[Paper Review] Molecules with ALMA at Planet-forming Scales (MAPS) XIX. Spiral Arms, a Tail, and Diffuse Structures Traced by CO around the GM Aur Disk

Jane Huang, Edwin A. Bergin+29 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation Studies174 references65 citations

TL;DR

This study presents high-resolution ALMA observations of 12CO J=2−1 emission around the GM Aur protoplanetary disk, revealing complex large-scale gas structures—including flaring spiral arms extending to 1200 au, a tail extending 1800 au southwest, and diffuse northern structures up to 1900 au—contrasting sharply with the axisymmetric millimeter continuum rings. These features suggest ongoing late infall of external envelope or cloud material, challenging the assumption of isolated, axisymmetric disk evolution and linking GM Aur’s morphology to environmental interactions seen in other Taurus disks like SU Aur and AB Aur.

ABSTRACT

The concentric gaps and rings commonly observed in protoplanetary disks in millimeter continuum emission have lent the impression that planet formation generally proceeds within orderly, isolated systems. While deep observations of spatially resolved molecular emission have been comparatively limited, they are increasingly suggesting that some disks interact with their surroundings while planet formation is underway. We present an analysis of complex features identified around GM Aur in $^{12}$CO $J=2-1$ images at a spatial resolution of $\sim40$ au. In addition to a Keplerian disk extending to a radius of $\sim550$ au, the CO emission traces flocculent spiral arms out to radii of $\sim$1200 au, a tail extending $\sim1800$ au southwest of GM Aur, and diffuse structures extending from the north side of the disk up to radii of $\sim1900$ au. The diffuse structures coincide with a "dust ribbon" previously identified in scattered light. The large-scale asymmetric gas features present a striking contrast with the mostly axisymmetric, multi-ringed millimeter continuum tracing the pebble disk. We hypothesize that GM Aur's complex gas structures result from late infall of remnant envelope or cloud material onto the disk. The morphological similarities to the SU Aur and AB Aur systems, which are also located in the L1517 cloud, provide additional support to a scenario in which interactions with the environment are playing a role in regulating the distribution and transport of material in all three of these Class II disk systems. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Motivation & Objective

• To investigate the morphology and kinematics of CO gas emission around the GM Aur protoplanetary disk at high spatial resolution to identify non-axisymmetric structures.
• To determine whether the observed large-scale gas features—such as spiral arms, a tail, and diffuse northern structures—are consistent with disk evolution models or external environmental interactions.
• To contrast the asymmetric CO gas structures with the axisymmetric millimeter continuum substructures, highlighting a disconnect between gas and dust morphologies.
• To explore the origin of these complex features by comparing with similar systems in the L1517 cloud, such as SU Aur and AB Aur, to assess the role of external infall in disk evolution.

Proposed method

• Acquired high-sensitivity, high-angular-resolution (≈40 au) 12CO J=2−1 data using ALMA in the MAPS Large Program.
• Constructed isovelocity contours for the Keplerian disk using a hydrostatic disk model with stellar mass M*=1.1 M⊙, gas pressure gradient, and temperature profile T(r) ∝ r−0.608.
• Applied a Keplerian mask (via the gofish and keplerian_mask Python packages) to isolate emission from the rotating disk, enabling separation from extended non-Keplerian features.
• Used an arcsinh color stretch and 5–35σ contour overlays to enhance visibility of faint emission in channel maps.
• Modeled gas density and pressure scale height using the disk surface density profile Σg(r) ∝ r−1 exp(−r/176 au) and H(r) ∝ r1.35.
• Computed line-of-sight velocities via vlos = vsys + vϕ cosϕ sin i, with i=53.21° and vsys=5.61 km s−1, mapping pixel positions to disk coordinates.

Experimental results

Research questions

• RQ1 What large-scale, non-axisymmetric gas structures are present in the CO emission around GM Aur at sub-arcsecond resolution?
• RQ2 How do the kinematics and morphology of the CO gas structures compare to the Keplerian rotation of the disk and the axisymmetric dust continuum rings?
• RQ3 What is the origin of the spiral arms, tail, and diffuse northern structures observed in the CO emission?
• RQ4 To what extent do the observed features suggest ongoing infall of external material, such as remnant envelope or cloud material?
• RQ5 How do the morphological similarities to SU Aur and AB Aur support a common environmental influence on disk evolution in the L1517 cloud?

Key findings

• The 12CO J=2−1 emission reveals flaring spiral arms extending out to radii of ∼1200 au, indicating non-Keplerian gas motions.
• A distinct tail extends ∼1800 au southwest of GM Aur, with emission offset in velocity and position from the main disk, suggesting a dynamical origin.
• Diffuse, extended structures are traced in the northern side of the disk up to ∼1900 au, coinciding with a previously identified dust ribbon in scattered light.
• The large-scale gas features are morphologically asymmetric and kinematically distinct from the Keplerian disk, which extends only to ∼550 au.
• The CO emission morphology contrasts strongly with the axisymmetric, multi-ringed millimeter continuum structure, indicating decoupling between gas and dust substructures.
• The morphological similarities to SU Aur and AB Aur—both in the L1517 cloud—support a scenario in which environmental infall plays a role in shaping gas distribution and transport in Class II disks.