On the Coronal Field Topology in Warm Stars: Is Procyon a Warm Hybrid?

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Abstract
The height up to which a coronal magnetic field line can remain closed is determined principally by the balance between the gas pressure, pg, and the magnetic pressure, pB. At an on-axis height, z, above an ideal dipole, the magnetic pressure falls off as z-6 in the far-field regime, where z exceeds the separation dAR between the poles of the stellar active region. Thus, for a given exponentially decreasing gas pressure, the smaller the dipole separation, dAR, the lower the transition point at which pg exceeds pB, where closed magnetic loops cannot be sustained. Because the maximum size of bipolar stellar active regions is likely to be limited by the depth of the convective envelope, this suggests that an increasing fraction of the field should open up for stars with progressively shallower convective envelopes. We discuss the available empirical evidence for this effect for F stars on or near the main sequence. This has direct implications for the mechanism 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. The explanation of the Linsky-Haisch dividing line proposed by Rosner and coworkers depends on a change in the dynamo mode and a consequent change in the surface-level size scale of active regions. For F-type stars 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 coronae. We also suggest that Procyon may be a "warm hybrid" star, an interpretation consistent with recent Hubble Space Telescope observations of an excess emission in the blue wings of transition-region lines and with Extreme-Ultraviolet Explorer measurements of its coronal temperature, structure, and density.