Carbonates in space - The challenge of low temperature data

Carbonatesinspace–

Thechallengeoflowtemperaturedata1

arXiv:0706.3963v2 [astro-ph] 18 Jul 2007

Th.Posch,A.Baier

Institutf¨urAstronomie,T¨urkenschanzstraße17,A-1180Wien,Austria

posch@astro.univie.ac.at

and

H.Mutschke

AstrophysikalischesInstitut,Schillerg¨asschen2-3,D-07745Jena,Germany

and

Th.Henning

Max-Planck-Institutf¨urAstronomie(MPIA),K¨onigstuhl17,D-69117Heidelberg,Germany

ABSTRACT

Carbonateshaverepeatedlybeendiscussedaspossiblecarriersofstardust

emissionbands.However,thebandassignmentsproposedsofarweremainlybasedonroomtemperaturepowdertransmissionspectraoftherespectivemin-erals.SinceverycoldcalcitegrainshavebeenclaimedtobepresentinprotostarsandinPlanetaryNebulaesuchasNGC6302,thechangesoftheirdielectricfunctionsatlowtemperaturesarerelevantfromanastronomicalpointofview.WehavederivedtheIRopticalconstantsofcalciteanddolomitefromre- ectancespectra–measuredat300,200,100and10K–andcalculatedsmallparticlespectrafordi erentgrainshapes,withthefollowingresults:i)Theab-sorptione ciencyfactorsbothofcalciteanddolomiteareextremelydependentontheparticleshapes.Thisisduetothehighpeakvaluesoftheopticalcon-stantsofCaCO3andCaMg[CO3]2.ii)Thefarinfraredpropertiesofcalciteanddolomitedependalsoverysigni cantlyonthetemperature.Below200K,apro-nouncedsharpeningandincreaseinthebandstrengthsoftheFIRresonancesoccurs.iii)Inviewoftheintrinsicstrengthandsharpeningofthe～44µmbandofcalciteat200–100K,theabsenceofthisband–inferredfromInfraredSpaceObservatorydata–inPNerequiresdusttemperaturesbelow45K.iv)Calcitegrainsatsuchlowtemperaturescanaccountforthe‘92’µmband,whileourdata