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Astron. Astrophys. 318, 60-72 (1997)

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1. Introduction

In recent years evidence has been accumulating that some stars have cool coronal condensations analogous to solar prominences. In many respects these features can be regarded as the remaining phenomenon to be confirmed for active stars which are routinely seen optically on the Sun. The present study involves an attempt to detect the presence of such material in a small sample of RS CVn binaries. In this first paper we present a detailed discussion of the techniques used and present the results for ER Vulpeculae.

The detection of cool material overlying the surface of a star is extremely difficult since in all but the most extremely active stars such material would manifest itself as a small additional absorption feature overlying the global absorption lines. This problem is also exacerbated by the fact that other regions such as plages or lower chromospheric structure, which may also be present, can actually fill-in the global absorption profile due to the increasing collisional domination of the source function for the activity sensitive transitions. An attempt at detecting such prominence-like material must therefore account for a variety of effects. To remove the effect of core-filling of active lines, the technique of spectral subtraction is often used. This technique essentially involves subtracting from the spectrum of the active star a synthetic spectrum representing this star in all other respects other than having an inactive chromosphere. The presence of plage-like or prominence-like material is often shown in subtracted spectra as excess emission or absorption features although it is difficult to assign such features unambiguously to absorbing material because they could equally well arise from intrinsic changes in the line-forming regions. Time series of spectra can to a certain extent alleviate this by providing evidence for features which evolve in a self-consistent manner. However the observation of active close binary systems which periodically undergo geometrical eclipses provides a powerful technique whereby absorption features can be unambiguously disassociated from the global absorption or emission lines. If the geometry is suitable, cool coronal material in the atmosphere of the eclipsing star can absorb the photospheric continuum radiation of the star behind.

The observational evidence for stellar prominences is still very sparse. AB Dor is a well studied single K0 dwarf star for which Robinson & Collier Cameron (1986) reported the presence of transient features in the H [FORMULA] line. Subsequent work on this star (Collier Cameron & Robinson 1989a; Collier Cameron & Robinson 1989b; Collier Cameron et al. 1990) has shown that the most consistent interpretation of these features is of prominence-like condensations of mainly neutral material trapped in corotation with the star by the stellar magnetic field. The variability in the H [FORMULA] line is consistent with a series of clouds crossing the stellar disk, and by solar analogy these are thought to consist of filamentary structures of cool material. Some other single stars are known to show clear evidence for large circumstellar prominences (Collier Cameron & Woods 1992). The main requirement seems to be the presence of magnetic loop structures extending far into the corona with field strengths sufficient to support the material against centrifugal ejection. Recent observations by Byrne, Eibe & Rolleston (1996) of HK Aqr revealed similar features which however appeared to form below the corotation radius.

In RS CVn binaries excess absorption features are commonly seen in subtracted spectra and arise due to prominence-like material occulting one of the component stars during eclipses (Hall & Ramsey 1992a). Hall et al. (1990) reported extensive observations of the RS CVn binary SS Boo in the Balmer lines as well as the Ca II H and K and Ca II IRT. An unusual excess absorption occurred near primary eclipse in the Balmer lines and this was attributed to an extended region of 4-4.6 [FORMULA]. It was further shown that the feature could not be associated with an accretion stream which was seen as an H [FORMULA] transient feature in similar observations of the UX Ari system by Huenemoerder, Buzasi & Ramsey (1989). In a follow up campaign Hall & Ramsey (1992b) surveyed 10 RS CVn systems and reported stable prominence-like material corotating with either the primary or secondary stars in eight of their targets. This study concluded that amongst the eclipsing RS CVn stars prominence material was a common feature. Buzasi, Huenemoerder & Ramsey (1991) made a study of the HR 1099 system in which spectral subtraction was performed on optical spectra in the H [FORMULA], H [FORMULA] and Ca II lines. When the excess line fluxes were plotted as a function of time a striking discontinuity was found such that the H [FORMULA] line flux dropped by about 30% for about a third of the orbit. During the drop the H [FORMULA] line appeared to be phase modulated whereas before it was not. These authors interpreted this as the disappearance of a large stellar structure which was perhaps prominence-like. In fact Buzasi (1989) concluded that extended prominences are the most reasonable explanation for the Balmer line emission from HR 1099.

Since the geometry of many of these inferred prominences exceeds the Roche lobes of the components it is reasonable to assume they are magnetically confined. This also lends credence to the hypothesis that they are analogous to solar prominences. Recently Hall & Ramsey (1994) have provided a first generation model for analysing the characteristics of prominence absorption in the subtracted spectra of RS CVn binaries. Even though this model is by no means sophisticated it generally produces physical parameters in broad agreement with the solar paradigm.

Less direct methods have been used to infer the presence of prominence material in RS CVn binaries. Buzasi (1989) developed an NLTE radiative transfer model to derive values of the Balmer equivalent width ratio of the H [FORMULA] and H [FORMULA] lines in subtracted spectra. A grid of models was used representing plage-like and prominence-like material over a range of appropriate temperatures, densities and optical depths. These results showed that low ratios of [FORMULA] 1-2 could be achieved in both plages and prominences viewed against the stellar disk but that values between [FORMULA] 3-15 could only be achieved in prominences seen off the limb of the star. Similar conclusions were given by Heasley & Mihalas (1976) who discussed detailed models of quiescent solar prominences. Observational confirmation of these predictions is given by Landman & Mongillo (1979) who observed high ratios in solar prominences seen at the limb, and by Chester (1991) who found ratios of [FORMULA] 2 in solar plage regions. In RS CVn binaries high Balmer line ratios are often found (Huenemoerder & Ramsey 1987; Huenemoerder & Barden 1986; Newmark 1990; Hall et al. 1990).

The results discussed above, if real, are significant in that previously the extended coronal regions of active stars were the domain of the X-ray and radio observers. They further suggest that there exists cospatial plasma with significant emission measure at both optical and X-ray wavelengths; probably consisting of fine threaded cool regions delineating magnetic loops permeating the hot corona. Since these structures ultimately represent the interface of the stellar atmosphere with the stellar wind they provide important insights into the physics of corona-wind interaction, the eruptive ejection of material into the ambient environment and hence angular momentum and mass loss effects. The unambiguous optical identification of coronal structures in active late-type stars, and in particular in active close binaries, is by no means conclusively proved. However the pursuit of this subject represents one of the last components of the solar paradigm for active stars.

In this paper we discuss in depth the validity of the spectral subtraction technique in the search for prominence-like material in active close binary stars, introduce our observational and analytical procedures and present the results for one of the stars in our survey.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998