The Spatial Variation of the Cooling Lines in the Reflection Nebula NGC7023

3.4 MB·English

by  J. Bernard-Salas

Astronomy&Astrophysicsmanuscriptno.ms4 (cid:13)c ESO2015
February2,2015
The Spatial Variation of the Cooling Lines in the Reflection Nebula
⋆
NGC7023
J.Bernard-Salas1,2,E.Habart2,M.Ko¨hler2,A.Abergel2,H.Arab2,3,V.Lebouteiller4,C.Pinto5,M.H.D.vander
Wiel6,G.J.White1,7,andM.Hoffmann2,8
1 DepartmentofPhysicalSciences,TheOpenUniversity,MK76AA,MiltonKeynes,UK
2 Institutd’AstrophysiqueSpatiale,Paris-Sud11,91405Orsay,France
3 SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218,USA
5 4 LaboratoireAIM,CEA/DSM-CNRS-UniversiteParisDiderot,DAPNIA/Serviced’Astrophysique,Saclay,France
1 5 Laboratoired’AstrophysiquedeMarseille,CNRS/INSUUniversite´deProvence,13388Marseille,France
0 6 InstituteforSpaceImagingScience,Departmentof Physics&Astronomy, UniversityofLethbridge, Lethbridge,ABT1K3M4,
2
Canada
n 7 SpaceScience&TechnologyDivision,TheRutherfordAppletonLaboratory,Chilton,DidcotOX110NL,UK
a 8 WolfsonBrainImagingCentre,Addenbrookes´Hospital,UniversityofCambridge,Cambridge,CB20QQ,UK
J
Receiveddate/Accepteddate
0
3
ABSTRACT

]
A Context.Thenorth-westphoto-dissociationregion(PDR)inthereflectionnebulaNGC7023displaysacomplexstructure.Filament-
likecondensationsattheedgeofthecloudcanbetracedviatheemissionofthemaincoolinglines,offeringagreatopportunityto
G
studythelinkbetweenthemorphologyandenergeticsoftheseregions.
. Aims.Westudythespatialvariationofthefar-infraredfine-structurelinesof[CII](158µm),[OI](63and145µm).Theselinestrace
h
thelocalgasconditionsacrossthePDR.Wealsocomparetheiremissiontomoleculartracersincludingrotationalandrovibrational
p
H ,andhigh-rotationallinesofCO.
- 2
o Methods.WeuseobservationsfromtheHerschel/PACSinstrumenttomapthespatialdistributionofthesefine-structurelines.The
r observed regioncoversasquareareaofabout 110′′x110′′ withanangular resolutionthatvariesfrom4′′ to11′′.Wecomparethis
t emissiontoground-basedandSpitzerobservationsofH ,Herschel/SPIREobservationsofCOlines,andSpitzer/IRAC3.6µmimages
s 2
a thattracetheemissionofpolycyclicaromatichydrocarbons.WeuseaPDRcodetomodelthe[OI]145µmline,andinferthephysical
[ conditionsintheregion.
Results. The [CII] (158µm) and [OI] (63 and 145µm) lines arise from the warm cloud surface where the PDR is located and

1 thegasiswarm,coolingtheregion.Wefindthatalthoughtherelativecontributiontothecoolingbudgetovertheobservedregionis
v dominatedby[OI]63µm(>30%),H contributessignificantlyinthePDR(∼35%),asdoes[CII]158µmoutsidethePDR(30%).Other
2
7 speciescontributelittletothecooling([OI]145µm 9%,andCO4%).Enhancedemissionofthesefar-infraredatomiclinestracethe
4 presenceofcondensations, wherehighexcitationCOrotationallinesanddustemissioninthesubmillimitrearealsodetected.The
7 [OI]mapsresolvethesecondensationsintotwostructures,andshowthatthepeakof [OI]isslightlydisplacedfromthemolecular
7 H emission. The size of these structures is around 8′′ (0.015 pc), and in surface cover about 9% of the PDR emission. We have
2
0 tested whether the density profile and peak densities derived in previous studies to model the dust and molecular emission, can
. predictthe[OI]145µmemission.Wefindthatthemodelwithapeakdensityof106 cm−3,and2×104−5cm−3intheoxygenemitting
1
region,predictsan[OI]145µmlinewhichisjust30%lessthantheobservedemission.Finally,wehavenotdetectedemissionfrom
0
[NII]122µm,suggestingthatthecavityismostlyfilledwithnon-ionisedgas.
5
1 Keywords.Infrared:general–photon-dominatedregion(PDR)–ISM:linesandbands–ISM:individualobjects–NGC7023
:
v
i
X
1. Introduction (430 pc), which is almost the average of the values given by
r vanLeeuwen(2007)andBenistyetal.(2013).
a
NGC7023 is a bright and well-known reflection nebula.
Although it is a relatively nearby object, its reported distance ItisexcitedbytheHerbigBebinarystarsystemHD200775
variessignificantlyintheliterature.UsingHipparcosparallaxes (Alecianetal., 2008), which is immersed in the cavity of the
vandenAnckeretal. (1997) and later on vanLeeuwen (2007) molecular cloud (Fuenteetal., 1992). The overall morphology
measuredadistanceof430+160pcand520±180pcrespectively, ofNGC7023isdrivenbytheactionoftheionisingstars,where
and Benistyetal. (2013) e−s9t0imated a distance of 320±51 pc the stellar outflow, now inactive, creates a biconical cavity in
based on revised orbital parameters and astrometry. In this pa- the shape of a butterfly of about of 1.5 pc× 0.8 pc in size
perweadoptthedistancegivenbyvandenAnckeretal.(1997) (Ko¨hleretal., 2014). The stars illuminate the walls of the cav-
ityinanalmostedge-onorientation.Attheedgeofthesewalls,
three PDRs are present (Fuente&Mart´ın-Pintado, 1997), and
Send offprint requests to: J. Bernard-Salas, e-mail:
[email protected] arelocatedat40′′northwest(hereinNW),70′′south-west(SW),
⋆ Herschel is an ESA space observatory with science instruments and 170′′ east of the stars. The NW PDR is the brightest, and
providedbyEuropean-ledPrincipalinvestigatorconsortiaandwithim- is oriented almost edge-on to the observer. Polarization ob-
portantparticipationfromNASA. servations in the near-infrared (Sellgrenetal., 1992), as well
1
Bernard-Salasetal.:CoolinglinesinNGC7023
as vibrational H (Lemaireetal., 1996) of the NW PDR re-
2
veal a filamentary morphology.Wittetal. (2006) find that this
morphology is the result of several condensations that are su-
perimposed along the line of sight, whose surfaces are di-
rectly illuminated by the star. The incident radiation field, in
units of the interstellar far-ultraviolet radiation field estimated
by Habing1, is G =2.6×103 (Chokshietal., 1988; Pillerietal.,
2012).However,highervaluesalsohavingbeenreportedinthe
literature (Fuenteetal., 1999). Deriving the incident radiation
field relies on several assumptions on the geometry and spec-
tral type of the star. For the rest of the paper, we will assume
the value of G =2.6×103 at the PDR front as in Chokshietal.
(1988)andPillerietal.(2012).
NGC7023 has been the subject of many studies in the lit-
erature,includingseveralspectroscopicstudiesin themid-and
far-infrared (FIR). The mid- and far-IR spectral regions con-
tain many important cooling lines and hold the fingerprints
of the dust emission. Fuenteetal. (2000) presented ISO/SWS
andLWSobservationsofNGC7023anddifferentiatedbetween
threeregions:1)thestarandthecavityformedbythestar,which
is filled with a low density gas, 2) the edges of the cavity that
definesthePDRs,and3)themoleculargas.Basedontheiranal- Fig.1.OverlayofthePACSfieldofviewonagroundbasedH2
ysisoftheatomicandmolecularlines([OI]63µm,[OI]145µm, 1-0 S(1) map at 2.12 µm (Lemaireetal., 1996). A 3×3 raster
[CII]158µm, H ), they find that the NW and SW PDRs have map was performed (see Sect. 2). The map covers an area of
2
similarexcitationconditions,withfilamentshavingahighden- approximately110′′x110′′ andis centredin the NW PDR. The
sity of n ∼106 cm−3, and a lower density of n ∼104 cm−3 in emissionfromtheionisingstars(HD200775),onthebottomleft
theregionbetweenfilaments.Werneretal.(2004)and,lateron, ofthefigure,issaturated.RAandDeccoordinatesareindegrees
Sellgrenetal. (2007) studied the variation of the dust features (J2000)
inthemid-IRspectrumofNGC7023usingtheSpitzer/IRSand
MIPSinstruments.TheyobservedvariationsintheNWandSW
PDRsoftheintensity,widths,andcentralwavelengthofspectral In this paper we present observations from the
featuresarisingfrompolycyclicaromatichydrocarbons(PAHs), Photoconductor Array Camera and Spectrometer (PACS,
asafunctionofthedistancetotheionisingsource(HD200775). Poglitschetal., 2010) of the [OI]63µm, [OI]145µm, and
Wittetal. (2006) found a good correlation between the opti- [CII]158µm linesinthe NWPDRof NGC7023.These obser-
cal emission from the dust, as traced by the R band (658 nm), vationsallowustoresolveindetailtheglobalshapeofthePDR,
withthatofro-vibrationalH2.UsingSpitzerdata,Flemingetal. althoughsubstructuressuchasthe filamentsobservedathigher
(2010)studiedtheH2 andPAHsemissioninthePDRsofthree spatial resolution (e.g., Lemaireetal., 1996; Fuenteetal.,
reflectionnebulae,includingNGC7023.Theyshowedevidence 1996),areunresolved.ThisstudycomplementsarecentSPIRE
forPAH de-hydrogenation,whichtheyconcludedissuggestive spectroscopic study of the molecular gas and dust emission
ofH2formationonPAHs. by our group (Ko¨hleretal., 2014). This paper is organised as
Recently,NGC7023hasbeenthetargetofseveralprograms follows: the observations and data reduction are described in
using the Herschel Space Observatory (Pilbrattetal., 2010), Sect. 2. In Sect. 3 the spatial morphology of the lines, their
vastly improving the spatial resolution of previous far-infrared comparison to the emission of CO and H , and the correlation
2
studies. Abergeletal. (2010) combined Spitzer and Herschel betweenthedifferentlineratiosasafunctionofthedistanceto
maps to study the spatial variations of the dust properties in theionisingsource,arediscussed.InSect.4thecoolingbudget,
the east PDR. Using a radiative transfer code they were able and the morphologyof the main cooling lines are analysed. In
toreproducethedifferentspatialvariationsofPAHs,verysmall Sect. 5 the results of the modelling are described. Finally, the
grains(VSGs),andlargedustgrainemission.Joblinetal.(2010) conclusionsaresummarisedinthefinalsection.
used the high spectral resolution provided by the HIFI instru-
ment to study the [CII]158µm line at several positions in the
nebula,includingacentralpositionintheNWPDR.Theyfound 2. ObservationsandDataReduction
that the emission of [CII]158µm line and PAHs arises from
the transition region between the atomic and molecular gas. Theobservationswerecarriedoutduringthesciencedemonstra-
Okadaetal. (2013) studied the photoelectric efficiency from tion phase (SDP) of Herschel on 7 January 2010, and are part
PAHs in a sample of six PDRs, including the East and NW of the ”Evolution of the Interstellar Medium” guaranteedtime
PDRs of NGC7023. Using Spitzer and Herschel/PACS obser- keyproject(observationID=1342191152,Abergeletal.,2010).
vation, they evaluate the photo-electric effect from PAHs and TheobservationsweretakenusingthePACSinstrumentinthe-
thecooling[OI]63and145µm,and[CII]158µmlinesinthree now decommissioned- wavelengthswitchingmode. Fourfine-
regions(cavity,interface,andmolecularregion)andfindthatre- structure lines were observed:[CII] at 158µm, [OI] at 63 and
gions with high fraction of ionised PAHs have a lower heating 145µm,and[NII]at122µm.Theobservationswerecentredon
efficiency. the NW PDR, with RA and DEC coordinates of 315.375◦and
60.178◦respectively(J2000).
1 TheHabingfieldcorrespondsto1.6×10−3 ergs−1cm−2wheninte- Theobservingstrategyandreductionmethodsfollowthatof
gratedbetween91.2and240nm(Habing,1968). ourearlierpaperon theOrionBar (Bernard-Salasetal., 2012).
2
Bernard-Salasetal.:CoolinglinesinNGC7023
We thussummarisethe main steps here,andreferto the above andthe[OI]63µmfluxinthemolecularregionagreewithin5%.
paperfordetails. The uncertainty in the Okadaetal. (2013) paper is of ∼30%,
TotracetheNWPDRa3x3rastermapwasperformed.The while in our work is typically <10%, and thus both values are
configurationat the time of the observationis shown in Fig. 1, consistentwithintheuncertainties.
wheretherastermapattheepochofobservationisoverlaidon In addition to the atomic lines presented in this paper, we
top of an H map of the regionmade with the Canada-France- compare in Section 4 their emission to other relevant cooling
2
Hawaii Telescope (CFHT). An additional map centred in the speciesofCO,H O,CH+,andC0.Inparticular,weusetheCO
2
East PDR (not shown in the figure) will be presented in a fu- 4<J <13transitionsintheSPIRErange,andtherotationallines
u
turepaper.Aminimumexposureconfigurationofonecycleand of H 0-0 S(1), S(2), and S(3) at 17.0, 12.3, and 9.7 µm from
2
repetitionperlinewasperformed.Rasterandpointlinestepsof the Spitzer data. We also use the 360 and 609µm lines of C0
23.5′′ werechosenandresultin Nyquistsamplingforthe lines (SPIRE).SincewedonotdetectanyCH+ andH Olinesinthe
2
in the red channel ([CII]158µm, [OI]145µm, [NII]122µm). SPIRErange,wederiveanupper-limittothetransitionsofthese
The purpose of the wavelength switching mode is to cancel speciesthatweredetectedintheOrionBar(Bernard-Salasetal.,
out the background by determining a differential line profile. 2012). These observationswere taken fromPillerietal. (2012)
This means that observations at an off-position are not needed forH lines,andKo¨hleretal.(2014)fortheSPIRElines.
2
to removethe backgroundin such mode. The present observa-
tions of NGC7023 were deemed to have calibration value by
the Herschel Science Center, and it was decided to obtain an 3. SpatialDistribution
additional set of observationsat the centre position so that the
3.1.CoolingLines
PDR would be observedwith a higher S/N. This was achieved
byincludinganoff-observationintheastronomicalobservation The three top panels in Fig. 2 show the observed distribu-
request(AOR), tobe performedafterthreerasterpositionsand tions of the [OI]63 and 145µm, and [CII]158µm lines. The
withzerooffsetrelativetothecentreofthemap.Thisresultedin exciting source, the binary system HD200775 (RA=315.404◦,
thecentreofthemapbeingobservedatotalof3times,withthe DEC=68.1633◦), illuminates the PDR from the lower left side
redundancy resulting in a higher S/N for that central position, of the figure (just outside the maps, see also Fig. 3). For each
whichiscentredinthePDR. panelthebeamsareshowninthelowerleftcornerandrepresent
The data were processed using version 6.0.3 of the reduc- sizes of 4.5′′, 8.8′′, and 11′′ from left to right respectively. In
tion and analysis package HIPE (Ott, 2010). Each raster posi- thisscale,andadoptingadistanceof430pctoNGC7023,10′′
tion, or footprint, consists of 5×5 spectral pixels (called spax- correspondstoaphysicalscaleof0.02pc.
els). HIPE producesa data cubefor eachfootprint,which con- ThePDRisdetectedinthethreelines.Theiremissionarises
tainsaspectrumforeachspaxel.Startingfromlevel1thecubes fromthesurfaceoftheclouds,wherethegasiswarm(∼700K-
were further processed, using proprietary tools, to correct for 70K, Fuenteetal., 1999; Flemingetal., 2010; Ko¨hleretal.,
minordriftingeffectsandfluxmisalignmentsbetweenscans.At 2014). There is also evidence for condensations of enhanced
this point the cubes are exported into the software PACSman emission within the PDR, where both [OI]63 and 145µm line
(Lebouteilleretal., 2012) to measure the line fluxes by fitting mapscanresolvetwostructures(redclumpsinthefigure).This
a Gaussian, and create the final map. The uncertainties in the isthefirsttimeinNGC7023thatsuchstructureshavebeenre-
line fluxes are small and on average amount to less than 5% vealed from the emission of the [CII]158µm and [OI]63 and
for the [CII] and [OI] lines. The relative flux accuracy be- 145µm lines within the PDR. These structuresor knotsare di-
tween spaxels is 10%2, and for the remaining of the paper we rectly illuminated by the star, and coincide with the presence
have adopted this value or the uncertainty from the fit given of highly-excited CO lines and dust emission (see next sec-
by PACSman, whichever is higher. Note that we do not de- tion).Theseclumpsarebetterdelineatedinthe[OI]145µmmap
tect the [NII]122µm line at any position in the map. The fi- (Figure 2), which reveals two distinct clumps, and a possibly
nalintegratedintensitymapsofthe[OI]63µm,[OI]145µm,and thirdfainterclump.Thesizeofthemainstructuresisaround8′′
[CII]158µmlinesareshowninFig.2. (0.015pcatthedistancetoNGC7023).Whileitispossiblethat
Wehavecomparedourmeasurementsofthe[CII]linewith theseclumpsmaynotbeyetberesolvedinourobservations,we
thefluxesmeasuredbyJoblinetal.(2010)usingtheHIFIinstru- findthattheseclumpscover9%ofthePDRsurfacearea,where
ment, andthe ISO/LWSmeasurementsby Fuenteetal. (2000). inthiscaseweconsiderthePDRareatobeany[OI]145µmflux
allcomparedtothelargestbeam.Thepositionusedforcompar- greaterthan0.9×10−7Wm−2 sr−1 (red,green,andbluecolours
ison(RA=315.385◦,DEC=68.1743◦)islocatedintheclumpof intheuppermiddle-panelofFig.2).
enhancedemissionandismarkedatthetoprightpanelofFigure The PDR is well delineated in the [OI]145µm line
2 with a plus symbol. All measurements agree within ∼10 %. (Fig. 2). Further away from HD200775, the emission of
Okadaetal. (2013) observed the same lines in the NW PDR. [OI]145µm drops to zero. The spatial distribution of the
Their observations consists of a single pointing, while our ob- [OI]63µm line is similar, although the emission is more ex-
servations perform a detail mapping of the region to show the tendedthanthatofthe[OI]145µmline.Thisisexpectedasthe
morphologyoftheselines,providingacompletecoverageofthe [OI]63µm line becomes optically thick at lower column den-
regionandallowingforbettercalibration.Wehavecomparedour sities than the [OI]145µm line, and once saturated, its emis-
fluxestothatofOkadaetal.(2013)inthecavity,interface,and sionwillbebroader(Cauxetal.,1999).The[CII]158µmemis-
molecularregion.Thesethreepositionsaremarkedwithsquares sion is even more extended, since it is easily excited. This
inthetoprightpanelofFigure2.Allvaluesagreewithin29%, was also seen in a similar study by our group on the Orion
wherethe[CII]158µmand[OI]145µmfluxesintheinterface, Bar (Bernard-Salasetal., 2012). The figure also shows that
the morphology of the [CII]158µm line is complex and
2 From the PACS spectroscopy performance structured. Using Herschel/HIFI high-resolution spectra of the
and calibration manual. This can be found at [CII]158µmline,Joblinetal.(2010)andBerne´ etal.(2014,in
http://herschel.esac.esa.int/twiki/bin/view/Public/PacsCalibrationWeb prep.)detectdifferentcomponentswithalargedistributionofve-
3
Bernard-Salasetal.:CoolinglinesinNGC7023
22.40 5.43 10.10
16.87 4.08 7.60
11.33 2.72 5.10
5.79 1.36 2.60
0.25 0.00 0.10
19.19 5.18 10.10
14.50 3.88 7.79
9.82 2.59 5.48
5.14 1.30 3.17
0.45 0.00 0.86
Fig.2.Theupperthreefiguresshowtheobservedimagesforthe[OI]63µm,[OI]145µm,and[CII]158µmlines.Thelowerpanels
showcombinationoflineswithcontourmapsover-plottedinwhite.Thebottomimageshavebeenconvolvedtothe158µm(largest)
beam,withthebeamsizeillustratedinthebottomleftofeachpanel.Thesquaresandplussymbolsintheupperrightpanelindicate
thepositionsusedtocomparethe[CII]158µm fluxtopreviousmeasurementsintheliterature(seeSect.2).Allmapsareinflux
unitsof10−7Wm−2sr−1.Theeighthighestcontoursareat100,91,82,74,65,56,47,and38%ofthepeakemission.
locities,indicatingthatbesidesthemainfilaments,otherregions 3.2.ComparisonwithCO,H,andPAHemission
2
arecontributingtothe[CII]emission.
Fig. 3 (left) displays a high-resolution CFHT image of the H
In the lower three panels of Fig. 2 we plot different com- 2
1-0S(1)molecularlineat2.12µm(Lemaireetal.,1996),which
binations of lines and their contours convolved to the largest
reveals in detail the filamentary structure of the region. Over-
beamsize(11′′).Itcanbeseenthatbothofthe[OI]linescorre-
plotted in white and red respectively are contours of the [OI]
late spatially very well with each other (lower left panel). The
145µm line and one of the excited rotational12CO line (J=12-
middle bottom panel shows that the [CII] emission also de-
11) measured in the SPIRE data by Ko¨hleretal. (2014). We
lineates the PDR but with a more complex structure than the
note that the images are not convolved as the purpose of this
[OI]145µm line. The peak emission is slightly displaced and
panel is to place the emission of [OI] 145µm line and warm
broader in the [CII] line than the [OI]145µm. This is bet-
COrelativetothefilaments.Therightpanelofthefigureshows
ter seen in the last panel where the concentric contours of the
the Spitzer H 0-0 S(3) 9.7µm map convolved to the 145µm
[OI]63µm line contrasts with the more irregular emission of 2
beam, with white [OI]145µm contours. The H 0-0 S(3) tran-
[CII],includingapeakofemissionwithaheart-shapestructure. 2
sition follows the same distribution as the 1-0 S(1), but being
purerotationallineitpeaksslightlybehind(∼1.9′′)the1-0S(1)
Finally, as mentioned in the previoussection, no [Nii] line transition. These observations show for the first time that the
at122µmwasdetectedinthePACSmap.The205µmlinewas [OI]145µmemissioninNCG7023isdisplacedwithrespectto
alsonotdetectedintheSPIREmaps(Ko¨hleretal.,2014).Using thebrightfilamentsthatareseeninH .Thereasonforthisisthat
2
HIFI observations that include the ionising stars, the Herschel since the H emission closely traces the UV field, its emission
2
WarmandDenseISMprogram(WADI,Ossenkopfetal.,2011) peaks at the edge of the PDR, while the [OI] 145µm is more
detectsionisedemissiononlyfromashellclosetothestars(pri- sensitive to both the gas density and temperature. Ko¨hleretal.
vate communication), an area just outside our observed map. (2014)haverecentlymappedtheemissionofhighexcitationCO
This could indicate that the gas in the cavity near the PDR is lines in the region. It can be seen that this highly excited CO
mostlyneutralandnotionised,andexplainswhywedonotde- emission(leftpanelinFig.3)islocatedatthesamepositionas
tectthe122µm linesince ourmapdoesnotincludethisregion the[OI]peak.Wenotethatthe12COJ=12-11linepeaksatthe
closetotheionisingstars.Alternatively,ifthedensityistoolow same position as the also observed 13CO J=7-6 line. Similarly,
inthecavity(Fuenteetal.,2000)itsemissionwillbetooweak although not shown, these condensations are also detected in
todetectinourobservations.Wehavederivedanupper-limitto Herschel/SPIRE and IRAM submillimitre maps that trace dust
thefluxforthislineof1.1×10−7Wm−2sr−1. emission(Ko¨hleretal.,2014).
4
Bernard-Salasetal.:CoolinglinesinNGC7023
5.0 14.7
3.6 11.0
2.2 7.4
0.8 3.7
-0.6 0.0
Fig.3. Left panel: Spatial correlation of the H 1-0 S(1) emission map at 2.12 µm (Lemaireetal., 1996) compared to the [OI]
2
(145µm)and12COJ=12-11(Ko¨hleretal.,2014)contourmapsinwhiteandredrespectively.Tobetterillustratethefilamentsthese
imageshavenotbeenconvolvedtothelargestbeam(inthiscase,CO).Thetwoorangesymbols(squares)markthepositionwhere
the high excitation CO lines are detected (A), and a nearby position with similar conditions but showing no highly excited CO
emission(B).ThesepositionsarediscussedinSect.3.2.Rightpanel:H 0-0S(3)mapat9.7µmconvolvedtothe[OI]145µmbeam
2
withitscontoursinwhite.Inbothpanelsthecolourbarintensitiesareinunitsof10−7Wm−2sr−1,wherethefourhighestcontours
havevaluesof5.4,4.7,3.9,and3.1for[OI]145µm,andof0.37,0.3,0.2,and0.1fortheCO.
It is interesting to explore the origin of this highly excited Because of the low ionisation potential required to ionised
COinrelationtotheatomiccoolinglineemission.Forthispur- carbon(11.3eV),the[CII]158µmlinecanarisefromboththe
pose, and to guide the discussion in the next section, we have neutral (i.e. PDR) and the ionised medium. To validate its use
selectedtworegions:onecoincidingwiththeCOJ=12-11peak asstarformationindicator,itisinterestingtocomparethe[CII]
(position A), and a nearby region with no high-J CO emission emissionwithotherknowntracersofthestarformationprocess,
(B).Wenotethatlow-JemissionisdetectedinpositionsAandB likePAHs(Peetersetal,2004).InFig.4wemakesuchcompari-
(Ko¨hleretal., 2014). Both positionsare labelled in Fig. 3. The sonwhereweplotthePAHemissiontracedbytheSpitzer/IRAC
[OI]145µm flux and the [OI]145µm/[CII]158µm ratio in po- 3.6µm band with the [CII]158µm contour map. Despite the
sition A is respectively 2.2 and 1.7 higher than in position B. the fact that the [CII]158µm line includes emission from dif-
Althoughsmall,thesedifferencesarereal(largerthantheasso- ferent velocity components, it can be seen that the [CII] line
ciated uncertainties in the [OI]145µm flux and ratio), and in- follows reasonably well the PAH emission, with the peak of
dicatevariationsinthephysicalconditionsbetweenthetwore- [CII]158µm emission (heart-shaped)coincidingwith the PAH
gionsconsidered.Suchdifferencescanbeachievedbychanges filament.Whicheverregionscontributetothe[CII]158µmemis-
in the radiation field and/or density of a factor ∼3 in standard sion,thesemustbe(mostly)PDR dominatedsinceittracesthe
PDR grid models (Kaufmanetal., 2006). How can such small PAHemission.
differences explain the presence of high-excited CO in posi-
tion A? The emission of highly excited CO can be explained
3.3.LineCorrelationsandRatios
bychangesinthegasdensityifacertainthresholdinthecondi-
tionsisreached.Ko¨hleretal.(2014)derivedadensityof5×104–
Different lines and line ratios have been plotted against each
106 cm−3 and a temperatureranging between 65–130K at po-
otherasafunctionofthedistancetotheionisingstarsHD20077
sition A, while at position B they found a lower density (104–
in Fig. 5. Panel a in this figure highlights the different regions
105cm−3)andT=80–120K.ThecriticaldensityoftheCOlines
we consider. These include regions in front of and behind the
(105–107cm−3forJ=4-3to13-12transitions)ishigherthanthe
PDR, which in subsequent panels are shown as blue and red
[OI] lines(∼105 cm−3).Thismeansthat,in the densityregime
dotsrespectively.InthefiguretheplussymbolsarePDRpoints
between 104−6 cm−3, the CO lines are more sensitive than the
with a flux higher than 75% of the [OI]145µm peak emission
oxygenlinestoanincreaseindensitywhichcouldboosttheCO
and are delineated by the black contour. Also shown are se-
emission while having little effecton the atomic line emission.
lectedpointsfromanadoptedcutacrossthePDRstartingfrom
It is thus possible that differences in densities in the two posi-
HD200775 (labeled numbers). The latter are shown with their
tionscouldthusexplainthepresenceofhighlyexcitedCOinthe
errorbarstoillustratethefluxuncertaintyindifferentregionsof
clumps.
themap.ThevariationinthefluxintensitiesacrossthePDRof
5
Bernard-Salasetal.:CoolinglinesinNGC7023
Fig.5.Intensityplotsfordifferentcombinationsoflinefluxes(in10−7Wm−2sr−1)andlineratiosrelativetootherlinesordistance
to HD200775. All fluxes have been convolved to the largest beam (11′′). The different regions are indicated in the convolved
[OI]145µmmapinthefirstpanelandarelabelledinthesecond,wherebluerepresentspointsinfrontofthePDR, andredthose
behindit. The peakemission in the PDR is delineatedby the blackcurvein the first panel(those with [OI]145µm fluxeshigher
than75%ofthepeakintensityoftheline),andareplottedasplussymbolsinthesubsequentpanels.Thereferencepointsforthe
adoptedcutarenumberedandshowninthefirstpanel(Sect.5),andhavebeenplottedasblackdotswitherrorbarsinsubsequent
panelstogiveanindicationoftheuncertainties.
the [CII]158µm, and [OI]63and 145µm lines is largerthan a causeofthemulti-component[CII]158µmemission,thereisa
factor10,whichishigherthanthoseobservedintheOrionBar broaderrelationintheratio.
(factor<7)forthesamelines(Bernard-Salasetal.,2012).
Panelsbandcshowrespectivelytherelationbetween[Cii] 4. Coolingbudget
and [OI]63 and 145µm lines. There is a broad correlation in
front of and after the PDR in both plots. The likely reason for The mid- and far-IR regions are home to the most important
the broad relation is the mixture in the line of sight of the dif- cooling lines in the PDR. Our Herschel observations, together
ferent components that give rise to the [CII]158µm line (see withSpitzerdata,haveenabledustoestimatetheimportanceof
Sect. 3). The relation in panel c is somewhat narrower than in these species to the cooling budget. We have thus derived the
panelb.Thiscouldbeattributedtothefactthattheemissionof contributionofthe[CII]and[OI]coolinglinesinthePDR,as
the[OI]63µmline,beingopticallythickerthanthe[OI]145µm, wellasotherrelevantatomicandmolecularlinesinthemid-and
is also extended and follows the [CII]158µm emission more far-IRincluding;CO,H2,andC0.H2andCOarethemostabun-
closely. dance molecular species in the ISM and contribute also to the
coolinginPDRs(Hollenbach&Tielens,1999).Tocalculatethe
Panel d shows a linear correlation between the [OI]63 and
budgetofthesespeciesweconsiderthe0-0S(1),S(2),andS(3)
145µmlinesbehindthePDR,andamoreparabolicrelationship
rotational lines of H at 17.0, 12.3, and 9.7 µm (Spitzer/IRS),
2
in front of it. In front of the PDR the gas is warmer and op- and CO 4≤ J ≤13 transitions (SPIRE). In the Orion Bar, C0,
tical depth effects become more important.Having an Einstein CH+ andH Oulinesare detectedin thePACS andSPIRE spec-
coefficient five times higher, the [OI]63µm line will be more 2
traandcontribute∼1%ofthetotalcoolingbudget(Habartetal.,
self-absorbedthanthe[OI]145µmline,anditsoverallemission
2010; Bernard-Salasetal., 2012). This is a small factor to the
willbemoreextended.Therefore,inthatregion[OI]145µmin-
totalcoolingbudgetintheOrionBar,butwehaveexploredtheir
creasesmorethanthe[OI]63µmlinedoes.
contribution in NGC7023 as well. We have detected the 370
In the last two panels in the figure (e and f), the and 609 µm lines of C0 in the SPIRE spectrum (Ko¨hleretal.,
[OI]145µm/[CII]158µmand[OI]145µm/[OI]63µmratiosare 2014). We donotdetectCH+ and H O transitionin theSPIRE
2
plotted respectively against the distance to HD200775. As we spectrumofNGC7023sohavederivedupper-limitsfortheirfar-
approachthePDRthedensityincreasesandbothlineratiosin- IRtransitionstoestimatetheircontribution.Forthiscomparison
creasesrapidly,peakingataround45′′.AfterthePDR,thetem- we choose the position of the [OI] emission peak in the PDR
perature dropsand both line ratios decrease. However, and be- (red knots in Fig. 2), and we convolve all the line emission to
6
Bernard-Salasetal.:CoolinglinesinNGC7023
with that of H . For H we consider the 0-0 rotational transi-
2 2
tionsS(1),S(2),andS(3)andthevibrationallinev=1-0S(1).All
mapshavebeenconvolvedto the[CII]158µmbeam.Thus,for
148.4 agivenpointinthemap,thetotalcoolingofthesefourspecies
amounts to 100%. Therefore this figure shows which species
dominatesthecoolingindifferentregions.Weignoredthecon-
tribution of C0, H O and CH+, but as we can see from Table
111.3 2
1,theircontributionisalreadynegligibleinthePDR,wherewe
wouldexpecttheircontributiontobestrongest.Ifwefirstlookat
eachmapindividually,weseethat[OI]63µmcontributesmore
in front of the PDR, and [CII]158µm outside it (in front and
74.2
behind). [OI]145µm and H contribute more in the PDR, but
2
with[OI]145µmcontributionbeinghigherinthesouth-eastre-
gion(lower-right).The[OI]145µmlineismoresensitivetothe
37.1 gasdensity than the H lines, and thuscontributesmore where
2
theclumpsarelocated.Readingthepercentagefromthecolour
bars,wecanalsocomparetherelativecontributionamongsteach
other.Ingeneral,[OI]63µmisthedominantcontributoroverthe
0.0
entireregion(>30%).However,[CII]158µm isalsoanimpor-
tantcoolantoutsideofthePDR(contributingabove30%),asis
H in the PDR (>35%).On the other hand, the contributionof
2
[OI]145µmdoesnotexceed12%inthePDRandislowerthan
5% in the rest of the region. This figure delineates the impor-
Fig.4. Comparison of the PAH morphology as traced by the
tanceof[OI]63µm,[CII]158µm,andH ascoolingagentsasa
2
IRAC 3.6µm band intensity in MJy/sr (taken from the Spitzer
functionoftheregion.
archive),with[CII]158µminwhitecontours.Indecreasingor-
derthecontourlevelsareat10.1,9.0,7.8,6.7,5.6,4.5,3.4,and
2.2×10−7Wm−2sr−1. 5. Modelling
Inthissectionourgoalistoexplorewhetherpreviouslyreported
thelargestbeam,inthiscasetheSPIRECO emission(25′′).In parametersusedtomodelthedustemissioninthePDR(density
profile,geometricalparameters),andCOlines(gasphysicalcon-
Table 1we quotethese valuestogetherwith thoseof the Orion
ditions), can also reproduce the [OI]145µm emission. This is
Bar(Bernard-Salasetal.,2012),whichisusuallyadoptedasthe
importantinourquesttofindamodelthatcanself-consistently
prototypicalPDR.
reproduce the dust, molecular, and atomic PDR components.
UsingHerschelphotometryandspectroscopy,thedustemission
Table1.CoolinglineemissionasapercentageatthePDRposi- andmolecularcontent(CO)of theNW PDRhasbeenrecently
tionandconvolvingtheemissionofallspeciestoa25beam. modelledby Arabetal. (2012) and Ko¨hleretal. (2014). In our
observationsthe[CII]158µmlineisamixtureofmanycompo-
NGC7023 OrionBar† nents,theemissionofthe[OI]63µmlineisopticallythick,and
[CII]158µm 19 8 the[NII]122µmwasnotdetected.Thus,attemptingtomodelthe
[OI]63µm 33 72 atomic componentfrom our observationswill serve to provide
[OI]145µm 9 10 basic physical conditions of the PDR based on just fitting the
[CII]+[OI] 61 90 [OI]145µm line. Thisis whywe preferinsteadto testwhether
H2 35 5 previouslyandmoredetailedmodellingstudiescanalsoexplain
CO 4 5 the[OI]145µmemission.
C0,H O,CH+ <1 <1
2 Inordertomodeltheemissionofthelinewehaveselecteda
cutoriginatingfromthe ionisingstarsand intersectingthe NW
†FromBernard-Salasetal.(2012).
PDRatapointofpeakdustemissionat250µm.Thisisthesame
cutusedin(Ko¨hleretal.,2014)forthemolecularlineanddust
emission.Theobservedemissionprofilesforthethreelines,to-
Table 1 shows that in the NW-PDR of NGC7023the cool- gether with that of the H 1-0 S(1) and CO J=12-11 lines, are
2
ingbudgetisdominatedbythe[CII]+[OI](61%),followedby showninFig.7(leftpanel).Theseprofilesareshownattheirnat-
H with35%,CO (4%),andC0 contributinglessthan1%.The uralresolutiontomaximisethespatialinformationwecanobtain
2
uncertainty in these percentages is around 20% of each value. fromthem.Itcanbeseenthat[OI]63and145µm,[CII]158µm,
Themaincontributorstothecoolingbudgetare[OI]63µmand and CO J=12-11 peak at the same position. Having the high-
H . Theoverallresultis similarto thatfoundin theOrionBar, estspatialresolution,theH profileresolvestwofilaments(see
2 2
wheretheatomiclinesdominatethecoolinglineemission,with Fig. 3), with the densestfilamentpeakingimmediatelyin front
[OI]63µmbeingthestrongest.However,themaindifferenceis ofthefine-structureandCOlinesrelativetotheexternalillumi-
thereducedroleofH intheOrionBar(5%)whencomparedto nation.The[CII]158µmlinehasthebroadestprofile,followed
2
NGC7023.Aspartofthe sameprogram,we havestudiednine by[OI]63µm.
other nearby PDRs and find that, like for NGC7023, H con- We have modelled the emission of the [OI]145µm line
2
tributestoabout30%(Bernard-Salasetal.inprep.). with the Meudon PDR radiative transfer code (version 1.4.4,
In Figure 6 we show the relative contribution by percent- LePetitetal.,2006;LeBourlotetal.,2012).Thiscodeassumes
age of the [OI]63 and 145µm, and [CII]158µm lines, along aplane-parallelslabofgasanddustthatisilluminatedbyanin-
7
Bernard-Salasetal.:CoolinglinesinNGC7023
45 12
37 9
29 6
21 4
13 1
52 60
43 48
35 36
27 25
18 13
Fig.6.Relativecoolingbudgetofthe[OI]63and145µm,[CII]158µmlines,andthe0-0S(1),S(2),S(3)H rotationaltransitions
2
andv=1-0S(1)inpercentage(%).Addingthecontributionofallfourspeciesforagivenpointinthemapamountsto100%.Other
speciesthatcancontributetothecoolingareignoredinthisfigure(seeSect.4).Inthefirstpanel,andtoguidetheeye,acontourof
thePDRastracedbythe[OI]63µmemissionisshown.Theimageshavebeenconvolvedtothelargestofthebeamsizesofthese
species,inthiscasethe[CII]158µmline,shownasasolidcircleinthe[CII]panel.Inaddition,andforcomparison,wealsoshow
thelargestCObeamsize(25′′)usedtocalculatethecoolingbudgetinTable1asadashedcircle.
cidentradiationfield,andresolvesiterativelytheradiativetrans- the PDR hasan approximatewidth of 20′′ and a lengthof 60′′
ferandthermalandchemicalbalancetocomputetheatomicand (astracedbythegreencoloursinthefigure),whichcorresponds
molecularemission in the cloud.As in the paperslisted above, to0.04and0.12pcrespectively.Frommodel1wefindthatthe
wehaveassumedanincidentradiationfieldofG∼2,600onthe depthisabout2timestheobservedlength,resultinginaflatex-
NWPDRat42”fromthestar(Chokshietal.,1988;Pillerietal., tendedgeometry.Formodel2thethedepthcomparesverywell
2012). The RADEX analysis of the CO lines (Ko¨hleretal., totheprojectedwidthresultinginacylindricgeometry.
2014), and the modelling of the dust emission are compatible
with two maximum gas densities of 105 and 106 cm−3. Thus, Thepredicted[OI]145µmintensitiesformodels1and2are
forourcomparisonweconsiderbothpossibilities.Thesestudies shownintherighthandpanelofFig.7,wherethelineemissivi-
derive a PDR length of 0.2 pc for a gas density 105 cm−3 and tieshavebeenconvolvedtothePACSbeamat145µm.Forthis
0.03 pc for a density of 106 cm−3, giving both a total column comparisonweconsidertheconditionsofthemodelsinthere-
density of about 1022cm−2, and we adopt these values in our gion where the predicted unconvolvedflux is greater than half
study.Weassumethedensityprofilederivedfromthedustanal- the maximum flux (48′′–52′′ from the binary system). In this
ysisandadjustedtostartat0.088pc(42′′)fromHD200775.The region model 1 gives n =104–8×104 cm−3 and T =90–530 K.
densityprofilesforthetwogasdensitiesweconsiderareshown Model2givesn=2×104–2×105cm−3andT =90–660K.Inboth
in the middle panelof Fig. 7. We hereafterrefer to these mod- modelsmostoftheemissioncomesfromaregionwithavisual
els as model 1 (n=105 cm−3) and and model 2 (n=106 cm−3), extinction A = 0.07–1.2. With a 10% uncertainty in the ob-
V
respectively. servedline,noneofthemodelsreproducesthe[OI]145µmpeak
intensity. Note that we did not attempt to fit the line, but this
Beforeweproceed,itisinterestingtoinvestigatewhetherthe additionalconstraintmayservetodiscriminatebetweenthetwo
depthofthePDRdeducedfromthepreviousstudies(Arabetal., models.Model1predictsanintensitynearlythreetimeshigher
2012; Ko¨hleretal., 2014) is compatible with the width and thantheobservations,whilemodel2underestimatetheemission
lengthofthe PDR we derivefromthe observations(spatialex- by just 30%. The maximumgas temperature in both models is
tentofthe [OI]145µm line).Inthe middle-toppanelofFig.2, higherthanthe gastemperaturederivedbyKo¨hleretal.(2014)
8
Bernard-Salasetal.:CoolinglinesinNGC7023
Fig.7. Left panel:Observednon-convolvedprofile of the atomic andmolecularlines as a functionof the distance to HD200775
alongthecutshowninFig.5anddescribedinSect.4.Middlepanel:Densityprofilesattwodifferentmaximumdensitiesusedby
Ko¨hleretal.(2014)intheiranalysisofthemolecularemission.Model1hasa peakgasdensityof105 andmodel2 of106 cm−3.
Right panel: Observed profile emission of the [OI]145µm line compared to the convolvedpredicted emission by Meudon PDR
models.
when modelling the CO J=13-12 to J=6-5 lines with RADEX at the edge of the PDR (Pillerietal., 2012). While there is a
(140K).Thehighertemperaturewefindisexpectedsincemod- cavity, the ionising sources are young and likely to have some
els predict that the [OI]145µm line originates from a slightly material around the star. This attenuation of the radiation field
warmerlayerthanthatoftheintermediateJCOexcitedlines. is likely caused by the envelopeof the star andthe dilutionef-
The fact that we find a model that can reproduce the dust fect.WealsoadoptISMabundances(Savage&Sembach,1996;
emission,as wellasroughlymatchingthe spatialprofileof the Meyeretal.,1997,1998).BetweenthestarandthePDRthereis
[OI]145µm line is an encouraging step towards a self consis- acavity.Thedensityprofileisadjustedtostartat0.023pc(11′′)
tentmodelthatexplainsboththegasanddust.However,while fromHD200775.Theintensityofthenitrogenlinesat122and
model 2 does not reproduce exactly the peak and width of the 205µmaresensitivetotheelectrondensityintheionisedregion,
[OI]emission,wemustbearinmindthatweareassumingasim- and on the incident radiation field, but we do not detect either
plemodelwithasimplegeometry.TheH observationsreveala line.Theupperlimitswehavederivedfortheselines(1.1×10−7
2
morecomplexstructurewithseveralfilaments.Alsothedensity and10−9Wm−2sr−1respectively)arecompatiblewithtwosce-
profile we use reflects a smooth PDR, while our own observa- narios for the cavity, one with an ionised region and a density
tionsclearlyrevealthepresenceofdensestructureinsideofthe n<100cm−3,andanotheronewithnoionisedregionandaden-
PDR.Ourcutisalsoarbitrary.Thesecomplexstructureswillaf- sityof500cm−3.
fectthe[OI]emissionprofile(width,peakposition).Giventhese
caveats,itisthereforeunsurprisingthatwedonothaveaperfect
6. SummaryandConclusions
fit. In fact, in order to explain the intensity peak of the excited
COemission,asteeperdensitygradientatthePDRedgeovera We have presented Herschel/PACS spatially resolved observa-
smallscaleisneeded,tohavebothalargeamountofwarmand tions of the [CII]158µm, [OI]63µm and 145µm lines of the
dense gas. The CO lines can be reproducedusing a high pres- NWPDRinNGC7023.Thishasenabledustostudytheemis-
sure model (P =108 K cm−3) and relatively small PDR width sion ofthese coolinglinesin relationtothe morphologyof the
anddepthof2–3′′(Joblinetal.inprep.).Suchamodelcanalso region.Wesummariseherethemajorfindings:
predictthe maximumpeak intensityof the [OI]145µm line. A
lowerpressureishoweverneededtoreproducethefullextentof 1. The emission of the atomic cooling lines trace the cloud
thePDRthatwehavepresentedinFig.2. surface that is directly illuminated by the binary system
Finally, and since the Meudoncode does nottakes into ac- HD200775.Here thegasiswarmer,andthese linesare as-
counttheionisedregion,wehavemodelledtheemissionofthe sociated with the filamentary structure revealed in higher-
[Nii]122 and 205 µm lines using the radiative transfer code resolutionH2 andPAHmaps.InthePDR,thepeakofcool-
Cloudy (Ferlandetal., 1998). We use the same density profile, ing line emission correspondsto the presenceof condensa-
peak density, and PDR length as in model3 2. For the radia- tions that are seen from high level rotationalCO lines, and
tion field, we assume a Kurucz spectrum (Kurucz, 1993) with Herschelandsubmillimetreemissionofdust.
T =15,000 K correspondingto the spectral type (B3V-B5V) 2. Bycomparingtheroleofdifferentcoolantswefindthatthe
eff
of the illuminating binary star (Alecianetal., 2008) but atten- [OI]63and145µmand[CII]158µmlinesaccountfor61%
uated in order to have an incident radiation field of G ∼2,600 of the cooling in the PDR, with [OI]63µm contributing to
0
33%of the emission. H also contributessignificantly with
2
3 Usingtheparametersofmodel1doesnotchangetheconclusion. 35%,COwith4%,andotheratomsandmolecules(C0,H2O,
9
Bernard-Salasetal.:CoolinglinesinNGC7023
CH+) contributingless than 1 percent. Lookingat the rela- a simple planar model to reproduce the [OI]145µm using
tivecontributionofthemaincoolingagentsovertheregion, parameters derived from modelling the dust and CO emission,
wefindthatwhilethe[OI]63µm dominatesthecooling,H this study offers an encouraging step forward in producing a
2
alsocontributessignificantlyinthePDR,and[CII]158µmis self-consistent model that can explain the atomic, molecular,
also important in front of and behind the PDR. This high- and dust emission. Moreover, spatially resolved studies like
lights the importance of [OI]63µm and H as a coolant in this can help us calibrate the use of the [CII]158µm and [OI]
2
thePDR,andof [CII]158µminlowerexcitedregions.Itis linestotracestarformation(Staceyetal.,1991;Meijerinketal.,
interesting to notice that while in the Orion Bar the atomic 2007), and understand the origin of the so called [CII] deficit
linesdominatethecooling,H contributestoonly5%. (Luhmanetal., 2003) in most luminous galaxies. Because of
2
3. The[OI]63and145µmmapsspatiallyresolvetheseconden- its brightness, the [CII]158µm line provides an unmatched
sation into two distinct structures. Furthermore, Herschel’s opportunity for redshift determinations and source diagnostics
high-angularresolutionshowsthatthe[OI]145µmemission offardistantsystems,especiallywithALMA.
peaks slightly farther away from the ionising stars than the
Acknowledgements. Wethankananonymousrefereeforusefulcommentsand
H emission which traces the edge of the PDR. This is ex-
2 suggestions. JBS wishes to acknowledge the support from a Marie Curie
pectedsince[OI]145µmismoresensitivetothegasdensity.
Intra-European Fellowship within the 7th European Community Framework
4. [OI]63 and 145µm and [CII]158µm peak at the same po- Program under project number 272820. MHDvdW is supported by CSA and
sition. The [CII]158µm line is more extended than the NSERC. We thank C. Joblin for useful comments and discussion. HCSS,
[OI]63and145µmlinesbecause[CII]158µmiseasilyex- HSpot, and HIPE are joint developments by the Herschel Science Ground
SegmentConsortium,consistingofESA,theNASAHerschelScienceCenter,
citedandpresentsamorecomplexstructure.Thisisconsis-
andtheHIFI,PACSandSPIREconsortia.PACShasbeendevelopedbyacon-
tent with HIFI observations of the region by (Joblinetal., sortium of institutes led by MPE (Germany) and including UVIE (Austria);
2010) which reveal different velocity components for the KULeuven,CSL,IMEC(Belgium);CEA,LAM(France);MPIA(Germany);
[CII]158µm emission, indicating that different regions are INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This devel-
opment has been supported bythe funding agencies BMVIT (Austria), ESA-
contributingtotheemissionofthisline.
PRODEX(Belgium),CEA/CNES(France),DLR(Germany),ASI/INAF(Italy),
5. Theemissionofhighlyexcited12COJ=12-11correlateswith
andCICYT/MCYT(Spain).
thepeakofemissionofthecoolinglines.Wefindthatemis-
sionfromtheatomiccoolinglinesandlineratiosatthispo-
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provide a means to understand how stellar radiation interacts Pilleri,P.,Montillaud,J.,Berne´,O.,&Joblin,C.2012,A&A,542,69
Poglitsch,A.,Waelkens,C.,Geis,N.,etal.2010,A&A,518,2
with the gas, and in the future it will be interesting to model
Savage,B.D.,&Sembach,K.R.1996,ARA&A,34,279
whether such clumps will be photo-evaporated or form stars.
Sellgren,K.,Uchida,K.I.,&Werner,M.W.2007,ApJ,659,1338
Given the complexityof the region, and that we have assumed Sellgren,Werner,M.W.,&Dinerstein,H.L.1992,ApJ,400,238
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