2008AJ....135.2291L

A line ratio of 3d-2p transition lines in boron-like spectra of Si X, S XII, Ar XIV, and Fe XXII has been investigated. Collisional-radiative model calculations reveal that the line ratio is sensitive to the electron density in ranges of n_e= 4.0x10⁷-3.0x10¹⁰/cm3, 4.0x10⁸-3.0x10¹¹/cm3, 3.0x10⁹-4.0x10¹²/cm3, and 2.0x10¹²-3.0x10¹⁵/cm3, respectively. This complements the K-shell diagnostics of helium-like ions. By a comparison between the predicted and the measured values, effective electron densities in the electron beam ion trap (EBIT) plasmas, performed by Lepson and collaborators at the Lawrence Livermore EBIT, are estimated to be n_e= 3.4^+0.8_–0.6x10¹⁰/cm3 and 5.6^+1.0_–1.1x 10¹⁰/cm3 for sulfur and argon plasmas, respectively. In the case of argon, a good agreement is shown with the actual electron density derived from an N VI K-shell spectrum. We further explore the 3d-2p transition lines of Si X and S XII in the stellar coronal spectra measured with the low energy transmission grating spectrometer combined with a high resolution camera on board the Chandra X-Ray Observatory. The constrained electron densities show a good agreement with those determined from C V and O VII K-shell spectra.