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Paper: Constraints on the Surface Magnetic Field Structure of Aldebaran (α Tauri, K5 III)
Volume: 448, 16th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun
Page: 1145
Authors: Harper, G. M.; Brown, A.; Redfield, S.
Abstract: The presence of both a stellar wind and the coronal proxy O VI 1032 & 1038 Å emission (Ayres et al. 2003; Dupree et al. 2005) shows that the outer atmosphere of the red giant Aldebaran (α Tauri) is influenced by magnetic fields. Exactly how these magnetic structures are organized is unknown but future thermal continuum observations with the Expanded Very Large Array (EVLA) and the Atacama Large Millimeter Array (ALMA) would provide powerful new insights into this thorny problem. Observations using these facilities would sample over three orders of magnitude in continuum optical depth and provide an area-averaged sweep through the wind and chromospheric layers. Here we examine how current 1-D time-independent models compare to existing centimeter and millimeter data. The 1-D chromospheric model of (McMurry 1999) embedded in the wind model of Robinson et al. (1998) predicts an excess of sub-mm emission. This excess may reflect that these wavelengths arise in pervasive and cold ∼ 2000 K structures, known from ultraviolet fluorescent CO studies. If this cold material lies beneath a magnetic canopy then the implied photospheric filling factor is < 0.04. The filling factor of the wind launch sites is difficult to constrain because the wind temperature is currently uncertain (≤ 20,000 K) and at chromosphere-like temperatures. The evolution of spectrally-resolved Ca II circumstellar absorption observed with the Harlan J. Smith 107″ at McDonald Observatory suggests the turbulence and wind acceleration in published models require revision. The filling factor parameter space can be constricted by future EVLA and ALMA data.
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