The shocked gas of the BHR71 outflow observed by Herschel: indirect evidence for an atomic jet

3.1 MB·

by  M. Benedettini

Astronomy&Astrophysicsmanuscriptno.benedettini (cid:13)cESO2017
January17,2017
The shocked gas of the BHR71 outflow observed by Herschel(cid:63):
indirect evidence for an atomic jet
M.Benedettini1,A.Gusdorf2,3,B.Nisini4,B.Lefloch5,S.Anderl5,6,G.Busquet7,C.Ceccarelli5,C.Codella8,S.
Leurini9,L.Podio8
1 INAF,IstitutodiAstrofisicaePlanetologiaSpaziali,viaFossodelCavaliere100,00133Roma,Italy
e-mail:[email protected]
2 LERMA,ObservatoiredeParis,PSLResearchUniversity,CNRS,UMR8112,75014,Paris,France
3 SorbonneUniversités,UPMCUniv.Paris6,UMR8112,LERMA,75005,Paris,France
4 INAF,OsservatorioAstronomicodiRoma,viadiFrascati33,00040,MontePorzioCatone,Italy
7
5 Univ.GrenobleAlpes,IPAG,F-38000Grenoble,France
1
6 CNRS,IPAG,F-38000Grenoble,France
0
7 InstitutdeCiènciesdel’Espai(IEEC-CSIC),CampusUAB,CarrerdecanMagrans,s/n,E-08193,Barcellona,Catalunya,Spain
2
8 INAF,OsservatorioAstonomicodiArcetri,LargoE.Fermi5,50125,Firenze,Italy

n 9 Max-Planck-InstitutfürRadioastronomie,AufdemHügel69,53121,Bonn,Germany
a
J Received;accepted

6
1 ABSTRACT

] Context.IntheBHR71region,twolow-massprotostarsIRS1andIRS2drivetwodistinguishableoutflows.Theyconstituteanideal
A laboratorytoinvestigateboththeeffectsofshockchemistryandthemechanismsthatledtotheirformation.
G Aims. We aim to define the global morphology of the warm gas component of the BHR 71 outflow and at modelling its shocked
component.
.
h Methods.WepresentthefirstfarinfraredHerschelimagesoftheBHR71outflowssystemintheCO(14–13),H O(2 –1 ),H O
2 21 10 2
p (2 –1 ) and[Oi] 145µmtransitions,revealingthepresenceofseveralknotsofwarm,shockedgasassociatedwiththefastoutflow-
12 01
- inggas.Intwooftheseknotsweperformedadetailedstudyofthephysicalconditionsbycomparingalargesetoftransitionsfrom
o
severalmoleculestoagridofshockmodels.
r
t Results.TheHerschellinesratiosintheoutflowknotsarequitesimilar,showingthattheexcitationconditionsofthefastmovinggas
s donotchangesignificantlywithinthefirst∼0.068pcoftheoutflow,apartattheextremityofthesouthernblue-shiftedlobethatis
a
expandingoutsidetheparentalmolecularcloud.Rotationaldiagram,spectrallineprofileandLVGanalysisoftheCOlinesinknot
[
1 Acomshpoawctth(1e8p(cid:48)(cid:48)r)e,swenacrmeo(fTtw=o1g7a0s0c–om2p2o0n0eKnt)s:wointheselxigtehntldyedlo,wcoelrdd(eTns∼ity80(nK(H)a2n)d=d2e×ns1e04(n–(H62×)1=043c×m1−035).–I4n×t1h0e6twcmo−b3r)igahntdesatnkonthoetsr
v (whereweperformedshockmodelling)wefoundthatH2andCOarewellfittedwithnon-stationary(young)shocks.Thesemodels,
3 however,significantlyunderestimatetheobservedfluxesof[Oi] andOHlines,butarenottoofaroffthoseofH2O,callingforan
4 additional,possiblydissociative,J-typeshockcomponent.
2 Conclusions. Ourmodellingindirectlysuggeststhatanadditionalshockcomponentexists,possiblyaremnantoftheprimaryjet.
4 Direct,observationalevidenceforsuchajetmustbesearchedfor.
0
Keywords. Stars:formation–ISM:jetsandoutflows–ISM:individualobjects:BHR71–Infrared:ISM
.
1
0
7
1. Introduction H CO, and H O (e.g. Bachiller et al. 2001; Garay et al. 1998,
1 2 2
2002; Benedettini et al. 2007; Arce et al. 2007; Codella et al.
:Intheearlyprotostellarstages,fastjetspoweredbythenascent
v 2010). Such enrichment reveals the activation of endothermic
istar,possiblysurroundedbyawideranglewind,stronglyinter- reactionsbyshockheatingandthemechanicalerosionandpro-
Xactwiththeparentalmediumthroughmolecularshocks,produc- cessingofgrains.Therefore,outflowshocksalsoofferaunique
ringaslowermovingmolecularoutflowcavity.Severalepisodes
laboratoryfortestingastrochemicalmodels.However,uptonow
a
ofbowshocksareusuallypresentwithintheoutflowlobes(e.g.
enough observations to allow detailed chemical studies that in-
Bachiller&PérezGutiérrez1997;Bourkeetal.1997;Leeetal.
cludemultiplemolecularspecieshavebeenmadeinonlyahand-
2006). In several outflows, fast and well collimated molecular
fulofoutflows.
bullets,tracinginternalshockswithinthejetarealsoseen(e.g.
One interesting target for studying shocks associated with
Gueth&Guilloteau1999;Nisinietal.2007;Tafallaetal.2010).
Outflow shocks significantly affect the gas chemical composi- outflows is the BHR71 system. Located at a distance of about
175pc(Bourkeetal.1995),theBokglobuleBHR71isawell-
tion,withabundanceenhancementsuptoseveralordersofmag-
knownexampleofanisolatedstarformationwithaclearbipo-
nitudeforvariousmolecularspeciessuchasSiO,CH OH,CS,
3 lar outflow close to the plane of the sky observable from the
(cid:63) HerschelisanESAspaceobservatorywithscienceinstrumentspro- southernhemisphere(seeFig.1).Theregioncomprisestwodis-
videdbyEuropean-ledPrincipalInvestigatorconsortiaandwithimpor- tinctoutflows(Bourkeetal.1997,2001;Pariseetal.2006)pow-
tantparticipationfromNASA. eredbytwoprotostellarsources,IRS1andIRS2whichhavelu-
Articlenumber,page1of16
A&Aproofs:manuscriptno.benedettini
Fig.1.DoubleoutflowofBHR71breakingoutfromitsparentalBokglobule.ThegreyscaleimageisfromtheDSS2-redacquiredwiththeUK
SchmidtTelescope((cid:13)c1993-1995bytheAnglo-AustralianTelescopeBoard).BlueandredcontoursarerespectivelyCO(3–2)andH 0–0S(5)
2
emission(G15).Blackstarsindicatethepositionofthetwoprotostars-IRS1andIRS2-thatdrivetheoutflows.Thewhiteandgreentriangles
indicatethepositionofthepeaksoftheHerschellineemissionandarelabelledfromAtoEinorderofdecreasingdeclination.Yellowrectangle
showstheapproximateextentofthelarge-scaleHerschel-PACSmaps.Thetwocyancrossesindicatethepositionsobservedwithadditionalpointed
Herschelobservations.
minosities of 13.5 and 0.5 L , respectively (Chen et al. 2008) thewarmgascomponentandtoderivegeneralindicationonits
(cid:12)
and are separated by ∼3400 AU (assuming a distance of 200 physicalcondition.Moreover,inthetwobrightestinfraredknots
pc, Bourke et al. 2001). A first view of the global energetics weperformedadetailedstudyoftheshockconditionsbyfitting
and physical conditions of the outflows have been derived by a large set of molecular transitions from different species with
low J COandH observations(Neufeldetal.2009;Giannini shockmodels.Thepaperisorganisedasfollows.InSect.2we
up 2
et al. 2011). Bright HH objects, HH320 and HH321 (Corporon present the new data acquired with Herschel and the reduction
&Reipurth1997),andseveralotherknotsofshockedgashave method.TheresultsandtheanalysisperformedontheHerschel
beenfoundintheoutflows.Theseknotsarousedagreatdealof data are described in Sects. 3 and 4. In Sect. 5 we present the
interest,andtheirshockconditionshavebeenstudiedinseveral deeper analysis of the large set of CO transitions available for
papers(Gianninietal.2004;Gusdorfetal.2011,2015,hereafter the brightest knot of the northern lobe. Shock models for this
G15).ThetwoHHobjectshavebeenimagedalsointheSiitran- knotananotherknotofthesouthernlobearepresentedinSect.
sitionat6711Å,indicatingthatatleastpartoftheoutflowsare 6.ThefinalconclusionsarereportedinSect.7.
dissociative(Corporon&Reipurth1997).Infact,suggestionsof
thepresenceofanatomicjethavebeenrecentlyfoundby[Oi]
observations(Nisinietal.2015).
2. Observationsanddatareduction
In this paper we present the first far infrared images of the
BHR71 outflows system observed with Herschel-PACS, map- We present observations carried out towards the BHR71 out-
ping the region of 200(cid:48)(cid:48) centred on the powering sources, plus flows system with the PACS (Poglitsch et al. 2010) and HIFI
somepointedspectraofCOtransitionsobservedwithHerschel- (deGraauwetal.2010)instrumentson-boardHerschel(Pilbratt
HIFI.Theimagestracethenot-yet-studiedwarmgas,atinterme- et al. 2010). For the scientific analysis we also make use of al-
diateexcitationconditionswithrespectthecoldergastracedby readypublishedHerschelandAPEXobservations.Inparticular,
lowJ COlines(Pariseetal.2006;G15)andthehotgastraced themapinthe[Oi]3P –3P line(Nisinietal.2015)andinthe
up 1 2
byH rotationallines(Neufeldetal.2009;Gianninietal.2011). CO(6–5) line(G15).AsummaryofthenewHerschelobserva-
2
Themainaimofthisworkistodefinetheglobalmorphologyof tionsispresentedinTable1.
Articlenumber,page2of16
Benedettinietal.:TheBHR71outflowobservedbyHerschel
Table1.ObservationstowardstheBHR71outflow.
220(cid:48)(cid:48)×100(cid:48)(cid:48),spatiallyNyquist-sampledmaps
Species transition E /k Frequency Wavelength Instrument FWHM Spectralresolution
up B
(K) (GHz) (µm) ((cid:48)(cid:48)) (kms−1)
H O (2 –1 ) 159.8 2773.977 108.07 PACS 9.4 318
2 21 10
[Oi] 3P –3P 326.6 2060.069 145.53 PACS 10.2 260
0 1
H O (2 –1 ) 80.1 1669.905 179.53 PACS 13.2 210
2 12 01
CO (14–13) 580.5 1611.794 186.00 PACS 13.2 195
47(cid:48)(cid:48)×47(cid:48)(cid:48) at9(cid:48).(cid:48)4spatialsamplingmaps
CO (22–21) 1397.3 2528.172 118.58 PACS 9.4 300
OH 2Π −2Π J = 5−− 3+ 120.7 2514.298 119.23 PACS 9.4 300
3 3 2 2
OH 2Π2 −2Π2J = 5+− 3− 120.7 2509.934 119.44 PACS 9.4 300
3 3 2 2
2 2
CO (20–19) 1109.2 2299.570 130.37 PACS 9.4 275
CO (18–17) 944.9 2070.616 144.78 PACS 10.2 260
[Oi] 3P –3P 326.6 2060.069 145.53 PACS 10.2 260
0 1
CO (14–13) 580.5 1611.794 186.00 PACS 13.2 195
Singlespectra
CO (14–13) 580.5 1611.794 186.00 HIFI 13.2 0.093
CO (9–8) 248.9 1036.912 289.12 HIFI 21.7 0.14
CO (5–4) 83.0 576.268 520.23 HIFI 39.0 0.26
13CO (5–4) 79.3 550.926 540.73 HIFI 40.8 0.27
2.1. Herschellinemaps Table2.Knotscoordinates.
The BHR71 outflows system was mapped with PACS in CO Knotslabel RA(J2000) Dec(J2000)
(14–13),H2O(221–110),H2O(212–101)and[Oi]3P0–3P1byus- A 12h01m36s.0 -65◦07(cid:48)30(cid:48).(cid:48)0
ing the line spectroscopy observing mode. The final extended B 12h01m32s.0 -65◦08(cid:48)09(cid:48).(cid:48)0
mapwasbuiltcombiningthree47(cid:48)(cid:48)×47(cid:48)(cid:48) partiallyoverlapping, C 12h01m36s.5 -65◦08(cid:48)09(cid:48).(cid:48)0
Nyquist-sampledsub-maps(seeFig.1).Datawerereducedwith IRS1 12h01m36s.6 -65◦08(cid:48)48(cid:48).(cid:48)5
the HIPE1 v9.0 package, where they were flat-fielded and flux- IRS2 12h01m34s.0 -65◦08(cid:48)44(cid:48).(cid:48)5
calibratedbycomparisonwithobservationsofNeptune.Theflux
D 12h01m36s.0 -65◦09(cid:48)29(cid:48).(cid:48)0
calibration uncertainty amounts to ∼ 20%, while the Herschel
E 12h01m37s.2 -65◦10(cid:48)27(cid:48).(cid:48)0
pointingaccuracyis∼2(cid:48)(cid:48).Post-pipelinereductionstepswereper-
formedtolocallyfitandremovethecontinuumemissionandto
constructintegratedlinemaps(seeFig2).
are averaged in order to increase the signal-to-noise ratio. The
observed lines are well resolved already at the WBS resolution
2.2. Herschelsinglepointing thereforeinourscientificanalysiswewillusethosespectra.Cal-
ibrationoftherawdataontotheT∗ scalewasperformedbythe
Two positions (shown in Fig. 1) were observed in additional in-orbitsystem,whilethespectrawAereconvertedtoaT scale
mb
spectral lines: CO (5–4), 13CO (5–4), CO (9–8), CO (14–13) byadoptingaforwardefficiencyof0.96andthemainbeameffi-
with HIFI and CO (14–13), (18–17), (20–19), (22–21), OH at cienciesgiveninRoelfsemaetal.(2012).
119 µm and [Oi] at 145 µm with PACS. The two positions In the northern pointing, the CO (14–13) line has been de-
correspond to local H2 peaks, namely the SiO knot (Gusdorf tectedbybothPACSandHIFI.Thereisadifferencebetweenthe
et al. 2011) in the northern lobe (R.A.(J2000) 12h01m36.00s, twomeasurementsof∼25%thatcanbeconsideredastheesti-
Dec.(J2000) -65◦07(cid:48)34(cid:48)(cid:48)) and knot 8 (Giannini et al. 2004) in mate of the intercalibration error between the two instruments.
the southern lobe (R.A.(J2000) 12h01m38.90s, Dec.(J2000) - Therefore, in the following analysis we assume a conservative
65◦10(cid:48)07(cid:48)(cid:48)). These observations were performed with a single erroroftheabsolutefluxesof30%thattakesintoaccountboth
pointingthereforeforeachlinetheHIFIdataconsistofasingle theinstrumentalcalibrationerrorandtheerrorofcrosscalibra-
spectrumwhilethePACSdataiscomposedby25spectracom- tionbetweenthedifferentinstruments.
bined in a 47(cid:48)(cid:48)×47(cid:48)(cid:48) map, not Nyquist-sampled with respect to
thepointspreadfunction(PSF).PACSdatawerereducedinthe
same way as the extended maps exposed in the previous sub- 3. Results
section by using the chopping-nodding pipeline v10.1.0 within
PACSlinemapsaredisplayedinFig.2.Itisworthnotingthatthe
HIPE.HIFIdata,boththeWideBand(resolution1.1MHz)and
PACSlinesarespectroscopicallyun-resolved,thereforeitisnot
HighResolution(resolution0.25MHz)Spectrographbackends
possibletoseparatethecontributionoftheredandblueoutflow
(WBS and HRS, respectively), were reduced with the standard
lobes.Tothisaim,inthesamefigurewealsoshowtheemission
product generation (SPG) v11.1.0 within HIPE. After consis-
oftheCO(6–5) lineobservedwithAPEX(G15),integratedover
tencycheck-ups,thehorizontalandverticalpolarisationspectra
theredandbluewingsthatshowstherelativepositionofthetwo
1 HIPEisajointdevelopmentbytheHerschelScienceGroundSeg- outflows.Northofthetwoexcitingsourcestherearetheredlobe
ment Consortium, consisting of ESA, The NASA Herschel Science oftheIRS1outflowandthebluelobeoftheIRS2outflow,and
Center,andtheHIFI,PACSandSPIREconsortia. the two lobes are spatially well separated. The situation of the
Articlenumber,page3of16
A&Aproofs:manuscriptno.benedettini
Fig.2.IntegratedlinemapsfromHerschel-PACSoftheinnerregionoftheBHR71outflowsystem.ThebottomrightpanelshowstheH 0-0S(5)
2
(Neufeldetal.2009)andthered(from0to20kms−1)andblue(from−20to−9kms−1)wingsoftheCO(6–5) emission(G15).Firstlevelisat
3σ,namely:2.1×10−5ergs−1cm−2sr−1 for[Oi] 63µm,3×10−6ergs−1cm−2sr−1 for[Oi] 145µm,5×10−6ergs−1cm−2sr−1 forCO(14–13),
7.5×10−6ergs−1cm−2sr−1 forH O179µm,4.5×10−6ergs−1cm−2sr−1 forH O108µm and6Kkms−1 forCO(6–5).Stepscorrespondsto8%
2 2
ofthemaximumofeachlineuptothe70%ofthemaximum.CyancirclesindicatethepositionofthepeaksoftheHerschellineemission,labelled
fromAtoEinorderofdecreasingdeclinationplustheprotostellarsourcesIRS1andIRS2.Thetwoyellowcrossesindicatethetwopositions
observedwiththeadditionalpointedHerschelobservations.TheblackcirclesinthebottomrightpartofeachpanelshowtheFWHMofthemap.
southern lobes is more complicated, with the fainter red lob of indecreasingvalueofdeclination.Afirstseriesofknots,namely
the IRS2 outflow overlapping the more extended and brighter A,C,DandEarewellalignedalonganorth-southstraightline
bluelobeoftheIRS1outflow. crossing IRS1 and have a separation of ∼ 40(cid:48)(cid:48) in the northern
lobe and ∼ 60(cid:48)(cid:48) in the southern lobe. They likely might indi-
The emission of the PACS lines appears to be clumpy with cate the direction of the driving jet emitted by IRS1. Knot B is
several individual peaks surrounded by a low level extended the only knot clearly associated with the blue lobe of the IRS2
emission.Thestrongestpeakisobservedtowardstheprotostel- outflow. No clear, strong peak along the straight line connect-
larsourceIRS1(Lbol=13.5L(cid:12)),theexcitingsourceofthemore ingBtoIRS2ispresentinthesouthernlobesuggestingthatin
extendedoutflow.ThesecondprotostellarsourceIRS2(Lbol=0.5 this lobe the emission in the PACS lines should be dominated
L(cid:12))isweakerbutstillvisibleasalocalmaximuminthemaps. by the blue lobe of the IRS1 outflow, as also suggested by the
WenamedthefiveknotslocatedintheoutflowlobesfromAtoE
Articlenumber,page4of16
Benedettinietal.:TheBHR71outflowobservedbyHerschel
CO (6–5) map. The emission peaks are spatially coincident in
all the observed PACS lines and are roughly located at the po-
sitionsofshockedspotspreviouslyidentifiedthroughH obser-
2
vations (Neufeld et al. 2009). We measured the position of the
knots from the peak of CO (14–13) emission, apart from knot
E whose coordinates are measured from the [Oi] 63 µm map.
InTable2thecoordinatesofthefiveoutflowknotsandthetwo
protostarsarelisted.
The additional single pointing observations with Herschel
PACS and HIFI were acquired towards knot A in the northern
lobe and about 15(cid:48)(cid:48) north-east of knot E in the southern lobe,
indicatedbytwocrossesinFigs.1and2.Thespectraofthefour
linesobservedwithHIFIareshowninFig.3.TowardsknotA,all
thefourlinesaredetectedwithgoodsignal-to-noiseratio,with
wingsextendingupto∼60kms−1,avelocityhigherthanprevi-
ouslyreported(∼45kms−1,G15).TheCO(5–4) linepresentsa
self-absorptionfeatureatthevelocityoftheenvironmentalcloud
(−4.5 km s−1, Bourke et al. 1995), observed in all CO lines up
to the (6–5) (G15). From the 12CO (5–4)/13CO (5–4) ratio we
measuredtheopticaldepthofthe12CO(5–4)linethatresultsto
be optically thin only in the red wing at (cid:51) (cid:62) 2 km s−1. The in-
Fig.3.PlotofthelinesobservedwithHIFItowardsknotAinthenorth-
tensity peak of the CO lines moves towards higher velocity for ernlobe(upperpanel)andabout15(cid:48)(cid:48) north-eastofknotEinthesouth-
lineswithhigher Jup,indicatingthatthelowandhigh Jup lines ernlobe(lowerpanel).Spectrawererebinnedatacommonspectralres-
are emitted from different layers of the shocked region. In the olutionof0.5kms−1.Intheinsetsazoominthelineswingsisshown.
southernlobepointingthelinesprofilesaredifferent,despitethe
bluewingsextendinguptoavelocity((cid:51) ∼ −60kms−1)similar
tothatobservedintheredlobe.Inparticular,noself-absorption 4. Analysisoftheextendedoutflow
and no peak shifting is observed in the southern tip of the blue
4.1. Thedistributionofthewarmgas
lobe. Moreover, the CO (14–13) was not detected and the line
ratioCO(9–8)/CO(5–4) islower,indicatingthattheexcitation The far-infrared PACS lines with excitation temperatures be-
condition at this position is lower than in knot A. This is also tween 80 and 580 K allow for the first time to have a com-
confirmedbythefactthatthenorthernpointingcorrespondstoa pleteviewofthedistributionofthewarmgascomponentinthe
peakofthehighexcitationtransitionsobservedwithPACSwhile BHR71outflowsystem,tracingtheintermediateexcitationcon-
thesouthernpointingisnotcentredonapeak.The12CO(5–4)/ ditionbetweenthecoldgastracedbylowJ COlines(Pariseet
up
13CO (5–4) ratio shows that also in the southern blue lobe the al.2006;G15)andthehotgastracedbyH (Neufeldetal.2009;
2
12CO (5–4) line is optically thin only in the wing, at v < −10 Gianninietal.2011).TheemissioninthePACSlinesshowsthe
kms−1.TheCO(9–8) lineinthispointofthesouthernlobeisat presenceofsevenbrightknotssurroundedbyalowlevelemis-
velocitybluerthansystemic((cid:51) < −4.5kms−1)whileintheCO sion. Two knots correspond to the two protostars that generate
(5–4) lineasmallredwingispresent.Thereareseveralpossible theoutflowssystemandtheotherfiveknotsareclosetoclumps
explanations for this. The first possibility is the larger beam of ofhightemperaturegas(T (cid:62)1000K)alreadyobservedinH ro-
2
the CO (5–4) line with respect to the CO (9–8), secondly, the tationaltransitions(seeFig.2).Theyarealsoassociatedwiththe
excitation condition of the gas in the red lobe of the IRS2 out- gasmovingatthehighestvelocityasshownbythecomparison
flowcouldbeinsufficiencytopopulatetheCO J =9level,and ofthe[Oi] 63µm mapwiththemapofthereddest(from10.5
up
athirdpossibliltyisthattheemissionatredvelocityoftheCO km s−1 to 40 km s−1) and bluest (from −30 km s−1 to −19.5
(9–8) lineisunderthesensitivitylevelofHIFIspectrograph.In km s−1) velocities of the CO (6–5) line wings (Fig. 5). They
allcaseswecanconcludethattheredlobeoftheIRS2outflow most likely represent shock spots where the fast jet that drives
does not contribute to the measured flux of the transitions with outflowsimpactagainstthelowerexpandinggas.
highexcitationlevels. SincethePACSlineshavecoarsespectralresolution,wecan-
not spectroscopically separate the contribution of the two out-
flowstothetotalemission.Thisisnotaproblemforthenorth-
Towardsthetwooutflowpositions(indicatedinFigs.1and ernlobessincethetwooutflowsarespatiallywellseparatedbut
2) we also acquired PACS maps of high J CO transitions, itcouldbeaproblemforthesouthernlobesthatpartiallyover-
up
namelyCO(14–13),CO(18–17),CO(20–19),CO(22–21),and lap.However,thespectralprofileoftheCOlinesobservedwith
of the OH 119 µm and [Oi] 145 µm lines. Towards knot A, HIFI at the tip of the southern lobe close to knot E, shows that
theCOlinesarebright,whiletheOHand[Oi] arelessintense the percentage of the emission at velocity redder than the sys-
(Fig.4).Allthelineshaveasimilar,partiallyresolved,roundish temic velocity (−4.5 km s−1) with respect to the total emission
form with the peak at the centre of the map and a deconvolved decreases for increasing J , being only the 3% for the CO (9–
up
circular size of 18(cid:48)(cid:48), indicating that they are emitted from the 8) transition.Thereforewecanexpectanevenlowerpercentage
same extended gas component. Towards the southern pointing fortheCO(14–13) lineandfortheotherPACSlinesfrom[Oi]
only a ∼ 3σ level CO (14–13) emission is detected while the and water, that trace the same warm gas component, as shown
otherlinesarenotdetected,confirmingonceagainthelowerex- from Fig. 2 for BHR71 and as also observed in other outflows
citationconditionofthisposition. (Santangelo et al. 2013; Busquet et al. 2014).In conclusion, at
Articlenumber,page5of16
A&Aproofs:manuscriptno.benedettini
Fig.4.MapsofthefourCOlines,OH119µm and[Oi] 145µm observedwithPACStowardsknotAintheredlobeoftheIRS1-drivenoutflow.
Firstlevelandlevelstepsare9×10−18 Wm−2,correspondingto3σ.Thecrossesmarkthecentralpositionofthe25spatialpixelsofthePACS
FOV.ThearrowinthetopleftpanelindicatesthedirectionoftheIRS1protostar.
the tip of the southern lobe the contribution of the red lobe of and the higher J CO (6–5) and (14–13) showing a roundish
up
the IRS2 outflow to the PACS lines is negligible. The situation structurewithspatiallycoincidentpeaks.
in the southern positions closer to the two protostars could be
different since in the uppest J available CO line, the CO (6– AdisplacementintheemissionpeakoftheCOlineswithin-
up creasing J wasalsoobservedtowardstheflowcontainingthe
5) observed with APEX, the emission in the red wing is still up
Herbig-HaroobjectHH54(Bjerkelietal.2014).InHH54,how-
significant.However,towardsthebrightknotDthesimilarmor-
ever, the gradient goes in the direction of the exciting source,
phology of the blue wing of the CO (6–5) and the PACS lines
that is, in the opposite direction with respect what we observe
(Fig.2)showsthateveninthisknotthecontributionofthered
in knot A of BHR71. Bjerkeli et al. (2014) interpreted the ob-
lobeoftheIRS2outflowtothePACSlinesshouldbenegligible.
serveddisplacementoftheCOlinespeakastheconsequenceof
thepresenceofanunder-dense,hotclumpupstreamoftheHH54
InSect.3wealreadynoticedthattowardsknotsAthepeak shockwave,tracedbyawelldefineddistinctgascomponentin
velocityoftheCOlinesmovestohighervelocityforlineswith thespectraofthelow J COlines.Althoughnodistinctveloc-
up
higher J .InFig.6wecomparethespatialmorphologyofCO itycomponentsarepresentintheCOspectraoftheBHR71out-
up
(3–2), CO (6–5) and CO (14–13) emission in knot A and we flow,theobserveddisplacementofthelocationoftheemission
find that the peak of the CO emission moves towards north for maximumofCOwithincreasing J inoursourcecouldbeat-
up
CO lines with higher J , with a separation of about 20(cid:48)(cid:48) be- tributedtodifferentlayersofshockedgasimpactinganambient
up
tween theCO (3–2) and theCO (14–13) peaks. Moreover, the medium with a significant stratification of particle density. The
peakofthehighestexcitationCOlineistheclosertothepeakof differenceinthemorphologyoftheCOlinesemissionbetween
H 0-0S(5)(E =4586K)emissiontracinghotgas.Thepeak knots A and D is probably due to the different environmental
2 up
shifting is along the straight-line connecting knots A, C and D conditionsinwhichthetwoshockshavebeenoriginatedand/or
andcrossingIRS1,suggestingthattheshiftislinkedtothebow togeometricaleffectsthatcouldchangetheangleofviewunder
shockgeneratedbytheimpactofthehigh-velocityjetagainstthe whichthetwoshocksareobserved.Infact,asclearlyvisiblein
slowermovingambientshell.ThesituationinthebrightknotD Fig. 1 that compares the outflow traced by CO (3–2) with an
ofthesouthernlobeisdifferent(seeFig.6),withthelowJ CO optical image of the region at 0.54 µm, the outflow is breaking
up
(3–2) line having the classical morphology of the cavity walls outfromitsparentalBokglobulewithmorethanhalfoftheblue
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Benedettinietal.:TheBHR71outflowobservedbyHerschel
Fig. 5. Map of the high velocity wings of CO (6–5) overlaid to [Oi]
63µm. The red wing (red line) is integrated from 10.5 km s−1 to 40
kms−1 andthebluewing(blueline)from-30kms−1 to-19.5kms−1.
COlevelsareat50%,70%and90%ofthemaximum.Thepositionof
thetwoprotostarsareindicatedwithatriangle.
southernlobeprojectedoutsidetheobscuredregionwhileinthe
red northern lobe only the very final part is projected outside Fig.6.MapoftheH 0-0S(5)line(grey-scalemap)comparedwithCO
the globule. Although it is more difficult to understand what is (3–2)(redline),CO2(6–5)(blackline)CO(14–13) (greenline).Upper
really happening on the rear side of the globule, the measured panel is centred on knot A, lower panel on knot D. Contours start at
muchlargermassandkineticenergyoftheredlobewithrespect 90%ofthepeakvalueinordertohighlightthepositionofthepeakof
thebluelobe(Bourkeetal.1997)confirmswhatFig.1suggests. the emission. The circles show the HPBW of the different lines. The
OurHerschel-PACSmapcoversthepartofthenorthernlobestill arrowsindicatethedirectionofIRS1.
fully embedded in the obscured and denser medium inside the
moleculardarkcloudwhilethefinalpartofthemappedportion
4.2. Lineratios
ofthesouthernlobeispropagatingoutsidethecloud.Inthisre-
spectknotsAandDareinadifferentenvironment:thenorthern
Withonlytwolinesperspeciesitisimpossibletoconstrainthe
knotAislocatedcompletelyinsidethecloudwhilethesouthern
physicalparametersoftheemittinggas.However,wecanderive
knot D is located at the border of cloud, likely in a less dense
some general indications on if and how the physical and exci-
medium. Moreover, while knot A is associated to only the red
tationconditionofthegaschangealongtheoutflow,comparing
lobeoftheIRS1outflowinthepositionofknotDtwodifferent
theratiosoflinesofthesamespeciesintheseveninfraredknots.
lobescoexist. In order to avoid different beam filling effects we smoothed all
Remarkably, towards knot A the peak of the high J CO the PACS maps at the common spatial resolution of 13(cid:48).(cid:48)2 and
up
correspondstothepeakofatomiclinessuchas[Oi] 63µm and extracted the brightness in the position of the seven Herschel
[Feii] 26 µm and of the H rotational lines (Fig. 7) as also ob- knots. In Fig. 8 we show the H O 179µm/108µm, the CO (6–
2 2
served in other bright, outflow shocks such as L1157-B1 and 5)/(14–13)andthe[Oi] 63µm/145µm ratiosinthesevenknots
HH54 (Benedettini et al. 2012; Bjerkeli et al. 2014). This im- alongtheoutflow.Theratiobetweenthetwowaterlinesisquite
plies that the presence of a dissociative and ionising shock but constant in all the knots but IRS1 where it is higher of about a
alsothatthejetmaterialisalreadypartlymolecular,ormolecules factoroftwo.AhigherH O179µm/108µm isanindicationofa
2
reformation is efficient after the passage of the shock front. A lowerparticledensityor/andlowerwatercolumndensity.IRS1
modeloftheshocktowardsknotAandDispresentedinSect.6. isalsotheknotwiththelowerCO(6–5)/(14–13)ratiowhichin-
Articlenumber,page7of16
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Fig.8.PACSlinesratiosofH O179µm/108µm (leftpanel),CO(6–5)/(14–13)(middlepanel)and[Oi] 63µm/145µm (rightpanel)intheseven
2
infraredknots.Arrowsrepresentlowerlimits.
E)wemeasuredaslightlylowerratio(15),indicatingthatinthe
latterpositionsthedensitycouldbelower.Itisworthnotingthat
the lowest ratio of 6.5 observed towards IRS2 is not compati-
blewithmodelpredictions.Unpredictedlow[Oi] 63µm/145µm
ratios have been already observed in several sources (Liseau et
al.2006)andinterpretedasduetoself-absorptionofthe63µm
line. This interpretation was corroborated by the recent obser-
vationofaspectroscopicallyresolved[Oi] 63µm linetowards
a high-mass source which shows a very prominent absorption
(Leurinietal.2015).Theself-absorptionofthe[Oi] 63µm line
couldbethereasonoftheextremelylow63µm/145µm ratioob-
servedtowardsIRS2.IndeedIRS2hasbeenclassifiedasaClass
0 protostar and it is likely at an earlier evolutionary stage with
respecttoIRS1(Chenetal.2008)thereforeitshouldbedeeply
embedded in its dusty envelope whose extinction capability is
stillefficientat63µm butitdoesnotaffectlinesathigherwave-
lengths.Wecannotexcludethepossibilitythatthe[Oi] 63 µm
linetowardsIRS1isalsopartiallyabsorbedthereforethedensity
inferredformthe[Oi] linesratioshouldbeconsideredalower
Fig. 7. Map of the H2 0-0 S(5) line compared with CO (14–13) (red limit.
line),[Oi] 63µm (greenline),[Feii]26µm (blueline)towardsknot
A.Contourlevelsis70%,80%and90%ofthelocalmaximaofeach
line.ThecirclesshowtheHPBWofthedifferentlines. 5. ModellingoftheCOlinesinknotA
ForthebrightestpointoftheredlobeoftheIRS1outflow(knot
dicatesamuchhighertemperature(T >1200K)forthissource. A) we have measurements of CO lines from J = 3 to J = 22
up up
Thisisnotunexpectedsincetowardsveryyoungprotostarsahot thatallowsustomakeadeepanalysisofthephysicalconditions
CO component was usually observed, traced by CO lines with presentinthispureshockposition.Theregionhasalreadybeen
excitation temperature higher than 1500 K (e.g. Dionatos et al. modelledinseveralpapers(Gusdorfetal.2011;G15),herewe
2013;vanKempenetal.2010;Karskaetal.2013).Inparticular, addthe newcontributionof thehigh J COlines observedby
up
towardsIRS1Karskaetal.(2013)detectedtheCO(24–23)line, PACS that trace a gas component not traced by the transitions
indicatinghightemperatureforthegassurroundingtheprotostar. modelledbypreviousworks.
The CO (6–5)/(14–13) ratio shows a decreasing trend from the
northernlobeknotstothesouthernlobeknotsandthisisindica-
5.1. Rotationaldiagram
tiveofalowertemperatureatthesoutherntipoftheoutflowthat
confirmsourresultsonthelowerexcitationconditionpresented Wecalculatedtherotationaltemperaturefromtherotationaldi-
intheprevioussections. agram of the CO lines, which gives a lower limit to the kinetic
Theratioofthetwo[Oi] linesgivesanestimateofthehy- temperatureifthegasisnotinlocalthermodynamicequilibrium
drogen particle density. We compared our observed ratios with (LTE). In order to enlarge our data set as much as possible we
theoreticallineintensityratioscalculatedfromthenon-LTEcode alsousedthelow J COlineobservedwithAPEXandSOFIA
up
alsousedinNisinietal.(2015)thatconsidersthefirstfivelevels (G15).WesmoothedalltheCOmapsatthecommonresolution
of oxygen. The three main peaks of the IRS1 outflow have ra- of24(cid:48)(cid:48).FourCOlines,namelyCO(5–4),CO(9–8),CO(11–10)
tiohigherthan22,indicativeofdensitybetween104–105cm−3 andCO(16–15),however,havebeenobservedonlywithasingle
while on sources and at the southern peak of the outflow (knot pointingandwithadifferentbeam(excepttheCO(11–10) that
Articlenumber,page8of16
Benedettinietal.:TheBHR71outflowobservedbyHerschel
Fig.9.RotationaldiagramofCOlines.ForthelowJ linesthederived
up
rotational temperature and column density are values averaged in the
24(cid:48)(cid:48) beam,assumingafillingfactor1.Forthehigh J linesasizeof
up
18(cid:48)(cid:48) isassumed.
wasobservedwithabeamof24(cid:48)(cid:48))andarenotusedinthefitting.
Inbuildingtherotationaldiagramwetookintoconsiderationthe
measuredsizeoftheCOemission,assumingthatlineswith J
up
from 3 to 11 are extended over the beam of 24(cid:48)(cid:48)while the lines
with J from14to22haveasizeof18(cid:48)(cid:48) (seeSect.3).Forthe
up
spectrally resolved lines observed with APEX and SOFIA the
linefluxwascalculatedintegratingtheemissioninthevelocity
rangebetween-4.5kms−1 and50kms−1 asinG15,whilefor Fig.11.BestfitoftheLVGmodelforCOlinesinknotAconsidering
two components. The parameters of the first component (dotted line)
the unresolved Herschel-PACS lines we used a Gaussian fit to
are:N(CO)=6×1016cm−2,size=24(cid:48)(cid:48),T=70K,n(H )=4×106cm−3:they
the line profile. We attribute to each line flux an uncertainty of 2
arethephysicalparametersofthelowexcitationgascomponentaver-
30%inordertotakeintoconsiderationtheintercalibrationerror agedinsidethe24(cid:48)(cid:48) beamcentredtowardstheknotAposition;thiscom-
betweendifferentinstruments.Therotationaldiagramisshown ponentis,however,moreextendedthanthebeam.Theparametersofthe
inFig.9. secondcomponent(dashedline)are: N(CO)=3×1015 cm−2,size=18(cid:48)(cid:48),
Two components are clearly present: a low excitation com- T=2200K,n(H )=4×104cm−3.Thesolidlinerepresentsthesumofthe
2
ponent fitting the low J lines and a higher excitation compo- twocomponents.
up
nentfittingthehigh J lines.Toderivethecolumndensityand
up
therotationaltemperatureofthetwocomponentswefittedtwo
sets of lines separately, grouping the lines measured with the has been found for the profile of the high Jup CO lines in the
same instrument, i.e. the APEX (J =3, 4, 6, 7) and the PACS B1 shock of the L1157 outflow (Lefloch et al. 2012) and also
lines(Jup=14,18,20,22),respectiveulpy.Indeedthepointatwhich theseauthorsattributetheemissionoftheCOlinewith Jup(cid:62)13
the slope of the fitting stray-line changes is between the two to shock excited gas. Another indication that the high Jup CO
sets of transitions. The best fit parameters are: T =68 K and lines are dominated by the high excitation gas related to the
rot
N(CO)=6×1016cm−2 forthelow J APEXlinesandT =267 shockisthatalsothelineprofileofSiO(Gusdorfetal.2011),a
up rot
K and N(CO)=6×1015cm−2 for the high J PACS lines. Note speciesintroducedinthegasphasebytheshockthroughsputter-
up
ingfromthedustgrainsorvaporisationingrain-graincollisions
thattheHIFIandtheSOFIAmeasurements,notusedinthefit-
(e.g.Guilletetal.2009)iswelldescribedbythesameexponen-
ting itself, are in agreement with the fitting results. It is worth
tiallaw(Fig.10).
noting that the low J lines, at least the ones with J ≤5, are
up up
notopticallythinatallvelocitiessothatcolumndensityderived
fromtherotationaldiagramrepresentsalowerlimitandthero-
5.2. LVGanalysis
tationaltemperatureanupperlimit.Thetworotationaltempera-
tures are similar to those found in other shock positions along We compared the CO lines emission with a grid of Large Ve-
protostellar outflows as L1157 and Cep E (Benedettini et al. locity Gradient (LVG) models calculated with the Ceccarelli
2012;Leflochetal.2012,2015). et al. (2003) code. The molecular data were taken from the
These two different gas components show a different line BASECOL2 database (Dubernet et al. 2013) and the line width
profile. In fact, the spectral profile of the CO (14–13) and CO (FWHMofthelineprofile)wassettoafixedvalueof9kms−1,
(16–15) lines is well described by a single exponential law asmeasuredintheHIFIspectra.Weconsiderthefirst41levels
I ((cid:51)) = I (0) exp(-(cid:51)/(cid:51) ) with the same slope (cid:51) = 10 km s−1 correspondingtoenergylevelsupto2000K.Theexploredrange
CO CO 0 0
forthetwolines(Fig.10),whilethelowerJ lineshaveamore inCOcolumndensityisfrom4×1015to1×1018cm−2,therange
up
complex profile indicating that different gas components con-
tribute to the total emission. A similar slope ((cid:51) = 12 km s−1) 2 http://basecol.obspm.fr
0
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A&Aproofs:manuscriptno.benedettini
Fig.10.ExponentialfitofthelineprofileforCOandSiOlinestowardsknotA.Thelinerepresentthefittothelineprofilewiththeexponential
lowI((cid:51))=I(0)exp(-(cid:51)/10kms−1).
Table3.ParametersoftheLVGfittingoftheCOlinestowardsknotAataresolutionof24(cid:48)(cid:48).
Component N(CO) size T n(H )
2
cm−2 (cid:48)(cid:48) K cm−3
cold 2×1016–6×1016 >24 ∼80 3×105–4×106
warm 3×1015–9×1015 18 1700–2200 2×104–6×104
in particle density is from 5×103 to 3×108 cm−3, the range in a slightly lower density (n(H ) = 2×104 – 6×104 cm−3). The
2
temperatureisfrom25to2000K. comparison between the observed CO line fluxes and the best
fitmodelisshowninFig.11.Thetemperatureofthewarmgas
ItisimpossibletofitalltheobservedCOlinesfrom J =3
up derived from CO is in agreement with the temperature derived
to J =22withasinglegascomponent,whileatwo-component
up fromtheH lines(Gianninietal.2011)confirmingthathighJ
modelgivesagoodfittothedata.Inordertoreducethefreepa- 2 up
CO lines and rotational H lines trace the same gas component
rameterswefixedalltheparametersthatarewellconstrainedby 2
asalsosuggestedfromthespatialcoincidenceoftheiremission
the observations. In particular, the size of the two components
(Fig.7).Thecolumndensityandtemperatureofthefirstcolder
can be measured from the maps: the low J component is ex-
up componentisinagreementwiththeestimatesderivedinthepre-
tended, that is, we assume a filling factor of 1, and the size of
vioussectionwiththerotationaldiagram,indicatingthatitisnot
thehigh J transitionsismeasuredfromthePACSmapstobe
up farfromLTE.Thetemperatureisalsocompatiblewiththelower
18(cid:48)(cid:48). The CO column density of the low J lines is well con-
up limitsofthekinetictemperatureofthelowerexcitationgasde-
strainedfromthefittingoftheAPEXlinesat N(CO)∼6×1016 rivedfromlowJ COlinesinotherpositionsalongtheoutflow
cm−2,avalueconsistentwiththatderivedfromtherotationaldi- (Pariseetal.200u6p).
agram.IfwefixthesethreeparametersthefittingwiththeLVG
grid of models constrains the other model parameters, as given
inTable3.ThefirstgascomponentfittingthelowJ linesisex-
up 6. Comparisonwithshockmodels
tended(size>24(cid:48)(cid:48)),cold(T ∼80K)anddense(n(H )=3×105
2
–4×106cm−3);thesecondcomponentfittingthehighJ lines Inthissection,wepresentcomparisonsofourobservationswith
up
is compact (size=18(cid:48)(cid:48)), warm (T = 1700 – 2200 K) and with outputs of the Paris-Durham shock code (Flower & Pineau des
Articlenumber,page10of16