Physics
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...4e..33g&link_type=abstract
The Ultraviolet Universe: Stars from Birth to Death, 26th meeting of the IAU, Joint Discussion 4, 16-17 August 2006, Prague, Cze
Physics
Scientific paper
The evolution over time of the magnetic activity and the resulting X-ray and UV coronal and chromospheric emissions of main-sequence dG, dK, and dM stars with widely different ages will be discussed. Young cool stars spin rapidly and have correspondingly very robust magnetic dynamos and strong coronal and chromospheric X-ray - UV (XUV) emissions. However, these stars spin-down with time as they lose angular momentum via magnetized winds and their magnetic generated activity and emission decrease. For example, the study of solar proxies shows that the young Sun was rotating more than ten times faster than today and had correspondingly very high levels of magnetic activity and very intense X-ray - UV (XUV) emissions. Studies of dK-dM stars over a wide range of ages and rotations show similar (but not identical) behavior. Particular emphasis will be given to discussing the effects that UV emissions have on the atmospheres and evolution of solar system planets as well as the increasing number of exoplanets found hosted by dG-dM stars. The results from modeling the early atmospheres of Venus, Earth and Mars using recently determined XUV irradiances and winds of the young Sun are also briefly discussed. For example, the loss of water from juvenile Venus and Mars can be explained by action of the strong XUV emissions and robust winds of the young Sun. We also examine the effects of strong X-ray and UV coronal and chromospheric emissions (and frequent flares) that dM stars may have on possible planets orbiting within their habitable zones (HZ) - located close to the low luminosity host stars (0.05 < 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. Furthermore, a large fraction of dM stars are very old (>5 Gyr), which present possibilities for the development of highly advanced modes of intelligent life on planets that may orbit them. This research is supported by grants from NASA and utilizes data from the IUE, FUSE, HST, EUVE, ROSAT, XMM, and the Chandra Missions. We are very grateful for this support.
Engle Scott G.
Guinan Edward F.
Ribas Ignasi
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