Very Blue Stars and Mass Segregation in the Core of M15

Abstract

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 (∼15% at m₃₄₆ ≃ 21), peaks at the turnoff and then drops continuously all the way to the detection limit at m₃₄₆ ≃ 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 at 4'.6 or ∼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 ∼7" to ∼1" from the center, but the presence of a small core of r_c ≃ 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.