Synchronous rotation in the long-period RS CVn binary BM camelopardalis

Abstract

Variability of the bright (V = 6~25) long-period RS CVn binary 12 Cam, subsequently named BM Cam, was discovered by Eaton et al. (1980). By attributing the variability to rotational modulation of the KO III star (seen alone in the spectrum), its surface darkened unevenly by starspot activity, they concluded that the rotation was synchronized with the orbital motion to within a few percent. The long period of BM Cam, 804174469 ± 04000003 according to Abt. et al. (1969), was then and still is the longest orbital period, among the chromospherically active binaries, in which synchronous rotation has been established. Here we present additional photometry obtained at 17 different observatories by the 16 observers collaborating with us, listed in Table I. Some observed in all three bandpasses of the UBV system but most observed in V only. Observations were obtained on a total of 509 nights, all with HR 1688 as comparison star. The light curve in V is shown in Figure l, where each point is a nightly mean of generally three individual differential measures, each corrected for differential atmospheric extinction and transformed differentially to V of the UBV system with known transformation coefficients. Along with the 1979 and 1980 light curves of Eato~ et al. (1980), we now have coverage for seven years. Inspection of Figure i shows beautifully that the light curve has undergone a gradual but dramatic metamorphosis. At the beginning in 1981 it was roughly sinusoidal with a period around 80 days, as Eaton et al. (1980) had found in 1979 and 1980. The first clear trace of a developing secondary minimum appeared in the 1982 light curve. By 1983 that secondary minimum had matured, making the light curve clearly asy~netric and not at all sinusoidal. In 1984 the light curve would be described best as roughly slnusoidal but with a period around 40 days. Most recently, in 1985, there were still two equally deep minima, separated by about 40 days, but the maxima in between were no longer equally bright. Meanwhile the amplitude, from maximum to minimum, varied from as large as 0~18 in 1981 to as small as 0~04 in 1985. The mean brightness also varied dramatically, from AV = +0~04 in 1981 to AV = -0~04 in 1985. The most reasonable interpretation of this metamorphosis involves two spot groups on a synchronously rotating star, at first situated close together in stellar longitude and acting in phase to produce a photometric period equal to the 80-day rotational period, later separating in longitude and acting out of phase to produre a photometric period equal to half the rotational period. For this reason we compiled a list of times of minimum light. These are given in Table II, where the first two are taken from Eaton et al. (1980), the rest were determined graphically from our 1981-1985 light curve, and the association with the two different spot groups is