The identification of krypton, xenon, and other elements of rows 4, 5 and 6 of the periodic table in the planetary nebula NGC 7027

Details The identification of krypton, xenon, and other elements of rows 4, 5 and 6 of the periodic table in the planetary nebula NGC 7027 The identification of krypton, xenon, and other elements of rows 4, 5 and 6 of the periodic table in the planetary nebula NGC 7027

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...283..593p&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 283, no. 2, p. 593-625

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Abundance, Asymptotic Giant Branch Stars, Computational Astrophysics, Krypton, Line Spectra, Novae, Planetary Nebulae, Spectrum Analysis, Stellar Evolution, Xenon, Chemical Evolution, Forbidden Transitions, Heavy Ions, Light (Visible Radiation), Near Infrared Radiation, Nuclear Fusion, Nuclides, Line: Identification - Planetary Nebulae: Noc 7027 - Planetary Nebulae: Abundances - Stars: Evolution (Agb) - Stars: Novae: Rs Oph, Rr Tel

Scientific paper

Elements Z > 30 are identified for the first time in a nebula. Their solar-system abundances by number relative to hydrogen range from 2 10-9 to 10-10.
About 3 dozen forbidden and fine-structure lines from 2 dozen ns2npκ ions (n = 4 for elements Z = , n = 5 for Z = 52-56, and n = 6 for Z = 82) are detected or suspected in the optical and near-infrared spectrum of the reference planetary nebula NGC 7027. The two strongest lines left unidentified for half a century in this nebula are [Kr iv] lines. [Kr IV] appears to be present in half the published spectra of planetary nebulae. Also the strongest line left unidentified in the spectra of the reference objects RS Ophiuchi and RR Telescopii is [Kr III].
A dozen ions belonging to elements Kr, Xe, Br, Se, Rb, and Ba are certainly or probably detected. There are marginal indications for the presence of lines from more ions of the same elements and some ions of Sr, Pb, and Y.
These detections unescapably lead to one (or a combination) of the two following consequences.
Either the effective collision strengths Ω for the transitions of the heavy ions 4pκ and 5pκ are, on average, at least 10 times larger than for the corresponding transitions of the light ions 2pκ and 3pκ, implying that resonances should be greatly and ubiquitously more important in heavy ions than in light ions. This interpretation of the observations, which may face difficulties, is open to atomic physics test. Alternatively the abundances of several nuclides of atomic mass A &sim 80-90, ˜ 130-140, and perhaps ˜207 are much larger in NGC 7027 than those determined from the many solar-system studies, confirming that the neutron- capture process is effective in stars progenitors of carbon- rich planetary nebulae and showing directly that the by-products of this nucleosynthesis are injected in the interstellar medium. In this preferred interpretation, planetary nebulae would gain a new important role in the chemical evolution of galaxies. They would provide new insights into final stages of stellar evolution since several key elements, undetectable in stars, are easily observed in nebulae.
The phenomenological theory of neutron capture and current ideas about the sites of heavy-element synthesis would seem to be at variance with three facts: (1) xenon, a predominantly r-element, was copiously synthesized in a low-mass star, (2) krypton and xenon, believed to be produced in rather different conditions, are enhanced in similar proportions, and (3) apparently, heavy elements are detected, whether they are r- or s-elements. However, coherence between observation and theory is restored after taking into account a selective locking of refractory heavy elements into dust grains. The enrichment of NGC 7027 in s-nuclides may approach a factor 100.

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