The Luminosity Function and Radial Profile of the Stellar Population in the Core of 47 Tucanae

Previous abstract Next abstract

Session 104 -- Globilar Clusters
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[104.17] The Luminosity Function and Radial Profile of the Stellar Population in the Core of 47 Tucanae

F. Paresce, G. De Marchi, R. Jedrzejewski, R. Gilliland (STScI), M.G. Stratta (Univ. of Rome)

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 narrow band F253M and F346M filters centered at 253~nm and 346~nm. A total of 511 stars down to the detection limit at $m_{346} \simeq 24$ in the $7^{\prime\prime} \times 7^{\prime\prime} $ field were accurately placed on a $m_{346}$ vs $m_{253}-m_{346}$ color magnitude diagram to characterize the stellar populations in the core. Approximately 100 objects above and to the right of the main sequence turn-off 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 $m_{346} \simeq 20$ 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. (1987). We interpret this premature drop in the number of stars below $\sim 0.7$ M$_\odot$ in the core as the effect of mass segregation due to two body relaxation. The position of 9 objects in the range $20< m_{346}<24$ and $-1.50$ is consistent with the expected location of a $\sim 0.5$ M$_\odot$ white dwarf cooling sequence and luminosity function. We have also analyzed very deep frames of the core of 47 Tuc obtained with the WFPC1 before refurbishment in the U band to study with the highest possible accuracy the radial profile of stellar density around the geometrical center of the cluster. This data set fully confirms and extends further the results published by Calzetti et al. (1993) that showed that the radial density profile of 47 Tuc is not consistent with a King model of core radius $25^{\prime\prime}$ extending all the way to the center but requires a central density enhancement of radius $\sim 8^{\prime\prime} = 0.02$ parsec superimposed on the former. This result provides fresh evidence that this cluster may have suffered substantially more dynamical evolution than expected heretofore.

Thursday program listing