[Paper Review] Molecules with ALMA at Planet-forming Scales. XX. The Massive Disk Around GM Aurigae

Kamber R. Schwarz, Jenny K. Calahan+26 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation Studies101 references45 citations

TL;DR

This study uses high-resolution ALMA and Herschel observations to model the gas and dust structure of the massive protoplanetary disk around GM Aurigae. It derives a total gas mass of ~0.2 M⊙, reveals strong CO depletion beyond 100 au, and finds gravitational instability near 70–100 au despite overall disk stability, offering insights into disk evolution and planet formation processes.

ABSTRACT

Gas mass remains one of the most difficult protoplanetary disk properties to constrain. With much of the protoplanetary disk too cold for the main gas constituent, H2, to emit, alternative tracers such as dust, CO, or the H2 isotopolog HD are used. However, relying on disk mass measurements from any single tracer requires assumptions about the tracer's abundance relative to \hh\ and the disk temperature structure. Using new Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program as well as archival ALMA observations, we construct a disk physical/chemical model of the protoplanetary disk GM Aur. Our model is in good agreement with the spatially resolved CO isotopolog emission from eleven rotational transitions with spatial resolution ranging from 0.15'' to 0.46'' (24-73 au at 159 pc) and the spatially unresolved HD J=1-0 detection from Herschel. Our best-fit model favors a cold protoplanetary disk with a total gas mass of approximately 0.2 solar masses, a factor of 10 reduction in CO gas inside roughly 100 au and a factor of 100 reduction outside of 100 au. Despite its large mass, the disk appears to be on the whole gravitationally stable based on the derived Toomre Q parameter. However, the region between 70 and 100 au, corresponding to one of the millimeter dust rings, is close to being unstable based on the calculated Toomre Q of <1.7. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Motivation & Objective

• To constrain the true gas mass of the massive protoplanetary disk around GM Aurigae, a key parameter difficult to measure directly.
• To resolve discrepancies between dust-derived and CO-derived gas mass estimates by modeling the disk's physical and chemical structure.
• To assess gravitational stability using the Toomre Q parameter, particularly in regions with observed dust rings.
• To investigate the role of CO freeze-out, photodissociation, and nonthermal desorption in shaping CO abundance profiles.
• To improve gas mass estimates by combining multi-tracer observations (CO isotopologs, HD, dust continuum) with a self-consistent disk model.

Proposed method

• Utilized spatially resolved ALMA observations of 11 CO isotopolog transitions (0.15–0.46′′ resolution) to map gas temperature and column density.
• Incorporated the unresolved HD J=1–0 line detection from Herschel to constrain the warm, inner disk gas mass.
• Combined archival ALMA CO data and new MAPS program observations to model the disk’s 3D density and temperature structure.
• Constructed a physical/chemical disk model that accounts for CO freeze-out in the midplane, photodissociation in the upper layers, and nonthermal desorption.
• Calculated the Toomre Q parameter using the modeled gas density and temperature to assess gravitational stability.
• Validated the model against millimeter dust continuum gaps and rings, linking kinematic instability to observed substructures.

Experimental results

Research questions

• RQ1 What is the true total gas mass of the GM Aurigae disk, and how does it compare to estimates from dust and CO alone?
• RQ2 How do CO abundance variations—especially depletion beyond 100 au—arise from physical and chemical processes?
• RQ3 Is the disk gravitationally stable, and could localized instability near 70–100 au explain the observed dust ring structure?
• RQ4 To what extent does nonthermal desorption restore CO to the midplane beyond the millimeter dust disk?
• RQ5 How do multi-tracer observations (CO, HD, dust) improve gas mass constraints compared to single-tracer methods?

Key findings

• The best-fit model yields a total gas mass of approximately 0.2 M⊙, with 32% of the disk mass at temperatures below 20 K.
• CO gas abundance is reduced by a factor of ~10 relative to ISM values within 100 au and by a factor of ~100 beyond 100 au, consistent with freeze-out and chemical processing.
• The HD emission traces only the inner 200 au, while the 12CO emission extends to 650 au, indicating a much more extended gas disk than traced by millimeter dust.
• The Toomre Q parameter dips below 1.7 (gravitational instability threshold) between 70 and 100 au, coinciding with a prominent millimeter dust ring, suggesting potential local instability.
• Despite this localized instability, the outer disk as a whole remains gravitationally stable, with Q > 1.7 across most of the disk.
• Nonthermal desorption is shown to replenish CO in the midplane beyond the outer dust ring, explaining residual CO emission in the outer disk.