Very blue stars and mass segregation in the core of M15

The core of the globular cluster M15 (NGC 7078) has been observed with the COSTAR-corrected Faint Object Camera on board the Hubble Space Telescope (HST), through medium-band ultraviolet filters centered at 255 nm and 348 nm. A total of about 1200 stars are detected in a region of size 7 '' x 7 '' located just west of the cluster center, to form a color-magnitude diagram extending down to magnitude 21.5 at 346 nm, or about 2 mag below the turnoff level, The luminosity function in the core, corrected for a small amount of photometric incompleteness (similar to 15% at m(346) similar or equal to 21), peaks at the turnoff and then drops continuously all the way to the detection limit at m(346) similar or equal to 22. We interpret the observed turndown below the turnoff as due to mass segregation in the cluster core. A comparison with the luminosity function obtained al 416 or similar to 3r(h) from the center indicates rough numerical consistency with the predictions of two-body relaxation. The projected density profile of stars brighter than the turnoff, corrected for differential photometric incompleteness due to crowding, follows a power-law increase with decreasing radial distance from similar to 7 '' to similar to 1 '' from the center, but the presence of a small core of r(c) similar or equal to 1.'' 8 cannot be excluded. It is, therefore, not possible to decide unequivocally whether M15 hosts a black hole in its core, or even if it is going through a dense collapse phase. Finally, the existence of a concentrated population of very blue stars, discovered with the aberrated HST, is confirmed. These objects appear bluer than the blue straggler sequence, with an average temperature of 25,000 K based on their photometry in the medium-band filters used here. Some hypotheses recently put forth to explain their nature are examined here and are found to be compatible with the observed properties, suggesting a dynamical origin for these stars.