Color–Magnitude Diagram of NGC 205 and Its RR Lyrae Variables

Abstract We present the deepest color–magnitude diagram (CMD) for NGC 205 in the F606W (≈ V ) and F814W (≈ I ) filters. Our archival Hubble Space Telescope/Advanced Camera for Surveys field is centered ∼3 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:mover> </mml:math> 8 northwest of the galaxy's center. The CMD shows the canonical features of a predominantly intermediate-to-old age stellar population. Among other features, the CMD reveals a prominent red clump as well as a blue horizontal branch. Notably absent is any significant signature of a young main sequence typically produced by a stellar population with an age less than a few Gyr. From the magnitude of the red-giant-branch tip, we derive a distance modulus of ( m − M ) o = 24.66 ± 0.05. When compared with α -enhanced theoretical isochrones, the color distribution of the red giant branch stars yields a peak metallicity value of [Fe/H] = −0.51 ± 0.10. Our photometry also reveals a sample of 479 RR Lyrae variable stars (443 ab-type and 36 c-type). Utilizing equations that relate the periods and amplitudes to the metallicities of these stars, we find a mean 〈[Fe/H]〉 = −1.45 ± 0.02 on the Zinn &amp; West scale for the purely old stellar population in NGC 205. The quoted uncertainty represents the standard error of the mean abundance value is consistent with that of the six genuine old globular clusters in NGC 205.