ON THE CORONAL FIELD TOPOLOGY IN WARM STARS - IS PROCYON A WARM HYBRID

The height up to which a coronal magnetic held line can remain closed is determined principally by the balance between the gas pressure, p(g ), and the magnetic pressure, p(B). At an on-axis height, z, above an ideal dipole, the magnetic pressure falls off as z(-6) in the far-fiel d regime, where z exceeds the separation d(AR) between the poles of th e stellar active region. Thus, for a given exponentially decreasing ga s pressure, the smaller the dipole separation, d(AR), the lower the tr ansition point at which p(g) exceeds p(B), where closed magnetic loops cannot be sustained. Because the maximum size of bipolar stellar acti ve regions is likely to be limited by the depth of the convective enve lope, this suggests that an increasing fraction of the held should ope n up for stars with progressively shallower convective envelopes. We d iscuss the available empirical evidence for this effect for F stars on or near the main sequence. This has direct implications for the mecha nism for the coronal dividing line for cool K-type giants proposed by Rosner and coworkers, in that a similar-albeit more gradual-transition should take place for warm F-type main-sequence and subgiant stars. T he explanation of the Linsky-Haisch dividing line proposed by Rosner a nd coworkers depends on a change in the dynamo mode and a consequent c hange in the surface-level size scale of active regions. For F-type st ars we expect a change in size scale but not a change in dynamo mode, which allows a separation of the effects of these two changes on hot c oronae. We also suggest that Procyon may be a ''warm hybrid'' star, an interpretation consistent with recent Hubble Space Telescope observat ions of an excess emission in the blue wings of transition-region line s and with Extreme-Ultraviolet Explorer measurements of its coronal te mperature, structure, and density.