Measuring the gas mass of the protoplanetary disk around HD 179218 with CO isotopologues

Nina Weichmann; Ke Zhang; Leon Trapman; Ao Zhang

AI summary · arXiv: 2611.04030v1 · astro-ph.EP

Measuring the gas mass of the protoplanetary disk around HD 179218 with CO isotopologues

Nina Weichmann, Ke Zhang, Leon Trapman, Ao Zhang

Original paper Submitted to arXiv on 02 Nov. 2026 16 pages, 10 figures, accepted for publication in AJ License: CC BY 4.0 Also in: fr

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The essentials

ALMA and NOEMA observations modeled with DALI yield a gas mass of about 8.1 × 10⁻³ solar masses for HD 179218, where CO remains gaseous throughout the disk.

Question

Can CO isotopologue emission provide a reliable gas-mass measurement in a disk where CO freeze-out and the resulting depletion are absent? What gas and dust distributions are consistent with the observations of HD 179218?

Method

The study analyzes ALMA and NOEMA observations of six 13CO, C18O, and C17O transitions, fitting the dust SED with RADMC-3D and modeling the CO emission with DALI. The gas mass and characteristic radius are constrained using optically thin C18O and C17O emission; the best-fitting model has a 90 au characteristic radius, a 50 au cavity with depletion factor 0.15, and a gas mass of (8.1 ± 0.8) × 10⁻³ solar masses.

Findings

Result The SED fit gives a fiducial dust mass of 3.0 × 10⁻⁵ solar masses for a maximum grain size of 1 mm.
See the passage in the paper
This gives $M_{\rm dust}=3.0\times 10^{-5}\,\mathrm{M}_{\odot}$, while $a_{\rm max}=1$ cm requires $6.0\times 10^{-5}\,\mathrm{M}_{\odot}$.
Result The best-fitting model has a gas mass of (8.1 ± 0.8) × 10⁻³ solar masses and a gas-to-dust ratio of 270 ± 28 at the fiducial dust mass.
See the passage in the paper
The error budget is instead dominated by the $\sim$10% absolute flux calibration, which is correlated across channels and propagates linearly to the optically thin line fluxes, giving a gas-to-dust ratio of $270\pm 28$ and $M_{\rm gas}=(8.1\pm 0.8)\times 10^{-3}\,\mathrm{M}_{\odot}$ at fixed dust mass.
Result The modeled midplane temperature stays above the CO freeze-out threshold across the disk, leaving CO at its full gas-phase abundance.
See the passage in the paper
The midplane gas temperature remains above the CO freeze-out temperature ($\sim$20 K) at all radii: it decreases from several hundred K in the inner disk to $\sim$30 K at the outer edge of the model (300 au), passing through $\sim$50 K at 50 au and $\sim$40 K at the gas-disk characteristic radius of 90 au.
Authors’ interpretation The gas disk is more extended than the millimeter dust emission, a difference the authors interpret as consistent with substantial inward dust drift.
See the passage in the paper
The resulting gas-to-dust size ratio of $\gtrsim$3 is consistent with substantial radial drift [61, e.g.,], and is insensitive to the dust opacity assumption.
Authors’ interpretation The authors propose that rapid pebble drift followed by photoevaporative clearing explains the gas cavity and compact dust distribution.
See the passage in the paper
We suggest that this structure records a two-step evolution: rapid radial drift followed by photoevaporation-driven clearing of the inner gas.

Limitations

The gas-to-dust ratio depends substantially on the assumed dust opacity and grain properties, so its uncertainty is not captured by the quoted formal error.
See the passage in the paper
The quoted uncertainties on the dust mass, gas-to-dust ratio, and gas mass reflect the formal uncertainty associated with the $\sim$10% continuum flux calibration (Section IV.4) and do not include systematic uncertainties associated with the adopted dust opacity, including the grain-size distribution, maximum grain size, composition, porosity, or uncertainties in the SED fit.
The available continuum size comparison does not uniquely quantify radial drift; resolved continuum profiles and dust-evolution modeling are needed.
See the passage in the paper
The compact millimeter dust emission relative to the gas indicates substantial dust evolution and is consistent with radial drift, but quantifying the inward transport would require resolved continuum profiles and models of growth, fragmentation, and drift.
A faint dust ring or alternative inner-disk structure cannot be excluded as an explanation for aspects of the inferred cavity.
See the passage in the paper
The absence of a detected ring at 50 au argues against this scenario, although a faint ring below our sensitivity, or a configuration in which small grains filter through the gap and re-grow in the inner disk, cannot be fully excluded.

Why it matters

The authors present HD 179218 as a benchmark showing that CO isotopologues can trace a substantial gas reservoir without freeze-out bias when the disk is warm enough to keep CO gaseous throughout. They argue that freeze-out and subsequent CO processing are major sources of the low CO-based mass estimates in colder disks.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: protoplanetary disks CO isotopologues gas mass CO freeze-out dust drift

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Comprehensive Summary
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Original abstract

The paper examines whether carbon monoxide isotopologues can provide reliable gas-mass estimates for protoplanetary disks in which CO freeze-out and the associated chemical processing are absent. The authors study HD 179218, a luminous Herbig Ae star whose disk is expected to be warm enough for CO to remain gaseous throughout. This makes the system a useful test case for CO-based mass estimates, which can substantially understate gas masses in colder disks when CO is depleted from the observable gas. The team combines ALMA observations of the ^13CO, C^18O, and C^17O J = 1–0 lines with NOEMA observations of the same isotopologues in J = 2–1. They fit the optical-to-millimeter spectral energy distribution (SED) using RADMC-3D to estimate the dust mass, then use the DALI thermochemical model to calculate the disk’s temperature, chemistry, and line emission. DALI models the isotopologues separately, accounting for their differing susceptibility to isotope-selective photodissociation. The authors compare models with the optically thin C^18O and C^17O emission to constrain the gas mass and disk size. They use ^13CO mainly to check the modeled disk structure because its optically thick emission is more sensitive to temperature than to the total gas column. For their fiducial dust model, which assumes a maximum grain size of 1 mm, the SED fit yields a dust mass of about 3.0 × 10^-5 solar masses. The best-fitting gas model has a characteristic radius of about 90 au and a gas mass of (8.1 ± 0.8) × 10^-3 solar masses. Given the adopted dust mass, this implies a gas-to-dust ratio of 270 ± 28, above the commonly assumed interstellar value of about 100. The model also requires a gas cavity extending to 50 au, where the gas surface density is depleted to 15 percent of the extrapolated outer-disk profile. The best-fitting surface-density index is 1.0. The modeled midplane temperature remains above the approximate 20 K CO freeze-out threshold throughout the disk: it is about 50 K at 50 au, 40 K at 90 au, and around 30 K at the model’s outer edge of 300 au. The authors therefore conclude that CO freeze-out does not significantly reduce the gas-phase CO abundance in this system. They argue that the inferred gas mass avoids the freeze-out-related bias affecting estimates for colder disks, while noting that isotope-selective photodissociation still occurs and is included in their modeling. Several observations are consistent with substantial inward drift of millimeter-sized dust. The millimeter continuum emission is concentrated within roughly 20–40 au, whereas the gas traced by ^13CO extends farther. The radius enclosing 68 percent of the ^13CO flux is about 120 au. The authors suggest that pebbles drifted inward before photoevaporation began clearing gas from the inner disk, which could explain why the dust lies inside the gas cavity and why no dust ring is detected at its edge. This evolutionary interpretation is plausible but not uniquely established; a faint ring or other explanations cannot be ruled out. The gas mass depends on the thermochemical model and the adopted CO chemistry, while the gas-to-dust ratio is particularly sensitive to dust opacity. Assuming a maximum grain size of 1 cm rather than 1 mm doubles the inferred dust mass and lowers the gas-to-dust ratio to about 135, while leaving the CO-based gas mass largely unchanged. The models also overpredict the optically thick ^13CO emission, possibly because they overestimate the temperature of the layer in which it originates.

Can Carbon Monoxide Reveal a Disk’s True Gas Mass?

Protoplanetary disks are the reservoirs of gas and dust from which planets form. Measuring how much gas a disk contains is therefore important—but not straightforward. Astronomers often use carbon monoxide (CO) emission as a tracer of gas. In cold disks, however, CO can freeze onto dust grains or be chemically altered, making the gas appear less massive than it really is.

In “Measuring the gas mass of the protoplanetary disk around HD 179218 with CO isotopologues,” Nina Weichmann, Ke Zhang, Leon Trapman, and Ao Zhang investigate a useful exception: a disk expected to be warm enough to keep CO in the gas phase. Their study asks whether different forms of CO can provide a reliable gas-mass estimate when freeze-out is not a major concern.

A Warm Disk as a Test Case

The target is HD 179218, a luminous Herbig Ae star surrounded by a protoplanetary disk. Its high temperatures make it a valuable test of CO-based mass measurements. If CO remains gaseous throughout the disk, then a low CO signal is less likely to be caused by freeze-out hiding the molecule from view.

The team observed three CO isotopologues—¹³CO, C¹⁸O, and C¹⁷O—in two rotational transitions. They used ALMA observations of the J = 1–0 lines and NOEMA observations of the J = 2–1 lines. These complementary measurements help reveal both the disk’s structure and the amount of gas it contains.

From Dust Emission to Gas Mass

The researchers first fitted the disk’s optical-to-millimeter spectral energy distribution, or SED, using RADMC-3D. For their reference dust model, which assumes grains grow to a maximum size of 1 millimeter, the fit gives a dust mass of about 3.0 × 10⁻⁵ solar masses.

They then used the DALI thermochemical model to calculate the disk’s temperature, chemistry, and predicted line emission. Importantly, DALI treats the isotopologues separately. This allows the model to account for isotope-selective photodissociation, a process that can destroy some isotopologues more readily than others when exposed to ultraviolet light.

The relatively optically thin C¹⁸O and C¹⁷O lines provide the main constraints on gas mass and disk size. The brighter ¹³CO emission is optically thick, so it is more sensitive to the temperature of the emitting layer than to the total gas column. The authors use it mainly to check whether the modeled disk structure is plausible.

A Gas-Rich Disk with an Inner Cavity

The best-fitting model has a characteristic disk radius of about 90 au and a gas mass of (8.1 ± 0.8) × 10⁻³ solar masses. Compared with the adopted dust mass, this implies a gas-to-dust ratio of 270 ± 28—higher than the commonly assumed interstellar value of about 100.

The model also includes a gas cavity extending inward to 50 au. Within that region, the gas surface density is reduced to 15 percent of what would be expected by continuing the outer-disk profile inward. The best-fitting surface-density profile has an index of 1.0.

Crucially, the modeled midplane stays warmer than the approximate 20 K threshold below which CO commonly freezes out. The temperature is about 50 K at 50 au, 40 K at 90 au, and around 30 K at the outer edge of the modeled disk, 300 au. The authors therefore conclude that freeze-out does not significantly lower the gas-phase CO abundance in HD 179218.

Dust and Gas Tell Different Stories

The millimeter continuum emission is concentrated within roughly 20–40 au, while the gas traced by ¹³CO extends farther out. About 68 percent of the ¹³CO flux comes from within a radius of approximately 120 au. This contrast suggests that millimeter-sized dust has moved inward relative to the gas.

One possible explanation is that solid particles, or pebbles, drifted inward before photoevaporation began clearing gas from the inner disk. That sequence could help explain why the dust lies inside the gas cavity and why there is no obvious dust ring at the cavity’s edge. The interpretation is plausible, but not definitive: a faint ring or another explanation cannot be ruled out.

What the Estimate Depends On

The result is a useful demonstration of CO isotopologues in a warm disk, but it is not free of modeling uncertainties. The gas mass depends on the thermochemical model and adopted CO chemistry. The gas-to-dust ratio is especially sensitive to assumptions about dust opacity and grain sizes.

For example, assuming a maximum grain size of 1 centimeter instead of 1 millimeter doubles the inferred dust mass. The CO-based gas mass changes little, but the gas-to-dust ratio falls to about 135. The models also predict more optically thick ¹³CO emission than is observed, possibly because they overestimate the temperature of the layer where that line originates.

Why the Study Matters

HD 179218 offers evidence that CO isotopologues can yield a meaningful gas-mass estimate when the disk is warm enough to avoid substantial CO freeze-out. The result also highlights an important caution: isotope-selective photodissociation, dust properties, and temperature structure still matter. For colder disks, where CO may be depleted from the observable gas, the same measurements can underestimate the true mass. Understanding those differences is essential when comparing disks and assessing how much material is available to form planets.

Created on 04 Nov. 2026

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Measuring the gas mass of the protoplanetary disk around HD 179218 with CO isotopologues

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Can Carbon Monoxide Reveal a Disk’s True Gas Mass?

A Warm Disk as a Test Case

From Dust Emission to Gas Mass

A Gas-Rich Disk with an Inner Cavity

Dust and Gas Tell Different Stories

What the Estimate Depends On

Why the Study Matters

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