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 c lusters M13 and M3. The clusters are almost identical in most respects , including chemical composition, but there are dramatic differences i n both the horizontal-branch (HB) and blue straggler populations. The M13 HE has a long blue tail extending 4.5 mag in V, reaching well belo w the level of the main-sequence turnoff. M3 has no such feature. M3 a nd M13 are thus an extreme case of the ''second-parameter problem'' in HB morphology. Also present in the M13 HE are two gaps similar to tho se seen in the clusters NGC 6752 and NGC 2808. M3 has a specific frequ ency of blue stragglers 3 times larger than that of M13. Our results i mply that neither age nor cluster density, two popular second-paramete r candidates, are likely to be responsible for the observed difference s.