Chromospheric modelling in late-type dwarfs. II - CES observations of active and quiescent stars

Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D.
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ESO Messenger (ISSN 0722-6691), no. 41, Sept. 1985, p. 18-22.

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The characteristics of the chromospheres of late-type dwarf stars were studied using the CoudeEchelle Spectrograph (CES) at ESO and the IUE. The study focused on the h and k Mg II lines of F and G dwarfs. The stability of the IUE spectra permitted calculations of averaged absorption features and the subtraction of the Mg II lines to leave intrinsic chromospheric and photospheric line characteristics. Stras with Mg emission cores are very similar in their chromospheres, while stars with Ca II cores vary greatly and exhibit more intense emissions. The higher activity has been linked to magnetic forces which produce spicules with diameters in the range of 1000 km. CES spectra on the Ca II core H lines in Epsilon Eri and Alpha Cen B, active and passive stars, respectively, show a 40 percent lower intensity in the quiescnet star than in the active star, which has an asymmetric core emission and a sharp central absorption with Ca IR triplet lines. The emission features in the active stars are similar to those of the active regions, photospheric spots, chromospheric plages and coronal structures observed in the sun.