Faint blue stars in the core of NGC 6397.

Abstract

The inner core of the globular cluster NGC 6397 within approximately 6 arcsecs of the geometrical centre has been investigated with high resolution deep images taken through several medium- and broad-band UV filters with the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST). Approximately 110 stars down to instrumental magnitude m₂₁₀ approximately equal to 23 were reliably identified in this region and located on a UV-U color magnitude diagram for the first time. The main sequence is well defined down to visual magnitude approximately equal to 6.3, corresponding to a mass of approximately 0.6 solar mass. Four faint blue stars are identified, with UV-U band coarse spectra compatible with black body temperatures in the range 10,000-20,000 K, whose fluxes show considerable variability over a 5 hour time interval. The four objects are all located inside the 90% confidence radius around the position of the possibly multiple low luminosity X-ray source C3 (Cool. 1993), and as such are strong candidates to the optical counterparts. The estimated L_X/L_V ratio suggests that these objects are cataclysmic variables.