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Mon. Not. R. Astron. Soc. 000, 1--?? (2002) Printed 16 September 2003 (MN L A T E X style file v2.2)
Millimeter observations of the IRAS 18162í2048 outflow:
evidence for cloud disruption around an intermediateímass
protostar. #
M. Benedettini 1 , S. Molinari 1 , L. Testi 2 , A. NoriegaíCrespo 3
1 CNR--Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere 100, 00133, Roma, Italy
2 INAFíOsservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3 SIRTF Science Center, California Institute of Technology, 220í6 Pasadena, CA 91125
Accepted . Received ; in original form
ABSTRACT
In order to study the morphology and dynamics of the molecular outflow associí
ated with IRAS 18162í2048, a wide area of #95 arcmin 2 around the source has been
mapped by means of CO and 13 CO (1í0) lines, and complemented by a map of a
smaller region surrounding the highímass object using the C 18 O(1í0) and CH 3 OH
(2 k í1 k ) and (3 k í2 k ) transitions. The lines profile reveals the presence of several veí
locity components among which two major line components at 11.9 and 12.8 km s -1
have been detected in all the tracers.
Simple morphological and energetic considerations led us to interpret the obserí
vations in a relatively straightforward scenario in which the powerful jet ejected by
IRAS 18162í2048 sets a big portion of the surrounding molecular cloud into motion.
The energy and momentum deposited by the flow breaks the cloud apart, shifting to a
blue velocity the northern region and to a red velocity the southern region, and giving
rise to a giant outflow. We calculated the physical parameters of the outflow, which
makes the IRAS 18162í2048 outflow, as one of the most massive (M=570 M# ) and
energetic (K>10 46 ergs) knowns. Despite the intrinsic di#culties in giving a precise
value of the age and the inclination angle of the flow, we used di#erent methods to
derive a reliable estimate. Our data show evidence in favor of a small inclination angle
(< 50 # ) and of a maximum outflow age of #10 6 yr.
C 18 O and CH 3 OH trace the dense core surrounding IRAS 18162í2048 and show
an elongated emission in the direction perpendicular to the outflow axis. Besides the
peak emission associated with the IRAS source, we found another peak at the position
RA(B1950)=18 h 16 m 20.2 s DEC(B1950)=í20 # 49 # 18 ## which coincides with a red near
infrared source. We provided evidence that this second peak may be surrounded by
a flattened rotating structure, suggesting that the newly discovered IR source can be
another site of recent star formation in this region.
Our analysis suggest that the powerful wind/outflow from the luminous stars
within the young cluster embedded in the GGD27 nebula are tearing apart the parental
molecular cloud. The IRAS 18162í2048 appears to be in the act of clearing the surí
rounding material on the verge of becoming an optically revealed young stellar cluster,
similar to those associated with Herbig Be stars.
Key words: ISM: individual: IRAS 18162í2048 (HH80/81) í ISM: jets and outflows
í stars: formation.
# Based on observations collected with SEST, the SwedishíESO
submillimeter telescope at La Silla (Chile)
1 INTRODUCTION
The early stage of the star formation process is characterí
ized by the presence of energetic mass ejections which iní
teract with the surrounding molecular cloud giving rise to
bipolar outflows. Molecular outflows are thought to be the
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# 2002 RAS

2 M. Benedettini et al.
mechanism through which the accreting central object loses
the exceeding angular momentum, and therefore, plays an
important role in stopping the gravitational collapse thus
setting the final mass of a young stellar object (YSO). More
than two decades of investigations have revealed that molecí
ular outflows are ubiquitously present in association with
lowímass protostars. The situation for the highímass proí
tostars, however, is less clear because of the intrinsic di#í
culties in observing this fast evolving objects. Recent sysí
tematic studies nevertheless indicate that # 80í90% of the
observed massive star forming region are associated with
outflows (Zhang et al. 2001; Beuther et al. 2002) and that
the massive outflows show physical characteristics similar,
apart from a scale factor, to those of lowímass outflows. This
suggests that a similar mechanism is at work in the formaí
tion of low and highímass stars (Beuther et al. 2002). While
this may be correct statistically, for a relationship between
the global flow energetics and the exciting source bolometric
luminosity, it is not clear that the detailed properties of high
mass flows are similar to those of the lowerímass counterí
parts. In particular highímass flows generally appear to be
less collimated and more turbulent than flows from lowímass
objects. Additionally the fast moving, high excitation and
highly collimated jet component usually observed in lowí
mass systems is much less important or absent in highímass
flows (Shepherd et al. 1997; Shepherd et al. 2003a). Imporí
tant di#erences in the physical structure of the flows have
also been observed at millimeter wavelengths (Molinari et
al. 2002). All these results suggest that simple scaling of
lowímass flow models may not be applicable (Richer et al.
2000; Shepherd 2003b). High angular resolution studies are
crucial in this respect, although up to know only a handful
of outflows from highímass young stellar objects have been
studied with the necessary detail: IRAS 20126+4104 (Cesaí
roni et al. 1997; 1999; Shepherd et al. 2000), G192 (Shepherd
et al. 1997), W75N (Shepherd et al. 2003a).
A relatively poorly studied massive flow is that powered
by the luminous (L=2½10 4 L# ) infrared source IRAS 18162í
2048 (Yamashita et al. 1989) at a kinematic distance of 1.7
Kpc (RodrÒÐguez et al. 1980). IRAS 18162í2048 is associated
with the infrared reflection nebula GGD 27 (Yamashita et
al. 1987), an ultracompact H ii region and a number of H2O
and OH masers (RodrÒÐguez et al. 1980). A CO bipolar outí
flow centered on the IRAS source and an elongated disk--like
structure perpendicular to the flow have been observed by
Yamashita et al. (1989). They derived a mass of 460 M#
and an age of 10 5 yr for the outflow, while for the disk the
density is in excess of 1.2½10 5 cm -3 and the mass is 560
M# . IRAS 18162í2048 also drives a highly collimated radio
jet composed of a multitude of knots (MartÒÐ et al. 1993)
which reach velocities of # 600í1400 km s -1 (MartÒÐ et al.
1995). The projected extent of the jet is # 5.3 pc, one of
the largest ever found. The HergibíHaro objects HH80 and
HH81 lie along the jet's path in the southern lobe and have
been studied in detail by Heathcote et al. (1998). They found
in HH80/81 exceptional high excitation conditions (indicatí
ing strong shocks), H# and [S II] line widths up to 600 km
s -1 and tangential velocity of # 350 km s -1 . HH80N defines
the northern tip of the radio jet and it is only revealed in
the far infrared and radio because of the high visual extincí
tion toward the denser cloud interior where the northern flow
lobe is advancing into. Molinari et al. (2001) have carried out
a study of the region by means of far infrared spectroscopy
using the Infrared Space Observatory (ISO), showing that
the shock velocities measured toward the HH objects are of
the order of 100 km s -1 only, indicating that the shocks may
arise at the interface between two fastímoving flows. They
also find evidence for a PhotoíDissociation Region induced
on the cavity walls by the UV radiation produced by the
shocks. Near and mid infrared observations of IRAS 18162í
2048 resolve it in a cluster of YSOs, with at least two massive
stars in an early evolutionary state: namely GGD27íILL and
IRS 7 (Aspin et al. 1994; Stecklum et al. 1997). GGD27íILL
has been identify as the exciting source of the radio jet and
the molecular outflow as well as the illuminating source of
the reflection nebula.
In this paper we present multiíline mapping observaí
tions, in CH3OH, CO and its isotopes, over a #95 arcmin 2
around IRAS 18162í2048 aimed at the study of the strucí
ture of the large scale molecular outflow and the dense core
and to investigate how the mass ejection from the high mass
forming object a#ects the parent molecular cloud.
2 OBSERVATIONS
We performed a multiíline observations of a reí
gion along the outflow excited by IRAS 18162í2048
(RA(B1950)=18 h 16 m 12.9 s DEC(B1950)=í20 # 48 # 49 ## ). The
observations were carried out with the SEST telescope
at La Silla (Chile) from 27 to 31 May 2001. The SESIS
100/150 GHz receiver has been coupled with the Acousto
Optical Spectrometers to observe simultaneously with high
(80 kHz) and low (1.4 MHz) resolution. We obtained a map
of 6.75½14.25 arcmin 2 in the CO (1í0) and CH3OH (3k í2k )
lines, while in the 13 CO (1í0), C 18 O (1í0) and CH3OH
(2k í1k ) transitions a smaller region has been covered (as
detailed in Table 1). The grid had a PA=í17 # and a 45 ##
spacing (about 1 beam) with the exception of the CH3OH
(2k í1k ) line map where the spacing was 22.5 arcsec. In
Table 1 we give the details of the observations: the observed
molecule and transition (Col. 1 and 2), the rest frequency
of the transition (Col. 3), the HPBW (Col. 4), the main
beam e#ciency (Col. 5), the typical system temperature
(Col. 6), the integration time in each map position (Col.
7), the spectral resolution of the 80 kHz (high resolution)
data (Col. 8) and the mapped area (Col. 9). To improve
the signal to noise ratio the spectra have been smoothed
at half the velocity resolution. The intensity calibration
was done with the classic chopper wheel method. The
antenna temperature has been corrected for the main beam
e#ciency (see Table 1) and the data is being presented in
units of mainíbeam brightness temperature.
3 RESULTS
3.1 Largeíscale Structures
We investigated the large scale structure mapping a wide
area of # 95 arcmin 2 around IRAS 18162í2048 by means of
CO and 13 CO (1í0) lines. The CO (1í0) integrated map is
shown in Fig. 1 while Figs. 2 and 3 show the spectra of the
CO and 13 CO (1í0) lines in di#erent positions of the mapped
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Table 1. List of the observed transitions and observing parameters.
Molecule Transition Frequency HPBW Beam e#. Tsys t int dv(80 kHz) mapped area
(GHz) (arcsec) (K) (s) (km s -1 ) arcmin½arcmin
CO 1í0 115.271 45 0.70 580 90 0.1107 6.75½14.25
13 CO 1í0 110.201 47 0.71 400 60 0.1157 6.00½12.75
C 18 O 1í0 109.782 47 0.71 420 60 0.1162 5.25½6.75
CH3OH 2 k í1 k 96.746 52 0.73 280 60 0.1318 2.25½3.75
CH3OH 3 k í2 k 145.113 34 0.66 290 30 0.0879 6.75½14.25
Figure 1. CO (1í0) integrated map. The contours start from
3 K km s -1 with spacing of 5 K km s -1 . The crosses indicate
the points of the map and the triangles the positions of IRAS
18162í2048 and the three HH objects HH80, HH81 and HH80N.
The three lines indicate the directions along which the spectra
shown in the following figures are taken from. The central line
corresponds to the direction of the radio jet.
area, moving from south to north along three directions parí
allel to the radio jet axis (i.e. the three straight line drawn
in Fig. 1). In the on source pointing a non--gaussian high
velocity wing emission is detected with total velocity width
at zero intensity of about 20 km s -1 (see Fig. 4). Thanks to
the high spectral resolution of the observations, we can idení
tify multiple components of line emission at most of the obí
served positions. Among these, two major line components
can be identified. These components show a similar intensity
in the immediate surrounding of IRAS 18162í2048 but beí
have di#erently as one moves away from the source along the
jet axis. In particular, in the northern part of the map the
blue velocity component prevails, while the red component
is predominant in the southern part of the map. The same
behavior has been observed in the 13 CO and C 18 O as can
be seen in Fig. 5 were the spectra of the three species are
overplotted for di#erent positions along the flow axis. We
anticipate that the two line components are also detected in
the CH3OH lines (see Fig. 6). We have measured the velocity
of these two major components using the C 18 O line because
Figure 2. Spectra of the CO (1í0) transition along the three
directions parallel to the flow axis as indicated by the straight
lines of Fig. 1. The o#set from the central position (coordinate
of IRAS 18162í2048) with respect to the flow axis (PA=í17 # ) is
given in the upper left corner in arcsec. The two vertical lines indií
cate the velocity positions of the two major outflow components:
v LSR=11.9 km s -1 and v LSR =12.8 km s -1 .
this tracer is less sensitive to the ambient component and is
a#ected less for opacity e#ects. The blue component is cení
tered at # 11.9 km s -1 and the red one at # 12.8 km s -1 .
The relative proximity of the two velocity components, preí
vents an accurate estimate of their line parameters for most
of the observed positions. Essentially it is impossible to reí
liably fit the overall line profile by means of two (or more)
gaussian contributions, without fixing an arbitrary FWHM.
The ratio between the brightness temperature of the
CO and 13 CO in the core of the (1í0) line ranges from #1
to #7, indicating that the CO (1í0) line is optically thick all
over the mapped area, thus a measurement of the excitation
temperature can be given from the line peak. Merging all
the CO spectra, we derive a mean temperature of # 12 K.
The ratio between the 13 CO and the C 18 O is about the
abundance ratio of the two isotopes so that both the lines
are optically thin. The column density of the CO is deduced
assuming an excitation temperature T=12 K (see Sect. 3.1)
and a CO abundance relative to H2 of 2.8½10 -4 (Cardelli
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4 M. Benedettini et al.
Figure 3. As in Fig. 2 for the 13 CO (1í0) transition.
Figure 4. CO (1í0) line in the onísource position with superimí
posed a two components gaussian fit.
et al. 1996). The optical depth is calculated from the ratio
between the CO and the 13 CO brightness temperatures in
each velocity channel. In those cases where the 13 CO data
are not available, the optical depth is deduced by a linear
extrapolation from the data of the neighboring positions.
Based on these measurements we obtain a total cloud mass
of 570 M# .
3.2 Emission Knots
Knots of CO (1í0) emission at various velocities, vLSR from
9.5 to 43 km s -1 , are clearly detected both along and aside
of the radio jet axis (see Figs. 7 and 8). In Table 2 we list
Figure 5. Spectra of the ground state transition of CO (dotted
line), 13 CO (longídashed line) and C 18 O (solid line, 3 times the
real value) in di#erent positions along the flow axis. For each
panel, the o#set in arcsec, from the central position with respect
to the flow axis (PA=í17 # ) is given in the left upper corner.
15 20 25 30 35
í0.5
0
0.5
1
v (km/s)
Figure 6. Spectra of the CH3OH 2 k í1 k line in di#erent posií
tion of the mapped area. The o#sets from the central position
with respect to the flow axis (PA=í17 # ) are 0 ## ,0 ## (solid line),
22.5 ## ,22.5 ## (dotted line) and 45 ## ,45 ## (longídashed line).
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The HH80í81 region 5
D L
G A
Figure 7. Velocity channel map of the CO (1í0) transition. The
velocity of each panel is indicated in the upper left corner. The
contour lines range from 1 to 23 K km s -1 with spacing of 1.5
K km s -1 . IRAS 18162í2048 is identify by a cross while the poí
sitions of HH80, HH81 and HH80N are indicated by triangles.
Knots of CO emission A, D, G, and L, with velocity between 9
km s -1 and 11 km s -1 , are also indicated . The filled circle in
the last panel shows the HPBW.
the coordinates of the emission peak of the identified knots,
which we label with a letter form A to L, together with
their line center velocity. We estimate an uncertainty of #
20 arcsec, which corresponds to the half spatial resolution
of the map. Knots B, C, F and I are spectroscopically well
identified and we can estimate their total mass and momení
tum (see Table 2 Cols. 5 and 6, respectively); these are only
lower limits for C, F and I since our observations did not
map them completely. Two of the CO knots are spatially
associated, within the quoted uncertainty, with radio spots
detected by MartÒÐ et al. (1993), namely C and H which
correspond to the radio sources 9 and 33, respectively. The
radio source 33 has been identified by MartÒÐ et al. as an
extragalactic source because of its negative spectral index
(#=í0.8‘0.1). The radio source 9 has a positive spectral iní
dex (#=0.9) but it is considered by MartÒÐ et al. (1993) as
a possible timeívariable object, in which case the derived
spectral index is meaningless. We note that the emission at
18.7 km s -1 of knot C is close to the position of HH80 even
if the emission of the knot C peaks more south. Knots A,
D, F, G, I and L have also been detected in the 13 CO (1í0)
line.
3.3 The Surroundings of IRAS 18162í2048
The surroundings of the massive source IRAS 18162í2048
have been mapped by means of high density tracers as
C 18 O and CH3OH. We detected the C 18 O (1í0) line and
five transitions of the CH3OH molecule, namely the 2-1 í
1-1E at 96.73939 GHz, 20 í10A + at 96.74142 GHz, 20 í10E
at 96.74458 GHz, 3-1 í2-1E at 145.09747 GHz and 30 í20
at 145.10323 GHz. Also in these lines we identify the two
E
H
C
F
B
Figure 8. Velocity channel map of the CO (1í0) transition. The
velocity of each panel is indicated in the upper left corner. Lowest
contour and contour interval is 0.4 K km s -1 . The filled circles
show the HPBW, the cross identifies the position of IRAS 18162í
2048 and the triangles indicate the position of HH80, HH81 and
HH80N. Knots of CO emission B, C, E, F, H, and I, with velocity
between 17 km s -1 and 43 km s -1 , are also indicated.
major line components, as in the CO and 13 CO tracers
(see Fig. 6). In the contour map of the C 18 O (1í0) line
(Fig. 9) and CH3OH 20 í10A + line (Fig. 10) we can see
that the integrated line emission is elongate in the east
direction along the plane orthogonal to the flow axis and
peaks #30 arcsec northíeast with respect to the IRAS
source, we will discuss this point in Sect. 4.2. A second
peak at # 100 arcsec southíeast the IRAS coordinate has
been detected at RA(B1950)=18 h 16 m 20.2 s DEC(B1950)=í
20 # 49 # 18 ## . Consulting the 2MASS quickílook images, we
found a red source in the same position of the second maxií
mum, visible only in the K band image. In Fig. 11 we show
the positionívelocity diagram for the (1í0) C 18 O line, along
the direction orthogonal to the jet axis and crossing the
IRAS source. We see two maxima on both sides and at equal
distance to the second peak at a velocity of 11.9 and 12.3
km s -1 , respectively. Also the positionívelocity plot of the
CH3OH 2k í1k lines (see Fig. 12) shows a structure similar
to that of the C 18 O with two methanol maxima in opposite
direction even if closer to the second peak. This feature will
be discussed in Sect. 4.2.
The methanol emission does not exhibit line wings, we
therefore attribute it to the dense core; contrary to the
expectations (e.g. Bachiller et al. 1995), we do not detect
a methanol component related with the shocked outflow.
Under the assumption of LTE condition, from the rotaí
tional diagram (Blake et al. 1987) we derive a Trot=7.5
K (see Fig.13), a value lower than the temperature of
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Table 2. Knots identified in the CO line map.
Knots RA(B1950) DEC(B1950) v Mass Momentum t dyn
h m s # # ## km s -1 M# M# km s -1 10 5 years
A 18 15 58 í20 47 46 10.6 -- -- --
B 18 16 07 í20 51 31 43.0 0.033 0.99 0.48
C 18 16 07 í20 53 53 18.7 >0.034 >0.21 4.2
D 18 16 13 í20 47 20 9.5 -- -- 2.4
E 18 16 17 í20 46 24 16.7 -- -- 3.1
F 18 16 18 í20 53 55 28.6 >0.632 >10.4 --
G 18 16 26 í20 44 20 10.3 -- -- 12
H 18 16 26 í20 48 00 17.0 -- -- --
I 18 16 27 í20 50 05 28.1 >0.208 >3.22 --
L 18 16 28 í20 42 00 10.1 -- -- 16
Figure 9. Contour plot of the C 18 O (1í0) transition. The coní
tours range from 0.42 (3#) to 7 K km s -1 with spacing of 0.5 K
km s -1 .
the molecular outflow, and a methanol column density of
N(CH3OH)=3½10 14 cm -2 . To calculate the methanol frací
tional abundance with respect to H2 we derive the H2 colí
umn density from the C 18 O data, since both molecules trace
the quiescent gas of the dense core. We derive a methanol
fractional abundance of X(CH3OH)=4½10 -8 . Both the roí
tational temperature and the fractional abundance are simií
lar to the values found in ambient gas close to young bipolar
outflows (Bachiller et al. 1995). In principle the methanol
abundance is expected to be enhanced in shocked regions
associated with outflows, however, our source, as well as
regions like Orion Hot Core, do not show any strong ení
hancement. It has been suggested that the CH3OH is not
abundant in regions where extreme high temperature proí
duces evaporation of the volatile CO molecules from the
grain mantles, preventing the formation of methanol (Caselli
et al. 1993).
Figure 10. Contour plot of the CH3OH 20 í1 0 A + transition.
The contours range from 0.4 (3#) to 4 K km s -1 with spacing of
0.4 K km s -1 .
Figure 11. Positionívelocity plot of the C 18 O (1í0) line in the dií
rection orthogonal to the flow axis and crossing the central source.
The contour levels range from 0.5 to 3.5 K by steps of 0.15 K.
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Figure 12. Positionívelocity plot of the CH3OH 20 í1 0 A + (left)
and 2-1 í1 -1E (right) lines in the direction orthogonal to the flow
axis and crossing the central source. The contour levels range from
0.3 to 1 K by steps of 0.05 K.
Figure 13. Rotational diagram for the CH 3 OH observed lines.
4 DISCUSSION
4.1 A molecular cloud breaking apart
As shown in the previous section, the wide mapped area surí
rounding the IRAS 18162í2048 source shows several velocity
components. The two major ones have been observed in all
the tracers and they cover most of the mapped area with
a coherent spatial distribution. They define two largeíscale
structures peaking at two di#erent positions almost along
the jet axis and on opposite sides with respect to IRAS
18162í2048 (see Fig. 5). We stress that these two compoí
nents represent the cloud in its entirety and not just highí
velocity tails of a cloud whose main body is at some interí
mediate systemic velocity.
An interpretation of our findings in terms of cloud roí
tation seems then unlikely. First of all the rotation would be
around an axis perpendicular to the axis of the observed raí
dio jet. If this was the case, one might observe a systematic
shifting of the line center in positions farther away from the
central source and the line width might be the same in all
the positions, but it is not what we observe. Furthermore, a
cloud rotation axis orthogonal to the flow axis is di#cult to
explain in the framework of standard star formation where
the contraction of a rotating cloud conserves the direction
of angular momentum (e.g. Shu et al. 1987; Olmi & Testi
2002) and forms YSO with jet parallel to the cloud rotation
axis.
A more plausible interpretation is that we may be lookí
ing at two di#erent clouds which are in relative motion. The
first scenario is one in which the two clouds are approaching
to each other, triggering the star formation process in the
region. However, in the case of star formation stimulated by
a cloudícloud collision, it is expected that the compressed
material will have a net angular momentum perpendicular
to the plane containing the clouds velocity vectors. This dií
rection is expected to be also the direction of the outflow
axis of the protostar(s) generated by the collision. In the
geometry derived from our observations, such axis should
be projected onto the plane of the sky in approximately an
eastíwest direction, i.e. orthogonally to the observed radio
jet.
A perhaps more intriguing possibility is suggested by
the spatial correspondence between the red(blue)íshifted
cloud and the red(blue)íshifted radio jet lobe, namely that
the powerful jet ejected by IRAS 18162í2048 sets a big porí
tion of the surrounding molecular cloud into motion. This
interpretation is supported by simple energy balance arguí
ment. The gravitational potential energy of the overall gas
material can be roughly estimated assuming that the obí
served mass is distributed in a sphere of uniform density and
radius of 400 arcsec, equal to the extent of the CO emission
each side of IRAS 18162í2048. In this case the potential ení
ergy U=(3GM 2 )/(5R)#5½10 45 ergs is less that the kinetic
energy of the moving gas (>10 46 ergs), so that the energy
and momentum deposited by the flow might be su#cient to
break the cloud apart. As a result the gas emission in the
northern region would be predominantly blueíshifted, while
the southern region would be predominantly redíshifted. If
this interpretation is correct the entire cloud could be seen
as a giant outflow, with the individual clouds being the two
lobes.
In addition to the two main components, we also idení
tify ten separate blue and red shifted CO knots. Three of
these, namely A, F, I are aside the axis of the flow. The
others seven, namely B, C, D, E, G, H, L are aligned along
the flow axis. In agreement with the velocity of the large
scale outflow all the knots (but E) lying in the northern
part of the map show blueíshifted velocity, while those lyí
ing in the southern part show redíshifted velocity. Therefore
it is reasonable to assume that these clumps are associated
with the outflow and to include their contributions into the
calculation of the mass and energy budget of the flow. High
velocity molecular clumps are often observed along the outí
flow of both low mass (e.g. L1448ímm (Bachiller et al. 1990))
and high mass stars (e.g. IRAS 20126+4104 (Shepherd et al.
2000)). On the other hand, infrared observations at high spaí
tial resolution have revealed a cluster of protostellar objects
in the region of the massive IRAS 18162í2048 source (Así
pin & Geballe 1992; Stecklum et al. 1997), and the observed
CO knots could also be an indication of the presence of other
cores/outflows less powerful with respect to the main one,
originated from the protostars of the cluster which cannot be
resolved by our observations because of their limited spatial
resolution.
Northíwest of HH80N we can see the signature of the
red and blue line wings of a CO outflow already discovered
by Girart et al. (2001) by means of interferometric observaí
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8 M. Benedettini et al.
Figure 14. A map of the CO (1í0) line wings: the blueshifted
component (dashed line) is integrated from 3 to 10 km s -1 and
the redshifted component (solid line) is integrated from 15 to 23
km s -1 . Lowest contour is 1 K km s -1 (3#) and the step is 3 K
km s -1 .
tion. They attribute the CO outflow to a local Class 0 object
whose formation has been triggered or at least speed up by
the outflow generated by IRAS 18162í2048. The emission
arising from this outflow is negligible with respect to the
main one and then we do not exclude this region for the
computation of the overall gas properties.
In the scenario described above where the overall obí
served gas is participating to the motion, we calculate the
physical parameters of the IRAS 18162í2048 outflow inteí
grating the line flux of the CO (1í0) transition over the whole
spatial extent of our map (6.75½14.25 arcmin 2 ) and over the
entire velocity range of the line (vLSR from 3 to 23 km s -1 )
and we also add the contributions of the knots at larger veí
locities (see Table 2). Fig. 14 shows the morphology of the
outflow, where for clarity we plot only the contribution of
the line wings: the blueíshifted emission is integrated beí
tween 3 and 10 km s -1 and the redíshifted one from 15
and 23 km s -1 . The momentum computed is 536 M# km
s -1 with a kinetic energy of 10 46 ergs. A rough estimate of
the age of the outflow can be derived from: i) the outflow
dynamical time t dyn=3½10 6 yr, calculated as the ratio beí
tween the mean size of the lobes in the CO (1í0) transition
and the mean velocity weighted with the mass and from ii)
the dynamical time of the most distant knot t dyn=1.6½10 6
yr. Both the methods give similar numbers so we assume
t dyn # 10 6 yr for the age of the outflow and we estimate
a mass loss rate ×
M=5.7½10 -4 M# yr -1 , a momentum flux
F=5.4½10 -4 M# km s -1 yr -1 and a luminosity L=8½10 -2
L# .
These estimates rely on the assumption that the gas
is ejected from the forming star and flows at a constant
velocity. However, if the massive outflow is generated by the
fragmentation and the acceleration of the hosting cloud, as
we discussed above, most of the lobe material is clearly not
ejected gas from the source and has not traveled along the
Table 3. Physical parameters of the outflow, not corrected for the inclination angle
for an inclination angle of 3 # (Col. 3) and 56 # (Col. 4).
Parameter i = 90 # i = 3 # i = 56 #
H2 column density (cm -2 ) 1.5½10 21 1.5½10 21 1.5½10 21
CO column density (cm -2 ) 4.2½10 17 4.2½10 17 4.2½10 17
Mass ( M# ) 570 570 570
Momentum (M# km s -1 ) 536 10242 647
Energy (ergs) 1½10 46 3.7½10 48 1.5½10 46
dynamical time (yr) 1½10 6 1½10 6 1½10 6
Momentum Flux (M# km s -1 yr -1 ) 5.4½10 -4 1½10 -2 6.5½10 -4
Mass Loss Rate (M#yr -1 ) 5.7½10 -4 5.7½10 -4 5.7½10 -4
Luminosity (L# ) 0.08 30 0.12
full extent of the lobe. In this case the dynamical time will be
an upper limit of the real age of the outflow. This also a#ects
all the physical parameters of the outflow which depend on
the time, namely the mass loss rate, the flux momentum and
the luminosity.
The derived outflow mass is a factor 1.24 larger than the
value that Yamashita et al. (1989) quoted from a smaller
CO (1í0) map (3½6 arcmin 2 ) and by calculating the optical
depth only from three pointings of 13 CO along the flow axis.
They did not identify the two major CO components and
calculated the outflow parameters by integrating the blueí
shifted gas over a velocity range of 6í10.5 km s -1 and the
redíshifted gas over 14.5 í19 km s -1 range, moreover they
assumed an excitation temperature of 40 K, 3.3 times higher
than our value. Only two of our CO knots are inside the area
mapped by Yamashita et al., namely D and E and they can
be identified also in the Fig. 2 of their paper. The momentum
and the energy we derived, assuming the same inclination
angle (i = 3 # ), are respectively a factor 1.9 and 47 higher
than the values given by Yamashita et al. (1989).
The outflow parameters we derived are not corrected
for the inclination of the flow axis with respect to the plane
of the sky. In fact, the measurement of the inclination angle
su#ers of large uncertainty and very di#erent estimates can
be found in the literature: Yamashita et al. (1989) deduced
i # 3 # from a geometrical model, while Heathcote et al.
(1998) deduced i # 56 # based on modeled positionívelocity
diagrams of the optical lines of the HH objects. Of course the
smaller the inclination angle is, the larger the correction of
the flow velocity is. In the case of i = 3 # , the outflow momení
tum would be p # 10 4
M# km s -1 the energy E=3.7½10 48
ergs, the momentum flux F=1½10 -2 M# km s -1 yr -1 and
kinetic luminosity L=30 L# while in the case of i = 56 # ,
p=647 M# km s -1 , E=1.5½10 46 ergs, F=6.5½10 -4 M# km
s -1 yr -1 and L=0.12 L# .
The comparison of our outflow map in the CO (1í0) line
(Fig. 14) with the models by Cabrit & Bertout (1986) indií
cates a small inclination angle (i<50 # ) and a wide opening
angle (# #40 # ). Comparing the values of the outflow physí
ical parameters corrected for the possible inclination angle
range (see Table 3) with the estimates of the same physical
quantities derived for the radio jet ×
M=6½10 -6 M# yr -1 ,
F=6½10 -3 M# km s -1 yr -1 and L=500 L# (MartÒÐ et al.
1995), we conclude that in the case of i # 3 # the jet power is
less to that required to put into motion the observed molecí
ular outflow while in the case of i # 56 # the jet is # 10 times
more powerful than the outflow. If we suppose that the ioní
c
# 2002 RAS, MNRAS 000, 1--??

The HH80í81 region 9
ized jet is the responsible for the acceleration of the outflow,
imposing the identity of the momentum flux of the jet and
the momentum flux of the molecular outflow, we derive an
inclination angle of 5 # . Despite a definite estimate of the
inclination of the outflow cannot be given, our data would
seem to favor a small inclination angle for various reasons:
i) a comparison of the morphology of the outflow with the
models of Cabrit & Bertout (1986) suggests an inclination
angle #50 # ; ii) the component of the velocity of the CO (1í
0) line along the line of sight is low (onísource FWZI=20 km
s -1 ) compared to the very high proper motion velocity of
the HH objects ( < = 600 km s -1 ) and the radio knots ( < = 1400
km s -1 ) of the flow; iii) the momentum and the energy of
the outflow corrected for the inclination angle, when comí
pared with the typical values derived for high mass YSOs
(Beuther et al. 2002; Zhang et al. 2001), seem more plausií
ble in the case of a small inclination angle; iv) if the radio
jet is the responsible for the acceleration of the outflow, the
inclination angle is # 5 # .
The properties that we derive for the IRAS 18162í2048
outflow, qualify it as one of the most massive, energetic and
extended outflows; the extreme excitation condition found
in the visible HH objects associated with it (Heathcote et al.
1998) do not contradict our results. In principle, one would
expect that the molecular gas associated to the high velocí
ity jet and HH objects has itself a high velocity. However we
have shown that from an energetic point of view, if the flow
is close to the plane of the sky, the jet could be the responí
sible for its acceleration and given the large mass involved,
the velocity of the large scale molecular outflow could be as
low as we observe. This outflow shows remarkable similarí
ities with respect to others high mass outflows (e.g. IRAS
20126+4104 (Shepherd et al. 2000), W75 N (Shepherd et al.
2003a)) in its low collimation degree and complex velocity
structure with presence of high velocity clumps.
All the observed lines show an intensity that drops o# in
the west side of the map, indicating that IRAS 18162í2048
is at the edge of its hosting cloud and that the red lobe of the
outflow, flowing in the southíwest direction, has broken out
of the cloud, while the blue lobe, flowing in the northíeast
direction, is propagating inside the cloud. This is confirmed
by the fact that the two southern HH objects, HH80 and
81, are visible in the optical while the northern counterpart,
HH80N, has not been detected at optical wavelengths.
4.2 The dense core
In spite of the di#erent critical densities, C 18 O and CH3OH
trace a similar structure for the dense core. Their integrated
emission (Fig. 9 and 10) is elongate in the eastíwest direcí
tion and peaks at # 30 arcsec northíeast with respect to
IRAS 18162í2048. Although the o#set is almost within the
uncertainty set by the beam size and 45 arcsec spacing of
the observations for the C 18 O, it is relevant for the methanol
which has been sampled at a resolution of 22.5 arcsec. Howí
ever, this o#set can be ascribed to the higher intensity of
the northern, blue component which shift the peak of the
integrated line flux to the northíeast direction (see Fig. 6).
A second peak at # 100 arcsec southíeast the IRAS
source has been observed in both C 18 O and CH3OH. The
2MASS database reports a red source in the same posií
tion, only visible in the Kíband image. The C 18 O positioní
velocity diagram obtained along the major axis of the deí
tected structure and across the peal position, shows two
maxima at the opposite sides of the peak, at a reciprocal disí
tance of #60 arcsec, (corresponding to 1.02½10 5 AU) and at
velocities of 11.9 and 12.3 km s -1 respectively (see Fig. 11).
A similar behavior is shown by CH3OH. This feature could
be interpreted as the signature of a rotating disk, however
the distance between the is too large to be attributed to a
disk. It is more likely that they are produced by velocity graí
dients inside the cloud suggesting a flattened rotating strucí
ture. Assuming a rigid rotation around the peak we derived
an angular velocity # 2.6½10 -14 s -1 , a value of the same
order of magnitude of that found by Olmi & Testi (2002) for
the Serpens molecular cloud. Photometric and spectroscopic
millimeter observations at higher angular resolution will be
necessary to study in more detail this area which is a likely
site of a star formation event in a very early stage.
4.3 Dispersal of Circumstellar Matter in IRAS
18162í2048
The case of IRAS 18162í2048 is particularly interesting as
this source shares the same luminosity as the most massive
of the Herbig Be systems and related young stellar clusí
ters (Testi et al. 1998; 1999) and of many intermediateí and
highímass protostellar systems (Molinari et al. 2002). This
source, however, appears in an intermediate stage between
the most embedded protostellar phase and the optically reí
vealed young stellar object. This is supported by the fact
that the young stellar cluster in IRAS 18162í2048 is still
deeply embedded into a dense molecular cloud, as for young
(protoí)stellar objects, while the estimated outflow dynamí
ical age is comparable with the ages of the youngest Herbig
Ae/Be stars (Testi et al. 1998). Thus the IRAS 18162í2048
system represent a very good candidate for the study of
the late stages of intermediateímass (protoí)stellar evoluí
tion and the clearing of circumstellar material.
Recently, Fuente et al. (2002) have studied the history
of mass dispersal around Herbig Ae/Be stars, and concluded
that 90% of the circumstellar material should be dispersed
by powerful outflows in the early stages of stellar evoluí
tion, before the objects become optically visible. Our findí
ings are in very good agreement with their conclusion, in
fact we find evidence for a very powerful outflow that at
t dyn # 10 5
- 10 6 yrs is dispersing the parental cloud surí
rounding IRAS 18162í2048. This picture is also consistent
with the morphology of the inner dense core which is eloní
gated perpendicular to the outflow axis. The situation of
IRAS 18162í2048 appears very similar to that of lower mass
systems where outflows appear to be the key factor in the
clearing of circumstellar material (see e.g. the case of B5--
IRS1 discussed in Velusamy & Langer (1998)).
5 CONCLUSIONS
We presented a multiwavelength spectroscopic mapping of
a wide area surrounding the IRAS 18162í2048 source: by
means of CO and 13 CO (1í0) transitions, a large scale bipoí
lar outflow has been observed while a dense core has been
traced by the C 18 O and CH3OH lines. All the tracers we
c
# 2002 RAS, MNRAS 000, 1--??

10 M. Benedettini et al.
have observed reveals the presence of two major line comí
ponents, defining two largeíscale structures with a coherent
spatial distribution. The blue component is centered at 11.9
km s -1 , and prevails in the northern part of the map while
the red one is centered at 12.8 km s -1 and prevails in the
southern part of the map. We suggest that the powerful jet
ejected by IRAS 18162í2048 has broken the surrounding
molecular cloud apart, giving rise to a giant outflow where
the two observed structures can be considered the two lobes.
The axis of the large scale molecular outflow is coincií
dent with the radio jet ejected by IRAS 18162í2048 and if
the inclination angle of the outflow is very low (i=5 # ), the
jet could have enough power to accelerate the flow. The total
mass put into motion is # 570 M# . The physical parameí
ters of the outflow we calculated (see Table 3) are a#ected
by two major sources of uncertainty. The first one derives
from the age of the outflow and the second one from the
inclination angle. Despite a precise value of the two quantií
ties can not be given, we used di#erent methods to derive
a reliable estimate. Our data show few evidences in favor of
a small inclination angle and of a maximum outflow age of
#10 6 yr.
C 18 O and CH3OH trace the dense core surrounding
IRAS 18162í2048 and show an elongated emission in the dií
rection perpendicular to the outflow axis. Besides the major
peak of the emission correspondent at the IRAS source, we
found a new peak at the position RA(B1950)=18 h 16 m 20.2 s
DEC(B1950)=í20 # 49 # 18 ## coincident with a red near iní
frared source. We provided evidence that this second peak
may be surrounded by a flattened rotating structure, sugí
gesting that the newly discovered IR source can be another
site of recent star formation in this region.
From our data IRAS 18162í2048 appears to be at an iní
termediate stage of its pre Main Sequence evolution, where
the powerful wind/outflow is breaking apart the parent
molecular cloud and dispersing the circumstellar material.
The high mass protostellar object is thus on the verge of beí
coming an optically visible young stellar cluster, similar to
those associated with the most massive Herbig Be systems.
ACKNOWLEDGMENTS
MB is grateful to Riccardo Cesaroni and Felipe MacíAuli#e
for their kind assistance during the observations at SEST.
ANC's research was partially supported by NASAíAPD
Grant NRA0001íADPí096 and by the Jet Propulsion Laboí
ratory, Caltech. This publication makes use of the data prodí
ucts from the Two Micron All Sky Survey, which is a joint
project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Techí
nology, funded by the National Aeronautics and Space Adí
ministration and the National Science Foundation.
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