Author(s): , ,
Institution(s): 1. NASA/Goddard Space Flight Center, 2. NRL
Stellar chromospheres and winds represent universal attributes of stars on the cool portion of H-R diagram. In this paper we derive observational constrains for the chromospheric heating and wind acceleration from cool evolved stars and examine the role of Alfven waves as a viable source of energy dissipation and momentum deposition. We use a 1.5D magnetohydrodynamic code with a generalized Ohm's law to study propagation of Alfven waves generated along a diverging magnetic field in a stellar photosphere at a single frequency. We demonstrate that due to inclusion of the effects of ion-neutral collisions in magnetized weakly ionized chromospheric plasma on resistivity and the appropriate grid resolution, the numerical resistivity becomes 1-2 orders of magnitude smaller than the physical resistivity. The motions introduced by non-linear transverse Alfven waves can explain non-thermally broadened and non-Gaussian profiles of optically thin UV lines forming in the stellar chromosphere of α Tau and other late-type giant and supergiant stars. The calculated heating rates in the stellar chromosphere model due to resistive (Joule) dissipation of electric currents on Pedersen resistivity are consistent with observational constraints on the net radiative losses in UV lines and the continuum from α Tau. At the top of the chromosphere, Alfven waves experience significant reflection, producing downward propagating transverse waves that interact with upward propagating waves and produce velocity shear in the chromosphere. Our simulations also suggest that momentum deposition by non-linear Alfven waves becomes significant in the outer chromosphere within 1 stellar radius from the photosphere that initiates a slow and massive winds from red giants and supergiants.