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

Abstract

V746Cas is known to be a triple system composed of a close binary with an alternatively reported period of either 25 d .4 or 27 d .8 and a distant third component in a 170 yr (62000 d) orbit. The object was also reported to exhibit multiperiodic light variations with periods from 0 d .83 to 2 d .50, 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 ∼ 16500(100) K), contributing 70 % of the light in the optical region, and a slightly cooler B tertiary (T eff 3 ∼ 13620(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 T super. conj. = HJD 2443838.78(81) + 25 d .41569(42) × E. (3) The intensity of the magnetic field undergoes a sinusoidal variation in phase with one of the known photometric periods, namely 2 d .503867(19), which we identify with the rotational period of the tertiary. (4) The second dominant photometric 1 d .0649524(40) period is tentatively identified with the rotational period of the broad-lined Btype 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 • < i 1 < 85 • even in the absence of binary eclipses, and we estimate the probable properties of the triple system and its components.

ARMADA. II. Further Detections of Inner Companions to Intermediate-mass Binaries with Microarcsecond Astrometry at CHARA and VLTI

“…McSwain et al (2007) obtained updated RV measurements on this spectroscopic binary and settled on a best-fit period of 25.4176 days after combining all data sets. Harmanec et al (2018) acquired additional velocities and determined several spectroscopic solutions that confirmed that the broad-lined component is the primary of the 25-day orbit. We acquired five new RVs of the broad-lined component (v i sin 137 = km s −1 ) with the 2 m AST, which are consistent with the 25-day SPEFO orbit of Harmanec et al (2018, Table 3).…”