Spectroscopic Analysis of the Companion to the Binary MSP PSR J1740-5340 in NGC 6397

摘要

By means of high-resolution spectra, we have measured radial velocities of the companion (hereafter COM J1740-5340) to the eclipsing millisecond pulsar PSR J1740-5340 in the galactic globular cluster NGC 6397. The radial velocity curve enables us to derive the mass ratio (MPSR/MCOM = 5.85 ± 0.13).The derived abundances are fully compatible with those of normal unperturbed stars in NGC 6397, with the exception of a few elements. The lack of C, in particular, suggests that the star has been peeled down to regions where incomplete CNO burning occurs, supporting a scenario where the companion is a turn-off star which has lost most of its mass.A detailed analysis of the H lines reveals that optically thin hydrogen gas resided outside the Roche lobe of COM J1740-5340. The line morphology suggests the presence of a steam of material going from the companion toward the neutron star; this material never reachs the neutron star surface, being driven back by the pulsar radiation, far beyond COM J1740-5340.The unexpected detection of strong He I absorption lines implies the existence of regions at T > 10,000 K, which are significantly warmer than the rest of the star, and reveals the existence of a region on the companion surface, heated by the millisecond pulsar flux.