Magnetic Flares on Asymptotic Giant Branch Stars

Noam Soker, Oranim, Tivon
Joel H. Kastner, Rochester Institute of Technology

© 2003 The American Astronomical Society

We thank an anonymous referee for comments which led us to clarify many points. We acknowledge support for this research provided by NASA/CXO grants GO0–1067X and GO2–3009X to RIT. N.S. acknowledges support from the US-Israel Binational Science Foundation and the Israel Science Foundation.

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We investigate the consequences of magnetic flares on the surface of asymptotic 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 ~2x10^29 ergs s^-1 and temperature ~10^7 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 (Refer to PDF file for exact formulas).