Modeling the Quasiperiodic Radial Velocity Variations of γ Draconis

Abstract

Abstract γ Draconis, a K5III star, showed radial velocity (RV) variations consistent with a 10.7 Jupiter mass planet from 2003 to 2011. After 2011, the periodic signal decayed, then reappeared with a phase shift. Hatzes et al. suggested that γ Dra’s RV variations could come from oscillatory convective modes, but did not fit a mathematical model. Here we assess whether a quasi-periodic Gaussian process—appropriate when spots with finite lifetimes trace underlying periodicity—can explain the RVs. We find that a model with only one quasiperiodic signal is not adequate: we require a second component to fit the data. The best-fit model has quasi-periodic oscillations with P 1 = 705 days and P 2 = 15 days. The 705 day signal may be caused by magnetic activity. The 15 day period requires further investigation.