X‐Rays from Hybrid Stars

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Abstract
The late-type giants and supergiants of the "hybrid chromosphere" class display signatures of cool (T 2 × 104 K) winds together with hot emission lines from species like C IV (T ~ 105 K). A survey of such stars by Reimers et al. using ROSAT reported numerous X-ray detections (T ~ 106 K), strengthening the (then heretical) idea that hot coronae and cool winds can coexist in luminous giants. However, several of the candidate sources were offset from the predicted stellar coordinates, calling into question the identifications. In an effort to secure better knowledge of the X-ray luminosities of the hybrids, the ROSAT fields from the Reimers et al. survey were reexamined, exploiting the USNO-A2.0 astrometric catalog to register the pointings to a few arcseconds accuracy. On the basis of positional mismatches, at least two of the previously reported detections of key hybrid stars—γ Dra (K5 III) and β Aqr (G0 Ib)—must be rejected. The new X-ray upper limits for these stars, combined with the remaining candidate detections (and nondetections) from the original survey, place the hybrids into the same "X-ray deficient" category as the "noncoronal" red giants like Arcturus (α Boo: K1.5 III) and Aldebaran (α Tau: K5 III). A few of the hybrid X-ray sources are exceptional, however. The archetype α TrA (K2 II-III), in particular, is securely detected in terms of positional coincidence, but its anomalous, contradictory coronal properties suggest that an unseen companion—a young hyperactive G dwarf—might dominate the X-ray emission.