CO emission tracing a warp or radial flow within ≲100 au in the HD 100546 protoplanetary disk

¹ School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
e-mail: c.walsh1@leeds.ac.uk
² Leiden Observatory, Leiden University, PO Box 9531, 2300 RA Leiden, The Netherlands
³ Astronomy Department, Wesleyan University, 96 Foss Hill Drive, Middletown, CT 06459, USA
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbackstrasse 1, 85748 Garching, Germany
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 7 June 2017
Accepted: 28 September 2017

Abstract

We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) images of ¹²CO J = 3−2 emission from the protoplanetary disk around the Herbig Ae star, HD 100546. We expand upon earlier analyses of this data and model the spatially-resolved kinematic structure of the CO emission. Assuming a velocity profile which prescribes a flat or flared emitting surface in Keplerian rotation, we uncover significant residuals with a peak of ≈7δv, where δv = 0.21 km s^-1 is the width of a single spectral resolution element. The shape and extent of the residuals reveal the possible presence of a severely warped and twisted inner disk extending to at most 100 au. Adapting the model to include a misaligned inner gas disk with (i) an inclination almost edge-on to the line of sight, and (ii) a position angle almost orthogonal to that of the outer disk reduces the residuals to <3δv. However, these findings are contrasted by recent VLT/SPHERE, MagAO/GPI, and VLTI/PIONIER observations of HD 100546 that show no evidence of a severely misaligned inner dust disk down to spatial scales of ~ 1 au. An alternative explanation for the observed kinematics are fast radial flows mediated by (proto)planets. Inclusion of a radial velocity component at close to free-fall speeds and inwards of ≈50 au results in residuals of ≈4δv. Hence, the model including a radial velocity component only does not reproduce the data as well as that including a twisted and misaligned inner gas disk. Molecular emission data at a higher spatial resolution (of order 10 au) are required to further constrain the kinematics within ≲100 au. HD 100546 joins several other protoplanetary disks for which high spectral resolution molecular emission shows that the gas velocity structure cannot be described by a purely Keplerian velocity profile with a universal inclination and position angle. Regardless of the process, the most likely cause is the presence of an unseen planetary companion.