The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry inthe V and I passbands of the recently discovered Local Group dwarf spheroidalgalaxy in Cetus. Our color-magnitude diagram extends from above the firstascent red giant branch (RGB) tip to approximately half a magnitude below thehorizontal branch (HB). Adopting a reddening of E(B-V) = 0.03, the magnitude ofthe RGB tip yields a distance modulus of (m-M)o = 24.46 +/- 0.14. Afterapplying the reddening and distance modulus, we have utilized the colordistribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7on 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 themean dereddened HB color yields (B-R)/(B+V+R) = -0.91 +/- 0.09, whichrepresents an HB that is redder than what can be attributed solely to Cetus'metal abundance. As such, Cetus suffers from the `second parameter effect' inwhich another parameter besides metallicity is controlling the HB morphology.If we adopt the conventional `age hypothesis' explanation for the secondparameter effect, then this implies that Cetus is 2-3 Gyr younger than Galacticglobular clusters at its metallicity.Comment: 13 pages, 8 figures, Accepted for publication in the March 10, 2002 Ap