Does Collinder 236 host a Cepheid calibrator?

David G. Turner$^1$, D. Forbes$^2$, P. J. T. Leonard$^3$, Mohamed Abdel-Sabour Abdel-Latif$^4$, Daniel J. Majaess$^1$, Leonid N. Berdnikov$^5$

1 - Department of Astronomy and Physics, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
2 - Department of Physics, Sir Wilfred Grenfell College, Memorial University, Corner Brook, Newfoundland A2H 6P9, Canada
3 - ADNET Systems, Inc., 7515 Mission Dr., Suite A100, Lanham, Maryland 20706, U.S.A.
4 - National Research Institute of Astronomy and Geophysics (NRIAG), Box 11242, Helwan, Cairo, Egypt
5 - Sternberg Astronomical Institute, 13 Universitetskij prosp., Moscow 119992, Russia

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.

Reference: MNRAS
Status: Manuscript has been accepted