4.2. Late-type Variable Giant Stars (~ 10⁸ km), Young Stars

Variations in optical and radio emission of old red giant stars (size ~ 10⁸ km) have been found. Mechanisms for such variations could involve radial pulsations and asymmetric dust distributions. Other less likely mechanisms of variability for red giant variable stars could involve magnetic fields, analogous to the solar activity cycle - however, the convection zones are extremely deep and the rotation rate slow, so a convective dynamo is unlikely to produce a significant surface magnetic field (Willson, 1988).

The polarization properties of young stars, including T-Tauri stars and young near IR stellar objects, has been reviewed by Bastien (1988). He found that models with dust aligned by magnetic fields have severe problems, while dust scattering models have little or no problems. These optical and near IR observations would say nothing on the magnetic fields there.

Observations of huge late-type supergiant stars, such as Rigel with a radius ~ 9 × 10⁷ km, have shown absorption components in the H spectral line. The absorption components vary in time and wavelength, suggestive of stellar rotation and of a large "magnetic loop" (~ 1 × 10⁸ km) in the corona near the equator, with coronal gas density ~ 5 × 10¹¹ cm^-3 and magnetic field B ~ 10 Gauss, and with photospheric surface gas density ~ 5 × 10¹² cm^-3 and magnetic field B ~ 25 Gauss (Israelian et al. 1997). Thus Rigel has an inhomogeneous circumstellar envelope with localized magnetic fields similar (but with larger sizes) to the dipolar magnetic regions near the equator of our Sun.