ALMA detection of continuum emission from GQ Lup B

Pietro Pirola; Stefano Facchini; Gabriele Cugno; Myriam Benisty; Tomas Stolker; Jaehan Bae; Ruobing Dong; Brendan P. Bowler; Ya-Lin Wu; Richard Teague; Pietro Curone; Felipe Alarcon

AI summary · arXiv: 2611.12504v1 · astro-ph.EP

ALMA detection of continuum emission from GQ Lup B

Pietro Pirola, Stefano Facchini, Gabriele Cugno, Myriam Benisty, Tomas Stolker, Jaehan Bae, Ruobing Dong, Brendan P. Bowler,

and 4 more authors

Ya-Lin Wu, Richard Teague, Pietro Curone, Felipe Alarcon

Original paper Submitted to arXiv on 09 Nov. 2026 12 pages, 11 figures, 2 tables including appendices. Accepted for publication in A&A License: CC BY 4.0

How to read this summary? Illustrated guide

The essentials

ALMA detects unresolved 0.89 mm emission at GQ Lup B, consistent with a compact dusty companion disk, but its emission mechanism and disk properties remain uncertain.

Question

Does GQ Lup B emit millimeter continuum that can be associated with a disk around the companion? What constraints does this detection provide on the disk’s size, dust mass, and relationship to the companion’s accretion?

Method

The authors combined ALMA Band 7 observations from two programs, comprising 16 execution blocks across 13 nights and five baseline setups, and applied self-calibration and visibility alignment before imaging. They tested the detection across imaging weightings and cleaning thresholds, then modeled the measured 0.89 mm emission under optically thin and optically thick assumptions to estimate dust mass and emitting radius.

Findings

Result The authors report a 5.8σ point-source detection at the companion’s location, with peak intensity 54.5 ± 9.4 μJy beam−1.
See the passage in the paper
The most significant detection ($5.8\sigma$) is obtained with robust = 2, with a peak intensity of $54.5\,\mu\rm Jy\,beam^{-1}$ and a 9.4 $\mu\rm Jy\,beam^{-1}$ rms calculated in the same emission-free annulus described in the previous section.
Result The emission is unresolved, giving an upper limit of 2.3 au on the dust emission radius.
See the passage in the paper
Therefore we adopt $\sim 2.3$ au as a conservative upper limit on the outer radius of the dust component of the disk.
Result Under the optically thin dust-emission assumption, the inferred dust mass is 0.0032 Earth masses.
See the passage in the paper
We find a mass of 0.0032 $M_{\oplus}$, less than half of and a quarter of the analog estimates we obtain for PDS 70 c and SR 12 c (0.006 and 0.012 $M_{\oplus}$), respectively.
Result Under the optically thick model, the flux corresponds to a smaller emitting radius of 0.20 au; the authors caution this may be a lower limit.
See the passage in the paper
We obtain an outer radius of 0.20 au (418 RJup), smaller than SR 12 c (0.34 au) and PDS 70 c (0.38 au, setting $\phi=1^{\circ}$).
Authors’ interpretation The short inferred depletion timescale may be reconciled with ongoing accretion if the companion’s orbit replenishes its gas reservoir by crossing the primary disk.
See the passage in the paper
This periodic interaction could continuously replenish the gas reservoir of the disk of GQ Lup B, naturally explaining the observed high accretion rate due to an increased inflow of material.

Limitations

A single-band measurement cannot distinguish thermal dust emission from other possible mechanisms, including free-free emission from accretion shocks.
See the passage in the paper
Only a multi-band characterization and thorough radiative transfer modeling can distinguish between different scenarios.
The disk’s exact mass and radius are not determined by the unresolved single-band detection and depend on modeling assumptions.
See the passage in the paper
To carry out our analysis, we had to make use of several assumptions (dust opacity, gas mass, inclination, radial extent, temperature), which often have no direct observational measurement.
The depletion-time interpretation is uncertain because the accretion rate may vary and depends on the extinction correction.
See the passage in the paper
First, the inferred $\dot{M}$ depends on the correction adopted for the extinction ($A_{V}\sim 2.3$ mag); previous work adopting lower estimates (Wu et al. 2015), inferred an accretion rate lower by an order of magnitude.

Why it matters

The detection adds a third millimeter-continuum detection of a gravitationally bound planetary-mass object and provides a benchmark for the sensitivity needed to find such faint disks. The authors argue that a larger sample and multiwavelength observations are needed to determine whether disks around substellar companions resemble scaled-down circumstellar disks or involve additional physical processes.

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: GQ Lup B; protoplanetary disks; ALMA; millimeter continuum; accretion

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper reports an ALMA search for millimeter continuum emission from GQ Lup B, a young, massive companion orbiting the T Tauri star GQ Lup A at a projected distance of about 110 astronomical units. Earlier infrared observations suggested that GQ Lup B has a dusty disk, but previous millimeter observations did not detect it. Millimeter emission can probe cooler dust and help constrain a disk’s material and extent. The authors investigate whether this emission can now be detected and what the observations reveal about the companion’s disk and accretion. The study combines ALMA Band 7 observations at approximately 0.89 millimeters from two observing programs conducted between July 2024 and June 2025. The data cover several baseline configurations, including observations with the Atacama Compact Array and the 12-meter array. The team calibrated and aligned the datasets, then applied self-calibration to improve image quality and sensitivity. They produced images using different weighting choices and tested whether the faint source might be an imaging artifact or contamination from the bright disk around the primary star. The authors detect a point-like source at GQ Lup B’s position, with a peak intensity of 54.5 ± 9.4 microjanskys per beam and a nominal significance of 5.8 sigma. The source is about 0.02 arcseconds from the companion’s position measured by GRAVITY astrometry. Its peak brightness is consistent across images with different angular resolutions, and tests of the residual images support the conclusion that it is not simply a sidelobe from the primary disk. The detection also lies near the position of the companion’s infrared emission. The authors therefore interpret it as emission associated with a disk around GQ Lup B, most likely thermal radiation from dust grains. The emission remains unresolved, even with the finest useful synthesized beam, approximately 29 by 25 milliarcseconds. At the system’s distance of 154.1 parsecs, this corresponds to an upper limit of about 2.3 astronomical units for the dust-emitting radius. Assuming optically thin dust emission, a mean dust temperature of about 56 kelvin, and an opacity appropriate for millimeter-sized grains, the authors estimate a dust mass of 0.0032 Earth masses. In a separate optically thick model, the observed flux is consistent with a much smaller emitting radius of about 0.20 astronomical units. These are conditional estimates, not direct measurements of the disk’s actual size or mass. The authors also compare the inferred disk reservoir with GQ Lup B’s reported accretion rate of about 10 to the minus 9.5 solar masses per year. Assuming a gas-to-dust mass ratio of 100 and a constant accretion rate, they estimate a depletion time of roughly 10,000 years—far shorter than the system’s estimated age of 3–5 million years. They suggest that the companion’s highly inclined orbit, which crosses the primary’s gas disk about twice during each roughly 1,000-year orbit, could replenish its gas reservoir. This explanation is plausible but is not demonstrated by the continuum measurement. The conclusions are limited by the single millimeter-frequency measurement and the source’s unresolved appearance. The inferred mass and radius depend on assumptions about temperature, dust opacity, optical depth, disk geometry, and the gas-to-dust ratio. The authors also cannot rule out a contribution from free-free emission associated with accretion shocks. Observations at multiple millimeter wavelengths and more complete radiative-transfer modeling are needed to distinguish emission mechanisms and better constrain the disk’s properties.

A first millimeter detection from GQ Lup B

Astronomers have detected faint millimeter-wavelength emission from GQ Lup B, a young, massive companion orbiting the T Tauri star GQ Lup A. The result, reported in “ALMA detection of continuum emission from GQ Lup B” by Pietro Pirola, Stefano Facchini, Gabriele Cugno, Myriam Benisty, Tomas Stolker, Jaehan Bae and colleagues, offers new evidence for dusty material around the companion.

GQ Lup B lies about 110 astronomical units from its host star in projection. Earlier infrared observations pointed to a dusty disk around the companion, but previous millimeter observations had not detected one. Millimeter light can reveal cooler dust and help researchers estimate how much material a disk contains and how far it extends. The new study asks whether that emission can be found—and what it might tell us about the companion’s disk and ongoing accretion.

Finding a faint point of light

The team combined ALMA Band 7 observations, taken at a wavelength of about 0.89 millimeters through two observing programs between July 2024 and June 2025. The data included multiple telescope configurations, from the Atacama Compact Array to ALMA’s 12-meter array. After calibrating and aligning the observations, the researchers used a technique called self-calibration to improve the images’ quality and sensitivity.

They also checked the signal using different image-weighting choices and investigated whether it could be an artifact or spillover from the much brighter disk around GQ Lup A. The source appears at the position of GQ Lup B, with a peak intensity of 54.5 ± 9.4 microjanskys per beam. That is a nominal detection significance of 5.8 sigma. Its position is about 0.02 arcseconds from the companion’s location measured by GRAVITY astrometry.

The measured brightness remains consistent across images made at different angular resolutions. Tests of the residual images also argue against the signal being a sidelobe from the primary star’s disk. The detection is close to the companion’s infrared emission as well. Taken together, these checks support the interpretation that the millimeter signal is associated with GQ Lup B, most likely as thermal emission from dust grains.

A disk too small to resolve

Although the signal is detected, the emitting source remains unresolved—even in the finest useful image, whose beam is approximately 29 by 25 milliarcseconds. At the system’s distance of 154.1 parsecs, the observations place an upper limit of about 2.3 astronomical units on the radius of the dust-emitting region. This is a limit, not a direct measurement of the disk’s true size.

The team estimates the dust mass in two ways, illustrating how strongly the answer depends on assumptions about the emission. If the dust is optically thin, with a mean temperature of about 56 kelvin and an opacity suited to millimeter-sized grains, the estimated dust mass is 0.0032 Earth masses. In a separate optically thick model, the observed brightness is consistent with a much smaller emitting radius of about 0.20 astronomical units. Neither model establishes the disk’s actual mass or radius on its own.

A short-lived reservoir?

The authors also consider how the inferred dust reservoir compares with GQ Lup B’s reported accretion rate of about 10 to the minus 9.5 solar masses per year. If the disk’s gas-to-dust mass ratio is 100 and the accretion rate remains constant, the available material would be depleted in roughly 10,000 years—much less than the system’s estimated age of 3–5 million years.

One possible explanation is that GQ Lup B replenishes its gas supply as it moves around its host. Its highly inclined orbit crosses the primary star’s gas disk about twice during each orbit, which lasts roughly 1,000 years. Those crossings could provide fresh material, but the continuum detection does not demonstrate that this process is occurring. It remains a plausible idea to investigate, rather than a conclusion established by these observations.

What remains uncertain

The finding is an important step, but a single millimeter-frequency measurement cannot fully identify the source of the emission or pin down the disk’s properties. Estimates of mass and size depend on the assumed dust temperature, opacity, optical depth, disk geometry and gas-to-dust ratio. The researchers also cannot rule out some contribution from free-free emission associated with accretion shocks.

Observations at multiple millimeter wavelengths could help distinguish thermal dust emission from other mechanisms and test the models. More complete radiative-transfer modeling would also improve estimates of the disk’s structure and material. For now, ALMA’s detection provides evidence for emission associated with GQ Lup B and sets valuable limits on its compact dusty environment—while leaving the disk’s exact scale, mass and replenishment history open to further study.

Created on 10 Nov. 2026

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ALMA detection of continuum emission from GQ Lup B

The essentials

Question

Method

Findings

Limitations

Why it matters

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Definitions

A first millimeter detection from GQ Lup B

Finding a faint point of light

A disk too small to resolve

A short-lived reservoir?

What remains uncertain

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