The optical thickness of stellar coronae in EUV lines

C.J. Schrijver (1), G.H.J. van den Oord (1), R. Mewe (2)

(1) Sterrekundig Instituut Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
(2) Laboratory for Space Research Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Astronomy & Astrophysics, 289, 1994, L23-L26


Stellar coronae are commonly assumed to be optically thin. Recent spectroscopic observations in the EUV, however, lead us to question the validity of this approximation for strong lines. We argue that scattering may significantly affect the strongest coronal lines in coronae composed of magnetic loops, possibly embedded in a hot stellar wind. Even if the average number of scatterings per photon in some coronal lines is only of order unity, the relative line strenghts and the line-to-continuum ratio can be significantly affected in non-symmetric inhomogeneous atmospheres: photons in weak lines and in the optically thin continuum escape without any scattering, but strong lines can be weakened or enhanced depending on the balance between outward traveling line photons that are scattered back toward the stellar surface (if not lost by branching), there destroyed by absorption, and downward traveling line photons that are scattered upward and escape. We draw attention to the fact that line scattering due to the non-negligible optical thickness in strong coronal lines can have severe implications for differential emission measure models and for abundance and density determinations, while it may serve as a diagnostic for the existence of tenuous hot winds.

keywords: Line: formation, Radiative transfer, Scattering, Stars: coronae, X-rays: stars

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