Hubble Space Telescope Ultraviolet Observations of the Cores of M3 and M13

We present preliminary results from Hubble Space Telescope (HST)/WFPC2 observations of the central regions of the of the Galactic globular clusters M13 and 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 populations. The M13 HB has a long blue tail extending 4.5 mag in V, reaching well below the level of the main-sequence turnoff. M3 has no such feature. M3 and 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 blue stragglers 3 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.

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.