Evolving Trio of Hybrid Stars: C 111

摘要

Hybrid stars represent the critical evolutionary state between coronal-like objects and the Alpha Ori-like objects. They assume a pivotal role in the definition of coronal evolution, atmospheric heating processes, and mechanisms to drive winds of cool stars. Our FUSE observations of 2 hybrid stars (Gamma Dra and Mu UMa) and one potentially hybrid object harboring a possible planet (Iota Dra), addressed the puzzling problem of the hybrid stage where apparently a warm wind/ warm corona coexist. Only 2 bright members of the hybrid class have been observed with FUSE to date: (alpha Aqr in the Cool Stars Team survey, and alpha TrA as a GI target). Our goal is to understand the behavior of the outer atmosphere in this intermediate stage to create a comprehensive picture of atmospheric evolution. In the hybrid phase, the large-scale magnetic dynamo activity decays and hydrodynamic processes assume importance. Some hot plasma is still confined close to the star by magnetic loops, yet the confining field is breaking open, the atmosphere can escape through these open field lines, and the diffuse corona may be warm. Rosner et al. (1995) argue that the nature of the stellar dynamo undergoes a topological transition between the two extremes, but it does not address the widely prevalent hybrid objects. There may well be a more extended and variable transition process. It remains for FUSE to identify the controlling parameters of the hybrid stars.