Cool Stars 14 - Submitted Abstract # 21
This version created on 05 October 2006

Structured Red Giant Winds with Magnetized Hot Bubbles and the X-ray
Dividing Line


Takeru Suzuki, University of Tokyo

By performing MHD simulations, we investigate mass loss of
intermediate/low-mass main sequence and red giant stars.  Alfven
waves, which are excited by the photospheric perturbations due to the
surface convections, travel outwardly and dissipate by nonlinear
processes to accelerate and heat stellar winds.  We dynamically treat
these processes in open magnetic field regions from the photospheres
to 25 stellar radii.  When the star evolves to slightly blueward of
the dividing line (Linsky & Haisch), the steady hot corona with
temperature ~ 10^6 K, suddenly disappears.  Instead, many hot (~ 10^6
K) and warm (>~ 10^5 K) bubbles are formed in cool (T <~ 2 x 10^4 K)
chromospheric winds because of thermal instability;  the RGB star wind
is not a steady stream but structured outflow.  The densities of the
bubbles which are supported by the magnetic pressure can be kept low
to reduce the radiative cooling so that the bubbles survive a long
time.  Even in the stars redward of the dividing line, hot bubbles
intermittently exist, which can be sources of UV/X-ray emissions from
hybrid stars.  Mass loss rates of RGB stars largely vary in time
because of many bubbles and blobs;  e.g.  mass loss rate of a 3 Msun
star with log g=1.4 varies from 10^{-10} to 5 x 10^{-7} (Msun/yr).
The wind velocity also rapidly decreases with the stellar evolution to
10-50km/s, slower than the surface escape velocity, because the wind
is practically accelerated from ~several stellar radii below which the
atmosphere is quasi-static.

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