Does Collinder 236 host a Cepheid calibrator?

TL;DR

This study investigates whether the Cepheid WZ Car is part of the open cluster Collinder 236, finding it is not a member due to distance and location discrepancies, but providing detailed cluster and Cepheid properties.

Contribution

The paper provides new UBV photometry and star counts for Collinder 236 and WZ Car, clarifying the cluster's parameters and the Cepheid's non-membership, along with revised period change analysis.

Findings

Collinder 236 is at 1944 pc, half the distance to WZ Car.

WZ Car is not a cluster member based on location and distance.

WZ Car's period is increasing at 8.27 s/yr, indicating a third crossing of the instability strip.

Abstract

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.