Electron-density-dependent extreme-ultraviolet intensity ratios from L-shell iron ions in the Frascati Tokamak Upgrade

We present measurements of density-sensitive extreme-ultraviolet line ratios from highly charged L-shell iron ions in the Frascati Tokamak Upgrade plasma at densities more than a factor of 2 higher than previously observed. These observations are directly applicable to recent measurements of emission from high-density regions of the atmospheres of W Ursae Majoris binary stars and DQ Herculis type cataclysmic variables. We compare our observations to collisional-radiative models with large atomic structure data sets computed with the HULLAC suite of codes. We find good agreement for nearly all cases where spectral blending does not compromise our data. We also compare the predictions of our HULLAC models with ratios given by the Brickhouse, Raymond, & Smith code; we find agreement to better than 20% between the two calculations independent of the size of the atomic model. Our results confirm observations of very high electron densities (>10¹⁴ cm^-3) in the coronae of 44i Boö and EX Hya.