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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[78.15] Fishing in the Coronal Graveyard

T. R. Ayres, A. Brown, G. M. Harper, P. D. Bennett (CASA), J. L. Linsky (JILA), K. G. Carpenter (GSFC), R. D. Robinson (CSC)

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.

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