AAVSO: American Association of Variable Star Observers

BVRI Photometry of CX Cephei (WR 151) (Abstract)

Volume 36 number 2 (2008)

Kate Hutton
Arne A. Henden
Michael Koppelman


(Abstract only) The CX Cephei system is a double-line spectroscopic, eclipsing binary, consisting of an O5V and a WN5 (Woft-Rayet star) component. It has the second shortest known period (2.12691d) among O + WR binaries. The primary eclipse is shallow (approx. 0.1 magnitude) and the secondary eclipse even shallower. There is other variability also, accounting for a total range of approx. 12.0 to 12.2 in Lipunova and Cherepashchuk 1982 (Sov. Astron. 26, 45–53) published photometry data from the 1980’s. Lewis et al. 1996 (ApJ 405, 312–326) published radial velocity data, showing that the WR star is in front at primary minimum. To this we add 340+ BVRI points over two years from the Sonoita Research Observatory (SRO), plus assorted time series from Sonoita (by HQA) and from Starhouse Observatory (by KMP). From our observations, we were able to, 1) refine the period given by Lipunova and Cherepashchuk, and show that, 2) there is intrinsic variability in addition to the eclipsing binary light curve, 3) that, unlike the 1980s, the secondary eclipse is now barely detectable, 4) there may or may not be additional “structure” in the eclipsing light curve, 5) and that the light curve varies with color. We see that the minima are pointed (eclipse not total), that the minima have a distinct beginning and end, although there are “shoulders” (ellipticity is important, but sky is seen between the stars at quadrature), that the eclipses are very shallow (low i, barely eclipsing). There is a dimward slope between phase 0.2 and 0.8, ranging from 2.5% in flux in B and V, to less than 1% in I. It seems unlikely that the WR core is substantially cooler than even an O5, so the primary minimum is expected to be at least partly an “atmospheric” eclipse caused by the WR wind (as Lipunova and Cherepashchuk modeled it), rather than the star itself. However, the primary minimum is one of the most stable features with time, so the wind opacity and configuration must not be responsible for observed changes. Changes in the secondary minima must be due to changes in what the O5 is eclipsing. We have not yet modeled all this!