Depletion and low gas temperature in the L183 prestellar core : the N2H+ - N2D+ tool

0.32 MB·English

by  Laurent Pagani

Astronomy&Astrophysicsmanuscriptno.lpagani (cid:13)c ESO2008
February5,2008
Depletion and low gas temperature in the L183 prestellar core:
+ + ⋆
the N H - N D tool
2 2
7
0 L.Pagani1,A.Bacmann2,S.Cabrit1,andC.Vastel3
0
2
1 LERMA&UMR8112duCNRS,ObservatoiredeParis,61,Av.del’ObservatoireF-75014Paris

n e-mail:[email protected]
a 2 UniversiteBordeaux1;CNRS;OASU;UMR5804,FloiracF-33270
J
3 Centred’EtudeSpatialedesRayonnements,9avenueduColonelRoche,BP4346,F-31029ToulouseCedex,France
9
2
Received31/10/2006;accepted26/01/2007

1
v ABSTRACT
3
2 Context.Thestudyofpre-stellarcores(PSCs)suffersfromalackofundepletedspeciestotracethegasphysicalpropertiesintheirverydense
8 innerparts.
1 Aims.WewanttocarryoutdetailedmodellingofN H+ andN D+ cutsacrosstheL183maincoretoevaluatethedepletionofthesespecies
2 2
0 andtheirusefulnessasaprobeofphysicalconditionsinPSCs.
7 Methods.Wehavedevelopedanon-LTE(NLTE)Monte-Carlocodetreatingthe1DradiativetransferofbothN H+ andN D+,makinguse
2 2
0
ofrecentlypublishedcollisionalcoefficientswithHebetweenindividualhyperfinelevels.Thecodeincludeslineoverlapbetweenhyperfine
/
h transitions. An extensive set of core models is calculated and compared with observations. Special attention is paid to the issue of source
p couplingtotheantennabeam.
- Results.Thebestfittingmodelsindicatethati)gasinthecorecenterisverycold(7±1K)andthermalizedwithdust,ii)depletionofN H+
o 2
doesoccur,startingatdensities5–7×105 cm−3 andreachingafactorof6+13 inabundance,iii)deuteriumfractionationreaches∼70%atthe
r −3
t corecenter,andiv)thedensityprofileisproportionaltor−1outto∼4000AU,andtor−2beyond.
s
a Conclusions.OurNLTEcodecouldbeusedto(re-)interpretrecentandupcomingobservationsofN2H+ andN2D+ inmanypre-stellarcores
: ofinterest,toobtainbettertemperatureandabundanceprofiles.
v
i
X Keywords.ISM:abundances–ISM:molecules–Radiativetransfer–ISM:structure–ISM:individual:L183–Line:formation
r
a
1. Introduction Among the above three species, H D+ is thus the only
2
one not to deplete. However, it has only one (ortho) transi-
Understandingstar formationis critically dependentuponthe
tion observable from the ground, that moreover requires ex-
characterisation of the initial conditions of gravitational col-
cellentsky conditions.The paraformgroundtransition at1.4
lapse,whichremainpoorlyknown.Itisthereforeofprimeim-
THzshouldnotbedetectableinemissionincoreswithT ≤
kin
portancetostudythepropertiesofpre-collapseobjects,theso- 10 K. Therefore H D+ is useful for chemical and dynamical
2
called pre-stellar cores (hereafter PSCs). As bolometers and
studies,butbringslittleinformationongasphysicalconditions.
infrared extinction maps have been unveiling PSCs through
NH inversionlinesat23GHzcanprovidekinetictemperature
3
their dust component, it has become clear that depletion of
measurements as long as higher lying non-metastable levels
moleculesonto ice mantlesis taking place inside these cores,
arenotsignificantlypopulated(Walsmley&Ungerechts1983).
preventing their study with the usual spectroscopic tools. In
However, because the critical density of the NH (1,1) inver-
3
mostPSCs,veryfewobservablespeciesseemtosurviveinthe sionlineisonly∼2000cm−3,thistracermayhavesubstantial
gasphaseinthedenseandcoldinnerparts,namelyN H+,NH ,
2 3 contribution from external, warmer layers, not representative
H2D+andtheirisotopologues(e.g.Tafallaetal.2002).Inafew ofthedensestpartsofPSCs.Thethirdspecies,N H+,appears
2
cases,itisadvocatedthatevenN-bearingspeciesalsodeplete
verypromising:ithasthestrongadvantageofhavingmmtran-
(e.g. B68: Bergin et al. 2002; L1544: Walmsley et al. 2004; sitions with critical densities in the range 0.5–70×106 cm−3
L183:Paganietal.2005,hereafterPPABC).
and intensehyperfinecomponentsforthe (J:1–0)line in typi-
calPSCconditions.Theratioofhyperfinecomponentsgivesan
Sendoffprintrequeststo:L.Pagani estimate of the opacityand excitationtemperatureof the line.
⋆ Basedonobservations madewiththeIRAM30-mandtheCSO Fitting both the hyperfineratiosand the (J:1–0) to (J:3–2) ra-
10-m.IRAMissupportedbyINSU/CNRS(France),MPG(Germany), tiowitharigorousNLTEmodelshouldthusbringstrongcon-
andIGN(Spain).
2 L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool
2 2
Fig.1.N H+ observations(greylines)comparedtoourbestradiativetransfermodel(seeFig4a-b).Eachcolumncorrespondsto
2
aspatialoffset(indicatedabovethetopbox)fromthemaindustpeak.ThetoprowcontainstheCSO10-mdata,theotherrows
theIRAM30-mdata.The(J:1–0)lineissplitbetweenthebottomrow(centraltriplet)andthenextrow(’red’triplet);theisolated
’blue’componentisnotshown.TherightmostcolumnrepresentsthefitforthecentralpositionwithaNLTEcodewithoutoverlap
forthesamecloudparameters.
straints on both T and n(H ). Collisional coefficients (with Weststripacrossthecore(centeredatα(2000)=15h54m08.5s,
kin 2
He)betweenindividualhyperfinesublevelshavebecomeavail- δ(2000)=−2◦52′48′′).Symmetriceasternandwesternspectra
ablerecently(Danieletal.2005).However,currentexcitation wereaveragedoutto±48′′togiveamorerepresentativeradial
models(Danieletal.2006a)donottakeintoaccountlineover- profile.A small anti-symmetricvelocityshiftof a few tensof
lap,whichlimitstheiraccuracy.Anotherquestionthatremains ms−1wasnoticedoneithersidesofthePSCcenteratdistances
is whether N H+ is indeed able to probe the central core re- beyond±30′′,possiblyindicativeofrotation(seeSection3.2).
2
gions,despitethedepletioneffectswhichhavebeenreported. This shift was compensated for when averaging eastern and
In this paper, we introduce a new NLTE Monte-Carlo 1D western spectra, in order not to artificially broaden the lines.
code1treatingN H+andN D+radiativetransferwithlineover- Beyond48′′,onlyeasternpositionsareconsidered,asthewest-
2 2
lap,andapplyittodetailedanalysisofthemainPSCinL183, ernsideiscontaminatedbyaseparatecore(Peak3;Paganiet
a clear-cut case of N H+ depletion (cf. PPABC). We demon- al. 2004). Lines of N H+ (J:1–0), (J:3–2) and of N D+ (J:1–
2 2 2
strate the capability of the modelto constrainphysicalcondi- 0),(J:2–1)and(J:3–2)were observedin Frequencyswitching
tions inside the PSC (temperature,density profile, abundance mode, with velocity sampling 30–50 ms−1 and T ranging
sys
anddepletion,deuteriumfractionation).Inparticularweshow from100K at3mmupto1000K at1mm.TheN H+ (J:2–1)
2
thatthegasisverycoldatthecorecenterandthermalizedwith lineat186GHzwasnotobserved,asitliesonly3GHzaway
thedust,andthatN D+appearsaveryusefultracerofphysical from the telluric water line at 183 GHz, hence its usefulness
2
conditionsintheinnermostcoreregions. toconstrainexcitationconditionswouldbelimitedbycalibra-
tionsensitivitytoeventinyskyfluctuations.Theproblemdoes
notapplytoN D+whichliesafactorof1.2lowerinfrequency.
2
2. Observationsandanalysis Spatialresolutionrangesfrom33′′at77GHzto9′′at279GHz.
ThemaindustcoreinL183(Paganietal.2004)wasobserved AdditionalCSO 10-mobservationsofN2H+ (J:3–2)wereob-
attheIRAM30-mtelescopeinNovember2003,July2004and tained in June 2004 at selected positions. Observations were
October 2006. Spectra were taken on a 12′′ grid in an East- done in Position Switch mode with the high resolution AOS
(48kHz sampling)anda T around600K. A majorinterest
sys
1 TheFortrancodeisavailableuponrequesttotheauthor
L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool 3
2 2
Table 1. Spectral parameters of each line observed towards the PSC center position (α(2000) = 15h54m08.5s, δ(2000)=
−2◦52′48′′) with V = 2.3672(2) kms−1 (from the NH (1,1) measurement): noise level, reference observing frequency,
LSR 3
intrinsiclinewidthofhyperfinetransitionsandtotalopacity(bothfromtheCLASSHFSroutine),relativevelocityandintensity
ofthemainhyperfinegroups(fromgaussianfits).Valuesinparenthesesare1σuncertaintiesonthelastdigit.Forcomparison,
theT∗ ratiosofhyperfinegroupsgivenbytheirstatisticalweightsarelistedinitalics.Thedetailedhyperfinestructureofthe3
R
speciesisdescribedinCasellietal.(1995)forN H+,Doreetal.(2004)forN D+ andKukolich(1967)forNH
2 2 3
N H+(J:1−0) N H+(J:3−2)(IRAM) N H+(J:3−2)(CSO)
2 2 2
Noise(1σ) : 28mK 59mK 64mK
Ref.frequency : 93.173764GHz 279.511832GHz 279.511832GHz
IntrinsicFWHM : 0.195(1)kms−1 0.18(2)kms−1 a0.28(4)kms−1
Totalopacity : 21.8(5) 10(2) 2(1)
Hyperfine V T∗ Area T∗ Ratio V T∗ Area V T∗ Area
rel. R R rel. R rel. R
Group (kms−1) (K) (Kkms−1) obs weights (kms−1) (K) (Kkms−1) (kms−1) (K) (Kkms−1)
1 −8.009(7) 2.05(3) 0.47(3) 0.932 0.428 −2.58(1) 0.17(6) 0.013(5) −2.63(2) 0.20(6) 0.025(8)
2 −0.611(8) 1.81(3) 0.42(3) 0.822 0.428 0.00(1) 0.50(6) 0.29(2) 0.00(1) 0.67(6) 0.37(2)
3 0.000(7) 2.20(3) 0.60(3) 1 1 4.70(8) 2.25(6) 0.014(5) 4.68(3) 2.12(6) 0.028(9)
4 0.956(7) 2.08(3) 0.54(3) 0.945 0.714
5 5.546(7) 2.12(3) 0.47(3) 0.964 0.428
6 5.983(8) 2.04(3) 0.53(3) 0.927 0.714
7 6.94(1) 1.26(3) 0.24(3) 0.573 0.143
N D+(J:1−0) N D+(J:2−1)b N D+(J:3−2)
2 2 2
Noise(1σ) : 31mK 26mK 43mK
Ref.frequency : 77.109616GHz 154.217182GHz 231.321917GHz
IntrinsicFWHM : 0.226(3)kms−1 0.185(2)kms−1 0.18(2)kms−1
Totalopacity : 4.7(3) 4.9(1) 1.5(7)
V T∗ Area T∗ Ratio V T∗ Area V T∗ Area
rel. R R rel. R rel. R
(kms−1) (K) (Kkms−1) obs weights (kms−1) (K) (Kkms−1) (kms−1) (K) (Kkms−1)
1 −9.697(8) 0.77(3) 0.15(1) 0.7 0.428 −5.35(2) 0.16(3) 0.137(5) −3.26(1) <0.12
2 −0.763(8) 0.76(3) 0.15(1) 0.691 0.428 −0.759(8) 0.21(3) 0.080(3) 0.000(5) 0.72(4) 0.186(8)
3 0.000(7) 1.10(3) 0.33(2) 1 1 0.000(1) 1.37(3) 0.513(3) 0.46(2) 0.19(4) 0.06(1)
4 1.146(8) 0.92(3) 0.25(2) 0.836 0.714 0.646(4) 0.40(3) 0.139(3) 2.47(3) 0.11(4) 0.028(7)
5 6.65(2) 0.55(3) 0.16(2) 0.5 0.428 2.5(1) 0.08(3) 0.02(2)
6 7.19(1) 0.90(3) 0.23(2) 0.818 0.714 2.97(2) 0.43(3) 0.08(2)
7 8.34(2) 0.34(3) 0.05(2) 0.309 0.143 3.57(5) 0.26(3) 0.16(3)
NH (1,1) NH (2,2)
3 3
Noise(1σ) : 74mK 76mK
Ref.frequency : 23.6944957GHz 23.7226332GHz
IntrinsicFWHM : 0.195(1)kms−1 0.20(1)kms−1
Totalopacity : 24.2(4) 0.1(7)
V T∗ Area T∗ Ratio V T∗ Area
rel. R R rel. R
(kms−1) (K) (Kkms−1) obs weights (kms−1) (K) (Kkms−1)
1 −19.504(6) 2.62(7) 0.89(4) 0.922 0.363 −0.005(5) 0.73(8) 0.152(7)
2 −7.814(5) 2.68(7) 0.69(3) 0.944 0.273
3 −7.255(8) 1.95(7) 0.57(3) 0.687 0.182
4 −0.17(1) 2.84(7) 1.15(9) 1 1
5 0.30(2) 2.78(7) 1.0(1) 0.979 0.636
6 7.465(9) 2.60(7) 0.72(5) 0.915 0.303
7 7.89(1) 1.91(7) 0.48(5) 0.673 0.152
8 19.318(9) 2.48(7) 0.62(4) 0.873 0.242
9 19.85(1) 1.72(7) 0.42(5) 0.606 0.121
a CSOobservationsaredonewitharesolutionaround0.1kms−1(ormore)whichexplainsthislargervalue
b Thehyperfinestructureistoocomplextobedetailedcompletely.Onlythemaingroupsofhyperfinetransitionsaregiven
4 L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool
2 2
toobservethe(J:3–2)linewiththeCSO isthatthebeamsize cylinderoflength6′,withitsaxisintheplaneofthesky.The
and efficiencyis very similar to the 30-mvalues for the (J:1– intensity distribution along the east-west section of the cylin-
0)line,thusalmostcancelingoutbeamcorrectionerrorsinthe der is taken as the emergent signal along the equator of our
comparisonbetween the two. It is thus a usefulconstraintfor spherical Monte-Carlo model. We then replicate these values
radiativetransfermodelling,eventhoughthehigherresolution alongthe(north-south)cylinderaxisbeforeconvolvingbythe
(9′′) 30-m (J:3–2) observations will be essential to constrain antenna beam. For N D+, the (unpublished) large-scale map
2
abundancesand physicalconditions in the innermostcore re- showsasmallernorth-southextent(≃2′)henceweuseacylin-
gions. Standard reduction techniques were applied using the derlengthof2′forconvolvingourmodel.
CLASSreductionpackage2. InFigs.1and2,weplottheobservationsinT∗ compared
R
ComplementaryobservationsofNH (1,1)and(2,2)inver- toourbestmodel(seeSection 3.4),convolvedbytheantenna
3
sion lines towards the PSC center were also obtained at the beamasexplainedabove.
new Green Bank 100-m telescope (GBT) in November 2006
withvelocitysamplingof20ms−1 andatypicalT of50K,
sys 3. Modellinganddiscussion
in Frequency Switch mode. The angular resolution (∼35′′) is
closetothatofthe30-minthelowfrequency(J:1-0)N2D+line. 3.1.Radiativetransfercode
The main beam efficiency of the GBT at 23 GHz is between
0.95and1,hencenocorrectionwasappliedtoputthespectra OursphericalMonte-CarlocodeisderivedfromBernes’code
onT∗ scale. (Bernes1979)andhasbeenextensivelytestedonmodelsofCO
R
Table 1 summarizesthe main observationalparametersof emissionfromdensecores(e.g.Pagani1998).Itwasrecently
the lines observed towards the core center: noise level of updated to take into account overlap between hyperfine tran-
the spectrum (rms), rotational line total opacity and intrinsic sitionsoccuringatcloseoridenticalfrequencies.Thisisdone
FWHM width of individualhyperfinetransitions (as fitted by by treating simultaneously the photon transfer for all hyper-
theCLASSHFSroutine,whichassumesequalexcitationtem- fine transitionsofthe same rotationalline (see also Appendix
perature for all sublevels), and relative velocity centroids and in Go´nzalez-Alfonso & Cernicharo 1993). Instead of choos-
intensitiesofthemainhyperfinegroupsinourspectra(derived ing randomly the frequency of the emitted photon, a binned
fromgaussianfits). Notethatthehyperfinegroupsare always frequencyvectorisdefinedforeachrotationaltransitioninside
broaderthantheintrinsiclinewidthderivedbyCLASS,dueto whichallhyperfinetransitionsarepositioned.Allbinsarefilled
opticaldeptheffectsand/orto thepresenceofadjacenthyper- with their share of spontaneously emitted photons (or 2.7K
finecomponentstooclosetobespectroscopicallyseparated. continuumbackgroundphotons)and, duringphotonpropaga-
To comparewith models,beamefficiencycorrectionis an tion, absorptioniscalculatedfor eachbin bysummingallthe
important issue. Since L183 (as well as other PSCs) is very hyperfinetransitionopacitiesatthatfrequency
extendedcomparedto the primarybeamof10′′–30′′, the T
mb
intensityscale is inadequate,as itwill overcorrectforsource- τ(ν)= κ(ν)ds= τφ(ν)= κφ(ν) ds
abnritgehntnnaescso3u(pclfi.nPgPaAnBdCth)u.sToovaevroeisdtitmhiastpertohbeletrmue,osbosuerrcveesdusrpfaecce- Z Xi i i Z Xi i i 
tra are correctedonlyformoonefficiency(∼T∗ scale), while where κi is the absorptioncoefficient,τi the opacityand φi(ν)
modelsareconvolvedbythefullbeampatternoRfthe30mtele- the local frequencyprofile of the ith hyperfine transition. The
scope(cf.Greveetal.1998)4,yieldingintensitiesonthesame total number of absorbed photons Io(ν)e−τ(ν) is then redis-
T∗ scale.FortheCSO,wealsocorrectthedataformooneffi- tributedamonghyperfinelevelsaccordingtotheirrelativeab-
R
ciency(η ≈0.8at279GHz)andasthedetailsofthebeam sorptioncoefficients
moon
pattern are not known, the model output is convolved with a
κφ(ν)
simplegaussianbeam.BecausetheCSOmainbeamcoupling dIi(ν)= iκ(iν) ×Io(ν)e−τ(ν)
isverygoodat1.1mm(η ≥0.7),theuncertaintyofthecor-
MB
rectionshouldbelimited. Overlap is treated for all rotational transitions. Statistical
Inordertotreatcorrectlythebeamcouplingtothesource, equilibrium,includingcollisions,isthensolvedseparatelyfor
wealsotakeintoaccountthefactthattheL183coreislocated eachhyperfineenergylevel.
within a larger-scale N H+ filament oriented roughly north- Collisionalcoefficientsareavailablefortransitionsbetween
2
south,asrevealedbypreviouslow resolutionmaps(PPABC). allindividualhyperfinelevels,butwerecalculatedwithHe as
Thiselongationismostlyconstantinintensityover≃6arcmin- acollisionerinsteadofH (Danieletal.2005).Wescaledthem
2
utes.Thereforeweapproximatethebrightnessdistributionasa up by 1.37 to correct for the difference in reduced mass, but
notethatthiscorrectionisonlyapproximate(seeWilleyetal.
2 http://www.iram.fr/IRAMFR/GILDAS/
2002andDanieletal.2006a).Thecodeworksforbothisotopo-
3 Teyssier et al. (2002) provide appropriate corrections for 30-m
logues.Asonecanprobablyneglectthevariationoftheelectric
data, but only for uniform circular disk sources, and for data taken
dipolemoment(Gerinetal.2001),theN D+ Einsteinsponta-
beforethe1998surfacereadjustment. 2
4 The last surface readjustment of the 30-m in1998 has not been neousemissioncoefficientsarederivedfromthoseofN2H+by
characterizedindetail,thereforewehavescaleddowntheerrorbeam simplyscalingthemdownby1.23.Deexcitationcollisionalco-
couplingcoefficientsgiveninGreveetal.(1998)toretrievethenew, efficients are kept the same as for N2H+ (following Daniel et
improvedbeamefficiencies al.2006b).
L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool 5
2 2
Fig.2. N D+ observations (grey
2
lines) compared to our best radia-
tive transfer model for the largest
N D+ abundance possible (dotted
2
histogram in Fig 4a). All data are
fromIRAM30-m.Onlythecentral
tripletisrepresentedforthe(J:1–0)
transition
Thelocallineprofileinour1Dcodetakesintoaccountboth velocityfieldofafewtensofms−1wasfurtherimposedbeyond
thermalandturbulentbroadening,aswellasanyradialandro- 3000AU (see Table 2) to reproducethe smallanti-symmetric
tationalvelocityfields.Inprinciple,rotationintroducesadevi- velocityshiftfromPSCcenterobservedatdistancesbeyond±
ation from spherical symmetry. However, for small rotational 30′′(cf.Section2).Theradialvelocitywaskeptto0inalllay-
velocities (less than half the linewidth, typically), the excita- ers.AsseeninFigs.1and2,thisapproximationalreadygives
tionconditionsdonotnoticeablychangewithinagivenradial aremarkablefittoobservedlineprofiles,andisthereforesuffi-
shell,asshownbyaprevious2–Dversionofthiscode(Pagani cienttoderivetheoveralltemperatureandabundancestructure.
&Bre´artdeBoisanger,1996).Thereforethe(muchfaster)1D We considered a variety of density profiles: single power
versionremainssufficientlyaccurate. lawprofilesρ∝r−1,ρ∝r−1.5,ρ∝r−2,aswellasbrokenpower
OurMonte-Carlomodelshowsthatexcitationtemperatures lawswithρ ∝r−1 outto4000AUfollowedbyr−2,orbyr−3.5.
amongindividualhyperfinetransitionswithinagivenrotational Forgoodaccuracy,thedensityprofilewassampledin330AU
linedifferbyupto15%,hencetheusualassumptionofasin- = 3′′ thick shells, i.e. 3 to 10 times smaller thanour observa-
gle excitationtemperatureto estimate the line opacity(e.g.in tionalbeamsizes.Inallcases,divergenceatr=0wasavoided
the CLASS HFS routine) is not fully accurate as already no- byadoptinginsider=990AU thedensityslopederivedfrom
ticed by Caselli et al. (1995). Neglecting line overlap affects the ISOCAM data (Pagani et al. 2004). The density profiles
differentially the excitation temperature of hyperfine compo- werethenscaledtogivethesametotalcolumndensityN(H )
2
nents when the opacity is high enough, mostly decreasing it, = 1.4×1023 cm−2 towards the PSC center, as estimated from
butalso increasingit in a few cases. Forexample,in our best theMAMBOemissionmap(Paganietal.2004).Theeffectof
model for the L183 PSC, the excitation temperaturesof indi- changingthetotalN(H )onthefitresultswillbediscussedin
2
vidualhyperfinecomponentschangebyupto10%.Asshown Section3.4.1.
in the last column of Fig. 1, a noticeable difference between Wethenconsideredavarietyoftemperaturesspanningthe
models with and without overlap appears in the (J:3–2) line range 6–10 K in the inner core regions. We also tested non-
shape. constanttemperaturelaws,increasingupto12Kintheouter-
mostlayersand/orintheinnermostlayers.
For each combination of density and temperature laws, a
3.2.Gridofmodels
range of N H+ abundance profiles was investigated.To avoid
2
Our core model has an outer radius of 13333 AU ≃ 2′, fixed a prohibitivenumberof cases, abundanceswere sampled in 6
bythemaximumwidth≃4′ofdetectableN H+emissioninthe concentriclayersonly,correspondingtothespatialsamplingof
2
east-westdirectionacrossthePSC,asseeninlargescalemaps ourspectra(exceptthelast,widerlayerwhichencompassesthe
(PPABC).We assumethatN H+ abundanceiszerooutsideof outermosttwooffsets),andmultiplemaxima/minimawerefor-
2
thisradius(duetochemicaldestructionbyCO). bidden.Thisleadtoatotalofabout40,000calculatedmodels
For all models, the microscopic turbulence is set to with 8 free parameters (6 abundances, 1 temperature profile,
∆v (FWHM)=0.136kms−1.Thiswasfoundsufficienttore- 1 density profile). Table 2 lists the values of H densities for
turb 2
producethewidthofindividualhyperfinegroups,andiscom- 4 density laws, together with the temperature and abundance
parable to the thermal width contribution. A small rotational profilesthatbestfittedthedataineachcase(cf.nextsection).
6 L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool
2 2
Table2. The4maindensitylawsinvestigatedinourmodels,andthecorrespondingtemperatureandabundanceprofilesgiving
the bestfit to N H+ data in eachcase. The leftmostmodelgavethe bestoverallfit(see Table 3) andwas used to computethe
2
synthetic spectra in Figs. 1 and 2. Its best N2D+ abundanceprofile is also listed. Parameters are held constantinside a given
radialshell,exceptfortherotationalspeedwhichislinearlyinterpolated.2.34(6)reads2.34×106.
ρ∝r−1,r−2(Bestmodel)a ρ∝r−1 ρ∝r−1.5 ρ∝r−2
Radius V.rot. Density T Abundance Abundance Density T Abundance Density T Abundance Density T Abundance
kin kin kin kin
A.U. (kms−1) (cm−3) (K) N H+ N D+ (cm−3) (K) N H+ (cm−3) (K) N H+ (cm−3) (K) N H+
2 2 2 2 2
330 0 2.34(6)b 7 2.4(−11) 2(−11) 2.34(6)b 7 7(−11) 3.30(6)c 7 1(−11) 3.46(6)c 9 1(−11)
660 0 2.05(6)b 7 2.4(−11) 2(−11) 2.05(6)b 7 7(−11) 2.89(6)c 7 1(−11) 3.03(6)c 9 1(−11)
990 0 1.55(6)b 7 8.5(−11) 4(−11) 1.55(6)b 7 7(−11) 2.19(6)c 7 7(−11) 2.29(6)c 9 3(−11)
1320 0 1.16(6) 7 8.5(−11) 4(−11) 1.16(6) 7 7(−11) 1.19(6) 7 7(−11) 1.46(6) 9 3(−11)
1650 0 9.27(5) 7 8.5(−11) 4(−11) 9.27(5) 7 7(−11) 7.74(5) 7 7(−11) 9.09(5) 9 3(−11)
1980 0 7.73(5) 7 8.5(−11) 4(−11) 7.73(5) 7 7(−11) 5.55(5) 7 7(−11) 6.10(5) 9 3(−11)
2310 0 6.62(5) 7 1.1(−10) 3(−11) 6.62(5) 7 1(−10) 4.20(5) 7 4(−10) 4.39(5) 9 1(−10)
2640 0 5.80(5) 7 1.1(−10) 3(−11) 5.80(5) 7 1(−10) 3.34(5) 7 4(−10) 3.54(5) 9 1(−10)
2970 0 5.15(5) 7 1.1(−10) 3(−11) 5.15(5) 7 1(−10) 2.74(5) 7 4(−10) 2.68(5) 9 1(−10)
3300 0.01 4.64(5) 7 1.1(−10) 3(−11) 4.64(5) 7 1(−10) 2.29(5) 7 4(−10) 2.20(5) 9 1(−10)
3630 0.01 4.21(5) 7 1.5(−10) 3(−11) 4.21(5) 7 1.5(−10) 1.95(5) 7 3(−10) 1.83(5) 9 6(−10)
3960 0.01 3.54(5) 7 1.5(−10) 3(−11) 3.86(5) 7 1.5(−10) 1.70(5) 7 3(−10) 1.46(5) 9 6(−10)
4290 0.02 3.01(5) 7 1.5(−10) 3(−11) 3.57(5) 7 1.5(−10) 1.49(5) 7 3(−10) 1.22(5) 9 6(−10)
4620 0.02 2.60(5) 7 1.5(−10) 3(−11) 3.31(5) 7 1.5(−10) 1.32(5) 7 3(−10) 1.07(5) 9 6(−10)
4950 0.02 2.26(5) 7 1.3(−10) 5(−12) 3.09(5) 7 5(−11) 1.18(5) 7 3(−10) 9.27(4) 9 4(−10)
5280 0.05 1.99(5) 7 1.3(−10) 5(−12) 2.90(5) 7 5(−11) 1.07(5) 7 3(−10) 8.5(4) 9 4(−10)
5610 0.05 1.76(5) 7 1.3(−10) 5(−12) 2.73(5) 7 5(−11) 9.67(4) 7 3(−10) 7.20(4) 9 4(−10)
5940 0.05 1.57(5) 8 1.3(−10) 5(−12) 2.58(5) 8 5(−11) 8.82(4) 7 3(−10) 6.34(4) 9 4(−10)
6667 0.05 1.27(5) 8 1(−10) ≤4(−12) 2.02(5) 8 2.5(−11) 8.10(4) 7 1.5(−10) 5.25(4) 9 4(−10)
7733 0.05 8.50(4) 9 1(−10) ≤4(−12) 1.57(5) 9 2.5(−11) 6.77(4) 7 1.5(−10) 4.55(4) 9 4(−10)
8867 0.05 6.50(4) 10 1(−10) ≤4(−12) 8.50(4)d 10 2.5(−11) 5.42(4) 7 1.5(−10) 3.46(4) 9 4(−10)
10000 0.02 5.20(4) 11 1(−10) ≤4(−12) 5.20(4)d 11 2.5(−11) 4.41(4) 7 1.5(−10) 2.68(4) 9 4(−10)
13333 0 3.45(4) 12 1(−10) ≤4(−12) 3.45(4)d 12 2.5(−11) 3.68(4) 7 1.5(−10) 1.70(4) 9 4(−10)
a ρ∝r−1(r<4000AU),ρ∝r−2(r>4000AU)
b ThefirstthreelayerdensitiesdonotfollowapowerlawbutthedensityprofilederivedfromtheISOCAMabsorptionmap(Paganietal.
2004)
c Sameasnote(b)above,butrescaledtomaintainaconstanttotalcolumndensityof1.4×1023cm−2(seetext).
d Forthepureρ∝r−1case,densityisnotfallingofffastenoughtoreachambientclouddensityandwecorrectforthisinthelast3layers.
3.3.Selectioncriteria component allows to discard solutions with overall identical
areabutdifferenthyperfinelineratios;2)areaofthe5central
Our best fit selection criteria are based on the use of several channels(=0.15kms−1)ofeachhyperfinecomponentforthe
χ2 and(reduced)χ2ν estimates.BecausetheN2H+ (J:1–0)data samelines(49values),inordertorejectself-absorbedprofiles,
havemuchmoreoverallweightthanthe(J:3–2)data(moreiso- i.e. line profileswhich have the same total area but are wider
latedhyperfinegroupsperspectrum,moreobservedpositions, andself-absorbed;3)totalareaofthe(J:3–2)CSOspectrumat
highersignal-to-noiseratio),computingasingleχ2 permodel (24′′,0′′)(1value).Thissinglemeasurementhelpstoconstrain
ν
ishardlysensitivetothequalityofthe(J:3–2)linefit.However, the density profile; 4) total area of the (J:3-2) 30-m spectrum
the (J:1–0) spectra being optically thick, they give little con- (1value)whichconstrainstheabundance(andthetemperature
straintsonthe temperatureandN2H+ abundanceoftheinner- to some extent) at the core center. These four measurements
mostlayers(withina6′′radius).Thoseareonlyconstrainedby areofcoursenotcompletelyindependentfromeachother,and
theIRAM(J:3–2)observations.TheCSO (J:3–2)spectrumat trying to improveone fit by changinga model parameter can
(24′′,0′′)also addsinterestinginformationonthedensitypro- degrade one or several of the others. We used χ2 for the first
ν
file. two(with41degreesoffreedom)andχ2 forthelasttwo(only
onemeasurement)sothatinallcases∆χ2 ≈1isequivalentto
Toconstrainthemodelswethusevaluatethegoodnessoffit
independentlyforthe(J:1–0)and(J:3–2)data,bycomputingχ2 a1σdeviation.Withthischoice,a3σvariationisequivalentto
∆χ2 ≈1.5forthefirsttwoand∆χ2=9forthelasttwo.
valueson4typesofmeasurements:1)fullareaofeachofthe7 ν
(J:1–0)hyperfinecomponents,ateachofthe7offsetpositions Afterhavingrunallofthemodelsinourgrid,wefirstused
(49values).Thismeasurementissensitivetotheglobaltemper- a single,globalχ2 combiningthefourtypesofmeasurements
ν
atureandabundanceprofiles.Fittingseparatelyeachhyperfine tovisualizetheglobalfitqualityofthevariousmodels.Fig.3
L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool 7
2 2
3.4.PhysicalconditionsintheL183maincore
3.4.1. Densityandtemperaturestructure
Our best density-temperature combination (circled 5-branch
starsymbolinFig.3)hasadensitylawofr−1 outtor≈4000
A.U.,consistentwiththeISOCAMprofile(Paganietal.2004)
andaslopeofr−2 beyond.Thetemperatureisconstantat7K
out to 5600 A.U. and increases up to 12 K at the core edge
(see Table 2). Further increasing the number of warmer lay-
ers failed, as well as introducingwarmer layers in the center.
Keeping the temperature constant at 7 K everywhere signifi-
cantlyworsensthefitofthe(J:3–2)lines(seeTable3).
The main uncertainty in our temperature determination
Fig.3.Globalχ2foralldensityprofilesandtemperaturecases. stems from the assumed total gascolumndensity throughthe
ν
An arrow indicates that the majorityof the modellayers is at core. Decreasing/increasingit by a factor 1.4 (the typical un-
thegiventemperaturebutthatouterlayershaverisingtemper- certaintyfoundbyPaganietal.2004)changesallvolumeden-
atures.Onlyone(circled)solutionsatisfies all4individualχ2 sitiesbythesamefactor.TorecoverthesameN H+ emission,
2
fitting(seetext) wefindthatthekinetictemperatureinourmodelsmustbein-
creased/decreasedby≃1K(respectively,whiletheN H+abun-
2
dance must be scaled accordingly to keep the same column
density).Since thedusttemperatureintheL183coreis7.5±
plotstheglobalχ2asafunctionoftemperature,forthevarious 0.5K(Paganietal.2004),gasanddustarethermalizedinside
ν
density laws that we investigated. Symbols with arrows indi- thiscorewithintheuncertainties.
cate models where temperature rises in the outermost layers. AdifferentresultwasfoundbyBerginetal.(2006)inthe
Foreachdensity-temperaturepair,weplotonlythesmallestχ2 B68PSC,whereouterlayersemittinginCOareconsistentwith
ν
obtainedbyadjustingtheabundanceprofile. akinetictemperatureof7–8KwhileNH measurementsindi-
3
It is readily seen that the best fits are found for relatively cate a higher temperature of 10–11K in the inner 40′′. This
low overall core temperatures ≃ 6.5–8 K. Hence, the (J:1–0) inwardincreaseingastemperaturewasattributedtothelackof
data alone (which dominate the global χ2) set a strong con- efficient CO cooling in the depleted core center, and requires
ν
straint on the core temperature: indeed, the combination of an order of magnitude reduction in the gas to dust coupling,
strongopacityandrelativelylowintensityinthislinerequires possiblyduetograincoagulation(Berginetal.2006).Suchan
lowexcitationtemperature.Becausethe(J:1–0)lineshouldbe effectisnotseeninL183,despitestrongCO depletionwithin
thermalizedinthedensecenter,thisrulesoutkinetictempera- the inner1′= 6600AU radius.Ourbest fittemperaturelaw is
turesabove8K(forouradoptedtotalcolumndensityof1.4× moreconsistentwiththethermo-chemicalevolutionofslowly
1023cm−2). contractingprestellarcoreswithstandardgas-dustcouplingco-
Tofurtherdiscriminateamongmodels,wethenlookedin- efficients,whichpredictslowgastemperatures≃6Knearthe
dividuallyatthefourqualityindicatorsdescribedabove,inpar- corecenter,increasingto≃14Knearthecoresurface(Lesaffre
ticularthoserelatedtothe(J:3-2)lines.Table3liststhese4val- etal.2005).
ues as well as the globalχ2 for variousmodelsselected from
ν
Fig. 3, namely: the best model fits for a given constant core 3.4.2. N H+ abundanceprofile
2
temperature (from 6 to 10 K), and the best model fits for a
given density law (with correspondingtemperatureand abun- Withourbestdensity-temperaturemodel,severalN H+ abun-
2
dancedistributionsgiveninTable2). danceprofilesgiveequallygoodfits(within<1σonall4qual-
It can be seen in Table 3 that for T = 10 K the (J:3– ity indicators). From all these acceptable abundance profiles,
kin
2) lines are not well fitted (4 and 5 σ deviationsfor the CSO we determined a median profile (plotted as a solid histogram
and IRAM lines respectively),hence such a high temperature inFig.4a),witherrorbarsrepresentingtherangeofacceptable
seemsruledouthere.Overall,itisratherdifficultto havelow valuesineachlayer(thoughnotanycombinationoftheseval-
χ2 values in both the (J:1-0) and the (J:3-2) indicators. Only uesdoesfittheobservations).Thismedianprofile,whoseval-
onedensity-temperaturecombination(indicatedbyboldfaces uesarelistedinTable2,isusedtocomputethefitdisplayedin
in Table 3) reaches low values in all 4 individual χ2ν and χ2, Fig.1.ThetotalN2H+columndensityis1.2±0.1×1013cm−2,
eachwithin1σoftheirminimumvalue,amongallthemodels comparable with the value reported by Dickens et al. (2000)
inTable3.Thiscombinationisreferredtoasour“bestmodel” andafactorof∼2belowthatinCrapsietal.(2005).
in the following, and its inferred physicalparameters are dis- The maximum N H+ abundance is 1.5+0.4 × 10−10, the
2 −0.3
cussedinthenextsection.ItcanalsobenoticedinTable3that same as found by Tafalla et al. (2004) in L1498 and L1517.
themodelwithT = 7K hasaslightlybetterglobalχ2 than However,thereisadefinitedropinabundanceintheinnerlay-
kin ν
thebestmodel(2.1comparedto2.3)butistobeeliminatedas ers.Thisdropisimposedbythefittothe(J:3–2)IRAMcentral
the(J:3–2)linesare2and3.5σoffthemark. spectrum (9′′ beam), which is quite sensitive to variations in
8 L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool
2 2
Table 3. Quality of fit evaluated individuallyon four differentsets of measurements, and globalχ2, for selected models from
ν
Fig.3(seetext).Adifferenceof1correspondsto1σinallcases.Valuesforthebestmodelareinboldface.Thelastrowsshow
theeffectofchangingonlythecentralN H+ abundanceinthisbestmodel
2
(J:1–0) (J:3–2)
modelparameters Totalareaχ2 5channelsareaχ2 CSOTotalareaχ2 IRAMTotalareaχ2 Globalχ2
ν ν ν
Bestfitforeachtemperature(adjustedinabundance)
T=6K (ρ∝r−1,r−2) 4.7 3.8 1.5 1.0 3.9
T=7K (ρ∝r−1,r−2) 1.9 2.4 12.6 4.1 2.1
T=8K (ρ∝r−1.5) 3.1 3.2 0.2 3.4 2.9
T=9K (ρ∝r−2) 4.5 5.6 7.5 6.8 4.7
T=10K (ρ∝r−2) 4.8 6.3 16.9 28.7 5.5
Bestfitforeachdensitylaw(adjustedinabundance)
ρ∝r−1,r−2 (T=7→12K) 2.4 2.7 0.3 0.0 2.3
ρ∝r−1 (T=7→12K) 3.7 3.8 1.2 2.9 3.4
ρ∝r−1.5 (T=7K) 3.4 3.2 0.1 0.1 2.9
ρ∝r−2 (T=9K) 4.5 5.6 7.5 6.8 4.7
VaryingthecentralN H+abundanceatr<660AUinthebestmodel
2
X(N H+)= 10−12 2.6 2.8 0.3 2.0 2.5
2
X(N H+)= 8×10−12 2.6 2.8 0.3 1.0 2.5
2
X(N H+)= 2.4×10−11 2.4 2.7 0.3 0.0 2.3
2
X(N H+)= 8×10−11 2.1 2.4 0.5 9.2 2.1
2
Fig.4.aN D+ andN H+ abundancesforthebestmodel.ForN H+,themedianabundanceprofileisplotted(thecorresponding
2 2 2
modeloutputisdisplayedin Fig.1).Errorbarsindicatetherangeofpossiblevalues(seetext).ForN D+,a rangeofvaluesis
2
given:Theupper,dottedhistogramgivesthebestfittothe(J:2-1)and(J:3-2)lines,andwasusedtocomputethemodelinFig.2.
Thedashedhistogramgivesthebestfittothe(J:1-0)and(J:2-1)lines.bdensityprofileofthebestmodelandN D+/N H+ ratio
2 2
range
the central abundanceof N H+, as illustrated in the last rows resultsforN D+wouldfavoravalueofatleast10−11(seenext
2 2
ofTable3.Incontrast,the(J:1–0)lineisopticallythickatthe section).
corecenterandtheχ2isthusnotsensitivetothisparameter(cf.
ν Comparedwiththemaximumabundance,thisgivesavol-
Table3).
umedepletionfactorof≃ 6+13 atthe corecenter.Themedian
−3
Exploringabroadrangeofabundanceprofiles,wefindonly abundance drop is less than (but marginally consistent with)
alimitedrangeofpossibleN H+abundancesintheinnerlayer that expected from simple geometrical arguments in PPABC,
2
of 6′′ radius (r < 660 AU): values above 4.5×10−11 always butitconfirmsthatthelevelingofN H+ intensityseenacross
2
producetoostrong(J:3–2)emissioninthecentralIRAMbeam thedustpeakisnotduetopureopacityeffects.Ascanbeseen
(χ2 > 1), while abundances below 10−12 give a (J:3–2) line fromFig.4a-b,depletionstartsatadensityof5–7×105 cm−3
marginallytooweak.Wethusderivearangeof2.4+2.1×10−11 and increases as density goes up, in agreement with the con-
−2.3
for the central N H+ abundance. However, we note that our clusions of PPABC. The abundancealso drops slightly in the
2
L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool 9
2 2
outermost regions, possibly due to partial destruction by CO.
Indeed, the outermost layers of the model have densities of a
few104 cm−3 whichisthelimitabovewhichCO startstode-
pleteinthissource(PPABC).
3.4.3. N D+ abundanceprofile
2
As seen in Fig.2, goodfits to the N D+ data may be obtained
2
withthesamedensityandtemperatureprofileasourbestmodel
for N H+, although it is difficult to reproducesimultaneously
2
theintensitiesofboth(J:1-0)and(J:3-2)lines.WeplotinFig.
4a the N D+ abundance profiles giving the best fit either for
2
(J:1-0)and(J:2-1)(dashedhistogram)orfor(J:2-1)and(J:3-2)
Fig.5.GBTNH (1,1)inversionlinetowardsthereferencepo-
(dotted histogram). The latter was used to produce Fig. 2. A 3
sition with CLASS NH3 fit (shifted by +3 K for clarity) and
better fit could be obtained with a temperature of 8K instead
fitresidual(shiftedby-0.6K).Thefityieldsatotalopacityof
of7K.Thismightindicatethatthecollisionalcoefficientswith
24.2(±0.4),andaglobalvelocityof2.3672(±0.0002)kms−1
HearesomewhatinaccuratetorepresentthosewithH .Indeed,
2
ithasbeenshownforNH thatcollisionalcoefficientswithHe
3
coulddifferbyafactorupto4withrespecttothosecomputed
withpara-H2(Willeyetal.2002).Asimilarproblemmayoccur 3.5.ComparisonwithNH3 temperatureestimates
here(seealsothediscussioninDanieletal.2006a).Therefore
Thelowkinetictemperatures≤8KinferredfromourN H+and
2
we consider that the temperatures are compatible within the
N D+ modelling are somewhat lower than previous tempera-
uncertaintiesonthecollisionalcoefficients. 2
ture estimates in L183 from NH inversion lines, which gave
3
valuesintherange9–10K(Ungerechtsetal.1980,Dickenset
The N2D+ abundance profile is quite different from that al.2000)upto12K(Swadeetal.1989).However,theseNH3
of N2H+ (cf. Fig. 4a). Its abundance is essentially an upper spectra were not obtained towards the PSC center itself, and
limitbeyond6000A.U.Itincreasessharplybyaboutanorder hadrelativelylowangularresolutionforthelasttwo.We thus
of magnitude in the regionbetween 600 and 4000 A.U., then briefly reconsider this issue using our NH spectra obtained
3
slightly drops by a factor of 2–2.5 in the core center, reach- atthePSCcenterposition,whichalsobenefitfromthehigher
ing an abundancebetween 1.3 and 2×10−11. The low optical resolutionandbeamcouplingofthenewGBT.TheNH spec-
3
depth of the line (τ = 0.84 for the strongest hyperfine com- tra were analyzed in the standard way, as discussed by Ho &
ponent,JFF′:123–012)allowstomeasurethecontributionofall Townes(1983)andWalmsley&Ungerechts(1983).Usingthe
layers to the emission and to determine with relatively little CLASSNH3fittingprocedure,wefoundatotalopacityof24
uncertaintytheabundanceprofile.Inparticular,forthechosen forthe(1,1)inversionline,andanexcitationtemperatureof5.5
densityandtemperatureprofiles,wefindthattheabundanceof Kassumingabeamfilling-factorof1(Figs.5&6).Makingthe
N2D+ inthecenterofthecloudcannotbebelow10−11.Hence, usual assumption of constant temperatureon the line of sight
we can set tighter constraints on the N2D+ abundance at the and of negligible populationin the non-metastablelevels, the
corecenterthanwaspossibleforN2H+. intensity ratio of the (2,2) to (1,1) main lines indicates a ro-
tation temperature of 8.4±0.3K which should correspond to
The N2D+/N2H+ ratio obtained by comparing the N2D+ a kinetic temperatureof 8.6±0.3K. Hence, NH3 emission to-
range of abundances to the median N H+ abundance profile wardsthePSCcenterindicatesanonlyslightlyhigherkinetic
2
is plotted in Fig. 4b. The deuteration ratio varies from an up- temperaturethanN2H+ (8.6±0.3K insteadof7±1K),almost
perlimit≤0.05–0.1awayfromthecoretoaveryhighfactorof equaltothatobtainedwithN2D+ (∼8K).Thediscrepancybe-
0.7±0.12inthedepletionregion.ThisislargerthanwhatTine tweenNH3andN2H+isthusnotaslargeasoriginallythought.
etal.(2000)reported48′′furthernorthinthesamesource,and Bothtracerspointtoverycoldgasinthecore,closetothermal
3–4timeslargerthanwhatCrapsietal.(2005)reporttowards equilibriumwiththedust.
thePSC.Deuteriumenrichmentisthusverystrongandcancer- Reasonsfora possibledifferencebetweenNH andN H+
3 2
tainlybelinkedtothestrongandextendedH D+ linedetected temperaturedeterminationsin L183includethe following:1)
2
towardsthissource(Vasteletal.2006).Ifweconsiderthefull thehighersensitivityofNH tothewarmerouterlayersofthe
3
range of possible valuesfor N H+ itself, the deuterationratio core, since NH inversionlines are much easier to thermalize
2 3
intheinnerlayerrangesfrom∼0.3to≥20.However,asD H+ (n ≈ 2000 cm−3) than N H+ lines. 2) a slight overestimate
2 crit 2
hasnotbeendetectedinthissourcedespiteitsstrongH D+line in the total column density towards the PSC (reducing it to
2
(Vasteletal.2006),anenrichmentabove1seemsunprobable, 1023cm−2,thetemperaturehastoberaisedto8Ktocompen-
suggesting that the central abundanceof N H+ is probably at sateforthedensitydecreaseandrecoverthesameN H+ emis-
2 2
least10−11. sion).3)systematicerrorsintroducedbytheuseofcollisional
10 L.Paganietal.:DepletionandlowgastemperatureintheL183prestellarcore:theN H+-N D+tool
2 2
A coefficients of N H+. We also thank an anonymous referee and
ul 2
C.M.Walmsleyforsuggestionswhichhelpedtoimprovethispaper.
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4. Conclusions
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3
whichalsoindicateverycoldgas(8.6±0.3K)towardsthe
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ListofObjects
4. We have found a noticeable depletion of N H+ by a fac-
2
torof6+13,andofN D+ byasmallerfactorof2–2.5.This ‘L183’onpage2
−3 2
smaller depletion is probably due to a strong (0.7±0.12)
deuterium fractionation, consistent with the detection of
H D+inthiscore.
2
5. N D+ should be a useful probe of the innermost core re-
2
gions, thanks to its low optical depth combined with its
strongenhancement.
Acknowledgements. WethanktheIRAMdirectionandstafffortheir
support, S.Le´onfor hisdedicated assistanceduring pool observing,
andF.Daniel forprovidingroutinestocompute thefrequencies and