Shocked water in the Cep E protostellar outflow

0.59 MB·

by  B. Lefloch

Astronomy & Astrophysics manuscript no. 16247 (cid:13)c ESO 2011
January 13, 2011
Shocked water in the Cep E protostellar outflow
B. Lefloch1,2, J. Cernicharo2, S. Pacheco1, and C. Ceccarelli1
1 UJF-Grenoble1/CNRS-INSU,InstitutdePlan´etologieetdAstrophysiquedeGrenoble(IPAG)UMR5274,Grenoble,
F-38041, France; e-mail: lefl[email protected]
2 LaboratoriodeAstrof´ısicaMolecular,DepartamentodeAstrof´ısica,CentrodeAstrobiologia,INTA,CtradeTorrejo´n
a Ajalvir, km 4, E-28850 Torrejo´n de Ardoz, E-28850 Madrid, Spain
Received : 3 December 2010; Accepted : 10 January 2011
ABSTRACT
1
Context. Previous far-infrared observations at low-angular resolution have reported the presence of water associated
1
with low-velocity outflow shocks and protostellar envelopes.
0
Aims. We want to elucidate the origin of water emission in protostellar systems.
2
Methods. The outflow driven by the intermediate-mass class 0 protostar CepE is among the most luminous outflows

n detected so far. Using the IRAM 30m telescope, we searched for and detected the p-H2O 313−220 line emission at
a 183GHz in the CepE star-forming core. The emission arises from high-velocity gas close to the protostar, which is
J unresolved in the main beam of the telescope. Complementary observations at 2(cid:48)(cid:48) resolution with the Plateau de Bure
interferometerhelpedestablishtheoriginoftheemissiondetectedandthephysicalconditionsintheemittinggas.The
2
waterlineprofileanditsspatialdistributionareverysimilartothoseofSiO.WefindthattheH Oemissionarisesfrom
1 2
warm (∼ 200K), dense ((1−2)×106cm−3) gas, and its abundance is enhanced by one to two orders of magnitude

with respect to the protostellar envelope.
]
A Results. We detect water emission in strong shocks from the high-velocity jet at 1000 AU from the protostar. Despite
the large beam size of the telescope, such emission should be detectable with Herschel.
G
Key words. Stars: formation - ISM: individual (CepE) - ISM: jets and outflows - ISM: molecules
.
h
p
-
o 1. Introduction line 3 − 2 at 183.3 GHz is an exception since it can
13 20
r beenhancedbythemasingeffecteveninregionsofmoder-
t Water is a key molecule not only for oxygen chemistry in
s ateexcitations.UsingtheIRAM30mtelescope,Cernicharo
a the interstellar gas, but also in the dynamical evolution etal.(1990,1994)detectedlarge-scalep-H O 3 −2 line
[ of star-forming regions, because it is one of the main gas 2 13 20
emissioningiantmolecularcloudsandstar-formingregions
coolants (Kaufman & Neufeld, 1996; Bergin et al. 1998).
1 and they showed that this line can be used to investi-
Whereas its abundance is very low in dark clouds, prestel-
v gate the spatial distribution of H O in protostellar envi-
7 larcores,andstarlessregions(fromlessthan10−8 toafew ronments at an angular resolution2comparable to the size
2 10−7; Bergin & Snell, 2002; Moneti et al. 2001), it can be of the protostellar cores (14(cid:48)(cid:48) at the IRAM 30m telescope).
3 greatlyenhancedinstar-formingregions,throughthesput-
Observing the low-mass protostellar outflows HH 7-11 and
2 teringoffrozenwaterfromgrainmantlesandthroughhigh-
L1448, Cernicharo et al. (1996) detected water line emis-
. temperature,sensitivereactionsinthegasphase(Elitzur&
1 sion arising from shocks associated with the Herbig-Haro
0 de Jong, 1978; Ceccarelli et al. 1996; Kaufmann & Neufeld objects in the outflows.
1 1996). It has been proposed that such processes are driven
In this Letter, we report on observations of the
1 by the heating of the nascent stars (hot core/corino re-
p-H O 3 −2 line in the star-forming core CepE with
: gion) or by shocks either in the accretion region in the in- 2 13 20
v the IRAM 30m telescope1. Located at 730pc, it is a rela-
ner protostellar core or in the outflow carrying away the
i tively nearby intermediate-mass Class 0 source of 100L
X angular momentum of the envelope. Previous observations (cid:12)
and a mass of 18M , intensively studied at mm and IR
r ofwaterlinesobtainedwiththeInfraredSpaceObservatory (cid:12)
a havesubstantiallyconfirmedthesetheoreticalexpectations wavelengths (Moro-Martin et al. 2001; Noriega-Crespo et
al. 2004; Lefloch et al. 1996, hereafter L96). The protostar
(see e.g. Giannini et al. 2001; Ceccarelli et al. 2000; van
drives an exceptionally powerful outflow, whose southern
Dishoeck 2004 for a review). However, many details are
lobeis terminatedby the Herbig-HaroobjectHH 377.Far-
stillmissingandtherelativeefficiencyofthesemechanisms
infrared observations at low spectral and angular resolu-
is not known, so that the origin of the water emission in
tion with ISO have revealed large amounts of water, which
protostellar regions remains a puzzle to be solved. One ob-
Gianninietal.(2001)interpretasarisingfromlow-velocity
stacleisindeedtherelativelylowspatialresolutionofthese
shocks,whosefar-infraredluminosityamountstoL∼3L .
observations. (cid:12)
Most of the H O lines cannot be observed from ground
2
because of strong atmospheric absorption. The para-H2O 1 Based on observations carried out with the IRAM 30m
Telescope. IRAM is supported by INSU/CNRS (France), MPG
Send offprint requests to: lefl[email protected] (Germany), and IGN (Spain)
2 B. Lefloch et al.: Shocked water in the CepE protostellar outflow
Fig.1.(left)Mapofthep-H O 3 −2 emissionbetween
2 13 20
−110and+70kms−1intheoutflowofCepEobtainedwith Fig.2. (left) Map of the velocity-integrated SiO emis-
the IRAM 30m telescope. Coordinates are in arcsec off- sion in the CepE jet obtained at 2(cid:48)(cid:48) resolution with the
sets with respect to the nominal position of the protostar. Plateau de Bure interferometer (PdBI; Lefloch et al. in
(right) H O and SiO J =8→7 emission detected towards prep.). The SiO J =2→1 emission integrated between
2
the protostellar source. The H O emission is averaged over +40 and +80kms−1 (−90 and −40kms−1) is drawn in
2
a region of 6(cid:48)(cid:48) × 6(cid:48)(cid:48) around the protostar. Intensities are solid (dashed) contours, superposed on the 3 mm con-
expressed in units of antenna temperature. The different tinuum emission map (grayscale and white contours).
components of the spectrum are marked with arrows. First contour and contour interval of the jet emission are
10−2 and 5 × 10−3 (10−2) Jy/beam for the redshifted
(blueshifted) emission. First contour and contour inter-
Observations of the millimeter lines of SiO observa- val of the 3mm continuum emission is 2×10−3Jy/beam.
tionsrevealthepresenceofstrongshocksalongtheoutflow The main beam of the IRAM 30m telescope at the fre-
(L96). Because it is usually undetected in the cold, qui- quency of the p-H2O 313−220 line is marked by a circle
escent molecular gas, SiO is a particularly good tracer of (dashed). The synthesized beam of the PdBI (2.4(cid:48)(cid:48)×2.0(cid:48)(cid:48))
shocksthatarestrongenoughtoreleaserefractoryelements is indicated as a filled ellipse in the bottom right cor-
fromgraincores.RecentHerschelobservationssuggestthat ner. (right) Comparison of the PdBI SiO J =2→1 and
SiOisalsoagoodtracerofthewater-emittingshockregions theaveragep-H2O 313−220 spectratowardstheprotostar.
(Lefloch et al. 2010; Nisini et al. 2010). We present obser- Intensities are expressed in units of antenna temperature.
vations of the high-excitation SiO J =8→7 line towards
the protostar.
2. Observations
2.1. Water observation tostar23h03m13.0s +61◦42(cid:48)21(cid:48)(cid:48) (J2000).Thetypicalrmsis
We observed the line of para-H O 3 →2 at 183.3 GHz 100mK per velocity interval of 2kms−1.
2 13 20
attheIRAM30mtelescopeinJanuary2009,nearthetran-
sitofCepEbetween61◦and65◦ofelevation.Bothreceivers
at 2mm were used simultaneously. Observations were car- 2.2. SiO observations
ried out in wobbler switching mode with a throw of 180(cid:48)(cid:48).
Filterbankswith1MHzspectralresolutionandanautocor- The SiO J =8→7 transition at 347.33006 GHz was ob-
relatorprovidingaspectralresolutionof1.25MHz(2km/s) servedtowardstheprotostarattheJCMTinAugust2005.
were used as spectrometers. Weather conditions were very The weather conditions were good with an average sys-
good and stable, with an atmospheric optical depth ≈1.10 tem temperature T ∼ 400K. Observations were carried
sys
along the line of sight. Calibration was carefully monitored out with a nutating secondary, adopting a throw of 60(cid:48)(cid:48).
every 4 minutes between two integration scans on CepE. The DBS was used as spectrometer, providing a spectral
Water vapor and system temperatures were found to re- resolution of 625kHz. The resolution was degraded to 3.75
mainstable,intherange1650−1800K.Anyfluctuationof MHz, corresponding to a final velocity resolution of about
the sky emissivity between the on and off positions would 3.2kms−1.AtthefrequencyoftheSiOline,themain-beam
result in a poor spectroscopic baseline and detection of the efficiency of the telescope is about 0.60 and the half-power
mesosphericwaterline,whichisnotthecaseinthepresent beamwidthis(cid:39)14.2(cid:48)(cid:48),verysimilartothetelescopeparam-
observations. Pointing was checked against nearby quasars eters of the p-H O observations. Complementary interfer-
2
every hour and has an accuracy of ≈2(cid:48)(cid:48). At the frequency ometric observations at 2(cid:48)(cid:48) resolution with the Plateau de
of the H O line, the main-beam efficiency of the telescope Bure in the SiO J =2→1 transition provide us with the
2
is 0.63 and the half-power beamwidth is 13.2(cid:48)(cid:48). We did a spatial distribution of the emission and its excitation con-
smalloversampledmapat6(cid:48)(cid:48) samplingofthesouthernlobe ditions.Theseobservationswillbepresentedanddiscussed
oftheoutflow,centeredonthenominalpositionofthepro- in detail in a forthcoming paper (Lefloch et al. in prep.).
B. Lefloch et al.: Shocked water in the CepE protostellar outflow 3
3. Results and discussion 1017cm−2, respectively (see also L96). Adopting a stan-
dard CO abundance in the gas, we then derive the source-
3.1. Water emission in CepE
averaged gas column-density N(H ) (cid:39) 2.5 × 1021cm−2.
2
The map of the p-H2O 313−220 emission in CepE is dis- We adopt these values of Tk and N(CO) for both water-
played in the left panel of Fig. 1. Emission is detected only emitting clumps at blue- and redshifted velocities.
towardstheprotostellarcore,inaregionunresolvedbythe The gas density and column density in the water-
mainbeamoftheIRAM30mtelescope.Inparticular,wedo emitting clumps were derived from modeling the H2O and
not detect any H O emission towards HH77, the southern SiO line brightness temperatures in the large-velocity gra-
2
bowshock at offset position (−5(cid:48)(cid:48), −15(cid:48)(cid:48)), associated with dient (LVG) approach. We adopted the collisional coeffi-
bright emission in the lines of SiO and H (L96). To im- cients of Faure et al. (2007) and Dayou & Balan¸ca (2006),
2
prove the signal-to-noise ratio of the data, we averaged the respectively. The gas density can be constrained reason-
emission of the water line over the inner core, in the range ablywellfromtheratiooftheSiOJ =8→7toJ =3→2
∆α = [1(cid:48)(cid:48);−5(cid:48)(cid:48)] ∆δ = [1(cid:48)(cid:48);−5(cid:48)(cid:48)]. The averaged spectrum is brightness temperatures. This ratio was computed using
displayed in the right panel of Fig. 1, and the final rms the SiO J =3→2 observations in the CepE protostellar
noise is 30mK (T∗) per interval of 2kms−1. core obtained by L96 (see also Fig. 3). We note that the
A
Two bright components (∼ 1K) with broad linewidths source is unresolved by the single-dish JCMT and IRAM
(∼ 10kms−1) are detected at about v ≈ −65 30m observations; therefore, we approximated the ratio of
lsr
and −45kms−1 in the direction of the protostar. Two the line brightness temperatures by the ratio of the main-
other, weaker, H O components are detected at approxi- beam brightness temperatures corrected for the difference
2
mately symmetric (redshifted) velocities v ≈ +34 and of main-beam solid angles R(8−7/3−2). A description
lsr
+50kms−1. We do not detect any emission at the veloc- of the radiative transfer code and the procedure applied to
ity of the protostellar envelope (v =−10.9kms−1; L96), dealwiththemasingeffectispresentedinCernicharoetal.
lsr
henceexcludinganycontributionfromthehotcorinotothe (1994) and Gonz´alez-Alfonso & Cernicharo (1993).
water emission. The presence of both blue- and redshifted The top right panel of Fig. 3 displays the variations of
components in the water line spectrum indicates that the R(8−7/3−2). The ratio varies little in the low-velocity
emission arises from the central protostellar region. This is gas and is about 0.2. It increases as a function of velocity
because there is barely any overlap in the plane of the sky and reaches values up to 0.7 and 1.5 in the redshifted and
betweentheblueandredshiftedwingsoftheCepEoutflow, blueshiftedgas,respectively.ThevariationsofR(8−7/3−2)
except in the central protostellar region as a consequence as a function of Tk and n(H2) were computed for a typical
of the limited angular resolution of the observations (see column density N(SiO) = 2×1014cm−2 and a linewidth
Figs. 1-2; also L96). ∆V = 10kms−1, and they are displayed in Fig. 3. With
The spectrum of the SiO J =8→7 emission in the di- the constraint Tk ≥ 100K, it comes out that n(H2) must
rectionoftheprotostarisdisplayedinFig.1andlooksvery be about 1.0×106cm−3 in order to account for the line
similartothep-H O 3 −2 linespectrum.Resemblance intensity ratio in the redshifted gas . The density in the
2 13 20
between both emissions is even more striking when com- blueshiftedgasisslightlyhigher≈2×106cm−3.Thesecal-
paring the water spectrum with the J =2→1 emission culations show that the ratio depends weakly on Tk above
observed with the PdBI (Fig. 2). This suggests that the 100K. For these values of n(H2) and the kinetic tempera-
emission of both tracers arises from the same region. ture estimated above, we estimate the source-averaged col-
Observations of the J =2→1 transition with the PdBI of umn density necessary to account for the J =8→7 line
thecentralprotostellarregions(Leflochetal.inprep)show intensityN(SiO)=(2.0−3.3)×1014cm−2 forgasat120K
that the SiO emission arises from two compact clumps, in the blue- and redshifted components. Under such phys-
in the blue- and redshifted gas, respectively, located at ical conditions both SiO transitions are optically thin, so
≈1.5(cid:48)(cid:48) from the protostar. They have a typical size of 2.5(cid:48)(cid:48) that R(8-7/3-2) depends weakly on the adopted value of
(2.3×1016cm), and a transverse size of 1(cid:48)(cid:48) (Fig. 2). Their the gas column density. From comparison with N(CO) and
similar physical properties (size, velocity, distance) suggest assuming a standard abundance [CO]/[H2]=10−4, we ob-
they are related to an episodic ejection phenomenon. In taintherelativeSiOabundanceintheshock:[SiO]/[H2](cid:39)
what follows, we assume that both the H O and the SiO 1.0×10−7, similar to the abundances measured in other
2
emissions have the same size. Class 0 protostellar outflows (Lefloch et al. 1998).
Thepara-water(source-averaged)columndensityN(p−
H O) was determined from modeling the line brightness
2
3.2. Physical conditions
temperature T . The latter was derived from the main-
B
The gas kinetic temperature Tk in the redshifted, high- beam brightness temperature TMB = TB ×ff, where the
velocity jet was previously determined by Hatchell et al. filling factor ff was obtained assuming that the H2O and
(1999) from JCMT observations of the CO J =4→3 and SiOJ =2→1emittingregionshavesimilarsizes,inagree-
J =2→1 lines. Unfortunately, their analysis is biased by mentwithSect.3.1.WefindTB =60−80Kand15−29Kin
a highly overestimated value of the main-beam efficiency theblueshiftedandredshiftedgas,respectively.Thebright-
ηMB of the J =4→3 observations, poorly known at that nesstemperatureofthep-H2O 313−220linewascomputed
time, leading to an underestimate of T (≈ 50K). Based for a wide range of physical conditions, adopting a typi-
k
on their observations and adopting a standard value con- cal linewidth ∆v = 10kms−1 : Tk between 50 and 200K,
sistent with the subsequent determinations from point-like n(H2) in the range 103−108cm−3 and N(p−H2O) in the
calibrators2,η =0.35,weobtainthejetkinetictemper- range3×1016 to1019cm−2.Theresultsofthecalculations
MB
ature and column density, T (cid:39)120K and N(CO)(cid:39)2.5× are presented in Fig. 4.
k
It comes out that the kinetic temperature T has to
k
2 http://www.jach.hawaii.edu/JCMT/spectral line/Standards/ be higher than 100K to account for brightness temper-
4 B. Lefloch et al.: Shocked water in the CepE protostellar outflow
in rather good agreement with this ”warm” solution and
inconsistent with the low-density ”hot” solution.
3.3. Shocked water in CepE
Crimier et al. (2010) determined the physical conditions
in the protostellar envelope of CepE envelope and found
n(H ) = 1.0×107cm−3 and T ∼ 50K, respectively, at
2 k
the location of the water-emitting clumps, i.e. 1000 AU
(1.4(cid:48)(cid:48)) from the protostar. No p-H O 3 −2 emission is
2 13 20
detected from the envelope at the 2σ level = 3K (T ).
B
From Fig. 4, we conclude that N(H O)<1016cm−2 in the
2
innerprotostellarenvelope,andthewaterabundanceisen-
hanced by 1 to 2 orders of magnitude in the high-velocity
clumps. The authors estimate a radius of ≈ 220 AU(0.3(cid:48)(cid:48))
for the hot corino region, and the density is predicted very
high∼108cm−3;again,nomasingeffectistobeexpected,
in agreement with our observations.
The water-emitting clumps are located along the high-
velocity jet, which suggests that they are closely related
with the jet (Fig. 2). One possibility is that these clumps
are tracing the jet/envelope interaction. However, the den-
sity of the protostellar envelope, as estimated by Crimier
et al. (2010) at the location of the water emitting-clumps
(n(H ) = 1.0 × 107cm−3), is about one order of magni-
2
tude higher than the density measured in the shocked gas
(106cm−3), in apparent contradiction with this hypothe-
sis. The structure of the envelope could be actually more
complex than the simple power-law distribution derived
by Crimier et al. (2010), as proposed that multiple proto-
starsareembeddedinsidethecore(Ladd&Hodapp,1997).
Fig.4.Predictionsofthep-H O 3 −2 lineintensity,in
2 13 20 Direct observational evidence for several condensations is
unitsofbrightnesstemperature,forgaswithdensityn(H )
2 still missing, however. Alternatively, we speculate that the
intherange103−108cm−3,temperatureintherange50−
water emission could arise from internal shocks in the en-
200K, for column densities of para-water equal to 0.003
trainedmaterialoftheflow.Interferometricobservationsat
(bottom curve), 0.01, 0.03, 0.1, 0.3, 1.0×1019cm−2 (top
subarcsec scale in the inner core (r <1000 AU) are needed
curve). The range of T −n(H ) determined in the water-
B 2 to determine the gas density and velocity field in order to
emitting clumps is delineated by rectangles.
determinethelocationandtheoriginofthewater-emitting
shocks, as well as their relation with the jet.
Simple calculations in the LVG approximation predict
thattheH Oemissionfromthehigh-velocityclumpsshould
2
atures of 60 − 80K. Our calculations indicate a p-H2O be easily detected in several ortho- and para-H2O transi-
column density in the range (0.3 − 2.0) × 1017cm−2 for tions with the high-resolution spectrometer HIFI onboard
Tk (cid:39) 200K. Assuming an ortho-to-para ratio of 3, we de- Herschel.Suchobservationswillallowustobetterconstrain
rive N(H2O) = (1.2−8.0)×1017cm−2, and using the H2 theshockphysicalconditions(seee.g.Flower&Pineaudes
column density derived in the high-velocity clumps, we ob- Forets, 2010).
tain the water abundance X(H O)=(0.5−3.2)×10−4. In
2
thisrangeofphysicalconditions,thelineopticaldepthτ is Acknowledgements. We thank an anonymous referee and the Editor
small≈−1andshowsevidencefortheweakmasingampli- for comments that helped to improve the manuscript a lot. This
ficationofp-H O 3 −2 line.Suprathermalexcitationof project was supported by the research grant SAB2009-0011 of the
2 13 20
SpanishMinistryofEducation.
the line would require much higher H O column densities.
2
Thisispossibleiftheemissionarisesfromasmallregionof
subarsec size (see e.g. Cernicharo et al. 1994) and is very References
sharp. The line T would then look weaker as a result of
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B. Lefloch et al.: Shocked water in the CepE protostellar outflow 5
Fig.3.(left)SiOemissionobservedintheJ =3→2(bot-
tom) and J =8→7 transitions towards the protostellar
source. Fluxes are expressed in units of main-beam tem-
perature. (right) Observed variations of the ratio of the
CO J =8→7 to J =3→2 brigthness temperatures R(8-
7/3-2) as a function of velocity (top) and predicted vari-
ations as a function of the gas density n(H ) and the ki-
2
netictemperatureT (bottom)foratypicalcolumndensity
k
N(SiO)=2×1014cm−2. The contour level R= 0.7, 1.5 are
drawn with a solid black line; the contours R= 0.5, 1.8 are
drawnwithadashedredlineandaccountforanuncertainty
of 30%.
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