Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

1.5 MB·English

by  Stephan R. McCandliss

ACCEPTEDFORPUBLICATIONINTHEASTROPHYSICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
MOLECULARANDATOMICEXCITATIONSTRATIFICATIONINTHEOUTFLOWOFTHEPLANETARYNEBULA
M27
STEPHANR.MCCANDLISS1,KEVINFRANCE2,ROXANAE.LUPU1,ERICB.BURGH3,KENNETHSEMBACH4,JEFFREYKRUK1,B.-G.
ANDERSSON1,ANDPAULD.FELDMAN1
Received2006November10;accepted2007January5
ABSTRACT
HighresolutionspectroscopicobservationswithFUSEandHSTSTISofatomicandmolecularvelocitystrat-
ificationin thenebularoutflowofM27challengemodelsfortheabundancekinematicsin planetarynebulae.
Thesimple pictureof a veryhighspeed (∼ 1000km s−1), highionization,radiationdrivenstellar windsur-
7
roundedbyaslower(∼10kms−1)mostlymolecularoutflow,withlowionizationandneutralatomicspecies
0
residing at the wind interactioninterface, is not supportedby the M27 data. We find no evidencefor a high
0
2 speedradiationdrivenwind.Insteadthereisafast(33–65kms−1)lowionizationzone,surroundingaslower
(. 33 km s−1) high ionization zone and, at the transition velocity (33 km s−1), vibrationally excited H2 is
n
intermixedwithapredominatelyneutralatomicmedium.ThegroundstateH ro-vibrationalpopulationshows
a detectableabsorptionfromJ′′.15andv′′ .3. Far-UVcontinuumfluoresce2nceofH isnotdetected,butLy-
J 2
mana (Lya )fluorescenceispresent. Wealsofindthediffusenebularmediumtobeinhospitabletomolecules
5
1 and dust. Maintaining the modest equilibrium abundance of H2 (NN((HH2I)) ≪ 1) in the diffuse nebular medium
requires a source of H , mostly likely the clumpy nebular medium. The stellar spectral energy distribution
2
1 showsnosignsofreddening(E(B−V)<0.01),butparadoxicallymeasurementsofHa /Hb reddeningfoundin
v theliterature,andverifiedhereusingtheAPODIS,indicateE(B−V)∼0.1. Wearguetheapparentenhance-
9 mentofHa /Hb intheabsenceofdustmayresultfromatwostepprocessofH ionizationbyLymancontinuum
2
3 (Lyc)photonsfollowedbydissociativerecombination(H +g →H++e−→H(1s)+H(nl)),whichultimately
4 producesfluorescenceofHa andLya . Intheopticallyth2inlimitat2theinferredradiusofthevelocitytransition
1
we find dissociation of H by stellar Lyc photons is an order of magnitude more efficient than spontaneous
0 2
7 dissociationbyfar-UVphotons. WesuggestthattheimportanceofthisH2 destructionprocessinHIIregions
hasbeenoverlooked.
0
/ Subjectheadings:atomicprocesses—ISM:abundances—(ISM:)dust,extinction—(ISM:)planetarynebu-
h lae: general—(ISM:)planetarynebulae: individual(NGC6853)—line: identification—
p
line: profiles—molecularprocesses—plasmas—(stars:) circumstellarmatter—(stars:)
-
o whitedwarfs—ultraviolet:ISM—ultraviolet:stars
r
t
s 1. INTRODUCTION of10,000K,weestimateane-foldlifetime≈230years,as-
a
suming electron impact dissociation of H at the rate given
: M27 (NGC 6853, the Dumbbell) exhibits a bi-polar mor- 2
v by Martinetal. (1998, g = 4.6 × 10−13 cm3 s−1). Nev-
phology, often associated with molecular hydrogen (H ) in- e−
i 2 ertheless there is a whole class of PNe with bi-polar mor-
X frared emission in planetary nebulae (PNe) (Kastneretal.
phologyin which infrared H emission is the defining char-
1996). High resolution spectra of the hot central star (CS), 2
r
acteristic (Kastneretal. 1996; Zuckerman&Gatley 1988).
a acquired with the Far Ultraviolet Spectroscopic Explorer
Zuckerman&GatleyconcludedthattheinfraredH emission
(FUSE), have revealed an unusually rich set of narrow H 2
2
inM27isshockexcited,basedonIRspectroscopicdiagnos-
absorptionfeaturesspanningtheentirespectralbandpass,an
tics. This result is considered somewhat surprising because
indication that the molecule is vibrationally excited. FUSE
the close presence of a hotstar suggeststhat far-UV contin-
carries no on-board source for wavelength calibrations and
uumexcitedfluorescenceoftheH maybeimportant.Forin-
consequentlyM27hasbeenobservednumeroustimesforthis 2
stance, Natta&Hollenbach (1998) calculate that continuum
purpose,resultinginahighsignal-to-noisedataset(seeFig-
pumped fluorescence processes dominate thermal processes
ure1andMcCandliss&Kruk2007). Theusefulnessofthese
inPNeevolutionarymodelsatlatetimes&5,000yrs.
linesforwavelengthcalibrationaside,wearepresentedwith
Herald&Bianchi(2002,2004)andDinersteinetal.(2004)
an interesting puzzle. What physical processes excite H in
2
havereportedexcitedH absorptionfeaturesinFUSEspectra
PNe and how does it survive in this high temperature and 2
ofahandfulofCSPNlocatedintheGalaxyandLargeMagel-
highlyionizedenvironment?
lanicCloud.Dinersteinetal.(2004)andSterlingetal.(2005)
AtfirstglanceitseemssurprisingtofindH inPNeatall.
2
For a typical electron density of 300 cm−3 and temperature haveusedFUSEtoexaminePNewithstrong,extendedH2in-
frared(IR)emission. TheyfindthedetectionofIRemission
Electronicaddress:[email protected] isnoguaranteeforfindingexcitedH2 inabsorptionandcon-
1DepartmentofPhysicsandAstronomy,TheJohnsHopkinsUniversity, cludethemolecularmaterialinthesesystemsisclumped.
Baltimore,MD21218. Many authors have discussed the clumped structures and
2CanadianInstituteforTheoreticalAstrophysics,UniversityofToronto,
dense knots observed in PNe, along with evidence for their
TorontoONM5S3H8
3SpaceAstronomyLaboratory,UniversityofWisconsin-Madison,1150 origin and chemical composition (c.f. O’Delletal. 2002,
UniversityAvenue,Madison,WI53706 2003; Hugginsetal. 2002; Bachilleretal. 2000; Coxetal.
4SpaceTelescopeScienceInstitute,Baltimore,MD21218. 1998; Meaburn&Lopez 1993; Reay&Atherton 1985, and
2 McCandlissetal.
6
LiF1a
4 LiF1b
2
0
−1) 6 SiC1b SiC 1a
Å
1 4
−
s
2 2
−
m
c 0
s
g
er 6
LiF2b
2
−1 4 LiF2a
0
1
x 2
(
x
u 0
l
F
0246 SiC2a SiC 2b
900 950 1000 1050 1100 1150 1200
Wavelength (Å)
FIG.1.—FluxoftheorbitalcoaddsfortheeightspectralsegmentstakenthroughtheHIRSspectrographaperture.Individualsegmentsarelabeled.Overplotted
isthemodelSEDwithoutextinction(upperline)andwithE(B−V)=0.005mag(slightlylowerline)andE(B−V)=0.05(lowestline)forastandardgalactic
extinctioncurve.E(B−V)=0.005magprovidesthebestmatchtotheSEDintheSiCchannels.
referencestherein). Thesestructuresareisolatedphotodisso- aryhistoryoftheclumps,whichshoulddependonwhenand
ciationregions(PDR)immersedinHIIregions,anditisnat- where they formed during the AGB → PN transition. The
uralto expectthem to be reservoirsof molecularmaterialin expectation is for dense clumps formed within an AGB at-
PNe, regardlessof the mechanismthat causesthem to form. mosphere to have more complex molecules, due to higher
Capriotti (1973) studied the dynamical evolution of a radia- dustextinctionandmolecularshielding,thanclumpsformed
tionboundedPNandfoundthatdenseneutralglobulesform laterinthePNphasewhenthestellarradiationfieldisharder
astheresultofagraduallyweakingradiationfieldinanout- andthe overalldensity lower. The abundanceof atomic and
wardpropagatingionizationfront.ARayleigh-Taylorlikein- molecularspecieswithrespecttoH isoffundamentalimpor-
2
stability develops and eventually an optically thick spike of tance to assessing the predictionsforthe chemicalevolution
neutralmaterialbecomesseparatedfromthefrontandforms inPNeclumps.Observationsofatomicandmolecularexcita-
ahighdensityglobulewheremoleculescanpresumablyform. tionstratificationintheoutflowprovideafossilrecordofthe
The formation of H at later stages, on dust grains and in AGB → PN transition containingclues to the origins of the
2
thehighelectrondensityenvironmentviatheH+H− →H clumps.
2
+ e− reaction, has been modeled by Aleman&Gruenwald Towardsthisendweexplorehighresolutionfar-ultraviolet
(2004)andNatta&Hollenbach(1998). Williams(1999)has absorption spectroscopy, provided by FUSE and the Space
discussed a shadowing instability, somewhat similar to that TelescopeImagingSpectrograph(STIS)onboardtheHubble
of Capriotti (1973), where small inhomogeneities perturb a Space Telescope (HST), to reveal the nebular outflow kine-
passingsupersonicionizationfront,causingacorrugationthat matics imprinted on the line profiles of a wide variety of
producesalargeneutraldensitycontrastfurtherdownstream. molecular and atomic species, including H2, CO, HI, CI -
Dysonetal.(1989)suggestedmolecularknotsinPNarerelic IV,NI-III,OI,OVI,SiII-IV,PII-V,SII-IV,ArI-IIand
SiO maser spots, which originate in the atmosphere of the FeII-III.Absorptionspectroscopysamplesmaterialdirectly
asymptoticgiantbranch(AGB)progenitor. Othershavepro- along the line-of-sight. We will sometimes refer to it as the
posedthatmoleculesmayalsooriginatefromrelicplanetary diffuse nebular medium to distinguish it from the extended
material,eitheraccretedandthenejectedorsweptupduring clumpy medium, offset from the direct line-of-sight, which
the AGB phase preceding the formation of the nebula (c.f. undoubtedlydiffersinphysicalandchemicalcomposition.
Wesson&Liu 2004; Rybicki&Denis 2001; Siess&Livio OurmainobjectiveistoquantifytheH excitationstate,its
2
1999; Livio&Soker 1983). Soker (1999) has discussed the abundancewith respect to HI, and its outflow velocity with
signaturesofsurvivingUranus/Neptune-likeplanetsinPNe. respect to other emitting and absorbing atomic and molecu-
Redmanetal. (2003) have emphasized the importance of lar species. We use thisinformation,alongwith the spectral
molecularobservationsasameanstoestablishtheevolution- energydistribution(SED)ofthecentralstar,toconstrainH
2
HotMolecularHydrogeninM27 3
0.3±0.06andastellarradiusR =0.055±0.02R . Theyfind
∗ ⊙
a bolometric magnitude M∗ = –1.67±0.37, which yields a
bol
3.0 luminosity for the central star of L =366 L . The radial
[N II] l 6584 velocityofthecentralstarinthehel∗iocentrics⊙ystemwasde-
terminedbyWilson(1953)tobeV =–42±6kms−1.
sys
2.5 Barker(1984)hasreportedontheanalysisoflineemission
from the nebula as derived from both ground based and In-
s
e ternationalUltravioletExplorer(IUE)spectra. Theemission
l
fi is typical of an HII region with an electron temperature of
ro 2.0 ≈10,000K, an electrondensityof≈ 300± 100cm−3, and
P [O III] l 5007
d an elevated metallicity with respect to solar in CNO. These
e parametersagreewell with those foundbyHawley&Miller
z
li 1.5 (1978)inasimilargroundbasedstudyofthenebula. Barker
a
m (1984)adoptsalinereddeningparameterofc=0.17(cisde-
or finedin§4.2.1). Theliteraturerevealsarangeofnebularline
N reddeningparameters,0.03≤c≤0.18(c.f.Miller1973;Kaler
k 1.0 1976; Cahn 1976; Barker 1984; Ciardulloetal. 1999). This
a H
e a translatesto0.02≤E(B−V)≤0.12,(usingc/E(B−V)=1.5,
P
Ciardulloetal.1999,Figure4).Pottaschetal.(1977)derived
0.5 E(B−V) = 0.10 ± 0.04 by removing a small (some would
sayimperceptible)2175A˚ bumpfromanIUEspectrum.This
valueis nearlythe same asthatgivenbyHarrisetal. (1997)
0.0 and Benedictetal. (2003), based on trigonometric distance
determinationsandabsolutevisualmagnitudeconsiderations.
−150 −100 −50 0 50
Cahnetal.(1992)reportc =0.04derivedfromtheratioof5
V (km s−1) GHzradiofluxdensitytoHrb absoluteflux.
hel
Velocity and position resolved emission line spectroscopy
FIG. 2.— NormalizedintensityprofilesofHa ,[OIII]l 5007and[NII] byGoudisetal.(1978)showedfaint[OI]and[NII]profiles
l 6584asafunctionofvelocity. Ha and[NII]l 6584weredigitizedfrom (beam size of 83′′) taken near the CS to be double peaked
Meaburn(2005)and[OIII]l 5007wasdigitizedfromMeaburnetal.(1992).
symmetrically around the radial velocity of the system. In
formationanddestructionprocessesinthenebula. theouterregionstheprofilesconvergedtosinglepeaksatthe
The analysis is supported by a variety of ancillary data systemicvelocity. Thebright[OIII]lines(beamsizeof30′′)
on M27. Longslit far-ultraviolet spectroscopy from a showedsimilarbehaviorbutwithalowerdoublepeakedsplit-
JHU/NASA soundingrocket(36.136UG)providesanupper ting. These observationsare consistent with the presence of
limittotheH2continuumfluorescence.Longslitopticalspec- two nested shells expanding with projected velocities of ≈
trophotometrywith the Double ImagingSpectrograph(DIS) 33and15kms−1 respectively. Meaburnetal.(1992)useda
at Apache Point Observatory(APO) yields the absolute flux fiberopticimagedissectorfedintoanechellespectrographto
oftheCSandthesurroundingnebularBalmerlineemission. closelyexaminetheOIIIvelocitystructureintheimmediate
Dwingeloo Survey 21 cm data helps constrain the velocity vicinityoftheCS(≈36′′North-South),andfoundquadruple
structureofHI.Webeginbyreviewingpreviousobservations peakedline profilesconsistentwith gasexpandingat veloci-
and physical properties of the nebula followed by a discus- ties of 31 km s−1 and 12 km s−1. Meaburn (2005) showed
sion of the various data sets. We then present the analysis, that Ha exhibits a filled-in asymmetric profile with a half-
discussion,conclusionsandsuggestionsforfuturework. widthhalf-maxiumof≈37kms−1,incontrasttothedouble
peakedemission exhibitedby [NII] l 6584in thevicinity of
2. PHYSICALPROPERTIESANDPREVIOUSOBSERVATIONSOF the CS. HeII l 6560 shows a much narrower profile and is
M27
consistentwith turbulentbroadeningalone. Examplesofthe
M27hasanellipticalshape≈8′×5′asseeninvisualpho- [NII] l 6584 and Ha profiles from Meaburn (2005, Figure
tographs(Burnham1978). DeepnarrowbandCCD imaging
3) are reproducedin the top and bottom panels of Figure 2.
ofHa +[NII]and[OIII]byPapamastorakisetal.(1993)has Themiddle panelof Figure2 showsOIII l 5007takenfrom
shown a faint halo of these emissions extending to 17′. Its
of Meaburnetal. (1992, Figure 6d). Note the base of these
bi-polarmorphologyismanifestinHa imagesasthe“Dumb- emissionfeaturesextendto≈±60kms−1.
bell” and in the 2.12 µm image of Kastneretal. (1996) as a
ObservationsbyHugginsetal.(1996)oftheCO(2–1)230
clumpy“bow-tie”alignedwiththesemi-minoraxis.Theends GHz emission lines at a position 68′′W and 63′′S from the
ofthebow-tieare≈3′ wide,thewaist≈1′ andthelength≈
CSalsoexhibitsimilardoublepeakedlineprofiles,withato-
6′(c.f.Kastneretal.1996;Zuckerman&Gatley1988). tal separation ≈ 30 km s−1. At this position they estimate
Napiwotzki (1999) determined the DAO CS parameters
a CO column density of 6.9 × 1015 cm−2 (assuming LTE
fromNLTEmodelatmosphereanalysisandevolutionarycon-
with5<T <150K).Ataposition10′′ dueWestoftheCS,
siderations (T = 108,600 ± 6800 K, log(g) = 6.7 ± 0.23, Bachillereetxal.(2000)showlinessplitby53±1kms−1(their
log(He/H) by number of –1.12 dex, and M = 0.56 ± 0.01
∗ Figure 4), yielding an expansion velocity of ≈ 27 km s−1
M ). Astrometric observations of the CS by Benedictetal.
⊙ withsymmetryaboutaheliocentricvelocityof–43kms−1(at
(2003)producedadistanceofd=417+−4695pc.Atthisdistance M27,Vhel =Vlsr –17.8kms−1),ingoodagreementwiththe
the ≈ 8′× 5′ ellipse is 1 pc × 0.6 pc and the 1025′′ diame- Wilson systemic velocity. They present a map of the region
ter of the halo is 2.1 pc. Benedictetal. give other physical (beamsize12′′),showingtheCOemissiontobeclumpyand
parametersas well,V =13.98±0.03,a total extinction A =
V
4 McCandlissetal.
Wavelength (Å)
900 1000 1100 1200 1300 1400
115500
110000
5500
00
−−5500
)
2−−110000
−
c
Brightness −1−2−1s s cm Å arcse 468 HI+CIII 975 NIII+HeII 990 bLy 1026 HeII 1085 CIII 1175 aLy 1216 CII 1335
g 2
r
e
6
1
−0 0
1
x 900 1000 1100 1200 1300 1400
(
Wavelength (Å)
FIG.3.— Top–rawlongslitfar-UVspectrumfromsoundingrocketexperiment36.136UG.Bottom–totalnebularbrightnessperA˚ summedoverthewhole
slit,excludingthecontinuumsource. Theredlineistheestimateddetectorbackground. ThereisnosignificantdustscatteringorH2 continuumfluoresence
emissionabovethisbackground.TheriselongwardofLya islikelycausedby2s–1s,2-photonemissionofrecombininghydrogen.Thedottedbluelineshows
theexpectedbrightnessforacolumnofN(2s)=4×108cm−2.
more-or-lesscoincidentwiththemolecularhydrogeninfrared in the v′′ = 2 levels in the presence of strong Lya emission.
emission observed by Kastneretal. (1996). Bachilleretal. Herald reports (private communication) that FUSE observa-
(2000)presenta pictureofan ionizedcentralregionwith an tionsoftheCSofNGC3132alsoexhibitexcitedH absorp-
2
electron density of about n ∼ 100 cm−3 surrounded by a tionlineswitharoughlythermalro-vibrationaldistributionof
e
ring of molecular clumps with densities ∼ 104 cm−3 under- ≈1750K.
going photodissociation. They suggest the clumps are sim-
ilar to the “cometary” shaped features observed in great de- 3. DATASETS
tailintheHelixnebula(c.f.O’Delletal.2002;Hugginsetal.
3.1. FUSE
2002; Meixneretal. 2005), althoughthe cometarymorphol-
ogy is not as prominent in the Dumbbell as it is in the He- Adescriptionofthedatasetandprocessingproceduresfor
lix. Meaburn&Lopez (1993) first noted (see also HST im- theFUSEspectraoftheCS(MASTobjectIDGCRV12336)
agesbyO’Delletal.2002)thatsomeclumpsappearasdark usedinthisstudycanbefoundinMcCandliss&Kruk(2007).
knotsagainsttheverybrightOIIIemissionthatsurroundsthe For a description of the detectors, channelalignmentissues,
centralregion. systems nomenclature and other aspects of the FUSE in-
McCandliss(2001)gaveapreliminaryanalysisofmolecu- strument, see Moosetal. (2000) and Sahnowetal. (2000).
lar hydrogenabsorptionin the FUSE spectra, identifing two Briefly,theCSwasobservednumeroustimesforobservatory
absorption systems, one blueward of the CS radial velocity wavelengthcalibration purposes. High signal-to-noise spec-
andoneredward.Thebluewardcomponenthaslinesoriginat- tra,acquiredintimetagmodethroughthehighresolutionslits
ingfromanexcitedelectronicgroundstate(X1S +),withro- (HIRS)oftheeightdifferentchannelsegmentshavebeenas-
g
tationallevels(J′′)upto15andvibrationallevels(v′′)atleast sembled. Data for each extracted channel segment consists
ashighas3. Manylinesoriginatingfromv′′=1and2arelo- of three 1-D arrays: wavelength (A˚), flux (ergs cm−2 s−1
catedlongwardofthegroundstatebandhead(v′ –v′′=0–0) A˚−1), and an estimated statistical error (in flux units). Fig-
at1108.12A˚,andassuchareunambiguousmarkersof“hot” ure 1 shows the eight individualchannelsegments and their
spectral range for all the coadded HIRS spectra. The high
H . In contrast, the redward component is much “cooler,”
2
showingnounusualexcitation(J′′ ≈4,v′′=0).Itisassoci- densityoftheH2 featuresisapparent.
max In the low sensitivity SiC channels the continuum signal-
atedwithnon-nebularforegroundgas.Herewewillrefinethe
to-noise is between 10 – 25 per pixel, while in the higher
original analysis, which was based on LWRS data acquired
sensitivity LiF channels it is 20 – 55. These signal-to-noise
underFUSEPITeamProgramP104.
Lastly we note, Lya fluorescence of H has recently been ratios are purely statistical and do not account for systemic
2
errors,suchasdetectorfixedpatternnoise. Theresolutionof
foundinthePNeM27andNGC3132byLupuetal.(2006).
the spectra changes slightly as a function of wavelength for
Thefluorescenceisadirectconsequenceofexcitedmolecular
eachchannelsegment.InmodelingtheH absorption(§4.1),
hydrogen,asthemechanismrequiresasignificantpopulation 2
wefindthatagaussianconvolutionkernelwithafullwidthat
HotMolecularHydrogeninM27 5
10−11
10−12
)
1
−
Å
1
−s 10−13
2
−
m
c
s
g
r 10−14
e
(
x
u
l
F
10−15
10−16
1000 10000
Wavelength (Å)
FIG. 4.— Log-logplotoftheM27centralstarSEDfrom900-20000A˚,asmeasuredbyFUSE,IUE,APOandtheopticalphotometryofTylendaetal.
(1991)(⋄),(Ciardulloetal.1999)(⊓⊔)andBenedictetal.(2003)(+).Overplottedinblack,purpleandredaretherespectiveunextinguished,lightlyextinguished
(E(B−V)=0.005)andheavilyextinguished(E(B−V)=0.05)stellarcontinuummodels.Breakbetween2000and3000A˚ indicatesagapinthestellarmodel.
halfmaximumof0.056A˚ at1000A˚ (spectralresolutionR≈ 3.2. Far-UVLongslitObservationsofM27fromJHU/NASA
18,000,velocityresolutiond V ≈17kms−1)providesagood SoundingRocket36.136UG
matchtotheunresolvedabsorptionfeaturesthroughoutmost
JHU/NASA sounding rocket mission 36.136 UG was
oftheFUSEbandpass.
launchedfromWhiteSandsMissileRangeLaunchComplex-
Closecomparisonofthewavelengthregistrationofoverlap-
36 on 14 June 1999 at 01:40 MST. Its purpose was to ob-
pingsegmentsrevealsisolatedregions,afewA˚ inlength,of
servethe hotCS of M27, providespatialinformationon the
slightspectralmismatch(∼afractionofaresolutionelement) excitation state of H in the nebula and investigate its dust
2
inthewavelengthsolutions. Consequently,combiningallthe
scatteringproperties.Thescienceinstrumentwasa40cmdi-
spectraintoonemasterspectrumwillresultinalossofreso- ameterDall–KirkhamtelescopewithSiCover-coatedAlmir-
lution. However,treatingeach channel/segmentindividually rors,feedingaRowlandcirclespectrographwitha200′′x12′′
increases the bookkeepingassociated with the data analysis. longslit, a 900 – 1400A˚ bandpassand an inverselinear dis-
Further, because fixed pattern noise tends to dominatewhen
persionof ≈ 20 A˚ mm−1. The basic configurationhasbeen
the signal-to-noise is high, there is little additional informa-
describedbyMcCandlissetal.(1994,2000)andBurghetal.
tion to be gained in analyzing a low signal-to-noisedata set
(2001)andtwosimilarmissionsusingthispayloadhaveflown
whenhighsignal-to-noiseisavailable. Forthesereasonswe
(Burghetal. 2002; Franceetal. 2004). This was the first
elected to form two spectra, each of which coversthe 900 –
flightforanewlyreconfiguredspectrographwithaholograph-
1190bandpasscontiguously,usingthefollowingprocedure.
icallycorrectedconcavegrating(McCandlissetal.2001),for
The flux and error arrays for the LiF1a and LiF1b seg-
improvingthe spatial resolution(3′′ spacecraftpointinglim-
ments were interpolated onto a common linear wavelength
ited) and a high QE KBr coated micro-channel plate with
scalewitha0.013A˚ bin,covering900–1190A˚.Theempty
double-delaylineanode(≈25µmresolutionelement),simi-
wavelength regions, being most of the short wavelength re-
lartothatusedbyMcPhateetal.(1999).
gion from 1000 A˚ down to the 900 A˚ and the short gap re- Thetargetwasacquiredat≈T+150s,atwhichtimepoint-
gion in between LiF1a and LiF1b, were filled in with most
ingcontrolwaspassedtoagroundbasedoperator,whoused
ofSiC2aandasmallportionofSiC2brespectively. Werefer a live video downlink of the nebular field to make real time
tothisspectrumass12. TheLiF2b,LiF2a,SiC1aandSiC1b pointingmaneuvers. At≈T+500sthetelescopewassealed
segments were merged similarily into a spectrum, s21. Ab- prior to reentering the atmosphere. The payload was recov-
sorption line analyses were carried out using the composite eredandpostflightcalibrationsweremadetosecureanabso-
s12ands21spectra(§4.1).5 lutecalibration.
Thepointingcorrectedtwodimensionalspectrumisshown
5 The s12 and s21 spectra are available through the H2ools website inthetoppanelofFigure3.CIIl 1335,CIIIl 977andl 1175,
(McCandliss 2003) along with the original complete M107 data product HeII+NIIl 1085,NIIIll 989–991andHILymanemission
(LWRS,MDRSandHIRS)processedbyJeffreyKruk(privatecommunica-
lines are evident along with the continuum spectrum of the
tion).
central star. The bottom panel shows the extracted nebular
6 McCandlissetal.
4.0
) 3.5
bH
3.0
/
2.5
aH
( 2.0 SW NE
1.5
0.7
−100 −50 0 50 100
0.6
)
bH 0.5
/

0.4
gH
( 0.3 SW NE
0.35 −100 −50 0 50 100
)
0.30
bH
/ 0.25

dH 0.20
( SW NE
0.15
−100 −50 0 50 100
Angular Offset from CSPN (")
position angle = 35°
FIG.5.— LongslitprofilesofHa /Hb ,Hb /Hg ,andHb /Hd .Intrinsiclineratioforagaswithatemperatureof10000Kandadensityof100cm−3isoverplotted
inblack.Thedatahavebeenbinnedupbytwo0.′′4pixels.AveragevaluesintheSWandNEregion,delimitedwiththeverticaldashedlines,areoverplottedin
red.ErrorbarsaredisplayedwithrespecttotheaverageintheSWregion.AlltheratiosareconsistentwithzeroextinctionexceptfortheNEHa /Hb ratio.This
resultsuggeststhatamechanism,otherthanextinctionbydust,iscausingtheHa /Hb ratiotodeviatefromtheexpectationsofrecombination.
spectrumintegratedoverthe lengthofthe slit, excludingthe tiongratingshaveinverselineardispersionsof1.85and2.26
stellarcontinuum. ThestrongLya emissionfeaturethatfills A˚ pixel−1respectively. Theusefulspectralrangesare3700–
theslitisdominatedbygeocoronalemission. Thereisahint 5400A˚ fortheblueand5300–9700A˚ forthered. Theplate
of HI 2s – 1s two photon emission starting at Lya and ris- scalesare0.′′42pixel−1 fortheblueand0.′′40pixel−1 forthe
ingtowards1400A˚.Thedottedbluelineshowstheexpected red.
brightnessforancolumnofN(H(2s)) =4×108 cm−2 (e.g. The hot sub-dwarf star BD +28 4211 was used as a spec-
Nussbaumer&Schmutz1984).Thereisnoevidenceforcon- trophotometric standard (Bohlinetal. 2001). This star was
tinuum pumped fluorescence of H2 or dust scattered stellar observedthroughthelarge5′′×300′′DISslitfor60seconds
continuum down to the detector backgroundlimit of ≈ 5 × at an airmass of 1.18. The M27 CS was observed shortly
10−17ergscm−2s−1 A˚−1arcsec−2indicatedbytheredline. thereafter for 200 seconds through the same aperture at an
airmass of 1.02. The CS and surroundingnebula were then
3.3. APODISLongslitObservationsofBalmerLineRatios observedthroughthenarrowest0.′′9×300′′ DISslitfor200
inM27 seconds, with the slit held fixed on the CS. The narrowest
It is curious that Hawley&Miller (1978) and Barker slitprovidescleanseparationofthe[NII]ll 6548,6584lines
(1984) both cited problems with their Ha /Hb ratios be- fromHa . Thepositionangleoftheslitwas35◦. Biases, flat
ing higher than the 2.9 ratio determined by Miller (1973). fieldsandemissionlinespectrawererecordedattwilight.and
Hawley&Miller(1978)suggestedsomeunidentifiedsystem- thedatawerereducedusingcustomIDLcode. TheCSabso-
atic error was affecting the ratio at the 10% level. Barker lutefluxisshowninFigure4andwillbediscussedin§4.2.
(1984)reachedasimilarconclusionandwentsofarastode- TheBalmerlinelongslitprofilescenteredontheCS,andra-
creasetheHa /Hb by13%beforeapplyinganextinctioncor- tioed to Hb , are shown in Figure 5 and will be discussed in
rection of c = 0.17. Both studies find ≈ 20% variations in §4.2.1. AnalysisofthefulldatasetfromtheAPOrunwillbe
Ha /Hb atdifferentlocationswithinthenebula. presentedelsewhere.
To more thoroughly investigate this phenomenon we re-
3.4. DwingelooHI21-cmObservationofM27
centlyacquiredanextensiveseriesoflongslitspectralscansof
the nebulawith the Double ImagingSpectrograph(DISver- Theshapesofatomichydrogenabsorptionprofilesarecom-
sionIII)attheARC3.5mtelescopeatApachePointObserva- plex, as they result from ensembles of intervening ISM ab-
tory(APO),duringthenightsof1–2July2006commencing sorptionsystems, separated in velocity space along the line-
at 01:15 MDT. DIS has blue and red spectra channels with of-sight.Determiningthecolumndensityanddopplerparam-
back-illuminated,13.5µmpixelMarconiCCDsina1028× eter for individual velocity components within these broad
2048 pixel2 format for recording data. The medium resolu- dampedand saturated absorptionprofilesis difficultwithout
HotMolecularHydrogeninM27 7
) )
1 1
− −
) )
1 1
− −
s s
m 1.5 m 1.5
k (l,b)=(60.5,−4.0) k (l,b)=(60.5,−3.5)
2 ( 1.0 2 ( 1.0
− −
m m
0.5 0.5
c c
0 0
2 2
0 0
1 0.0 1 0.0
( (
HI HI
N −150 −100 −50 0 50 N −150 −100 −50 0 50
V (km s−1) V (km s−1)
hel hel
) )
−1 −1 M27 (l,b)=(60.8,−3.7)
) )
1 1
− −
s s
m 1.5 m 1.5
k k
2 ( 1.0 2 ( 1.0
− −
m (l,b)=(61.0,−4.0) m (l,b)=(61.0,−3.5)
0.5 0.5
c c
0 0
2 2
0 0
1 0.0 1 0.0
( (
HI HI
N −150 −100 −50 0 50 N −150 −100 −50 0 50
V (km s−1) V (km s−1)
hel hel
FIG.6.— NearestneighborHI21cmemissionspectrafromtheDwingloospectralsurveyatlas.Thesespectrawereusedtoformaminimumspectrumanda
distance-weightedmeanspectrum.
a priori information of the line-of-slightvelocity structures. a five bin boxcar average and subtracted from a similarily
One way to gain this information is to examine H I 21 cm smoothed spectrum formed from a distance weighted aver-
emission data such as found in the Atlas of Galactic Neu- age of the same 4 nearest neighbors. This process is shown
tralHydrogen(Hartmann&Burton1997, also knownas the inFigure7. TheresultingsubtractionwasfitwithnineGaus-
Dwingeloosurvey). The high spectralresolutionof the data sian profiles spread between –99 <v < 24 km s−1. The
hel
isexcellentforlocatingvelocitycomponents,providesanup- integratedemissioncolumndensities,rmsvelocitywidths(b
perlimittotheamountofneutralhydrogenintheforeground, values),andheliocentricvelocitiesaregivenincolumns2,3,
and serves as a starting point for absorption line modeling. and4ofTable2.
Theatlas, in galacticcoordinates(l,b) with(0.◦5)2 cells, has Column5 showsthecolumndensitiesthatwe adoptedfor
avelocityspacingof1.03kms−1. Thepositionscovermost the model of the Lyman series absorptions as discussed in
oftheskyandthevelocitycoveragespans–450≤v ≤400 §4.3. Absorptionlinecomponentstotheblueofthesystemic
lsr
kms−1 inlocal-standard-of-restcoordinates. Theintensityis velocity at –42 km s−1 occur within the expansion velocity
giveninantennatemperaturepervelocity(K(kms−1)−1),but inferredfromtheopticalemissionlinesandcanplausiblybe
canbeconvertedtoaHIcolumndensity,undertheassump- associatedwiththenebularexpansion.Iftheabsorptioncom-
tionofopticallythinemission,bymultiplyingwithaconver- ponentstotheredof–42kms−1 areassociatedwiththeneb-
sionfactor(0.182×10−19cm−2,Hartmann&Burton1997). ulathey wouldhaveto be infalling. Theyaremore likelyto
ThegalacticcoordinatesofM27arel =60.◦84,b=–3.◦70. belocatedintheforeground.
The neutral portion of the nebula should be at least as large
as the Ha halo detected by Papamastorakisetal. (1993) (& 3.5. STIS
0.◦28) and since the Dwingeloo half power beam width is
TheE140MspectrumofM27(o64d07020_x1d.fits),taken
slightly larger than the atlas grid, it is reasonable to expect
fromtheMultimissionArchiveatSpaceTelescope(MAST),
some signal from the nebula to appear in the four nearest
neighborpoints, i.e. atlas coordinates(l = 61.◦, b = –3.◦5) (l was acquired for HST Proposal 8638 (Klaus Werner – PI).
=60.◦5,b=–3.◦5)(l =61.◦,b=–4.◦)and(l =60.◦5,b=–4.◦). These data are a high level product consisting of the ex-
tracted one-dimensionalarrays of flux, flux error and wave-
The column density profiles of HI as a function of velocity
length for the individual echelle orders. The data were ac-
forthesefourgridpoints,inheliocentricvelocitycoordinates,
quiredthroughthe0.′′2×0.′′2spectroscopicapertureforanex-
areshowninFigure6.
posuretimeof2906sandwerereducedwithCALSTISver-
To reduce the non-local contributions we constructed a
sion 2.18. The spectral resolution of the E140M is given in
nearest neighbor minimum spectrum under the assumption
the STIS data handbook (version 7.0 Quijano 2003) as R ≈
thatnon-localcontributionsarewidespreadanduniform.This
48,000(d V = 6.25kms−1). Theintrinsiclineprofileforthe
assumption has been checked by examining an area out to
±1.◦5, which showed similar structures, typified by strong 0.′′2×0.′′2 aperture has a gaussian core with this width, but
peaks near 0 km s−1 and smaller peaks near -85 km s−1. there is non-negligible power in the wings. Comparison of
theFUSEandSTISwavelengthscalesforthe CSrevealeda
Thenearestneighborminimumspectrumwassmoothedwith
discrepantoffset.Theestablishmentofaselfconsistentwave-
8 McCandlissetal.
)
1
− 2.0
)
1
−
s
m 1.5
k
(
2 1.0
−
m
c
0 0.5
2
0
1
( 0.0
HI
N
−150 −100 −50 0 50
V V
) 1 2 34 sysgr 5 6 7 8 9
1
− 2.5
)
1
− 2.0
s
m 1.5
k
( 1.0
2
−m 0.5
c 0.0
9
1
0 −0.5
1
( −1.0
HI
N
−150 −100 −50 0 50
V (km s−1)
hel
FIG.7.— Top–Minimumspectrumissubtractedfromtheangulardistanceweightedmeanspectrumtoremovewideanglebackgroundemission.Verticalaxis
iscolumndensityperkms−1. Horizontalaxisisheliocentricvelocity. Bottom–Resultingspectrumcontainsbothforegroundandsomebackgroundemission.
Ninecomponentgaussianfitisshownalongwiththefitresiduals(offsetby-5×1018 cm−2). SeeTable2fortotalcolumns,velocitycentroids, anddoppler
parametersderivedfromgaussianfits.
lengthscaleisdiscussedin§4.5. tionalstates,alongwiththemonotonicvariationintransition
strengthwithincreasinguppervibrationallevel,makesidenti-
4. DATAANALYSIS
ficationofdiscretevelocitycomponentsstraightforward.The
Our primary purpose is to quantify the excitation state of highlinedensitybecomesanadvantage,virtuallyguarantee-
theH2 inthediffusenebularmedium. Thecolumndensities ingafewlinesforagivenrotationalstatewillbeunblended.
ofthe individualro-vibrationalstateswere determinedusing This allows column densities to be determined by a straight
curve-of-growthanalysis.Wedetectessentiallyimperceptible forward measurement of the equivalent widths for use in a
reddeningofthestellarSEDoveradecadeofwavelengthcov- curve-of-growthanalysis.
erage, in contrast to the reddening determinations using the Wehaveidentifiedtwodistinctcomponentswithheliocen-
Balmerdecrementmethodasreportedinthe literature(§2). tric velocities of –75 and –28 ± 2 km s−1. The blueward
We show new high precision longslit Balmer decrement ra- velocitycomponentis the more highly excited, allowingthe
tios (Ha /Hb , Hg /Hb , and Hd /Hb ) where Ha /Hb appears to constructionofcurves-of-growthforallthero-vibrationallev-
be reddened in certain regions of the nebula, but the higher els with J′′ ≤ 11 and v′′≤ 1 of the ground electronic state
orderdecrementsappearunreddenedeverywhere.Apotential X1S +.6 The redward componenthas no extraordinaryexci-
g
resolution of the reddening conundrum will be discussed in tation with measurable absorption only found for J′′ ≤ 3 in
§5.4. v′′ = 0. Figure 8 shows a small region around the (v′–v′′) =
We also producean estimate of the total atomic hydrogen
(4–0)band near 1050A˚ where the “blue” and “red” velocity
columndensityinthenebulatoenableadiscussionofH for-
2 componentsare identified. Blueward absorptions for (v′–1),
mationanddestructionprocesses.Anupperlimitisplacedon
(v′–2)and(v′–3)inthisregionarealsodisplayed.
thecolumndensityofCOinthediffusemediumandananal-
Asemi-autonomousmethodwasdevelopedtomeasurethe
ysis of the excitation of CI fine structure lines will provide
equivalentwidthsofthelines,basedinpartonthecompletely
a diagnostic of nebular pressure and insight into the H exi-
2 autonomousmethodusedbyMcCandliss(2001).Allthelines
tation mechanism. Absorption line profiles of high and low
for a given velocity offset and rotational state were located
ionizationspeciescomparedtotheneutralprofilesrevealthe
withinasummationintervalinitially13–15pixelswide.The
ionizationstratificationoftheoutflowandprovideavelocity
pixelsadjacenttoeithersideofthesummationintervalwere
descriptionofthenebularkinematics.
usedtodefinecontinuumpoints,throughwhichastraightline
4.1. Curve-of-GrowthfortheMolecularHydrogen wasfittoserveasthemodelforthecontinuuminthesumma-
AbsorptionComponents tionregion.Thedegreeofblendingwasassessedinteractively
It seems with a first glance at Figure 1 that determining 6Forrotationalandvibrationalquantumnumbersinatransitionleadingto
the column densities for the ro-vibrational transitions and anupperelectronicstate(e.g. eitherB1S +u orC1P u)fromthegroundstate
identifyingvelocitycomponentswill be daunting. However, (X1S +)theconventionistodesignatetheupperstatewithasingleprime(J′,
g
the regular spacing of H2 lines arising from common rota- v′)andthelowerstatewithadoubleprime(J′′,v′′).
HotMolecularHydrogeninM27 9
10 Q4:4− 3 R3:16− 3 P3:16− 3 P4:4− 3
1−1 Å) 8 R4:2− 2 P2:2− 2 Q3:2− 2 P3:12− 2 R5:2− 2 R4:12− 2 P3:2− 2 Q4:2− 2
12−2− ergs cm s 46 P5:9− 1 R0:4− 0 R0:4− 0 R9:11− 1 R1:4− 0 R3:8− 1 R1:4− 0 R9:7− 0 P7:10− 1 P12:10− 0 P11:0− 0 P1:4− 0 P1:4− 0 R6:9− 1 Q12:0− 0 R2:4− 0 P11:9− 0 R2:4− 0 P3:8− 1 P7:6− 0 P5:5− 0 R1:0− 1 R0:0− 1 P8:1− 1 R8:10− 1 P10:8− 0 R2:0− 1 Q9:1− 1 P2:4− 0 R4:8− 1 P2:4− 0
−
0
1
x
(
x
u
l 2
F
0
1049 1050 1051 1052 1053
Wavelength (Å)
FIG. 8.— RegionneartheH2(4–0)bandidentifyingthehighexcitationvelocitycomponentat–33kms−1(blue)andthelowexcitationcomponentat+14
kms−1(red)asmeasuredintheCSrestframe.FUSEs12ands21spectraareoverplottedinorangeandgreenrespectively.
by plotting the spectrum surroundingeach individualline in
a (-150, 50) km s−1 interval and overplottingall the known TABLE1
atomicandmolecularfeatures.Lineswithevidenceofblend- DERIVEDNEBULARH2X1S +g COLUMN
ingwererejectedfromthe curve-of-growth. Examinationof DENSITIES*
the lines for blends providedan opportunityto fine-tune the
summation interval and continuum placement. The equiva- J′′ lolgoNg((cvm′′−=2)0) lnincoegs lolgoNg((cvm′′−=2)1) lnincoegs
lent width measurements were carried out on both the s12
and s21 spectra (see § 3.1), using the same summation and 0 15.0 2 13.6 3
1 15.9 11 14.4 5
continuumintervals.Allowancewasmadefortheslightlocal
2 15.7 8 14.2 3
mismatches in velocity scale that exist between the s12 and 3 16.3 4 14.6 9
s21 spectra, by locating the minimum within the integration 4 15.8 4 14.1 10
regionandadjustingtheintervalaccordingly. 5 16.4 8 14.6 12
6 15.7 8 14.1 5
Initialcurves-of-growthwereconstructedfromtheequiva-
7 15.8 5 14.5 11
lentwidthmeasurementsusingthewavelengthsandoscillator 8 15.2 7 13.9 2
strengthsfromAbgralletal.(1993a,b). Werequiredatleast2 9 15.2 11 14.3 4
10 14.6 13 13.5 2
unblendedlinesforthesecurves. Linesfromtransitionswith
11 14.8 5 13.8 2
J′′ ≥ 11and v′′ ≥ 2 are detected, buttheyare weak and too
few to constructa reliable curve-of-growth. Independentc 2 *Systematicerror=±0.1dex.
fittingswereperformedforeachcurvebyvaryingthedoppler Thedopplerparameteris6.5±0.5kms−1.
parameterover2–10kms−1andthelogarithmofthecolumn
density (in cm2) over 13 – 18 dex. The doppler parameters
of-growthfortheindividualrotationalstatesfromthes12and
thus derived varied from 4 to 8 km s−1 and the logarithmic
s21spectralmeasurementswerecombined(toppanelofFig-
columndensitiesrangedover≈13.5–17dex.
ure9). Thecolumndensitiesfortheindividualro-vibrational
Comparingresultsfromthes12ands21measurementsre-
levels are listed in Table 1 along with the number of lines
vealed a few disagreements for the doppler parameter (and
usedforeachcurve-of-growth. The errorsare dominatedby
thus column density) derived for the same rotational state.
placementofthecontinuum. Experimentswereperformedto
This produced an inconsistent modulation in the ortho-para
gauge the effect of systematic continuum offsets on the de-
ratio, expected to be ≈ 3:1 for the odd to even rotational
rivedcolumndensities. We foundthecolumndensitieswere
states. The inconsistencies were traced to curves-of-growth
consistent to within ± 0.1 in the dex for offsets within the
with the most saturated lines and lowest number of points.
localcontinuumsignal-to-noise.
Restrictingthealloweddopplerparametersinthefittoeither
ThetotalcolumndensityforthehotH componentat–75
6 or 7 km s−1 (the most common values) resolved the col- 2
kms−1isN(H )=7.9±2×1016cm2withadopplervelocity
umndensitydiscrepancyandresultedinconsistentortho-para 2
of 6.5 ± 0.5 km s−1. Curve-of-growthanalysis on the cold
modulation. It also reduced the scatter when all the curves-
non-nebularcomponent, for which we wish only to identify
10 McCandlissetal.
−3.5
1015
−4.0
) gJ1014 T=4080 K
/
l/ )
W −4.5 m−2
( T=2040 K
g (c
o NJ
l
1013
−5.0 T=4080 K
1012
−5.5
6 7 8 9 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
log [Nfl ] (log(cm−1)) Level Energy (eV)
FIG. 9.—Upper–curves-of-growthfordopplerparametersof6and7kms−1alongwiththedata. Adopplerparameterof6.5kms−1wasadopted. Lower
–columndensities,normalizedbylevelstatisticalweight,asafunctionofthelevelenergy. Thev′′=0,0≤J′′≤11columnsareshownwith+marks,andthe
v′′=1,0≤J′′≤11columnsareshownwith*marks.Overplotted(solidline)isasingletemperatureBoltzmanndistributionforT =2040K.Therearestatically
significantdeviationsfromthesingletemperaturepopulation,althoughthehigher7≤J′′≤11statesareinagreement. Thelower0≤J′′≤6statesappearto
haveaflatterslopeindicativeofatemperatureafactoroftwohigherasthedashedlinesindicate.
the strength of its aborptionfeatures, yielded a total column We note how the modeling by Spitzer&Zweibel (1974)
density of 1.3 ± 5 × 1017 cm2 with a doppler velocity of 5 shows qualitatively how the “curvature” of the population
kms−1. Therotationaldistributioniswellapproximatedwith density can go from “concave” to “convex” as the parame-
atemperatureof200K. terschangefroma low temperture,density andphotoexcita-
In the bottom panel of Figure 9 we show the population tionrateenvironmenttoeitherahighphotoexcitationrateor
density of the ro-vibrationallevels as a functionof levelen- ahighdensityenvironment. However,theydonotattemptto
ergy. Weuse(+)markstoindicatethecolumnsforJ′′ inthe exploretemperaturesinexcessof1000K.
v′′ = 0 state and (*) marks to indicate the columnsfor J′′ in
the v′′ = 1 state. The straight solid line is the best fit single 4.2. CentralStarSpectralEnergyDistributionand
temperaturemodel(T =2040K),assumingaBoltzmanndis- Extinction
tribution of level populations. This model is not successful, SED modelswith CS parameterstaken fromthe literature
as there are statistically significant deviations from a single provide a fairly precise match to the stellar continuum ob-
temperatureBoltzmanndestribution. served by FUSE. This allows us to determine the line-of-
The variation of population density as a function of level sightextinction,andprovidesareasonablestellarcontinuum
energyisquitedifferentfromwhatisusuallyobservedinthe to use for assessing the success of our model of atomic and
coldISM.Typicallythefirsttworotationallevelsareconsis- H absorption. The combined SED and absorption model
2
tent with a rather steep slope with a temperature T01 ∼ 80 has been used by McCandliss&Kruk (2007) to identify 63
K while the higher rotational levels flatten, giving the ap- photospheric,nebularandnon-nebularabsorptionfeaturesof
pearance of a higher “temperature” of several hundred de- ionizedandneutralmetals,lurkingamidtheseaofhydrogen
greesormore(Spitzer&Cochran1973). Theexcesscolumn features.
in the J > 1 levels has long been thought to be caused by In Figure 4 we show a log-log plot of the CS SED from
far-ultraviolet continuum fluoresence, but recently Gryetal. 900–20000A˚,asmeasuredusingFUSE,IUE,andtheAPO
(2002)hasquestionedthishypothesis. Theyfavoratruetwo
DIS,alongwiththe opticalphotometry(Tylendaetal. 1991;
temperaturemodelfortheISM.
Ciardulloetal. 1999; Benedictetal. 2003). Over-plotted is
Regardless, here we have the opposite case. In each vi-
a synthetic stellar flux interpolated from the grid of Rauch
brational level v′′ = 0, 1 the trend is for the lower rota-
(2003) withlogg =6.5, T= 120,000K, andratioofH/He =
tional states to have flatter slopes (higher temperature) than
10/3by mass. This modelincludesno metals and is consis-
the higher rotational states. Reproduction of the energy
tentwith,althoughslightlyhotterthan,thequantitativespec-
level distribution found here will be an interesting chal-
troscopyof Napiwotzki(1999). It is slightly coolerthanthe
lengeforH excitationmodels(c.f.Spitzer&Zweibel1974;
2 determinationofTraulsenetal.(2005). Thetemperatureand
Black&Dalgarno 1976; Shull 1978; vanDishoeck&Black
gravityadoptedforourmodelhasslightlylesspressurebroad-
1986; Sternberg&Dalgarno1989; Draine&Bertoldi1996).
ening and is a better match to the observed Lyman lines to-