A Closer Look at the Alpha Persei Coronal Conundrum

Abstract

A ROSAT survey of the Alpha Per open cluster in 1993 detected its brightest star, mid-F supergiant Alpha Persei: the X-ray luminosity and spectral hardness were similar to coronally active late-type dwarf members. Later, in 2010, a Hubble Cosmic Origins Spectrograph SNAPshot of Alpha Persei found far-ultraviolet coronal proxy SiIV unexpectedly weak. This, and a suspicious offset of the ROSAT source, suggested that a late-type companion might be responsible for the X-rays. Recently, a multi-faceted program tested that premise. Groundbased optical coronography, and near-UV imaging with HST Wide Field Camera 3, searched for any close-in faint candidate coronal objects, but without success. Then, a Chandra pointing found the X-ray source single and coincident with the bright star. Significantly, the SiIV emissions of Alpha Persei, in a deeper FUV spectrum collected by HST COS as part of the joint program, aligned well with chromospheric atomic oxygen (which must be intrinsic to the luminous star), within the context of cooler late-F and early-G supergiants, including Cepheid variables. This pointed to the X-rays as the fundamental anomaly. The over-luminous X-rays still support the case for a hyperactive dwarf secondary, albeit now spatially unresolved. However, an alternative is that Alpha Persei represents a novel class of coronal source. Resolving the first possibility now has become more difficult, because the easy solution -- a well separated companion -- has been eliminated. Testing the other possibility will require a broader high-energy census of the early-F supergiants.