The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy

Abstract

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.