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Digging Deeper in the Coronal Graveyard

Soft X-ray detections of stellar coronae (T ~ 106 K) are rare in the giant branch redward of ~K1 III. We have conducted a less direct—but more sensitive—search using the Hubble Space Telescope Goddard High-Resolution Spectrograph to probe for far-ultraviolet proxies of the hot coronal gas in representative "noncoronal" red giants. In every target so far examined, we find weak but statistically significant Si IV emission, as well as, commonly, C IV and, in some cases, N V. Si IV is not affected by the CNO anomalies produced by the first dredge-up, which can deplete the carbon abundance and weaken C IV. In the low-activity giants, the λ1393 component of the Si IV doublet must be corrected for sharp absorptions, which we believe are caused by carbon monoxide in overlying cool material. The normalized flux ratios (ℛ ≡ f/fbol) of Si IV and X-rays among the "coronal" yellow giants (lying just blueward of the "noncoronal" zone) fall on a uniform track, ℛX ~ℛ2Si IV. In the noncoronal zone, however, the Si IV index is nearly constant (ℛSi IV ~10−8), independent of ℛX (which ranges from ~10-8 to ≲10-10). The mechanism that diminishes X-ray activity in the red giants is highly sensitive to an as yet unidentified stellar property. Photoelectric absorption by cool gas might play a more important role than previously suspected, particularly if hot magnetic loops are partly or completely buried in the chromosphere.