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 ,a nd Ovii are applied to the X-ray spectra of Capella and Procyon measured with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f), intercombination (i), and the resonance (r) lines of the helium-like triplets are used. The (logarithmic) electron densities (in cgs units) derived from the f=i ratios for Capella are < 9:38 cm 3 for O vii (2 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 for C v (f=i =1 :48 0:34), while for Procyon we obtain 9:28 +0:4 9:28 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 C v (f=i =0 :48 0:12). These densities are quite typical of densities found in the solar active regions, and also pressures and temperatures in Procyon's and Capella's corona at a level of T 10 6 K are quite similar. We nd no evidence for densities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little dierence in the physical properties of the layers producing the C v ,N vi ,a nd Ovii emission. Assuming the X-ray emitting plasma to be conned in magnetic loops, we obtain typical loop length scales of LCapella 8 LProcyon from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite dierent. The total mean surface fluxes emitted in the helium- and hydrogen-like ions are quite similar for Capella and Procyon, but exceed typical solar values by one order of magnitude. We thus conclude that Procyon's and Capella's coronal lling factors are larger than corresponding solar values.