A Photometric Study of the Outer Halo Globular Cluster NGC 5824

Abstract

Multi-wavelength CCD photometry over 21 years has been used to produce deep color-magnitude diagrams (CMDs) together with light curves for the variables in the Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are identified, bringing the total to 47, of which 42 have reliable periods determined for the first time. The CMD is matched using BaSTI isochrones with ages of 13 Gyr, and reddening is found to be E(B-V)=0.15+/- 0.02; using the period-Wesenheit relation in two colors, the distance modulus is {(m-M)}<SUB>0</SUB>=17.45+/- 0.07 corresponding to a distance of 30.9 Kpc. The observations show no signs of populations that are significantly younger than the 13 Gyr stars. The width of the red giant branch does not allow for a spread in [{Fe}/{{H}}] greater than σ =0.05 {dex}, and there is no photometric evidence for widened or parallel sequences. The V,{c}<SUB>{UBI</SUB>} pseudo-CMD shows a bifurcation of the red giant branch that by analogy with other clusters is interpreted as being due to differing spectral signatures of the first (75%) and second (25%) generations of stars whose age difference is close enough that main-sequence (MS) turnoffs in the CMD are unresolved. The cluster MS is visible against the background out to a radial distance of ∼17 arcmin. We conclude that NGC 5824 appears to be a classical Oosterhoff Type II globular cluster, without overt signs of being a remnant of a now-disrupted dwarf galaxy.