2011MNRAS.414..634J

An emission line at ≃ 1319Å is one of the strongest unidentified lines in the ultraviolet spectra of cool dwarf stars. In most lists of solar and stellar lines it is identified as a transition in N I, although its intensity would then be anomalous and the wavelength does not precisely fit that expected for N I. The line is also observed in giant stars but becomes very weak in supergiants, relative to photoexcited lines of neutral atoms. The measured wavelength of the line in stellar spectra is 1318.94 ±0.01 Å. Observations of giant stars provide further information that shows that this line is not due to N I. It is proposed that the line is due to a decay from the 3p³(²D°)3d ¹D°₂ level of S I, above the first ionization limit. The previous tentative assignment of this upper level to an S I line at ≃ 1309.3Å then needs to be revised. The 1309.3-Å line is identified here for the first time in an astrophysical source. The 3d ¹D°₂ level could, in principle, be populated by collisions from nearby autoionizing levels that are shown to have large number densities, through population by low-temperature dielectronic capture. Spin–orbit interaction with the autoionizing 3d ³D°₂ level might also lead to dielectronic capture into the 3d ¹D°₂level. A line at 1309.87 Å observed in cool giant stars is identified as a transition in P II, pumped by the O I resonance lines.