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Astron. Astrophys. 353, 666-690 (2000)

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1. Introduction

Calcium is one of the best represented and best studied elements in the visible spectrum of the cool F, G and K stars. Lines of calcium cover a wide variety of excitation conditions and line strengths, and especially accurate atomic parameters are available. The dramatic improvement brought about by modern silicon array detectors has led investigators to focus more on the red and near infrared part of the spectrum. The infrared triplet lines of ionized calcium (Ca II IRT) at 8498, 8542 and 8662 Å are quite prominent features in this region of the spectrum of late type stars. On the one hand, the profiles of the extended wings of these very strong lines have been shown to be a good signature of the temperature distribution in the atmosphere of the stars. On the other hand, their central intensity is quite sensitive to the degree of activity of their outer layers. G. Cayrel de Strobel and her collaborators (to which I am priviliged to belong) systematically made use of the latter property, albeit in a purely qualitative way. In view of this, they usually record high quality spectra of the relevant spectral region for each star under investigation. Over recent years, G. Smith, J.J. Drake and collaborators showed that the Ca II IRT could be used for a more quantitative discussion of stellar atmospheric properties. A more quantitative use of our good quality observations thus seems advisable. Therefore, I re-examined the solar case as a reference for further quantitative high resolution spectroscopic stellar studies. The data relevant to the study of the Ca II IRT being quite scattered in the literature, it appeared useful to write this paper as a vade mecum for those wishing to use these lines as an element of diagnosis of the cool stellar atmospheres. The relevant literature is reviewed in some detail in Sect. 2. Contamination by telluric lines and continuum location problems are discussed in Sect. 3. In Sect. 4 synthetic profiles of the Ca II IRT lines in the solar flux spectrum are presented for a set of two empirical solar model atmospheres and three theoretical constant-flux model solar photospheres. The perturbation by neighbouring hydrogen Paschen lines is discussed for the first time in quantitative terms. Figures illustrating the response of the line profiles to changes in the basic stellar photospheric parameters can be found in Sect. 5. Finally, in Sect. 6, existing scattered data of diffferent sources on the central depths of the Ca II IRT lines is gathered and an improved relationship is established between this data and other quantitative chromospheric activity indicators obtained from observations of the emission reversals in the Ca II resonance H and K lines.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999