The Astrophysical Journal, 659:1265Y1290, 2007 April 20 A # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. s-PROCESS ABUNDANCES IN PLANETARY NEBULAE Brian Sharpee,1 Yong Zhang,2, 3, 4 Robert Williams,2 Eric Pellegrini,5 Kenneth Cavagnolo,5 Jack A. Baldwin,5 Mark Phillips,6 and Xiao-Wei Liu 3 Received 2005 May 6; accepted 2006 December 4 ABSTRACT The s-process should occur in all but the lower mass progenitor stars of planetary nebulae, and this should be re- flected in the chemical composition of the gas that is expelled to create the current planetary nebula shell. Weak for- bidden emission lines are expected from several s-process elements in these shells and have been searched for and in some cases detected in previous investigations. Here we extend these studies by combining very high signal-to-noise ratio echelle spectra of a sample of PNe with a critical analysis of the identification of the emission lines of Z > 30 ions. Emission lines of Br, Kr, Xe, Rb, Ba, and Pb are detected with a reasonable degree of certainty in at least some of the objects studied here, and we also tentatively identify lines from Te and I, each in one object. The strengths of these lines indicate enhancement of s-process elements in the central star progenitors, and we determine the abundances of Br, Kr, and Xe, elements for which atomic data relevant for abundance determination have recently become available. As representative elements of the ‘‘light’’ and ‘‘heavy’’s-process peaks, Kr and Xe exhibit similar enhancements over solar values, suggesting that PN progenitors experience substantial neutron exposure. Subject headings:g ISM: abundances — nuclear reactions, nucleosynthesis, abundances — planetary nebulae: general Online material: machine-readable table, tar file 1. INTRODUCTION IR fine-structure nebular emission from postYFe peak ions As remnants of stars that have evolved through the asymptotic (Dinerstein 2001; Sterling & Dinerstein 2004) and from far-UV giant branch (AGB) phase, most planetary nebulae (PNe) are be- resonance-line absorption by s-process elements in the interven- lieved to consist of material that has undergone nuclear process- ing nebular shell seen in Far Ultraviolet Spectroscopic Explorer ing in the precursor star via the s-process. The analysis of nebular (FUSE) spectra of their central stars (Sterling el al. 2002; Sterling emission from elements that have experienced nucleosynthesis in & Dinerstein 2003). They detected IR emission from Se and Kr in the parent star provides valuable information for stellar models. roughly 50% of their sample PNe from which abundances of up However, the detection of emission lines from ions enhanced to 10 times solar were derived for these elements. The FUSE spectra revealed Ge iii absorption in five PNe for which Ge en- by the s-process has been hampered by their weakness and by Y uncertainties in the atomic data needed for the analysis, includ- hancements of 3 10 times solar were deduced, clear evidence ing line wavelengths. that central star AGB progenitors are major sites of s-process An initial attack on this problem was made a decade ago by element production. Pe´quignot & Baluteau (1994, hereafter PB94), who obtained a As part of our ongoing program to detect and identify the deep optical spectrum of the high-ionization PN NGC 7027. Us- weakest lines in PNe at visible wavelengths down to levels sig- < À5 the intensity ing the best atomic data available for the energy levels of the nificantly below that of the continuum, viz., 10 of H , we have obtained very high signal-to-noise ratio (S/N) more prominent ionization stages of elements in the fourth and spectra at high spectral resolution of some of the higher surface fifth rows of the periodic table, they identified a number of postYFe brightness nebulae. Some fraction of the weaker lines observed peak emission lines in the nebula. From the observed line in- Y tensities they concluded that the elements Kr and Xe, the latter are likely to originate from post Fe peak elements; therefore, we have used the automatic line identification routine EMILI (Sharpee normally a predominantly r-process element in stars of solar met- et al. 2003), supplemented by recent energy level data for heavier allicity, were enhanced in NGC 7027 by factors of 20 relative to their initial formation abundances, presumably roughly solar. elements, to assist in the search for such lines. Since the publi- cation of PB94, new calculations have been made for sponta- They also detected Ba ii and [Br iii] emission at intensities in- neous emission coefficients (Bie´mont et al. 1995) and collision dicating that Ba could be enhanced whereas Br might be depleted strengths (Scho¨ning 1997; Scho¨ning & Butler 1998) for fourth- relative to solar values, subject to uncertainty due to poorly known and fifth-row elemental ion transitions. In this study we use these excitation cross sections. newer atomic parameters to compute ionic and overall elemental Dinerstein and collaborators have subsequently pursued the abundances for Br, Kr, and Xe in order to make comparisons study of s-process abundances in PNe through surveys to detect with their abundances in H ii regions and the Sun and to derive s-process enrichment factors relevant to the study of s-process 1 SRI International, Menlo Park, CA 94025. 2 Space Telescope Science Institute, Baltimore, MD 21218. nucleosynthesis in the progenitor stars. 3 Department of Astronomy, Peking University, Beijing 100871, China. 4 Current address: Department of Physics, University of Hong Kong, Hong 2. OBSERVATIONS AND DATA REDUCTION Kong, China. Our present sample of objects consists of four PNe (IC 2501, 5 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824. IC 4191, NGC 2440, and NGC 7027) that satisfy the most im- 6 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, portant criteria for the detection of weak emission lines in having Chile. (1) high surface brightnesses, (2) low expansion velocities, and 1265 1266 SHARPEE ET AL. Vol. 659 TABLE 1 Journal of Observations Dates Integration Times Object Telescope (UT) (s) Slit Center NGC 7027........................ KPNO 4 m 2002 Jun 19Y22 34 ; 1200, 9 ; 300, 1 ; 120, 4 ; 60, 6 ; 30 3.500 W, 0.5 00 N IC 2501 ............................ LCO 6.5 m 2003 Feb 12Y13 6 ; 1800, 1 ; 300, 4 ; 60 200 E IC 2501 ............................ LCO 6.5 m 2003 Feb 25Y26 15 ; 1800, 2 ; 60 200 E IC 4191 ............................ LCO 6.5 m 2003 Feb 12Y13 5 ; 1800, 1 ; 1200, 1 ; 300, 1 ; 60 200 E IC 4191 ............................ LCO 6.5 m 2003 Feb 25Y26 10 ; 1800, 1 ; 300, 3 ; 60 200 E NGC 2440........................ LCO 6.5 m 2003 Feb 12Y13 11 ; 1800, 1 ; 300, 1 ; 60 1400 E (3) except for NGC 7027, relatively small internal dust extinc- NGC 7027 was observed on four nights in 2002 June with tion. The latter two criteria produce sharper lines and higher peak the Mayall 4 m Telescope at Kitt Peak National Observatory intensities relative to the continuum. (KPNO) using the Cassegrain echelle spectrograph. Because we We obtained spectra of the PNe IC 2501, IC 4191, and were interested in detecting faint 40Y50 km sÀ1 FWHM emis- NGC 2440 during two observing runs of two nights each in sion lines over the widest possible wavelength range, rather than early 2003 using the Las Campanas Observatory (LCO) Baade detailed measurements of the line profiles, we used the short 6.5 m telescope with the MIKE echelle spectrograph. Similar UV camera with the 79.1 groove mmÀ1 echelle grating, cross- instrumentation setups were used during the two runs. MIKE is a dispersing grating 226-1 in first order, and a GG-475 order sep- dual-beam spectrograph that simultaneously measures separate arating filter. This gave full wavelength coverage from 4600 to red and blue spectra, giving useful data over the continuous 9200 8 at 20 km sÀ1 FWHM (k/Ák ¼ 15;000) resolution wavelength ranges 3280Y 4700 and 4590Y7580 8 at resolutions with our 200 slit width, with partial coverage out to 9900 8.On (k/Ák) of 28,000 and 22,000, respectively, for the 100 slit width each night we offset to the same position with the slit at P:A: ¼ we used. 145 andcentered0.500 north and 3.500 west of the central star, A series of long and short exposures were taken of each nebula which is the brightest part of the PN shell in H emission. Most on one or more nights, typically adding up to a period of 2Y3hr of the observing time was spent obtaining sequences of 1200 s at a time spent observing each object. Because MIKE is used exposures that added up to a total of 680 minutes of integration without an image rotator at a Nasmyth focus, the orientation of time. We also took a number of shorter exposures 30, 60, 120, the slit with respect to each nebula rotated by a large amount dur- and300sinlengthtomeasurebrightemissionlinesthatwere ing these series of exposures. The central star was placed along a saturated on the longer exposures. line perpendicular to the spectrograph slit so that the slit center The data for all four PNe were reduced using standard proce- was roughly midway between the central star and outer edges of dures with IRAF-based reduction packages in the same manner the nebula, and then the telescope was guided to keep the star in that has been described in detail in our discussion of comparable that same position as seen on the acquisition/guide TV. The result spectra of the PN IC 418 that we obtained previously with the was that during the course of the observations the slit swept out Cerro Tololo Inter-American Observatory 4 m echelle spectro- an arc about the central star, so that the final spectrum integrates graph (Sharpee et al.