1. The Abundances of Light Neutron- Capture Elements in Planetary Nebulae Nick Sterling NASA Goddard Space Flight Center June 19, 2007 Collaborators: Harriet Dinerstein (University of Texas-Austin), T. R. Kallman (NASA/GSFC)

2. Nucleosynthesis and Dredge-Up in AGB Stars Thermally-pulsing AGB –Double shell structure –He-shell flashes → Thermal pulses Third Dredge-up –Convective Envelope dredges up material in intershell region C, n-capture elements From Busso et al. 1999, ARA&A, 37, 239 s-process nucleosynthesis –“Slow” n-captures (relative to  - decay lifetime) on Fe peak seed nuclei, during interpulse phase –Protons mixed into C-rich region → 13 C “pocket” – 13 C( ,n) 16 O neutron source –Intermediate-mass stars: 22 Ne( ,n) 25 Mg –Produce ~1/2 of the Z > 30 isotopes in Universe

3. Brief History Light (Z=30-40) elements most abundant Easiest to detect Low cosmic abundances (< 4×10 -9 relative to H in Solar System) Péquignot & Baluteau (1994) NGC 7027: Kr (Z=36), Xe (Z=54) detected, perhaps others Dinerstein (2001) 2.199, 2.287  m lines seen in several PNe spectra Identified as [Kr III] and [Se IV] (Z=34) Sterling et al. (2002); Sterling & Dinerstein (2003) Detection of Ge III (Z=32) in UV spectra of 6 PNe; enriched in 4 Sharpee et al. (2007) Deep, high resolution optical spectra of 4 PNe Br (Z=35), Kr, Rb (Z=37), Xe, Ba (Z=56), Pb (Z=82) lines GRAND TOTAL: 11 objects with n-capture element abundances

4. The Survey First large-scale survey of n- capture elements in PNe CoolSpec on 2.7-m at McDonald Observatory [Kr III] 2.199, [Se IV] 2.287  m lines 120 PNe –81 detections of Kr/Se Survey mode: R=500 Line Blends –H 2 3-2 S(3) 2.201, H 2 3-2 S(2) 2.287  m –H 2 in 20-25% of sample targets –High resolution setting (R=4400) Splits H 2 /[Kr III] Estimate H 2 contribution to 2.287  m

5. Se and Kr Abundances Ionization corrections from grid of photoionization models –Cloudy (Ferland et al. 1998); XSTAR (Kallman & Bautista 2001) Enrichment factors –Relative to Ar for Type I PNe, O for others Possible O depletion in Type I objects (from HBB) Range of abundances –[Kr/(O, Ar)]: –0.05 to 1.9 dex (mean: 0.98) –[Se/(O, Ar)]: –0.5 to 0.9 dex (mean: 0.31) –Enrichments → s-process, third dredge-up ~44% of targets enriched Increases number of PNe with known n-capture element abundances by factor 10!

6. Correlations Type I and Bipolar PNe –Intermediate-mass progenitors –Little to no s-process enrichment –Confirmed by KS tests –Interpretation: Severe dilution into massive AGB envelopes Less massive intershell zones relative to lower-mass AGB stars [WC] PNe –s-process enrichments similar to other PNe –Despite C-rich central stars likely enriched in n-capture elements From Sterling & Dinerstein 2007, ApJS, in press (astro-ph/0706.1074)

7. Correlations (cont.) Positive correlation with C/O –C dredged up during TP-AGB along with n-capture elements –Observed in AGB, post-AGB stars Binary Central Stars –Binary interactions can truncate AGB phase (Izzard 2004) → C, s-process enrichments not as large –Only 14 PNe in sample with direct evidence of binary central stars Little to no s-process enrichment Not conclusive (progenitor mass, etc. can also result in small enrichments) From Sterling & Dinerstein 2007, ApJS, in press (astro-ph/0706.1074)

8. Final Remarks More details: –Sterling et al. 2007, ApJS, 169, 37 (Reprints available) –Sterling & Dinerstein 2007, ApJS, in press (astro-ph/0706.1074) Recently detected in extragalctic PNe –Dinerstein et al. poster (#12) Improvements to abundances –Uncertainties in Se, Kr abundances: factor of 2-3 for most PNe Atomic data uncertainties, large ICFs (only one ion detected) –New atomic data calculations First six ions of Se, Kr, Xe AUTOSTRUCTURE (Badnell 1986) Will be added to atomic databases of Cloudy, XSTAR New ICFs derived –New deep, high-resolution optical spectra s-process enriched PNe Search for additional ions of Se and Kr, lines of Xe Reduces magnitude, uncertainties of ICFs Background image credits: Ragnar & Einar Johnskås ( http://auroraborealis.vika.no/) http://auroraborealis.vika.no/ NASA/Hubble Heritage team (AURA/STScI, http://hubblesite.org/newscenter/archive/)