A study of the brightness variations in the red supergiant BC Cygni

Abstract

The variability of the type C semi-regular (SRC) M3.5 Ia supergiant variable BC Cyg is examined with reference to measurements of its photographic B magnitude derived from 866 archival plates in the Harvard and Sternberg collections as well as eye estimates of its visual V magnitude made by members of the AAVSO. BC Cyg is the brightest member of the young open cluster Berkeley 87, so it has a well established reddening, distance, and age. A discrete Fourier analysis was performed on the BC Cygni light curve, as well as on individual subsets of the data. The analysis was made to study the fundamental periods of variability in this red supergiant variable. BC Cyg exhibits interesting features in its century-long baseline of brightness variations that relate to fundamental mode envelope pulsation as well as evolution: an 0 m .5 increase in <B> over the past century in conjunction with a steady decrease in pulsation period from ~697 d in 1900 to 688 d in 2000. Despite the increase in [left angle bracket] B [right angle bracket], the star's luminosity appears to have decreased over the past century, presumably as a result of stellar evolutionary effects. A detailed examination of a well sampled portion of the star's light curve, data spanning the interval HJD 2442000 to 2449000, by means of non-linear least squares analysis indicates that only one periodicity (686 days) exists in the observations. No secondary peridicity can be detected, to within the constraints of observational uncertainty.