WEAK TURBULENCE IN THE HD 163296 PROTOPLANETARY DISK REVEALED BY ALMA CO OBSERVATIONS

Abstract

Turbulence can transport angular momentum in protoplanetary disks and\ninfluence the growth and evolution of planets. With spatially and spectrally\nresolved molecular emission line measurements provided by (sub)millimeter\ninterferometric observations, it is possible to directly measure non-thermal\nmotions in the disk gas that can be attributed to this turbulence. We report a\nnew constraint on the turbulence in the disk around HD 163296, a nearby young A\nstar, determined from ALMA Science Verification observations of four CO\nemission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions).\nThe different optical depths for these lines permit probes of non-thermal\nline-widths at a range of physical conditions (temperature and density) and\ndepths into the disk interior. We derive stringent limits on the non-thermal\nmotions in the upper layers of the outer disk such that any contribution to the\nline-widths from turbulence is <3% of the local sound speed. These limits are\napproximately an order of magnitude lower than theoretical predictions for\nfull-blown MHD turbulence driven by the magneto-rotational instability,\npotentially suggesting that this mechanism is less efficient in the outer\n(R>30AU) disk than has been previously considered.\n