AQ Col (EC~05217-3914) is one of the first detected pulsating subdwarf B (sdB) stars and has been considered to be a single star. However, its periodic pulsation timing variations indicate that AQ Col may not be a single star. We present pulsation period variations observed over twenty-four years and derived orbital characteristics these would imply if these were a consequence of AQ Col being a pulsating hot subdwarf in a long-period binary. The derived orbital period is P = 486.0 days. In the sdB star binary evolution scenario, a Roche lobe overflow channel results in long period (450 < P < 1400 d) for sdB + Main Sequence (MS) binaries. However the derived orbital eccentricity of the system is 0.424, which is too large for a typical long period sdB+MS system. The Skymapper u – z vs. z – WISE W1 diagram is incompatible with sdB+MS binary systems, and suggests the system contains a white dwarf or other hot and faint object. The expected radial velocity amplitude of AQ Col due to this orbital motion is ~15 km/s. However, the radial velocity amplitude differences obtained from spectroscopy show that the amplitude could be more than ~300 km/s, which indicates the possibility that AQ Col also has a short period companion with orbital period of ~1 day. Therefore, the AQ Col system may be a triple star system. Because such systems have not yet been studied in detail, AQ Col may offer unique insight into the production of sdB stars and this system deserves continued time-series and spectroscopic monitoring.
Reference:
Orbital Characteristics of the AQ~Col (EC~05217-3914) System, T. Otani (1), A. E. Lynas-Gray (2 and 3, 4), D. Kilkenny (4), C. Koen (5), T. von Hippel (1), M. Uzundag (6), M. Vuckovic (6), C. M. Pennock (7), R. Silvotti (8) ((1) Department of Physical Sciences and SARA, Embry-Riddle Aeronautical University, (2) Department of Physics and Astronomy, University College London, (3) Department of Physics, University of Oxford, (4) Department of Physics and Astronomy, University of the Western Cape, (5) Department of Statistics, University of the Western Cape, (6) Instituto de Fisica y Astronomia, Universidad de Valparaiso, (7) Lennard-Jones Laboratories, Keele University, (8) INAF-Osservatorio Astrofisico di Torino), submitted to ApJ, arXiv:2109.05394