The chemical composition of the peculiar companion to the millisecond pulsar in NGC 6397
E. Sabbi, R. G. Gratton, A. Bragaglia, F. R. Ferraro, A. Possenti, F. Camilo, N. D'Amico
Abstract
We present the chemical composition of the bright companion to the millisecond pulsar J1740-5340 in NGC 6397, based on high resolution spectra. Though the large rotation velocity of the star broadens the lines and complicates the analysis, the derived abundances are found fully compatible with those of normal unperturbed stars in NGC 6397, with the exception of a few elements (Li, Ca, and C). The lack of C suggests that the star has been peeled down to regions where incomplete CNO burning occurs, favouring a scenario where the companion is a turn-off star which has lost most of its mass. In addition we found an unexpected large Li abundance, which suggests that fresh 7Li has been produced on the stellar surface.
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