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Magnetic Flares on Asymptotic Giant Branch Stars
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MAGNETIC FLARES ON ASYMPTOTIC GIANT BRANCH
STARS
Noam Soker1,2 and Joel H. Kastner1
ABSTRACT
We investigate the consequences of magnetic flares on the surface of asymp-
totic giant branch (AGB) and similar stars. In contrast to the solar wind, in
the winds of AGB stars the gas cooling time is much shorter than the outflow
time. As a result, we predict that energetic flaring will not inhibit, and may
even enhance, dust formation around AGB stars. If magnetic flares do occur
around such stars, we expect some AGB stars to exhibit X-ray emission; indeed
certain systems including AGB stars, such as Mira, have been detected in X-rays.
However, in these cases, it is difficult to distinguish between potential AGB star
X-ray emission and, e.g., X-ray emission from the vicinity of a binary companion.
Analysis of an archival ROSAT X-ray spectrum of the Mira system suggests an
intrinsic X-ray luminosity ~ 2 × 1029 erg s?1 and temperature ~ 107 K. These
modeling results suggest that magnetic activity, either on the AGB star (Mira
A) or on its nearby companion (Mira B), is the source of the X-rays, but do not
rule out the possibility that the X-rays are generated by an accretion disk around
Mira B.
Subject headings: stars: mass loss — stars: winds, outflows — X-rays: ISM —
stars: AGB — stars: magnetic fields
1. INTRODUCTION
Asymptotic giant branch (AGB) stars represent the final stages of evolution for inter-
mediate mass (~ 1 ? 8M⊙) stars, just before these stars (potentially) generate planetary
nebulae. Such stars have exhausted their core hydrogen, have completed core helium and
1Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial
Dr., Rochester, NY 14623; jhk@cis.rit.edu
2Department of Physics, Oranim, Tivon 36006, ISRAEL; soker@physics.technion.ac.il
– 2 –
H-shell burning phases, and are now burning H and He in concentric shells around the ex-
hausted stellar c
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