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In the context of the Hubble Space Telescope UV Survey of Galactic GlobularClusters (GCs), we derived high-precision, multi-band photometry to investigatethe multiple stellar populations in the massive and metal-poor GC M 15. Bycreating for red-giant branch (RGB) stars of the cluster a 'chromosome map',which is a pseudo two-colour diagram made with appropriate combination ofF275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads aroundtwo of the three already known stellar populations. These spreads cannot beproduced by photometric errors alone and could hide the existence of (two)additional populations. This discovery increases the complexity of themultiple-population phenomenon in M 15. Our analysis shows that M 15 exhibits afaint sub-giant branch (SGB), which is also detected in colour-magnitudediagrams (CMDs) made with optical magnitudes only. This poorly-populated SGBincludes about 5% of the total number of SGB stars and evolves into a red RGBin the m_F336W vs. m_F336W-m_F814W CMD, suggesting that M 15 belongs to theclass of Type II GCs. We measured the relative number of stars in eachpopulation at various radial distances from the cluster centre, showing thatall of these populations share the same radial distribution within statisticuncertainties. These new findings are discussed in the context of the formationand evolution scenarios of the multiple populations.

monthly notices of the royal astronomical society

The Hubble Space Telescope UV Legacy Survey of Galactic Globular  Clusters - XIV. Multiple stellar populations within M 15 and their radial  distribution