HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF CANDIDATE YOUNG GLOBULAR-CLUSTERS AND STELLAR ASSOCIATIONS IN THE RECENT MERGER REMNANT NGC-3921

New, high-resolution images of NGC 3921 obtained with the Wide Field a nd Planetary Camera 2 of the refurbished HST are presented. NGC 3921 i s the remnant of two merged disk galaxies and thought to be a 0.7+/-0. 3 Gyr old protoelliptical. A search for star clusters has yielded 102 candidate globular clusters (GC) and 49 separate, more extended stella r associations (A) brighter than V=26.0, the estimated completeness li mit (corresponding to M(v)=-8.5 for H-0=75 km s(-1) Mpc(-1)). Most of these objects are newly discovered. Among the best-observed objects [V less than or similar to 25, error sigma(V-I)less than or equal to 0.1 5], the majority are blue and have remarkably uniform colors, with med ian (V-I)(Cousins)=0.65 for GCs and 0.56 for As. The color indices of the GCs yield estimates of 250-750 Myr for the median age, depending o n the adopted metallicity (1.0-0.2 Z.), and suggest a prolonged format ion period (several 100 Myr) roughly in agreement with the estimated d uration of the merger. The most luminous candidate GCs have absolute m agnitudes M(v)=-12 to -13 and are predicted to fade by 3-4 mag over th e next 15 Gyr, whence they will then have luminosities typical of lumi nous old GCs. The median half-light radius of candidate GCs is R(eff)l ess than or similar to 5 pc, compared with a median R(eff)=3 pc for Mi lky Way globulars. Since from their colors most of these clusters appe ar to be dozens of core crossing times old, they must be gravitational ly bound. The luminosity functions of both the GCs and the As are well represented by a single power law, phi(L)dL proportional to L(-2.1+/- 0.3)dL (for M(v) less than or equal to-8.5). Various ents suggest that the progenitors of these objects may have been normal Giant Molecular Clouds rather than Supergiant Molecular Clouds. The projected radial distribution of GCs is strongly centrally concentrated and follows clo sely the V-light profile of the galaxy (similar to r(1/4) law), sugges ting that these clusters or their progenitors experienced the same vio lent relaxation as did the average luminous matter of the two merging galaxies. In contrast, the radial distribution of As appears relativel y flat, We estimate that the total number of GCs in NGC 3921 has incre ased by greater than or similar to 40% during the merger, from about 2 70+/-60 GCs before to greater than or similar to 380 GCs after the mer ger. The inferred specific GC frequency is S-N greater than or similar to 0.7 at present and will, barring further mergers or significant cl uster destruction, increase to S-N greater than or similar to 1.4, 1.8 , and 2.9 over the next 2, 5, and 15 Gyr as the galaxy fades. Hence, w ithin less than or similar to 7 Gyr NGC 3921 may begin to appear as a normal elliptical with a GC population typical for Es in low-density e nvironments. (C) 1996 American Astronomical Society.