The Road to Accurate Trans-Iron Element Abundances in Ionized Nebulae

Details The Road to Accurate Trans-Iron Element Abundances in Ionized Nebulae The Road to Accurate Trans-Iron Element Abundances in Ionized Nebulae

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0307s&link_type=abstract

American Astronomical Society, AAS Meeting #212, #3.07; Bulletin of the American Astronomical Society, Vol. 40, p.184

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We present initial results from a program that will dramatically improve the accuracy of trans-iron element abundance determinations in planetary nebulae (PNe) and other ionized nebulae, through new atomic data determinations and multi-wavelength observations. Neutron(n)-capture elements (atomic number Z>30) can be produced in PN progenitor stars by s-process nucleosynthesis during the asymptotic giant branch phase. Abundance determinations of Ge (Sterling et al. 2002, ApJ, 578, L55), Se, and Kr (Sterling & Dinerstein 2008, ApJS, 174, 158) from ultraviolet and infrared spectroscopy are uncertain by factors of 2 or 3 for most PNe. These uncertainties arise from: (1) the detection of only one ion of each element, leading to large and uncertain ionization correction factors (ICFs); and (2) the lack of atomic data governing the ionization balance of these elements.
We present calculations of photoionization cross-sections and recombination rate coefficients for the four most widely observed n-capture elements in ionized nebulae (Ge, Se, Kr, and Xe), using the atomic structure codes AUTOSTRUCTURE (Badnell 1986, J. Phys. B, 19, 3827) and DARC (Ballance & Griffin 2006, J. Phys. B, 40, 3617). To calibrate these calculations, we have experimentally measured absolute photoionization cross-sections at the Advanced Light Source synchrotron radiation facility, and present results for Se and Xe ions. These new atomic data will be incorporated into state-of-the-art photoionization models to derive robust ICFs for these elements. In addition, we have obtained deep, high resolution optical spectra of 14 s-process enriched PNe. Combined with IR spectra, these measurements enable multiple Se, Kr, and Xe ions to be detected, which reduces the magnitude and uncertainties in their ICFs (and consequently the derived elemental abundances). We will apply these results to determine the n-capture element abundances in PNe more accurately than previously possible, and to study the details of s-process nucleosynthesis in their progenitor stars.

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