Measuring the gas mass of the protoplanetary disk around HD 179218 with CO isotopologues
Authors: Nina Weichmann, Ke Zhang, Leon Trapman, Ao Zhang
Abstract
Carbon monoxide (CO) isotopologues are widely used to estimate protoplanetary disk gas masses, but their reliability is often compromised by CO freeze-out and subsequent chemical processing. The disk around the luminous Herbig star HD~179218 ($L_* \sim 100\,L_\odot$, $M_* \sim 3.0\,M_\odot$) offers a rare test case of CO-based mass measurements. Its CO snowline lies beyond the outer disk edge, and thus CO remains in the gas phase throughout. We present ALMA and NOEMA observations of six CO isotopologue lines ($^{13}$CO, C$^{18}$O, and C$^{17}$O, $J$=2--1 and 1--0). Fitting the spectral energy distribution yields $M_{dust} \approx 3 \times 10^{-5}\, M_\odot$. Modeling CO isotopologue emission with the thermochemical code DALI yields the gas-disk characteristic radius ($R_{c} \approx 90$~au) and the gas mass ($M_{gas} \approx 8 \times 10^{-3}\, M_\odot$). These results yield a gas-to-dust ratio of $\sim$270 for our fiducial dust opacity ($a_{\rm max} = 1$~mm), above the canonical interstellar value of $\sim$100. The thermal structure of our best-fitting model confirms that CO never freezes out in this disk, so the elevated ratio is not an artifact of CO depletion; together with the compact millimeter dust disk relative to the gas disk, it is consistent with substantial inward radial drift of millimeter dust. In addition, we find a gas cavity inside 50\,au that encloses the compact millimeter dust disk---the inverse of a typical transition disk---which we attribute to photoevaporative clearing of the inner gas after the pebbles drifted inward. HD~179218 thus provides a clean benchmark for studying the gas mass distribution and gas--dust interaction in protoplanetary disks, free from the CO-depletion biases that affect colder disks.
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This study models CO emission in debris disks to distinguish primordial gas from gas released by collisions, rather than measuring a protoplanetary disk's gas mass.
Abstract
The detection of gas in debris disks raises the question of whether this gas is a remnant from the primordial protoplanetary phase, or released by the collision of secondary bodies. In this paper we analyze ALMA observations at 1-1.5" resolution of three debris disks where the $^{12}$CO(2-1) rotational line was detected: HD131835, HD138813, and HD156623. We apply the iterative Lucy-Richardson deconvolution technique to the problem of circumstellar disks to derive disk geometries and surface brightness distributions of the gas. The derived disk parameters are used as input for thermochemical models to test both primordial and cometary scenarios for the origin of the gas. We favor a secondary …
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Abstract
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This study infers gas-to-dust ratios in planet-forming regions from stellar abundances, rather than fitting disk CO emission to measure gas mass.
Abstract
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Abstract
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This study tests whether primordial gas can explain gas-rich debris disks, whereas the paper measures the gas distribution in a protoplanetary disk.
In short
Thermochemical models support a primordial origin for CO-rich debris disks under dust-poor, weak-ionisation conditions, but predict neutral-carbon emission more extended than observed.
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Thermochemical constraints on a primordial-origin of gas-rich debris disksH. Mitani, W. Ooyama, R. Nakatani et al. · 2026 · Earth and Planetary Astrophysics · AI summary available
This study tests whether primordial gas can explain gas-rich debris disks, whereas the paper measures the gas distribution in a protoplanetary disk.
In short
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This study observes warm H2 and cooler CO in pre-main-sequence and debris disks, rather than using CO isotopologues to determine one disk's gas mass.
Abstract
We present ISO-SWS observations of H2 pure-rotational line emission from the disks around low and intermediate mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. We detect `warm' (T ~ 100-200 K) H2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~1E-4 Solar mass up to 8E-3 Solar mass, and can constitute a non-negligible fraction of the total disk mass. Complementary single-dish 12CO 3-2, 13CO 3-2 and 12CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T ~ 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Kep…
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ALMA Observations of the HD~110058 debris diskAntonio S. Hales, SebastiÁn Marino, Patrick D. Sheehan et al. · 2022 · Solar and Stellar Astrophysics · cited 16 times · AI summary available
This study uses ALMA and radiative-transfer modeling to characterize gas and dust in a debris disk, rather than measuring a protoplanetary disk's gas mass.
Abstract
We present Atacama Large Millimeter Array (ALMA) observations of the young, gas-rich debris disk around HD110058 at 0.3-0.6\arcsec resolution. The disk is detected in the 0.85 and 1.3~mm continuum, as well as in the J=2-1 and J=3-2 transitions of $^{12}$CO and $^{13}$CO. The observations resolve the dust and gas distributions and reveal that this is the smallest debris disk around stars of similar luminosity observed by ALMA. The new ALMA data confirm the disk is very close to edge-on, as shown previously in scattered light images. We use radiative transfer modeling to constrain the physical properties of dust and gas disks. The dust density peaks at around 31~au and has a smooth outer edge …
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