Abstract

UBV CCD photometry for 7217 stars in the loose Galactic globular cluster IC 4499 is presented. The new colour-magnitude diagram (CMD) reaches 2mag below the main-sequence turn-off (TO), yielding the first view of the subgiant branch and the upper main sequence. Beyond confirming the already known overall morphology of the horizontal and giant branches, these new data lead to the following results. (i) The cluster interstellar reddening, independently obtained from the two-colour diagram, is E(B-V)=0.25+/-0.05. (ii) The mean metallicity derived via different photometric observables measured in the CMD turns out to be [Fe/H]=-1.75+/-0.20, about 0.2-0.3 dex lower than that obtained via spectroscopy. (iii) 64 candidate blue stragglers (BSS) have been identified. They are more concentrated than the subgiant stars of similar brightness (at <~3sigma). (iv) Coupling the apparent luminosity of the horizontal branch, V_HB=17.65+/-0.04, with V_TO=20.90+/-0.10 [and (B-V)_TO=0.657+/-0.003], one gets DeltaV^TOHB=3.25+/-0.12, a value much lower than 3.55+/-0.09 found by Buonanno, Corsi & Fusi Pecci for 18 well-studied clusters. With the adopted metallicity, this suggests that IC 4499 is younger (by 3-4 Gyr) than most globular clusters of similar metallicity. This conclusion is further supported by differential comparisons of the main branches in the CMDs of a wide set of reference clusters. (v) Together with Pal 12, Ruprecht 106, Arp 2 and Terzan 7, this is the fifth Galactic globular cluster showing significant evidence for large age differences in the medium-far Galactic halo.