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Astron. Astrophys. 337, 757-771 (1998)

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1. Introduction

The most extreme examples of magnetic activity phenomena are generally found in rapidly-rotating late-type stars. Current theories to explain the origin of magnetic phenomena are based on the interaction between plasma and magnetic fields. The two physical processes of convection and rotation are believed to play a fundamental role in the generation of stellar magnetic fields through a dynamo mechanism (Parker 1970). In this scenario rapid rotators among the convective, late-type stars are expected to show signs of stronger activity. This may be not valid, however, for stars at the bottom of the main sequence. Recent studies of rotation and activity in very late-type dwarfs (Basri & Marcy 1995) suggest a significant change in the rotation-activity connection for the latest-type stars.

Rapid rotation is also a very important factor in order to resolve and investigate spatial features in stars. High rotational velocities ensure that the chromospheric line profiles are rotationally dominated, favouring the detection of emission and absorption transients associated with plage and prominence phenomena, respectively.

Observations of RE 1816+541 were first aimed at seeking evidence for stellar prominences in this object, as part of a project to investigate the nature of this kind of phenomena. Although there are several examples of rapidly-rotating late-type stars showing signs of prominence activity (Cameron & Robinson 1989a,b; Collier-Cameron & Woods 1992; Jeffries 1993; Byrne, Eibe & Rolleston 1996), our understanding of stellar prominences and their possible implications is still poor. Detailed studies were carried out only in two well known cases: the K dwarf AB Dor (Cameron & Robinson 1989a,b) and the M dwarf HK Aqr (Byrne, Eibe & Rolleston 1996, Eibe 1997, van den Oord, Eibe & Byrne 1998, hereafter VEB98). Prominences in AB Dor occur near the co-rotation radius and may be formed as condensation at the top of magnetic loops that extend beyond this radius, where the effective gravity is directed outwards. This condensation mechanism constitutes the basis of a model proposed by Collier-Cameron (1988) to explain prominence clouds formation in rapidly-rotating stars. Under negative effective gravity, condensed material could dissipate by diffusion across the field lines, leading to stellar angular momentum loss. Consequently, it was first thought that prominence clouds could contribute significantly to AB Dor's rotational braking rate.

This result would have important implications for angular momentum evolution studies. Investigations by Stauffer et al. (1984, 1985) showed that rotational braking on G and K type rapid rotators of young open clusters occurred too rapidly to be explained by conventional angular momentum loss theories, via a stellar wind. It was therefore interesting to investigate whether prominence clouds could account for the observed spin-down time-scales. However, later work to estimate cloud masses and lifetimes in AB Dor (Collier-Cameron et al. 1990) implied that they would not be very efficient as a braking mechanism. Moreover, in recent work by Collier-Cameron & Jianke (1994) it is concluded that conventional wind models may be adjusted to explain the rotational evolution of G and K dwarfs in young clusters, without introducing new braking mechanisms.

Prominences in HK Aqr have turned to be fundamentally different to those found in AB Dor, since they have been mainly found well below the co-rotation radius. In this case it is not possible to apply Collier-Cameron's model (1988) and, additionally, the role of prominence clouds in rotational braking is also dubious.

By analysing further examples of late-type rapid rotators it is possible to obtain more observations of stellar prominences and discriminate between different models. Results may not only provide a more complete view of the phenomenon but also have important consequences in studies of angular momentum evolution.

The star RE 1816+541 is similar to HK Aqr in many respects. This paper contains the main results derived from an extensive analysis of its H[FORMULA] line variability. These include the study of prominence clouds and discrete bright regions, the latter being ascribed to solar-like plage regions.

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998