RR Lyrae Luminosity Differences between Oosterhoff Group I and II Cluster Systems and the Origin of the Oosterhoff Dichotomy

Abstract

We present a comparative study of the Oosterhoff II cluster M2 and the Oosterhoff I cluster M3. Both have similar metallicities, [Fe/H] = -1.62 for M2 and -1.66 for M3, but very different horizontal-branch (HB) morphologies (B-R)/(B + V + R) = 0.92 for M2 and 0.08 for M3. A period shift analysis and main-sequence fitting show that RRab variables in M2 are about 0.2 mag brighter than those in M3. Comparisons of the M2 period shift with Oosterhoff I clusters NGC 3201 and NGC 7006 also yield similar results, while a comparison between M2 and the Oosterhoff II cluster NGC 5986 reveals that the RR Lyrae luminosities are very similar. The luminosity difference is thought to be due to the evolutionary effect described in 1990 by Lee, Demarque, & Zinn: the M2 RRab variables have evolved away from the zero-age horizontal branch (ZAHB), while most M3 RRab variables lie near the ZAHB. A comparison of the mean period change rates of two clusters supports this hypothesis. Our relative age estimation using the difference in color between the base of giant branch and turn-off point shows that M2 is about 2 Gyr older than M3. Our result strongly suggests that the Oosterhoff dichotomy is due to age differences between Oosterhoff group I and II. This is consistent with the idea that the global second parameter is age.