The chromospherically active binaries are detached binary systems with cool components characterized by strong chromospheric, transition region, and coronal activity. The RS CVn systems have at least one cool evolved component whereas both components of the BY Dra binaries are main sequence stars (Fekel et al. 1986).
In this series of papers we try to study the chromosphere of this kind of extremely active stars using the information provided by several optical spectroscopic features that could be used as chromospheric activity indicators. The simultaneous observations of different lines, that are formed at different height in the chromosphere (from the region of temperature minimum to the higher chromosphere), are of special interest for stellar activity studies since they provide very useful information about this stellar region. Ideally, simultaneous observations should be performed at all wavelengths in order to develop a coherent 3-D atmosphere model. In practice, simultaneous observations of several activity indicators are rare and tend to focus on the same small number of extremely active systems.
The best way to obtain the active-chromosphere contribution to some spectral line in the chromospherically active binaries is to subtract the underlying photospheric contribution using the spectral subtraction technique (subtraction of a synthesized stellar spectrum constructed from artificially rotationally broadened, radial-velocity shifted, and weighted spectra of inactive stars chosen to match the spectral types and luminosity classes of both components of the active system under consideration).
The emissions in the Ca II H & K resonance lines are the most widely used optical indicators of chromospheric activity, since their source functions are collisionally controlled and represent an extremely important cooling mechanism. In chromospheric active binaries the subtraction of the photospheric flux in this spectral region has been recently applied using the spectral subtraction (see Montes et al. 1995c, 1996a and references therein).
The H
line is also an important chromospheric activity
indicator, but it is only in emission above the continuum in very active stars,
and in less active star only a filled-in absorption line is observed.
So, to infer chromospheric activity level the spectral subtraction is needed
(see Montes et al. 1994; 1995a,b,d, and references therein;
Lázaro & Arévalo 1997).
A similar behaviour is observed in the other Balmer lines
(Hall & Ramsey 1992; Montes et al. 1995d).
Recently, the spectral subtraction technique has been used in other lines as the Ca II IRT, Mg I b, Na I D1, D2, and He I D3 lines (Gunn & Doyle 1997; Gunn et al. 1997). The Ca II IRT lines are formed deeper in the atmosphere and are thus sensitive probes of the temperature minimum region. The Na I D1, D2 lines are collision dominated and are good indicators of changes in the lower chromosphere. The Mg I b triplet lines are formed in the lower chromosphere and the region of temperature minimum and they are good diagnostics of photospheric activity (Basri et al. 1989). The He I D3 line has been largely ignored as activity indicator; however it could be a valuable probe of stellar activity and the observation of this line in emission supports the detection of flare like events (Zirin 1988).
In this first paper we focus
our study on the analysis of the extensively used H
chromospheric activity indicator together with simultaneous
observations of the less studied He I D3 and
Na I D1, D2 spectral features
in a sample of 18 northern active binary systems
selected from "A Catalog of Chromospherically Active Binary Stars
(second edition)" (Strassmeier et al. 1993, hereafter CABS).
By using the spectral subtraction technique,
we have determined the excess emission in these lines and
we have computed absolute chromospheric fluxes in H.
The primary aim of this study is analyse in detail the
excess H emission
and to study the subtracted H line profile, especially in
some extremely active stars which exhibit broad wings.
Moreover, we try to understand the behaviour of the
He I D3 and Na I D1, D2 lines as
chromospheric activity indicators taking into account the advantage
that we simultaneously know the behaviour of the
chromospheric excess H emission in these systems.
In forthcoming papers we will analyze in detail several
optical spectroscopic features using echelle spectroscopy
in order to determine the effects of stellar activity on spectral
lines originating at different heights in the chromosphere.
Another of our goals is to obtain information about the presence of extended
matter (prominence-like structures) in the chromospheric active binaries
using simultaneous H and H
observations
at near-eclipse orbital phases.
In Sect. 2 we give the details of our observations and data reduction.
In Sect. 3 we describe the individual results of
H, Na I D1, D2, and He I D3
line observations of our sample.
Finally in Sect. 4 we discuss our results.
94: Fernández-Figueroa et al. (1994), 95a: Montes et al. (1995a), 95b: Montes et al. (1995b), 95c: Montes et al. (1995c), 96a: Montes et al. (1996a).
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