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 ) .
Adopting a reddening of E ( B - V ) = 0.03 , the magnitude of the RGB tip yields a distance modulus of ( m - M ) _ { 0 } = 24.46 \pm 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 \sim 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 \pm 0.09 , which represents an HB that is redder than what can be attributed solely to Cetus ’ metal abundance .
As such , Cetus suffers from 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 .