X-ray and extreme-ultraviolet emission from the coronae of Capella

The primary objective of this work is the analysis and interpretation of co ronal observations of Capella obtained in 1999 September with the High Ener gy Transmission Grating Spectrometer on the Chandra X-ray Observatory and t he Extreme Ultraviolet Explorer (EUVE). He-like lines of O (O vii) are used to derive a density of 1.7 x 10(10) cm(-3) for the coronae of the binary, consistent with the upper limits derived from Fe xxi, Ne ix and Mg xi line ratios. Previous estimates of the electron density based on Fe xxi should b e considered as upper limits. We construct emission measure distributions a nd compare the theoretical and observed spectra to conclude that the corona l material has a temperature distribution that peaks around 4-6 MK, implyin g that the coronae of Capella were significantly cooler than in the previou s years. In addition, we present an extended line list with over 100 featur es in the 5-24 Angstrom wavelength range, and find that the X-ray spectrum is very similar to that of a solar flare observed with SMM. The observed to theoretical Fe xvii 15.012-Angstrom line intensity reveals that opacity ha s no significant effect on the line flux. We derive an upper limit to the o ptical depth, which we combine with the electron density to derive an upper limit of 3000 km for the size of the Fe xvii emitting region. In the same context, we use the Si iv transition region lines of Capella from HST/Godda rd High-Resolution Spectrometer observations to show that opacity can be si gnificant at T = 10(5) K, and derive a path-length of approximate to 75 kin for the transition region. Both the coronal and transition region observat ions are consistent with very small emitting regions, which could be explai ned by small loops over the stellar surfaces.