission peaks along the blueshifted outflow lobe correspond to successive ejection events. Given the differences in the velocities of the bulk emission at the successive peaks (see, for instance, Fig. 9), it results that B0, B1, and B2 correspond to events which are progressively older. In Sect. 3.1 we estimated the ages of B0, B1, and B2 to be 4500, 7000, and 15000 yr, respectively. The outflow axis can then be considered as a kind of time axis, and the chemical composition of the gas around B0, B1, and B2 could be considered as that of a shock which is rapidly evolving in time. We next discuss some of our results in the context of some recent time-dependent chemical theoretical models.
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Figure 11: SiO emission for the same intervals as in Fig. 2. First contour and step are 0.8 K kms-1 for the red lobe and 0.5 for the blue one. The thick lines (also shown in Fig. 2) represent different directions of the outflow. |
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The chemistry of sulfur-bearing molecules in interstellar shocks
has been described by Pineau des Forêts et al. (1993). More
recently, Charnley (1997) has developed a model
to explain the sulfur chemistry of hot cores at several temperatures
which in principle can also be applied to a shock wave heating the
gas. In both models the abundance of H2S is first enhanced by
several orders of magnitude. Pineau des Forêts et al. (1993) suggest that the passage of the shock wave
induces the following sequence of ion-neutral reactions leading to
the formation of H2S: S + H3+
SH+ + H2;
SH+ + H2
H2S
;
H2S
H3S+ + H, finally followed by the
dissociative recombination reaction: H3S+ + e
S + H. Neutral-neutral reactions with
energy barriers could also be effective producing H2S
in the hot dense gas:
S + H
H2S.
Charnley (1997)
assumes that the sulfur content of grain mantles is predominantly in
the form of H2S (as suggested by near-IR spectroscopy toward
massive protostars), and that these molecules are directly ejected to
the gas phase. In any event, once present in the gas, H2S gives
rise to a series of hot phase chemical reactions to produce SO,
SO2, CS, OCS, and H2CS. For instance, the models by
Charnley (1997) predict a sharp decrease in the
H2S abundance after
104 yr because oxidation with O and OH
produces SO first and SO2 afterwards. These trends can in
principle be compared to our observations of L1157. The estimate of
the abundance ratio SO2/H2S is 0.7 in peak B1, and 1.4 in peak
B2 (see also BP97), and the SO2/SO ratio is <1 in B1 and >1 in
B2. These values are in general agreement with the sequence H2S
SO2 predicted by the
models. No significant differences are observed for CS between peak B1
and B2. This is also consistent with the trend found in the chemical
models, in which CS remains more or less constant with time. Note
also the dynamical timescales of shocks B1 and B2 are of the order of
magnitude required by models.
CH3OH is expected to be released in the gas phase due to direct desorption from grain mantles. A model of the gas-phase chemistry in regions where ice mantles contaning CH3OH and other alcohols are evaporated from ice mantles was presented by Charnley et al. (1995).Once liberated, CH3OH is expected to disappear after a few 104 yr due to depletion back onto the grains, or to gas phase ion-molecule reactions, which produces complex organic molecules like CH3OCH3 and HCOOCH3 (see van Dishoeck & Blake 1998, and references therein). The high abundances of CH3OH observed in the southern lobe of L1157, comparable to those of hot cores like Ori-KL, means that the outflow is very young, and that the strong interaction between the shock wave and the quiescent medium is taking place right now. The enhancement of methanol does not last long once the underlying jet ceases or changes its direction (Sandford & Allamandola 1993). This could explain the absence of this emission at farther distances in the northern lobe.
During the passage of the shock wave, sputtering of the grains can
produce high abundances of gas phase Si or Si-bearing molecules in the
post-shocked gas (Caselli et al. 1997). However, as SiO is a
very refractory molecule, it depletes quickly onto the grains, with a
timescale
104 yr (e.g. Mikami et al. 1992). Schilke et al. (1997) suggest that SiO can be destroyed more
efficiently by reacting with OH: SiO + OH
SiO2+ H, since OH is expected to be very abundant in the post-shock gas,
where it is produced from the destruction of the water formed in the
shock. Bergin et al. (1998) have found that water ice mantles
are easily formed in the layer of water-rich gas processed by shocks
with velocities in excess of 10 kms-1.
Similarly to what happens with the water molecules,
SiO2 is expected to condensate very quickly onto the grains mantles.
Models predict that SiO is transformed into SiO2 with a
timescale of the order of 104 yr, which is very similar to the
timescale of SiO depletion. In any case (oxidation or depletion), SiO
is expected to disappear from the gas phase in a few
104 yr. This is in general agreement with the present
observations. Similar conclusions were drawn from previous works in
which the evolution of SiO in outflows was studied from an
observational point of view (Martín-Pintado et al. 1992;
Codella et al. 1999, and references therein).
The evolution of other molecular species is much more uncertain. Formaldehyde (H2CO) could rapidly be produced in gas phase if the abundance of CH3OH is high enough, as indicated by laboratory experiments (e.g. Bernstein et al. 1995). However, solid H2CO has been observed in the vicinity of young stellar objects (Schutte et al. 1996) suggesting that direct ejection from grain mantles is also a plausible mechanism to enhance its gas phase abundance. It seems anyway that the chemistries of H2CO and CH3OH are closely linked.
The cases of CN and HCO+ are of high interest, since
these molecules are observed to be more enhanced toward the
youngest shocks. It seems that, after being enhanced in the
shock, there are mechanisms that rapidly destroy these
molecules. As suggested by BP97, CN could be efficiently
destroyed by its reaction with atomic oxygen which is
indeed expected to be abundant in the shocked region. But,
unfortunately, the activation barrier of the reaction CN +
O
CO + N is unknown. On the other hand,
if water and methanol are abundant in the gas phase, as
seems very likely in the shock, the ions H3O+ and
CH3OH2+ are expected to be formed quickly, and to
produce HCO+ by gas phase reactions. Afterwards,
HCO+ could be efficiently destroyed by dissociative
recombination since the electronic abundance is presumably
enhanced in the shocked region.
The shock chemistry of HCO+ has been modelled recently
by Bergin et al. (1998) in the context of their
detailed study of the evolution of H2O and O2 in shocks.
Similar processes
to those that control the HCO+ abundance could
account for the behaviour of N2H+ which is not
observed at high velocities.
In spite of the much effort which is being devoted during the last years to study theoretically the influence of shocks on the chemistry, the present discussion illustrates that many details remain poorly known. In fact, the classical gas-phase chemical problem is made even more complex in the presence of shock waves by the action of shocks on the mantles and cores of the dust grains. From a series of studies (e.g. Flower & Pineau des Fôrets 1994, 1995; Flower et al. 1996) it is clear nowadays that sputtering, both thermal in high-velocity J-shocks and non-thermal in slower C-shocks, is responsible for injecting refractory and volatile species into the gas. Much chemical modelling which take into account these processes is being carried out recently (e.g. Pineau des Fôrets 1993; Schilke et al. 1997; Caselli et al. 1997; Bergin et al. 1998) and should continue in the future to provide details on the chemical processes taking place in shocks.
We finally point out that, as discussed by Houde et al. (2000a), the presence of a magnetic field is expected to cause a dramatic spatial segregation between neutrals and ions. This is because under some circumstances the magnetic forces on the ions can dominate over collisional processes generating a different pattern for the motion of ions and neutrals. This effect clearly depends on the relative orientation of the magnetic field and the bipolar outflow axis, and it requires a significant misalignement of both axis to be effective. But it seems to explain ion spectral line profiles with narrower line widths and lacking high velocity wings (when compared to the line profiles of the neutral species) similar to those that we observe for N2H+, and in a lesser extent for HCO+. Clearly, observations of different outflows presenting different geometric configurations are necessary to check the importance of this effect (Houde et al. 2000b, 2000c).
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Figure 12:
Precession model predictions overlayed on the
integrated SiO map of Fig. 10 (left) and the CO map of Fig. 1
(right). The thick sinusoidal line
indicates the actual ejecta position, and the thin
sinusoidal lines indicate ejecta positions at times equal
to 0.80, 0.60,... the present time. The model parameters
are jet velocity of 100 kms-1, period of 5000 yr,
outflow age of 4000 yr and a precession angle of 15
![]() |
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Bipolar outflows have been an essential element for the classification of young stellar objects (YSOs). As the circumstellar material around a YSO is dispersed by the action of its outflow, the YSO spectral energy distribution (SED) evolves systematically, and this allows to group YSOs in Classes 0, I, II, and III (Lada 1991; André et al. 1993). However, it is clear nowadays that this classification is too schematic, and it results - for instance - that under the label of "Class 0'' sources (the presumed youngest "sub-millimeter protostars'') we find a rather hetereogeneous assemblage of YSOs. The inclination of the outflow (and that of the disk/protostar system) with respect to the line of sight makes difficult the distinction among objects which could be intrinsically different.
Indeed the main obstacle when studying the evolution of
YSOs is the identification of a reliable time indicator.
An attempt to
describe YSO evolution in a unified way with main-sequence
stars was done by Myers and Ladd (1993) by intruducing the
"bolometric temperature''
,
but this parameter
does not seem enough to classify Class 0 sources, which
have all
values in a tight range of 40 to 70 K.
As discussed previously by Bachiller & Tafalla (1999), outflows change dramatically during the first stages of the protostellar life, and could again be helpful to refine the classification of YSOs. These authors described a preliminary time sequence for outflows based on well known objects and in the chemical changes that are expected to induce in the surrounding molecular material. The first stage of this sequence is defined by jet-like outflows at extremely high velocities (EHV). These outflows are very highly collimated (collimation factor >10), and are composed of molecular "bullets'' that appears as secondary EHV components in the spectra. The abundance of SiO appears highly enhanced at EHVs. The exciting source is a Class 0 protostar. Examples of this kind of outflows are L1448-mm (Bachiller et al. 1990), IRAS03282+3035 (Bachiller et al. 1991), and SVS13B (Bachiller et al. 1998).
At the end of the sequence described by Bachiller &
Tafalla (1999) there are the outflows from Class I
sources in which a clear shell structure surrounding an
evacuated cavity with a reflection nebula has been
developed. Outflows from Class I sources are much less
collimated and have a much lower mechanical power
efficiency (
)
than those from Class 0
sources (Bontemps et al. 1996; Bachiller 1996).
Class I outflows are associated with prominent Herbig-Haro objects
and with observable kinematic perturbations in the
surrounding dense core, but with no significant chemical
anomalies. The prototype of such outflows is L1551/IRS5
(Moriarty-Schieven et al. 1989).
Between these two extreme types of young outflows, there is a
kind of outflows for which we propose here the denomination
of "chemically active outflows''. These are less
collimated and their terminal velocities are lower than in
the phase before. There is no evidence for "bullets'' in
the spectra, but the maps show regions with large amounts
of warm (100 K) gas. Very strong
chemical anomalies are observed, with SiO, CH3OH,
H2CO, and H2O lines exhibiting prominent wings. The
prototype of such outflows clearly is L1157, but other
prominent example is BHR71 (Bourke et al.
1997; Garay et al. 1998). The
exciting source of these outflows is a Class 0, as in the
phase before, but since the outflow seems more evolved (and
presents a wider opening angle at the base) it is
reasonable to assume that the driving sources of these
outflows are slightly more evolved than those of the
highly-collimated EHV flows.
There are outflows of similar collimation and velocities than the chemically active outflows, but with much smaller chemical anomalies, as if the shocks have already cooled down. Examples of such outflows are L483 (Tafalla et al. 2000) and L1527 (MacLeod et al. 1994). The exciting sources of these outflows seem in the transition from Class 0 to I (see e.g. Tafalla et al. 2000) suggesting that these are the link from the "chemically active outflows'' to the Class I outflows.
This classification scheme needs to be more firmly established by the means of observations of a complete sample of outflows in different evolutionary stages. Such a study is under way, and we expect to provide additional results in the next future. In any event, what results clear from the previous discussion is that the chemical anomalies induced by outflows provide a powerful tool to refine the classification of young protostars. We finally point out that L1157, as the most conspicuous representative of chemically active outflows, also provides a unique opportunity to study chemical processes that can be compared to those operating in other astrochemical circumstances, such as hot cores, photo-dominated regions, or comets. For instance, Bocklee-Morvan et al. (2000) from a comparison of the abundances in L1157 with those found in comet C/1995 Hale-Bopp were able to study the link between the cometary and the interstellar material. A more detailed investigation of the molecular abundances in L1157, including very rare species, is underway and will be reported elsewhere.
We have conducted detailed mapping observations of the L1157 outflow in several molecular lines including CO. The new data have provideded important results on the structure and chemistry of the outflow that are summarised below.
1. New CO maps have enabled us to characterise the physical
properties of the molecular outflow. The outflow has a total
length of 0.8 pc and contains a mass of 0.6
.
We found that there are
notable asymmetries between the northern and southern lobe
in terms of their sizes and physical parameters, but the morphology
clearly display a repetitive S-shaped
point-symmetry indicating the presence of an underlying
precessing jet.
2. We have found evidence for 3 or 4 independent episodes
of mass ejection from the central source. The kinematical timescale
of the oldest episodes is 15000 yr, and the time
elapsed between successive episodes is a few 103 yr.
3. From several maps in different molecular spectral lines we conclude that those of C3H2, DCO+ and N2H+ trace only the central condensation (along the axis of a hypothetical disk) and are not observed in the outflow. These are the best lines to further study the structure and kinematics of the material surrounding the protostar. Such a study would require higher angular and spectral resolution than that of the data presented here.
4. A chemical study of the blueshifted lobe show strong gradients in the distribution of the emission from different molecules. The emission gradients are due to chemical stratification and not only to excitation effects. The chemical stratification is shown to be a consequence of the chemical evolution of the outflowing gas. These observations, together with other recent studies of Class 0 sources (Blake et al. 1994, 1995; van Dishoeck et al. 1995), illustrate the potential of the time-variable chemistry to study the first stages of evolution of bipolar outflows.
5. Given the resolution of the present maps, and the rapidity with which the chemistry is expected to evolve, we cannot study in detail if different molecules are generated in different parts of the bow-shocks. However some trends are observed: SiO seems to be produced at the shock head or at the precursor, whereas other molecules like CH3OH, HCN or HC3N could be produced at the shock wings by turbulent entrainment. The chemical and spatial segregation has to be studied in individual bow shocks. This requires mapping of the blue lobe of L1157 with high angular resolution in different molecular lines that cover a wide range of the chemical processes operating at the shocks.
6. The observations of SiO and broad NH3 emission (Bachiller et al. 1993), CO (Zhang et al. 2000) and H2 emission (Davis & Eislöffel 1995) of the red lobe clearly indicate that shocks are present in the northern lobe also. However, most molecular lines present weaker intensities than toward the south lobe suggesting that important physical and chemical differences exist between both lobes. Such differences could be due to the structure of the molecular cloud before the star formation activity started.
7. L1157 can be considered as the prototype of a particular category of outflows from Class 0 sources for which we propose the denomination of "chemically active outflows''. These outflows seem to be more evolved than highly-collimated outflows with "bullets'' (such as L1448-mm), but less evolved than other outflows from objects which are in transition to the Class I (such as L483).
Acknowledgements
This paper benefited from helpful comments and suggestions of an anonymous referee. The authors are also grateful to Drs. Asunción Fuente and Claudio Codella for extensive and fruitful discussions. This research have been partially supported by Spanish DGES grants PB96-0104 and AYA2000-927.