Abstract
Emission lines of singly charged ions populate many astrophysical spectra. However, the interpretation of the line intensities (usually line ratios) often depends on the transition rates of the decays of very long-lived low-lying levels. For example, the line ratio of two electric-dipole forbidden transitions in the 3s 2 3p 3 ground configuration of singly ionized sulfur (ion S + , spectrum S II) has been interpreted in terms of a density diagnostic for planetary nebulae, i.e., for densities in the order of 10 4 cm − 3 . The predicted lifetimes of the 2 D 3 / 2 , 5 / 2 o levels are in the order of one hour. Modeling indicates that a 10% uncertainty of the lifetime determination in this case corresponds to a 50% uncertainty of the density diagnostic. The available theoretical lifetime predictions scatter by much more than 10%. Considerations about an experimental approach are presented with the goal of instigating a measurement of the actual level lifetimes.
Highlights
In the middle of the 19th century, astrophysical observations revealed several spectral lines that could initially not be reproduced in the laboratory
Some lines observed only in the solar corona during solar eclipses were assigned to the hypothetical element ‘Coronium’, and the emission lines that dominated the spectra of planetary nebulae were assumed to indicate yet another hypothetical element, ‘Nebulium’ [1,2]
He explained the non-observation of these lines in the laboratory versus the observation in astrophysical objects such as planetary nebulae by the extremely low particle density of the latter which permitted radiative decays even of extremely low transition rates (Einstein A values) (electric-dipole ‘forbidden’ transitions, i.e., mostly magnetic dipole (M1), electric quadrupole (E2) transitions, magnetic quadrupole (M2)
Summary
In the middle of the 19th century, astrophysical observations revealed several spectral lines that could initially not be reproduced in the laboratory. Bowen recognized in the 1920s that the photon energies of the ‘nebulium’ lines matched fine structure intervals in various low-charge ions of ordinary elements such as nitrogen and oxygen (spectra N II, O II, O III) [4,5,6,7,8] He explained the non-observation of these lines in the laboratory versus the observation in astrophysical objects such as planetary nebulae by the extremely low particle density of the latter which permitted radiative decays even of extremely low transition rates (Einstein A values) (electric-dipole ‘forbidden’ transitions, i.e., mostly magnetic dipole (M1), electric quadrupole (E2) transitions, magnetic quadrupole (M2). The following presentation describes the case of S II in more detail and how an eventual experiment might proceed
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