Astronomy and Astrophysics, volume 367, 282-296 (2001/2-3)
Helium-like triplet density diagnostics. Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon.
NESS J.-U., MEWE R., SCHMITT J.H.M.M., RAASSEN A.J.J., PORQUET D., KAASTRA J.S., VAN DER MEER R.L.J., BURWITZ V. and PREDEHL P.
Abstract (from CDS):
Electron density diagnostics based on the triplets of helium-like CV, NVI, and 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 <9.38cm–3 for OVII (2σ upper limit) (f/i=4.0±0.25), 9.86±0.12cm–3 for NVI (f/i=1.78±0.25), and 9.42±0.21cm–3 for CV (f/i=1.48±0.34), while for Procyon we obtain 9.28+0.4–9.28cm–3 for OVII (f/i=3.28±0.3), 9.96±0.23cm–3 for NVI (f/i=1.33±0.28), and <8.92cm–3 for CV (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∼106K are quite similar. We find no evidence for densities 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, NVI, and OVII emission. Assuming the X-ray emitting plasma to be confined in magnetic loops, we obtain typical loop length scales of LCapella≥8LProcyon from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite different. 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 filling factors are larger than corresponding solar values.