The optical thickness of stellar coronae in EUV lines

Details The optical thickness of stellar coronae in EUV lines The optical thickness of stellar coronae in EUV lines

Sep 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...289l..23s&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 289, no. 1, p. L23-L26

Physics

44

Optical Thickness, Radiative Transfer, Scattering, Stellar Coronas, Stellar Winds, Ultraviolet Spectra, Abundance, Branching (Physics), Photon Absorptiometry, Photon Density

Scientific paper

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 strengths 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-neglible 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.

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