The Ca II infrared triplet as a stellar activity diagnostic

Free access article

Issue		A&A Volume 466, Number 3, May II 2007


Page(s)		1089 - 1098
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20065588

A&A 466, 1089-1098 (2007)
DOI: 10.1051/0004-6361:20065588

II. Test and calibration with high resolution observations

I. Busà¹, R. Aznar Cuadrado², L. Terranegra³, V. Andretta³, and M. T. Gomez³

¹ Istituto Nazionale di Astrofisica - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
    e-mail: ebu@oact.inaf.it
² Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
    e-mail: aznar@mps.mpg.de
³ Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
    e-mail: [andretta;gomez;terranegra]@na.astro.it

(Received 11 May 2006 / Accepted 24 January 2007)

Abstract
Aims.We report on our analysis of the high resolution spectra ( $R\approx86\,000$ ) of a sample of 42 late-type active stars (with measured ${\log{R'_\mathrm{HK}}}$ spanning from ${\approx}{ -}3$ to ${\approx}{ -}5$ ) acquired with the Italian 3.6 m Telescopio Nazionale Galileo (TNG) using the SARG spectrometer in the 4960-10 110 Å range. The high quality of the spectra and the good activity-level coverage allow us to measure two different chromospheric indicators that can be derived from the Ca II infrared triplet (Ca II IRT) lines: the residual equivalent width (EQW) and the chromospheric indicator ${R_{\rm IRT}}$ . The aim of this work is determine and test the best way of deriving activity-level information and errors from the Ca II IRT lines, in preparation of the GAIA Cornerstone mission by ESA, by which the Ca II IRT spectral range will be spectroscopically observed for millions of stars.
Methods.The ${R_{\rm IRT}}$ index is calculated for each observed star as the difference between the calculated NLTE photospheric central intensity and the observed one. The residual EQW, ${\Delta W_\mathrm{IRT}}$ , is calculated as the area of the positive profile obtained as the difference between the calculated NLTE photospheric and the observed profiles. We correlate ${\log{R'_\mathrm{HK}}}$ with ${R_{\rm IRT}}$ and the ${\Delta W_\mathrm{IRT}}$ .
Results.This analysis indicates that Ca II IRT lines are good chromospheric diagnostics. We find that both ${\Delta W_\mathrm{IRT}}$ and the ${R_{\rm IRT}}$ quantities can be used as chromospheric indicators, although the former exhibits a tighter correlation with the ${\log{R'_\mathrm{HK}}}$ index. Furthermore, we find that the total chromospheric excess EQW in the Ca II IRT is almost linearly correlated with the excess in the Ca II H & K doublet, as estimated through the ${\log{R'_\mathrm{HK}}}$ index.

Key words: stars: atmospheres -- radiative transfer -- line: formation -- stars: chromospheres -- stars: activity

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