Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L120 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014603 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Water cooling of shocks in protostellar outflows
Herschel-PACS map of
L1157![[*]](/icons/foot_motif.png)
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. Yildiz7 -
B. Delforge32,17 - J. Desbat9,33
- W. A. Hatch29 -
I. Péron34,32,17 - R. Schieder35
- J. A. Stern29 -
D. Teyssier36 - N. Whyborn37
1 - INAF - Osservatorio Astronomico di Roma, Via di Frascati 33, 00040
Monte Porzio Catone, Italy
2 - INAF - Istituto di Fisica dello Spazio Interplanetario, Area di
Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
3 - INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125
Firenze, Italy
4 - Department of Radio and Space Science, Chalmers University of
Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
5 - Department of Physics and Astronomy, Johns Hopkins University, 3400
North Charles Street, Baltimore, MD 21218, USA
6 - IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá
de Henares, Spain
7 - Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden,
The Netherlands
8 - Max Planck Institut for Extraterestrische Physik, Garching, Germany
9 - Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux,
France; CNRS/INSU, UMR 5804, Floirac, France
10 - Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
11 - Department of Astronomy, The University of Michigan, 500 Church
Street, Ann Arbor, MI 48109-1042, USA
12 - California Institute of Technology, Division of Geological and
Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
13 - School of Physics and Astronomy, University of Leeds, Leeds LS2
9JT, UK
14 - Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA.
Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain
15 - LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de
l'Observatoire, 75014 Paris, France
16 - Astronomical Institute Anton Pannekoek, University of Amsterdam,
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
17 - Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO
Box 9010, 6500 GL Nijmegen, The Netherlands
18 - Department of Physics and Astronomy, Denison University,
Granville, OH, 43023, USA
19 - University of Waterloo, Department of Physics and Astronomy,
Waterloo, Ontario, Canada
20 - SRON Netherlands Institute for Space Research, PO Box 800, 9700
AV, Groningen, The Netherlands
21 - National Research Council Canada, Herzberg Institute of
Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
22 - Department of Physics and Astronomy, University of Victoria,
Victoria, BC V8P 1A1, Canada
23 - Centre for Star and Planet Formation, Natural History Museum of
Denmark, University of Copenhagen,
Øster Voldgade 5-7, 1350 Copenhagen, Denmark
24 - Department of Physics and Astronomy, San Jose State University,
One Washington Square, San Jose, CA 95192, USA
25 - Department of Astronomy, Stockholm University, AlbaNova, 106 91
Stockholm, Sweden
26 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS
42, Cambridge, MA 02138, USA
27 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
28 - Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA
29 - Department of Physics and Astronomy, University of Calgary,
Calgary, T2N 1N4, AB, Canada
30 - Department of Physics and Astronomy, University College London,
Gower Street, London WC1E6BT, UK
31 - Kapteyn Astronomical Institute, University of Groningen, PO Box
800, 9700 AV, Groningen, The Netherlands
32 - Institute Laboratoire d'Etudes du Rayonnement et de la Matire en
Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
33 - CNRS/INSU, UMR 5804, B.P. 89, 33271 Floirac cedex, France
34 - Institute Institut de Radioastronomie Millimetrique, IRAM, 300 rue
de la Piscine, 38406 St Martin d'Heres, France
35 - KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str.
77, 50937 Köln, Germany
36 - European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva
de la Caada, Madrid, Spain
37 - ALMA
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 m
transition obtained toward the young outflow L1157.
Methods. The 179 m 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 m
emission is spatially correlated with emission from H2 rotational
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 H2O peak is also observed at
the location of the proto-star, where none of the other molecules have
significant emission. The absolute 179
m 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
,
about 40% of the cooling due to H2 and 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 physical variations and the temporal evolution of protostellar outflows, thus representing a very powerful 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 cooling and of variations in its abundance with shock properties and 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 outflow gas components (e.g., Franklin et al. 2008; Bjerkeli et al. 2009). All these facilities, however, had poor spatial resolution (i.e. greater than 80
In this framework, a sample of YSO outflows will be surveyed
in different water lines by the PACS and HIFI instruments onboard the Herschel
satellite, as part of the key program WISH (Water In
Star-forming-regions with Herschel).
This paper presents the first results obtained from this survey,
consisting of a PACS map of the H2O 2
12-101
179
m
line covering the outflow of the protostar L1157-mm, obtained during
the Herschel science demonstration phase.
The 179
m
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
representing an 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,
,
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 observations planned
for 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 m.
The L1157 outflow region (of about
)
was covered by 3 individual PACS raster maps, arranged along the
outflow
axis. Each map consists of
PACS frames acquired
in steps of 40
.
The instrument is a
pixel array providing a spatial sampling of 9.4
/pixel,
while the spectral resolution at 179
m is
(i.e.,
210 km s-1).
The data were reduced with HIPE 2.0. Additional IDL routines were
developed to construct a final integrated and continuum-subtracted line
map.
Flux calibrations used calibration files obtained by ground tests that
remain very uncertain at the time of paper writing, especially for
extended sources. To evaluate the flux uncertainty, 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
erg s-1 cm-2 sr-1.
![]() |
Figure 1:
Continuum subtracted PACS map of the integrated H2O
179 |
Open with DEXTER |
3 Results and comparison with other tracers
Figure 1 presents the PACS map of the 179










The strongest water peak is located at the position of the
B1 knot, which is known to be
the most chemically active of the L1157 spots (e.g. Bachiller &
Perez Guitierrez 1997;
Benedettini et al. 2007;
Codella et al. 2010).
This knot at near-IR wavelengths appears as a bow shock with intense H2
2.12 m
emission (Davis & Eislöffel 1995) and has a
significant H2 column density
enhancement (Nisini et al. 2010). Although
the spatial resolution of the present observations prevents us from
completely resolving the bow shock structure, the observed morphology
at the B1/B0 positions suggests that water emission is mainly localized
at the bow apex and eastern wing. A similar morphology has been
observed for molecules such as SiO, NH3, and CS
(Benedettini et al. 2007;
Tafalla & Bachiller 1997), 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 induced 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 emission will be investigated by dedicated Herschel
observations, but we note here the interesting evidence that no other
molecule exhibits 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 protostellar envelope is unlikely to be the origin
of the on-source H2O emission, since
the two molecules should desorb at similar temperatures. The
non-detection of the H2 0-0 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 H2O 179 m/H2 17
m
ratio in the range of that observed along the outflow, we estimate that
Av
on-source should be
150 mag
to be able to explain the H2 line
non-detection.
Alternatively, C-type shocks with very high pre-shock densities (
106 cm-3)
and velocities between 20 and 40 km s-1
are 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 produce the observed 179








![]() |
Figure 2:
LVG theoretical predictions of the 179 |
Open with DEXTER |







In the figure, observations are indicated as boxes that take into
consideration the uncertainty of a factor of about 1.5 in the
179 m
flux, estimated by comparing with the ISO observations
(Sect. 2).
Theoretical curves were derived as a function of the o-H2O
column density, 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
cm-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 excitation temperature of this
line with respect to the 179
m line.
To evaluate the effect of different temperature components along the
line of sight on the ratio of the two considered transitions,
Fig. 2
also plots the theoretical predictions assuming a temperature
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 values that
consistently fit
the H2 rotational 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 transitions.
Several general conclusions can be drawn from the inspection
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 ()
0.1-0.2,
which suggests that the emission is 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 1016 cm,
i.e., about 1/10 of the PACS spatial resolution at D=440 pc.
The observed 179
m/557 GHz
ratios, ranging between 10 and 20, are consistent with N(H2O)
2-
cm-2 (assuming a
km s-1
from the 557 GHz line width).
The H2 column densities, averaged
within the PACS emitting areas, were measured from
the H2 mid-IR rotational lines and results in
cm-2
in
both regions covered by the B and R observations. The water abundance
in the unresolved clumps is therefore estimated to be
(H2O)/N(H2)
(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
m
emission is of
the order of
,
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 10-6
and 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
m
emission and SWAS observations, Benedettini et al. (2002)
derived a water abundance for the warm shocked gas of
,
thus in the lower range of values estimated in the present analysis.
However, the ISO observations
did not cover the entire L1157 outflow 179
m emission,
and the inferred ISO 179
m/SWAS 557 GHz ratio was
underestimated by about a factor of 2.
Table 1: Estimated water abundances.
Given the considered conditions, the 179



5 Conclusions
We have presented a PACS spectral map of the H2O 179


Additional Herschel PACS/HIFI observations of the 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.
AcknowledgementsThis program is made possible thanks to the HIFI guaranteed time and the PACS instrument builders.
References
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Footnotes
- ... L1157
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important partecipation from NASA.
- ... Herschel
- http://www.strw.leidenuniv.nl/WISH/
All Tables
Table 1: Estimated water abundances.
All Figures
![]() |
Figure 1:
Continuum subtracted PACS map of the integrated H2O
179 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
LVG theoretical predictions of the 179 |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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