Institute for Advanced Study Astrophysics Seminar

Magnetic activity in the Sun manifests itself as sunspot activity, flares, and reversals in the CaII H and K lines, and is believed to originate in the transition region between the convective and radiative zones. Lower- mass stars become fully convective at about 0.3 solar masses, and must produce magnetic fields via a fully-convective dynamo. Magnetic activity in stars is studied using the incidence and variability of chromospheric emission lines, of radio emission and of X-ray emission. The enormous photometric and spectroscopic stellar data base produced by the SDSS has been used to measure the incidence of chromospheric activity with spectral tyoe and age, the rate of flares and the correspondence with chromospherically active stars, the short-time-scale variability, and the effect of binary membership. Stellar activity is present at a low level, about 10%, of late type stars and abruptly rises to almost 100% at spectral type M4, corresponding to the mass at which stars become fully convective. Variability on the several minute time scale also rises towards the latest M-type stars, and shows an approximately exponential dependence on amplitude except for the largest-amplitude events (flares). Dwarf M stars which are binary companions to white dwarfs show an increased incidence of activity, but this appears to be confined to close pairs, former common-envelope binaries, suggesting the regeneration of the dynamo by angular momentum transfer.