EUV spectroscopy of cool stars. I. The corona of α Centauri observed with EUVE. R

We perform an emission measure analysis of spectra of the coronae of the cool star binary α Cen (A: HD 128620 (G2 V) + B: HD 128621 (K1 V)) as observed with the Spectrometers of the Extreme UltraViolet Explorer (EUVE). These observations, covering the wavelength range of 60A up to 800A with a resolution of λ/{DELTA}λ=~160-320, constrain the coronal temperature structure between about 10^5^K up to roughly 10MK, while some additional line and continuum information is available to extend that coverage from several tens of thousands of Kelvin up to several tens of millions of Kelvin, although the solution is poorly constrained in the extended range. We performed a re-calibration of the wavelength scale for each of the three instrumental pass bands by comparing the model list of emission lines with the spectra of α Cen and eight other cool stars. The high signal-to-noise ratio of the α Cen spectrum yielded indications for a Lorentzian rather than a Gaussian line profile for the MW passband. The observed spectrum was decomposed into a linear combination of spectra emitted by isothermal plasmas in thermal equilibrium, using the SPEX plasma emission code developed in Utrecht. The resulting differential emission measure distribution shows: a) emission from plasma below ~5MK with a broad peak around ~3MK and with a tail extending down to about 0.5MK, b) very little emission from plasma between 0.1MK and 0.5MK, c) emission from plasma below 0.1MK, and d) possibly a hot component exceeding several tens of million of Kelvin which, however, may (in part) have another origin than a hot coronal plasma component. We propose that the hot component may be an artifact of the corona being close to unit optical thickness for resonant scattering in the strongest spectral lines, so that part of the line photons can be destroyed by being scattered towards the stellar surface while the plasma remains optically thin for continuum photons. Diagnostics for the electron density n_e_, using Fe X, XII, XIII, and XIV lines, yields values in the range 2-20x10^8^cm^-3^ at T=1-2MK.