Water cooling of shocks in protostellar outflows

Full text

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A&A 518, L120 (2010)
DOI: 10.1051/0004-6361/201014603
c ? ESO 2010
Astronomy
&
Astrophysics
Special feature
Herschel: the first science highlights
Letter to the Editor
Water cooling of shocks in protostellar outflows
Herschel-PACS map of L1157?
B. Nisini1, M. Benedettini2, C. Codella3, T. Giannini1, R. Liseau4, D. Neufeld5, M. Tafalla6, E. F. van Dishoeck7,8,
R. Bachiller6, A. Baudry9, A. O. Benz10, E. Bergin11, P. Bjerkeli4, G. Blake12, S. Bontemps9, J. Braine9, S. Bruderer10,
P. Caselli13,3, J. Cernicharo14, F. Daniel14, P. Encrenaz15, A. M. di Giorgio2, C. Dominik16,17, S. Doty18, M. Fich19,
A. Fuente6, J. R. Goicoechea14, Th. de Graauw20, F. Helmich20, G. Herczeg8, F. Herpin9, M. Hogerheijde7, T. Jacq9,
D. Johnstone21,22, J. Jørgensen23, M. Kaufman24, L. Kristensen7, B. Larsson25, D. Lis12, M. Marseille20, C. McCoey19,
G. Melnick26, M. Olberg4, B. Parise25, J. Pearson28, R. Plume29, C. Risacher20, J. Santiago6, P. Saraceno2,
R. Shipman20, T. A. van Kempen26, R. Visser7, S. Viti30,2, S. Wampfler10, F. Wyrowski27, F. van der Tak20,31,
U. A. Yıldız7, B. Delforge32,17, J. Desbat9,33, W. A. Hatch29, I. Péron34,32,17, R. Schieder35, J. A. Stern29,
D. Teyssier36, and N. Whyborn37
(Affiliations are available in the online edition)
Received 31 March 2010 / Accepted 23 April 2010
ABSTRACT
Context. The far-IR/sub-mm spectral mapping facility provided by the Herschel-PACS and HIFI instruments has made it possible to obtain, for
the first time, images of H2O emission with a spatial resolution comparable to ground based mm/sub-mm observations.
Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from
young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of
this program, presenting a PACS map of the o-H2O 179 μm transition obtained toward the young outflow L1157.
Methods. The 179 μm map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin
water observations of the same source that allow us to constrain the H2O abundance and total cooling.
Results. Strong H2O peaks are localized on both shocked emission knots and the central source position. The H2O 179 μm emission is spatially
correlated with emission from H2rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission
peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH3. A strong H2Opeak is also
observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 μm intensity and its
intensity ratio to the H2O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact
clumps, spatially unresolved by PACS, having a H2O abundance of the order of 10−4. This testifies that the clumps have been heated for a time
long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total H2O cooling is ∼10−1L?, about 40% of the
cooling due to H2and 23% of the total energy released in shocks along the L1157 outflow.
Key words. stars: formation – ISM: jets and outflows – ISM: molecules
1. Introduction
Among the main coolants in molecular shocks, water is the
tracer most sensitive to physicalvariationsand the temporalevo-
lution of protostellar outflows, thus representing a very power-
ful probe of their shock conditions and thermal history (e.g.,
Bergin et al. 1998). Water emission and excitation in shocks
were studied extensively for the first time with ISO, the first
space facility with spectroscopic capabilities in the mid- and
far-IR. ISO surveyed the water emission in a large sample of
outflows from young stellar objects (YSOs), providing a global
statistical picture of the importance of water in the outflow cool-
ing and of variations in its abundance with shock properties and
?Herschel is an ESA space observatory with science instruments
provided by European-led Principal Investigator consortia and with im-
portant partecipation from NASA.
ages (see e.g., Nisini 2003; van Dishoeck 2004). Following ISO,
the SWAS and Odin facilities made it possible to observe the
ortho-H2Ofundamental line at 557 GHz, providing important
constraints on the water abundance and kinematics in the cold
outflowgas components(e.g.,Franklinet al. 2008; Bjerkeli et al.
2009). All these facilities, however, had poor spatial resolution
(i.e. greater than 80??), which did not allow one to locate the ori-
gin of the water emission nor study variationsin abundancesand
excitation within individual flows.
In this framework, a sample of YSO outflows will be sur-
veyed in different water lines by the PACS and HIFI instru-
ments onboard the Herschel satellite, as part of the key program
WISH (Water In Star-forming-regionswith Herschel1). This pa-
per presents the first results obtainedfromthis survey,consisting
of a PACS map of the H2O 212−101179 μm line covering the
1http://www.strw.leidenuniv.nl/WISH/
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A&A 518, L120 (2010)
outflow of the protostar L1157-mm, obtained during the
Herschel science demonstration phase. The 179 μm line is the
transition connecting the lower two back-bone levels of ortho-
H2O. It is therefore one of the brightest water lines expected in
collisionally excited conditions,thus representingan ideal tracer
of the water distribution in shocked regions. L1157 is a well
known outflow driven by a low mass class 0 object (L1157-mm,
Lbol ∼ 8.3 L?, D = 440 pc, Froebrich 2005). It is considered
to be the prototype of chemically active flows, given the large
number of different species detected in its shocked regions (e.g.,
Bachiller & Perez-Gutierrez 1997). This paper is presenting the
first of several observationsplannedfor this source by the WISH
team.
2. Observations
Observations were performed on 26 October 2009 with the
PACS instrument (Poglitsch et al. 2010) onboard the Herschel
Space Observatory (Pilbratt et al. 2010) in line spectroscopic
mode, with the grating centred on the H2O 212−101 line at
179.527 μm. The L1157 outflow region (of about 6?× 2?) was
covered by 3 individual PACS raster maps, arranged along the
outflow axis. Each map consists of 3 × 3 PACS frames acquired
in steps of 40??. The instrument is a 5 × 5 pixel array provid-
ing a spatial sampling of 9.4??/pixel, while the spectral resolu-
tion at 179 μm is R ∼ 1500 (i.e., ∼210 kms−1). The data were
reduced with HIPE 2.0. Additional IDL routines were devel-
oped to construct a final integrated and continuum-subtracted
line map. Flux calibrations used calibration files obtained by
groundtests that remain very uncertain at the time of paper writ-
ing, especially for extended sources. To evaluate the flux uncer-
tainty, we compared with the three measurements performed by
the ISO satellite along the outflow (Giannini et al. 2001). To do
that, we performed aperture photometry of the line emission in
the PACS map within the 80??ISO circular beam. The ratio of
PACS to ISO fluxes ranges between 1.1 and 1.8 at the three
positions: we adopt this as the uncertainty in our quantitative
analysis. The typical rms noise across the map is of the order of
2 × 10−6ergs−1cm−2sr−1.
3. Results and comparison with other tracers
Figure 1 presents the PACS map of the 179 μm line emission. In
the same figure, the H2O map is overlaid with contours of the
emission from the H20–0 S(1) (Neufeld et al. 2009), CO 2–1
and SiO 3–2 (Bachiller et al. 2001) transitions. The water map
exhibits several emission peaks corresponding to the positions
of previously-known shocked knots, labelled as B0-B1-B2 for
the south east blue-shifted lobe, and R0-R-R2 for the north west
red-shifted lobe, following the nomenclature of Bachiller et al.
(2001). These emission knots represent the actual working sur-
faces of a precessing and pulsed jet and are thus associated with
the present location of the active shock regions. With respect
to CO, H2O emission appears more localized, having a less
prominent diffuse component. About 60% of the total 179 μm
flux is found within 30??apertures centered on the knots. This
could be partly related to the line excitation: the 179 μm line
excitation temperature is ∼80 K above the o-H2O ground state
(compared to the 17 K for CO 2–1), and the critical density of
its upper level is above 108cm−3for T<∼500 K. It may how-
ever also be a consequence of the specific conditions needed to
ensure a significant production of water. The H2O abundance is
indeed significantly higher only in shocks strong enough to re-
lease the water ice located on grain mantles by sputtering and
grain-grain collisions or to activate the gas-phase reactions that
convert the gas-phase oxygen into water. Both these processes
becomeefficientat shockvelocitiesvs>∼15kms−1(Caselli et al.
1997; Jiménez-Serra et al. 2008; Kaufman & Neufeld 1996). In
this respect, we note that the H2O emission peaks correspond
rather closely to both the position and the relative intensity of
the H2 rotational emission (with the H2O 179 μm/H2 17 μm
ratio in the range ∼(2−3) × 10−2for all the H2 peaks). Peaks
of low-J H2pure rotational lines are associated with warm gas
(with T ∼ 300−500 K) excited in low velocity non-dissociative
shocks that are tracers of regions in which a high H2O abun-
dance is expected. Other molecules are known to have strongly
enhanced abundances in shocks. One of the most well studied
of these molecules is SiO, for which Fig. 1 shows that, like wa-
ter, its emission is very localized around the shocked knots. A
similar behavior is found for other molecules, such as NH3and
CH3OH (Bachiller et al. 2001; Tafalla & Bachiller 1995).
The strongest water peak is located at the position of the
B1 knot, which is known to be the most chemically active of the
L1157spots(e.g.Bachiller&PerezGuitierrez1997;Benedettini
et al. 2007; Codella et al. 2010). This knot at near-IR wave-
lengths appears as a bow shock with intense H22.12 μm emis-
sion (Davis & Eislöffel 1995) and has a significant H2column
density enhancement (Nisini et al. 2010). Although the spatial
resolution of the present observations prevents us from com-
pletely resolving the bow shock structure, the observed mor-
phology at the B1/B0 positions suggests that water emission is
mainly localized at the bow apex and eastern wing. A similar
morphologyhas been observed for molecules such as SiO, NH3,
andCS (Benedettiniet al.2007; Tafalla&Bachiller1997),while
other shock produced molecules, such as CH3OH, noticeably
have emission localized on the bow western wing (e.g. Codella
et al. 2009). This behavior probably relates to an asymmetry in
the excitation conditions along the bow structure, most likely in-
duced by the jet precession or the propagation of shocks in an
inhomogeneous medium.
Strong, spatially unresolved, water emission is also detected
on-source. This localized emission can originate in different
components, including shocks impacting on a dense medium at
the jet base, the infalling protostellar envelope,or emission from
a UV-heated outflow cavity, as discussed in van Kempen et al.
(2010) for the HH46-IRS case. The precise origin of this emis-
sion will be investigatedbydedicatedHerschel observations,but
we note here the interesting evidence that no other molecule ex-
hibits significant emission at the central position. In particular,
the non-detection of strong emission from molecules such as
CH3OH indicates that grain ice mantle evaporation in the pro-
tostellar envelope is unlikely to be the origin of the on-source
H2O emission, since the two molecules should desorb at simi-
lar temperatures. The non-detection of the H20–0 S(1) line at
the central position is also remarkable. This may be caused by
the heavy extinction close to the central source. Assuming an
intrinsic H2O179 μm/H217 μm ratio in the range of that ob-
served along the outflow, we estimate that Avon-source should
be>∼150 mag to be able to explain the H2 line non-detection.
Alternatively, C-type shocks with very high pre-shock densi-
ties (≥106cm−3) and velocities between 20 and 40 kms−1are
expected to have a large H2O/H2 cooling ratio (Kaufman &
Neufeld 1996).
4. Water abundance and total cooling
To constrain the range of water column densities that could pro-
duce the observed 179 μm emission, we consider the SWAS
and Odin observations of the H2O 110−101557 GHz (538 μm)
line observed in this outflow (Franklin et al. 2008; Bjerkeli
et al. 2009). Given the large size of the apertures of these two
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B. Nisini et al.: Water cooling of shocks in protostellar outflows
Fig.1. Continuum subtracted PACS map of the integrated H2O 179 μm emission along the L1157 outflow. Offsets are with respect to the
L1157-mm source, at coordinates α(2000) = 20:39:06.2, δ(2000) = +68:02:16. The different emission peaks are labelled following the nomencla-
ture adopted by Bachiller et al. (2001) for individual CO peaks. The same map is shown in the other panels with overlays of other tracers, namely
H20–0 S(1) at 17 μm (Neufeld et al. 2009), CO 2–1, and SiO 3–2 (Bachiller et al. 2001). The spatial resolution of these images are ∼11??, for H2
and CO, and 18??for SiO. Note that the H2observed region does not cover the B2 and R2 shocked peaks.
Fig.2. LVG theoretical predictions of the 179 μm line brightness versus
the 179 μm/557 GHz line ratio, compared with observed values. See
text for the details.
instruments relative to the PACS spatial resolution, we evalu-
ate here only properties averaged over large outflow regions. In
particular, we consider the Odin observations acquired towards
the blue (B) and red (R) outflow lobes at offsets (+29??, −52??)
and (−21??, +121??) (Bjerkeli et al. 2009). The 179 μm/557 GHz
intensity ratios are obtained by diluting the PACS observations
to the 126??Odin resolution. The same procedure was adopted
for the SWAS observation that encompasses almost the entire
L1157 PACS mapped region with its 3.5?× 5.0?elliptical aper-
ture.Figure2presentslargevelocitygradient(LVG)predictions,
assuming a slab geometry, of the 179 μm line brightness versus
the 179 μm/557 GHz line ratio, compared to the observations
combined above. The absolute brightnesses are those averaged
within an area enclosing 90% of the total PACS emission in-
side each considered Odin/SWAS aperture.These emitting areas
are 5.9 × 10−8, 8.0 × 10−8, and 2.7 × 10−7sr for the R, B, and
the SWAS apertures, respectively. The line intensity derived in
this way was considered to be a lower limit to the true 179 μm
brightness if the PACS emission originates in a clumpy medium,
of which the clump size is smaller than the Herschel diffraction
limit at 179 μm.
In the figure, observations are indicated as boxes that take
into consideration the uncertainty of a factor of about 1.5 in the
179 μm flux, estimated by comparing with the ISO observations
(Sect. 2). Theoretical curves were derived as a function of the o-
H2O columndensity, using the RADEX code (Van der Tak et al.
2007) assuming temperature and density conditions measured
from the H2 Spitzer observations or ground-based millimeter
observations (Nisini et al. 2010, 2007; Mikami et al. 1992). The
temperature is between 300 and 500 K and the density is in the
range 1−5×105cm−3, the blue lobe being on average colder and
denser than the red lobe. Part of the 557 GHz emission can arise
from a gas colder than these assumed values, given the lower ex-
citation temperature of this line with respect to the 179 μm line.
To evaluate the effect of differenttemperature componentsalong
the line of sight on the ratio of the two considered transitions,
Fig. 2 also plots the theoretical predictions assuming a temper-
ature stratification where the column density in each layer at a
given T varies as T−b(Neufeld & Yuan 2008). A minimum and
maximum temperature of 100 K and 4000 K, respectively are
assumed, and b values between 2 and 4, i.e., the range of val-
ues that consistently fit the H2rotational lines (Neufeld et al.
2009). These curves give the same range of predicted values
as the single T curves, indicating that contributions from high-
temperature gas do not significantly affect the considered transi-
tions.
Several general conclusions can be drawn from the in-
spection of Fig. 2. Firstly, the data are consistent with model
predictions only if we assume that the real emitting areas are
smaller than those estimated from the PACS map. In particular,
agreement with the theoretical curves is found for covering
factors (fc) ∼0.1–0.2, which suggests that the emission is
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A&A 518, L120 (2010)
Table 1. Estimated water abundances.
Ra
Tb
K
300
500
n(H2)b
cm−3
3 × 105
1 × 105
N(oH2O)
1016cm−2
3–9
2–4
X(H2O)b
10−4
0.6–3
0.8–2
Ac
sr
B
R
14–20
10–14
∼5.2 × 10−9
∼9.4 × 10−9
Notes.(a)179 μm/557 GHz ratio within the Odin aperture.(b)See text
for references and assumptions on T,n and N(H2).(c)Effectiveemission
area that reconciles the observed and predicted 179 μm line intensity
within the Odin aperture.
concentrated on some unresolved emission knots that together
do not fill an area larger than a few tens of arcsec. This is not
unexpected, since interferometric mm observations illustrate
the extreme clumpiness of the shocked gas, individual knots
being of sizes of a few arcsec each (e.g., Benedettini et al. 2007;
Lefloch et al. 2010). We note that the typical length scale for
planar C-type shocks at the considered densities is of the order
of 1016cm, i.e., about 1/10 of the PACS spatial resolution at
D = 440 pc. The observed 179 μm/557 GHz ratios, ranging be-
tween 10 and 20,are consistent with N(H2O) ∼ 2–9 × 1016cm−2
(assuming a Δv = 15 kms−1from the 557 GHz line width). The
H2 column densities, averaged within the PACS emitting areas,
were measured from the H2mid-IR rotational lines and results
in ∼5 × 1019cm−2in both regions covered by the B and R
observations. The water abundance in the unresolved clumps is
therefore estimated to be ∼N(H2O)/N(H2)×fc ∼ 0.6−3 × 10−4
(with a H2O o/p ratio of 3). Table 1 reports in more detail the
range of values derived in each considered aperture. The total
mass of the shocked gas involved in the 179 μm emission is
of the order of 5 × 10−3M?, which is only a small fraction
(∼1/100) of the total mass of the outflow estimated from
CO observations (e.g. Bachiller et al. 2001). Lefloch et al.
(2010) show that H2O components with different velocities
are discernible in the 557 GHz data acquired by HIFI in a 40??
beam centred on the L1157-B1 knot. They separately analyse
the different velocity components, confirming that small filling
factors are required to explain their observations and finding
that the component of higher velocity is the one exhibiting the
water abundance of the order of 10−4. Lower H2O abundance
values, between 10−6and 10−5, were estimated using only the
SWAS and Odin 557 GHz emission, assuming that the 557 GHz
emission originates in the same cool gas traced by the low-J
CO emission, thus a gas with a larger covering factor and
lower temperature than considered here (Neufeld et al. 2000;
Franklin et al. 2008; Bjerkeli et al. 2009). Combining ISO-
179 μm emission and SWAS observations, Benedettini et al.
(2002) derived a water abundance for the warm shocked gas of
∼5 × 10−5, thus in the lower range of values estimated in the
present analysis. However, the ISO observations did not cover
the entire L1157 outflow 179 μm emission, and the inferred
ISO179 μm/SWAS557 GHz ratio was underestimated by about
a factor of 2. Given the considered conditions, the 179 μm
line contributes to about 30–40% of the water emission in the
outflow: the total estimated H2O luminosity is ∼8−9 × 10−2L?,
which is about 40% of the total H2shock luminosity (0.2 L?,
Nisini et al. 2010) and about 23% of the total shock cooling
in the L1157 outflow, if we also consider the contributions
given by CO and [Oi] derived from ISO observations by
Giannini et al. (2001). The high water abundance estimated
in the present analysis is consistent with predictions of non-
dissociative shock models, in which water is mainly produced
by endothermic reactions, activated at T >∼ 300 K, where all
the available gas-phase oxygen is converted into H2O, or by the
sputtering of icy grain mantles behind the shock. According to
Bergin et al. (1998), the time needed to complete this process is
of the order of 103yr, for T = 400 K. This is comparable to the
shock timescales estimated from H2observations of individual
emission knots of the L1157 outflow (Nisini et al. 2010), thus
supporting the idea that the water in this outflow has had time to
reach its maximum allowed abundance.
5. Conclusions
We have presented a PACS spectral map of the H2O 179 μm
transitionobtainedtowardtheL1157protostellaroutflow.Strong
water emission peaks have been found at the location of
previously-knownshockedspotsandcorrelatewell withH2mid-
IR rotationallines, as well as other importantshock tracers, such
as SiO and NH3. The absolute 179 μm intensity and the in-
tensity ratios with respect to the previously-observed 557 GHz
line,indicatethatthewateremissionoriginatesinwarmcompact
clumps, spatially unresolved by PACS, that have a H2O abun-
dance of the order of 10−4. The total H2O cooling has been
estimated to be of the order of 8−9 × 10−2L?, representing
about 40% of the cooling due to H2and 23% of the total energy
released in shocks along the L1157 outflow.
Additional Herschel
PACS/HIFI
L1157 outflow are planned by the WISH program. These will
enable us to investigate variations in the water abundance within
the outflow and correlate these with kinematical information.
observations of the
Acknowledgements. This program is made possible thanks to the HIFI guaran-
teed time and the PACS instrument builders.
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B. Nisini et al.: Water cooling of shocks in protostellar outflows
1INAF - Osservatorio Astronomico di Roma, Via di Frascati 33,
00040 Monte Porzio Catone, Italy
e-mail: nisini@oa-roma.inaf.it
2INAF - Istituto di Fisica dello Spazio Interplanetario, Area di
Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma,
Italy
3INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125
Firenze, Italy
4Department of Radio and Space Science, Chalmers University of
Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
5Department of Physics and Astronomy, Johns Hopkins University,
3400 North Charles Street, Baltimore, MD 21218, USA
6IGN Observatorio Astronómico Nacional, Apartado 1143, 28800
Alcalá de Henares, Spain
7Leiden Observatory, Leiden University, PO Box 9513, 2300 RA
Leiden, The Netherlands
8Max Planck Institut for Extraterestrische Physik, Garching,
Germany
9Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux,
France; CNRS/INSU, UMR 5804, Floirac, France
10Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
11Department of Astronomy, The University of Michigan, 500 Church
Street, Ann Arbor, MI 48109-1042, USA
12California Institute of Technology, Division of Geological and
Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
13School of Physics and Astronomy, University of Leeds, Leeds LS2
9JT, UK
14Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA.
Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid,
Spain
15LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av.
de l’Observatoire, 75014 Paris, France
16Astronomical InstituteAnton Pannekoek, University of Amsterdam,
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
17Department ofAstrophysics/IMAPP,
Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
18Department of Physics and Astronomy, Denison University,
Granville, OH, 43023, USA
RadboudUniversity
19University of Waterloo, Department of Physics and Astronomy,
Waterloo, Ontario, Canada
20SRON Netherlands Institute for Space Research, PO Box 800, 9700
AV, Groningen, The Netherlands
21NationalResearch Council
Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7,
Canada
22Department of Physics and Astronomy, University of Victoria,
Victoria, BC V8P 1A1, Canada
23Centre for Star and Planet Formation, Natural History Museum
of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350
Copenhagen, Denmark
24Department of Physics and Astronomy, San Jose State University,
One Washington Square, San Jose, CA 95192, USA
25Department of Astronomy, Stockholm University, AlbaNova, 106
91 Stockholm, Sweden
26Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
MS 42, Cambridge, MA 02138, USA
27Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69,
53121 Bonn, Germany
28Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA
29Department of Physics and Astronomy, University of Calgary,
Calgary, T2N 1N4, AB, Canada
30Department of Physics and Astronomy, University College London,
Gower Street, London WC1E6BT, UK
31Kapteyn Astronomical Institute, University of Groningen, PO Box
800, 9700 AV, Groningen, The Netherlands
32Institute Laboratoire d’Etudes du Rayonnement et de la Matire en
Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP,
Paris, France
33CNRS/INSU, UMR 5804, B.P. 89, 33271 Floirac cedex, France
34Institute Institut de Radioastronomie Millimetrique, IRAM, 300 rue
de la Piscine, 38406 St Martin d’Heres, France
35KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77,
50937 Köln, Germany
36European Space Astronomy Centre, ESA, PO Box 78, 28691
Villanueva de la Caada, Madrid, Spain
37ALMA
Canada,HerzbergInstitute of
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