Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line

Abstract

By performing MHD simulations, we investigate the mass loss of intermediate-and low-mass stars from MS to RGB phases. Alfvén waves, which are excited by the surface convections, travel outwardly and dissipate by nonlinear processes to accelerate and heat the stellar winds. We dynamically treat these processes in open magnetic field regions from the photospheres to '25 stellar radii. When the stars evolve to slightly blueward positions of the dividing line (Linsky & Haisch), the steady hot corona with temperature T % 10 6 K suddenly disappears. Instead, many hot ($10 6 K) and warm (k10 5 K) bubbles are formed in cool (T P 2 ; 10 4 K) chromospheric winds because of thermal instability; the red giant wind is not a steady stream but structured outflow. As a result, the mass-loss rates,Ṁ , largely vary in time by 3Y 4 orders of magnitude in the RGB stars. Supported by magnetic pressure, the density of hot bubbles can be kept low to reduce the radiative cooling and to maintain the high temperature a long time. Even in the stars redward of the dividing line, hot bubbles intermittently exist, and they can be sources of UV/soft X-ray emissions from hybrid stars. Nearly static regions are formed above the photospheres of the RGB stars, and the stellar winds are effectively accelerated from several stellar radii. Then, the wind velocity is much smaller than the surface escape speed because it is regulated by the slower escape speed at that location. We finally derive an equation that determinesṀ from the energetics of the simulated wave-driven winds in a forward manner. The relation explainsṀ from MS to RGB and can play a complementary role to the Reimers formula, which is mainly for more luminous stars.

Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER

“…On the other hand, the spectropolarimetric observations of Aurière et al (2010) detected no signature of magnetic fields in Aldebaran, implying that the surface-averaged field strength is much smaller than the 1 G in the model. An extension of the MHD simulations of Suzuki (2007) with weaker magnetic fields and weakly ionized plasma, as well as the inclusion of the MOLsphere component in the model of Airapetian et al (2010) would be crucial for comparison with the observed data. McMurry & Jordan (2000) propose an alternative scenario, in which the hot chromospheric gas is associated to acoustic shocks.…”

Nonlinear long-wavelength torsional Alfvén waves

“…Cranmer 2009) and stellar (e.g. Charbonneau & MacGregor 1995;Suzuki 2007) winds and coronal heating (e.g. Ofman 2005).…”

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

“…They may be related to large convective cells predicted to be present in cool luminous stars (Schwarzschild 1975;Freytag et al 2002) or alternatively to thermal instability taking place in the outer atmosphere. For example, the magnetohydrodynamical simulation of Suzuki (2007) for red giant branch (RGB) stars, which are much less luminous than RSGs, shows that thermal instability leads to "structured" stellar winds with many bubbles of hot gas (∼10 5 K) embedded in cool winds (∼1−5 × 10 3 K). For cooler RSGs, the formation of molecules may also promote such thermal instability, and in particular, CO is an important coolant in the atmosphere of late-type stars (Cuntz & Muchmore 1994).…”