Millimeter Dust Emission and Planetary Dynamics in the HD 106906 System

HD 106906 is is a short-period stellar binary, host to a planet, HD 106906b, at a separation of 737 au, and a debris disk extending to ~100 au, which has been imaged at optical and radio wavelengths. Only a few known systems host a debris disk and a directly imaged planet, and it is particularly rare for the planet to be located outside the debris belt. The debris disk is edge on and highly asymmetric at optical wavelengths. We resolve the structure in the disk at a wavelength of 1.3 mm with the Atacama Large Millimeter/submilliter Array (ALMA) at a resolution of 0.”31 (30 au). Our observations support a disk that is symmetric in surface brightness at this wavelength, with moderate evidence for a stellar offset. To constrain HD 106906b’s orbit, we use dynamical theory to compare astrometric constraints on the orbit (Ngyuen et al. 2021) with scattered light morphology and the disk morphology we observed. The apparent stellar offset in the image plane might indicate an eccentric disk, although a visibility domain analysis shows that the offset is not statistically significant. Furthermore, a truly eccentric disk would result in asymmetrical surface brightness as well, which we do not observe. A planet with an orbit sufficiently eccentric to induce disk eccentricity should also induce inclination in the disk material, resulting in a vertically puffy or warped disk (Nesvold et al. 2017), which is not consistent with recent GPI imaging that shows a vertically thin disk (Crotts et al. 2021). In order to confirm our application of dynamical theory, we simulated disk-planet interactions for a selection of planet orbital parameters drawn from the astrometric constraints using an N-body code, REBOUND (Rein et al. 2012). Comparisons of simulated disks with ALMA observations prefer an unperturbed disk over disks with induced eccentricity within the parameter space allowed by the astrometric constraints on the planet’s orbit. The observed stellar offset can be explained with a circular disk, as the high mass and separation of HD 106906b would cause some offset between the center of mass of the system and the location of the stellar binary.