The role of the star formation on the nitrogen abundance evolution

We analyze the evolution of nitrogen resulting from a set of spiral and irregular galaxy models computed for a large number of input mass radial distributions and with various star formation efficiencies. We show that our models produce a nitrogen abundanc

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a**FULLTITLE**ASPConferenceSeries,Vol.**VOLUME**,**YEAROFPUBLICATION****NAMESOFEDITORS**TheevolutionroleofthestarformationonthenitrogenabundanceM.Moll´a,1,J.M.V´ lchez,2A.I.D´ az,3andM.Gavil´an.3Abstract.Weanalyzetheevolutionofnitrogenresultingfromasetofspiralandirregulargalaxymodelscomputedforalargenumberofinputmassradialdistributionsandwithvariousstarformatione ciencies.Weshowthatourmodelsproduceanitrogenabundanceevolutioningoodagreementwiththeobservationaldata.Di erencesinthestarformationhistoriesoftheregionsandgalaxiesmodeledareessentialtoreproducetheobservationaldataintheN/O-O/Hplaneandthecorrespondingdispersion.1.TheprimaryNitrogenWhenobservations,speciallythemetal-poorgalaxydata,areplotedintheplaneN/OvsO/H,itisevidentthataprimaryNcontributionmustexist.ApplyingtheclassicalClosedBoxModel,thepointsmaybelimitedbythreelinesasthoseplottedinFig.1a:1)ThisonecalledNS,whichshowstheevolutionarytracksofN/OwhenNneedaseedofOtobecreated:NpO∝O.2)ThisonecalledNPthatappearwhenNiscreateddirectlyfromHorHe:NpO=constant.3)NS+NPwhenbothcontributionsthereexist,whichseemstobetheapparentbehaviorreproducedbythedata.Itis,however,necessarytotakeintoaccountthreefactorsnotincludedinthatsimplescenario:1)themeanlifetimesofstars,2)thestarformatione ciencyortheexistenceofdi erentstarformationhistoriesorstarformationratesindi erentgalaxiesorobjects,and3)themetallicitydependentofstel-laryields.IftheNPisejectedbylowandintermediatemass(LIM)stars,itwillappearintheinterstellarmediumveryabruptelyafteratimedelay,whichmeanswhenO/Hhasalreadyreachedacertainvalue.Obviously,thislevelwillbehigherorlowerdependingonthemean-lifetimeofthesestars.Butitalso

dependsonthee ciencytoformstars:forahighe ciencytheNPwillappearlaterinthetime,orathigherO/H,thanforalowone.Bothfactsproduceacertaindispersionintheresultingabundances.Besidesthatthemetallicitydependentstellaryieldsproducetrackselongatedincomparisonwiththeoneproducedwhenonlyavalueofyieldisused(fordetailsseeMoll´aetal.2006).

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