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1、UnidentifiedInfraredEmissionBandsintheDiffuseInterstellarMedium1Kin-WingChan2,3,T.L.Roellig2,T.Onaka4,M.Mizutani4,K.Okumura5,I.Yamamura6,T.Tanab´e7,H.Shibai8,T.Nakagawa6,andH.Okuda9ABSTRACTUsingtheMid-InfraredSpectrometeronboardtheInfraredTelescopeinSpaceandthelo
2、w-resolutiongratingspectrometer(PHT-S)onboardtheInfraredSpaceObservatory,weobtained820mid-infrared(5to12µm)spectraofthediffuseinterstellarmedium(DIM)intheGalacticcenter,W51,andCarinaNeb-ularegions.Thesespectraindicatethattheemissionisdominatedbytheunidentifiedinf
3、rared(UIR)emissionbandsat6.2,7.7,8.6,and11.2µm.Therelativebandinten-sities(6.2/7.7µm,8.6/7.7µm,and11.2/7.7µm)werederivedfromthesespectra,andnosystematicvariationintheseratioswasfoundinourobservedregions,inspiteofthefactthattheincidentradiationintensitydiffersbya
4、factorof1500.Comparingourresultswiththepolycyclicaromatichydrocarbons(PAHs)modelfortheUIRbandcarriers,PAHsintheDIMhavenosystematicvariationintheirsizedistribution,theirdegreeofdehydrogenationisindependentofthestrengthofUVradiationfield,andtheyaremostlyionized.Th
5、efindingthatPAHsintheDIMwithlowUVradiationfieldstrengtharemostlyionizedisincompatiblewithpasttheoreticalstudies,inwhichalargefractionofneutralPAHsispredictedinthiskindofenvironment.Aplausibleresolutionofthisdiscrepancyisthattherecombinationcoefficientsforelectronan
6、dlargePAHpositiveionarebyatleastanorderofmagnitudelessthanthoseadoptedinpasttheoreticalstudies.Becauseoftheverylowpopulationofneutralstatemolecules,arXiv:astro-ph/0008335v122Aug20001BasedonobservationswithISO,anESAprojectwithinstrumentsfundedbyESAmembersstates(
7、especiallythePIcountriesFrance,Germany,theNetherlands,andtheUnitedKingdom)andwiththeparticipationofISASandNASA.2NASAAmesResearchCenter,MS245–6,MoffettField,CA94035-10003presentaddress:DepartmentofAstronomy,UniversityofTokyo,Bunkyo-ku,Tokyo113-0033,Japan,kwc@astr
8、on.s.u-tokyo.ac.jp4DepartmentofAstronomy,UniversityofTokyo,Bunkyo-ku,Tokyo113-0033,Japan5CommunicationsResearchLaboratory,Koganei,Tokyo184-8795,Japan6InstituteofSpaceandAstr
