The globular cluster ω Centauri and the Oosterhoff dichotomy

CCD observations obtained by the OGLE team for 128 RR Lyrae variables in omega Centauri have been analyzed. The period-luminosity and period-amplitude plots indicate that, in addition to fundamental (RRab) and first overtone (RRc) pulsators, the omega Centauri RR Lyrae population seems to include second overtone (RRe) and possibly third-overtone pulsators. The mean period for the 59 RRab stars is 0(d).649, for the 48 RRc stars, it is 0(d).383, and for the 21 RRe stars, it is 0(d).304. The mean periods derived for the RRab and RRc stars are typical values for an Oosterhoff type II (OoII) cluster. Nevertheless, the period-amplitude plot also shows that some of the RR Lyrae variables have "Oosterhoff type I" (OoI) characteristics. Most of the second-overtone variables exhibit nonradial pulsations similar to those recently detected in some of the RR Lyrae variables in the clusters M55 and M5, in the Galactic bulge, and in the LMC. Relative luminosities derived for the RRc variables from Fourier coefficients correlate with the observed apparent magnitudes. Masses for the RRe stars have been calculated from Fourier coefficients. A comparison of the derived masses for RRc stars in the four OoII dusters omega Cen, M15, M55, and M68 indicates that the masses of the RRc stars in M15 and M68 are almost 0.2 M., greater than those in the other two. Since M15 and M68 have a high frequency of RRd stars among their first-overtone pulsators, while none have been identified in omega Cen or M55, this suggests that the double-mode pulsation phenomenon may be associated with mass. Among the RRc variables in omega Cen, the OoII variables have lower derived masses and higher luminosities than the OoI variables. An application of the period-density law to pairs of OoI and OoII RRab stars selected according to their position in the period-amplitude plot also indicates that the OoII variables in general have lower masses and higher luminosities. These findings support the hypothesis that the RR Lyrae variables in OoII systems are evolved horizontal-branch stars that spend their zero-age horizontal-branch phase on the blue side of the instability strip.