RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy

We have surveyed four of the globular clusters in the Fornax dwarf galaxy (clusters 1, 2, 3 and 5) for RR Lyrae stars, using archival F555W and F814W Hubble Space Telescope observations. We identify 197 new RR Lyrae stars in these four clusters, and 13 additional candidate horizontal-branch variable stars. Although somewhat restricted by our short observational baseline, we derive periods and light curves for all of the stars in the sample, and calculate photometric parameters such as mean magnitudes and colours. This is the first time that RR Lyrae stars in the Fornax globular clusters have been quantitatively identified and measured. We find that the Fornax clusters have exceptionally large specific frequencies of RR Lyrae stars, in comparison with the galactic globular clusters. It is likely that Fornax 1 has the largest specific frequency measured in any globular cluster. In addition, the Fornax clusters are unusual in that their RR Lyrae populations possess mean characteristics intermediate between the two Oosterhoff groups defined by the galactic globular clusters. In this respect the RR Lyrae populations in the Fornax clusters most closely resemble the field populations in several dwarf galaxies. Fornax 5 has an unusually large fraction of RRc stars, and also possesses several strong RRe (second overtone pulsator) candidates.

With a large sample of horizontal-branch variable stars available to us, we revise previous measurements of the horizontal-branch morphology in each cluster. The Fornax clusters most closely resemble the `young' galactic halo population defined by Zinn in that their horizontal-branch morphologies are systematically redder than many galactic clusters of similar metallicity. We also confirm the existence of the second-parameter effect among the Fornax clusters, most markedly between clusters 1 and 3. The edges of the instability strip are well defined in several of the Fornax clusters, and we are able to make measurements of the intrinsic V-I colours of these edges. Finally, we determine foreground reddening and distance estimates for each cluster. We find a mean distance modulus to the Fornax dwarf of (m-M)₀= 20.66 +/- 0.03 (random) +/-0.15 (systematic). Our measurements are consistent with a line-of-sight depth of ~8-10 kpc for this galaxy, which is in accordance with its dimensions as measured in the plane of the sky. This approximately spherical shape for Fornax is incompatible with tidal model explanations for the observed high internal stellar velocity dispersions in many dwarf spheroidal galaxies. Dark matter dominance is suggested.