Modeling the NeixTriplet Spectral Region of Capella with theChandraandXMM‐NewtonGratings

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Abstract
High resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the HETGS (155ks), LETGS (219ks), and RGS (53ks) for comparative analysis focusing on the NeIX triplet at around 13.5A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the HEG spectrum, including many from FeXIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code (APEC). The model uses an emission measure distribution derived from FeXV to FeXXIV lines. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of NeIX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low density limit (n_e<2 10^10 cm^-3); however, the electron temperature indicated by the NeIX G-ratio is surprisingly low (~2MK) compared with the peak of the emission measure distribution (~6MK). Models show that the NeIX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.Comment: 14 pages, 13 figures, accepted by AP
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