September 13, 2013, 12:00 pm - 1:00 pm

September 13, 12:00 pm - 1:00 pm

Evolution of Stellar Atmospheres: From the Sun to a Red Giant

V. Airapetian, Sigma Space and NASA/GSFC

All stars in the H-R diagram later than spectral class F5 possess a convective zone that provides a reservoir of mechanical energy to drive activity in layers above the photosphere and to shape its atmospheric structures. Cool stars on the main sequence are characterized by compact chromospheres/transition regions and extended coronae transitioning into stellar winds. As the star exhausts its hydrogen fuel and enters into a giant/supergiant phase of its life, its atmospheric structures change dramatically showing signatures of bloated chromospheres and compact coronae. What heating mechanisms are responsible for such a drastic transition from a "dwarf" chromosphere of a cool dwarf star into a "giant" chromosphere of an evolved giant? How does a "giant" corona of a dwarf star evolve into a "dwarf" corona" of a giant star?
In this talk we present a unified picture of the evolution of stellar atmospheric structures. Our 2.5D magnetohydrodynamic (MHD) simulations show the dynamics of the emergence of magnetic flux into the atmospheres of giant stars forming compact active regions. We then compare these results to the flux dynamics in the solar atmosphere. These simulations suggest that as the surface gravity becomes smaller and the magnetic field weaker as a star evolves, the magnetic flux cannot be transported high enough into the atmosphere to form an extended corona. Instead, it forms highly compact loops in the lower layers of stellar chromospheres heated by Alfven waves to coronal temperatures. Finally, we will discuss the results of our MHD simulations of "bloated" red giant chromospheres created by dissipation and momentum deposition of upward propagating Alfven waves generated in the stellar photospheres.