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Astron. Astrophys. 340, L15-L17 (1998)

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1. Introduction

It is now well established that emission line ratios can be used as powerful electron temperature and density diagnostics for many astrophysical plasmas; the Solar transition region/corona (Dere, 1978); early and late type stars (Jordan, 1988 and Shore & Sanduleak 1984); planetary nebulae (Keyes, Aller & Feibelman 1990); H II regions (Fich & Silkey 1991); Symbiotic stars (Schmid & Schild, 1990). See Keenan (1996) for a recent review. The large dynamic range (0.1-0.00003) of the Be-like O V (1213.90 Å/1218.34 Å) (2s2 1S0 [FORMULA] 2s2p 3P2 [FORMULA] 2s2 1S0 [FORMULA] 2s2p 3P1) line ratio and the fact that it is temperature insensitive makes it potentially a very useful density diagnostic over the [FORMULA] density range (see for example Keenan et al. 1995).

The emission lines of O V have been observed in a variety of astronomical objects including the Sun and late-type stars (Doschek 1996; Maran et al. 1994), supernova remnants (Cornet et al. 1995) and from hot gas around an eclipsing neutron star (Anderson, Wachter & Margon, 1994). The O V (1218.35 Å) intercombination line has been observed in various solar features by Skylab (Doschek 1996) and the High Resolution Telescope and Spectrograph (Brekke 1993), as well as in gaseous nebulae (Doschek & Feibelman 1993). Both forbidden and intercombination lines are located in the wings of the strong Lyman-[FORMULA] feature, but the forbidden line is far more difficult to detect, due to it's faintness. Sandlin, Brueckner & Tousey (1977) tentatively identify the line in some solar limb spectra from Skylab , and more recently McKenna et al. (1997) have detected the line in a high resolution HST spectrum of RR Tel, with a wavelength separation from the intercombination line of 4.62 [FORMULA] 0.12 Å.

In this paper, we present high spectral resolution, high signal-to-noise spectra of the quiet Sun in the wavelength region of the O V intercombination and forbidden lines. The observations were obtained with the Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) on-board the Solar and Heliospheric Observatory (SOHO). We use recent level population calculations to derive the forbidden to intercombination line ratio and estimate the electron density for the emitting region. The potential usefulness of these lines for calculating electron densities in the the Sun, as well as other astronomical objects, is demonstrated.

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998