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Friday, October 13, 2:10-3:00 p.m, CUE 203
Edward Guinan
The Effects of the Young Sun on the Evolution of Planetary Atmospheres and the Development of Life
The Nasty Things the Young Sun Did to Its Closest Planets
Abstract
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The evolution over time of the magnetic activity and the resulting X-ray and
UV coronal and chromospheric emissions of main-sequence G-, K-, and M-type
stars with widely different ages will be discussed. Young solar-type and
cooler stars spin rapidly and have correspondingly very robust magnetic
dynamos and very strong coronal and chromospheric X-ray-to-Ultraviolet (XUV)
emissions as well as strong plasma outflows (winds). However, these stars
spin down with time, as they lose angular momentum via magnetized winds, and
their magnetic generated activity and XUV emissions 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 strong XUV emissions. Studies of K-M dwarfs over a wide
range of ages and rotations show similar (but not identical)
behavior. Particular emphasis will be given to discussing the effects that XUV
emissions have on the atmospheres and evolution of solar system planets as
well as the increasing number of exoplanets found hosted by G-M dwarfs. 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. We also examine the effects of strong X-ray and UV coronal
and chromospheric emissions (and frequent flares) that young dM stars may have
on possible planets orbiting within their habitable zones located close
to the low luminosity host stars. Dwarf M stars make
fascinating targets for further study because of their long nuclear lifetimes,
deep convective zones, efficient dynamos and strong XUV emissions when they
are young. Furthermore, a large fraction of M dwarfs are very old, which
presents possibilities for the development of highly advanced modes of
intelligent life on planets that may orbit them.
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