The stellar population in the core of M15

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Session 50 -- Globular Clusters
Display presentation, Wednesday, 9:20-6:30, Pauley Room

[50.10] The stellar population in the core of M15

G. De Marchi \& F. Paresce (STScI)

The inner core of the globular cluster M15 within $\sim 2^{\prime\prime}$ of the geometrical center has been explored with high resolution images taken through several broad-band UV filters with the Faint Object Camera on board the Hubble Space Telescope. Approximately 210 stars in this region down to a $5 \sigma$ detection limit of m$_{220} = 21.5$ were reliably identified and located on a UV--U color magnitude diagram for the first time. A majority of stars (about $70 \%$ of the total) observed this way lie well above the expected main sequence turn-off of this cluster and below the sparsely populated horizontal branch. The extension of the main sequence above the turn-off separates this population in two roughly equal components situated to the right and left of this line. Most of the former must be classical blue stragglers while the rest belong to a new, as yet unidentified, population of very blue stars. Possibilities include, but are not restricted to, well mixed single stars, subdwarfs, and helium white dwarfs. Similar objects are also found just outside the core out to $\sim 6^{\prime\prime}$ from the center but the brighter, presumably more massive ones, are sharply confined to the core itself. The measured excess of bright blue stars and the relative deficiency of bright red giants in the core are consistent with the blue inward color gradient measured from the ground and imply that dynamical evolution can significantly affect the stellar population in the very dense central regions of a high concentration globular cluster like M15.

Wednesday program listing