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[TiII] and [NiII] emission from the strontium filament of eta Carinae

We study the nature of the [TiII] and [NiII] emission from the so-called strontium filament found in the ejecta of eta Carinae. To this purpose we employ multilevel models of the TiII and NiII systems which are used to investigate the physical condition of

Mon.Not.R.Astron.Soc.000,()Printed5February2008

A(MNLTEXstyle lev1.4)

[Tiii]and[Niii]emissionfromthestrontium lamentof

ηCarinae

M.A.Bautista1,H.Hartman2,T.R.Gull3,N.Smith4,K.Lodders5

deF´ sica,IVIC,P.O.Box21827,Caracas1020A,VenezuelaObservatory,LundUniversity,Box43,SE-22100Lund,Sweden

3Code667,NASAGoddardSpaceFlightCenter,GreenbeltMD,USA

4CenterforAstrophysicsandSpaceAstronomy,UniversityofColorado,389UCB,Boulder,CO80309,USA

5PlanetaryChemistryLaboratory,Dept.ofEarthandPlanetarySciences,WashingtonUniversity,St.Louis,MO63130,USA

2Lund1Centro

arXiv:astro-ph/0606275v1 12 Jun 2006

ABSTRACT

Westudythenatureofthe[Tiii]and[Niii]emissionfromtheso-calledstrontium lamentfoundintheejectaofηCarinae.Tothispurposeweemploymultilevelmod-elsoftheTiiiandNiiisystemswhichareusedtoinvestigatethephysicalconditionofthe lamentandtheexcitationmechanismsoftheobservedlines.FortheTiiiion,forwhichnoatomicdatawaspreviouslyavailable,wecarryoutabinitiocalcu-lationsofradiativetransitionratesandelectronimpactexcitationratecoe cients.Itisfoundthattheobservedspectrumisconsistentwiththelinesbeingexcitedinamostlyneutralregionwithanelectrondensityoftheorderof107cm 3andatemper-aturearound6000K.Inanalyzingthreeobservationswithdi erentslitorientationsrecordedbetweenMarch2000andNovember2001we ndlineratiosthatchangeamongvariousobservations,inawayconsistentwithchangesofuptoanorderofmagnitudeinthestrengthofthecontinuumradiation eld.Thesechangesresultfromdi erentsamplingsoftheextended lament,duetothedi erentslitorientationsusedforeachobservation,andyieldcluesonthespatialextentandopticaldepthofthe lament.TheobservedemissionindicatesalargeTi/Niabundanceratiorelativetosolarabundances.ItissuggestedthattheobservedhighTi/Niratioingasiscausedbydust-gasfractionationprocessesanddoesnotre ecttheabsoluteTi/NiratiointheejectaofηCarinae.WestudythecondensationchemistryofTi,NiandFewithinthe lamentandsuggestthattheobservedgasphaseoverabundanceofTiislikelytheresultofselectivephoto-evaporationofTi-bearinggrains.Somemechanismsforsuchascenarioareproposed.

Keywords:atomicdata–atomicprocesses–line:formation–stars:abundances–stars:EtaCarinae

1INTRODUCTION

ηCarinaeisthemostluminousstarinourpartoftheGalaxyandindeedisoneofthemostmysteriousknowntous(Davidson&Humphreys1997).Itissurroundedbycom-plexejectacreatedinseveraloutburstsofthestarandanoutgoingmassivewindwithperiodiceventsevery5.54years(Damineli1996).Thelargestmassejectionoccurredduringtheso-calledGreatEruptionobservedinthe1840s–whichthestarsurvived.Inthatevent,ηCarinaereleasedabout1049.6ergsofkineticenergycontainedinmorethan10M⊙ofmaterial(Smithetal.2003),whichhassinceexpandedtoformthebipolarHomunculusnebulaweseetoday(Gaviola1950;Smith&Gehrz1998;Morseetal.2001).Despitethehotandveryluminouscentralstar,thecircumstellarneb-ulaiscoolenoughtoproducearemarkablyrichemission

cRAS

spectrumoflinesfromneutralandlowionizationspecies

likeFe+andNi+(Davidsonetal.2001;Zethson2001;Zeth-sonetal.2001),line-of-sightabsorptionfromseveralneutralandsinglyionizedatoms(Gulletal.2005),andevenemis-sionfromH2andverycooldust(Smith2002a;Smithetal.2003).Inadditiontobeingdenseandcool,theejectaofηCarinaearenitrogenrich,representingashesoftheCNOcycle(Davidsonetal.1982,1986;Smith&Morse2004;Verneretal.2005).Itisthehighdensity,low-excitation,andCNOabundancesoftheejectathatmaketheHomunculusauniquelaboratoryamongcircumstellarnebulae,havingaprofounde ectontheresultsdiscussedinthispaper.InFebruary1999aseriesofSTISobservationsofηCari-naewasmadeofaregionabout1.5”northwestofthestar(Zethsonetal.2001).Thespectrumfromthisregionwassigni cantlydi erentfromthenebularemissionspectra

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  • eta carinae
  • strontium
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  • filament
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