Coronal Density Diagnostics with Si X: Chandra LETGS Observations of Procyon, α Centauri A and B, Capella, and ϵ Eridani

Electron density diagnostics based on the line intensity ratio of Si X are applied to the X-ray spectra of Procyon, α Cen A and B, Capella, and ϵ Eri measured with the Low Energy Transmission Grating Spectrometer combined with the High Resolution Camera on board the Chandra X-Ray Observatory. The ratio R1 of the intensities of the Si X lines at 50.524 and 50.691 Å is adopted. A certain temperature effect in R1 appears near the low-density limit region, which is due to the contamination of the Si X line at 50.703 Å. Using the emission measure distribution model derived by Audard and coworkers for Capella and emissivities calculated with the Astrophysical Plasma Emission Code model by Smith and coworkers, we successfully estimate the contributions of the Fe XVI lines at 50.367 and 50.576 Å (73% and 62%, respectively). A comparison between the observed ratios and theoretical predictions constrains the (logarithmic) electron densities for Procyon to be 8.61 cm-3, while for α Cen A and B, Capella, and ϵ Eri they are 8.81, 8.60, 9.30-0.48, and 9.11 cm-3, respectively. The comparison of our results with those constrained by the triplet of He-like carbon shows good agreement. For normal stars, our results display a narrow uncertainty, while for active stars, a relatively larger uncertainty due to contamination from Fe XVI lines is found. Another possible reason may be the uncertainty of the continuum level, since the emission lines of Si X become weak for active stars. For ϵ Eri, an electron density in the C V-forming region was first estimated through Si X emission.