THE O-V 1371.29-ANGSTROM 1218.35-ANGSTROM EMISSION-LINE RATIO IN SOLAR AND STELLAR SPECTRA

New calculations of the theoretical electron-density-sensitive emissio n-line ratio R = I(2s2p P-1-2p(2) D-1)/I(2s(2) S-1-2s2p P-3(1)) = I(13 71.29 Angstrom)/I(1218.35 Angstrom) in O v are presented, which are si gnificantly different from those deduced previously, principally due t o the adoption of improved atomic data in the present analysis. Electr on densities estimated from the current diagnostics, in conjunction wi th observed values of R measured from solar spectra obtained by the S0 82B spectrograph on board Skylab and by the High Resolution Telescope and Spectrograph (HRTS) on board a sounding rocket flight, are found t o be in good agreement with densities determined from Line ratios in s pecies formed at similar electron temperatures to O v, such as O IV an d N IV. These results provide experimental support for the accuracy of the diagnostic calculations presented in this paper, and hence the at omic data used in their derivation. The observed value of R for the M0 Ve flare star AU Mic, determined from observations made by the Goddard High Resolution Spectrometer on board the Hubble Space Telescope, is within 4 per cent of the expected low-density limit, implying that log N-e less than or equal to 10.4 for the O v emitting region of the AU Mic atmosphere. This is similar to density estimates made for AU Mic u sing other methods.