Soft X-ray emissions of Si IX in Procyon

An analysis of $n=3 \to 2$ transition lines of carbon-like silicon reveals that some ratios of line intensities are sensitive to the electron density. The ratio between two group of $3d\to2p$ transition lines at 55.246 \AA and 55.346 \AA is a good $n_{\rm e}$-diagnostic technique, due to its insensitivity to the electron temperature. Using this property, a lower limit of the density of 0.6$\times10^8$cm$^{-3}$ is derived for Procyon, which is consistent with that constrained by C V and Si X emissions. Significant discrepancies in ratios of $3s\to2p$ lines to $3d\to2p$ lines between theoretical predictions and observed values, are found, by the spectral analysis of Procyon observed with the {\it Chandra} High Resolution Transmission Grating spectra. The difference exceeding a factor of 3, cannot be explained by the uncertainty of atomic data. The opacity effect is also not a choice as reported by Ness and co-workers. For the $3s\to2p$ line at 61.611 \AA, present work indicates that the large discrepancy may be due to the contamination from a S VIII line at 61.645 \AA . For the lines at 61.702 and 61.846 \AA, we suggest that the discrepancies may be attributed to contaminations of unknown lines.