Cracking the Conundrum of F-Supergiant Coronae

TL;DR

This study investigates the unique high-energy emission properties of early-F supergiants, revealing they form a distinct coronal class with elevated X-ray/FUV ratios, likely due to thinner outer atmospheres and weaker ionization effects.

Contribution

It identifies early-F supergiants as a separate coronal class with distinctive high-energy emission characteristics, advancing understanding of stellar atmospheric behavior.

Findings

Early-F supergiants have higher X-ray/FUV ratios than G supergiants.

They align with low-activity late-type dwarfs in X-ray/FUV behavior.

Thinner outer atmospheres may cause their unique emission properties.

Abstract

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.