Article

The CHESS spectral survey of star forming regions: Peering into the protostellar shock L1157-B1. II. Shock dynamics

Astronomy and Astrophysics (Impact Factor: 4.38). 07/2010; 518(3). DOI: 10.1051/0004-6361/201014630
Source: OAI

ABSTRACT

Context. The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and chemical processes are strongly interdependent.

Aims. We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program “Chemical HErschel Surveys of star forming regions” (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the excitation conditions in the shock region.

Methods. The CO 5-4 and o-H2_O 1_(10)–1_(01) lines have been detected at high-spectral resolution in the unbiased spectral survey of the HIFI-band 1b spectral window (555–636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window help establish the origin of the emission detected in the main-beam of HIFI and the physical conditions in the shock.

Results. Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We find that the molecular emission arises from two regions with distinct physical conditions : an extended, warm (100 K), dense (3 × 10^5 cm^(-3)) component at low-velocity, which dominates the water line flux in Band 1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (>400 K) gas of moderate density ((1.0–3.0) × 10^4 cm^(-3)), which appears to dominate the flux of the water line at 179μm observed with PACS. The water abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8 × 10^(-7) up to 8 × 10^(-5). The properties of the high-velocity component agree well with the predictions of steady-state C-shock models.

Full-text

Available from: Bertrand Lefloch
A&A 518, L113 (2010)
DOI: 10.1051/0004-6361/201014630
c
ESO 2010
Astronomy
&
Astrophysics
Herschel: the first science highlights
Special feature
Letter to the Editor
The CHESS spectral survey of star forming regions: Peering i nto
the protostellar shock L1157-B1
II. Shock dynamics
B. Lefloch
1
,S.Cabrit
2
,C.Codella
3
,G.Melnick
4
, J. Cernicharo
5
,E.Caux
6
, M. Benedettini
7
, A. Boogert
8
,P.Caselli
9
,
C. Ceccarelli
1
,F.Gueth
10
, P. Hily-Blant
1
, A. Lorenzani
3
,D.Neufeld
11
,B.Nisini
12
, S. Pacheco
1
, L. Pagani
2
,
J. R. Pardo
5
,B.Parise
13
,M.Salez
2
,K.Schuster
10
,S.Viti
12,14
, A. Bacmann
1,15
, A. Baudry
15
,T.Bell
16
,E.A.Bergin
17
G. Blake
16
, S. Bottinelli
6
, A. Castets
1
, C. Comito
13
, A. Coutens
6
, N. Crimier
1,5
,C.Dominik
18,19
,K.Demyk
6
,
P. Encrenaz
2
, E. Falgarone
2
,A.Fuente
20
, M. Gerin
2
, P. Goldsmith
21
,F.Helmich
22
, P. Hennebelle
2
, T. Henning
23
,
E. Herbst
24
, T. Jacq
15
,C.Kahane
1
,M.Kama
18
,A.Klotz
6
,W.Langer
21
,D.Lis
16
,S.Lord
16
,S.Maret
1
, J. Pearson
21
,
T. Phillips
16
, P. Saraceno
7
, P. Schilke
13,25
, X. Tielens
26
,F.vanderTak
19
,M.vanderWiel
19
, C. Vastel
6
, V. Wakelam
15
,
A. Walters
6
,F.Wyrowski
13
,H.Yorke
21
, R. Bachiller
20
, C. Borys
16
,G.DeLange
22
,Y.Delorme
5
,C.Kramer
25,27
,
B. Larsson
28
,R.Lai
28
,F.W.Maiwald
21
, J. Martin-Pintado
5
, I. Mehdi
21
, V. Ossenkopf
25
,P.Siegel
21
,
J. Stutzki
24
, and J. H. Wunsch
13
(Aliations are available in the online edition)
Received 31 March 2010 / Accepted 30 April 2010
ABSTRACT
Context.
The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright
bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and
chemical processes are strongly interdependent.
Aims.
We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program
“Chemical HErschel Surveys of star forming regions” (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the
excitation conditions in the shock region.
Methods.
The CO 5-4 and o-H
2
O1
10
1
01
lines have been detected at high-spectral resolution in the unbiased spectral surve y of the HIFI-band
1b spectral window (555–636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window
help establish the origin of the emission detected in the main-beam of HIFI and the physical conditions in the shock.
Results.
Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We fi nd that the molecular emission
arises from two regions with distinct physical conditions : an extended, warm (100 K), dense (3 × 10
5
cm
3
) component at low-velocity, which
dominates the water line flux in Band 1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (>400 K)
gas of moderate density ((1.03.0) × 10
4
cm
3
), which appears to dominate the flux of the water line at 179 μm observed with PACS. The w ater
abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8×10
7
up to 8 ×10
5
. The properties
of the high-velocity component agree well with the predictions of steady-state C-shock models.
Key words. stars: formation ISM: jets and outflo ws ISM: individual objects: L1157
1. Introduction
Shocks in protostellar outflows play a crucial role in the
molecular cloud evolution and star formation by transferring
momentum and energy back to the ambient medium. There is
mounting evidence that these shocks often involve a magnetic
precursor where ionic and neutral species are kinematically de-
coupled. Magneto-hydrodynamical (MHD) shocks are impor-
tant not only for the cloud dynamics, but also for the chemi-
cal evolution through temperature and density changes, which
favors the activation of endothermic reactions, ionization, and
dust destruction through sputtering and shattering in the ion
neutral drift zone. These various processes lead to abundance
Herschel is an ESA space observatory with science instruments
provided by European-led Principal Investigator consortia and with im-
portant participation from NASA.
enhancements up to several orders of magnitude, as reported
for various molecular species in “chemically active” outflows
(Bachiller et al. 2001). Conversely, the magnetic field and the
ionization fraction play an imp ortant role in controlling th e size
and the temperature of the ion-neutral drift zone. Because of the
interplay between the dynamics and chemistry, the physics of
MHD shocks requires a comprehensive picture of both the gas
and dust physical conditions in the compressed region itself.
Along with H
2
,H
2
O and CO are two key-molecules pre-
dicted to dominate the cooling of MHD shocks (Kaufman &
Neufeld 1996). The abundance of H
2
O in protostellar regions
can be greatly enhanced in shocks, even of moderate velocity.
This occurs both from the sputtering of frozen water from grain
mantles and through high-temperature sensitive reactions in the
gas phase (Elitzur & de Jong 1978; Elitzur & Watson 1978;
Bergin et al. 1998). Multi-transition o bservations of th ese two
Article published by EDP Sciences Page 1 of 5
Page 1
A&A 518, L113 (2010)
molecules therefore serve as good probes of shock regions with
various excitation conditions, and can be used to set stringent
constraints on MHD shock models (Flower & Pineau des Forets,
2001).
The heterodyne instrument HIFI onboard Herschel allows us
to study with unprecedented sensitivity the chemical and dynam-
ical evolution of protostellar shocks, at spectral and angular res-
olutions comparable to the best ground-based single-dish tele-
scopes. This is the main goal of the spectral survey of L1157-B1,
carried out in the guaranteed time key-project CHESS.
The source L1157-mm is a low-mass Class 0 protostar lo-
cated at a distance estimated between 250 pc (Looney et al.
2007) and 440 pc (Viotti 1969). It drives a spectacular bipolar
outflow, which has been studied in detail at millimeter and far-
infrared wavelengths. Mapping of the southern lobe of L1157
with the Plateau de Bure Interferometer (PdBI) reveals two limb-
brightened cavities (Gueth et al. 1996), each one terminated
by a strong bow shock, dubbed “B1” and “B2” respectively
(Fig. 1), which are likely the re sult of episodic ejection events
in a precessing, highly collimated jet. The spatial and kinemati-
cal structure of B1 has been modelled in great detail by various
authors, making it the archetype of protostellar bowshocks in
low-mass star-forming regions and the testbed of MHD shock
models (Gusdorf et al. 2008a,b).
Here, we report on the emission lines of CO and H
2
Ode-
tected in the low-frequency band of HIFI in the course of the
CHESS spectral survey. From comparison with complementar y
observations, we discuss the origin of the emission, and based on
a simple modelling of the source, we derive the water abundance
in the shock region.
2. Observations and results
A full coverage of the band 1b at 20
h
39
m
10.2
s
+68
01
10.5

(J2000) in the bowshock B1 was carried out with the HIFI het-
erodyne instrument (de Graauw et al. 2010) on board of the
Herschel Space Observatory (Pilbratt et al. 2010) during the
performance verification phase on 2009 August 1. The corre-
sponding dataset is OBS_1342181160. The HIFI band 1b (from
555.4 to 636.2 GHz) was covered in double beam switching.
Both polarizations (H and V) were observed simultaneously. The
receiver was tuned in double sideband (DSB) with a total inte-
gration time of 140 min. In order to obtain the best possible data
reconstruction, the survey was acquired with a degree of redun-
dancy of 4. The wide band spectrometer (WBS) was used as
spectrometer, providing a frequency resolution of 0.5 MHz.
The data were processed with the ESA-supported package
HIPE (Ott et al. 2009). Fits file from level 2 were then cre-
ated and transformed into GILDAS format
1
for baseline sub-
traction and subsequent sideband deconvolution. The spectral
resolution was degraded to 1 MHz in the final single sideband
(SSB) dataset. The calibration for each receiver (H and V) is bet-
ter than 2–3%. The relative calibration between both receivers is
also rather good, with a dierence in intensity of about 4%. The
overall calibration uncertainty is about 7%, except for the strong
CO line presen t in the band (see below).
Two strong lines dominate over the molecular transitions de-
tected in the spectral band: the fundamental line of water in
its ortho state o-H
2
O1
10
1
01
at 556.936069 GHz and the CO
5-4 line at 576.276905 GHz (Fig. 1). The final rms noise is
13 mK. We adopted the theoretical telescope main-beam e-
ciency η
mb
= 0.723, and a main-beam size of 39

(HPFW) in
1
http://www.iram.fr/IRAMFR/GILDAS
Fig. 1. (left) Southern outflow lobe of L1157 in CO 1-0 (greyscale and
black contours) and in SiO 2-1 (white contours) as observed at the PdBI
(Gueth et al. 1996, 1998). A black square marks t he nominal position of
bowshock L1157-B1 observed with HIFI. The HIFI main-beam is rep-
resented with a black circle. (right) Panel of the CO 5-4 (bottom)and
o-H
2
O1
10
1
01
(centre) line spectra obtained in band 1b of HIFI. For
both lines, we show (dashed) a magnified and spectrally smoothed view
of the emission. Intensities are expressed in units of antenna temper-
ature.(top)H
2
O spectrum with fitted low-velocity component (LVC),
high-velocity component (HVC), absorption feature and summed tted
spectrum.
the whole band. Unless indicated, intensities are expressed in
units of antenna temperature T
A
.
The CO 5-4 transition is detected with an intensity of 9 K
(T
A
) and a linewidth of 5 km s
1
. We notice a weak absorption
feature in the line profile in the redshifted gas over a wide ve-
locity range, which may partly arise fr om cloud contamination
in the reference position. The intensity in the blue wing of the
CO line diers by as much as 20% between both polarizations.
This eect is not observed towards the H
2
O line. Its origin is not
understood at the moment.
The fundamental o-H
2
O1
10
1
01
line is detected with an
intensity of 0.9K (T
A
) at the peak. It is characterized by a
broad linewidth 15 km s
1
. The line displays an absorption
dip at v
lsr
=+2.9kms
1
and a broad redshifted wing extend-
ing up to +8kms
1
. The broad linewidth of the H
2
O spec-
trum could be fit with three Gaussian velocity components, a
low-velocity, a high-velocity, and an absorption component. The
low (high) velocity component peaks at v
lsr
= 0.58 km s
1
(7.86 km s
1
); the linewidth and peak intensity derived from
the fit are 9.56 km s
1
and 0.77 K (13.72 km s
1
and 0.29 K), re-
spectively. The absorption component was fit by a narrow line
Gaussian (ΔV = 1.38 km s
1
)ofamplitude0.48 K centered at
v
lsr
=+2.9kms
1
. The fit of the individual components and the
resulting fit to the water spectrum is displayed in Fig. 1.
Overall, the H
2
O and CO emission are detected in the same
velocity range. The high sensitivity of the HIFI observations
permits the detection of emission from the entrained gas up to
v
lsr
= 30 km s
1
, i.e. about 10 km s
1
higher than was pre-
viously known from ground based observations. However, line
profiles dier noticeably and the ratio of the H
2
O/CO54 line
intensities increases with increasing velocities from about 0.2 in
the amb ient gas up to 0.9 at v
lsr
= 25 km s
1
(Fig. 3).
Page 2 of 5
Page 2
B. Lefloch et al.: CHESS survey of L1157-B1 : Shock dynamics. II.
Fig. 2. (left) Velocity-integrated CO 6-5 emission maps of the low-
and high-velocity components. LVC (HVC) emission is represented in
greyscale and thin dashed contours (white contours); rst contour and
contour interval are 3σ and 1σ (10% of the peak ux), respectively,
Contours at half-power are drawn in thick. (right) Same for SiO 2-1
observed at the PdbI. The HIFI main-beam is represented with a black
circle.
All the other molecular tracers detected in the HIFI b and
show a pronounced break in the line profile at v
lsr
≈−7.2kms
1
(Codella et al. this volume). This is also observed in the CO 6-
5 spectrum of B1 obtained by us at the CSO, as part of com-
plementary observations to help analyse the CHESS data. The
maps of the whole southern lobe of L1157 in CO 3-2 and 6-5
obtained at the CSO in June 2009, with 24

and 14.5

respec-
tively, will be discussed in detail in a forthcoming paper (Lefloch
et al. 2010, in prep.).
Below, we define the region with v
lsr
< 7.25 km s
1
as the
high-velocity component, hereafter HVC (see also Codella et
al), and the region with v
lsr
> 7.25 km s
1
) as the low-velocity
component (LVC). As we discuss below, these two velocity com-
ponents are characterized by dierent spatial extents and excita-
tion conditions.
3. Discussion
3.1. Origin of the emission
Due to its relatively high energy above the ground state (E
up
=
116 K) the CO 6 -5 transition is a good probe of the warm regions
where H
2
O can evaporate from grain mantles and be released in
the gas phase. SiO 2-1, observed at the PdBI at 2.5

resolution
is a particularly good tracer of shocks strong enough to release
refractory elements in the gas phase, because it is usually unde-
tected in the cold, quiescent molecular gas.
The overall SiO emission is strongly peaked at the position
of B1, which appears as a region of 15

size located at the
apex of the cavity. Interferometric maps of the southern lobe
(Figs. 12) reveal extended emission along the eastern wall of
the cavity (the low-velocity wing of the bow) and downstream of
B1, at velocities close to systemic, both blue and red. By com-
paring these data with IRAM 30m observations (Bachiller et al.
2001), we checked that unlike the HVC, a fraction of the flux
emitted in the LVC is actually missed in the PdBI data, which
is direct evidence for extended emission. This is consistent with
the CO 6-5 data (Fig. 2). The low-velocity gas emission is lo-
cated in the wake of B1, reaching 40

North from the apex.
The area of the LVC amounts to 1/3 of the HIFI beam (see
Fig. 2). Interestingly, the PACS map of the H
2
O 179 μm line
reveals large-scale emission, spatially coinciding with SiO 2-1
emission in the outflow (Nisini et al. 2010).
Fig. 3. (bottom left) Comparison of the o-H
2
O1
10
1
01
line profile with
the SiO 2-1 emission observed at the PdBI, averaged over the HIFI
beam. (top left) Variations of the SiO 2-1 / H
2
O line ratio as a func-
tion of velocity. (bottom right) Comparison of the o-H
2
O1
10
1
01
and
CO 5–4 line profiles. (top right) Variations of the H
2
O /CO 5-4 line
ratio as a function of velocity, smoothed to a resolution of 2 km s
1
.
In any case, there is definitely much less molecular gas emis-
sion associated with the western wall of the cavity (Benedettini
et al. 2007). We therefore expect an asymmetry in the H
2
Ospa-
tial distribution, similar to that observed in many other tracers
such as CS or HCN, as shown by the PACS map of the 179 μm
H
2
O line (Nisini et al. 2010).
We note an excellent agreement between the H
2
Oandthe
low-excitation SiO 2-1 line profiles (Fig. 3 ) in the high-velocity
range, with a constant SiO 2-1 / H
2
O line ratio 0.8 between –20
and 7kms
1
. This has important implications. First, this con-
stant ratio in the range of the HVC suggests that both emissions
most likely arise from the same region and that the emissions are
optically thick. In that case, the low intensities measured in the
high-velocity component (a few tenths of K) point to a small size
extent. This is direct evidence that the H
2
O emission detected
fills only partly the HIFI b eam. Indeed, the bulk of the SiO HVC
originates from a small region of 4

× 12

in B1 (Fig. 2), cor-
responding to a filling factor ff 0.03 in the HIFI main-beam.
Second, if silicon comes from grain erosion, the SiO profile is
predicted to be much narrower than H
2
O because it takes a long
time for Si to oxidize into SiO, so SiO comes only from the cold
postshock, as discussed by Gusdorf et al. (2008b). The similarity
of the SiO and H
2
O line profiles suggests that SiO forms more
extensively in the shock than predicted by oxidation of sputtered
Si atoms. As it can be released in the gas phase even at low ve-
locities in the shock, SiO is present in the gas phase over the full
width of the shock wave.
In summary, we find strong observational evidence that the
emission from the HVC and LVC arises from regions of dier-
ent physical extent. The size of the HVC appears definitely much
lower than the LVC ( ff 0.03 and 0.3, respectively). It is true
however that the present determinations are uncertain. HIFI ob-
servations of the high-excitation lines of CO and H
2
O will make
it possible to better establish appropriate filling factors for these
components.
3.2. Physical conditions
We first estimated the physical conditions from the emission
detected in the CO 3-2, 5-4, and 6-5 transitions both in HVC
and LVC. We modelled each velocity component as a sim-
ple uniform slab, adopting the size (filling factor ) estimated
above. Calculations were done in the large-velocity gradient
approach, using the CO collisional coecients determined by
Flower (2001) for ortho-H
2
collisions in the range 5 K–400 K.
For temperatures beyond 400 K, the collisional coecients were
extrapolated adopting a temperature dependence of
T/400 K.
Page 3 of 5
Page 3
A&A 518, L113 (2010)
Table 1. Observed and tted parameters of the H
2
O and CO 5-4 lines and prediction for the 179 μmH
2
O line flux.
H
2
O 557 GHz CO 5-4 CO 6-5
1
CO 3-2
1
Size N(CO)
2
n(H
2
)
2
T
2
X(H
2
O)
3
F(179 μm)
(Kkms
1
)(Kkms
1
)(Kkms
1
)Kkms
1
(

)(cm
2
)(cm
3
)(K)(cm
2
)(Wcm
2
)
LVC 7.83 45.4 29.2 40.4 25 8.0(16) (1.0-3.0)(5) 100 0.8(-6) 4.2(-20)
HVC 4.28 3.98 1.34 3.11 7 5.0(16) (1.0-3.0)(4) 400 0.8(-4) 7.1(-20)
Notes. The CO fluxes in the HVC and LVC are integrated in the velocity interv als [–30; –7.25] and [–7.25; +11.0], respectively. The H
2
Ofluxes
are derived from a multiple Gaussian fit to the line profile. Intensities are expressed in units of antenna temperature (T
A
)Kkms
1
.
(1)
From CSO
observations smoothed down to the resolution of the HIFI observations;
(2)
determined from LVG analysis of the CO emission;
(3)
from comparison
of LVG-deri ved N(o H
2
O) with N(CO), assuming a water OPR of 3 and an abundance [CO]/[H
2
] = 10
4
.
Fig. 4. Best-fit solution to the LVG modelling of CO line temperatures
in the 3–2, 5–4 and 6–5 transitions for both HVC (left)andLVC(right)
components, respectively. The contour of the observed CO 5–4 inte-
grated line area (T Δv), corrected for main-beam dilution, is drawn in a
dashed line, as well as the 5–4/3-2 and 6–5/3–2 line ratios (solid and
dotted lines, respectively). Thin lines delineate the uncertainties in the
observed values.
Both components appear to have the same gas column den-
sity N(CO) 10
17
cm
2
(Fig. 4). We found the high-velocity
gas component to be unambiguously associated with hot gas
(T > 350 K) of moderate density 3.0 × 10
4
cm
3
, whereas
the low-velocity component arises from gas at a lower temper-
ature (100 K) and higher density (3.0 × 10
5
cm
3
). The tem-
perature estimated for the extended component agree reasonably
well with other determinations from NH
3
and CH
3
CN (Tafalla
& Bachiller 1995; Codella et al. 2009).
With the physical conditions derived from the CO analy-
sis, we modelled the integrated intensity and the line profile of
the o-H
2
O1
10
1
01
transition as well as the reported PACS-
measured 179 μmH
2
O line intensity (10
19
Wcm
2
, Nisini
et al. 2010) to compute the total ortho water abundance in each
velocity component. We used a radiative transfer code in the
large-velocity gradient approach (and slab geometry) detailed in
Melnick et al. (2008), taking into account an ortho to para ra-
tio (OPR) of 1.2 for H
2
, as derived from Spitzer (Neufeld et al.
2009). Here, we assume the absorption component at +2.9km
is due to foreground gas unrelated to L1157-B1. Together, the
two components of the o-H
2
O1
10
1
01
line produce a total H
2
O
2
12
1
01
179 μm line flux of 1.1 × 10
19
Wcm
2
.Forthe
temperature range derived from our CO analysis, we estimated
ortho-H
2
O column densities of (4.0 5.0) × 10
14
cm
2
and
(2.53.0) × 10
16
cm
2
for the LVC and the HVC, respectively.
Assuming an OPR of 3, we derived the H
2
O abundance from
comparison with the gas column densities estimated from CO
(see Table 1). We obtained an abundance ratio [H
2
O]/[CO]
0.8 in the high-velocity gas, which is consistent with previous
results from ODIN (Benedettini et al. 2002) and agrees reason-
ably well with the predictions of steady-state C-shock models for
this set of physical parameters (shock velocity V
s
20 km s
1
,
pre-shock density n(H
2
) = 5×10
3
cm
3
; Gusdorf et al. 2008a,b).
An interesting prediction of our model is that the HVC con-
tribution to the 179 μm flux dominates over the LVC contribu-
tion (see last column in Table 1). The higher temperature of this
component drives the neutral-neutral reactions that eciently
form H
2
O, and the higher shock velocity can more eciently
remove water from grain mantles (see Melnick et al. 2008), re-
sulting in the much greater ortho-H
2
O column density than in the
LVC. Comparison with NH
3
also suggests that the water produc-
tion in the HVC is strongly dominated by high-temperature reac-
tions (see Codella et al.). The higher ortho-H
2
O column density
is what produces the higher 179 μm line flux from this compo-
nent. Consistent results are obtained by Nisini et al. (2010) based
on a 179 μm PACS map and previous ODIN and SWAS observa-
tions of the o-H
2
O1
10
1
01
line, assuming one single physical
component dominates the water line emission in the HIFI beam.
Follow-up observations of the higher-excitation lines of CO and
H
2
O with HIFI will help us constrain more accurately the physi-
cal conditions of each velocity component (density, temperature)
and more generally in the shock.
Acknowledgements. HIFI has been designed and built by a consortium of
institutes and uni versity departments from across Europe, Canada and the
United States under the leadership of SRON Netherlands Institute for Space
Research, Groningen, The Netherlands and with major contributions from
Germany, France and the US. Consortium members are: Canada: CSA,
U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA,
MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio
Astrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK;
Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología
(CSIC-INTA). Sweden: Chalmers University of Technology MC2, RSS &
GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm
Uni versity Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA:
Caltech, JPL, NHSC. HIPE is a joint development by the Herschel Science
Ground Segment Consortium, consisting of ESA, the NASA Herschel Science
Center, and the HIFI, PACS and SPIRE consortia.
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Page 5 is available in the electronic edition of the journal at http://www.aanda.org
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Page 4
B. Lefloch et al.: CHESS survey of L1157-B1 : Shock dynamics. II.
1
Laboratoire d’Astrophysique de Grenoble, UMR 5571-CNRS,
Université Joseph Fourier, Grenoble, France
e-mail: lefloch@obs.ujf-grenoble.fr
2
Observatoire de Paris-Meudon, LERMA UMR CNRS 8112.
Meudon, France
3
INAF, Osservatorio Astrofisico di Arcetri, Firenze, Italy
4
Center for Astrophysics, Cambridge MA, USA
5
Centro de Astrobiología, CSIC-INTA, Madrid, Spain
6
CESR, Universi Toulouse 3 and CNRS, Toulouse, France
7
INAF, Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
8
IPAC, NASA Herschel Science Center, CalTech, USA
9
School of Physics and Astronomy, University of Leeds, Leeds, UK
10
Institut de Radio Astronomie Millimétrique, Grenoble, France
11
Johns Hopkins University, Baltimore MD, USA
12
INAF, Osservatorio Astronomico di Roma, Monte Porzio Catone,
Italy
13
Max-Planck-Institut für Radioastronomie, Bonn, Germany
14
Department of Physics and Astronomy, University College London,
London, UK
15
Univ ersité de Bordeaux, Laboratoire d’Astrophysique de Bordeaux,
France; CNRS/INSU, Floirac, France
16
California Institute of Technology, Pasadena, USA
17
Univ ersity of Michigan, Ann Arbor, USA
18
Astronomical Institute Anton Pannekoek”, University of
Amsterdam, Amsterdam, The Netherlands
19
Department of Astrophysics/IMAPP, Radboud University
Nijmegen, Nijmegen, T he Netherlands
20
IGN Observatorio Astronómico Nacional, Spain
21
Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
22
SRON, Groningen, The Netherlands
23
Max Planck Institut für Astronomie, Heidelberg Germany Ohio
State University, Columbus, OH, USA
24
Physikalisches Institut, Universität zu Köln, Köln, Germany
25
Leiden Observatory, Leiden University, Leiden, The Netherlands
26
IRAM, Granada, Spain
27
Department of Astronomy, Stockholm University, Stockholm,
Sweden
28
Northrop Grumman Aerospace Systems, Redondo Beach, CA
90278, USA
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  • Source
    [Show abstract] [Hide abstract] ABSTRACT: 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 um transition obtained toward the young outflow L1157. The 179 um 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 water abundance and total cooling. Strong H2O peaks are localized on both shocked emission knots and the central source position. The H2O 179 um 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 um 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 water cooling is ~10^-1 Lo, about 40% of the cooling due to H2 and 23% of the total energy released in shocks along the L1157 outflow. Comment: Accepted for publication in Astronomy and Astrophysics (Herschel special issue)
    Full-text · Article · May 2010 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We present the first results of the unbiased survey of the L1157-B1 bow shock, obtained with HIFI in the framework of the key program Chemical HErschel Survey of Star forming regions (CHESS). The L1157 outflow is driven by a low-mass Class 0 protostar and is considered the prototype of the so-called chemically active outflows. The bright blue-shifted bow shock B1 is the ideal laboratory for studying the link between the hot (~1000-2000 K) component traced by H2 IR-emission and the cold (~10-20 K) swept-up material. The main aim is to trace the warm gas chemically enriched by the passage of a shock and to infer the excitation conditions in L1157-B1. A total of 27 lines are identified in the 555-636 GHz region, down to an average 3σ level of 30 mK. The emission is dominated by CO(5-4) and H2O(110-101) transitions, as discussed by Lefloch et al. in this volume. Here we report on the identification of lines from NH3, H2CO, CH3OH, CS, HCN, and HCO+. The comparison between the profiles produced by molecules released from dust mantles (NH3, H2CO, CH3OH) and that of H2O is consistent with a scenario in which water is also formed in the gas-phase in high-temperature regions where sputtering or grain-grain collisions are not efficient. The high excitation range of the observed tracers allows us to infer, for the first time for these species, the existence of a warm (≥200 K) gas component coexisting in the B1 bow structure with the cold and hot gas detected from ground. Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA.Table 1 is only available in electronic form at http://www.aanda.org
    Full-text · Article · Jul 2010 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: High resolution line spectra of star-forming regions are mines of information: they provide unique clues to reconstruct the chemical, dynamical, and physical structure of the observed source. We present the first results from the Herschel key project "Chemical HErschel Surveys of Star forming regions", CHESS. We report and discuss observations towards five CHESS targets, one outflow shock spot and four protostars with luminosities bewteen 20 and 2 × 10 5 L : L1157-B1, IRAS 16293-2422, OMC2-FIR4, AFGL 2591, and NGC 6334I. The observations were obtained with the heterodyne spectrometer HIFI on board Herschel, with a spectral resolution of 1 MHz. They cover the frequency range 555−636 GHz, a range largely unexplored before the launch of the Herschel satellite. A comparison of the five spectra highlights spectacular differences in the five sources, for example in the density of methanol lines, or the presence/absence of lines from S-bearing molecules or deuterated species. We discuss how these differences can be attributed to the different star-forming mass or evolutionary status.
    Full-text · Article · Oct 2010 · Astronomy and Astrophysics
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