Soft X-ray emissions of Si IX in Procyon

An analysis of n = 3 → 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 → 2p transition lines at 55.246 Å and 55.346 Å is a good ne-diagnostic technique, due to its insensitivity to the electron temperature. Using this property, a lower limit of the density of 0.6×10cm is derived for Procyon, which is consistent with that constrained by C V and Si X emissions. Significant discrepancies in ratios of 3s → 2p lines to 3d → 2p lines between theoretical predictions and observed values, are found, by the spectral analysis of Procyon observed with the 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 → 2p line at 61.611 Å , present work indicates that the large discrepancy may be due to the contamination from a S VIII line at 61.645 Å . For the lines at 61.702 and 61.846 Å , we suggest that the discrepancies may be attributed to contaminations of unknown lines. PACS: 95.20.Jg; 97.10.Ex; 95.85.Nv