The origin and evolution of RR Lyrae stars of high metal abundance

Abstract

Examination of the mean space motion and metal abundance (Fe/H) of the metal-rich RR Lyrae stars, i.e., those with Preston index ..delta..s equal to or less than 2, suggests that they descend from red giants of the old disk population. The probability that an old disk giant will later become an RR Lyrae star is only 1/200 that of its halo-population counterpart. Metal-rich RR Lyraes of the field show a strong preference for slow or nondetectable period changes. One thus thinks it unlikely that the relatively parsimonious production of metal-rich RR Lyraes, per old disk giant progenitor, can be attributed to unusually rapid evolution through the instability strip. Model zero-age horizontal-branch (ZAHB) stars with solar composition (X = 0.70, Y = 0.28, Z = 0.02) and helium core mass M/sub c/ = 0.45 M/sub sun/ were constructed for a mass range 0.49 to 0.60 M/sub sun/, and were evolved through the stage of core helium burning. Initial evolution of all models proceeded rapidly blueward along the ZAHB, the slowest part of all tracks being located at the ''blue hooks,'' beyond which the relative inactivity of the hydrogen shell source leads to contraction of the core and expansion of the envelope.more » Evolutionary time scales near the ''hooks'' were found to agree with those required by the slow period changes of metal-rich RR Lyraes. The models suggest that metal-rich RR Lyraes can be mapped from old disk giants only if the latter lose about 0.5 M/sub sun/ and lie in a very limited mass range (..delta..M approximately equal to 0.02 M/sub sun/).« less