90 Years of Orbital Timing Measurements of KIC 9832227

The interpretation of time variations of the 0.46 d orbital period of the contact binary star system KIC 9832227 have been the subject of recent research. Molnar et al. (2017, ApJ, 840, 1) presented photometry and spectroscopy spanning 1999-2016 showing a period derivative that was negative and increasing in magnitude. They ruled out period changes due to changing surface features or third bodies and suggested the period change may indicate an imminent merger. Socia et al. (2018, ApJL, 864, L32) presented archival timing from 2003 and revised timing from 1999 that together implied a positive period derivative in the time range 1999-2010, effectively ruling out the merger hypothesis. Kovacs et al. (2019, A&A, 631, A126) suggested the timing data may be modeled with a third body in a 14-year period along with a fixed negative period derivative.

We present here a significant extension of the timing data for KIC 9832227. They are extended forward to 2021 with observations from the Calvin Observatory and back to the 1930s using the Harvard DASCH digitized plate stacks. The data show modulation on an approximately 25-year time scale with no long-term period derivative. The modulations are not strictly periodic and are inconsistent with any third body model.

With other interpretations ruled out, we are now investigating whether the timing modulations may be a consequence of variations in magnetic field strength over a starspot cycle, a mechanism developed by Applegate (1992, ApJ, 385, 621). Volschow et al. (2018, A&A, 620, A42) show the relevance of this mechanism has not yet been established in any binary system, so it would be significant if it can be shown in this case. While the theory of stellar magnetic fields cannot be developed from first principles, observation of modulation of the surface magnetic field strength correlated with timing modulation would be strong evidence. Molnar et al. (2017) reported significant spot activity in the Kepler light curves. Subsequent observations have shown changes in the average field strength. The first test of this hypothesis will be if the change in field strength reverses when the sign of the period derivative next flips. This work was supported by NSF grant 1716622.