Elements In Planetary Nebulae Research Articles

Neutron-capture Elements in Planetary Nebulae: First Detections of Near-infrared [Te iii] and [Br v] Emission Lines* †

Abstract We have identified two new near-infrared (NIR) emission lines in the spectra of planetary nebulae arising from heavy elements produced by neutron-capture reactions: [Te iii] 2.1019 μm and [Br v] 1.6429 μm. [Te iii] was detected in both NGC 7027 and IC 418, while [Br v] was seen in NGC 7027. The observations were obtained with the medium-resolution spectrograph Espectrógrafo Multiobjeto Infra-Rojo (EMIR) on the 10.4 m Gran Telescopio Canarias at La Palma, and with the high-resolution Immersion GRating INfrared Spectrometer (IGRINS) on the 2.7 m Harlan J. Smith telescope at McDonald Observatory. New calculations of atomic data for these ions, specifically A-values and collision strengths, are presented and used to derive ionic abundances of Te2+ and Br4+. We also derive ionic abundances of other neutron-capture elements detected in the NIR spectra, and estimate total elemental abundances of Se, Br, Kr, Rb, and Te after correcting for unobserved ions. Comparison of our derived enrichments to theoretical predictions from asymptotic giant branch (AGB) evolutionary models shows reasonable agreement for solar metallicity progenitor stars of ∼2–4 M ⊙. The spectrally isolated [Br v] 1.6429 μm line has advantages for determining nebular Br abundances over optical [Br iii] emission lines that can be blended with other features. Finally, measurements of Te are of special interest because this element lies beyond the first peak of the s-process, and thus provides new leverage on the abundance pattern of trans-iron species produced by AGB stars.

Abundances of Iron-Group Elements in Planetary Nebulae and Consequences for Chemical Enrichment

AbstractWe have developed a method for determining elemental Fe-group abundances in planetary nebulae using an infrared emission line of Zn, the least refractory Fe-group species. Many planetary nebulae, particularly those of the Milky Way’s thick disk and bulge, display subsolar [Fe/H] (as inferred from Zn) although their abundances of α elements such as O, S, and Ar are nearly solar. We discuss the implications for determining enhancements of species synthesized by the progenitor star during the AGB (e.g.,s-process products), and for galactic chemical evolution in view of the metallicity dependence of AGB nucleosynthetic yields.

The star formation rate in the Milky Way: Results from stars and planetary nebulae

AbstractThe star formation rate (SFR) is still a poorly known characteristic of the Milky Way, especially concerning the possibility of an irregular SFR, compared to a constant rate. Some recent results based on the distribution of dwarf stars with chromospheric ages suggest at least two major bursts in the past 10 Gyr, while other investigations are consistent with an approximately constant SFR. The SFR also shows important spatial variations, particularly concerning the radial variations along the galactic disc. In this work, we investigate two different problems relative to the galactic SFR: (i) We estimate the star formation rate in the galactic disc based on the age distribution of the planetary nebula central stars (CSPN), and compare these results with previous investigations based on dwarf stars. The CSPN ages were derived on the basis of five different methods, involving the observed nebular metallicities and kinematical properties; (ii) We derive radial abundance gradients from several elements in planetary nebulae, and compare these results with recent determinations based on younger objects, such as HII regions and cepheid variables. Since the gradients are linked to the formation process of the galactic disc, we can estimate the spatial variation of the SFR. Preliminary results indicate that at least one major star formation burst is obtained, as well as a relatively smooth variation of the SFR along the galactocentric radius.

Heavy elements in planetary nebulae: A theorist's gold mine

AbstractObservations of planetary nebulae have revealed a wealth of information about the composition of heavy elements synthesized by the slow neutron capture process (the s process). In some of these nebulae the abundances of neutron-capture elements are enriched by factors of 10 to 30 times the solar value, indicating that these elements were produced in the progenitor star while it was on the asymptotic giant branch (AGB). In this proceedings we summarize results of our recent full s-process network predictions covering a wide range of progenitor masses and metallicities. We compare our model predictions to observations and show how this can provide important insights into nucleosynthesis processes occurring deep within AGB stars.

New atomic data for trans-iron elements and their application to abundance determinations in planetary nebulae1This review is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Investigations of neutron(n)-capture element nucleosynthesis and chemical evolution have largely been based on stellar spectroscopy. However, the recent detection of these elements in several planetary nebulae (PNe) indicates that nebular spectroscopy is a promising new tool for such studies. In PNe, n-capture element abundance determinations reveal details of s-process nucleosynthesis and convective mixing in evolved low-mass stars, as well as the chemical evolution of elements that cannot be detected in stellar spectra. Only one or two ions of a given trans-iron element can typically be detected in individual nebulae. Elemental abundance determinations thus require corrections for the abundances of unobserved ions. Such corrections rely on the availability of atomic data for processes that control the ionization equilibrium of nebulae (e.g., photoionization cross sections and rate coefficients for various recombination processes). Until recently, these data were unknown for virtually all n-capture element ions. For the first six ions of Se, Kr, and Xe — the three most widely detected n-capture elements in PNe — we are calculating photoionization cross sections and radiative and dielectronic recombination rate coefficients using the multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE. Charge transfer rate coefficients are being determined with a multichannel Landau–Zener code. To calibrate these calculations, we have measured absolute photoionization cross sections of Se and Xe ions at the Advanced Light Source synchrotron radiation facility. These atomic data can be incorporated into photoionization codes, which we will use to derive ionization corrections (hence abundances) for Se, Kr, and Xe in ionized nebulae. Using Monte Carlo simulations, we will investigate the effects of atomic data uncertainties on the derived abundances, illuminating the systems and atomic processes that require further analysis. These results are critical for honing nebular spectroscopy into a more effective tool for investigating the production and chemical evolution of trans-iron elements in the Universe.

The Abundances of Light Neutron‐Capture Elements in Planetary Nebulae. II.s‐Process Enrichments and Interpretation1

We present the results of a large-scale survey of neutron(n)-capture elements in Galactic planetary nebulae (PNe), undertaken to study enrichments from s-process nucleosynthesis in their progenitor stars. From new K-band observations of over 100 PNe supplemented by data from the literature, we have detected the emission lines [Kr III] 2.199 μm and/or [Se IV] 2.287 μm in 81 of 120 objects. We determine Se and Kr elemental abundances, employing ionization correction formulae derived in the first paper of this series. We find a significant range in Se and Kr abundances, from near solar (no enrichment) to enhanced by >1.0 dex relative to solar, which we interpret as self-enrichment due to in situ s-process nucleosynthesis. Kr tends to be more strongly enriched than Se; in 18 objects exhibiting both Se and Kr emission, we find that [ Kr/Se ] = 0.5 ± 0.2. Our survey has increased the number of PNe with n-capture element abundance determinations by a factor of 10, enabling us for the first time to search for correlations with other nebular properties. As expected, we find a positive correlation between s-process enrichments and the C/O ratio. Type I and bipolar PNe, which arise from intermediate-mass progenitors (>3-4 M☉), exhibit little to no s-process enrichments. Finally, PNe with H-deficient Wolf-Rayet central stars do not exhibit systematically larger s-process enrichments than objects with H-rich nuclei. Overall, 44% of the PNe in our sample display significant s-process enrichments (>0.3 dex). Using an empirical PN luminosity function to correct for incompleteness, we estimate that the true fraction of s-process enriched Galactic PNe is at least 20%.

The Abundances of Light Neutron‐Capture Elements in Planetary Nebulae. I. Photoionization Modeling and Ionization Corrections

We have conducted a large-scale survey of 120 planetary nebulae (PNe) to search for the near-infrared emission lines [Kr III] 2.199 μm and [Se IV] 2.287 μm. The neutron (n)-capture elements Se and Kr may be enriched in a PN if its progenitor star experienced s-process nucleosynthesis and third dredge-up. In order to determine Se and Kr abundances, we have added these elements to the atomic databases of the photoionization codes Cloudy and XSTAR, which we use to derive ionization correction factors (ICFs) to account for the abundances of unobserved Se and Kr ions. However, much of the atomic data governing the ionization balance of these two elements are unknown, and have been approximated from general principles. We find that uncertainties in the atomic data can lead to errors approaching 0.3 dex in the derived Se abundances and up to 0.2-0.25 dex for Kr. To reduce the uncertainties in the Kr ionization balance stemming from the approximate atomic data, we have modeled 10 bright PNe in our sample, selected because they exhibit emission lines from multiple Kr ions in their optical and near-infrared spectra. We have empirically adjusted the uncertain Kr atomic data until the observed line intensities of the various Kr ions are adequately reproduced by our models. Using the adjusted Kr atomic data, we have computed a grid of models over a wide range of physical parameters (central star temperature, nebular density, and ionization parameter) and derived formulae that can be used to compute Se and Kr ICFs. In the second paper of this series, we will apply these ICFs to our full sample of 120 PNe, which comprises the first large-scale survey of n-capture elements in PNe.

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 -process calculations indicate that it should be produced with Kr at detectable levels.

The Abundances of Light Neutron-Capture Elements in Planetary Nebulae

We present preliminary results from a large-scale survey of the neutron( n )-capture elements Se and Kr in Galactic planetary nebulae (PNe). These elements may be produced in PN progenitors by s -process nucleosynthesis, and brought to the stellar envelope by third dredge-up (TDU). We have searched for [Kr iii] 2.199 and [Se iv] 2.287 $\mu$ m in 120 PNe, and detected one or both lines in 79 objects, for a detection rate of 66%. In order to determine abundances of Se and Kr, we have added these elements to the atomic database of the photoionization code CLOUDY, and constructed a large grid of models to derive corrections for unobserved ionization stages. Se and Kr are enriched in $\sim$ 73% of the PNe in which they have been detected, and exhibit a wide range of abundances, from roughly solar to enriched by a factor of 10 or more. These enrichments are interpreted as evidence for the operation of the s -process and TDU in the progenitor stars. In line with theoretical expectations, Kr is more strongly enhanced than Se, and the abundances of both elements are correlated with the carbon abundance. Kr and Se are strongly enhanced in Type I PNe, which may be evidence for the operation of the $^{22}$ Ne neutron source in intermediate-mass AGB stars. These results constitute the first broad characterization of s -process enrichments in PNe as a population, and reveal the impact of low- and intermediate-mass stars on the chemical evolution of trans-iron elements in the Galaxy.

Neutron-Capture Elements in Planetary Nebulae: Identification of Two Near-Infrared Emission Lines as [K[CLC]r[/CLC] [CSC]iii[/CSC]] and [S[CLC]e[/CLC] [CSC]iv[/CSC

I propose that two previously unidentified lines at 2.199 and 2.287 μm seen in the spectra of planetary nebulae (PNs) are fine-structure transitions of Kr+2 and Se+3. These are the cosmically most abundant elements with Z > 32. The ionic stages and originating energy levels of the observed transitions are expected to be significantly populated under nebular conditions, and these elements—especially the noble gas Kr—are unlikely to be strongly depleted out of the gas phase into grains. Furthermore, their concentrations can be enhanced by s-processing, which occurs in the interiors of the PN progenitor stars. The observed line strengths are consistent with modest (factor of a few) overabundances. In support of the identification of the 2.199 μm line as [Kr III] 3P1-3P2, PNs that display this line also show [Kr III] 1D2-3P2 6828 A emission. I identify the 2.287 μm line as [Se IV] 2P3/2-2P1/2. These identifications suggest a new range of possibilities for line identifications in the infrared. In addition, the observability of emission lines from n-capture species introduces a novel approach for studying advanced evolutionary stages and nucleosynthesis in the progenitor stars and for more fully delineating the role of PNs as agents of galactic chemical enrichment.

Emission from an Inhomogeneous Plasma: Line Intensities and Determination of Elemental Abundances in Gaseous Nebulae with Fluctuations of Te and ne

A method is proposed for determining the abundances of chemical elements in planetary nebulae based on allowance for the actual distribution functions of errors in measuring line intensities. Fluctuations both in temperature and in mass density of a nebula are taken into account in the proposed method. The results of a determination of C and O abundances and of the amplitudes of temperature and density fluctuations are given.

Electron excitation of heavy elements in Planetary Nebulae

Modern observational equipment makes it possible to identify extremely weak metal lines in deep surveys of the optical and near-IR region of the electromagnetic spectrum. In particular, collisionally excited lines from heavy ions (Z > 30) have been recently identified in the spectrum of NGC 7027 (Péquignot & Baluteau 1994). However, detailed quantitative analyses of the observed lines have not been feasible due to the lack of accurate collisional data for the heavy elements.

Iron condensation and the formation of planetary nebulae

view Abstract Citations (9) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Iron condensation and the formation of planetary nebulae. Scalo, J. M. ; Shields, G. A. Abstract The observed degree of iron depletion in planetary nebulae (PN) is used as a constraint on PN formation in an attempt to distinguish between an 'ejection' model and a 'wind' model. In the ejection model the entire shell is ejected in a single event; the wind model envisions PN formation as resulting from a relatively steady phase of rapid mass loss over an extended period. Proposed explanations for the existence of dust in PN are reviewed, and the condensation of iron in PN is considered, along with processes that may account for the observed small fraction of iron that remains in the gas. The results indicate that condensation of refractory elements in PN is to be expected in either model and that only grain-grain collisions leading to shattering and partial vaporization can successfully explain the gas-phase iron abundances. This process is shown to be effective only in the wind model. Publication: The Astrophysical Journal Pub Date: March 1979 DOI: 10.1086/156874 Bibcode: 1979ApJ...228..521S Keywords: Cosmic Dust; Iron; Planetary Nebulae; Stellar Evolution; Abundance; Collision Parameters; Condensation; Drift Rate; Stellar Models; Vaporizing; Astrophysics; Planetary Nebulae:Formation; Planetary Nebulae:Models full text sources ADS | data products SIMBAD (4)

The Ionization of Planetary Nebulae

The ionization of the most abundant elements in planetary nebulae has been determined for a number of models of nebulae at different epochs in their expansion. The values used for the temperatures and radii of the central stars and the sizes and densities of the shells have come from Seaton's evolutionary sequence. The ionizing radiation field has been taken from model atmosphere calculations of the central stars by Gebbie and Seaton, and Böhm and Deinzer. Emission-line fluxes have been calculated for the models and compared with observations of planetary nebulae by O'Dell, Osterbrock's group, and Aller and his collaborators. Results indicate that the central stars have strong He+ Lyman continuum excesses, similar to those predicted by Gebbie and Seaton. The mean abundance determinations for the nebulae made by Aller are confirmed, with the exception of nitrogen, which appears to be 3 or 4 times more abundant than his value. It is also seen that the electron temperatures of the nebulae are higher than previous theoretical determinations, providing better agreement with empirically derived values.

The Ionisation Equilibrium for Heavy Elements

Calculations are being made of the distribution of the ions of heavy elements in planetary nebulae. Initial work has been concentrated on the central or He2+ zone of planetaries. The optical depths of ions of C, N, O, and Ne have been computed using ground state ionisation cross-sections and using approximations which should be substantially better than hydrogenic. A comparison has been made between the combined optical depth of the heavy elements and the optical depth of He+ in the far ultraviolet. The optical depths of the heavy elements in this spectral region may become significant, but a reasonable first approximation to the radiation field may be obtained by neglecting the absorption of all ions except He+. The distribution of the ions of the heavy elements has been calculated on this assumption.