EUV Spectroscopy of cool stars I. The corona of alpha Cen observed with EUVE

R. Mewe (1), J.S. Kaastra (1,2), C.J. Schrijver (3), G.H.J. van den Oord (3), F.J.M. Alkemade (1,4)

(1) Laboratory for Space Research Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
(2) Laboratory for Space Research Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands
(3) Sterrekundig Instituut Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
(4) National Institute of Public Health and Environmental Protection, P.O. Box 1, 3720 BA Bilthoven, The Netherlands

Astronomy & Astrophysics, 1994 (in press)

Abstract

We perform an emission measure analysis of spectra of the coronae of the cool star binary alpha Cen (A:HD 128620 (G2V) + B: HD 128621 (K1V)) as observed with the Spectrometers of the Extreme Ultraviolet Explorer (EUVE). These observations, covering the wavelength range of 60Å up to 800Å with a resolution of lambda/Delta-lambda ~160-320, constrain the coronal temperature structure between about 1E+05 Kelvin up to roughly 10 million Kelvin, 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 ill constrained in the extended range. We performed a re-calibration of the wavelength scale for each of the three instrumental passbands by comparing the model list of emission lines with the spectrum of alpha Cen, combined with spectra of eight other cool stars. The high signal-to-noise ratio of the alpha 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 ~5 MK with a broad peak around ~3 MK and with a tail extending down to about 0.5 MK, b) very little emission from plasma between 0.1~MK and 0.5~MK, c) emission from plasma below 0.1 MK, 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 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. Electron density diagnostics based on Fe X, Fe XII, Fe XIII, and Fe XIV lines, yields values in the range 2-20E+08 cm-3 at T = 1-2 MK.

keywords: stars: coronae, X--rays: stars, stars: activity, stars: late--type, stars: abundances, stars: individual: alpha Cen

Back to the Abstracts Index Page