The molecular environment of the massive star forming region NGC 2024: Multi CO transition analysis

0.43 MB·English

by  M. Emprechtinger

Astronomy&Astrophysicsmanuscriptno.aa2024 (cid:13)c ESO2009
January21,2009
The molecular environment of the massive star forming region
9
0 NGC 2024: Multi CO transition analysis
0
2
n Emprechtinger M.1,Wiedner, M.C.1,Simon,R.1,Wieching, G.1,2,Volgenau, N.H.1,⋆,Bielau,F.1,Graf,U.U.1,
a Gu¨sten,R.2,Honingh,C.E.1,Jacobs, K.1,Rabanus, D.1,3,Stutzki,J.1,andWyrowski,F.2
J

5
1 1 I.PhysikalischesInstitut,Universita¨tzuKo¨ln,Zu¨lpicherStr.77,50937Ko¨ln,Germanye-mail:[email protected]
2 Max-PlanckInstitutfu¨rRadioastronomie,AufdemHu¨gel69,53121Bonn,Germany

] 3 EuropeanSouthernObservatory,AlonsodeCordova3107,Vitacura,Casilla19001,Santiago,Chile
A
G
. ABSTRACT
h
p
Context.Sites of massive star formation have complex internal structures. Local heating by young stars and kinematic processes, such as
-
o outflowsandstellarwinds,generatelargetemperatureandvelocitygradients.Complexcloudstructuresleadtointricateemissionlineshapes.
r COlinesfromhighmassstarformingregionsarerarelyGaussianandshowoftenmultiplepeaks.Furthermore,thelineshapesvarysignificantly
t
s withthequantumnumberJup,duetothedifferentprobedphysicalconditionsandopacities.
a Aims.Thegoalofthispaperistoshowthatthecomplexlineshapesof12COand13COinNGC2024showingmultipleemissionandabsorption
[
features,whichvarywithrotationalquantumnumber Jcanbeexplainedconsistentlywithamodel,whosetemperatureandvelocitystructure
1 arebasedonthewell-establishedscenarioofaPDRandthe′′Blistermodel′′.
v Methods.Wepresentvelocity-resolvedspectraofseven12COand13COlinesrangingfromJ =3toJ =13.Wecombinedthesedatawith
up up
7 12COhigh-frequencydatafromtheISOsatelliteandanalyzedthefullsetofCOlinesusinganescapeprobabilitycodeandaone-dimensional
7 fullradiativetransfercode.
2 Results.WefindthatthebulkofthemolecularcloudassociatedwithNGC2024consistsofwarm(75K)anddense(9·105 cm−3)gas.An
2 additionalhot(∼300K)component,locatedattheinterfaceoftheHIIregionandthemolecularcloud,isneededtoexplaintheemissionofthe
.
1 high-JCOlines.Deepabsorptionnotchesindicatethatverycoldmaterial(∼20K)existsinfrontofthewarmmaterial,too.
0 Conclusions.AtemperatureandcolumndensitystructureconsistentwiththosepredictedbyPDRmodels,combinedwiththevelocitystructure
9 ofa′′Blistermodel′′,appropriatelydescribestheobservedemissionlineprofilesofthismassivestarformingregion.ThiscasestudyofNGC
0 2024showsthat,withphysical insightsintothesecomplex regionsandcarefulmodeling, multi-lineobservations of12COand13COcanbe
:
v usedtoderivedetailedphysicalconditionsinmassivestarformingregions.
i
X
Keywords.Stars:formation–HIIregions–NGC2024–Methods:observational–Submillimeter
r
a
1. Introduction Themassivestarformingregionweselectedforourstudy
is the HII region NGC 2024 and its associated molecular
High-mass star forming regions are very complex. The inter-
cloud, which is located at a distance of 415 pc (Anthony-
action of OB stars, which are often deeply embedded, with
Twarog 1982). It is a part of Orion B, a well-studied giant
the surrounding molecular cloud via radiation and outflows
molecularcloud(e.g.Maddalenaetal.1986,Ladaetal.1991,
and condensations of cold gas, still largely unaffected by the
Krameretal.1996,Mitchelletal.2001).
star forming activities, lead to a complex density and tem-
A possible scenario for the three-dimensionalstructure of
perature structure, causing intricate shapes of the observed
the NGC 2024 region was proposed by Barnes et al. (1989),
emission lines. Some CO lines in such regions (e.g., M 17,
whocombinedobservationsfromopticaltoradiowavelengths
Stutzki & Guesten et al. 1990, W3, Kramer et al. 2004 and
toconstructamodelofthecloud(Fig.1).Intheirmodel,the
Mon R2, Giannakopoulou et al. 1997) have multiple peaks,
HIIregionsitsinfrontofthebulkofthecloudbutispartlyob-
mostlikelyduetoabsorptionbyforegroundmaterial.Theline
scuredintheopticalbyaveryprominentridgeofcoldmolec-
shapesvarysignificantly,dependingontherotationalleveland
ular material. This geometry provides an explanation for the
theobservedisotopes,whichtracedifferentregimesofphysical
complexlineshapes,whichmayconsistofcontributionsfrom
conditionsandopticaldepths.
fore- and background regions, including self-absorption. The
Sendoffprintrequeststo:M.Emprechtinger ionizing sources of NGC 2024 are invisible at optical wave-
⋆ Currentaddress:CaliforniaInstituteofTechnology,OwensValley lengthsbecausetheyareobscuredbythedustridge.IRS2b,a
RadioObservatory,BigPine,CA93513USA lateOorearlyBstar,hasbeenidentifiedastheprimaryioniz-
2 EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024
This velocity shift between background and foreground
componentcanbeexplainedbya′′BlisterModel′′(Israel1978,
Zuckerman1973). In sucha model,an OBstar is assumedto
be located close to the surface of the molecular cloud form-
ing an HII region. The ionization frontof such an HII region
movesslowlyintothiscloudestablishingahighpressuregra-
dient.Becauseofthispressuregradienttheionizedgasmoves
awayfromthemolecularcloud.Thereforehydrogenrecombi-
nationlinesappearatnegativeline ofsightvelocityoffsetsof
about3 km/s relative to the molecular lines, if the HII region
islocatedatthenearsideofthemolecularcloud(Israel1978).
InthecaseofNGC2024theionizedmaterialcannotflowinto
space,butpushesontheforegroundmaterial.Wethereforeex-
pect that the foregroundcomponentis at somewhatlower ve-
locitiesthanthebulkofthemolecularmaterial.Theassumption
thatthevelocitystructureofNGC2024isindeedcausedbya
′′Blister′′ is buttressedby the fact thatthe H109αrecombina-
tion line appears at 7 km/s (Israel 1978), and thus even more
blueshifted.
Fig.1. Schematic view of the geometry of NGC2024. C18O 2-1 and C17O 2-1 emission observed by Graf et
Abbreviationsare asfollows:DMC= densemolecularcloud,
al.(1993),followsthe1.3mmcontinuumverywellandpeaks
EMC = extended molecular cloud, IF=ionization front and τ
closetothefarinfraredsourcesdetectedbyMezgeretal.(1988
= high optical depth cloud (optical dust bar). Adapted from &1992).ContrarytotheC18OandC17Olines,themapofthe
Barnesetal.(1989)byGianninietal.(2000). integrated intensity of optically thicker 12CO 7-6 line shows
features similar to those seen in the 6 cm continuum map
(Crutcher et al. 1986), which traces the ionized gas. This in-
ing source of NGC 2024through observationsin the near in- dicatesthatthe12CO7-6mainlyoriginatesfromaphotodom-
frared (Bik et al. 2003). The extinction (Av) through the dust inatedregion(PDR)atthesurfaceofthemolecularcloudand
ridgealongthelines-of-sighttostarsinsidetheHIIregionare notfromembeddedprotostellarobjects.Theexcitationcondi-
intherangeof15to25mag.Furthermore,theHIIregionisex- tions in this PDR were studied using the integrated intensity
pected to expand into the molecular cloud and to trigger star of far infrared line emission observed with the ISO satellite
formation.Indeedsomeprotostellarcondensations(FIR1to7) (Giannini et al 2000). They studied [NII], [NIII], and [OIII]
havebeendetectedclosetotheHIIregion(Mezgeretal.1988 linesfromtheHIIregionandhigh-JCOlines,[CII],and[OI],
&1992).Theirmasses,derivedfromsub-millimetercontinuum which originate(in part)fromthe PDR. Comparisonwith the
emission,rangefrom1.6to5.1M⊙(Visseretal.1998).Because predicted line ratios of a PDR model (Burton et al. 1990) re-
thesemassescorrespondtovisualextinctionsbetween270mag vealedadensityof≈ 106 cm−3andaUVfieldof3·104 G at
0
and 870mag and the opticaldepth of the dustridge at the near thesurfaceofthemolecularcloud.
side of the H II region is only 15-25mag, it is very likely that PDRmodelsforhighdensitiesandhighUV fields,asrel-
these condensationsare located in the dense molecular cloud evant for NGC 2024, predict a column density for hot CO
(DMC)behindtheHIIregion.Massiveoutflowshavebeende- (> 100 K) of the order of 1016 − 1017 cm−2 (see, Ro¨llig et
tectedclosetothesourceFIR5(e.g.,Richer1990,Sanders& al. 20071).High-JCO lines(inthispaperwerefertoalllines
Willner1985)givingadditionalevidenceforongoingstarfor- with frequencies> 1 THz as high-J)wouldbe emitted exclu-
mation. sivelyfromsuchhotanddensematerial,becauseoftheirhigh
Detailed investigations of the molecular cloud associated criticaldensity(∼107cm−2)andhighenergyoftheupperlevel
with NGC 2024 were performed by Graf et al. (1990, 1993), (E ≥250K).
up
whostudiedmultiple12COlines(uptoJ=7-6)anditsisotopo- In this paper we combine twelve transitions of 12CO and
logues. They derived temperatures of 23.5 K and 67.4 K for 13CO,rangingfromJ =3toJ =19totracemolecularmate-
up up
the foregroundand the backgroundcomponent,as defined by rial NGC 2024 under very different physical conditions. The
Barnes et al. (1989), respectively. The corresponding column goal of this paper is to find, based on the above mentioned
densitiesare5.4·1022cm−2(Av =56mag)and2.0·1023cm−2 scenarios (′′Blister Model′′ and PDR) a model for the veloc-
(Av = 210 mag)). Furthermore they found different velocities ity,densityandtemperaturestructureofNGC2024,whichre-
of9km/sand11km/sfortheforegroundandthebackground producesthe observedline intensities and line shapes consis-
component,respectively.Thepeakvelocitiesofthemaincom- tently.ForthispurposeweobservedsevenCOlinesspectrally
ponentofthe12COlines,vLSR = 13km/s,differsignificantly resolvedwhicharepresentedinsection3.Insection4,wefirst
fromthoseoftheopticallythinlines(e.g.,ofC18O),whichare examine the physical conditions of the gas, which emits the
at a velocity of 11 km/s. Graf et al. (1993) suggested thatthe
bluesideofthe12COlinesareabsorbedbythedustridgeata 1 All of their results are available under
velocityof9km/s. http://www.ph1.uni-koeln.de/pdr-comparison
EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024 3
high-JCOlines,usinganescapeprobabilitycodeandcompare Table1.Positionsofthe12COJ=13-12observations.The(0,0)
theresultwiththeexpectationfromPDRmodels.Theoutcome position is α = 5h 41m 44.18s, δ = −1◦ 55′ 38.0′′ (J=2000,
oftheescapeprobabilitycalculationsareusedasinputofafive Mezgeretal.1988).
componentradiativetransfermodel(section4.2).Aninterpre-
tationofthemodelisgiveninsection5.
Position ∆α[′′] ∆δ[′′]
FIR5 0.0 0.0
2. Observations IRS3 3.3 16.0
#1 8.8 28.6
2.1.Observationsof12COJ= 13−12atAPEX
#2 13.8 41.9
IRS2 24.6 68.5
The 12CO 13-12(ν = 1496.922909GHz, Mu¨ller et al. 2001)
observations were carried out on November 22, 2005 at
the APEX 12 m telescope (Gu¨sten et al. 2006), located on
Llano de Chajnantor, Chile. We used the CO N+ Deuterium Observatorium fu¨r Submillimeter Astronomie (KOSMA), lo-
Observations Receiver (CONDOR, Wiedner et al. 2006), a catedonGornergrat,Switzerland.Thebeamsizes(HPBW)of
heterodyne receiver that operates at THz frequencies (1.25- theseobservationsare∼50′′and82′′at690GHzand345GHz,
1.53THz).The typicaldoublesideband(DSB) receivernoise respectively.Tomapthecentralpartofthemolecularcloudas-
temperature was between 1500 K and 1900 K. The mean sociatedwithNGC2024,weusedadual-channelSISreceiver
atmospheric transmission at zenith during the observations (Graf et al. 1998) with typical DSB receiver noise tempera-
was ∼ 0.2. As a backend, we used the APEX Fast Fourier turesof100and400K fortheJ=3-2andJ=6-5lines,respec-
TransformSpectrometer(FFTS),whichhasanintrinsicresolu- tively. As backends, we used two acousto optical spectrome-
tionof60kHz. ters(AOS).TheMediumResolutionSpectrometer(MRS)has
Theexpecteddiffractionlimitedmainbeamsizeofa12m abandwidthof0.3GHzandaresolutionof360kHz.TheLow
telescopeat1.5THzis4.3′′. Becausetheaccuracyofthepri- ResolutionSpectrometer(LRS)hasabandwidthof1GHzand
mary surface derived from planet observations at frequencies aresolutionof1150kHz.Theforwardefficiency,F = 0.93,
eff
between 350 GHz and 1500 GHz (with CONDOR) is 18 µm was determined by skydips. For the J=3-2 observations, we
(Gu¨sten et al. 2006), we expect some of the power to be di- used beam efficiencies (B ) of 0.59 and 0.62 for the 12CO
eff
rectedintoanerrorbeam.Weassumethattheerrorbeamisap- andthe13COline,respectively.FortheJ=6-5observations,the
proximately80′′,whichisderivedfromtheapproximatesizeof beamefficiencieswere0.40and0.48,respectively.
theindividualpanelsofthetelescope(∼70cm).Todetermine
the efficiency for the calibration to the main beam tempera-
2.3.Observationsofmid-JCOlineswithNANTEN2
ture (T ), we measured the continuum of Moon and Mars
MB
and compared the observed values with models. These mea- Simultaneous observations of 12CO 7-6 (ν = 806.65 GHz)
surements lead to beam efficiencies of 0.4 and 0.09-0.11 for and 12CO 4-3 (ν = 461.04 GHz) were carried out with the
theMoonandMars,respectively.Thedifferentbeamefficien- NANTEN2 4m telescope at Pampa La Bola, Chile. We ob-
ciesarisebecauseMarsisaboutone-fourththesizeoftheerror taineda2′×2′map,whichwascenteredonNGC2024IRS3.
beam (18.2′′ on the date of the observations), and thus these The observations were carried out in December 2007, using
observations suffer from beam dilution. A more detailed dis- the dual-channel 460/810 GHz test receiver, which had DSB
cussionofthebeamsizesandcouplingefficienciesisgivenin receiver temperatures of ∼ 750 K and ∼ 250 K for the up-
Volgenauetal.(2008)andWiedneretal.(2006). per andthe lowerchannel,respectively.Two AOS with band-
Since NGC 2024 is an extended source, clearly larger widthsof1GHzandchannelwidthsof∼ 560kHzwereused
than the error beam (Graf et al. 1993, Kramer et al. 1996), asbackends.Thebeamsizes(HPBW)oftheobservationswere
we use a beam efficiency of 0.4 for the calibration to T . 25′′ and37′′ forthe 12CO 7-6and12CO 4-3observations,re-
MB
Pointing and focusing were determined from observations of spectively.Thebeamefficienciesare0.5and0.45for460and
Mars,andweestimateapointingaccuracybetterthan7′′.We 810 GHz, respectively, and a forward efficiency of 0.86 was
observed 12CO J=13-12 towards five positions, along a line measuredatbothfrequencies.ThepositionofIRS3wastaken
from FIR 5 via IRS 3 to IRS 2. The coordinates of FIR 5 tobeα = 5h 41m 44.40s,δ = −1◦ 55′ 22.8′′ (J=2000,Mezger
are α = 5h 41m 44.18s, δ = −1◦ 55′ 38.0′′ (J=2000, Mezger et al. 1992). Pointing was checked on IRc2 in Orion A right
et al. 1988). The offsets of the other positions with respectto beforetheobservationsandisexpectedtohaveanaccuracyof
FIR5arelistedinTab.1. <7′′.
2.2.Observationsofmid-JCOlineswithKOSMA 3. ObservationalResults
In addition to the 12CO 13-12 transitions, we used archival, To be able to comparethe observedlines with each other,we
so far unpublished, maps of 12CO 6-5 (ν = 691.5 GHz), convolvedthemapsobservedwiththeKOSMAtelescopeand
12CO 3-2 (ν = 345.7 GHz), 13CO 6-5 (ν = 661.1 GHz) theNANTEN2telescopetoaspatialresolutionof80′′,whichis
and 13CO 3-2 (ν = 330.6 GHz). The observations were con- thespatialresolutionoftheJ=3-2spectraaswellastheapprox-
ductedbetweenJanuary27andFebruary21998attheKo¨lner imate size of the error beam of the 12CO 13-12 observations.
4 EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024
Since70%-80%oftheradiationisexpectedtocomefromthe Table2.Heretheintegratedintensities,v and∆vofthelines
LSR
errorbeamatthesehighfrequencies,weconsidered80′′asthe showninFig.2arelisted.Becauseofthecomplexlineshapes,
spatial resolutionof the 12CO 13-12observationsas well. An especially from the lower-J 12CO lines, the v and ∆v have
LSR
analysisofthemapsofthelower-Jlineswillbegiveninasub- been determinedby using the first and second momentof the
sequentpaper. spectra,respectively.
ThespectraareshowninFig.2andthelineparametersare
listed in Tab. 2. While most of the lines have a Gaussian line
profile (12CO 13-12, 13CO 6-5) or only relatively weak blue Position R Tmbdv VLSR ∆v
[Kkm/s] [km/s] [km/s]
shifted shoulders (12CO 7-6, 12CO 6-5, 13CO 3-2), the 12CO
12CO13−12
4-3 and 3-2 lines show complex line shapes with absorption
FIR5 120±10 12.23±0.3 3.34±0.5
notches,enhancedemissionfromtheredshiftedwing,andan
IRS3 109±10 12.75±0.3 3.92±0.5
additionalbumpat∼4.5km/s.
#1 127±10 13.50±0.3 4.51±0.5
The emission of the J=3-2 and J=6-5 transitions of 12CO
#2 57±10 13.17±0.3 3.54±0.5
and 13CO are fairly uniform in line intensity and shape at all
IRS2 <30 – –
fivepositions,andtheintegratedintensityoftheselinesdrops
12CO7−6
towards IRS 2 only by ∼ 30%. This uniformity is expected,
IRS3 229±10 11.5±0.2 4.5±0.5
sincetheseparationbetweenthepositionsofFIR5andIRS2
12CO6−5
isonly73′′,whichisontheorderofthespatialresolutionofthe
FIR5 288±10 12.0±0.15 7.5±0.5
observations.The12CO13-12linepeakstowardsthesouthern
IRS3 289±10 11.9±0.15 7.2±0.5
threepositions(FIR5,IRS3andposition#1)anddeclinesno-
#1 279±10 11.9±0.15 7.0±0.5
ticeablytowardsthenorth,indicatingthathigh-JCOemission #2 265±10 11.9±0.15 7.2±0.5
originatesfromarathercompactregionaroundIRS3. IRS2 235±10 11.8±0.15 7.5±0.5
Because of the different and complex line shapes of the 13CO6−5
12CO and 13CO lines, we give the centroid velocity of the FIR5 159±5 10.9±0.15 2.8±0.5
spectra in the following. The velocities of the 13CO lines are IRS3 157±5 10.9±0.15 2.7±0.5
∼ 11 km/s, whereas the velocity of 12CO 3-2 and 12CO 6-5 #1 149±5 10.8±0.15 2.6±0.5
is ∼ 12 km/s. The J=13-12 line of 12CO is found at a veloc- #2 138±5 10.7±0.15 2.5±0.5
ity of ∼ 13 km/s, although there is a variation in the veloc- IRS2 106±5 10.5±0.15 2.3±0.5
ity of about 1 km/s with position. The velocity of the 13CO 12CO4−3
linesis uniformthroughoutall five positions,whereasthe ve- IRS3 257.8±10 11.5±0.2 11.8±0.3
locityof12CO3-2and12CO6-5declinesfromsouthtonorth. 12CO3−2
Thisisduetoaredshiftedoutflowdetectedinthesetwolines, FIR5 213±10 11.9±0.15 10.2±0.2
whichisstrongestatthesouthernpositionsandreachesveloc- IRS3 202±10 11.9±0.15 11.3±0.2
ities up to ∼ 20 km/s. This outflow was detected previously #1 186±10 11.6±0.15 10.7±0.2
by Sanders& Willner (1985). The lack of a blueshifted wing #2 171±10 11.0±0.15 8.55±0.2
IRS2 155±10 10.7±0.15 6.52±0.2
in our spectra is consistent with previous observations (e.g.,
Richer 1990 & Richer et al. 1992). One explanationgiven by 13CO3−2
Richeretal.(1989)isthattheoutflowexistsinaverycomplex FIR5 146±5 10.9±0.1 3.36±0.2
IRS3 144±5 10.8±0.1 3.42±0.2
regionclosetotheionizationfrontoftheHIIregion.Northof
#1 136±5 10.8±0.1 3.34±0.2
its origin lies verydense gas seen in 1.3mm map (Mezgeret
#2 127±5 10.8±0.1 3.24±0.2
al.1992)andHCO+ (Richeretal.1989).Anymaterialejected
IRS2 109±5 10.7±0.1 2.93±0.2
bytheprotostarinthisdirectionmightberetardedbydensegas
and/ordestroyedbystrongUV-radiation.
The 12CO 13-12 data displayed in Fig. 2 are binned to a
resolutionof0.48km/s,whichissufficientlyhigh,becausethe ponent (24 K) is lower than the temperature of 67 K of the
line width of the J=13-12line is on the orderof 2.5 km/s. At background(Grafetal.1993).Thus,theforegroundcomponent
thisresolutionwedetectedthelineinfouroutoffivepositions addsanabsorptionfeaturecenteredat9.2km/stothe12CO6-
withanS/Nratio>3σ.AtthepositionofIRS2,wecanclaim 5lineemittedfromthebackgroundcomponent.Therefore,the
onlyatentativedetectionof7.5±3K. centroidvelocityofthespectraappearsredshiftedwithrespect
The 12CO 6-5 line is not symmetric, but shows a shoul- to the backgroundcomponent. In 13CO 6-5, no signs of self-
derat itslower velocityside. Its velocityof ∼ 12 km/sis ap- absorptionappear,andthusitscentroidvelocitycoincideswith
proximately 1 km/s redshifted with respect to the 13CO 6-5 thev ofthebackgroundcomponent.
LSR
line. The difference in velocity can be explained by the two- The profile of the 13CO 3-2 line looks similar to the
component model, suggested by previous studies (Barnes et 12CO6-5 line, but it does not appear redshifted and lies at a
al.1989,Grafetal.1993).Inthismodel,theforegroundcom- velocityof∼11km/s.Theshapeofthe12CO3-2linesisdom-
ponent (τ in Fig. 1) is assumed to be at a lower v than inatedbydeepabsorptionnotchesatvelocitiesof9.8km/sand
LSR
the background component (9.2 km/s and 11.1 km/s, respec- 11.9km/s,whicharecausedbymateriallocatedinfrontofthe
tively). Furthermore, the temperature of the foregroundcom- HII region.Thealternativescenario thatthe emission is com-
EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024 5
IRS 2 Pos. 2 Pos. 1 IRS 3 FIR 5
75
2
K] 50 -1
3
[MB25 O 1
T 0 C
2
1
75
K] 50 12CO 7-6 -5
6
[B25 O
M C
T 0 2
1
75
K] 50 -5
6
[B25 O
M C
T 0 3
1
75
K] 50 12CO 4-3 -2
3
[B25 O
M C
T 0 2
1
75
K] 50 -2
3
[B25 O
M C
T 0 3
1
5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20
V [km/s] V [km/s] V [km/s] V [km/s] V [km/s]
LSR LSR LSR LSR LSR
Fig.2. Spectra of CO emission frompointedobservationsin NGC 2024.The spectra are all convolvedto a resolutionof 80′′,
whichisapproximatelytheresolutionoftheobserved12CO 3-2andthe13CO 3-2spectraaswellastheresolutionoftheerror
beam of the 12CO 13-12 observations. The spectra are taken towards the positions (north to south left to right) as denoted in
Tab.1.Thegreyspectra,overlaidon12CO6-5and12CO3-2atISR3,arethe12CO7-6andthe12CO4-3spectra,respectively.
posed of three individual, equally strong (∼ 25 K), velocity velocityofthe12CO7-6(11.5km/s)lineisinbetweentheve-
shifted cloud components is unlikely, because these compo- locity of 12CO 6-5 (∼ 12 km/s) and 13CO 6-5 (∼ 11 km/s).
nentsarenotseeninmostoftheotherlines.Afourth,weaker If these velocity differencesare indeed due to foregroundab-
peak (∼ 3 K) can be seen at 4.6 km/s. 12CO 2-1 observations sorption,thevelocitydispersionofthe12CO7-6absorbingma-
(Krameretal.1996)revealedthatthiscomponentextendsfur- terial is significantlylower than the velocitydispersionof the
ther to the north-east and seems to be kinematically distinct material which absorbs 12CO 6-5. The minimum, most likely
fromthemaincomponent. caused by the foreground absorption, lies at 9.5 km/s, which
is0.3km/sblue-shiftedwithrespecttotheabsorptionnotches
Thespectraof12CO7-6and12CO4-3,convolvedtoares-
olutionof80′′,aresuperimposedonthe12CO6-5and12CO3- seenin12CO4-3and12CO3-2.
2 in Fig. 2, respectively. The 12CO 4-3 spectrum looks very
much like the one of 12CO 3-2. The two absorption notches,
Grafetal.(1993)observedNGC2024in12CO7-6aswell,
the4.6km/scomponent,andtheredshiftedoutflowcanbeseen
using the UKIRT telescope, which has a similar spatial reso-
in both spectra. Even the intensities are nearly identical. The lution. Their spectrumat FIR 5, 16′′ south of IRS 3, shows a
only difference is that the emission from the outflow, i.e., at
v of12.8km/s,clearlyredshiftedwithrespecttoourobser-
v >12km/s,isslightlystrongerin12CO4-3. LSR
LSR vations. However the frequency stability of the Laser system
The blueshiftedshoulderof the 12CO 6-5spectrum, inter- that was used as local oscillator in these early measurements
pretedasselfabsorption,appearssimilarbutmorepronounced was about1 MHz (∼ 0.4km/s).Furthermore,the UKIRT ob-
in12CO7-6.However,the12CO 7-6lineisnarrower,because servations were made in a double beam switch mode with a
theoutflowsignalisweakerthanin12CO6-5.12CO7-6seems chopthrowof 3′. Thustheir off-positionmighthave beennot
to suffer much more from absorption and what is seen as a completely clean, leading to a different apparent line shape.
shoulderin 12CO 6-5appearsasa secondpeakhere.The dif- Possible pointingerrorscannotbe the reasonfor differentve-
ferent strengths of the absorption of 12CO 7-6 and 12CO 6-5 locitiesreportedinGrafetal.(1993)andthiswork,becauseno
requiresa stronggradientin the excitationofthose lines. The bigvelocitygradientswithpositionareseeninNGC2024.
6 EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024
4. ModelingResults emissioncomponentsandself-absorptioncorrectly.Withsuch
aradiativetransfermodel,wefittedthelineshapesandinten-
The aim of this investigation is to show that the major fea-
sitiesofallseven12COand13COlinesobservedattheposition
tureofthecomplexshapesandintensitiesofallobserved12CO
ofIRS3andtheintegratedintensitiesof12CO withJ > 13,
and13COlinescanbeexplainedconsistentlywithaphysically up
simultaneously.The12COlinewithJ >13arethesamedata
plausiblescenario,whichisbasedonthe′′Blistermodel′′ and up
weusedintheprevioussection.Thesedatahavebeenobserved
thePDRscenario(seesection1).
atthepositionofFIR5,whichlies16′′southofIRS3,asepara-
tionmuchlowerthanthespatialresolutionofourobservations.
4.1.EscapeProbabilityCodeResults We used SimLine (Ossenkopf et al. 2001), a 1-dimensional,
spherical radiative transfer code. SimLine computes the pro-
In this section we examine the physical properties of the
filesofmolecularrotationallinesforanarbitrarydensity,tem-
high-J CO lines using an escape probability code (Stutzki &
peratureandvelocitydistribution,specifiedasasetofdiscrete
Winnewisser 1985). By comparing the column density of hot
layers, by integrating the radiative transfer equation numeri-
COrequiredtoexplaintheobservedhigh-JCOemissionwith
cally.Thepopulationoftheindividuallevelsofamoleculeare
thecolumndensityexpectedbyPDRmodelswe check,ifthe
computedbysolvingthefullsystemofbalanceequationsitera-
assumption of a PDR-scenario is consistent. Furthermore we
tively.Bysettingtheinnerradiusofthemodeltoavalue,which
usetheresultsofthismodelsasafirstguessforamoresophis-
ismuchlargerthanthebeamsizetimesthedistanceoftheob-
ticatedfivecomponentmodel(section4.2),whichreducesthe
servations,weusedSimLineinaquasiplane-parallelway.
freeparameterofthismodel.
In addition to the new 12CO 13-12 spectra of NGC 2024
presented in this work, the integrated CO intensities from 4.2.1. Constraintsonthemodels
J =14toJ =17observedwiththeISOsatellitetowardsFIR5
up up
(Gianninietal.2000)weretakenfromliterature.The12CO13- A close look at the data in Fig. 2 and Table 2 reveals an ap-
12 line does not show any sign of absorption or emission at parent contradiction in the line strengths at vLSR = 11 km/s.
∼9,5km/s.Thus,weassumethattheemissionof12CO13-12, The strong (∼ 50 K) 13CO 6-5 emission, which indicates a
aswellasoftheotherhigh-JCOlines,ispurelydeterminedby largecolumndensityofhotmoleculargas,beliestherelatively
the materiallocatedbehindthe HII region.Therefore,we can weak (< 15 K) 12CO 13-12 emission at the same velocity.
useanescapeprobabilitycode(Stutzki&Winnewisser1985) The possibility that the 12CO 13-12 line is absorbed by the
tomodeltheemissionof12CO J ≥ 13.Inthiscode,theemit- foreground gas can be ruled out, since the estimated column
ting gas is treated as an isothermal cloud with homogeneous density (NH2 = 2· 1022 cm−2, which corresponds to Av=20-
density.Foragivensetofkinetictemperatures(T ),H den- 25mag, Bik et al. 2003) is orders of magnitudes too low for
kin 2
sities(n(H )),and12COcolumndensitiespervelocityinterval the derived temperatures (∼ 25 K) to cause this absorption
2
(N(CO)/∆v), the emitted line strengths (main beam tempera- (Graf et al. 1993). Relatively strong 12CO 13-12 emission at
tures and integrated intensities), as well as the optical depths avLSR of∼ 13km/s,wherenootheropticallythinlineshows
(τ)atthelinecenterarecalculated. an emission peak, suggests an error in the frequency calibra-
Tomatchtheobservedintensitiesofthehigh-Jlines,a12CO tion. The origin of such an error is puzzling, given that par-
column density of 9.5±0.7·1016 cm−2 in a velocity interval ticularcarewastakentocheckcalibrationduringtheobserva-
∆v = 4 km/s, i.e., the width of the 12CO 13-12 line, is re- tions.ImmediatelypriortoobservingNGC2024,weobserved
quired. This column density is in good agreement with PDR Orion FIR 4 to check telescope pointing, and the 12CO 13-
models(seesection1).Constraintsonthegastemperatureand 12 emission had the expected velocity (Wiedner et al. 2006,
H -density, however, are looser; many solutions are possible. Kawamura 2002, Wilson et al. 2001). The uniformity of the
2
The two best fit solutions (both with a χ2 of 2.9) yield a H2- vLSR throughoutthe 13CO 6-5 map also diminishes the likeli-
density of 1.3 · 106 cm−3 and a temperature of 250 K and hoodthattheanomalous12CO13-12velocityisduetoapoint-
n =3·105cm−3andT=410K,respectively.Allmodelswith ingerror.Moreover,allsoftware-setvalues(e.g.,restfrequency
H2
aχ2lowerthan10haveincommonthatT3·nisapproximately and sky frequencyof the line) were checkedcarefully,but no
constant. errorwasfound.Despitethisscrutinyweareforcedtoassume
Ifweassumethatthegasisdistributeduniformlythrough- thatthevLSR of12CO13-12and13CO6-5areboth10.9km/s,
outthe(80′′)beam(andtheareafillingfactorisone),thenthis togetareasonablesolutionforourmodel.Thisassumptionis
layerofthePDRhasathicknessof5·1014to1.5·1015cm(= buttressed by unpublished high-J CO observations (Marrone,
33-100AU). At a distance of 415 pc, 100 AU correspondsto priv.comm.),whichalsoindicateavLSRof∼11km/s.
0.25′′,whichindicatesthatthishotcomponentisindeedathin In addition to the previoustwo componentmodels we in-
layeratthe,possiblyclumpy,surfaceofthemolecularcloud. troduceda hot, dense, and thin gas component,located at the
interfaceoftheHIIregionandtheDMC(Fig.1).Anindepen-
dentestimationofthephysicalconditionsofthishotmaterial,
4.2.FullRadiativeTransferModel
which is illuminated by OB stars within the HII region, has
To get a more completepicture of the source, which explains beenderivedwiththeescapeprobabilitycode.Inthecaseofthe
also the complex line shapes of the lower-J lines we have to foregroundmaterial,wehavetoassumeseveralcomponentsas
useamultilayerradiativetransfermodel,whichtreatsmultiple well.First,wehavetoadopttwocomponentsofcoldgasatdif-
EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024 7
Table 3. Physical properties of a 5-componentradiative transfer model of NGC 2024. Properties with a range of values vary
continuouslywithdepth.
Component T[K] n(H )[cm−3] r[cm] N(H )[cm−2] v [km/s] ∆v [km/s]
2 2 LSR turb
B1 75 9·105 8.0·1016 7.2·1022 11.0 1.8
B2 75-330 9·105-2·106 6.2·1013 8.9·1019 11.0 1.8
F1 330-40 1·105 8.9·1014 8.9·1019 9.3 1.3
F2 40-30 1·105−3·104 1.7·1017 1.04·1022 9.3-9.7 1.3-1.1
F3 30-15 5·104 5.9·1015 2.9·1020 12.1 1.8
ferentvelocities,asboththe12CO4-3andthe12CO3-2spectra as mentioned above. The v of F2 shows a gradient from
LSR
showtwoabsorptionnotches.Inaddition,itisreasonablethat 9.3km/sto9.7km/stowardstheobserver,andthev ofF3
LSR
materiallocated at the interfaceof the foregroundcomponent is 12 km/s. The total column density of the foregroundcom-
andtheionizedgasisheatedaswell,andthusweintroduceda ponent (F1+F2+F3;N(H ) = 1.08·1022 cm−2) correspond
2 tot
layerofhotgasonthissideoftheHIIregion,too.Furthermore, to an optical extinction of ∼ 11mag, which is slightly lower
the totalcolumndensityofall componentslocatedin frontof than the 15 to 25mag found by Bik et al. (2003). To convert
theHIIregionisrestrictedbythemeasurementsofAvtowards thecolumndensityintoA weusedtherelationN [cm−2] =
v H2
theobscuredstars,whichtranslatesintoatotalH columnden- 0.94×1021A (mag).However,sincethelightofsomeofthese
2 v
sityof2·1022cm−2. starsmightbe extinctedbydustin the vicinityof thestar,the
lowervalueofA isprobablymoreappropriate.
v
4.2.2. BestFitModel:Reproducingtheobservedline
profiles
The result of the modeling is listed in Tab. 3. As mentioned
above,fivecomponents(threeforegroundandtwobackground
components)wereappropriatetogetareasonablesolutionfor ]1e-07
1
theobservedspectra.Eachthesefivecomponentsisdescribed 2-s
-m
by5parameters(T,n(H2),r,vLSR and∆v)andthusthemodel W
contains 25 parameters. However, since the hot foreground W [
component(F1)isdeterminedbythetemperatureandcolumn
densityofthehotbackgroundcomponent(B2)andthedensity,
v and∆vofthebulkoftheforegroundmaterial(F2),these 1e-08
LSR
fiveparametersarenotfree.Inaddition,wesetv and∆vof
LSR
the backgroundcomponents(B1 and B2) to the same values,
0 5 10 15 20
and thus the number of free parametersis 18. But even these J
up
18 parameter are not fully free. The total column density of
the foregroundcomponents(F1+F2+F3)) must correspondto Fig.3. Integrated line intensity of 12CO versus Jup. The ob-
an A of 10-25mag to match the IR-observation, the tempera- served data (full circles) include all 12CO lines from this
v
tureandthecolumndensityofthehotbackgroundcomponent workandobservationswiththeISOsatellitefromGianniniet
al.(2000).Thesolidlineshowstheresultsofthefullradiative
(B2)hastobeconsistentwiththepredictionofthePDRmod-
els, andthe velocitydifferencebetweenthe backgroundcom- transfercodeSimLine
ponentsandthe bulkof the foregroundmaterialmustbe neg-
ative and in the range of a few km/s in order to be explained
The complex shapes of the emission lines, especially the
by the blister model.Thereforethe numberof effectivelyfree
deepabsorptionnotchesin12CO3-2,requiredepth-dependent
parametersis∼15.
gradientsin the foregroundsubcomponents.F2 containsmost
Thebackgroundmaterialwasdividedintotwosubcompo- of the foreground material (> 96 % of the mass). It consists
nents,denotedbyB1andB2.B1consistsofwarm(T= 75K) ofa1.54·1017 cmdeep,40Kwarmgaslayer,whosedensity
anddensegas(n(H2)= 9·105 cm−3)withahighcolumnden- decreaseslinearlyfrom105cm−3to3·104cm−3.Subsequently
sity(N(H2) = 7.2·1022 cm−2).B2isathinlayerwithasteep the F2 gas density stays constant, but the temperature drops
temperature and density gradient. At the surface of the cloud to 30 K within the next 1.4·1016 cm. Simultaneously to the
the temperature reaches 330 K and the density is as high as temperaturedecrease,thev shiftsfrom9.3km/sto9.7km/s.
LSR
a2n·d10a6FcWm−H3M.BoofththBe1tuanrbduBle2nthvaveleoacivtyeloofci∆tyvotufrbv=LSR1.8=k1m1/ksm. /s Fw3ithisaaccoonmstpaanrtatdivenelsyitythionf(5N·H120=4 c2m.9−·31an0d20acmte−m2p)ecroamtupreonthenatt
Theforegroundcomponentwasdividedintothreesubcom- dropsfrom 30 K to 15 K. The v of F3 is 12.2 km/s. F3 is
LSR
ponents,F1,F2,andF3,toexplainthetwoabsorptiondipsin responsibleforoneofthetwoabsorptiondipsin12CO3-2,as
the 12CO 4-3 and 12CO 3-2 spectra and for physical reasons, wellastheabsorptionofthe12CO6-5intensity.
8 EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024
TheF1componentrepresentstheradiationheatedcounter-
cpoamrtpoofnBe2notdnothesennoeatrcshidanegoeftthheeHmIoIdreelgeidonli.nTeheimsaisdsdioitnioansallognags K] 40 12CO 13-12
[
astheH2 columndensityislowerthan4.6·1020 cm−2,i.e.as mb 20
longas itscolumndensityis smaller than5 times the column T 0
density of B2. Because the existence of such a componentis
physically reasonable,we assumed that the interface between
thebulkof theforegroundmaterial(F2)andtheHIIregionis ] 60 12CO 7-6
heatedsimilar to the interfaceatthe background,i.e., column K
density and maximum temperature of F1 and B2 are similar. [ 40
b
m
DensityandvelocityofF1arethesameasfoundforF2. T 20
ThegeometricarrangementofthecomponentsB1,B2,F1,
and F2 is well defined by our modeling and the temperature- 0
and density trend of these components are shown in Fig. 5. 60
12
] CO 6-5
However, the location of F3 is not clear, apart from residing K
40
[
infrontoftheHIIregion.Thefactthatthiscomponentiscold
b
andnotblueshiftedwithrespecttothebackgroundcomponent m 20
T
indicatesthatthiscomponentmightbeclosesttotheobserver.
Acomparisonbetweentheobservedspectraandthemodel 0
resultsisshowninFig.4.Giventheknowncomplexityofthe
source and the limits of a five-component model, the model K] 40 13CO 6-5
spectra fit the observations quite well. Two spectral features [

b
that were not includedin the modelfit are the redshiftedout- m 20
T
flow,whichcanbeseenin12CO6-5,12CO4-3and12CO3-2,
andthecomponentatv ∼4.5km/s.Nevertheless,themain 0
LSR
features of the five emission lines can be explained with our
40 12
rathersimpleassumptions. K] CO 4-3
InFig.3weshowtheintegratedintensityofthe12COlines [

versus Jup. Especially for the low- and mid-J CO lines the mb 20
model matches the observations quite well. At J > 10 the T
up
scatteroftheobserveddataincreases,possiblyduetoobserva- 0
tionaluncertainties.
] 12
K CO 3-2
5. Physicalinterpretationanddiscussionofthe [ 20
b
modelresults m
T
The model discussed above, successfully explaining the ob-
0
servedlinesincludingtheprofilesofthevelocityresolvedlines,
is the simplest model scenario satisfying the constraints im- K] 40 13CO 3-2
posedbytheblisterandPDRpicture.
[
The B1 component is equivalent to the DMC in Fig. 1. b
m 20
Because the properties of B1 are mostly based on the fits of T
lines with J < 5, only the warm gas of the DMC is repre-
up 0
sented. Cold gas, which is deeper inside of the DMC, would
5 10 15
have no effect on the modeled lines, either because the up-
per levels are not populated ( 12CO 13-12, 13CO 6-5), or the v [km/s]
LSR
linesarealreadyopticallythick(12CO7-6and12CO6-5).Thus,
thecolumndensityofB1 isjustthelowerlimitfortheDMC. Fig.4. Observed(black)and model(red)spectra of 12CO 13-
ObservationsofC18O 2-1andC17O 2-1(Grafetal.1993) re- 12,12CO6-5,13CO6-5,12CO3-2and13CO3-2.Theobserved
vealed a three times lager H2 column density than what we spectrumofthe12CO13-12linewasshiftedby-2.2km/s(see
foundhere.TheB2componentrepresentsthethin,hotinterface text).
betweentheHII-regionandthemoleculargas.Duetothehigh
UV radiationemittedbytheionizingsource(s)ofNGC2024,
thegasattheinterfaceisstronglyheated,whichmayresultin here,butitrepresentsthe counterpartofB2 atthe foreground
temperaturesupto330K. componentalbeitatlower density.Forthe othertwo subcom-
The three subcomponents F1, F2, and F3 together repre- ponents located in frontof the HII region, F2 and F3, we as-
senttheforegroundmaterial,τinFig.1.TheF1subcomponent sumethatmatterisdistributeduniformlyacrosstheareaofthe
is, as mentioned above, not traced by the lines we observed beam.Interstellarmatter,however,showsclumpystructureon
EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024 9
densities are solved self-consistently (Ossenkopf et al. 2001).
Inthepreviousstudy,anH columndensityofthebackground
2
componentof2.0·1023 cm−3 wasfound,whichisthreetimes
morethanwhatwefoundhere.Thisdifferencearisesbecause
they included rarer isotopes, e.g., C18O and C17O, which are
opticallythinandtracemateriallocateddeepinsidethecloud.
Most of this materialis hiddendue to the high opticaldepths
inthe12COand13COlinesweobserved.Thetemperaturede-
termined by Graf et al. (1993), 67.4 K, is in good agreement
with the 75 K determined in our work. Also the kinematic
parameters Graf et al. (1993) report (v = 11.1 km/s and
LSR
∆v = 1.8 km/s) match ourresults. Since they do notobserve
highrotationaltransitionsof CO (nolineswithJ > 7),they
up
Fig.5. Temperature and density structure of NGC 2024. The
havenoinformationaboutthehotinterfaceregion.
observeris locatedto the right.The componentF3 resides on
Giannini et al. (2000) give a CO column density of 2 −
the near side of the HII region, but its exact position is not 5·1018 cm−2, which correspondsto an H column density of
2
clear and it might be physically disconnected from the other 2.1 − 5.3 · 1022 cm−2. This column density, which is about
components.
50%lowerthanthetotalcolumndensitywefound(N =
H2,total
8.3·1022cm−2),wasderivedusinganescapeprobabilitycode.
The fact that Giannini et al. fitted only a single, isothermal
all scales. In NGC 2024, substructures on scales down to the gascomponentmightexplainthedifferentresults.Inparticular
resolution of previous observations were detected (Kramer et
thebackgroundmaterialwillbeunderestimatedinsuchanap-
al.1996,Ladaetal.1991,seeSection1).Therefore,weexpect proachdue to self absorptioneffects. The kinetic temperature
aclumpystructurewithinthebeamaswell,whichwouldcause
they give(110-130K) is in the rangeof the valueswe found,
amuchmorecomplexstructureoftheforeground.Suchsmall
butbecauseoftheirisothermalapproachhardtocomparewith
scale structures must be taken into account for more detailed
ourresults.
modeling. For example, dense, cold clumps, smaller than the
For the foregroundcomponents, we found a total H col-
2
FWHM of the beam, which mainly affect the 12CO 3-2 line, umn density of 1.07·1022 cm−2, which correspondsto an A
v
butduetotheirlowtemperatureleavethe12CO6-5linerather
of 11.4. This is lower than the visual extinction of 15 to 25
unaffected,areapossibleexplanationforthenarrowabsorption
foundtowardsseveralstellarobjectsinsidetheHIIregion(Bik
notchesobservedin12CO3-2and4-3,whereasthelineshape
etal.2003).However,duetothelow spatialresolutionofour
of 12CO 6-5 is much smoother. This might be especially true
observations,thebeam-averagedcolumndensitymightbewell
forthethinandcoldcomponentF3.However,observationsat
lower than the values obtained from individual stars. The H
2
higherspatialresolutionwouldbenecessarytoverifythishy-
columndensityoftheforegroundcomponentgivenbyGrafet
pothesis.
al. (1993) is a factor of two larger than our value. They also
Thevelocitydifferencebetweenthebackgroundgasandthe foundatemperatureof23.5K,i.e.,slightlylowerthanourre-
bulkoftheforegroundmaterialis1.5km/s.Assumingthatthe sults (30-40 K), which might be caused by the fact that they
velocitydifferenceiscausedbythepressureoftheionizedgas, assumedLTEconditions.
accordingtothe′′Blistermodel′′thisvelocityshiftcorresponds
to a totalmomentumper unitarea of5670gcm−1 s−1, which
6. Conclusion
hastobetransferredtotheF2component.Wecalculatedthein-
ternalpressureoftheionizedgas,whichpushestheforeground Wepresenttheobservationsofseven12COand13COlinesfrom
component,usingmeasuredelectrontemperature(8160K)and J = 3 to J = 13 towards NGC 2024, a well know mas-
up up
electrondensity of ∼ 103 cm−3 (Odegard1985). Such a pres- sive star forming region in Orion. The shapes of these lines
sure has to act for approximately 2 · 105 years to accelerate rangefromalmostGaussian(13CO6-5,12CO13-12)tohighly
theforegroundcomponenttoitscurrentvelocity.However,the complex,multiplepeaked(12CO 3-2, 12CO 4-3),whichindi-
mass of the F1 componentis highly unknown,and Te and ne cates a complex internalstructure of NGC 2024. In our anal-
may have changed in the past, so that this simple calculation yses we also included the integrated intensities of 12CO lines
maybesomewhatdebatable,butitshowsthattheobservedve- withJ ≥14(Gianninietal.2000).
up
locitystructurecanbeindeedexplainedbyablistermodel. We modeled the high-J CO lines, which all seem to be
optically thin, using an escape probability code (Stutzki &
Winnewisser1985)andfoundthattheselinesareemittedfrom
5.1.Comparisonwithothermodels
ahot(> 250K),dense(> 3·105 cm−3),andthin(∼ 100AU)
Thepropertiesofthebackgroundcomponentsagreefairlywell layer, which is most likely located at the interface of the HII
with the results obtained by Graf et al. (1993). The main dif- regionandthemolecularcloud.
ference between their model and the present one is that Graf WeconstructedamodelforNGC2024using1Dradiative
etal.(1993)assumedLTEconditions,whereasintheSimLine transfer code SimLine (Ossenkopf et al. 2001). The velocity
code,thebalanceequationsforalllevelpopulationsandenergy structureofthismodelisbasedontheprinciplesoftheBlister
10 EmprechtingerM.etal.:TheMolecularEnvironmentofNGC2024
modelandthetemperaturesandcolumndensitiesofthecom- Churchwell,E.,Smith,L.F.,Mathis,J.,etal.1978,A&A,70,719
ponentsare constrainedby the PDR scenario.Thismodelex- Crutcher,R.M.,Henkel,C.,Wilson,T.L.,etal.1986,ApJ,307,302
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well.Inourmodel,thebulkofthemoleculargasresidesatthe Giannakopoulou, J,Mitchell,G.F.,Hasegawa, T.I.,etal.1997, ApJ,
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Graf,U.U.,Genzel,R.,Harris,A.I.,etal.1990,ApJ,358,49
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Acknowledgements. Thispublicationisbased ondataacquired with
mitted
theAtacamaPathfinderExperiment(APEX).APEXisacollaboration
Wiedner,M.C.,Wieching,G.,Bielau,F.,etal.2006,A&A,454,33
between the Max-Planck-Institut fr Radioastronomie, the European
Wilson,T.L.,Muders,D.,Kramer,C.,etal.2001,ApJ,557,240
SouthernObservatory,andtheOnsalaSpaceObservatory.
Zuckerman,B.1973,ApJ,183,863
TheKOSMA3mobservatoryisadministratedbytheInternational
Foundation Gornergrat &Jungfraujoch. Theuniversities ofCologne
and Bonn are operating jointly the KOSMA 3m telescope and, to-
gether with the university of Nagoya, the NANTEN2 observatory.
NANTEN2 is financially supported in part by a Grant-in-Aid for
ScientificResearchfromtheMinistryofEducation, Culture,Sports,
ScienceandTechnologyofJapan(No.15071203)andfromJSPS(No.
14102003andNo.18684003),andbytheJSPScore-to-coreprogram
(No.17004)andbyspecialfundingfromtheLandNRW.
The CONDOR receiver was built by the Nachwuchsgruppe of
the Sonderforschungsbereich 494, which is funded by the Deutsche
Forschungsgemeinschaft(DFG).
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