Brian David Sharpee

s-PROCESS ABUNDANCES IN PLANETARY NEBULAE

The s-process should occur in all but the lower mass progenitor stars of planetary nebulae, and this should be reflected in the chemical composition of the gas that is expelled to create the current planetary nebula shell. Weak forbidden 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 ratioechellespectraof asampleof PNewithacriticalanalysisoftheidentificationof theemissionlinesof Z > 30ions. 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,andXe,elementsforwhichatomicdatarelevantforabundancedeterminationhaverecentlybecomeavailable. Asrepresentative elementsofthe‘‘light’’and‘‘heavy’’s-processpeaks,KrandXeexhibitsimilar enhancementsover solar values, suggesting that PN progenitors experience substantial neutron exposure. Subject headingg ISM: abundances — nuclear reactions, nucleosynthesis, abundances — planetary nebulae: general

Introducing EMILI: Computer-aided Emission Line Identification

The identification of spectral lines can be a tedious process requiring the interrogation of large spectroscopic databases, but it does lend itself to software algorithms that can determine the characteristics of candidate line identifications. We present here criteria used for the identification of lines and a logic developed for a line identification software package called EMILI, which uses the v2.04 Atomic Line List as the basic line database. EMILI considers all possible database transitions within the wavelength uncertainties for observed optical emission lines and computes an approximate intensity for each candidate line. It searches for other multiplet members that are expected to be seen with each candidate line, and rank-orders all of the tentative line identifications for each observed line based on a set of criteria. When applied to the spectra of the Orion Nebula and the planetary nebula IC 418, EMILIs recommended line IDs agree well with those of previous traditional manual line assignments. The existence of a semiautomated procedure should give impetus to the study of very high signal-to-noise spectra, enabling the identification of previously unidentified spectral lines to be handled with ease and consistency.

THE CHANGING BLAZHKO EFFECT OF XZ CYGNI

New CCD photometry has been obtained for the RR Lyrae variable star XZ Cygni. An analysis of old and new photometry confirms earlier results that XZ Cyg exhibits the Blazhko effect and that its Blazhko period has changed over time. These changes in the Blazhko period are anticorrelated with observed changes in the primary period of XZ Cyg. During the first half of the 20th century, XZ Cyg had a Blazhko period of approximately 57.4 days. Beginning in 1965, its primary period underwent a steep decline in several steps. Coincidentally, its Blazhko period increased to about 58.5 days. In 1979, the primary period suddenly increased again. After an interval in which the Blazhko effect was small, the Blazhko effect reestablished itself, with a period of approximately 57.5 days. When its Blazhko period is near 57.5 days, XZ Cyg has also shown a tertiary period of 41.6 days. We confirm that there is evidence for a longer 3540 day period in photometry obtained during the first half of the 20th century. XZ Cyg is compared with three other RR Lyrae stars that also appear to show changing Blazhko periods. The observed changes in the length of the Blazhko period of XZ Cyg constrain possible explanations for the Blazhko effect. In particular, they argue against any theoretical explanation that requires that the Blazhko period be exactly equal, or directly proportional, to the rotation period of the star.

Chemical origins of the Mars ultraviolet dayglow

Airglow optical emissions from planetary atmospheres provide remotely observable signatures of atmospheric composition, energy deposition processes, and the resulting chemical reactions. We may one day be able to detect airglow emissions from extrasolar planets. Reliable interpretation requires quantitative understanding of the energy sources and chemical mechanisms that produce them. The ultraviolet dayglow observations by the Mariner 6 and 7 (1969) and Mariner 9 (1971-72) motivated numerous modeling studies and laboratory experiments. The most obvious source reaction is photodissociation and photoionization of ambient CO2, which is known in the laboratory to produce the four strong dayglow emitting states: hv + CO2 --> O(1S), CO(a3pi), CO2(+)(A2pi(u) & B2sigma u(+)). (1) If this simplest of models were sufficient, then the high altitude dayglow emissions would all share the same scale height, which would be that of CO2. The few Mariner dayglow observations provide weak statistics. Addition of 4 months of Mars Express dayglow data, and including radio occultation and mass spectrometry data from other missions, have made the analyses and conclusions more robust. The CO(a3pi) and CO2+(B2sigma u(+)) dayglow altitude profiles are consistent with Reaction (1). In contrast, the O(1S) dayglow scale heights are much larger and are consistent with source Reaction (2): O2(+) + e(-) --> O(1S) (2). Both sets of scale heights change with respect to solar activity roughly as suggested by modeling studies.

Independent emission and absorption abundances for planetary nebulae

Emission-line abundances have been uncertain for more than a decade due to unexplained discrepancies in the relative intensities of the forbidden lines and weak permitted recombination lines in PNe and H II regions. The observed intensities of forbidden and recombination lines originating from the same parent ion differ from their theoretical values by factors of more than an order of magnitude in some of these nebulae. In this study we observe UV resonance line absorption in the central stars of PNe produced by the nebular gas and from the same ions that emit optical forbidden lines. We then compare the derived absorption column densities with the emission measures determined from ground-based observations of the nebular forbidden lines. We find for our sample of PNe that the collisionally excited forbidden lines yield column densities that are in basic agreement with the column densities derived for the same ions from the UV absorption lines. A similar comparison involving recombination line column densities produces poorer agreement, although near the limits of the formal uncertainties of the analyses. An additional sample of objects with larger abundance discrepancy factors will need to be studied before a stronger statement can be made that recombination line abundances are not correct.

Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418

We present high signal-to-noise ratio echelle spectra of the compact high surface brightness, low-ionization planetary nebula (PN) IC 418. These reveal 807 emission lines down to intensities less than 10-5 that of Hβ for which we determine widths and relative intensities. We show that line profiles are a valuable parameter for making line identifications and in constraining the excitation mechanism of the lines. We present evidence that indicates that many supposed high-level recombination lines may in fact be excited by a process other than recombination. We contend from the detection of dielectronic recombination lines that their relatively low intensities argue against their making a significant contribution to level populations of the heavy ions in this object. Following similar analyses of other PNe we find that IC 418 shows a small discrepancy in ion abundances derived from forbidden versus recombination lines of the heavy elements.

Measurements of the Singly Ionized Oxygen Auroral Doublet Lines λλ7320, 7330 Using High-Resolution Sky Spectra

The wavelengths of the individual [O II] 2s22p3 2D-2s22p3 2P λ7320 and [O II] 2s22p3 2D-2s22p3 2P λ7330 auroral doublet component lines have been measured directly in the nightglow for the first time, from high-resolution spectra obtained with the High Resolution Echelle Spectrophotometer (HIRES) spectrograph on the Keck I telescope at the W. M. Keck Observatory. Specifically, we find doublet splittings of 1.077±0.003 A and 1.080±0.003 A at λ7320 and λ7330, respectively, with the former significantly larger than the often quoted and utilized value of 0.8 A from Moore and in line with the National Institute of Standards and Technology (NIST) (1.07 A) as well as more recent astrophysical observations of the lines in planetary nebulae, including 1.07 A from De Robertis, Osterbrock, & McKee and 1.09 A from Barnett & McKeith. Our results suggest, however, that adjustments of +0.124 A and +0.131 A should be made to current NIST wavelengths for the blue and red components of the λ7320 doublet respectively, while our wavelengths of the λ7330 doublet components are little changed from current NIST values. The observed intensity ratio of λ7320/λ7330 from these measurements agrees with the theoretical value calculated under conditions of thermally populated 2Po levels.

B,V photometry of variable stars in the northeast arm of the Small Magellanic Cloud

B and V photometry has been obtained for variable stars in the northeast arm of the Small Magellanic Cloud (SMC). Periods and light curves have been determined for 237 periodic variables, including 201 Cepheids, 68 of which are newly discovered. Fundamental-mode Cepheids and first-overtone–mode Cepheids are generally well separated in the BV color-magnitude diagram, with the latter having bluer mean colors than the former. The Cepheid period-color relationship for this outlying SMC field is indistinguishable from that seen in more centrally located SMC fields and is bluer than theoretical predictions. The red edge to the populated portion of the instability strip shifts to bluer colors for fainter Cepheids. There is support from our sample for a previously reported steepening in the slope of the period-luminosity relation for fundamental-mode Cepheids near a period of 2 days. The Cepheids of the northeast arm may be closer to us than are those of the main body of the SMC, but the difference is smaller than or equal to about 4 kpc, comparable to the tidal radius of the SMC.

Abundances of s -process elements in planetary nebulae: Br, Kr & Xe

We identify emission lines of post-iron peak elements in very high signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement with theories of

The Changing Blazhko Effect of XZ Cygni

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