A Search for Nitrogen-Enhanced Metal-Poor Stars

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1、DRAFTVERSIONFEBRUARY5,2008PreprinttypesetusingLATEXstyleemulateapjv.04/21/05ASEARCHFORNITROGEN-ENHANCEDMETAL-POORSTARS12JENNIFERA.JOHNSONDominionAstrophysicalObservatory,HerzbergInstituteofAstrophysics,NationalResearchCouncil,5071WestSaanichRd.,Victoria,

2、BCV9E2E7,CanadaFALKHERWIGTheoreticalAstrophysicsGroup,LosAlamosNationalLaboratory,LosAlamos,NM87545,USATIMOTHYC.BEERSDepartmentofPhysics&Astronomy,CSCE:CenterfortheStudyofCosmicEvolution,andJINA:JointInstituteforNuclearAstrophysics,MichiganStateUniversit

3、y,EastLansing,MI48824,USAANDNORBERTCHRISTLIEBHamburgerSternwarte,UniversitätHamburg,Gojenbergsweg112,D-21029Hamburg,GermanyDraftversionFebruary5,2008ABSTRACTTheoreticalmodelsofverymetal-poorintermediate-massAsymptoticGiantBranch(AGB)starspredictalargeove

4、rabundanceofprimarynitrogen.Theverymetal-poor,carbon-enhanced,s-process-richstars,whicharethoughttobethepollutedcompanionsofnow-extinctAGBstars,providedirecttestsofthepredictionsofthesemodels.Recentstudiesofthecarbonandnitrogenabundancesinmetal-poorstars

5、havefocusedonthemostcarbon-richstars,leadingtoapotentialselectionbiasagainststarsthathavebeenpollutedbyAGBstarsthatproducedlargeamountsofnitrogen,andhencehavesmall[C/N]ratios.WecallthesestarsNitrogen-EnhancedMetal-Poor(NEMP)stars,anddefinethemashaving[N/F

6、e]>+0.5and[C/N]<-0.5.Inthispaper,wereportonthe[C/N]abundancesofasampleof21carbon-enhancedstars,allbutthreeofwhichhave[C/Fe]<+2.0.IfNEMPstarsweremadeaseasilyasCarbon-EnhancedMetal-Poor(CEMP)stars,thenweexpectedtofindbetweentwoandsevenNEMPstars.Instead,wefo

7、undnoNEMPstarsinoursample.Therefore,thisobservationalbiasisnotanimportantcontributortotheapparentdearthofN-richstars.Our[C/N]valuesareinthesamerangeasvaluesreportedpreviouslyintheliterature(-0.5to+2.0),andallstarsareindisagreementwiththepredicted[C/N]rat

8、iosforbothlow-massandhigh-massAGBstars.Wesuggestthatthedecreasein[C/N]fromthelow-massAGBmodelsisduetoenhancedextra-mixing,whilethelackofNEMPstarsmaybecausedbyunfavorablemassratiosinbinariesorthedifficultyofmasstransferinbin