Thermochemical constraints on a primordial-origin of gas-rich debris disks

This paper investigates whether the gas in CO-rich debris disks around A-type stars could be primordial material retained from the protoplanetary phase rather than secondary gas released from volatile-rich solids. These disks persist for tens of millions of years, challenging conventional gas-dispersal timescales. Although secondary-origin models can explain strong CO emission, they often predict excessive neutral carbon. The authors therefore test whether long-lived primordial remnants can simultaneously reproduce substantial CO reservoirs, low neutral-carbon-to-CO mass ratios, and the weak HCO⁺ emission implied by observational non-detections. The study post-processes existing one-dimensional disk-evolution models for a two-solar-mass star at ages of 20, 30, and 40 million years using the chemical and radiative-transfer code Cloudy. The underlying evolutionary model assumes that small dust grains are depleted, suppressing far-ultraviolet-driven photoevaporation and prolonging gas survival. Gaussian vertical density profiles are constructed from the evolutionary models, but temperatures are prescribed rather than calculated self-consistently. The authors examine external irradiation by the interstellar radiation field and internal irradiation by the host star separately. They vary dust-to-gas mass ratios from 10⁻² to 10⁻⁴, grain populations, and cosmic-ray ionisation rates from 10⁻¹⁷ to 10⁻¹⁹ s⁻¹. Synthetic CO images are also generated with radmc-3d using the observing geometry of HD 121617 as an illustration, rather than as an object-specific fit. Dust-poor, externally irradiated models retain substantial molecular gas and CO. At the standard cosmic-ray ionisation rate, molecular hydrogen masses decrease from approximately 615 Earth masses at 20 million years to 108 Earth masses at 40 million years, while CO masses decline from 1.67 to 0.287 Earth masses. Both the main CO isotopologue and ¹³CO exhibit optically thick J = 3–2 emission across roughly 10–300 astronomical units. Consequently, observational estimates that assume optically thin emission could substantially underestimate the CO reservoir. The models also produce low integrated neutral-carbon-to-CO mass ratios, generally below 0.1 in dust-poor cases, and radial CO intensities of the same order of magnitude as those observed. HCO⁺ provides the strongest constraint on ionisation. Dust-poor models with the standard cosmic-ray ionisation rate predict HCO⁺ J = 2–1 luminosities of 3.4–7.3 × 10²⁴ erg s⁻¹, exceeding observational upper limits of 1.1–3.6 × 10²³ erg s⁻¹. Reducing the ionisation rate to 10⁻¹⁹ s⁻¹ lowers HCO⁺ masses by factors of approximately 11–34, to around 2 × 10⁻⁸ Earth masses. Predicted luminosities then fall to 1.6–3.4 × 10²³ erg s⁻¹, overlapping the range of observational upper limits but not satisfying every source-specific constraint. CO and neutral carbon are much less sensitive to this reduction in ionisation. The principal observational mismatch concerns spatial extent: the representative model produces neutral-carbon emission beyond 300 astronomical units, whereas comparison observations indicate distributions confined to about 100 astronomical units. Hydrogen-abundance constraints in individual systems introduce additional tensions. An extreme dust-depletion experiment, with a dust-to-gas mass ratio of 10⁻⁸, converts much of the molecular hydrogen into atomic hydrogen and raises the CO-to-molecular-hydrogen number ratio to approximately 0.006. This result illustrates that a low molecular hydrogen abundance does not necessarily exclude residual primordial hydrogen. The authors conclude that a primordial origin remains chemically viable but is not established. Important limitations include prescribed temperatures, the lack of coupling between chemistry and disk evolution, separate treatment of internal and external irradiation, omission of radial external irradiation, and simplified isotope chemistry. Stellar photodissociation modelling is particularly uncertain: an approximate spectral correction reduces HCO⁺ masses in internally irradiated models by more than two orders of magnitude. Multidimensional, self-consistent models and improved observations of neutral carbon, molecular ions, and atomic and molecular hydrogen are needed to distinguish primordial, secondary, and hybrid origins.

Could Gas-Rich Debris Disks Be Leftovers from Planet Formation?

Planet formation is usually described as a race against time. Young stars begin surrounded by disks of gas and dust, but much of that gas disappears within a few million years. Some debris disks around A-type stars, however, contain substantial carbon monoxide, or CO, tens of millions of years later. Where does this surprisingly persistent gas come from?

In “Thermochemical constraints on a primordial-origin of gas-rich debris disks,” H. Mitani, W. Ooyama, R. Nakatani, R. Kuiper, and T. Hosokawa investigate whether these systems could retain gas from their original planet-forming disks. Their results keep that possibility alive, while revealing important tensions between the models and observations.

Two Origins for an Unexpected Gas Reservoir

One explanation is that the gas is secondary: it was released relatively recently from volatile-rich solids through collisions or other destructive processes. Such models can explain strong CO emission, but they often produce too much neutral carbon, which accumulates as CO molecules are broken apart by radiation.

The alternative is primordial gas that survived from the protoplanetary phase. This requires disks to retain material much longer than conventional gas-dispersal timescales suggest. The researchers ask whether such remnants could satisfy three observational clues simultaneously: abundant CO, little neutral carbon relative to CO, and weak emission from the molecular ion HCO⁺.

Building a Chemical Test

The team used existing one-dimensional disk-evolution models for a star twice the Sun’s mass, examining snapshots at 20, 30, and 40 million years. Those models assume a depletion of small dust grains. With fewer small grains, far-ultraviolet-driven photoevaporation is suppressed, allowing gas to survive longer.

The researchers then applied the chemical and radiative-transfer code Cloudy to calculate the gas chemistry. They constructed Gaussian vertical density profiles, but prescribed temperatures rather than calculating them self-consistently. External radiation from the interstellar environment and internal radiation from the host star were examined separately.

The study varied dust-to-gas mass ratios from 0.01 to 0.0001, grain populations, and cosmic-ray ionisation rates. Synthetic CO images were also produced using radmc-3d and the viewing geometry of HD 121617. This was an illustration of the models’ appearance, not a tailored fit to that system.

Plenty of CO, Without Too Much Carbon

The dust-poor, externally irradiated models retained substantial molecular reservoirs. At the standard cosmic-ray ionisation rate of 10⁻¹⁷ per second, molecular hydrogen declined from about 615 Earth masses at 20 million years to 108 Earth masses at 40 million years. Over the same interval, CO decreased from 1.67 to 0.287 Earth masses.

These models also generally produced integrated neutral-carbon-to-CO mass ratios below 0.1. That is a promising result because excessive neutral carbon is a challenge for many secondary-origin explanations. Predicted radial CO intensities were also of the same order of magnitude as observed values.

A hidden reservoir behind bright emission

Both the main CO isotopologue and ¹³CO showed optically thick J = 3–2 emission across roughly 10–300 astronomical units. When emission is optically thick, adding more gas does not translate straightforwardly into a proportionally brighter signal. Consequently, mass estimates based on an optically thin assumption could substantially underestimate the true CO reservoir.

HCO⁺ Becomes the Critical Test

The strongest chemical constraint came from HCO⁺, whose abundance is sensitive to ionisation. At the standard cosmic-ray rate, dust-poor models predicted HCO⁺ J = 2–1 luminosities of 3.4–7.3 × 10²⁴ erg per second. Observational upper limits were much lower: 1.1–3.6 × 10²³ erg per second.

Reducing the ionisation rate to 10⁻¹⁹ per second lowered HCO⁺ masses by factors of approximately 11–34. Predicted luminosities then fell to 1.6–3.4 × 10²³ erg per second. These overlap the range of observational upper limits, although they do not satisfy every source-specific constraint. CO and neutral carbon changed much less, making HCO⁺ a particularly useful diagnostic.

Where the Primordial Picture Still Struggles

The clearest mismatch was spatial. A representative model produced neutral-carbon emission beyond 300 astronomical units, while comparison observations showed distributions confined to about 100 astronomical units. Hydrogen-abundance constraints in individual systems create additional tensions.

An extreme experiment, with a dust-to-gas mass ratio of 10⁻⁸, added another wrinkle. Much of the molecular hydrogen became atomic hydrogen, raising the CO-to-molecular-hydrogen number ratio to about 0.006. A low molecular hydrogen abundance therefore does not necessarily mean that primordial hydrogen is absent.

A Viable Possibility, Not a Verdict

The calculations support chemical viability, not proof of primordial survival. Temperatures were prescribed, chemistry was not coupled to disk evolution, and internal and external irradiation were treated separately. Radial external irradiation was omitted, while isotope chemistry was simplified. Stellar photodissociation is especially uncertain: an approximate spectral correction reduced HCO⁺ masses in internally irradiated models by more than two orders of magnitude.

Resolving the gas’s origin will require multidimensional, self-consistent models and better observations of neutral carbon, molecular ions, and both atomic and molecular hydrogen. For now, primordial remnants remain a credible possibility alongside secondary production and hybrid origins.