Helium-like triplet density diagnostics - Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon

Electron density diagnostics based on the triplets of helium-like C V, N VI , and O VII are applied to the X-ray spectra of Capella and Procyon measure d with the Low Energy Transmission Grating Spectrometer (LETGS) on board th e Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f), intercombination (i), and the re sonance (r) lines of the helium-like triplets are used. The (logarithmic) e lectron densities (in cgs units) derived from the f/i ratios for Capella ar e < 9.38 cm(-3) for O VII (2<sigma> upper limit) (f/i = 4.0 +/- 0.25), 9.86 +/- 0.12 cm(-3) for N VI (f/i = 1.78 +/- 0.25), and 9.42 +/- 0.21 cm(-3) f or C V (f/i = 1.48 +/- 0.34), while for Procyon we obtain 9.28(-9.28)(+0.4) cm(-3) for O VII (f/i = 3.28 +/- 0.3), 9.96 +/- 0.23 cm(-3) for N VI (f/i = 1.33 +/- 0.28), and < 8.92 cm(-3) for Cv (f/i = 0.48 +/- 0.12). These den sities are quite typical of densities found in the solar active: regions, a nd also pressures and temperatures in Procyon's and Capella's corona at a l evel of T <similar to> 10(6) K are quite similar. We find no evidence for d ensities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little difference in the physical properties of the layers producing the Cv, N VI. and O VII emission. Assumi ng the X-ray emitting plasma to be confined in magnetic loops, we obtain ty pical loop length scales of L-Capella greater than or equal to 8 L-Procyon from the loop scaling laws, implying that the magnetic structures in Procyo n and Capella are quite different. The total mean surface fluxes emitted in the helium- and hydrogen-like ions are quite similar for Capella and Procy on, but exceed typical solar values by one order of magnitude. We thus conc lude that Procyon's and Capella's coronal filling factors are larger than c orresponding solar values.