LowLow--metallicitymetallicitystars and stars and the Rthe R--processprocessGrant VolleGrant VolleUniversity of Washington Department of PhysicsUniversity of Washington Department of PhysicsPHYS 554PHYS 5547 December 20077 December 2007OverviewOverview►►Galactic halo contains old starsGalactic halo contains old stars►►RR--process abundance ratios nearly the same process abundance ratios nearly the same for all stars studied, and the solar systemfor all stars studied, and the solar system►►Single RSingle R--process site?process site?►►ThThand U as chronometers?and U as chronometers?►►Constraints on RConstraints on R--process modelsprocess modelsWhich Stars?Which Stars?►►Low Low metallicitymetallicity[Fe/H] < [Fe/H] < --22Notation: [X/Y] = Notation: [X/Y] = log(Nlog(NXX/N/NYY))starstar--log(Nlog(NXX/N/NYY))sunsunFormed early in galactic historyFormed early in galactic history►►RR--process richprocess rich[[EuEu/Fe] > 1/Fe] > 1[[Ba/EuBa/Eu] < ] < --0.70.7►►Located in globular clusters, galactic haloLocated in globular clusters, galactic halo►►Also one faraway galaxyAlso one faraway galaxyNeutron CaptureNeutron Capture►►SS--process fairly wellprocess fairly well--understoodunderstoodFollows Follows ββ--stabilitystabilityOccurs in asymptotic giantsOccurs in asymptotic giants►►Halo stars formed before ISM had much sHalo stars formed before ISM had much s--process process isotopesisotopes►►RR--process very complicated, many exotic nuclei process very complicated, many exotic nuclei formed.formed.Neutron density most important parameterNeutron density most important parameter►►Some elements produced in both processes Some elements produced in both processes ––ss--process must be subtracted for SS abundances for process must be subtracted for SS abundances for these.these.RR--process abundances (main)process abundances (main)From Kratz, et alRR--process abundances (weak)process abundances (weak)From Johnson & BolteRR--process Sitesprocess Sites►►Main componentMain componentA > 130A > 130Type II supernovaeType II supernovae(maybe) colliding neutron stars(maybe) colliding neutron stars►►Weak componentWeak componentA < 130, esp. A < 130, esp. SrSr, Y, , Y, ZrZrLarge spread in abundances for metalLarge spread in abundances for metal--poor poor starsstarsPossibly multiple sitesPossibly multiple sitesSupernovae?Supernovae?Requires lower neutron density than main Requires lower neutron density than main componentcomponentRR--process computationsprocess computationsFrom Kratz, et alComputational ModelsComputational Models►►Larger neutron density = more highLarger neutron density = more high--A A productsproducts►►Best available models cannot predict both Best available models cannot predict both main and weak components simultaneouslymain and weak components simultaneously►►Main model parameters are neutron density Main model parameters are neutron density and and freezeoutfreezeouttemperaturetemperature►►Main process occurs at 23<log Main process occurs at 23<log nnnn<28, weak <28, weak process at 20<log process at 20<log nnnn<23<23ChronometersChronometers►►LongLong--lived radioactive isotopes (lived radioactive isotopes (ThTh, U) produced , U) produced only in ronly in r--processprocessCan predict age of star (sort of) from Can predict age of star (sort of) from ThTh/U, /U, 235235U/U/238238U U ratiosratiosU abundance measurements difficult, so U abundance measurements difficult, so Th/EuTh/Euis is generally more usefulgenerally more usefulGives stellar ages of 10Gives stellar ages of 10--15Gyr, typical uncertainty of 15Gyr, typical uncertainty of 4Gyr4GyrSome uncertainty from nuclear decay modelsSome uncertainty from nuclear decay models►►Th/PbTh/Pbmay be a good chronometermay be a good chronometer►►Star age puts lower limit on age of the galaxyStar age puts lower limit on age of the galaxyReferencesReferences►►Cowan, J.J., Cowan, J.J., et alet al., 1999, ., 1999, Astrophysical Astrophysical JournalJournal521521, 194, 194►►Cowan, J.J., Cowan, J.J., et alet al., 2006, in ., 2006, in International International Symposium on Nuclear AstrophysicsSymposium on Nuclear AstrophysicsIXIX►►Johnson, J.A. and Johnson, J.A. and BolteBolte, M., 2002, , M., 2002, Astrophysical JournalAstrophysical Journal579579, 616, 616►►KratzKratz, K.L., , K.L., et alet al., 2007, ., 2007, Astrophysical Astrophysical JournalJournal662662, 39, 39►►ProchaskaProchaska, J.X., , J.X., et alet al., 2003, ., 2003, NatureNature423423, 57, 57►►QianQian, Y.Z. and , Y.Z. and WasserburgWasserburg, G.J., 2001, , G.J., 2001, Physics ReportsPhysics Reports333333, 77,