197 9ApJ. . .232. .7403 + + The AstrophysicalJournal,232:740-746,1979September15 © 1979.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. formed instellaratmospheres(HoyleandWickrama- for theirgenesis.Breakdownofevenlargermolecules dense interstellarcloudshasledtovarioussuggestions singhe 1977),recombinationoninterstellargrains number ofmoleculesaremostreadilyexplainedonthe the chemistryoftheseclouds.TheH/Dratiosa (Herbst andKlemperer1976;DalgarnoBlack (Watson andSalpeter1972a,6;AllenRobinson basis ofion-moleculereactions(Watson1976).The provides directevidencethationsplaysomerolein vation ofCH,HCODCONHandD centrations ofCHandionsarerequired their observedrelativeabundances,althoughhighcon- observation oflinearmoleculescontaininglongchains in ourlaboratoryandelsewhere.Itisintendedtoshow Turner (1975)tosuggestthation-moleculereactions branched orcycliccompoundspromptedFerteland of unsaturatedcarbonatomsandthefailuretodetect to accountfortheabsoluteabundances. cyanoacetylenes HCN,N(Krotoetal. formation ofobservedmolecules.Asaparticular for thesynthesisoflargemoleculesbasedonmeasure- play majorroles. and HCN(MacLeod1978),isconsistentwith observation ofmethylacetylene(HC)andthe example weshowthattheschemecanaccountfor plausible, butnotnecessarilyexclusive,routestothe molecule reactionsinvolves successive bondformation ments oftheratesion-moleculereactionsobtained 1976; Dalgarno1976)havebeenproposed.Theobser- 1977), andhomogeneousion-moleculereactions 1978; Churchwell,Winnewisser,andWalmsley1978), 2 2 357 9 43 The recentdetectionoflarge,complexmoleculesin In thepresentpaperweproposeageneralscheme In thisscheme,synthesisof largemoleculesbyion- © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + AN ION-MOLECULESCHEMEFORTHESYNTHESISOFHYDROCARBON-CHAIN based onratesofion-moleculereactionsmeasuredinourlaboratoryandelsewhere.C-Cbond formation isdominatedbyreactionsinvolvingthemustabundantionsC,CHH2and example, weshowthattheschemecanaccountforobservationofmethylacetylene,methyl cyanide, andthecyanoacetylenesHCN,N,isconsistentwiththeir CH andthemostabundantneutralsCH*,where1PA(CH),producessimilarresults higher protonaffinity,e.g., Proton transferfromtheiontoaneutralhaving most familiaroftheneutralizationprocesses,will discussed insomedetailbyOppenheimerandDalgarno tive recombination.Finally, theionmayhydrogenate leaves thehydrogencontent unchanged,asdoesradia- and generallyproceedsatthecollisionrate(Bohme (1974), 2 1975). Chargetransfertospeciesoflowionization 32 Neutralization mayoccurinanumberofdifferent + CH +NH->,(3) + 234 + CH^ +-^C*(1) n2+ 1 CH+ Fe->Fe , (4) 2 2 + CH +e^CH.(2) 23 197 9ApJ. . .232. .7403 + + <14±5) + 1 + + + 13 + + + 9 on neutralizationthroughhydrideiontransferofthe In principlelargernegativeionfragments(e.g.,CH“) type are seldomobservedwhenother,energeticallyallowed, could alsobetransferred,butinpracticesuchprocesses neutralization channelsareavailable. 31- dense clouds.CcanformCHionsbyradiative association withH: starts withCand,whichareabundantspeciesin to be10"*at90K(Herbst,Schubert,and The rateconstantforthisreactionhasbeencalculated transfer toCatomsfromsomeprotonatedspecies producing CH(Dalgarno1976),suchasproton Certain 1977).Otherprocessesmaybeoperativein (H,HON HCO)whoseprotonaffinity 3 tive association: is lessthanthatofcarbon,e.g., CH andbutisendothermicfor. 2 Further hydrogenationofCHmayoccurbyradia- 2 at 50Konthebasisoftheirmeasurementsrate constant forreaction(8)maybeashigh4x10“ Smith andAdams(1978)haveestimatedthattherate atom areexpectedtobeCandCH.Neutralization of thethree-bodyassociationCHwithH. clouds (Rydbeck,Elldér,andIrvine1973;Turner of CH*ionswillleadtoneutralspeciescontaining However, thedominantionscontainingonecarbon (Fox andJennings1978).Theremaybeotherroutes Zuckerman 1974;Robinsonetal.1974),ashasCH one carbonatom.CHhasbeenobservedininterstellar to theformationofCH,suchasreaction 32 which wehaveobservedtoarateconstantof 4.7 xlO". 23 react withsingle-carbonneutralstoproducedouble- 3 3 32 carbon ions 4 4 and units ofcmmolecule“sfor second-orderreactions. + The generalschemeissummarizedinFigure1.It 1 Hydrogenation byreaction(1)isexothermicfor The dominantsingle-carbonionsCandCHcan 3 Throughoutthispaper,rate constants willbegivenin © American Astronomical Society • Provided by theNASA Astrophysics Data System + + CH+ CH*->CH* H.(11) H +C0->CHHCO,(9) + 32+1 2 324 CH +A.(5) + 23 + C+ CH^QH*..^H(10a) CH +H—>CHhv.(8) + + 325 C +H—CHhv.(6) C +H->CH.(7) 2 32 a) C-CBondFormation + ->CH*_ +(10b) 2 ION-MOLECULE SCHEME + + + + + + + + + + + frame. The rateconstantsforthesereactionshavebeen large forreactionswiththefreeradicals. Table 1).Itislikelythattherateconstantswillalsobe measured onlywithCHandarefoundtobefast(cf. ions willthenbeCHandalongwith endothermic forCH.Themajordouble-carbon is fastandleadstoCHbutslowfor reactions analogousto(11),e.g., can continuetocondensewithCH*neutralsby which reaction(1)isalsoendothermic.TheCions CH andformedbyreaction(11)for and trons orchargetransferwithCHmayproducethe The CHionisstabletowardfurtherhydrogenation. 4 Neutralization ofCHbyrecombinationwithelec- 23 23 carbon atomatatimebyanalogous reactionsbetween observed interstellarmolecule,methylacetylene,HC. 2 ions withneutralsmayalso beimportant(Smithand CH* ionsandCH*neutrals. Alternatively,conden- 2 245 C andCHions.Radiative recombinationofCH sation reactionscanoccurbetween CH*neutralsand 34 3 35 43 n 3 3 n + Fig. 1.—Limitedschemeforthebuildupofhydrocarbon Hydrogenation byreaction(1)forCandH The hydrocarbonskeletoncancontinuetobuildone 2 + + CH +CH(12) CH +CH->(13a) 2345 2435 + ^CH +.(13b) 342 741 197 9ApJ. . .232. .7403 742 + + + + + + neutrals orofCHionswith.Asmentioned the generalschemerepresentedinFig.1. tively abundantneutralspecies ;ithasbeenobserved Adams 1978)and,ifso,canbeconsideredaspartof earlier, CHandarerelativelystableions. these ions,isrelativelyinert toattackbyfreeradicals Acetylene, whichcanbeformedbyneutralizationof at atimebyreactionsofCHandwith in circumstellarcloudsabsorption (Ridgwayetal. or byOandNatomsshould thereforebearela- + nx2 1976). Anumberofion-molecule reactionsinvolving + + 23 23 + + + + ch + CH +ch—> + + CH +HCN- 23 2 + b a d ch +ch e 2 ch +ch ch +ch-> f 6 + ch + + CH +HCN- 24 + CH +HCN- 32 234 HCN +CH 32 3 cn +h- 2 ch + 4 Note.—Rate constantsat300Kforthetotallossofreactantion. ch +nh SmithandAdams1977. Huntress1977. The backbonecanalsobebuiltuptwocarbonatoms Fehsenfeld1976. + 2 34 Resultsinthislaboratoryusingtheflowingafterglowtechnique. + Freemanetal.191%. Resultsinthislaboratoryusingselectedionflowtube(Vlachos etal.1979). 3 cn +nh- cn +ch■ ch + + 23 + 24 + 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System C +HCN- + + ch + + 2 2 c +h C +HCN- + cn +h- 2 2 C +CH- c +nh- ch +n- 4 3 3 c +h- 2 Reaction I + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ■HCN +HCN ■ hcn+c -CH +HCN -hcn +ch ■ hcn+ ■ products ■ ch+nh ■ hcn+ch •HCN +H ■CH +CN •HCN +C ■CN +H CH +h CH + CH + ch + CH + CH + CH + 2 23 3 2 23 23 23 23 2 2 3 CH + CH + CH + CH + ch + CH + ch + HCN + CH + nh +c 2 23 HCN +H 2 25 NH +CH HCN + 35 2 23 HCN+ chnh 342 HCN +H 32 352 CN +H 3 products 42 43 chhcn 432 HCN + 2 52 chcn 3 CH +HCN 2 42 42 2 3 2 2 3 2 2 23 SCHIFF ANDBOHME Rate Constants a a a a a a a a a e a a 6 e e e -0.1 -0.3 -0.6' 0.29 0.71 0.37 0.79 0.21 0.63 0.47 0.50 0.03 0.80 0.20 0.84 0.5 \ 0.16 0.41 0.59 0.5 j 0.1/ 0.9 \ 1.0 TABLE 1 Product Distribution e to befast,e.g., CH havebeenstudiedinthelaboratoryandfound b b b b 2 b b b b b b e e -0.05 0.33 / 0.67 \ 0.78 0.22 0.75 0.03 0.22 0.70 \ 0.20 0.10/ 0.05 0.90 :::} + + + + CH +->QH(15a) CH +^ H,(16) CH3+ H^. (18) 23 234 2 325 c CH + H ,(17) f 325 0.80 \ 0.2° / 0.5 ±0.2' 0.85 0.15 + C +H->C,(14) 23 } + ->CH +,(15b) 42 e < 0.0001 ab ab ab ab ab ab ae ab a e abe a ab d a e ae e a6f e e e e e No reaction,0.01 0.96, 1.2 0.78, 1.7 0.22, 0.17,0.2° 1.12, 1.4 0.84, 1.0,0.80° 0.25, 0.72 2.7° 1.45, 1.2,,1.6° 1.2, I 1.3° 2.3, 2.3,2.3 1.41 0.83, 2.2 0.067 No reaction 2.0 3.2, 3.5 2.5 0.053, 0.39,0.71 2.5 0.0044 1.8 0.89 1.3 9-3 x 10cms Rate Constant Vol. 232 197 9ApJ. . .232. .7403 + + + + + + tions arealwaysincompetitionwithneutralization,the between ionsandneutrals,eachcontainingmorethan two carbonatoms.However,sinceion-moleculereac- involving eitherthemostabundantions,C,CH bond formationshouldbedominatedbyreactions decrease withthenumberofcarbonatoms.ThusC-C No. 3,1979 nitrogen-containing compoundsrecentlyobserved, steady-state abundancesofbothionsandneutralswill heteronuclear bondformation. discussed aboveandasanextensionoftheschemeto both asexamplesofthegeneralsyntheticscheme CH* andCH.Theseprocessesaretheonesrepre- CH, andorthemostabundantneutrals, sented inFigure1. NH hasbeenshowntobe(cf.Table1) The sameproductionhasbeenobservedinthe Electron recombinationofthisionleadstothe observed neutralHCN. reaction (reaction [21c])intheirexperimentatapressureof which haverelativeprobabilitiesof7:1:2atapressure Smith andAdams(1977)tohavethreechannels: 0.2 torrandroomtemperatureledSmithAdams 3 for radiativeassociation, complex mightbequitelongandthattherateconstant of 0.2torrHe.Dissociativerecombination (1978) tosuggestthatthelifetimeofCH•NH HCN probablyleadstoHCNformation. 2 23 at 20K,mightbecomparabletoÆ.Recombination 3 of thisionmightproduceHCN. has threechannels: and thatthereactionofCNwithHisimmeasurably slow. ThereactionofCNwithCHisalsoslowand 3 4 21a 2 24 + + + In principle,condensationreactionscanalsooccur We willnowconsiderpossibleroutestosomeofthe An importantchannelinthereactionofCwith The reactionofCHwithNHhasbeenshownby The observationofalargethree-bodychannel We havefoundthatCreactsrapidlywithHCN, 3 © American Astronomical Society • Provided by theNASA Astrophysics Data System b) FormationofOrganonitrogenCompounds + + + CH +NH—>HCN(21a) CN +CH->H HCN (23a) 342 243 CH +NH->hv 3 + + + C +NH->HN.(19) CH +N->-HCN.(20) 32 32 C +HCNNH,(22) 2 + + + + —>NH +CH(21b) ->HCN +H.(23c) ->HCN +H(23b) -^CH-NH3, (21c) 42 2 23 3 ION-MOLECULE SCHEME + + + + + _9 10 1 10 -6 reaction (23a)andbyelectronrecombinationofthe The reactionleadstothere-formationofHCNby in reaction(23b)maybeaminorsourceoftheobserved interstellar ion. these reactionssupporttheconclusionofHartquist cyanoacetylene HCCN.Thelowrateconstantsfor HCN formedinreaction(23c).Itisalsoasourceof relatively largerateconstantandtwochannels: CH and.NeutralizationofNformed and Dalgarno(1978)thatCNmaybeanabundant Again, HCNisre-formedandC-Cbondformation to reaction(19).ThusreactionsofCNleadmainly The netresultofreactions(22)and(25)isequivalent used astherawmaterialforsynthesisoforgano- larger organonitrogencompounds. to HCNformationandarenotmajorsourcesof occurs. reaction betweenCHandNHatapressureof nitrogen compounds.Huntress(1977)didnotfindany however, thattheassociationreaction bination oftheioncouldleadtoobservedmethyl bination counterpart, 2.0 x10,whichsuggeststhattheradiativerecom- 2 2 cyanide (CHCN)molecule. 23 and twoapproximatelyequallyprobablechannels: may alsohavealargerateconstant.Electronrecom- occurs withaneffectivesecond-orderrateconstantof 2 report arateconstantof7.1x10"andratio agreement withthoseofHuntress,butwealsofind k7J x0.02.Ourresultsareinmuchcloser Huntress (1977)tohavearateconstantof5.3x10" is some10timesfasterthan reactions(27a)and(27b). that ataHepressureof0.3torrthethree-body Freeman, Harland,andMcEwan(1978),however, reactionatthatpressure wasfound tobe3.9x10", The effectivesecond-orderrate constantfortheoverall associative channel 2 3 ~10 torr.Atapressureof~0.5torrHewefind, 3 227h + + + The reactionofCNwithH,however,hasa The reactionofCNwithNHisfast: HCN formedbyanyoftheaboveprocessesmaybe The reactionofHCNwithCHwasfoundby 2 23 2 + + CH +HCN->-hv,(26b) + 3 CN +NH->HHCN.(25) + CH +HCN->HN(27a) 23 + 23 CN +H->HCN(24a) + 23 CH +HCN^• HCN (27c) 2 CH +HCN^•(26a) 3 + + ->HCN +.(24b) -^HCN +H.(27b) 23 2 743 197 9ApJ. . .232. .7403 + + 9 + + 6 8 interstellar temperatures.Neutralizationoftheions which suggeststhattheradiativerecombination tion ofsuccessivemembersthecyanopolyacetylene 744 formed byreactions(27a)and(27c)couldproduce counterpart ofreaction(27c)mightbeeffectiveat cyanoacetylene, HCN. type CH,3wherenisanevennumber, series. OneinvolvestheadditionofHCNtoions followed bydissociativerecombinationoftheproton- reactants followedbyelectronrecombinationofthe ated cyanoacetylene.Radiativerecombinationofthese product ionis,ofcourse,analternativebutcanbe considered tobepartofthesamescheme. formed byone-stepreactionsofthetype as opposedtothesequentialreactionsinvolvingCH*. 3 tions withotherneutralmolecules,X,whichleadto competitive stepsareasfollows:removalofCH n2 products otherthancyanoacetylenes.Asimilarscheme by electron-ionrecombinationorexothermicreac- to havearateconstantof1.41x10". could beinvokedinvolvingCHinplaceof. Reaction (29)hasbeenobservedforCHandfound acetylenes resultingfromthisscheme.Iftherate constants forneutralizationoftheprotonatedcyano- for theabundanceratio,R,ofsuccessivecyano- acetylenes areapproximatelyequal,then Under steady-stateconditions The abundanceofCHindensecloudshasnotbeen The ratio/c//:9isoftheorder100.abundance present inamountsapproaching 10"«.Thusthe expected toberelativelystableandcouldwell determined, butforthereasonsgivenabove,itis Substitution thengives n2 two-body reactionsarelikely withthesemoleculesand could beclosetounity.The mostabundantneutrals, of electronsistheorder10"h(Guélinetal.1977). second termontheright-hand sideofequation(30) n32 2 H andCO,arenotincluded inXsincenoexothermic 2 e2a 2 At leasttworoutescanbesuggestedfortheforma- The higherhydrocarbonionsaremostreadily This routeissummarizedinFigure2,wherethe It ispossibletoderiveanapproximateexpression © American Astronomical Society • Provided by theNASA Astrophysics Data System + [HCN]^[HC+[CH =n +12 + + + + CH +HCNN,(28b) CH +HCN->N,(28a) [HCN] ~[HC[CH* + n32 +1 n2 +1 n +32 CH +(29a) [CH ] n2 + n +2 ^■9a[CH] C2H 2 MX] k[e] e + fc9a[CH] 2n UX] +k[e] e + CH +(29b) n +23 SCHIFF ANDBOHME (30) + + + with H0tobeimmeasurablyslow.Ifreactions the CHions.Wehavefoundreactionof tribution. Theobservedratios(Churchwelletal.; also slow,thefirsttermmaynotmakealargecon- other relativelyabundantneutralssuchasOandNare Kr otoetal.1978)areestimatedtobeoftheorder preceding memberoftheseries: acetylene involvesadditionofCHortothe tion ofthehighercyanoacetylene,whileion-molecule not protontransferaretheonlycompetitiveprocesses. reactions ofthisionwithotherneutrals,Y,whichdo as representedinFigure3.Electron-ionrecombination transfer. Thissuggeststhatreactions(31a)and(31b) et al.Freemanreportthattheydidnotobserve or protontransferoftheproductionleadstoforma- 2 any additionofCHtoHCNbutonlyrapidproton n2 Reaction (31c)hasalsobeensuggestedbyChurchwell 1-4. 23 23 The otherrouteforformationofagivencyano- + ch ch 2 2 + + + Fig. 3.—Buildupofcyanoacetylenes throughCH2 Fig. 2.—BuildupofcyanoacetylenesthroughC2H2 2 HCN +CH->,(31b) HCN +CH->hv,(31a) HCN +CH->,(31c) n +123 n +123 n +123 g+ePT e, -Y e, X ' ^CH3N-’+*HC 23 chHN+ Ch+l 62,3 ^*-HCN 4¿3 *HCN—*.HC 27 t 25 HCN HCN- 27 23 C2 ^2 HCN 02^ e ,PT 0 * «JL'■'7 ^ HCN■ 5 ,+ -ílEJL H CN. HCN 3 C Hi C Hi CHy\ Vol. 232 197 9ApJ. . .232. .7403 + 4-3 + + + No. 3,1979 between CH2andHCNwhilewefindittobea proton transfertobethemajorchannelforreaction are unlikely.However,thesameauthorsalsofind to deriveexpression(30),whenappliedthisscheme minor channelrelativetoreaction(31a). give fortheabundanceratiosofsuccessivecyano- acetylenes they observedHCNandN,is3x10cm. density oftheTaurusMolecularCloud1,inwhich The firstterminbracesshouldbeclosetounity. 2 with observations.Inpractice,ofcourse,synthesis densities [CH*]/[CH^1.5,sothatexpression Churchwell etal.haveestimatedthatthetotalnumber either ofthesuggestedroutes,steady-statetreatment the cyanoacetylenescouldinvolvecombinationsof (32) wouldalsoyieldvaluesofRwhichareconsistent the relativeabundanceofagivencyanoacetylene.For both routes.Onecanalsomakeacrudeestimateof Mitchell etal.(1978)havecalculatedthatforthese gives tion oftheseequationsthengives 35 and 2 For thereasonsgivenabove,k[e]£A:Y].Combina- investigated, HCN/HCNiswithinafactorof10 unity inSgrB2.Ourschemethereforerequiresthat unity. Krotoetal.(1978)alsoestimatetheratiotobe i.e., ofionswhichreactwithHCNatclosetoclassical the abundanceofCHbesimilartothatZ, rates. AlthoughthereactionsbetweenHCNand atomic ionsHe,CandShavenotyetbeen ey 3 Allen, M.,andRobinson,G.W.1977,Ap./.,212,396. 3 Bohme, D.K.1975,inInteractionsofIonswithMolecules, 22j3 Dalgarno, A.1976,inAtomicProcessesandApplications,ed. Churchwell, E.,Winnewisser,G.,andWalmsley,C.M.1978, 3 Dalgarno, A.,andBlack,J.H.1976,Kept.Progr.Phys.,39, Fehsenfeld, F.C.1976,Ap.J.,209,638. Freeman, C.G.,Harland,P.W., andMcEwan,M.J.1978, Fox, K.,andJennings,D.E.1978, Ap.J.{Letters),226,L43. Fertel, J.H.,andTurner,B.E. 1975, Ap.Letters,16,61. Guélin, M.,Langer,W.D.,Snell, R.L.,andWooten,H.A. Steady-state approximations,similartothoseused Morris etal.(1976)reportthatinsome17sources ed. P.Ausloos(NewYork:Plenum). Astr. Ap.,OH,\?>9. Holland), p.111. P. G.BurkeandB.L.Moiseiwitsch(Amsterdam:North- 673. Ap Letters19133 1977, Ap.J.{Letters),217,LI65. © American Astronomical Society • Provided by theNASA Astrophysics Data System + R = [H,3CN] 23 + [HCN] rTT^ xnfc[e][H,3CN] 3 e23 + + [HCN] _fc[CH,] 3272 [HCN] MZ] + ~ k[e]+Y] ^ fc[HCN][CH,3] ey 272 + jcivn [CH,3 ]J’ 2 ION-MOLECULE SCHEME (32) REFERENCES +5 4-3 + + 6 + + + + 6 + + + 3- 46 9 5 tions ofMitchell,Ginsburg,andKuntz(1978)givethe to believethattheywillnotoccur.Themodelcalcula- ratio ofCHtotheseionsbeabout8x10"for n =10cm.Theydonotcalculateabundancesfor reaction ofCHwith.InspectionTable1 studied inthelaboratory,thereisnoapriorireason reveals thatreactionsofCandCHwith recently reestimatedCH^3x10“«andCH CH. Intheirmodelisformedonlybythe efficiency. Mitchell,Huntress,andPrasad(1979)have with CHandCHarethemainlossprocessesfor other CH*radicalsgiveCHandwithhigh ions canbecalculatedtooftheorder107that regions withinagivencloud(Churchwelletal.). CH and,thentheconcentrationofthese These regionsmaywellbethoseinwhichthedensities of C.Itshouldbenotedthatthecyanoacetylenesare 2 observed inrelativelyfewcloudsandevensmall 1 x10"«.Ifelectronrecombinationandreaction 2 turn impliesahighdensityofCH,suggestionalso 34 made byChurchwelletal. of CHandareparticularlyhigh.Thisin 42 23 inclusion wouldincreasetheproductionofcyano- (1976) haveindicatedthattheCNabundancesare could haveusedCNinplaceofHCN.Morrisetal. of thecyanoacetylenes.Bothschemessuggestedabove 42 23 acetylenes bymorethanafactorof2.Mitchell, similar tothoseofHCN,soitisdoubtfulthatits o an additionallinkage(cf.Fig.3)betweenHCiN laboratory. currently attemptingtostudythisreactioninthe and HCNbyreactionwithacetylene.Weare Huntress, andPrasad(1979)haverecentlysuggested 23 low densitiesininterstellarclouds.Thesmallerinter- tions oftheorder10"cm,whilelargeonessuch assumptions whichshouldbeexaminedinviewofthe stellar molecules(e.g.,NH,HCN)haveconcentra- 2 23 years, which,althoughlarge,donotprecludethe reaction times(1/evalues)rangefrom10to3x typical rateconstantsof10"thecorresponding as HCNhaveconcentrationsofabout10".For attainment ofsteadystatewithinthelifetime cloud. 2n + 2n +3 Hartquist, T.W.,andDalgarno,A.1978,Proc.GREYNOG Herbst, E.,Schubert,J.G.,andCertain,P.R.1977,Ap.J., Herbst, E.,andKlemperer,W.1976,Phys.Today,29,32. 3 Huntress, W.T.,Jr.1977,Ap.J.Suppl.,33,495. 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