Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions

26 Pages·2017·1.84 MB·English

Comparative study of CH+ and SH+ absorption lines
observed towards distant star-forming regions
Benjamin Godard, E. Falgarone, M. Gerin, D.C. Lis, M. de Luca, J.H. Black,
J. R. Goicoechea, J. Cernicharo, D.A. Neufeld, K. M. Menten, et al.
To cite this version:
Benjamin Godard, E. Falgarone, M. Gerin, D.C. Lis, M. de Luca, et al.. Comparative study of CH+
and SH+ absorption lines observed towards distant star-forming regions. 2012. ￿hal-00662960￿
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Astronomy&Astrophysicsmanuscriptno.Godard2011˙SHp-CHp˙v17˙print c ESO2012
(cid:13)
January25,2012
+ +
Comparative study of CH and SH absorption lines observed
⋆,⋆⋆
towards distant star-forming regions
B.Godard1,E.Falgarone2,M.Gerin2,D.C.Lis3,M.DeLuca2,J.H.Black4,J.R.Goicoechea1,J.Cernicharo1,D.A.
Neufeld5,K.M.Menten6,M.Emprechtinger3
1 DepartamentodeAstrof´ısica,CentrodeAstrobiolog´ıa,CSIC-INTA,Torrejo´ndeArdoz,Madrid,Spain
2 LERMA,CNRSUMR8112,E´coleNormaleSupe´rieure&ObservatoiredeParis,Paris,France
3 CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
4 DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaSpaceObservatory,43992Onsala,Sweden.
5 TheJohnsHopkinsUniversity,Baltimore,MD21218,USA
6 MPIfu¨rRadioastronomie,Bonn,Germany.
Received8July2011/Accepted19January2012
Abstract
Aims.TheHIFIinstrumentonboardHerschelhasallowedhighspectralresolutionandsensitiveobservationsofground-statetransi-
tionsofthreemolecular ions:themethylidynecationCH+,itsisotopologue 13CH+,andsulfanyliumSH+.Becauseoftheirunique
chemicalproperties,acomparativeanalysisofthesecationsprovidesessentialcluestothelinkbetweenthechemistryanddynamics
ofthediffuseinterstellarmedium.
Methods.The CH+, 13CH+, and SH+ lines are observed in absorption towards the distant high-mass star-forming regions (SFRs)
DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51, and towards two sources close to the Galactic centre, SgrB2(N) and
SgrA*+50.Allsightlinessamplethediffuseinterstellarmatteralongpathlengths ofseveralkiloparsecsacrosstheGalacticPlane.
Inordertocomparethevelocitystructureofeachspecies,theobservedlineprofilesweredeconvolvedfromthehyperfinestructure
oftheSH+transitionandtheCH+,13CH+,andSH+spectrawereindependentlydecomposedintoGaussianvelocitycomponents.To
analysethechemicalcompositionoftheforegroundgas,allspectraweredivided,inasecondstep,intovelocityintervalsoverwhich
theCH+,13CH+,andSH+columndensitiesandabundanceswerederived.
Results.SH+ isdetectedalongallobservedlinesofsight,withavelocitystructureclosetothatofCH+ and13CH+.Thelinewidth
distributions of the CH+, SH+, and 13CH+ Gaussian components arefound to be similar. These distributions have thesame mean
( ∆υ 4.2kms 1)andstandarddeviation(σ(∆υ) 1.5kms 1).Thismeanvalueisalsoclosetothatofthelinewidthdistribution
− −
ohfthie∼CH+ visibletransitionsdetectedinthesolarn∼eighbourhood. Weshowthatthelackofabsorptioncomponentsnarrowerthan
2kms 1 isnotanartefactcausedbynoise:theCH+,13CH+,andSH+ lineprofilesarethereforestatisticallybroaderthanthoseof
−
mostspeciesdetectedinabsorptionindiffuseinterstellargas(e.g.HCO+,CH,orCN).TheSH+/CH+columndensityratioobserved
inthe components located away fromthe Galacticcentrespans twoorders of magnitude and correlates withtheCH+ abundance.
Conversely,theratioobservedinthecomponentsclosetotheGalacticcentrevariesoverlessthanoneorderofmagnitudewithno
apparentcorrelationwiththeCH+abundance.TheobserveddynamicalandchemicalpropertiesofSH+andCH+areproposedtotrace
theubiquitousprocessofturbulentdissipation,inshocksorshears,inthediffuseISMandthespecificenvironmentoftheGalactic
centreregions.
Keywords.Astrochemistry-Turbulence-ISM:molecules-ISM:kinematicsanddynamics-ISM:structure-ISM:clouds
1. Introduction terunderstandingofitsdynamical,thermal,andchemicalevolu-
tion.Awidevarietyofdiatomicandtriatomicmolecularspecies
Studyingthediffusephasesoftheinterstellarmedium(ISM)is hasalreadybeenobservedinthediffusemedium.Moreover,its
essential,notonlybecausetheycontainalargepartofthetotal chemicalcompositionhas now been probedin the solar neigh-
gasmassofthecoldISMandaretheprecursorsofdenseclouds, bourhoodthroughUV (e.g. Shull & Beckwith 1982;Crawford
butalsobecausetheyharbourthefirststepsofinterstellarchem- & Williams 1997;Snow et al. 2000;Rachfordet al. 2002;Gry
istry.SincethedetectionofthefirstdiatomicmoleculesCN,CH, etal. 2002;Lacouretal. 2005),visible (e.g.Crane etal. 1995;
andCH+(seereferencesinthereviewofSnow&McCall2006) Gredel1997;Thorburnet al. 2003;Weselak etal. 2008;Maier
throughtheir narrow visible absorptionlines, the improvement etal. 2001),andradio(e.g.Haud& Kalberla 2007;Lisztet al.
of theobservationaltechniquesandinstrumentshasprovideda 2006andreferencestherein)spectroscopy,andtheinnerGalaxy
more comprehensiveview of the diffuse ISM and led to a bet- materialthroughsubmillimetre and radio(millimetre, centime-
tre) spectroscopy (e.g. Koo 1997; Fish et al. 2003; Nyman &
⋆ BasedonobservationsobtainedwiththeHIFIinstrumentonboard Millar 1989; Greaves & Williams 1994; Neufeld et al. 2002;
theHerschelspacetelescopeintheframeworkofthekeyprogrammes Plumeetal.2004).
PRISMASandHEXOS.
⋆⋆ Herschel is an ESA space observatory with science instruments The Herschel space mission has broadened this investiga-
providedbyEuropean-ledPrincipalInvestigatorconsortiaandwithim- tion,givingaccessto thefullsubmillimetredomain,whichhas
portantparticipationfromNASA. allowedthedetectionofmanymolecularspeciesthatcouldnot
1
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
bedetectedfromthegroundbeforebecauseofthehighopacity 2. Observationsanddatareduction
oftheatmosphere.IntheframeworkoftheHIFIkeyprogramme
2.1.Observingconditions
PRISMAS (PRobing InterStellar Molecules with Absorption
line Studies) many hydridessuch as HF (Neufeldet al. 2010b;
Sonnentrucker et al. 2010), OH+, H O+(Neufeld et al. 2010a;
2
Gerin et al. 2010a), CH (Gerin et al. 2010b), NH, NH , NH
2 3
(Perssonetal.2010),andCH+ (Falgaroneetal.2010)werede-
tected in absorption against the strong continuum emission of
distantstar-formingregions,providingforthe firsttimea good
censusoftheselighthydridesintheinnerGalaxy.
Of all molecules targeted by PRISMAS, the methylidyne
cation CH+ is particularly interesting because its presence in
the diffuse ISM remains one of the most intriguing puzzle in
astrophysics. The CH+ abundances predicted by steady-state,
UV-dominated, PDR-type (PhotoDissociation Regions) chemi-
cal models are several orders of magnitude lower than the ob-
servedvalues,becausealltheCH+ formationpathwaysthatare
alternativeto thehighlyendothermicC+ +H CH+ +H re-
2
→
action,areinefficientinbalancingitsfastdestructionbyhydro-
genation. Indriolo et al. (2010) recently showed that the upper
limit on the CH+/CH+ abundance ratio observed towards Cyg
3
OB2 can only be reproduced in diffuse molecular clouds with
Figure1.OriginalSH+ spectra(double-sideband antenna temperature
extremeconditions(i.e. fH2 < 0.2,orT > 1000K).Sofar,the TA beforeremovingthestandingwaves)observedtowardsSgrA*+50,
solutiontothispuzzlehasbeentoinvokeregionsofthediffuse W31C, W33A, and W49N in the horizontal polarization and for the
gaswhereawarmchemistryisactivatedbythereleaseofsupra- threedifferentLOsettings(inblack,red,andblue).Formoreclaritythe
thermal energy in the cold ISM induced by low-velocity mag- redandbluecurveshavebeenshiftedfromtheblackcurveby0.2and
netohydrodynamic(MHD)shocks(Draine&Katz1986;Pineau 0.4K(fortheSgrA*+50data),andby0.1and0.2K(fortheW31C,
desForeˆtsetal.1986),Alfve´nwaves(Federmanetal.1996),tur- W33A,andW49Ndata).
bulentmixing(Xieetal.1995;Lesaffreetal.2007),orturbulent
dissipation (Falgarone et al. 1995; Joulain et al. 1998; Godard
et al. 2009).Indrioloet al. (2010)claimed that observationsof
The observations were carried out from March to October
theexcitedlevelsofCH+areabletoprovidethecluesnecessary
3 2010 and in April 2011 towards the eight submillimetre back-
tofavouronetheoryovertheother.
groundcontinuumsourceslistedinTable1(withtheirGalactic
Apotentiallyrelatedproblemistheexistenceofsulfanylium coordinates, their distance from the Sun, and their measured
(SH+)inthediffusegas.Becausethehydrogenationreactionof single-sideband continuum temperature Tc in K at 830 and
S+ has an endothermicity twice as high as that of C+, a mea- 530 GHz). Using the dual-beam switch (DBS) m∼ode (with
surementoftheSH+/CH+abundanceratioshouldprovidevalu- ∼a throwat 3’ fromthe source)and the wide-bandspectrometer
ableinsightsabouttheregionswhereCH+isformed.Soughtfor (WBS) of Herschel/HIFI (see Roelfsema et al. 2012 for a de-
without success in the local diffuse ISM in absorption against tailed description of the propertiesand performancesof HIFI),
nearby stars in the UV domain since 1988 (Millar & Hobbs weobserved
1988; Magnani & Salzer 1989, 1991), this molecular ion was
the J = 1 0 absorption lines of CH+ and 13CH+, in the
recently detected by Menten et al. (2011) through its ground-
• ←
upperandlowersidebandsofband3a,
staterotationaltransitioninthesubmillimetrerangeobservedin
andtheF = 3/2 1/2,5/2 3/2,and3/2 3/2hyper-
absorption towards the Galactic centre line of sight SgrB2(M)
• ← ← ←
finecomponentsoftheN,J =1,2 0,1absorptionlineof
withtheAPEXtelescope.
SH+,inthelowersidebandofband←1a.
In this paper, we report the detection of SH+, CH+, and
13CH+ towardsthe sixdistantstar-formingregionsDR21(OH), The data obtained towards SgrB2(N) were part of the full
G34.3+0.1,W31C, W33A, W49N, and W51, and the Galactic HIFI spectral scan performed by the HEXOS key programme;
centresightlines1 SgrA*+50andSgrB2(N),andwe performa the corresponding double-sideband spectra were deconvolved
crossanalysisoftheobservedpropertiesofthosethreespecies. into single-sideband spectra, including the continuum (Comito
The observations, obtained in the framework of the key pro- & Schilke 2002). Towards the other sources, the observations
grammesPRISMASandHEXOS,arepresentedinSect.2.The were performed in the framework of the PRISMAS key pro-
methodsusedfortheanalysisandtheresultsobtainedareshown gramme;toseparatespectralfeaturesfromupperandlowerside-
inSects.3and4,respectively.WeconcludethisworkinSect.5 bands of the WBS spectrometer, each transition was observed
with a discussion on the chemical and dynamicalpropertiesof usingthreeslightlydifferentsettingsofthelocaloscillator(LO)
thegasseeninabsorption.Thecomparisonwiththemodelpre- frequency,adaptedtoinducearelativevelocityshiftof 30km
dictions will be the subject of a forthcoming paper (Godard et s 1betweenthetwosidebands.Thespectroscopicparam∼etersof
−
al.,inprep.). theobservedlinesarelistedinTable2,alongwiththermsnoise
levels relative to the single-sideband continuum intensities ob-
tained with an on-sourceintegrationtime rangingfrom1 to 20
1 TheSgrA*+50sightline(GCM-0.02-0.07) corresponds tothe50 min.Inthesefrequencyranges,theWBSresolutionof1.1MHz
bkrmigsh−t1suclbomudillliomceattreedsionurthcee(vDicoiwnietylloeftaSlg.r1A9*9,9)w.hichisknowntobea caonrdre13spCoHn+dstrtaonvseitlioocnisty,arnesdooluftion0s.5o7fk∼m0.s361kfomrsth−1efSoHr+thteraCnHsi+-
−
∼
2
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
Table1.Propertiesofbackgroundsources.
Source RA(J2000) Dec(J2000) l b Da R T (K)b T (K)b
G c c
(h)(m)(s) ( )()( ) ( ) ( ) (kpc) (kpc) (830GHz) (530GHz)
◦ ′ ′′ ◦ ◦
SgrA*+50 174550.2 -285953 359.97 -0.07 8.4 0.1 0.4 0.2
SgrB2(N) 174719.9 -282218 0.68 -0.03 8.4 0.1 8.0 2.9
G34.3+0.1 185318.7 011458 34.26 0.15 3.8 5.8 2.3 0.7
W31C 181028.7 -195550 10.62 -0.38 4.8 3.9 1.9 0.5
W33A 181439.4 -175200 12.91 -0.26 4.0 4.7 0.6 0.2
W49N 191013.2 +090612 43.17 +0.01 11.5 7.9 2.4 0.8
W51 192343.9 +143031 49.49 -0.39 7.0 6.6 2.7 1.0
DR21(OH) 203901.0 422248 81.72 0.57 1.0 8.4 1.3 0.4
a ForG34.3+0.1,W31C,W33A,andW49N,thesourcedistancesD,andthesubsequentGalactocentricdistancesR ,aretakenfromFishetal.
G
(2003)andPandianetal.(2008),whoresolvedthekinematicdistanceambiguity.Distanceuncertaintiesareoftheorderof0.5kpc.
b UncertaintiesonthecontinnuumtemperaturesT areoftheorderof10%(Roelfsemaetal.2012).
c
Table2.CH+X1Σ+,13CH+X1Σ+,andSH+X3Σ spectroscopicparametersfortheobservedpurerotationaltransitions.Numbersinparenthesisare
−
powerof10.
Transition g ν a A ∆υ b I c σ/T d
u 0 h h l c
(GHz) (s 1) (kms 1) SgrA*+50SgrB2(N)DR21(OH) G34 W31C W33A W49N W51
− −
CH+ 1-0 3 835.137504 5.83(-3) 1.1(-2) 9.8(-3) 1.5(-2) 1.9(-2)5.2(-2)2.0(-2)1.7(-2)1.7(-2)
13CH+ 1,1/2-0,1/2 2 830.215004 5.83(-3) 0.59 0.50
13CH+ 1,3/2-0,1/2 4 830.216640 5.83(-3) 0.00 1.00 5.5(-2) 5.5(-3) 1.5(-2) 2.0(-2)1.5(-2)2.9(-2)1.4(-2)2.0(-2)
SH+ 1,2,3/2-0,1,1/2 4 526.038722 7.99(-4) +5.26 0.56
SH+ 1,2,5/2-0,1,3/2 6 526.047947 9.59(-4) 0.00 1.00 5.0(-2) 1.5(-2) 2.9(-2) 2.5(-2)9.0(-3)2.4(-2)1.1(-2)1.5(-2)
SH+ 1,2,3/2-0,1,3/2 4 526.124976 1.60(-4) -43.93 0.11
a CH+ 13CH+ andSH+linefrequenciesarefromAmano(2010)andSavageetal.(2004),respectively.
b VelocityshiftsofthehyperfinecomponentsoftheSH+(1,2 0,1)line,relativetotheF =5/2 3/2transition,thereferencehyperfine
← ←
transitioninthetext.
c Intensitiesofthehyperfinecomponentscomputed(atT )asg A /g A ,relativetoachosenreferencehyperfinetransition(desig-
ex → ∞ u ul u0 ul0
natedbytheindex0inthepreviousformula).
d σ/T isthermsnoiseleveldividedbythecontinuumintensitiesofthespectraattheresolutionof1.1MHz.
l c
tions,andtheHerschelHPBWis26”at835GHzand41”at526 oftheSH+ absorptionfeatures(seeFig.1),theresultingspectra
GHz. weresensitivetotheFHFinputparameters:severalplausibleso-
The data were calibrated with hot and cold blackbodies lutionswerefounddependingonthenumberofsub-bandstaken
(Roelfsema et al. 2012), reduced using the standard Herschel intoaccountforthefit,thenumberofsinewavestoremove,and
pipeline to Level 2, and subsequently analysed using the the use of a frequencymask. We estimated a maximalerror of
Herschel Interactive Processing Environment2 (HIPE v5.1, Ott 50%onthedeepestabsorptionfeaturesofG34.3+0.1andW51,
etal.2010).ThefinalanalysiswasperformedwiththeGILDAS- 30 % on those of W33A, and W31C and 10 % on the others.
CLASS90software3(Pety2005),andasetofFortran95numeri- Thelattervalueiscomparabletothatduetotheuncertaintieson
calroutinesthatwedeveloped.Whilethesignalsmeasuredinthe thebeamefficiencyandthesidebandgainratio(Roelfsemaetal.
two orthogonalpolarizations that were obtained with the three 2012).
LOsettingsagreedexcellentlyforboththeCH+ and13CH+ line For each transition and both polarizations, we obtained an
observations,theSH+spectra,displayedinFig.1,exhibitstand- average spectrum by combining the data from the three obser-
ing waves (SW) of identified origin4 and removed using the vationswithdifferentLOsettings.Becauseweareinterestedin
HIPE sine wave fitting task FitHifiFringe (FHF). Since the in- thevelocitystructureandthepropertiesoftheabsorbinggas,the
ferreddetectedopacitiesofSH+arelow,andsincetheperiodand spectrainbothpolarizationswerenormalizedtotheirrespective
theamplitudeofthewavesaresimilartothesizeandthedepth continuumtemperature.As in Falgaroneetal. (2010),we used
the saturatedshape ofthe CH+ absorptionlineprofilestomea-
surethesidebandgainratiosRat835.1375GHz,definedasthe
2 Seehttp://herschel.esac.esa.int/HIPE download.shtmlformorein-
ratioofthecontinuumtemperaturesinthelowerandupperside-
formationaboutHIPE.
bands.Forallspectrawithsaturatedabsorptionlines,wefound
3 See http://www.iram.fr/IRAMFR/GILDAS for more information
R 0.9 1andR 0.7 0.8inthehorizontalandverticalpolar-
aboutGILDASsoftwares. ∼ − ∼ −
izations,respectively.We finallyusedthesevaluesat835GHz,
4 MostoftheobservedSWhaveaperiodof 90 100MHz,iden-
∼ − andR = 1at830GHzand526GHz,tocombinethedatafrom
tified by Roelfsema et al. (2012) as reflections occuring between the
mixer focus and the cold and hot black bodies. For two spectra ob- bothpolarizations,andobtainthefinalaveragespectra(withrms
servedtowards W49Nwehadtoremoveanadditional standing wave noise levels given in Table 2) shown in Fig. 2 as functions of
withaperiod 150MHz. the LSR (local standardof rest) velocity.This figure illustrates
∼
3
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
the quality of the baselines over the large bandwidth for most 3. Analysisofthelineprofiles
ofthespectra.Thestrongemissionlinesdetectedinthespectra
showninFigs.1and2wereidentifiedastheHCO+(6 5)and 3.1.Multi-Gaussiandecomposition
→
H2CO(6 5)transitionsnear535.1GHzand525.6GHz,three As in Godard et al. (2010), the decomposition of the spectra
→
methanol lines near 829.9 GHz and 830.3 GHz, and the SO2 in velocitycomponentswas deducedthrougha multi-Gaussian
emissionbandnear835GHz,emittedbytheSFRsthemselves. fitting procedure that we developed, based on the Levenberg-
Marquardtalgorithm,whichtakesadvantageoftheinformation
carriedbythehyperfinestructureofagiventransition.Thisalgo-
2.2.DeconvolutionoftheSH+hyperfinestructure rithmisappliedtoadjust,intheleast-squaressense,theminimal
numberofGaussiansrequiredtodescribethedatawithintheob-
Unfortunately (see Table 2), the velocity shifts ∆υ associated
h servationalerrorswithoutintroducinganysystematiceffect.The
withtheSH+hyperfinetransitionsaresmallerthantheobserved
number of Gaussians was increased, for instance, when we vi-
velocity ranges of the SH+ absorption spectra, and prevent us
sually spotted serpentine curves - characteristic of poor fits in
fromperforminga directcrosscomparisonofthe velocitypro-
the line wings - in the residuals. Thus, for each transition, the
filesof theSH+, CH+, and13CH+ lines. To solvethisproblem,
observednormalizedlineprofile(line/continuum)iswritten
wedevelopedanumericalproceduretoextractthesignalassoci-
aetqeudatwioitnhsefoacrhτrh(yυp),eorfivneretthraenesnittiioren,absosolvripntgiotnhveefloolcloitwyidnogmsaeitno,f TT(υc) =exp−XjN=c1XkN=h1Ih(k)τ0(j)e−21(cid:20)υ−υ0σ(j0)−(j∆)υh(k)(cid:21)2, (2)
whereτ , υ , andσ are the usualGaussian fitparameters.All
0 0 0
Nh spectraweredecomposedindependentlyfromoneanother,with-
Ih(k)τr(υ ∆υh(k))= ln[T(υ)/Tc], (1) out imposing any constraints on the Gaussian parameters. The
Xk=1 − − choiceoftheinputparameters,namelytheinitialvaluesofυ0(j)
and σ (j) for each velocity components j, was guided by the
0
comparisonofthedifferentlinesobservedtowardseachsource.
whereN , I ,τ ,andT(υ)/T arethenumberofhyperfinetran-
h h r c To correctlydeterminethe opacityof weak absorptionfeatures
sitions,theintensitiesrelativetothestrongesthyperfinecompo-
blendedwithsaturatedlines,asobservedintheCH+spectra,we
nent, the opacity of the reference hyperfine transition, and the
appliedanempiricalmodelconstrainedbythewingsofthesatu-
normalized line profile (line/continuum), respectively. The re-
ratedlineprofiles.Theresultsofthemulti-Gaussiandecomposi-
sultingspectraareshowninFig.3,and,asanexample,theout-
tionprocedureandtheassociatederrorsontheGaussianparam-
come of the hyperfine decomposition code, applied to the ab-
etersaregiveninTableA.1ofAppendixA,andtheresultingfits
sorptionlinesobservedtowardsSgrB2(N)andW49N,isshown
and models of the saturated line profiles are displayed in Figs.
inFig.4.Thisfigureillustratestheexcellentagreementbetween
A.1-A.8.Becauseweaimtocomparethekinematicsignatures
the originaldata (inblack)andthe spectra rebuiltafterdecom-
oftheCH+,13CH+,andSH+spectra,wediscussbelowtherelia-
position(ingreen).
bilityoftheextractedGaussiancomponentsinviewoftheerrors
The13CH+ J =1 0linealsoexhibitsaspin-rotationsplit- ontheirparameters.
←
ting (Amano 2010), although the associated F = 3/2 1/2
←
and F = 1/2 1/2 transitions are separated by only 1.636
MHz( 0.59km←s 1)andarethereforetooclosetobeindividu- 3.2.Validityandself-consistencyofthemulti-Gaussian
−
allyres∼olvedgiventhesignificantvelocitydispersionofthegas. decompositions
Thishyperfinestructureinducesasystematicbroadeningofthe
Sincethedecompositionalgorithmallowsthedetectionofcom-
absorption velocity components that depends on their FWHM
ponentswith veryweak centralopacities, the numericalproce-
∆υ :for∆υ varyingbetween2and10kms 1thebroaden-
real real − duremayconvergeuponGaussiancomponentswhoserealityis
ingranges5between4%and0.1%.Sincethiserrorisfarsmaller
questionable. To keep only the most reliable velocity compo-
than that imputable to the rms noise levels of the 13CH+ spec-
nentsforoursubsequentanalysisofthe linewidths,we applied
tra (see Sect. 3), we chose to ignore the hyperfine structure of
thefollowingdetectioncriterion:anyGaussiancomponentwas
13CH+ inthefollowinganalysis.
considered real if its Gaussian parameters simultaneously ver-
The spectra shown in Fig. 3 are highly structuredand have ify ∆υ > 3σ(∆υ) and τ > 2.5σ(τ), where σ(∆υ) and σ(τ) are
thefollowingremarkableproperties:(1)thankstothehighsen- theerrorson∆υ andτ respectively.Theresultingconfirmedor
sitivity of the HIFI receiver, the SH+ ion is seen in absorption uncertainGaussiancomponents(inthefollowingC-components
alongeverylineofsight;(2)allhydridelinesaredetectedinab- andU-components)areindicatedinTableA.1(incolumns4,8,
sorption,andwithinthelimitsimposedbythesignal/noise(S/N) and12),andinFigs.A.1-A.8(solidredanddashedbluecurves)
ratio, CH+ 13CH+ and SH+ absorptions are detected over the ofAppendixA.
wholevelocityrangeoftheforegroundmatteralongeachlineof Intotalwefoundthat25C-componentsaresimultaneously
sight;(3)althoughtheopacityratiosvaryfromonelineofsight observedin atleast two molecularspectra towardsDR21(OH),
to anotherand from onevelocity rangeto another,the velocity G34.3+0.1,W31C,W33A,W49N,andW51.Whencompared,
structureofSH+ issimilartothoseofCH+ and13CH+.Itisthe the positions of these componentsare found to agree with one
similarity and differences of these absorption line profiles that anotherwithin their respective error for 18 of them, within 0.5
arethefocusofthepresentstudy. kms 1 for6ofthem,andwithin1.3kms 1 for1ofthem;sim-
− −
ilarly, out of the 17 common C-components observed towards
SgrA*+50 and SgrB2(N), 10 are found to agree with one an-
5 Thisresultonthelineprofilebroadeningisderivedfromtheanaly- otherwithin 1 km s−1, 6 within 2 kms−1, and1 within 3.5km
sisof1760syntheticspectratakingintoaccountthehyperfinestructure s−1.ExceptfortheSH+componentsat-126kms−1observedto-
of13CH+(linestrengthandvelocitystructure,Amano2010) wardsSgrA*+50theseshiftsinthecentralpositionsareatleast
4
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
fourtimessmallerthanthecorrespondingGaussianlinewidths. componentsofagivenlineofsight.Inaddition,sincethemost
Moreover, many of the U-components have corresponding C- intenseSH+ componentsaresaturatedinCH+,andbecausethe
componentsinotherspecies,indicatingthatthefittingprocessis moderateS/Nratioofthe13CH+ andSH+datapreventsusfrom
robustandthattheselectionmethodissevere. observingthe low-opacitystructuresdetectedin theCH+ spec-
Theseconcordancescombinedwiththestrictselectiononthe tra, the respective distributions do not correspond to the same
GaussianparameterssuggestthatallC-componentsarerealde- velocitycomponents.Itisthereforeremarkablethatthedistribu-
tectionsandnotartefactscausedbynoiseorthestandingwaves tionsoftheCH+,13CH+,andSH+widthsoftheGaussianveloc-
removingprocedure. itycomponentsaresosimilar,evenidenticalwithinthestatistical
uncertainty. They exhibit a common pattern: a peak (hereafter
called P1),observedtowardsallthe sources, definedbya first-
3.3.ComparisonoftheGaussianlinewidths (mean) and a second- (standard deviation) order moments6 of
4.2 0.2kms 1 and1.5 0.1kms 1,respectively,andanex-
In Fig. 5, we display the distributions of linewidths associated − −
± ±
with the C-components extracted from the CH+, 13CH+, and tendedtail (hereaftercalled P2), observedonly on the Galactic
SH+ spectra. To emphasize the differences observed along the centresightlines(leftpanels),with∆υ-valuesupto20kms−1.
Galactic centre sightlines (l 0), these distributionsare com- Becausethe linesofsightsample kiloparsecsofinterstellar
putedforallsightlines(leftpa∼nels),andforthel,0sightlines material in the Galaxy, the patterns P1 and P2 result from the
only(rightpanels).Asacomparison,thelinewidthdistributions small-scale dynamics of the production processes of CH+ and
of the HCO+ groundstate radio transition observedby Godard SH+,theturbulentdynamicsofthediffuseISM,andtheGalactic
et al. (2010) towards W31C, W49N, W51, and G34.6 are dis- dynamics.Thecomparisonbetweentheleftandright(d)and(e)
playedinpanels(a),andthoseofCHandCH+visibletransitions panelsofFig.5showsthatthefirst-andsecond-ordermoments
observedathighspectralresolution( 0.3kms 1)in thelocal of the ∆υ distributions are the same for the componentsof the
diffuse medium by Crane et al. (1995∼)are show−n in panels (c) Galactic ISM along the l , 0 sight lines and the visible CH+
of Fig. 5. Lastly, panels (d) and (e) of Fig. 5 display the first- lines sampling the solar neighbourhood (defined by first- and
and second-ordermomentsof the ∆υ-distributionsissued from second-ordermoments of 4.3 0.4 km s−1 and 1.85 0.3 km
thecombinedCH+,13CH+,andSH+data(redsquares)andfrom s−1,respectively).Becausethel±atterisunaffectedbythe±Galactic
theCH+dataobservedinthesolarneighbourhood(bluecircles). dynamics,thissimilaritysuggeststhatthepeakP1resultsfrom
Inpanels(a),thehistogramofHCO+linewidthsisnarrowerand thedynamicsoftheformationprocessesofCH+ andSH+ con-
peaks at lower values than those of the submillimetre lines of volvedwiththatoftheturbulenceofthediffusegas.Becausethe
CH+,13CH+,andSH+.Conversely,thehistogramofthevisible tailofthe∆υdistributionsisobservedonlyontheGalacticcen-
CH+ data, characterisingthe local diffuse interstellar matter, is tresightlines,andbecausethehighabsorptiondips(∆υ>8km
very similar to that of the submillimetre data. We discuss the s−1) observedon the l , 0 sight lines can be decomposedinto
validityandthesignificanceofthesecomparisonsbelow. manynarrowcomponents(asperformedbyGodardetal.2010
with the HCO+ spectra), we conclude that P2 is caused by the
To demonstratethattheabsenceofnarrowvelocitycompo-
nentsintheCH+,13CH+,andSH+spectraisrealandnotduetoa Galacticdynamicsonly.
Lastly we note that while the first- and second-order mo-
limitationofourextractionalgorithm,wehavederivedthemini-
ments of peak P1 both agree with those of the ∆υ distribution
mumwidthofaGaussiancomponentofopticaldepthτ thatcan
0
beextractedfromagivenprofilecharacterisedbyanoiseσ at obtainedby Crane et al. (1995)within the statistical uncertain-
thevelocityresolutionδυ.Becauseσ scalesinverselywithτthe ties,theyalsosubstantiallydifferfromthoseoftheCH+ ∆υdis-
τ
tributionderivedinthesolarneighbourhoodbyPanetal.(2004),
squarerootofthevelocityresolution,acomponentdetectedata
3στlevelnecessarilyverifies awnhdo0fi.4ndfi0r.s0t3-aknmdsse1c.oAnds-oprrdoepromseodmbeynPtsanofe3t.a3l.±(200.0045)k,mthes−se1
−
±
∆υ>δυ[3σ /τ ]2. (3) differences may originate from their profile fitting method: (1)
τ 0
the number of CH+ components and their position were con-
While the noise level of the SH+ spectra towards SgrA*+50, strainedbytheobservationsofotherspecies,possiblyunrelated
DR21(OH),G34.3+0.1,andW33Aforbidsustoextractcompo- chemically(e.g.CN,CO,Ca),and(2)amaximalvalueof∆υof
nentswithlinewidthsmallerthan1kms 1,thatofalltheother 5.8kms 1 wasset.
− −
spectra is small enough to allow us to detect narrow velocity
structuresdowntotwovelocitychannels(0.72kms 1 forCH+
−
and13CH+,and1.14kms 1 forSH+).Thereforethescarcityof 4. Analysisoftheintegratedopacities
−
componentsin the first bin [0 ; 2 km s−1] of the CH+, 13CH+, Since the multi-Gaussian decomposition of CH+, 13CH+, and
andSH+ histogramsisnotanoiseartefact. SH+ absorptionprofilesprovidessets of C-componentsthatdo
Now, can we compare these linewidth distributions with
not always strictly coincide in velocity and width, and sets of
one another? Because the S/N ratios of HCO+ (see Table. 1 of
U-componentsthatsometimesclearlycorrespondtorealabsorp-
Godardetal.2010)andCH+(seeTable.2)profilesarehighand
tionbutwithwidthanddepthpoorlyconstrained,oursubsequent
comparable,thosetwosetsofspectraaredecomposedwiththe
analysisandcomparisonofthecolumndensitiesandabundances
samelevelofdetection.Thenoisethereforeaffectsthestatistics
ofthesespeciesisbasedonintegralsofopacitiescomputedover
ofthecomponentlinewidthsatthesamelevel.Thesameistrue
broadvelocityintervalscorrespondingtomarkedabsorptionfea-
forthelinesofSH+and13CH+,whichhavecomparable,though
turescommonto alllines. Thesebroadintervals(typically5 to
poorer,S/Nratios.Finally,becauseCH+ and13CH+ necessarily 20kms 1)aregiveninthetwofirstcolumnsofTable3.Thecol-
−
bear the same dynamical signatures, all distributions displayed
inFig.5canbecomparedwithoneanother. 6 Theuncertaintiesonthemomentsofthedistributionsarethestatis-
Werecallthatallabsorptionspectrahavebeendecomposed ticalstandarderrorscomputedforthefirst-andsecond-ordermoments
intoGaussiansindependentlyfromoneanother,withoutimpos- asσ/√N andσ/√2N,respectively,whereσisthestandarddeviation
ingthesamevelocitycentroidsorwidthtothedifferentvelocity andNthesizeofthesample.
5
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
umndensitiesgivenincolumns4,5,and6ofTable3arederived and W51 are given in columns 9 and 10 of Table 3. Towards
assumingasingleexcitationtemperatureof4.4Kfor12CH+and SgrA*+50 and SgrB2(N), because most of the gas appears to
13CH+, and of 3.0K forSH+ (see AppendixB). Followingthe beassociatedwiththeGalacticcentreenvironment(Rodriguez-
method set in Sect. 3.2 to keep only the most reliable absorp- Fernandez et al. 2006), a 13C/12C abundance ratio of 20 is as-
tionfeatures,a3σdetectionlevelwasadopted.Throughout,ev- sumed everywhere except for the velocity interval -20 to +30
erymeasurementbelowthislevelisconsideredasalowerlimit. kms 1,avelocitydomainwheretheabsorptionfeaturesarealso
−
Conversely, we adopt a conservative lower limit of 2.3 on the associatedwithgasintheGalacticplane,andwhereweusetwo
opticaldepthfor the saturatedvelocityintervals(Neufeldet al. alternativevalues,20and60,tobrackettheresult.
2010b). Theuncertaintiesgivenin Eq. 4 correspondto the standard
deviationofthebestleast-squaresfitperformedbyMilametal.
(2005) on the CN, CO, and H CO data. They do not take into
4.1.Variationofthe12C/13CisotopicratioacrosstheGalactic 2
accounttheuncertaintyonR duetotherandommotionofinter-
disk G
stellarclouds.Todoso,wereliedontheanalysisoftheHIemis-
sion in the first Galactic quadrant by Elmegreen & Elmegreen
(1987):theyfoundthatalargepartofthemassofthediffusegas
is distributedinto 200pc superclouds,separatedalongspiral
114400 arms by 1.5 kpc. ∼Their one-dimensional internal velocity dis-
this work
persion of 5.3 km s 1 is the main sourceof uncertaintyon R .
Milam et al (2005) − G
112200
6.2 R + 18.7 Taking into accountthe (l,v) location of the absorbing gas ob-
G
servedtowardseachsource(seetheequationofR inthecaption
110000 G
ofTable3),weobtainamaximaluncertaintyofabout30%on
C)C)
1313N(N( 8800 RtoGta.lWunhceenrtcaoinmtybionfeadbowuitth50th%eoenrrtohres1g2iCve/n13CinrEatqio. 4a,ndwtehefinsudba-
C) / C) / 6600 sequentN(12CH+)givenincolumns9and10ofTable3.
22
11
N(N(
4400
4.2.ComparisonofthecolumndensitiesofCH+andSH+
2200
00
00 22 44 66 88 1100 1122 1144 1166 1188
RR ((kkppcc))
GG GC los
other los
Figure6. N(12C)/N(13C) column density ratio as a function of the 111eee+++111444
galactocentric distance R . The red squares are from this work, and
G
thereforeevaluatedwithmeasurementsoftheN(12CH+)/N(13CH+)col-
umndensityratio.Thebluecirclesarefrompreviousmeasurementsof -2-2-2m)m)m)
tchoeluNm(n12dCeNns)i/tNy(r1a3tCioNs)(,MNi(la12mCOet)/aNl.(2103C05Oa),ndanrdefeNr(eHnc122eCsOth)e/rNei(nH)12w3ChOile) +++) (c) (c) (c 111eee+++111333
HHH
thegreencurvecorrespondstoalinearleast-squaresfitappliedtothese SSS
N(N(N(
data.
111eee+++111222
In Fig. 6, we display the 12C/13C column density ratio de-
rivedfromthepresent12CH+and13CH+dataasafunctionofthe 111eee+++111111
111eee+++111222 111eee+++111333 111eee+++111444
Galactocentricdistance RG (givenin columns7 and 8 of Table NNN(((CCCHHH+++))) (((cccmmm---222)))
3)whichwascomputedassumingaflatGalacticrotationcurve.
We comparedthese resultsto those deducedfrompreviousob-
Figure7.SH+columndensityasafunctionoftheCH+columndensity
servationsofCN,CO,H COandtheirrespectiveisotopologues
2 per broad velocity interval (see Table 3). The open squares and open
(Milametal.2005).Wefoundthatthefivefirmvalues(indicated
circles are from the present analysis and from Menten et al. (2011),
in Table 3) derivedfromthe simultaneousdetectionsof 12CH+
respectively. Theblueandpurplepointsarefromtheabsorptionlines
and13CH+absorptionlinesoverthesamevelocityrangearecon- observed along the Galacticcentre sight lines(SgrA*+50, SgrB2(N),
sistentwiththosederivedfromtheneutralspecies.The25lower andSgrB2(M))andalongtheothersightlines(DR21(OH),G34.3+0.1,
limitsinferredfromsaturated12CH+ lines(dashedsymbols)are W31C,W33A,W49N,andW51),respectively.Theblackdashed,dot-
alsoconsistentwithMilametal.(2005).Thisresultnotonlysug- ted,anddashed-dottedcurvesindicateN(SH+)/N(CH+)ratiosof0.01,
geststhattheisotopicratiosmeasuredwithionsandneutralsare 0.1,and1,respectively.
notsubstantiallyinfluencedbychemicalfractionationprocesses,
butitalsovalidatestheuseoftheempiricalrelation
12C/13C=6.2( 1.0)R +18.7( 7.4) (4) Fig.7 displaysthe SH+ andCH+ columndensitiesinferred
± G ± foreachbroadvelocityinterval.TheCH+columndensitieswere
found by Milam et al. (2005) to infer the 12CH+ column den- derived either from the 12CH+ profile, where unsaturated, or
sities from those measured in the 13CH+ spectra. The 12C/13C from13CH+ and the aboveisotopic ratio in the othercase. The
ratio and N(12CH+) computed from Eq. 4 for the velocity in- data set includes the results of the present study and those ob-
tervalstowardsDR21(OH),G34.3+0.1,W31C,W33A,W49N, tainedbyMentenetal.(2011)towardsSgrB2(M).WhileMenten
6
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
et al. (2011) reported the detection of two absorption compo- 2003). Marshall et al. (2006) have measured the near infrared
nents at N(CH+) 3 1012 and N(SH+) 1013 cm 2, we colour excess in large areas of the inner Galaxy (l < 100 ,
− ◦
removedthose poi∼ntsfr×omFig. 7 because th∼e 13CH+ spectrum b < 10 ) to obtain the visible extinctions (A 1|0|A ), pro-
◦ V K
| | ∼
inthecorrespondingvelocityrangeclearlyexhibitscontamina- viding an estimate of the total hydrogencolumn density along
tion of the absorption features by a strong and broad emission thelinesofsight.However,becauseofthelowresolutionofthe
line.Last,wenotethatthefewpointsat N(CH+) < 1013 cm 2 2MASS extinction analysis ( 15 arcmin), the uncertainty on
−
correspondtothefaintest12CH+ absorptionfeatures.Sincethey N (computedas the standard∼deviationof the extinctionmea-
H
arenotdetectedinSH+,thesepointsweaklyconstrainoursub- sured along the four closest lines of sight surrounding a given
sequentanalysisoftheSH+/CH+ratio. source)canbeimportant,andisashighas50%forDR21(OH),
TakingintoaccounttheupperandlowerlimitsshowninFig. W31C,andW33A.ThelastcolumnsofTable4showthatthese
7,wefindthatN(CH+)andN(SH+)spanmorethantwoorders twoindependentmeasurementsofN differbylessthan25%.
H
ofmagnitudeandthattheSH+/CH+columndensityratiovaries
from less than 0.01 to 1. Interestingly, the N(SH+)/N(CH+)
ratios observed towards SgrB2(M), SgrB2(N), and SgrA*+50 5. Resultsanddiscussion
are very similar, with a mean value of 0.28 0.02, 9 times
higherthanthattowardsDR21(OH),G34.3+0±.1,W31C∼,W33A, 5.1.Chemicalpropertiesofthegasseeninabsorption
W49N,andW51, N(SH+)/N(CH+) 0.03 0.007.Theabove
differenceinthemheanSH+/CH+ratiio∼ismuc±hlargerthanthe50
%uncertaintyonthecomputed12C/13Cratio.Moreover,weob-
tainthesamedifferenceofvaluesifweuseonlythefewpoints
detectedin12CH+.Itisthereforeunlikelythatthisdifferencecan
be ascribed to an uncertainty on the 12CH+/13CH+ ratio. Last, 111
Daflon&Cunha(2004)foundthatcarbonandsulfurhavesim-
ilar abundance gradients across the Galactic disk (with slopes
of-0.037and-0.040dexkpc−1,respectively).Consequently,the +++)))
differenceobservedintheSH+/CH+columndensityratiosmea- CHCHCH 000...111
sured on the l 0 and l , 0 sight lines is most likely tracing N(N(N(
∼ +++)/)/)/
variations of both the physical and chemical conditions of the HHH
SSS
diffusegassampledineachcase. N(N(N(
000...000111
Finally, with a correlation coefficient of 0.1, no evident
chemicalrelationshipseemstostandoutfromFig.7,asurpris-
ing finding in view of the fact that CH+ and SH+ are clearly GC los
linked by their dynamics(see Sect. 3.3). However,this lack of other los
000...000000111
correlationappliestothecolumndensities.Inthefollowing,we 111eee---000888 111eee---000777
discussthepropertiesoftheabundancesrelativetohydrogen,a NNN(((CCCHHH+++)))///NNN
HHH
discussionthatrequirestheknowledgeofN .
H
Figure8. N(SH+)/N(CH+) column density ratio as a function of the
4.3.Estimationofthehydrogencolumndensitiesinthe CH+ mean abundance. The CH+, SH+, and H column densities are
computedfromCH+,13CH+,SH+,HI,HF,andCHopacitiesintegrated
broadvelocityintervals
over thevelocityintervalsgiveninTables3and4.Theblueandpur-
To (1) estimate the mean molecular abundances that are to be ple points are from the absorption lines observed along the Galactic
compared with the chemical model predictions (Godard et al., centresightlines(SgrA*+50andSgrB2(N))andalongtheothersight
inpreparation),and(2)toestablisha possiblerelationbetween lines (DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51), re-
theCH+ andSH+ abundances,itisessentialtoestimatethehy- spectively. The redline corresponds toa least-squares fit of the latter
data.
drogen column density N in the broad velocity intervals over
H
which N(CH+) and N(SH+) are measured.Sincethe molecular
fractionofthegaswhereCH+ andSH+ aredetectedislow(0.4
onaverage,seeAppendixC),bothatomicandmolecularhydro- In Fig. 8, we display the N(SH+)/N(CH+) column density
genHIandH areneededtoestimateN . ratioasafunctionoftheCH+ meanabundance(withrespectto
2 H
The method consists in separately evaluating the column the total hydrogen column density N ) integrated over the ve-
H
densities of HI and H , using the VLA observations of the locity intervals givenin Table 4. Because Menten et al. (2011)
2
λ21 cm absorption line of H (Koo 1997; Fish et al. 2003; useda differentmethodthanwe didtoestimate theH column
2
Dwarakanathetal.2004;Pandianetal.2008;Langetal.2010) densities7thedataobtainedtowardsSgrB2(M)arenotincluded
and a relevant tracer for the molecular hydrogen because H in this plot. We found that both the mean abundances and the
2
is not directly observable. Then, N = N(HI) + 2N(H ). In abundanceratiosvarybytwoordersofmagnitudeinthediffuse
H 2
AppendixC we discussthe validity ofusing the HIFI observa- ISMsampledbythoselinesofsight.Inaddition,Fig.8reveals
tionsofCHandHFtocomputetheH columndensities.Ifavail- majordifferencesbetweentheresultsobtainedonthel 0and
2
able, HF is preferentiallyused in the followingsection to infer thel , 0sightlines.WhiletheCH+ meanabundancess∼panthe
N(H )andtheensuingCH+,13CH+,andSH+meanabundances. same range of values in both cases, the SH+/CH+ ratio mea-
2
If not, N(H ) is derived from CH, assuming a HF/CH mean sured towards SgrA*+50 and SgrB2(N) shows no correlation
2
abundanceratioof0.4(avaluedefinedwithalargestandardde-
viationof0.25),deducedfromcolumns5and6ofTable4.Last, 7 InMentenetal.(2011)theH columndensitiesarededucedfrom
2
wale. (c2o0m1p0a),refrtohmesethveaaluneaslyosfisNoHftothteho2sMeAinSfeSrrseudr,vaesyi(nCGuotrdiaertdaelt. 1th9o9s6e).ofHCO+,assumingN(HCO+)/N(H2)=5×10−9(Lucas&Liszt
7
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
with N(CH+)/N . Conversely, the points corresponding to the theirrapiddestruction,theCH+ andSH+ abundancespredicted
H
observationsperformedonthel,0sightlinesexhibitatrend by UV-dominated chemistry are very low. Fig. 9 displays the
CH+ and SH+ relative abundances computed with two mod-
N(SH+)/N(CH+) 0.09 108 N(CH+)/N −1.4, (5) els of PhotoDissociation Regions (PDR)9 illuminated on one
H
∼ h × i side as functions of the shielding from the ISRF AV. For the
with a correlation coefficient of 0.8. These are the results that physicalconditionsofthediffusegas,weobtainN(CH+)/N
H
have to be compared with the predictions of chemical models 1.8 10 11 and N(SH+)/N 1.6 10 13, two to four order∼s
− H −
× ∼ ×
appliedtothediffuseISM. ofmagnitudelowerthantheobservedvalues,andwithacorre-
spondingabundancerationeverhigherthan0.01.
5.2.CarbonandsulfurchemistriesinthediffuseISM
5.2.2. Alternativemodels
5.2.1. UV-drivenchemistry
In the diffuse interstellar medium sampled by the l , 0 sight
lines,theonlyproductionpathwaysefficientenoughtobalance
thefastdestructionofCH+ andSH+ are
11ee--1100 C++H CH++H ∆E/k=4640K, and (8)
2
→
CCHH++
S++H SH++H ∆E/k=9860K. (9)
HH 11ee--1111 2 →
nn
X)/X)/ Since these reactions are highly endothermic, it has been pro-
n(n(
e e 11ee--1122 posedthathighCH+ andSH+ abundancesarethesignaturesof
cc
anan shockwavespropagatingthroughtheISM(Draine1986;Millar
dd
unun etal.1986).ComparingthepredictionsofHDandMHDshocks
abab 11ee--1133 with the CH+ column density observedtowards ζ Oph, Draine
e e
relativrelativ 11ee--1144 SSHH++ (c1a9se86in) awnhdicPhinCeaHu+daensdFSorHeˆ+tsfeotrmal.m(1a9in8l6y)tfharvoouugrhediothne-nMeuHtrDal
n = 50 cm-3 friction.Includingthe sulfurchemistryMillar etal. (1986)and
H
n =300 cm-3 PineaudesForeˆtsetal.(1986)predictedaSH+/CH+abundance
H
11ee--1155 ratio increasing from 0.01 to 0.4 for a shock speed increasing
11ee--0033 11ee--0022 11ee--0011 11ee++0000
from 9 to 16 km s 1, a transverse magnetic field of 5µG and
AA −
VV a preshockdensityof 20 cm 3. These resultsagreeexcellently
−
withourobservations.
Figure9.PredictionsoftwoPDRmodelsforgasdensitiesnH =50and Anotherscenario,alternativetotheshockwaves,istheTDR
300 cm 3.TheCH+(inred)andSH+(inblue)relativeabundancesare
− (turbulentdissipationregions)model.Inthismodeltheturbulent
displayedasfunctionsoftheshieldingA fromtheinterstellarradiation
V energy is dissipated in many small-scale magnetized vortices
fieldforaslabofgasilluminatedononesideonly.
inwhichtheionizedandneutralfluidsdecouple.Dissipationis
causedbybothion-neutralfrictionandviscousdissipationatthe
edgeofthevortices.ComparingthepredictionsofTDRstoob-
InachemistryentirelydrivenbytheUV-radiationfieldand servationsofCH,CH+,OH,andHCO+ inthelocaldiffusegas,
thecosmicrayparticles,thehydrogenationchainsofcarbonand Godard et al. (2009) also favoured models in which the dissi-
sulfur,andthesubsequentproductionsofCH+ andSH+ areini- pation is dominated by the ion-neutral friction and where the
tiatedbytheradiativeassociationsofC+ andS+ withmolecular production of CH+ and SH+ via reactions 8 and 9 is triggered
hydrogen, by the ambipolar diffusion. An analysis of the SH+/CH+ ratio
obtainedintheframeworkoftheTDRmodelwillbepresented
C+ + H2 → CH+2 + γ and (6) inaforthcomingpaper(Godardetal.inprep.).
S+ + H SH+ + γ, Whilethescenariosofshocksandvorticescouldequallyap-
2 → 2 plytogassampledbythel 0sightlines,analternativechem-
∼
tworeactionswithlongtimescales:2 106yr f 1 50 cm 3/n icalprocessmaybeatworkthere.Largeamountsofthediffuse
× H−2 (cid:16) − H(cid:17) gasdetectedalongSgrA*+50,SgrB2(N),andSgrB2(M)belong
Hanedrb1st×et1a0l.81y9r8f9H−)2,1w(cid:16)5h0ercemf−3/nisHt(cid:17)h,eremspoelecctiuvlealryfr(aHcteirobnstde1fi9n8e5d; tvoadtheedCbyenatrsatrloMngolXec-rualyarraZdoinaetio(CnMfieZld).,BhiegchaCusHe+thaendCMSHZ+iasbpuenr--
as f = 2n(H )/n . In comHp2arison,CH+ and SH+ are mainly dancescouldbeduetoregionswhereC++ andS++ ionsco-exist
H2 2 H
destroyedbyhydrogenationanddissociativerecombination8, withH andformCH+ andSH+ bythereactions
2
CH+ + H CH+ + H and C+++H CH++H+, and (10)
2 → 2 (7) 2 →
SH+ + e S + H,
− → S+++H2 SH++H+, (11)
→
twoprocesseswithshorttimescales:1yr f 1 50 cm 3/n and
5yr (T/100K)0.72(1.38 10−4/xe )(50 cmH−−23/(cid:16)nH), re−specHti(cid:17)vely. a(1s1p)roobptoasinededbbyyLCahnegneret(1al9.7(82)0.0U3)s,inAgbethleethailg.h(2r0a0te8)offoruenadcttihoant
× −
Becauseofthelackofefficientproductionpathwaystobalance
9 The Meudon PDR model employs a one-dimensional chemical
8 Because the hydrogenation of SH+ is highly endo-energetic: codeinwhichaslabofgaswithagivendensityprofileisilluminated
∆E/k=6380K. bytheambientinterstellarradiationfield(LePetitetal.2006).
8
B.Godardetal.:ComparativestudyofCH+andSH+absorptionlines.
thepredictedSH+columndensityformoleculargassurrounding Board, Stockholm University - Stockholm Observatory; Switzerland : ETH
anactivegalacticnucleusistwoordersofmagnitudehigherthan Zurich,FHNW;USA:CalTech,JPL,NHSC.BG,EF,MG,andMDLacknowl-
thatpredictedbyUV-dominatedchemistry.Sinceratemeasure- edgethesupportfromtheCentreNationaldeRechercheSpatiale(CNES),and
from ANRthrough the SCHISMproject (ANR-09-BLAN-231). BG, JC, and
mentsofreaction(10)pointtolowervalues,thischemicalpro-
JRGthanktheSpanishMICINNforfundingsupportthroughgrants,AYA2009-
cess couldaccountforthe high meanSH+/CH+ abundancera- 07304andCSD2009-00038
tioderivedtowardsSgrA*+50,SgrB2(N),andSgrB2(M)(about
nine times higher than that obtained along the other Galactic
sightlines,seeSect.4.2).
The above results are in line with the detection of large References
amounts of warm diffuse gas recently identified with the 3µm
Abel,N.P.,Federman,S.R.,&Stancil,P.C.2008,ApJ,675,L81
absorptionlinesofH+(Okaetal.2005;Geballe&Oka2010)in Abia,C.,Cunha,K.,Cristallo,S.,etal.2010,ApJ,715,L94
3
theCMZ.Thetemperature( 200 300K)ofthelow-density Allen,M.M.1994,ApJ,424,754
gas phase is found to be con∼siderab−ly higher than that of typi- Amano,T.2010,ApJ,716,L1
Brown,A.&Balint-Kurti,G.G.2000,J.Chem.Phys.,113,1870
caldiffuseclouds,anduniquetotheGalacticCMZ(Gotoetal.
Chen,D.,Gao,H.,&Kwong,V.H.2003,Phys.Rev.A,68,052703
2008). They also corroborate the specific dynamics of molec- Comito,C.&Schilke,P.2002,A&A,395,357
ular clouds associated with the Galactic centre regions (e.g. Crane,P.,Lambert,D.L.,&Sheffer,Y.1995,ApJS,99,107
Rodriguez-Fernandezetal.2006). Crawford,I.A.1995,MNRAS,277,458
Crawford,I.A.&Williams,D.A.1997,MNRAS,291,L53
Cutri,R.M.,Skrutskie,M.F.,vanDyk,S.,etal.2003,2MASSAllSkyCatalog
ofpointsources.,ed.Cutri,R.M.,Skrutskie,M.F.,vanDyk,S.,Beichman,
6. Summaryandperspectives
C.A.,Carpenter,J.M.,Chester,T.,Cambresy,L.,Evans,T.,Fowler,J.,Gizis,
J.,Howard,E.,Huchra,J.,Jarrett,T.,Kopan,E.L.,Kirkpatrick,J.D.,Light,
We have presented the analysis of Herschel/HIFI observations
R.M.,Marsh,K.A.,McCallon,H.,Schneider,S.,Stiening,R.,Sykes,M.,
oftheground-statetransitionsofCH+,13CH+,andSH+,allde- Weinberg,M.,Wheaton,W.A.,Wheelock,S.,&Zacarias,N.
tected in absorption against the submillimetre dust continuum Daflon,S.&Cunha,K.2004,ApJ,617,1115
of distantstar-formingregionsand the Galactic centre sources, Danks,A.C.,Federman,S.R.,&Lambert,D.L.1984,A&A,130,62
Dowell,C.D.,Lis,D.C.,Serabyn,E.,etal.1999,inAstronomicalSocietyof
SgrA*+50andSgrB2(N).Thevelocityrangeoverwhichtheab-
thePacificConferenceSeries,Vol.186,TheCentralParsecsoftheGalaxy,
sorptionfeaturesaredetectedcorrespondstodiffuseortranslus-
ed.H.Falcke,A.Cotera,W.J.Duschl,F.Melia,&M.J.Rieke,453
centenvironments.Thedeconvolutionofthehyperfinestructure Draine,B.T.1986,ApJ,310,408
embedded in the SH+ 1,2 0,1 spectra, and the independent Draine,B.T.&Katz,N.1986,ApJ,310,392
decomposition of the absor−ption domains in Gaussian velocity Dwarakanath,K.S.,Goss,W.M.,Zhao,J.H.,&Lang,C.C.2004,Journalof
AstrophysicsandAstronomy,25,129
components allowed us to identify many velocity components
Elmegreen,B.G.&Elmegreen,D.M.1987,ApJ,320,182
persightlineandtoperformacrosscomparisonofthedynami- Falgarone,E.,Godard,B.,Cernicharo,J.,etal.2010,A&A,521,L15+
calandchemicalsignaturesofthosethreespecies. Falgarone,E.,PineaudesForeˆts,G.,&Roueff,E.1995,A&A,300,870
This study provides the following main results. (1) The Federman, S.R.,Rawlings, J.M.C.,Taylor,S.D.,&Williams, D.A.1996,
linewidth distributions of CH+, 13CH+, and SH+ are found to MNRAS,279,L41
Federman,S.R.,Strom,C.J.,Lambert,D.L.,etal.1994,ApJ,424,772
be similar and likely trace the kinematics of the chemical pro-
Fish,V.L.,Reid,M.J.,Wilner,D.J.,&Churchwell,E.2003,ApJ,587,701
duction processes of these species convolved with that of the Geballe,T.R.&Oka,T.2010,ApJ,709,L70
turbulent and Galactic dynamics of the diffuse ISM. (2) These Gerin,M.,deLuca,M.,Black,J.,etal.2010a,A&A,518,L110+
linesarebroad( 4.2kms 1),similartothosefoundinvisible Gerin,M.,deLuca,M.,Goicoechea,J.R.,etal.2010b,A&A,521,L16+
∼ − Godard,B.,Falgarone,E.,Gerin,M.,Hily-Blant,P.,&deLuca,M.2010,A&A,
absorption lines in the solar neighbourhood, and broader than
520,A20+
those of HCO+ and CN along the same lines of sight. (3) The Godard,B.,Falgarone,E.,&PineauDesForeˆts,G.2009,A&A,495,847
SH+/CH+ abundanceratiocoversabroadrangeofvaluesfrom Goto,M.,Usuda,T.,Nagata,T.,etal.2008,ApJ,688,306
0.01 to more than 1, shows higher values in warmer environ- Greaves,J.S.&Williams,P.G.1994,A&A,290,259
Gredel,R.1997,A&A,320,929
ments (such as the Galactic centre clouds), and appears to be
Gredel,R.,vanDishoeck,E.F.,&Black,J.H.1993,A&A,269,477
proportionalto (N(CH+)/N ) 1.4 in the diffuse gas sampledby
H − Gry,C.,Boulanger,F.,Nehme´,C.,etal.2002,A&A,391,675
thel,0sightlines.(4)AsforCH+,theSH+abundancescannot Hammami,K.,OwonoOwono,L.C.,&Sta¨uber,P.2009,A&A,507,1083
bereproducedbyUV-drivenchemistryinthediffusegas. Haud,U.&Kalberla,P.M.W.2007,A&A,466,555
The unique properties of the carbon and sulfur chemistries Herbst,E.1985,ApJ,291,226
Herbst,E.,Defrees,D.J.,&Koch,W.1989,MNRAS,237,1057
supporttheframeworkofawarmchemistrytriggeredbyturbu-
Indriolo,N.,Oka,T.,Geballe,T.R.,&McCall,B.J.2010,ApJ,711,1338
lentdissipation(eitherinshocksorintensevelocityshears)that Jenniskens,P.,Ehrenfreund,P.,&De´sert,F.1992,A&A,265,L1
selectivelyenhancestheproductionofSH+forwhichtheforma- Joulain,K.,Falgarone,E.,PineaudesForets,G.,&Flower,D.1998,A&A,340,
tionendothermicityisthehighest.Adetailedcomparisonofthe 241
Kerr,F.J.&Lynden-Bell,D.1986,MNRAS,221,1023
TDRmodelpredictionswiththeseobservationalresultswillbe
Koo,B.1997,ApJS,108,489
giveninaforthcomingpaper(Godardetal.inprep.).
Lacour,S.,Ziskin,V.,He´brard,G.,etal.2005,ApJ,627,251
Lambert,D.L.,Sheffer,Y.,&Crane,P.1990,ApJ,359,L19
Acknowledgements. Wearemostgratefultotherefereeforprovidingconstruc- Lang,C.C.,Goss,W.M.,Cyganowski,C.,&Clubb,K.I.2010,ApJS,191,275
tivecommentsandhelpinginimprovingthecontentofthispaper.HIFIhasbeen Langer,W.D.1978,ApJ,225,860
designedandbuiltbyaconsortiumofinstitutesanduniversitydepartmentsfrom LePetit,F.,Nehme´,C.,LeBourlot,J.,&Roueff,E.2006,ApJS,164,506
acrossEurope,CanadaandtheUnitedStates(NASA)undertheleadership of Lesaffre,P.,Gerin,M.,&Hennebelle,P.2007,A&A,469,949
SRON,NetherlandsInstituteforSpaceResearch,Groningen,TheNetherlands, Lim,A.J.,Rabada´n,I.,&Tennyson,J.1999,MNRAS,306,473
andwith majorcontributions from Germany,France andthe US.Consortium Liszt,H.&Lucas,R.2002,A&A,391,693
members are : Canada: CSA, U. Waterloo; France : CESR, LAB, LERMA, Liszt,H.S.,Lucas,R.,&Pety,J.2006,A&A,448,253
IRAM; Germany : KOSMA, MPIfR, MPS; Ireland : NUI Maynooth; Italy : Lucas,R.&Liszt,H.1996,A&A,307,237
ASI,IFSI-INAF,OsservatorioAstrofisicodiArcetri-INAF;Netherlands:SRON, Magnani,L.&Salzer,J.J.1989,AJ,98,926
TUD; Poland : CAMK, CBK; Spain : Observatorio Astrono`mico Nacional Magnani,L.&Salzer,J.J.1991,AJ,101,1429
(IGN),CentrodeAstrobiologia;Sweden:ChalmersUniversityofTechnology Maier,J.P.,Lakin,N.M.,Walker,G.A.H.,&Bohlender,D.A.2001,ApJ,553,
- MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space 267
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