Advances in Atomic Data for Neutron-Capture Elements
N. C. Sterling, M. C. Witthoeft, D. A. Esteves, P. C. Stancil, A. L. D. Kilcoyne, R. C. Bilodeau, A. Aguilar
aa r X i v : . [ a s t r o - ph . S R ] S e p Planetary Nebulae: An Eye to the FutureProceedings IAU Symposium No. 283, 2012***NAME OF EDITORS*** c (cid:13) Advances in Atomic Data forNeutron-Capture Elements
N. C. Sterling † , M. C. Witthoeft , . D. A. Esteves , P. C. Stancil A. L. D. Kilcoyne , R. C. Bilodeau , , and A. Aguilar Department of Physics and Astronomy, Michigan State University, 3248 Biomedical PhysicalSciences, East Lansing, MI 38824-2320, USA, email: [email protected] NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA Department of Astronomy, University of Maryland, College Park, MD 20742, USA JILA, University of Colorado, Boulder, CO 80309-0440, USA Department of Physics and Astronomy and the Center for Simulational Physics, Universityof Georgia, Athens, GA 30602-2451, USA The Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,USA Western Michigan University, MS 5252, 1903 W. Michigan Ave., Kalamazoo, MI 49008, USA
Abstract.
Neutron( n )-capture elements (atomic number Z > s -process nucleosynthesis, have been detected innearly 100 PNe. This demonstrates that nebular spectroscopy is a potentially powerful toolfor studying the production and chemical evolution of trans-iron elements. However, significantchallenges must be addressed before this goal can be achieved. One of the most substantialhurdles is the lack of atomic data for n -capture elements, particularly that needed to solve fortheir ionization equilibrium (and hence to convert ionic abundances to elemental abundances).To address this need, we have computed photoionization cross sections and radiative and dielec-tronic recombination rate coefficients for the first six ions of Se and Kr. The calculations werebenchmarked against experimental photoionization cross section measurements. In addition, wecomputed charge transfer (CT) rate coefficients for ions of six n -capture elements. These ef-forts will enable the accurate determination of nebular Se and Kr abundances, allowing robustinvestigations of s -process enrichments in PNe. Keywords. atomic data, planetary nebulae: general, stars: AGB and post-AGB
Neutron( n )-capture elements can be produced in low- and intermediate-mass stars( ∼ ⊙ ), the progenitors of planetary nebulae (PNe), during the asymptotic giantbranch (AGB) phase via slow n -capture nucleosynthesis (the “ s -process”; Busso et al.1999). Nebular spectroscopy is a promising new tool for investigating n -capture nucle-osynthesis and the chemical evolution of trans-iron elements, providing access to ele-ments not detectable in AGB stars and to classes of stars (e.g., intermediate-mass stars,4–8 M ⊙ ) whose photospheres are obscured by heavy mass-loss during the AGB and post-AGB stages of evolution. The detection of n -capture element emission lines in the opticaland near-infrared spectra of nearly 100 PNe (Sharpee et al. 2007, Sterling & Dinerstein2008) demonstrates the potential of nebular spectroscopy for the study of these species.The lack of atomic data for processes governing the ionization balance of n -captureelements presents the primary challenge in honing nebular spectroscopy into an effectivetool for studying trans-iron elements. These data are needed because typically only one ortwo ions of n -capture elements have been detected in individual PNe, and unobserved ions † NSF Astronomy and Astrophysics Postdoctoral Fellow n -capture elements in PNe. Using the AUTOSTRUCTURE atomic structure code (Badnell2011), we computed multi-configuration distorted-wave PI cross sections and RR and DRrate coefficients for the first six Se and Kr ions (Sterling & Witthoeft 2011, Sterling 2011).In nearly all cases, DR is the dominant recombination mechanism, with rate coefficientsat 10 K exceeding those of RR by as much as 1–2 orders of magnitude.Our calculations were benchmarked against experimental absolute PI cross sectionsmeasured at the Advanced Light Source synchrotron radiation facility at Lawrence Berke-ley National Laboratory in California (Sterling et al. 2011, Esteves et al. 2011a,b). Thesemeasurements were conducted with the merged-beams method on the Ion Photon Beamsapparatus, with typical accuracies of 20–30%.Based on comparison to experimental measurements and the sensitivity of our results toorbital radial scaling parameters, we estimate the direct PI cross sections to have uncer-tainties of 30–50% for most Se and Kr ions. RR rate coefficients are uncertain by n -capture ele-ments that have been detected in PN spectra (Ge, Se, Br, Kr, Rb, and Xe), using theLandau-Zener and Demkov approximations (Sterling & Stancil 2011). These approxima-tions generally are accurate to within a factor of three for transitions with large ratecoefficients, and an order of magnitude for weaker ones (Butler & Dalgarno 1980).We are expanding the atomic database of the photoionization code Cloudy (Ferlandet al. 1998) up to Kr, using the newly determined atomic data. We will use Cloudy tocompute a grid of models for deriving robust ionization corrections for unobserved Se andKr ions, enabling much more accurate abundance determinations than currently possible.Moreover, we will test the sensitivity of abundance determinations to uncertainties in theatomic data, illuminating the atomic processes and species that require further analysis.Such tests are illustrative of the abundance uncertainties of lighter elements with atomicdata derived in a similar manner (especially iron-peak elements). References
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