The chemical composition of the peculiar companion to the millisecond pulsar in NGC 6397^*

¹ Dipartimento di Astronomia Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
² INAF–Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
³ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40126 Bologna, Italy
⁴ INAF–Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra, Italy
⁵ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
⁶ Dipartimento di Fisica Università di Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy

Corresponding author: E. Sabbi, sabbi@bo.astro.it

Received: 19 June 2003
Accepted: 19 September 2003

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.