IR photometry of the symbiotic star BF Cyg: Detection of the red giant’s ellipsoidal brightness variability

Abstract

We present the results of our IR photometric observations of the classical symbiotic star BF Cyg acquired in 1978–2003. The variability range in the J and K bands was ∼0.2^m. A periodic component in the cool star’s brightness variations is clearly visible, its period being half the orbital one and its J amplitude being ∼0.15^m. This component is associated with the ellipsoidal shape of the red giant, which model calculations show fills its Roche lobe. This is required in order to reproduce ellipsoidal brightness variability with such a large amplitude: the calculated amplitude for a red giant filling 90% of its Roche lobe is half the observed value. At the same time, it was not possible to confidently chose the optimum component-mass ratio, q = M _giant /M_hot, and orbital inclination, i, from possible values in the ranges q = 2–4, i = 70°–90°. Including the contribution from the hot radiation sources (the hot component and ionized envelope), which vary with a period equal to the orbital period, has a considerable influence on the estimated parameters associated with the red giant’s ellipsoidal brightness variations, and this contribution cannot be neglected. The deviations of the observed from the calculated light curve are irregular, with the rms deviation being σ(O-C) ≈ 0.04^m.