Activity as the Main Explanation of Light Variations of o And — Towards a Rotational Modulation Model

Extract

Observations carried out for 5 years on o And show that 65 to 85% of its light variations can be described by a double wave: A sin (2πt/P₁ + ϕ₁) + B sin (2πt/P₂ + ϕ₂) + C with P₁ ≈ 1.6 d = 2P₂. When determined independently, P₁ and P₂ are always found in a 2:1 ratio (within 1%), while they can vary together by as much as 4%. The peak to peak amplitudes of this double-wave fit lay between 40 and 140 mmag (and can even be reduced to less than 10 mmag - our 1987 observations). The rest of the light variations do not show any permanent period or behavior, although a ∼ 2.3 d. (i.e. ≈ 3P₁/2) period is frequently detected. Sometimes a marginal ∼ 6 d. period or time constant has been detected.

In spite of the quality of our photometric data, the precision on the periods and amplitudes obtained over a few nights is never increased by longer observations: our phase diagrams show significant irregular displacements around the average double-wave analytical solution if we include longer data strings (Fig. 1). This phenomenon was already apparent in our 1992 study (Sareyan et al., 1998): the star shows real irregular behaviour superimposed onto its double-wave “mean” light curve; these changes may show up as a progressive, or sometimes abrupt, modification of the shape of the double-wave light curve (Fig. 1).