Compact CO emission and no evidence of radial drift

ALMA observations of the faintest planet-forming disks in Lupus

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁴ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁵ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁶ Joint ALMA Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁷ University of Virginia (UVA), 1827 University Avenue, Charlottesville, VA, USA
⁸ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁹ Universidad de Sevilla, ETSI, Camino de los Descubrimientos, 41092 Sevilla, Spain
¹⁰ INAF OA Arcetri, Largo Enrico Fermi, 5, 50125 Firenze FI, Italia
¹¹ Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 December 2025
Accepted: 3 April 2026

Abstract

Context. A large fraction of the planet-forming disks surveyed by ALMA show faint CO emission, which is commonly interpreted as an indication of severe CO depletion. However, disks can be faint for multiple reasons, including by having their emission unresolved spatially, which may result in their size being overestimated, making their flux appear faint. The limited sensitivity of previous observations prevented us from determining whether this scenario can indeed account for the observed faint CO emission for radially compact disks, hindering our understanding of disk evolution and planet formation in most of the disk population.

Aims. We present new ALMA observations targeting ¹²CO (J = 3–2) and ¹³CO (J = 3–2) in 17 of the faintest planet-forming disks in Lupus. We aim to test the feasibility of the compact disk scenario as a plausible explanation for compact disks with faint CO isotopolog emission.

Methods. Our sample contains 17 disks observed with ALMA in Band 7 at the moderate angular resolution of 0_⋅^′′25 (≈20 au radius at 160 pc, the median distance of the sample), approximately one order of magnitude deeper than the available archival ALMA data where ¹²CO and ¹³CO were not detected. We used line stacking techniques to enhance the signal-to-noise ratio and extract the CO fluxes when possible. Finally, we compared the CO line luminosities with a grid of physical-chemical models of extended and compact disks and computed the disk dust and CO sizes.

Results. We detected ¹²CO and ¹³CO emission in ten disks. Four disks were detected only in ¹²CO, and three disks were not detected in either of the two isotopologs.

Conclusions. The observations indicate that some of these disks are consistent with being intrinsically compact and optically thick, in both ¹²CO and ¹³CO. This scenario offers an alternative explanation to the commonly accepted hypothesis of significant CO depletion. The derived gas radii further support this interpretation (R_CO ≤ 40 au), suggesting that a significant fraction of disks may be born intrinsically small, as is also indicated by recent Class 0/I surveys. Furthermore, the resulting gas-to-dust size ratios reveal no clear signs of dust evolution, suggesting that these compact disks are not drift-dominated.