A Spatially Resolved Vertical Temperature Gradient in the HD 163296 Disk

We analyze sensitive, sub-arcsecond resolution ALMA science verification observations of CO emission lines in the protoplanetary disk hosted by the young, isolated Ae star HD 163296. The observed spatial morphology of the ¹²CO J = 3-2 emission line is asymmetric across the major axis of the disk; the ¹²CO J = 2-1 line features a much less pronounced, but similar, asymmetry. The J = 2-1 emission from ¹²CO and its main isotopologues have no resolved spatial asymmetry. We associate this behavior with the direct signature of a vertical temperature gradient and layered molecular structure in the disk. This is demonstrated using both toy models and more sophisticated calculations assuming non-local thermodynamic equilibrium conditions. A model disk structure is developed to reproduce both the distinctive spatial morphology of the ¹²CO J = 3-2 line as well as the J = 2-1 emission from the CO isotopologues assuming relative abundances consistent with the interstellar medium. This model disk structure has τ = 1 emitting surfaces for the ¹²CO emission lines that make an angle of ~15° with respect to the disk midplane. Furthermore, we show that the spatial and spectral sensitivity of these data can distinguish between models that have sub-Keplerian gas velocities due to the vertical extent of the disk and its associated radial pressure gradient (a fractional difference in the bulk gas velocity field of >~ 5%).