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

Abstract

We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) images of 12 CO 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 spatiallyresolved 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.

Asymmetric mid-plane gas in ALMA images of HD 100546

“…These values are consistent with most reported position angles from previous sub-mm observations of the disc with ALMA; Walsh et al (2014) Panić et al (2014) observed the disc with the MID-infrared Interferometric instrument (MIDI) on the VLT and find a position angle of 145±5 • , their inclination is considerably higher however because their observations probe the inner wall of the disc at 11 au. There is growing evidence for a warp in the disc (Quillen 2006;Panić et al 2010;Walsh et al 2017) which would cause inclination to deviate radially.…”

“…For example there has been recent evidence of warps from ALMA observations, e.g. this disc (Walsh et al 2017) and T Tauri star AA Tau (Loomis et al 2017). A warp of the inner disc can lead not only to shadowing of the outer disc but potentially even a break of the disc into two distinct planes.…”

Scattered light shadows in warped protoplanetary discs

“…The inner disc casts unique shadows on the outer disc, constraining the relative inclination between the two (Min et al 2017). Relative inclinations have been measured from 30 • for DoAR 44 (Casassus et al 2018) to 45 • for AA Tau (Loomis et al 2017), 70 • for HD 142527 (Marino et al 2015), 72 • for HD 100453 Benisty et al (2017) and up to 80 • for HD 100546 (Walsh et al 2017). Numerical simulations suggested that such an arrangement may be generated through gravitational interaction with an inclined binary.…”

Rocking shadows in broken circumbinary discs

“…Narrow lane shadows have been observed in scattered light observations of a number of protoplanetary discs (Marino et al 2015;Walsh et al 2017;Benisty et al 2018;van der Plas et al 2019). These indicate a large (∼ 70 − 90 • relative misalignment between the inner and outer disc (Marino et al 2015;Walsh et al 2017;Pinilla et al 2018).…”