WHITE DWARFS AND MASS SEGREGATION IN THE CORE OF 47 TUCANAE

摘要

The core of the galactic globular cluster 47 Tucanae was observed by the Faint Object Camera on the refurbished Hubble Space Telescope for the first time through medium-band F253M and F346M filters centered at 255 and 348 nm. A total of 442 stars down to the detection limit at m_(346) approx= 24 in the 7″ x 7″ field were accurately placed on a m_(346) versus m_(253) — m_(346) magnitude diagram to characterize the stellar populations in the core. Approximately 100 objects above and to the right of the main-sequence turnoff are the same as those classified earlier with the aberrated HST, but the rest below it are new objects seen now for the first time thanks to the substantial improvement in sensitivity. The new objects form a well-defined main sequence whose luminosity function turns over dramatically at m_(346) approx= 19 well before the completeness limit and well before the end of the slowly increasing luminosity function for the outer fields measured from the ground by Hesser et al. We interpret this premature drop in the number of stars below ～0.8 solar mass in the core as the effect of mass segregation due to two-body relaxation. The position of nine objects in the range 20 < m_(346) < 24 and -1.5 < m_(253_) — m_(346) < 0 is consistent with the expected location of a ～0.5 solar mass white dwarf cooling sequence and luminosity function.