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E. Guinan, S. Engle (Villanova University), I. Ribas (IEEC - Spain), G. Harper (University of Colorado - Boulder)
The evolution over time of the magnetic actvity and resulting X-ray and FUV coronal and chromospheric emissions of main-sequence dG, dK, and dM stars will be discussed. Young stars of these types spin rapidly and slow down over time as they lose angular momentum via magnetized winds. For example, the study of solar proxies shows that the young Sun was rotating more than 10x faster than today and had correspondingly high levels of magnetic activity and very intense X-ray and FUV (XUV) emissions. Studies of dK-dM stars over a wide range of ages and rotations shows similar (but not identical) behavior. Differences may arise from the increasing depth of the convective zone with increasing spectral type from dG to dM. Particular emphasis will be given to discussing the effects that X-ray and FUV radiations from dG-dM stars have on the atmospheres and evolution of the increasing number of planets hosted by these stars. Also discussed are recent results from modeling the early atmospheres of Venus, Earth and Mars using recently determined XUV irradiances and winds of the younger Sun. For example, the loss of water from juvenile Venus and Mars can be explained by these models. We also examine the effects of strong X-ray and FUV emissions (and frequent flares) that dM stars have on possible planets orbiting within their habitable zones (HZ) - located close to the low luminosity host stars (0.1 - HZ - 0.4 AU). Dwarf M stars make interesting targets for further study because of their deep convective zones, efficient dynamos and strong XUV emissions. Also, they are very numerous and have long main-sequence lifetimes (20+ Gyr). Furthermore, a large fraction of dM stars are in fact very old (5+ Gyr), which present possibilities for the development of highly advanced modes of intelligent life.
This research is supported by NASA FUSE grants: NAG5-12125, NNGG04GC76G, NNG04GO38G, and HST Grant GO-101114 which we gratefully acknowledge.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.