破解F-超巨星日冕之谜
Cracking the Conundrum of F-Supergiant Coronae
摘要：
钱德拉X射线和HST对三颗早期F超巨星的远紫外线（FUV）观测为之前的一个谜团提供了新的线索，这个谜团涉及到英仙座α星（HD 20902：F5 Ib）。暖超巨星是一个中等强度的硬日冕（T~ 10 MK）X射线源，但其“次日冕”Si IV 139 nm（T~ 80，000 K）辐射比具有类似高能性质的早G超巨星弱十倍。阿尔法每X射线的过量推测归因于一个接近的过度活跃的G矮星同伴，它可能已经逃脱了之前的注意，迷失在明亮的恒星的眩光中。然而，随后的专门多波长成像活动未能找到任何证据，解决了二次。高能量的Alpha Per二分法的起源于是转移到：（1）一个未解决的伴星;或（2）内在的日冕行为。探索第二种可能性，本计划发现早期F超巨星似乎确实属于一个独特的日冕类，其特征是X射线/FUV比值升高，尽管在其短暂的X射线“高态”中与造父变星有一些相似之处。“值得注意的是，早期F超巨星现在被认为与晚型矮星定义的X射线/FUV序列的低活动端对齐，这表明相对于G超巨星的不相交行为可能归因于F型上较薄的外部大气，就像在矮星中一样，但在这种情况下，可能是由于过于温暖的光球导致的“电离阀”效应减弱。
英文摘要：
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.
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