Improved model of the triple system V746 Cassiopeiae that has a bipolar magnetic field associated with the tertiary ^⋆

¹ Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
e-mail: hec@sirrah.troja.mff.cuni.cz
² Astronomical Institute, Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic
³ Department of Physics and Astronomy, The College of Charleston, Charleston, SC 29424, USA
⁴ University of Ege, Department of Astronomy & Space Sciences, 35 100 Bornova – İzmir, Turkey
⁵ Institute of Physics, Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Praha 8, Czech Republic

Received: 22 February 2016
Accepted: 30 August 2017

Abstract

V746 Cas is known to be a triple system composed of a close binary with an alternatively reported period of either 254 or 278 and a distant third component in a 170 yr (62 000 d) orbit. The object was also reported to exhibit multiperiodic light variations with periods from 083 to 250, on the basis of which it was classified as a slowly pulsating B star. Interest in further investigation of this system was raised by the recent detection of a variable magnetic field. Analysing spectra from four instruments, earlier published radial velocities, and several sets of photometric observations, we arrived at the following conclusions: (1) The optical spectrum is dominated by the lines of the B-type primary (T_{eff 1} ~ 16 500(100) K), contributing 70% of the light in the optical region, and a slightly cooler B tertiary (T_{eff 3} ~ 13 620(150) K). The lines of the low-mass secondary are below our detection threshold; we estimate that it could be a normal A or F star. (2) We resolved the ambiguity in the value of the inner binary period and arrived at a linear ephemeris of . (3) The intensity of the magnetic field undergoes a sinusoidal variation in phase with one of the known photometric periods, namely 2503867(19), which we identify with the rotational period of the tertiary. (4) The second dominant photometric 10649524(40) period is tentatively identified with the rotational period of the broad-lined B-type primary, but this interpretation is much less certain and needs further verification. (5) If our interpretation of photometric periods is confirmed, the classification of the object as a slowly pulsating B star should be revised. (6) Applying an N-body model to different types of available observational data, we can constrain the orbital inclination of the inner orbit to ~60°<i₁< 85° even in the absence of binary eclipses, and we estimate the probable properties of the triple system and its components.