Vol. 609
In section 6. Interstellar and circumstellar matter

Direct mapping of the temperature and velocity gradients in discs

The authors present a new, empirical method for locating the CO-isotopologue emitting surfaces from high spectral and spatial resolution ALMA observations of protoplanetary disks. They have applied this method to the disk surrounding IM Lupi, and present the first model-independent measurements of the radial and vertical gradients of temperature and velocity in a protoplanetary disk. The derived disk structure is consistent with that of an irradiated self-similar disk, in which the temperature increases and the velocity decreases towards the disk surface. The authors have also mapped the vertical CO snow line, which is located at approximately one gas scale-height at radii between 150 and 300 au, with a CO freeze-out temperature of 21 ± 2 K. In the outer disk (>300 au), the velocity rotation field becomes significantly sub-Keplerian, in agreement with the expected steeper pressure gradient. This should result in a very efficient inward migration of large dust grains, explaining the lack of millimeter continuum emission beyond 300 au.