Hst Uv Observations of the Cores of M3 and M13

We present preliminary results from HST/WFPC2 observations of the central regions of the of the Galactic globular clusters M13 & M3. The clusters are almost identical in most respects including chemical composition, but there are dramatic differences in both the horizontal branch (HB) and blue straggler (BSS) populations. The M13 HB has a long blue tail extending 4.5 mag in V, reaching well below the level of the main sequence turn-off. M3 has no such feature. M3 & M13 are thus an extreme case of the “second parameter problem” in HB morphology. Also present in the M13 HB are two gaps similar to those seen in the clusters NGC 6752 and NGC 2808. M3 has a specific frequency of BSS three times larger than that of M13. Our results imply that neither age nor cluster density, two popular second parameter candidates, are likely to be responsible for the observed differences. Subject headings: globular clusters: individual(M3,M13)—stars: horizontal-branch— ultraviolet: stars—stars: evolution Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555 Osservatorio Astronomico di Bologna, via Zamboni 33, I-40126 Bologna, ITALY Stazione Astronomica di Cagliari, 09012 Capoterra, ITALY Space Telescope Science Institute, Baltimore, USA Astronomy Dept, University of Virginia, P.O.Box 3818, Charlottesville, VA 22903-0818 Laboratory for Astronomy & Solar Physics, Code 681, NASA/GSFC, Greenbelt MD 20771