Plaskett's Star: an examination of the TESS photometric data

HD 47129 (“Plaskett's Star”) is an O+O spectroscopic binary reported by Linder et al to have a short orbital period of approximately 14.4 d. Grunhut et al. (2013) reported the secondary component of this star to be strongly magnetic and follow up work (Grunhut et al., in prep.) determined the secondary star's rotational period to be 1.21551 d through magnetic modulation. This system is particularly significant because it is the only known close binary containing a magnetic O-type star. Furthermore, the rapid rotation of the magnetic star suggests there exists some mechanism that has rejuvenated the star's angular momentum. It has been proposed that past mass transfer may be responsible for this, raising questions about mass transfer as the origin of its magnetic field and suggesting a connection to stellar mergers as the origin for some magnetic fields.

The TESS satellite observed this system from Dec 15, 2018 to Jan 6, 2019, providing new high-cadence, high-precision photometry for this system. To analyze this new data, we have developed a robust prewhitening method employing a two-stage routine for simultaneously fitting many frequencies. In addition to the new TESS data, we examined the archival CoRoT photometry (first analyzed by Mahy et al. 2011). These datasets are both roughly 1 month in length with comparable precision and were acquired 10 years apart. We analyzed both datasets independently as well as in tandem.

Both datasets reveal the presence of strong modulation consistent with the rotation period of the magnetic star and its first harmonic as well as the weaker presence of several higher harmonics. The CoRoT dataset also exhibits modulation at the orbital period of the system and its harmonic. Individual analysis yields strong variability at 0.623, 0.286, 0.125, and 0.696 c/d for the TESS timeseries and 0.369, 0.648, and 0.146 c/d for the CoRoT timeseries. Finally, a combined analysis of the datasets demonstrates that the principal modulation at 1.645 c/d is stable over the 10 years separating the datasets.

Ongoing work involves understanding the origin of the non-rotational non-orbital frequencies, evaluating their stability, and leveraging these results to better understand the properties and evolutionary history of the system