2002ApJ...567..915S

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in the V and I passbands of the recently discovered Local Group dwarf spheroidal galaxy in Cetus. Our color-magnitude diagram extends from above the first-ascent red giant branch (RGB) tip to approximately half a magnitude below the horizontal branch (HB). Given a reddening of E(B-V)=0.03, the magnitude of the RGB tip yields a distance modulus of (m-M)₀=24.46±0.14. After applying the reddening and distance modulus, we have utilized the color distribution of RGB stars to determine a mean metal abundance of [Fe/H]=-1.7 on the Zinn & West scale, with an intrinsic internal abundance dispersion of ∼0.2 dex. An indirect calculation of the HB morphology of Cetus based on the mean dereddened HB color yields (B-R)/(B+V+R)=-0.91±0.09, which represents an HB that is redder than what can be attributed solely to Cetus's metal abundance. Thus, Cetus is affected by the ``second-parameter effect'', in which another parameter besides metallicity is controlling the HB morphology. If we adopt the conventional ``age hypothesis'' explanation for the second-parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic globular clusters at its metallicity.