Evolution of Stellar Coronae: 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 physical 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 as a cool stars moves from the main-sequence to a red giant phase. The results of our 2.5D magnetohydrodynamic (MHD) simulations suggest the dynamics of the emergence of magnetic flux into the atmospheres of cool stars is strongly dependent on surface gravity and the magnetic field. We simulate the dynamics of emergence of magnetic field in the Sun, a sug-giant and a giant star. Our simulations suggest that as the surface gravity becomes smaller and the magnetic field weaker as the 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 bloated stellar chromospheres and heated by Alfven waves to coronal temperatures.
- Publication:
-
American Astronomical Society Meeting Abstracts #223
- Pub Date:
- January 2014
- Bibcode:
- 2014AAS...22342401A