The Magnetic Lives of Giant Stars

After two decades of study by high-energy missions---beginning with HEAO-1 and Einstein, and extending more recently to ROSAT, EUVE, ASCA and BeppoSAX---the evolution of coronal magnetic activity of evolved stars has come into sharper focus. We present a synthesis of ideas advanced to explain the various coronal peculiarities of late-type giants, ranging from the ``X-ray deficiency'' of the F--G0 III Hertzsprung gap stars, the ``rapid braking zone'' in the yellow giants (mid-G), and the ``coronal graveyard'' redward of K0 III. The main source of the diversity of coronal behavior among the giant stars is the wide range of main sequence progenitors whose post-MS evolution converges in the relatively small region of the H--R diagram appropriately called the red giant ``clump.'' Common threads are: relic magnetospheres from hot-star predecessors; the tension between direct convectively generated magnetic flux (the so-called ``magnetic carpet'' fields on the Sun) and the large scale rotation-catalyzed ``dynamo;'' activity-assisted mass loss and spindown; and coronal rejuvenation (through spin-up via internal redistribution of angular momentum, or engulfment of a substellar companion). The new generation of X-ray missions---AXAF and XMM---can test many of these ideas through moderate-resolution spectroscopy of the coronal plasmas, to complement ongoing high-resolution dissection of the subcoronal layers by HST (earlier GHRS, now STIS) and soon FUSE. This work was supported by grant NAG5-3226 from NASA to the University of Colorado.