The Structure of the Outer Atmosphere of Cool Stars

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Arcturus, K Giants

Scientific paper

Models of the K2 III star Arcturus computed with the latest model atmosphere code and line lists, those of scATLAS9, and with the latest values for the stellar parameters predict twice as much flux in the violet and near ultra-violet spectral regions as is observed. The models also predict line profiles in these regions that are much too strong. The addition of an approximately gray continuous absorption opacity in this spectral region that is approximately equal to that which is already included in scATLAS9 and that has the same depth distribution as n_{rm H} eliminates both the flux and the line profile discrepancy. A comparison of the violet spectrum of Arcturus with that of other K2 III standard stars indicates that this additional opacity is a general feature of early K giants. Although we have not identified this hitherto undiscovered source of opacity, we present evidence that it is molecular in origin. The addition of this continuous opacity in the violet spectral region decreases the relative strength of the predicted Ca II K line. We present a chromospheric model of Arcturus based on this line with the additional opacity included. The value of T_{ rm min} in this model is 300 K lower and lies at a column mass density that is almost an order of magnitude smaller than that of the only other chromospheric model of Arcturus in the literature. As a result, proposed chromospheric heating mechanisms and thermally bifurcated models of Arcturus need to be re-considered.

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