Fe XXI emission line ratios as electron temperature diagnostics for the coronae of cool stars

Abstract

Theoretical Fe xxi electron temperature and density sensitive emission line ratios are presented for

$$R_1 = I\left( {2s^2 2p^2 {}^3P_1 - 2s2p^3 {}^5S} \right)/I\left( {2s^2 2p^2 {}^3P_0 - 2s^2 2p^2 {}^3P_1 } \right)$$

and

$$R_2 = T\left( {2s^2 2p^2 {}^3P_1 - 2s2p^2 {}^1S} \right)/T\left( {2s^2 2p^2 {}^3P_0 - 2s^2 2p^2 {}^3P_1 } \right)$$

.

A comparison of these with observational data for the 1973 December 17 solar flare, obtained by the Naval Research Laboratory's S082A and S082B spectrographs on board Skylab, reveals reasonably good agreement between theory and observation for R ₁[= I(242.07 $\mathop \Lambda \limits^c$)/I(1354.08 Å)], which provides some support for the accuracy of the atomic data adopted in the line ratio calculations. However, our results for R ₂ indicate that the emission feature in flare spectra at 338.06 Å, previously identified as the Fexxi 2s ²2p ^{2 3} p ₁ − 2s ²2p ^{2 1} S line, is not due to this species but may be the Ca v 3p ^{4 3} p ₁ − 3p ³3d ³ P ₂ transition. The potential of using R ₁ as an electron temperature diagnostic for the Fe xxi line emitting regions of cool star coronae, through a combination of Extreme Ultraviolet Explorer and Hubble Space Telescope observations, is briefly discussed.