Multiwavelength optical observations of chromospherically active binary systems - V. FF UMa (2RE J0933+624): a system with orbital period variation

Abstract

Context. This is the fifth paper in a series aimed at studying the chromospheres of active binary systems using several optical spectroscopic indicators to obtain or improve orbital solution and fundamental stellar parameters. Aims. We present here the study of FF UMa ( 2RE J0933+ 624), a recently discovered, X- ray/ EUV selected, active binary with strong Hα emission. The objectives of this work are, to find orbital solutions and define stellar parameters from precise radial velocities and carry out an extensive study of the optical indicators of chromospheric activity. Methods. We obtained high resolution echelle spectroscopic observations during five observing runs from 1998 to 2004. We found radial velocities by cross correlation with radial velocity standard stars to achieve the best orbital solution. We also measured rotational velocity by cross- correlation techniques and have studied the kinematic by galactic space- velocity components ( U, V, W) and Eggen criteria. Finally, we have determined the chromospheric contribution in optical spectroscopic indicators, from Ca II H & K to Ca II IRT lines, using the spectral subtraction technique. Results. We have found that this system presents an orbital period variation, higher than previously detected in other RS CVn systems. We determined an improved orbital solution, finding a circular orbit with a period of 3.274 days. We derived the stellar parameters, confirming the subgiant nature of the primary component (M_P = 1.67 M_[circled dot] and R sin i_P = 2.17 R_[circled dot]) and obtained rotational velocities (v sin i), of 33.57 +/- 0.45 km s^-1 and 32.38 +/- 0.75 km s^-1 for the primary and secondary components respectively. From our kinematic study, we can deduce its membership to the Castor moving group. Finally, the activity study has given us a better understanding of the possible mechanisms that produce the orbital period variation.