Shadows cast by precessing annuli as evidence for an inclined planet in HD 139614

Abstract

We analyse scattered-light images at multiple epochs of the protoplanetary disc HD139614 taken with GPI and SPHERE instruments. We show that the azimuthal distribution of the surface brightness of the inner bright ring ($13-21$au) varies with time (likely caused by a structure in the inner-disc at $\sim1$au) and constrain the timescale of this variation to be $<11$months. We show that inner arc ($31-44$au) is likely a spiral and tightly wound, measuring a pitch angle of $6.1\pm1.2^\circ$. Additionally, we construct a physical model to explain the broad shadow across $\sim2/3$ of the outer disc and the azimuthally asymmetric bright ring (which has a shadow not aligned with the broad shadow). We model the disc using 1D warp equations with a fast radiative transfer code to produce scattered-light images. We setup the disc as three annuli separated by gaps with an inclined perturbing planet between the inner two annuli. The planet causes the inner annuli to tilt and precess independently casting broad shadows across the outer (un-tilted) annulus. These shadows combine to produce a shadow across $\sim2/3$ of the outer disc and an asymmetric bright ring which matches the observations of HD139614. The precession of the annuli is periodic (so the model matches the observations multiple times) with long precession time-scales ($8.4\times10^3$ and $7.2\times10^4$years for the inner and middle annulus respectively) so the shadows are effectively static over accessible observational time-scales.