The two-dimensional pressure structure of the HD 163296 protoplanetary disk as probed by multi-molecule kinematics

¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, Milano 20133, Italy
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands

^★ Corresponding author; viviana.pezzotta@unimi.it

Received: 29 June 2024
Accepted: 6 January 2025

Abstract

Gas kinematics is a new and unique way to study planet-forming environments by an accurate characterization of disk velocity fields. High angular resolution ALMA observations allow deep kinematical analysis of disks, by observing molecular line emission at high spectral resolution. In particular, rotation curves are key tools for studying the disk pressure structure and efficiently estimating fundamental disk parameters, such as mass and radius. In this work we explore the potential of a multi-molecule approach to gas kinematics to provide a 2D characterization of the HD 163296 disk. From the high quality data of the MAPS Large Program we extracted the rotation curves of rotational lines from seven distinct molecular species (¹²CO, ¹³CO, C¹⁸O, HCN, H₂CO, HCO⁺, C₂H), spanning a wide range in the disk radial and vertical extents. To obtain reliable rotation curves for the HCN and C₂H hyperfine lines, we extended standard methodologies to fit multi-component line profiles. We then sampled the likelihood of a thermally stratified model that reproduces all the rotation curves simultaneously, taking into account the molecular emitting layers ɀ(R) and disk thermal structure T(R, ɀ). From this exploration, we obtained dynamical estimates of three fundamental parameters: the stellar mass M_⋆ = 1.89 M_⊙, the disk mass M_d = 0.12 M_⊙, and the scale radius R_c = 143 au. We also explore how rotation curves, and consequently the parameter estimates, depend on the adopted emitting layers: the disk mass proves to be the most affected by these systematics, yet the main trends we find do not depend on the adopted parameterization. Finally, we investigated the impact of thermal structure on gas kinematics, and show that the thermal stratification can efficiently explain the measured rotation velocity discrepancies between tracers at different heights. Our results show that such a multi-molecule approach, tracing a large range of emission layers, can provide unique constraints on the (R, ɀ) pressure structure of protoplanetary disks.