Evolution of Stellar Coronae
Andrea Dupree (Harvard-Smithsonian Center for Astrophysics)
Nancy Brickhouse (Harvard-Smithsonian Center for Astrophysics)
Two single cool stars, Beta Dra and Alpha TrA offer a
contrast in the structure and evolution of their outer
atmospheres. Alpha TrA is a hybrid object,
from a class of cool luminous stars originally identified
based on C IV emission accompanied by absorption features
indicating a massive stellar wind and circumstellar material.
The atmosphere of Beta Dra appears to be similar to the Sun
with high temperature emission, and lacking a massive wind.
Spectra of these stars obtained with RGS-XMM/Newton show
high temperature emission T≈107K in their steady (non-flaring)
state. A 1-T fit to the spectra suggests that the corona of
Beta Dra is slightly hotter than that of Alpha TrA. Abundances
of O, Ne, and Fe are 'solar' in Beta Dra, but O and Ne are
enhanced with respect to solar in Alpha TrA. The density
sensitive ratio of O VII transitions suggests that Alpha TrA
has a lower density than Beta Dra. These spectra demonstrate
that slowly-rotating giants and supergiants can have coronae
hotter than the Sun. These spectra also give the first evidence
that coronal structure changes, becoming cooler and less dense
as luminous stars evolve to the hybrid phase.