  Chandra X-ray and HST far-ultraviolet (FUV) observations of three early-F
supergiants have shed new light on a previous puzzle involving a prominent
member of the class: Alpha Persei (HD20902: F5Ib). The warm supergiant is a
moderately strong, hard coronal (T~10MK) X-ray source, but has ten times weaker
"sub-coronal" Si IV 139 nm (T~80,000 K) emissions than early-G supergiants of
similar high-energy properties. The Alpha Per X-ray excess speculatively was
ascribed to a close-in hyperactive G-dwarf companion, which could have escaped
previous notice, lost in the glare of the bright star. However, a subsequent
dedicated multi-wavelength imaging campaign failed to find any evidence for a
resolved secondary. The origin of the Alpha Per high-energy dichotomy then
devolved to: (1) an unresolved companion; or (2) intrinsic coronal behavior.
Exploring the second possibility, the present program has found that early-F
supergiants do appear to belong to a distinct coronal class, characterized by
elevated X-ray/FUV ratios, although sharing some similarities with Cepheid
variables in their transitory X-ray "high states." Remarkably, the early-F
supergiants now are seen to align with the low-activity end of the X-ray/FUV
sequence defined by late-type dwarfs, suggesting that the disjoint behavior
relative to the G supergiants might be attributed to thinner outer atmospheres
on the F types, as in dwarfs, but in this case perhaps caused by a weakened
"ionization valve" effect due to overly warm photospheres.
