The emission line near 1319 A in solar and stellar spectra

TL;DR

This paper identifies a previously unknown sulfur I emission line near 1319 A in solar and stellar spectra, revising earlier line assignments and providing new insights into atomic transitions in astrophysical sources.

Contribution

It proposes that the 1319 A line is due to a decay from a new S I level above the ionization limit, revising previous identifications and discovering a new P II line at 1309.87 A.

Findings

The 1319 A line is from a new S I level, not N I.

The 1309.3 A line is identified as a transition in S I.

A new P II line at 1309.87 A is observed in giant stars.

Abstract

An emission line near 1319 A is one of the strongest unidentified lines in the ultraviolet spectra of cool dwarf stars. In most line lists it is identified as a transition in N I, although its intensity would then be anomalous and the observed wavelength does not fit precisely that expected for N I. The line is also observed in cool giant stars. The measured wavelength of the line in stellar spectra is 1318.94 (+,- 0.01) A. Observations of giant stars provide further evidence that this line is not due to N I. It is proposed that this line is a decay from a previously unknown level in S I, which lies above the first ionization limit. This is identified with the 3d singlet D (odd parity) term. The previous tentative assignment of this term to the S I line at 1309.3 A then needs to be revised. The 1309.3 A line has been identified here for the first time in an astrophysical source. The singlet D (odd parity) level could, in principle, be populated by collisions from nearby autoionizing levels that have large number-densities, through population by di-electronic capture. Spin-orbit interaction with the autoionizing triplet D (odd parity) term might also lead to di-electronic capture into the singlet D (odd parity) level. A line at 1309.87 A observed in cool giant stars is identified as a transition in P II, pumped by the O I resonance lines.