[Paper Review] CO Line Emission Surfaces and Vertical Structure in Mid-Inclination Protoplanetary Disks

Charles J. Law, Sage Crystian+19 more|arXiv (Cornell University)|May 3, 2022

Astrophysics and Star Formation StudiesPhysics and Astronomy133 references57 citations

TL;DR

This study maps CO line emission surfaces in 10 mid-inclination protoplanetary disks using high-resolution ALMA archival data, revealing vertical gas temperature structures and emission heights (z/r ≈ 0.1–0.5) that trace 2–5× the gas pressure scale height (Hg). It confirms strong diversity in vertical structure, links emission height to disk flaring and stellar mass, and establishes CO emission surfaces as empirical tracers of Hg with potential for dynamical mass calibration.

ABSTRACT

High spatial resolution CO observations of mid-inclination (30-75{\deg}) protoplanetary disks offer an opportunity to study the vertical distribution of CO emission and temperature. The asymmetry of line emission relative to the disk major axis allows for a direct mapping of the emission height above the midplane, and for optically-thick, spatially-resolved emission in LTE, the intensity is a measure of the local gas temperature. Our analysis of ALMA archival data yields CO emission surfaces, dynamically-constrained stellar host masses, and disk atmosphere gas temperatures for the disks around: HD 142666, MY Lup, V4046 Sgr, HD 100546, GW Lup, WaOph 6, DoAr 25, Sz 91, CI Tau, and DM Tau. These sources span a wide range in stellar masses (0.50-2.10 M$_{\odot}$), ages (${\sim}$0.3-23 Myr), and CO gas radial emission extents (${\approx}$200-1000 au). This sample nearly triples the number of disks with mapped emission surfaces and confirms the wide diversity in line emitting heights ($z/r\approx0.1$ to ${\gtrsim}0.5$) hinted at in previous studies. We compute radial and vertical CO gas temperature distributions for each disk. A few disks show local temperature dips or enhancements, some of which correspond to dust substructures or the proposed locations of embedded planets. Several emission surfaces also show vertical substructures, which all align with rings and gaps in the millimeter dust. Combining our sample with literature sources, we find that CO line emitting heights weakly decline with stellar mass and gas temperature, which, despite large scatter, is consistent with simple scaling relations. We also observe a correlation between CO emission height and disk size, which is due to the flared structure of disks. Overall, CO emission surfaces trace ${\approx}2$-$5 imes$ gas pressure scale heights (H$_{ m{g}}$) and could potentially be calibrated as empirical tracers of H$_{ m{g}}$.

Motivation & Objective

• To map the three-dimensional vertical distribution of CO emission in mid-inclination protoplanetary disks using high-spatial-resolution ALMA data.
• To derive radial and vertical gas temperature profiles from CO line intensity, assuming local thermodynamic equilibrium (LTE).
• To determine the relationship between CO line emitting height (z/r) and physical disk properties such as stellar mass, disk size, and gas pressure scale height (Hg).
• To assess whether CO emission surfaces can serve as empirical tracers of the gas pressure scale height (Hg) in protoplanetary disks.
• To investigate correlations between CO emission structure and millimeter dust substructures (rings and gaps).

Proposed method

• Used ALMA archival data at high angular resolution and sensitivity to produce line and continuum image cubes for 10 disks.
• Applied the disksurf algorithm to extract CO line emission surfaces from position-velocity data, measuring emission height (z) as a function of radius (r).
• Computed radial and 2D temperature profiles using line+continuum cubes under LTE assumptions, with brightness temperature as a proxy for local gas temperature.
• Defined a characteristic z/r as the median emission height within a radius cutoff (rcutoff = 0.8×rtaper), excluding regions with turnover or low sensitivity.
• Compared emission surface morphology with millimeter dust continuum structures to identify alignment with rings and gaps.
• Combined sample with literature sources (n=15 total) to analyze trends in z/r with stellar mass, disk size, and temperature.

Experimental results

Research questions

• RQ1 What is the vertical distribution of CO emission in mid-inclination protoplanetary disks, and how does it vary across different systems?
• RQ2 How do CO line emission surfaces relate to the gas pressure scale height (Hg), and can they be calibrated as empirical tracers of Hg?
• RQ3 Is there a correlation between CO emission height (z/r) and disk flaring, stellar mass, or gas temperature?
• RQ4 Do vertical substructures in CO emission align with dust rings and gaps in the millimeter continuum?
• RQ5 What insights do CO emission surfaces provide for dynamical mass estimation and planet-disk interaction diagnostics?

Key findings

• The sample of 10 disks nearly triples the number of protoplanetary disks with mapped CO emission surfaces, revealing a wide diversity in emission height with z/r ranging from ≈0.1 to ≳0.5.
• CO emission surfaces trace approximately 2–5× the gas pressure scale height (Hg), supporting their use as empirical proxies for Hg in disk structure modeling.
• A weak but significant anti-correlation is observed between z/r and stellar mass, with lower-mass stars showing more vertically extended emission, consistent with theoretical scaling relations.
• A correlation between CO emission height and disk size is confirmed, driven by the flared geometry of disks, with larger disks exhibiting higher emission surfaces at larger radii.
• Vertical substructures in CO emission are observed in several disks and align spatially with dust rings and gaps in the millimeter continuum, suggesting a connection between gas and dust structures.
• Local temperature dips or enhancements are detected in a few disks, some of which coincide with dust substructures or proposed planet locations, indicating potential links to planet-induced perturbations.