1995A&A...302..438S

We analyze the coronal EUV spectra of seven cool stars, solar-like single stars and components of RS CVn-like binaries, as observed with the Spectrometers of the Extreme Ultraviolet Explorer (EUVE). The observations cover the wavelength range of 60A up to 800A with a resolution of λ/{DELTA}λ≃160-320. The data constrain the coronal temperature structure between several hundred thousand Kelvin up to roughly 10 million Kelvin through a differential emission measure analysis. The resulting differential emission measure distributions show distinct features from source to source, but the common properties are a) a relatively weak emission from coronal plasma below about 1MK, b) a dominant component somewhere between 2MK and about 10MK, often peaking at solar-like coronal temperatures of 2 to 4MK, and c) in all cases but χ¹ Ori a very hot component in the formal solution with a temperature exceeding several tens of million of Kelvin. This hot tail in the differential emission measure distribution may reflect, as discussed in this paper, one or several of the following sources or processes: a real hot component, a reduced coronal abundance of heavy elements, or scattering in some of the strongest coronal lines with subsequent photon destruction upon impact on the lower, dense atmosphere. Coronal electron densities of brightly emitting regions are constrained by an analysis of ratios of density-sensitive iron lines. Strengths of Fe XIX-Fe XXII lines (corresponding to a temperature range of T=6-11MK) for α Aur, AU Mic (of which the spectrum is dominated by a large flare), ξ UMa, and σ Gem suggest typical electron densities in the range n_e∼10¹²-10¹³cm^–3. Cooler Fe X and Fe XII-Fe XIV lines (T=1-2MK) in the case of α CMi suggest n_e∼10⁹-10¹⁰cm^–3. In general, the electron densities of the hot 5-15MK components are some three orders of magnitude larger than typical of the solar-like component around 2MK; the volume filling factors of the hot components are therefore expected to be substantially smaller than those of the cooler component.