Rate of period change $\dot{P}$ for a Cepheid is shown to be a parameter that
is capable of indicating the instability strip crossing mode for individual
objects, and, in conjunction with light amplitude, likely location within the
instability strip. Observed rates of period change in over 200 Milky Way
Cepheids are demonstrated to be in general agreement with predictions from
stellar evolutionary models, although the sample also displays features that
are inconsistent with some published models and indicative of the importance of
additional factors not fully incorporated in models to date.Comment: Published in PASP (March 2006); TeX source & figures now provide
ABSTRACT. The evolutionary changes in pulsation period for the Cepheid Polaris are reinvestigated using archival observational material (radial velocities, photometry, and eye observations) over the interval 1844 to the present, including new photometry for the star obtained in [2003][2004]. The star's pulsation period increased at a rate of 4.5 s yr Ϫ1 during that interval, with the exception of a brief hiatus between 1963 and 1966, when it suddenly decreased, possibly as a result of a brief reduction in average stellar radius amounting to Ϫ0.055%. At roughly the same time, the pulsation amplitude of Polaris underwent a marked change. Prior to 1963 the V amplitude was in excess of about 0.1 mag, possibly decreasing at a rate of 0.019 mag century Ϫ1 . Following the hiatus of 1963-1966, the pulsation amplitude underwent a sharp decline and now appears to be erratic on a cycle-to-cycle basis, always smaller than 0.05 mag. The rapid rate of period increase for Polaris is consistent with a first crossing of the Cepheid instability strip, while the hiatus of 1963-1966 and sudden decrease in pulsation amplitude thereafter suggest that the star may have left the instability strip for first crossers at that time, leaving it near the center of the instability strip for Cepheids in higher crossing modes.
Photoelectric UBV photometry and star counts are presented for the previously
unstudied open cluster Collinder 236, supplemented by observations for stars
near the Cepheid WZ Car. Collinder 236 is typical of groups associated with
Cepheids, with an evolutionary age of (3.4+-1.1)x10^7 years, but it is 1944+-71
pc distant, only half the predicted distance to WZ Car. The cluster is reddened
by E(B-V)~0.26, and has nuclear and coronal radii of rn~2 arcmin (1.1 pc) and
Rc~8 arcmin (4.5 pc), respectively. The Cepheid is not a member of Collinder
236 on the basis of location beyond the cluster tidal radius and implied
distance, but its space reddening can be established as E(B-V)=0.268+-0.006
s.e. from 5 adjacent stars. Period changes in WZ Car studied with the aid of
archival data are revised. The period of WZ Car is increasing, its rate of
+8.27+-0.19 s yr^(-1) being consistent with a third crossing of the instability
strip.Comment: Accepted for publication (MNRAS
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.