The Curious Case of the Alpha Persei Corona: A Dwarf in Supergiant's Clothing?

Alpha Persei (HD 20902: F5 Iab) is a luminous, nonvariable supergiant located at the blue edge of the Cepheid instability strip. It is one of the brightest coronal X-ray sources in the young open cluster bearing its name, yet warm supergiants as a class generally avoid conspicuous high-energy activity. The Cosmic Origins Spectrograph on the Hubble Space Telescope has recently uncovered additional oddities. The 1290–1430 Å far-ultraviolet (FUV) spectrum of α Per is dominated by photospheric continuum emission, with numerous superposed absorption features, mainly stellar. However, the normal proxies of coronal activity, such as the Si iv 1400 Å doublet (T ∼ 8 × 104 K), are very weak, as are the chromospheric C ii 1335 Å multiplet (T ∼ 3 × 104 K) and O i 1305 Å triplet. In fact, the Si iv features of α Per are not only narrower than those of later, G-type supergiants of similar LX/Lbol, but are also fainter (in LSi iv/Lbol) by two orders of magnitude. Further, a reanalysis of the ROSAT pointing on α Per finds the X-ray centroid offset from the stellar position by 9'', at a moderate level of significance. The FUV and X-ray discrepancies raise the possibility that the coronal source might be unrelated to the supergiant, perhaps an accidentally close dwarf cluster member; heretofore unrecognized in the optical, lost in the glare of the bright star.