Thursday, September 26, 2013, 3pm
Location: Room 1110 of the Nguyen Engineering Building
Vladimir Airapetian
Sigma Space
Corporation, NASA/GFSC,
and SPACS/GMU
Evolution of Stellar Atmospheres:
From the Sun to a Red Giant
Abstract
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.