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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,
Hot coronae (T\sim 10^6 K) are thought to be rare among single giant stars to the right of the ``Linsky--Haisch dividing line'' near K0 in the H--R diagram. K and M giants are such slow rotators that absence of dynamo generated magnetic activity would be natural. Nevertheless, \gamma Dra (K5 III) unexpectedly was detected in FUV coronal proxies---hot lines Si IV \lambda1393 and C IV \lambda1548---by HST\,/GHRS during Science Verification, and subsequently was discovered as a faint X-ray source in a deep ROSAT\,/PSPC pointing. Is \gamma Dra anomalous, or is the lack of coronal detections among the K giants simply a matter of insufficient sensitivity?
We have used the GHRS low resolution mode to search for additional examples of hot lines among inactive single red giants. Si IV provides a clean diagnostic of subcoronal material because it falls near the peak sensitivity of the G140L mode and does not suffer from abundance depletions that can affect C IV in red giants. X-ray/Si IV ratios are such that HST\/ can reach to much fainter limiting ``coronal'' magnitudes than even very deep ROSAT\/ pointings. In every target so far examined, we find weak---but statistically significant---Si IV emission. These include: the ancient red giant Arcturus (\alpha Boo: K1 III), recorded at the end of Cycle 5; and \epsilon Crv (K2.5 III) and \epsilon Sco (K2 III) observed in Cycle 6.
X-ray/Si IV ratios of red giants (for which measurements, or upper limits, of both diagnostics are available) fall on a uniform track, extending downward from active K0 ``Clump'' giants like \beta Ceti all the way to Arcturus itself, in the depths of the ``coronal graveyard.'' The systematic behavior argues that magnetic dynamo action continues even when long term angular momentum loss has slowed the stellar spin to a crawl.
This work was supported by grant GO-06066.01-94A from STScI.
