CO gaps in cold transition disks around low-mass stars

A. A. Rota; N. van der Marel; E. F. van Dishoeck; T. Muto

doi:10.1051/0004-6361/202659073

AI summary · arXiv: 2611.09449v1 · astro-ph.EP

CO gaps in cold transition disks around low-mass stars

A. A. Rota (Leiden Observatory, the Netherlands), N. van der Marel (Leiden Observatory, the Netherlands), E. F. van Dishoeck (Leiden Observatory, the Netherlands), T. Muto (Kogakuin University, Japan)

Original paper · DOI 10.1051/0004-6361/202659073 Submitted to arXiv on 06 Nov. 2026 Published in Astronomy & Astrophysics (09 Nov. 2026) Accepted on 08 Jun. 2026 Accepted for publication in A&A License: CC BY 4.0 Also in: fr

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The essentials

ALMA observations of three low-mass-star transition disks reveal dust cavities, CO emission within them, little inner dust, and high estimated inner-disk gas-to-dust ratios.

Question

How are gas and dust distributed inside and around the cavities of cold transition disks orbiting low-mass K- and M-type stars? The authors also compare their gas temperatures and inferred inner-disk gas-to-dust ratios with those of other disks and consider what companions could explain the structures.

Method

The study analyzes ALMA Band 6 continuum observations at about 0.02 arcsec resolution and 12CO, 13CO, and C18O J=2–1 observations at about 0.1 arcsec resolution for three inclined disks around stars of spectral types M1, K5, and K2. The authors measure radial profiles and cumulative-flux radii, estimate brightness temperatures and upper limits on inner dust mass, and infer gas surface densities from stellar accretion rates assuming alpha = 10^-3.

Findings

Result All three disks have dust cavities of about 35–40 au, while CO emission is detected inside the dust cavities.
See the passage in the paper
the dust rings are located in the three disks at a similar distance from the host star – 39 au, 35 au, and 40 au for Sz 111, J1633, and J1608, respectively.
Result The 12CO emission peaks closer to the star than the less abundant isotopologues, consistent with differing optical depths.
See the passage in the paper
The 12CO emission peaks closer to the star than both the 13CO and C18O, likely reflecting the effect of different optical depths of CO isotopologues according to their different abundances (e.g., Dong et al. 2017; Leemker et al. 2022).
Result No inner dust emission is detected, and the inferred gas-to-dust ratios at 1 au are approximately 10^5.
See the passage in the paper
The gas-to-dust ratios at 1 au are thus $\sim 10^{5}$.
Authors’ interpretation The authors find no significant difference in brightness temperature between their disks and comparison transition or full disks, suggesting luminosity alone does not determine gas temperature.
See the passage in the paper
This suggests that the gas temperature is not only regulated by the stellar luminosity, but also by additional parameters related to the detailed disk structure and chemistry, such us the presence of an inner disk, the fraction of large grains, or the depth of the gas cavity (e.g., Leemker et al. 2022).
Authors’ interpretation The disk structures could be explained by super-Jovian companions on eccentric orbits, but this remains unconfirmed.
See the passage in the paper
This suggests that the observed disk structures can be explained by super-Jovian companions on eccentric orbits, but further modeling and deeper high-resolution imaging, such as will be possible with ELT-METIS, are needed to confirm this conclusion.

Limitations

The three-disk sample and nonuniform resolution of comparison observations limit the strength of the temperature comparison.
See the passage in the paper
New ALMA observations of the CO isotopologues, with a uniform spectral and spatial resolution for all disks, are thus crucial for performing a detailed comparison of the brightness temperature profiles.
The observations do not resolve the CO gap, so companion-mass estimates rely on a proxy and an extrapolation beyond the mass range studied by the relation used.
See the passage in the paper
Since the resolution of our observation is insufficient to resolve the CO gap, $R_{\text{CO}}$ cannot be measured directly.
The proposed eccentric-companion explanation requires further modeling for confirmation.
See the passage in the paper
The observed disk structures could be explained by super-Jovian companions on eccentric orbits, but more detailed modeling is needed to confirm this conclusion.

Why it matters

The authors argue that low-mass-star transition disks do not show the expected differences in CO brightness temperature or inferred inner gas-to-dust ratio compared with other disks. They emphasize individual-target thermochemical modeling or multi-transition analyses to constrain cavity structure and caution that companion explanations require further modeling.

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: transition disks; protoplanetary disks; ALMA; gas-to-dust ratio; giant planets

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper investigates the distribution of gas and dust within and around the cavities of three transition disks orbiting relatively low-mass K- and M-type stars. Transition disks have large central dust cavities, often interpreted as evidence that massive companions clear gaps and trap millimeter-sized dust farther out. The authors examine whether these cooler, inclined systems differ from better-studied transition disks around more luminous stars, and what the observations reveal about gas flows through the cavities and possible companions. The study presents new ALMA Band 6 observations of Sz 111, J1633, and J1608, located about 140–160 parsecs away. The stars have spectral types M1, K5, and K2, luminosities of 0.21–1.9 times the Sun’s luminosity, and accretion rates of roughly 10⁻¹⁰–10⁻⁹ solar masses per year. The continuum was imaged at approximately 0.02–0.03 arcseconds, corresponding to a few astronomical units. The carbon monoxide isotopologue lines (¹²CO, ¹³CO, and C¹⁸O, all in the J = 2–1 transition) were imaged at about 0.1 arcseconds, or roughly 20 astronomical units. The authors measured deprojected radial profiles, line brightness temperatures, cumulative-flux disk sizes, and upper limits on inner-disk dust. They also estimated gas surface densities from the stellar accretion rates and used a published scaling between dust-ring and CO-gap radii to infer possible companion masses. All three systems have a central cavity in the millimeter continuum, surrounded by a narrow, approximately symmetric dust ring. The ring peaks at about 35–40 au in each disk, and no inner dust disk is detected. Gas emission extends into the dust cavities, with ¹²CO generally peaking closer to the star than the less abundant isotopologues, consistent with differences in optical depth. The ¹²CO disks extend to about 206–307 au, substantially farther than the dust disks; on average, the gas radius is about 2.3 times the dust radius. J1633 has the narrowest dust ring. Sz 111 and J1608 show a shoulder in their outer profiles near 90 au, which may indicate a second gap that these observations do not resolve. The CO velocity patterns are consistent with Keplerian rotation, and the spectra show no strong foreground absorption. The ¹³CO brightness-temperature peaks occur at roughly 24–30 au and reach about 21–32 K across the three targets. These temperatures show no clear difference from those of other transition or full disks, including disks around more luminous stars, despite expectations that the lower-luminosity systems would be cooler. The authors therefore caution that luminosity alone may not determine gas temperature; disk structure, chemistry, inclination, and optical-depth effects may also play a role. No inner continuum emission is detected, with dust-mass limits below 0.003 Earth masses in Sz 111, 0.02 in J1633, and 0.0006 in J1608. However, the stellar accretion rates indicate that gas remains close to the stars. Assuming a viscosity parameter of 0.001, the inferred gas-to-dust ratios at 1 au are at least about 10⁴–10⁵; the paper’s overall conclusion places them at around 10⁵. The estimated companion masses exceed 15 Jupiter masses in all three systems, but these estimates are highly uncertain. The adopted relation was calibrated only up to 15 Jupiter masses and does not account for eccentric orbits, which may affect gap structure at high companion masses.

Peering into the cavities of cold transition disks

Transition disks have a striking feature: a broad central cavity in their millimeter-wavelength dust emission, surrounded by a ring. One popular explanation is that a massive companion—such as a giant planet or brown dwarf—clears a path through the disk, while larger dust grains collect beyond the gap. But dust is only part of the story. Gas can continue to flow through the cavity, feeding the star and potentially revealing how the gap was formed.

In “CO gaps in cold transition disks around low-mass stars,” A. A. Rota, N. van der Marel, E. F. van Dishoeck, and T. Muto examine three such disks around relatively low-mass K- and M-type stars. Their observations offer a closer look at how gas and dust are arranged in these cooler systems—and test whether their properties differ from transition disks around brighter stars.

Three disks, observed in detail

The targets are Sz 111, J1633, and J1608, located about 140–160 parsecs from Earth. Their host stars range from spectral type M1 to K2, with luminosities between 0.21 and 1.9 times that of the Sun. All three stars are still accreting material from their disks, at rates of roughly one ten-billionth to one-billionth of a solar mass per year.

The team used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe dust emission and three carbon monoxide isotopologues: ¹²CO, ¹³CO, and C¹⁸O. The dust images resolve structures just a few astronomical units across, while the gas observations resolve features on scales of about 20 au. Together, these measurements let the researchers compare the cavity, the surrounding ring, and the more extended outer disk.

Dust rings and gas-filled cavities

Each disk has a central cavity in its millimeter continuum emission, encircled by a narrow, nearly symmetric dust ring. The rings peak at about 35–40 au. No inner dust disk is detected in any of the three systems, but the gas tells a different story: CO emission reaches into the dust cavities. The ¹²CO emission generally peaks closer to the star than the less abundant isotopologues, as expected because the different lines trace gas with different optical depths.

The gas disks also reach much farther out than the dust disks. In ¹²CO, their measured radii span about 206–307 au, averaging roughly 2.3 times the dust-disk radius. This mismatch is a reminder that a sharp edge in millimeter dust emission does not mark the outer edge of the whole disk.

There are hints of additional structure beyond the main rings. Sz 111 and J1608 show a shoulder in their outer profiles near 90 au. It could signal a second gap, though the observations cannot resolve that feature well enough to confirm its nature. J1633, meanwhile, has the narrowest dust ring of the three.

What the gas temperatures reveal

The CO velocity patterns are consistent with disks rotating around their stars in Keplerian motion, and the spectra show no strong foreground absorption that would dominate the signal. The ¹³CO brightness-temperature peaks fall at about 24–30 au and reach roughly 21–32 kelvin across the sample.

Those temperatures are not clearly lower than measurements in other transition disks or in full disks, including disks around more luminous stars. That result is somewhat unexpected: lower-luminosity stars might be expected to heat their disks less. The comparison suggests that stellar luminosity alone does not set the observed gas temperature. Disk structure, chemistry, viewing angle, and the optical depth of the emission may all matter too.

Hidden dust, ongoing accretion

Although the observations reveal no inner dust emission, the stars’ ongoing accretion shows that material is reaching their inner regions. The limits on dust mass inside the cavities are very low: less than 0.003 Earth masses for Sz 111, 0.02 for J1633, and 0.0006 for J1608. In other words, a cavity can look nearly empty in millimeter-sized dust while still allowing gas to move inward.

Using the measured stellar accretion rates, and assuming a viscosity parameter of 0.001, the researchers estimate that the gas-to-dust ratios near 1 au are at least around 10⁴–10⁵, with an overall estimate near 10⁵. These are indirect estimates rather than direct measurements, but they underline how strongly dust and gas can differ in the innermost disk.

Possible companions—and important caveats

The team also used a published relationship between dust-ring radii and CO-gap radii to estimate the masses of possible companions. The inferred masses exceed 15 Jupiter masses in all three systems. Such objects could be very massive planets or brown dwarfs, but the estimates are highly uncertain. The relationship was calibrated only up to 15 Jupiter masses and does not account for eccentric orbits, which may change the shape and width of a gap.

The study therefore does not establish that companions of those masses are present. Instead, it shows how observations of both gas and dust can narrow the possibilities while exposing the limits of current estimates. The cavities are dust-poor, gas remains inside them, and the stars continue to accrete. Better-resolved observations and more complete models will be needed to determine what is carving these gaps—and how material crosses them.

Created on 09 Nov. 2026

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CO gaps in cold transition disks around low-mass stars

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Peering into the cavities of cold transition disks

Three disks, observed in detail

Dust rings and gas-filled cavities

What the gas temperatures reveal

Hidden dust, ongoing accretion

Possible companions—and important caveats

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