Direct mapping of the temperature and velocity gradients in discs. Imaging the vertical CO snow line around IM Lupi

TL;DR

This paper introduces an empirical method to directly measure the vertical and radial temperature and velocity gradients in protoplanetary discs using high-resolution ALMA data, revealing detailed disc structure and the CO snow line.

Contribution

It presents a model-independent framework for mapping the emitting surfaces of CO isotopologues in discs, providing direct measurements of physical gradients and the CO snow line location.

Findings

The CO snow line is at about one gas scale height with a temperature of 21±2 K.

Outer disc regions show sub-Keplerian velocities, indicating pressure gradients.

The measured temperature above the snow line drops to approximately 15 K in outer regions.

Abstract

Accurate measurements of the physical structure of protoplanetary discs are critical inputs for planet formation models. These constraints are traditionally established via complex modelling of continuum and line observations. Instead, we present an empirical framework to locate the CO isotopologue emitting surfaces from high spectral and spatial resolution ALMA observations. We apply this framework to the disc surrounding IM Lupi, where we report the first direct, i.e. model independent, measurements of the radial and vertical gradients of temperature and velocity in a protoplanetary disc. The measured disc structure is consistent with an irradiated self-similar disc structure, where the temperature increases and the velocity decreases towards the disc surface. We also directly map the vertical CO snow line, which is located at about one gas scale height at radii between 150 and 300 au, with a CO freeze-out temperature of $21\pm2$ K. In the outer disc ($> 300$ au), where the gas surface density transitions from a power law to an exponential taper, the velocity rotation field becomes significantly sub-Keplerian, in agreement with the expected steeper pressure gradient. The sub-Keplerian velocities should result in a very efficient inward migration of large dust grains, explaining the lack of millimetre continuum emission outside of 300 au. The sub-Keplerian motions may also be the signature of the base of an externally irradiated photo-evaporative wind. In the same outer region, the measured CO temperature above the snow line decreases to $\approx$ 15 K because of the reduced gas density, which can result in a lower CO freeze-out temperature, photo-desorption, or deviations from local thermodynamic equilibrium.