First detection of CF+ towards a high-mass protostar

11.6 MB·English

by  S. Fechtenbaum

Astronomy&Astrophysicsmanuscriptno.aa24718-14 (cid:13)cESO2015
January23,2015
LettertotheEditor
+
First detection of CF towards a high-mass protostar
S.Fechtenbaum1,2,S.Bontemps1,2,N.Schneider1,2,T.Csengeri3,
A.Duarte-Cabral4,F.Herpin1,2,andB.Lefloch5
1 Univ.Bordeaux,LAB,UMR5804,F-33270,Floirac,France
e-mail:[email protected]
2 CNRS,LAB,UMR5804,F-33270Floirac,France
3 Max-Planck-InstitutfürRadioastronomie,AufdemHügel69,53121Bonn,Germany
4 SchoolofPhysicsandAstronomy,UniversityofExeter,StockerRoad,ExeterEX44QL,UK
5 LaboratoireAIMParis-Saclay,CEA/IRFU-CNRS/INSU-UniversitéParisDiderot,CEA-Saclay,Gif-sur-YvetteCedex,France
5
Received—,—;accepted——,—-
1
0
2 ABSTRACT

n Aims.WereportthefirstdetectionoftheJ=1-0(102.6GHz)rotationallinesofCF+(fluoromethylidyniumion)towardsCygX-N63,
a
ayoungandmassiveprotostaroftheCygnusXregion.
J
Methods. This detection occurred as part of an unbiased spectral survey of this object in the 0.8−3 mm range, performed with
2 theIRAM30mtelescope.Thedatawereanalyzedusingalocalthermodynamicalequilibriummodel(LTEmodel)andapopulation
2 diagraminordertoderivethecolumndensity.
Results.Thelinevelocity(–4kms−1)andlinewidth(1.6kms−1)indicateanoriginfromthecollapsingenvelopeoftheprotostar.
] WeobtainaCF+columndensityof4×1011cm−2.TheCF+ionisthoughttobeagoodtracerforC+andassumingaratioof10−6for
R
CF+/C+,wederiveatotalnumberofC+of1.2×1053withinthebeam.Thereisnoevidenceofcarbonionizationcausedbyanexterior
S sourceofUVphotonssuggestingthattheprotostaritselfisthesourceofionization.Ionizationfromtheprotostellarphotosphereis
. notefficientenough.Incontrast,X-rayionizationfromtheaccretionshock(s)andUVionizationfromoutflowshockscouldprovide
h
alargeenoughionizingpowertoexplainourCF+detection.
p
Conclusions.Surprisingly,CF+hasbeendetectedtowardsacold,massiveprotostarwithnosignofanexternalphotondissociation
-
o region(PDR),whichmeansthattheonlypossibilityistheexistenceofasignificantinnersourceofC+.Thisisanimportantresult
r thatopensinterestingperspectivestostudytheearlydevelopmentofionizedregionsandtoapproachtheissueoftheevolutionofthe
st innerregionsofcollapsingenvelopesofmassiveprotostars.Theexistenceofhighenergyradiationsearlyintheevolutionofmassive
a protostarsalsohasimportantimplicationsforchemicalevolutionofdensecollapsinggasandcouldtriggerpeculiarchemistryand
[ earlyformationofahotcore.

1 Keywords. stars:formation,protostars,massive,outflows,individual:CygX-N63
v
9
3
1. Introduction CygX-N63 has been determined by Motte et al. (2007) to
4
be one of the brightest and most compact 1.2mm cores in the
5
CygnusXregion(Schneideretal.2006)atadistanceof1.4kpc
0
.Theformationofmassivestarsisstillnotwellunderstood.Im- (Rygletal.2012).Itisverylikelyamassiveprotostarinitsearli-
1
portantquestionsarerelatedtoaccretionratesrequiredtoform estphaseofformation(Bontempsetal.2010).Withanenvelope
0
5massivestarsinthemonolithiccollapsescenario,andtotheevo- massof44M(cid:12)within2500AU,andaluminosityof340L(cid:12),itis
lution of the inner regions of the collapsing cores where the themostmassiveandyoungestprotostardetectedinCygnusX,
1
:disksformandstellarfeedbackoccurs(e.g.Yorke&Sonnhalter andisrecognizedasaClass0massiveprotostardrivingapower-
v2002;Duarte-Cabraletal.2013).Forhighaccretionrates(10−4 fulCOoutflow(Duarte-Cabraletal.2013).Itdoesnotyetexcite
Xito10−3M(cid:12)/yr),theprotostarshavelargeradii(10to100R(cid:12))be- anUCHIIregionanddoesnotcontainadevelopedhotcoreand
foretheycontracttoreachthemainsequencewhilestillaccret- could thus be a rare example of a massive protostar in its pre-
r
aing(e.g.Hosokawa&Omukai2009).Itisroughlywhenthepro- UCHIIregionandpre-hotcorephase.Thisexceptionalobjectis
tostarsreachthemainsequencethattheystarttodevelopultra- one of the best known examples of a dense core in monolithic
compact (UC HII) regions which may then regulate and possi- collapsewithnosignofsub-fragmentationfrom0.1pcdownto
bly stop further accretion, limiting the final mass of the stars. 500AU,althoughitcouldstillformaclosebinary.
Massive protostars also form hot cores with large fractions of Molecular lines are important probes of the dense gas from
collapsinggasradiativelywarmedover100K.Theyreleaseinto whichstarsformandhenceprovideimportantinsightforunder-
thegasphasemolecularspeciespreviouslyfrozenongrainsand standingtheearliestphasesofstarformation.Oneofthemajor
driveawarmgaschemistry.Itisnotclearwhetherthishotcore coolinglinesofthewarminterstellarmediumisthefar-infrared
phase occurs before or at the same time as the development of finestructurelineofionizedcarbon(C+)at158µm.Itwasrec-
anUCHIIregion.Byopeninglargeinnercavities,outflowsand ognized(Neufeldetal.2005)thatinthemoleculargasphaseof
UCHIIregionsmayhelptoheatupalargefractionofcollapsing the interstellar medium, HF is the main fluor reservoir and that
envelopes,leadingtowell-developedhotcores. CF+ could be a good indirect tracer of C+, which is not reach-
Articlenumber,page1of6
A&Aproofs:manuscriptno.aa24718-14
ablefromground-basedfacilities.Thefirst(andonly)detections
ofmm-transitionsofCF+ towardstheOrionBar(Neufeldetal. 11 CF+ 1-0
2006)andtheHorseheadnebula(Guzmanetal.2012)confirmed 8
these predictions and indicate that CF+ can trace C+ in photon
5
dominatedregions(PDRs). K]
HerewepresentthefirstdetectionofCF+towardsaprotostar m 2
obtainedwithina0.8to3mmunbiasedspectralsurveyemploy- T [ 0
ingtheIRAM30mtowardsCygX-N63.InSect.2theobserva- -2
tions are detailed while results are provided in Sect. 3. Sect. 4
discussestheoriginofCF+detectionanditsimplications. -5
-8
-40 -30 -20 -10 0 10 20 30
Velocity [km/s]
2. Observationsanddatareduction
We carried out an unbiased spectral survey with the IRAM 30 100 CF+ 2-1
m telescope on Pico Veleta, Spain, towards the source CygX- 80
N63,locatedat(RA,Dec) =(20h40m05.2s,41◦ 32(cid:48) 12.0(cid:48)(cid:48)).
J2000
60
CygX-N63isanisolatedsingleobjectandcanthereforebeob-
K]
served without confusion within the beam of the IRAM 30m m40
(up to 30(cid:48)(cid:48) at 3mm). The observations were obtained between T [
20
September2012andJanuary2014usingthewobblerswitching
mode to assure good baselines, and covering so far the whole 0
1,2,and3mmbands(80−270GHz).TheEMIRreceiverwas -20
connected to the FTS backend at 195 kHz with settings, sepa-
ratedby3.89GHztocoverthebandswitharedundancyof2.In -40 -30 -20 -10 0 10 20 30
Velocity [km/s]
ordertodistinguishghostsfrombrightlinesintherejectedside-
band, we observed each frequency as pairs of settings shifted Fig. 1. Spectra of the CF+ J = 1→0 and 2→1 lines on a main beam
by20MHz.ThedatawerereducedwiththeGILDASpackage1. brightnesstemperaturescale.Thebluecurvesarethemodeledspectra
Afterazeroth-orderbaseline,allspectrawereaveragedandcon- oftheCF+ linesandtheredcurveisthemodelforCF+ andCH CHO
3
vertedintomain-beamtemperature(T )usingtheforwardand together.
mb
main-beam efficiencies (Feff and Beff) listed in Table 1 leading
to typical rms noise levels of 2.9, 4.2, and 8.6 mK for the 3, 2,
and1mmband,respectively.
4. Discussionandconclusion
+
4.1. FirstdetectionofCF towardsamassiveprotostar
3. Analysisandresults
The CF+ ion is stable and is the second fluorine reservoir after
+
3.1. LineidentificationanddetectionofCF (1-0) HFinPDRs.Itsformationrouteissimple(Neufeldetal.2005)
TheanalysiswasperformedusingCASSIS2andtheCDMS3cat- F+H2→HF+H and HF+C+→CF++H,
alogue.Weherereportacleardetection(6.2σ)oftheJ=1→0 wherebothHFandC+areabundant,i.e.attheinterfaceHI−H2
lineofCF+at102.59GHzatv =−4km/s.Theonlyotherline andmoregenerallywhereionizationisstrongenoughtoproduce
ofCF+(J=2→1at205.17GHLzS)Rinourspectralcoveragecannot C+.CF+shouldbeanexcellentproxyforC+inpartiallyionized
befirmlydetectedbecauseofblendingwithalineofCH CHO regions. CF+ has so far been detected only towards two bright
3
(11 -10 )(seeFig.1). PDRs:theOrionBarandtheHorseheadnebula.TheOrionBar
1110 1100
isanintensePDRpartlyseenedge-onleadingtoalargecolumn
densityofCF+ (Neufeldetal.2006).TheHorseheadnebulaeis
3.2. Totalcolumndensityandexcitationtemperature a weaker PDR, but the interface is seen almost perfectly edge-
on with a significant limb-brightening increasing the observed
Using an excitation temperature of 10 K as in Neufeld et al. columndensityofCF+(Guzmánetal.2012).
(2006)andGuzmánetal.(2012),assuminglocalthermodynam-
Thepresentdetectionisthefirstdetectiontowardsanobject
icalequilibrium(LTE)andopticallythinlines,wederiveatotal
CF+ column density of 4×1011cm−2 (see Appendix A for de- whichismostlikelynotaPDR(seeSect.4.2).Itisaninteresting
findingbecauseitthenpointstotheexistenceofasourceofC+
tails).Takingthe(2→1)linetentativedetectionasanupperlimit,
insidetheCygX-N63core.Acolumndensityof4×1011cm−2in
weobtainamaximumexcitationtemperatureof∼20K.
a25(cid:48)(cid:48) FWHMbeamcorrespondstoatotalnumberofCF+ ions
Thelinevelocityof–4kms−1 isexactlytherestvelocityof of1.2×1047.WiththeabundanceratioCF+/C+ oftheorderof
the protostar (Bontemps et al. 2010), which suggests an origin
10−6 at high density and high UV field (see Fig. 4 in Neufeld
fromtheprotostar.Thestudyoflineprofilessuggestsacorrela- etal.2006),itconvertstoatotalnumberofC+of1.2×1053.We
tion between the line width and the spatial origin of molecules
discussbelowthedifferentpossibilitiestoexplainthisamountof
(Fechtenbaum et al. in prep.). The observed full width at half C+indirectlytracedbyCF+.
maximum (FWHM) of 1.6 km s−1 points to an origin from the
envelope.
4.2. APDRfrontdetectedatthedistanceofCygnusX?
1 http:/www.iram.fr/IRAMFR/GILDAS/
2 http://cassis.irap.omp.eu/ SinceCygX-N63issituatedintherichCygnusXcomplexhost-
3 http://www.astro.uni-koeln.de/cgi-bin/cdmssearch ing a large number of OB stars, the detected CF+ could poten-
Articlenumber,page2of6
S.Fechtenbaum etal.:FirstdetectionofCF+towardsahigh-massprotostar
Table1.Observationparameters,GaussianfitsresultsandLTEmodelresults.
Line Frequency Eup/k Aij Feff Beff Beam VLSR FWHM Tpeak Noise
GHz K s−1 arcsec kms−1 kms−1 mK mK
CF+(1-0) 102.587533 4.9235 4.82×10−6 0.94 0.79 25.5 -4.1±0.5a 1.6±0.5a 10.5a 2.3
CF+(2-1) 205.170520 14.7703 4.62×10−5 0.94 0.64 11.4 -4.1b 1.6b <62c 14.7
aGaussianfitresults
bFixedequaltotheJ=1-0lineparameters
cUpperlimit.LTEmodelingwitha25”sourcesize(beamsize)andanexcitationtemperatureof10KgivesTpeak=24mK.
4.3. AninnerCIIregiontowardsCygX-N63?
CygX-N63isanOBstarinformation.Itisorwillsoonbecome
astrongionizingsourceandastrongsourceofC+.Interestingly
enough,however,CygX-N63isnotyetanUCHIIregion.Ithas
not been detected in free-free emission with the VLA. CygX-
N63 only coincides with a 2.3σ peak of 0.14 mJy at 8.4GHz
(see Appendix B). Since carbon has a lower ionization energy
(11.3ev)thanhydrogen(13.6ev),aC+ regioncanexistbefore
thedevelopmentofanHIIregion.
Withaluminosityof340L ,CygX-N63canhostaB5−B6
(cid:12)
mainsequencestarofphotospherictemperature15000Kwitha
carbonionizingflux(between11.3and13.6ev)of2.63×1045s−1
(Diaz-Miller et al. 1998). Assuming a r−2 law and having 44.3
M inside a FWHM of 2500 AU (Duarte-Cabral et al. 2013),
(cid:12)
wegetanH densityof1.95×1010×(r/100AU)−2cm−3.Using
2
the recombination rate of the UMIST database (McElroy et al.
2013),assumingallcarboninatomicformandthationizationis
dominated by carbon, we can estimate the thickness of the CII
region (see Appendix C for details). For this estimate we also
neglect the effect of dust extinction on the ionizing radiation.
Even with these strong assumptions the above carbon ionizing
Fig.2. IRAM30mbeamat102GHz(greencircle)overlaidonaRGB fluxcanionizeathinlayerofonly0.1AUforaninternalradius
(8,4.5,and3.6µm)SpitzerIRACimagetowardsCygX-N63.Contours of100AU(AppendixC).Suchathinlayerwouldonlycontain
showthe250µmdustemissionfromHerschel(Hennemannetal.2012). 2.6×1050 ionizedcarbons,i.e.∼ 500timeslessthanobserved.
No prominent PAH emission (red, 8µm) is detected towards CygX- Toincreasetheefficiencyoftheionization,theionizingradiation
N63.
needs to reach lower density layers where the recombination is
lessefficientsothattherelativelyweakionizingradiationisable
to keep a large amount of carbon ionized. The typical required
tially originate from the PDR at the surface of the clump host- H densityis4×107cm−3 (seeAppendixC).Suchadensityis
2
ingCygX-N63,excitedbythemostnearbymassivestars.How-
expected at large radii, of the order of 2000 AU, which cannot
ever,CygX-N63islocatedinaquietpartofCygnusXwhereno
beeasilyreachedbyinnerUVradiationbecauseoftheexpected
nearbyOstarscanbedetected(theprojecteddistancetothecen-
largedustextinctionfromtheinnerenvelope.SinceCygX-N63
tralregionsoftheCygnusOB2clusteris∼20-30pc).Incompar-
drives an outflow (Duarte-Cabral et al. 2013), its envelope has
ison,theOrionBarPDRissituatedatlessthan0.5pcfromthe
bipolarcavities,andshouldhaveadisk-likestructureinthecen-
O stars of the Orion nebula cluster. To detect such a weak CF+
tral regions that reduces extinction in the equatorial directions.
line in Cygnus X, a PDR has to be bright and should therefore Itisdifficult,however,toexpectareducedenoughextinctionfor
exhibitstrongPAHemissioninthemid-IR.InFig.2showinga
ionizingradiationtoreach2000AU,whichisclosetothephys-
three-colorSpitzerimage,oneseesthattheCygX-N63coreac-
icalsizeoftheenvelope(seeBontempsetal.2010).
tuallycoincideswithoneofthedarkestpartsoftheregionwith
nosignofPAHred(8µm)emissionintheobservedbeam.Only
somegreen(4.5µm)weakextendedfeaturesareseenprecisely 4.4. X-rayionizationfromanaccretionshock?
coincidingwithCygX-N63.ItdoesnottraceaPDRfrontandis
most probably due to outflow shock 4 from the strong CO out- CygX-N63 is actually a young protostar (Class 0) with a lumi-
flowdrivenbyCygX-N63(seeSect.4.5).Thereisthereforeno nositydominatedbyaccretion.Anaccretionshockcreatesahot
hintoftheexistenceofabrightPDRatthesurfaceoftheclump plasmawhichradiateshalfoftheaccretionpoweroutward.This
whichwouldexplaintheobservedstrongsourceofC+. plasma has a typical temperature of 106K and mostly radiates
inextremeUV,butalsoinX-rays.X-raysionizegasmoreeffi-
cientlythandoesphotosphericradiation(e.g.Montmerle2001),
4 TherearenoPAHfeaturesat4.5µm.Suchstrongexcessesat4.5µm and are also less affected by extinction than UVs (Ryter 1996)
areusuallyreferredtoasEGOs(extendedgreenobjects)andaredueto andcouldthusionizecarbondeeperintheenvelopedecreasing
H lineemissioninoutflowshocks(e.g.Cyganowskietal.2008). theextinctionproblemdiscussedintheprevioussection.Alarge
2
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A&Aproofs:manuscriptno.aa24718-14
fractionofX-rayionizationoriginatesfromlocallyemittedUV 4.6. ACIIregionwithoutHIIregion
radiation(possiblydowntocarbonionizingphotons)asaresult
In the two possible scenarios discussed above (Sects. 4.4 and
oflocaldepositionofenergyfromphotoelectricelectronsgener-
4.5)theproducedUVradiationshouldionizefirsthydrogenand
atedbyX-rays(Krolik&Kallman1983;Glassgoldetal.2000).
ThishastheinterestingeffectthatX-rays,especiallyhigh-energy create an HII region which has, however, not been detected in
X-rays, can propagate deep in the envelope, being less affected free-freeemission(Fig.B.1).Fordensitiesandsizesinvolvedin
the accretion shock case the free-free emission has to be opti-
byextinctionthanUVs,anddepositlocallytheirenergyasUV
callythick.Wethenestimatethatthe5σupperlimitinfree-free
ionizingradiation.ThetotalluminosityofCygX-N63couldrep-
resentupto7.2×1046s−1ionizingphotonsforcarbon,assuming of0.3mJywouldcorrespondtoanemittingregionof∼ 20AU
radius(seeAppendixD).TheCionizedregionisexpectedtobe
thatalltheradiatedenergyisdedicatedtoCionization,whichis
moreextendedthantheHIIregionandgiventheveryuncertain
obviouslythemostoptimisticview.Usingthesameassumptions
geometryanddensitydistribution,20AUcouldbeconsideredto
as in Sect. 4.3, we find that the carbon ionized layer for an in-
be roughly compatible with the 100AU scale discussed above.
nercavityof100AUwouldthenbe2.6AUandwouldcontain
6.8×1051 ionized carbons, i.e. ∼ 20 times less than observed. In the outflow shock scenario, for a dissociative outflow shock
withajetvelocityof200km/sandn =3×10−3cm−3,thefree-
Thisisstillabitlow,butthediscrepancyisreducedcomparedto H
Sect.4.3.Thelocaldensityrequiredtokeep1.2×1053 carbons freeemissionisopticallythinwithτff =5×10−4.Foranionizing
ionized would be 1.1 × 109cm−3, i.e. ∼ 20 times lower than fluxof2×1044s−1,usingarecombinationrateof3×10−13ne−s−1
(Spitzer1978)andforfullionization(n =3×10−3cm−3),one
the density at 100 AU in the pure spherical case. If the reach- e−
getsEM =6.6×1056cm−3leadingto0.94mJyat8.4GHz(see
abledensitycouldbeabitreduced,forinstanceatthesurfaceof V
AppendixD).Thisisonlythreetimeslargerthanthe0.3mJyup-
theoutflowcavities,asintheirradiatedoutflowwallscenarioof
perlimitwiththeVLAwhichcouldbeseenasastillacceptable
Brudereretal.(2009),theagreementcouldperhapsbereached,
discrepancygiventhelargeuncertaintiesintheaboveestimates.
providing that a significant fraction of the available energy can
Wecanthereforeconcludethatinboththeaccretionandoutflow
be transfered to carbon ionization as discussed above. The es-
shocksscenariosaCIIregioncanbedetectedinCF+,whileno
timate of the possible X-ray luminosity which could increase
free-free emission has been detected in the accompanying HII
byordersofmagnitudeduringaccretionbursts(seeMontmerle
2001andreferencestherein)andtheoverallefficiencyofX-ray region.
ionization at R ∼ 100AU accounting for X-ray absorption in
such a complex geometry are difficult to assess precisely. We 4.7. Anewprobeoftheearlyevolutionofmassiveprotostars
canonlyconcludethattheionizingpowerexpectedfromanac-
cretionshockmayreachtherightorderofmagnitudetoexplain Inconclusion,thefirstdetectionofCF+towardsamassiveproto-
ourCF+detection. starmayprovideuswithanewtoolforinvestigatingtheionizing
powerofaccretionandoutflowshocks.IncontrasttoC+ inthe
far-IR,CF+inthemillimeterrangecanbeobservedathighspa-
4.5. Outflowshocks? tialresolutionwithinterferometer.Itcouldoffertheopportunity
toimagethedevelopmentofC+regionseveninthecaseofnon-
The last possible source of ionization is outflow shocks whose detectionofany HIIregion.Theindicationofapossiblestrong
energyisextractedfromtheinnerregionsofthecollapsingen- X-rayand/orUVionizationalsohasimportantimplicationsfor
velope. With a CO velocity of 40 km/s and an outflow force theearlychemicalevolutionincollapsingenvelopesinthecon-
of 2.9×10−3M km/syr−1 (Duarte-Cabral et al. 2013), we get textoftheformationofahotcore.
(cid:12)
an outflow luminosity of 9.6L which could represent up to
(cid:12) Acknowledgements. WethankV.WakelamforfruitfuldiscussionsandN.La-
2 × 1045s−1 carbon ionizing photons. Young jets are expected
garde for reducing VLA data. This work is supported by the IdEx Bordeaux
to radiate their energy through shocks. Three knots of 4.5 µm fundingandbytheprojectSTARFICH,fundedbytheFrenchNationalResearch
emission (shocked H ) are indeed located in projection at only Agency(ANR).ADCacknowledgesfundingfromtheFP7ERCstartinggrant
2
∼ 3.5(cid:48)(cid:48) from CygX-N63 (Fig. 2). These shocks should radi- projectLOCALSTAR.
ate in UV (van Kempen et al. 2009). For an average projection
angle of 57◦, the shocks would be then located at a radius of
9000AU from the protostar where the expected n density in
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AppendixA: CASSISLTEmodeling
CASSISprovidesaJythonscriptwhichminimizestheχ2tofind
thebestparameterstoadjusttheobservations.Withaspectrumi
ofNpoints,theχ2isderivedas
(cid:88)N (I −I )2
χ2 = obs,j model,j ,
i rms2+cal2(I −I
j=1 i i obs,j cont,j)2
whereI andI arerespectivelytheobservedandthemod-
obs model
eledintensities,I isthecontinuumintensity,cal thecalibra-
cont i Fig.B.1.VLAimageat8.4GHzofthesameregionthaninFig.2with
tionuncertaintyforthespectrumiandNisthetotalnumberof colorscalefrom-0.1to0.7mJy/beam.Thedashedellipseindicatesthe
pointsofthespectrum.Thereducedχ2isthecalculatedas
VLA primary beam and the green circle shows the IRAM 30m beam
at 102 GHz. The rms in this image stays large, of the order of 0.06
1 N(cid:88)spectra χ2 mJy/beam,duetostrongsidelobesoriginatingfromtoDR22,astrong
χ2red = Nspectra i=1 NNind −idof, rsaoduitoh)s.ource(acompactHIIregion)outsidetheimagedfield(∼5arcmin
withN thenumberofspectra,N thenumberofindepen-
spectra ind
dentpoints,anddofthedegreeoffreedom. AppendixC: Carbonionizationequilibrium
Theadjustableparametersarethecolumndensity,tempera-
ture, FWHM, size of the source, and vLSR. In the case of CF+, Atequilibriuminsphericalgeometryfromaninitialradiusri to
thecolumndensityvariesbetween1010 and1014 cm−2 with500 the radius of the CII region (where mostly all carbons are ion-
steps;theotherparameterswerefixed.Thetemperaturewasset ized)rCII theequilibriumofcarbonionizationcanbeexpressed
to10K,asinNeufeldetal.(2006)andGuzmánetal.(2012).The as
FWHM was measured on the spectrum as 1.6 km/s. No source QCII =(cid:90) rCIIkrecnc+ne−4πr2dr,
size was assumed, so we kept the 30 m beam size of 25”. The
velocityofthesourceisaround-4km/s.CASSISalsotakesinto ri
where Q is the total rate of ionizing photons (in s−1), k
account the beam size variation as a function of the frequency. CII rec
The obtained χ2 is 3.22 and the column density is (4 ± 1) × is the carbon recombination rate that we take as equal to
red 2.36×10−12(T/300)−0.29exp(−17.7/T) cm3s−1 (UMIST 2012
1011cm−2.
TheV andFWHMderivedfortheJ=1-0linearethen database McElroy et al. 2013), and nc+ and ne− are the densi-
used to prLeSdRict the intensity of the J = 2 - 1 line. We detected ties of C+ and of electrons. For a large range of gas temper-
ature (10 to 4000 K), k stays within a factor of less than 2
202CH CHOlinesinthewholecoverageandusedapopulation rec
3 aroundtheadoptedvalueof1.74×10−12s−1(rangingfrom1.08
diagram to derive a mean excitation temperature of 40 K and
to2.80×10−12s−1withthemaximumobtainedat∼60K).
a column density of 3 × 1013 cm−2. These values were used to
IfcarbonionizationdominatesandassumingC+/Hequalto
modeltheCH CHOlinesasshowninthebottompanelofFig.
3 thestandardC/Hratioof1.6×10−4 (ifmostofthecarbonisin
1.
atomicform),wehavene− = nC+ = 1.6×10−4×nH.Assuming
ar−2 densitylawandhaving44.3M insideaFWHMof2500
(cid:12)
AppendixB: VLAobservations AU (Duarte-Cabral et al. 2013), we here get an H2 density of
1.95×1010×(r/100AU)−2cm−3.Forthissimplemodel,weget
CygX-N63 was observed on 27 April 2003 with the 27 anten- the following expression of the extent of the ionized region in
nasoftheVLAinDconfigurationat8.4GHz(bandX)(project theinnerenvelope,
AB1073). The image shown in Fig. B.1 corresponds to a total
integration time of 25 mins spread over 4.5 hr (track sharing r Q
r = i with η= CII ,
technique to improve UV coverage). It has been cleaned with CII 1−ηr 2.85×1050s−1
i
thecleanalgorithmofAIPS.Theresultingrmsisequalto60.2
µJy/beam. andtheradiiexpressedinAU.ForthecaseofSect.4.3(Q =
CII
InthedirectionofCygX-N63,aweakpeakof0.14mJy,i.e. 2.63×1045s−1),onegetsaverysmallvalueof9.4×10−6 forη.
at a level of 2.3σ, is found. Below 3σ it cannot be considered For r = 100AU, r is then equal to 100.1 AU. Only the very
i CII
as apossible detection, especiallybecause theside lobes (large firstlayersareionizedforvaluesofQ significantlylowerthan
CII
stripesintheimageofFig.B.1)areroughlyofthesameintensity. ∼1050s−1.
Articlenumber,page5of6
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Theionizationequilibriumcanalsobeexpressedas
(cid:90) rCII
QCII = krecne−dNc+
ri
with Nc+ being the number of ionized carbons. It shows that
k n isthefractionofionizedcarbonsthatrecombinespersec-
rec e−
ond.Forn = 1.6×10−4 ×n ,wecanthenderivethetypical
e− H
density(assumingconstantdensity)requiredtokeepanamount
ofionizedcarbonNc+ ionizedunderanionizingrateQCII.Here
istheexpressionofthisdensitynCII forouradoptednumerical
H
values:
Q
nCII =3.6×1015 CII cm−3.
H Nc+
ForthecaseofSect.4.3with QCII = 2.63×1045s−1 and Nc+ =
1.2×1053,onegetsnCII =7.9×107cm−3.
H
AppendixD: Centimetricfree-freeemission
As discussed in Sect. 4.6, any ionized gas is expected to emit
free-freeemission.
The centimetric flux of free-free emission in the optically
thin case can be expressed as a function of EM , the emission
V
measure(seeAndré1987;Curieletal.1987),
(cid:18) S (cid:19) (cid:18) EM (cid:19)(cid:18) T (cid:19)−0.35(cid:18) ν (cid:19)−0.1(cid:18) d (cid:19)−2
ν =1.41 V e
mJy 1057cm−3 104K 8.4GHz 1.4kpc
with
(cid:90)(cid:90)(cid:90)
EM ≡ n2dV.
V e
The optical depth is expressed as a function of the linear emis-
(cid:82)
sionmeasureEM (≡ n2dl),
L e
(cid:18) EM (cid:19)(cid:18) T (cid:19)−1.35(cid:18) ν (cid:19)−2.1
τ ∼1.22 L e ,
ν 1027cm−5 104K 8.4GHz
which is equal to ∼ 8.8×106 for n = n = 2.2×109cm−3,
e− H
l = 100AU (see Sect. 4.4), 104K (the opacity is even larger
for lower temperatures) and at 8.4GHz (frequency of the VLA
observationfromAppendixB).
Intheopticallythickcasetheemissiondependsonthearea
ofemissionexpressedinphysicalsurfaceA ifdistancescaling
em
isincluded,
(cid:18) S (cid:19) A (cid:18) T (cid:19) (cid:18) ν (cid:19)2(cid:18) d (cid:19)−2
ν =2.60 em e ,
mJy (100au)2 104K 8.4GHz 1.4kpc
whichisequalto0.3mJyfor A = πR 2 withR = 19.2AU
em em em
(Sect.4.4).
Curiel et al. (1987, 1989) also expressed the expected flux
andopticaldepthinthecaseofadissociativeshockasafunction
ofpre-shockdensityn andshockvelocityV (forV (cid:38)60km/s),
0 s s
(cid:18) n (cid:19)(cid:18) V (cid:19)1.68(cid:18) T (cid:19)−0.55(cid:18) ν (cid:19)−2.1
τ ∼5.22×10−4 0 s e ,
ν 104cm−3 100km.s−1 104K 8.4GHz
whichisfoundequalto5×10−4 forn = 3×10−3cm−3,V =
0 s
200km/s(seeSect.4.5),104Kandat8.4GHz.
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