CO depletion in infrared dark clouds

¹ Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
⁴ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904-4325, USA
⁵ Department of Astronomy, Yale University, New Haven, CT 06511, USA
⁶ INAF Osservatorio Astronomico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁷ California Institute of Technology, Pasadena, CA 91125, USA
⁸ Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, Madrid, Spain
⁹ Laboratory for the study of the Universe and eXtreme phenomena (LUX), Observatoire de Paris, 5, place Jules Janssen, 92195 Meudon, France
¹⁰ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching bei München, Germany
¹¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

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

Received: 4 September 2025
Accepted: 28 October 2025

Abstract

Context. Infrared dark clouds (IRDCs) are cold, dense structures that are likely representative of the initial conditions of star formation. Many studies of IRDCs employ CO to investigate cloud dynamics, but CO can be highly depleted from the gas phase in IRDCs, which affects its fidelity as tracer. The CO depletion process is also of great interest in astrochemistry because CO ice in dust grain mantles provides the raw material for the formation of complex organic molecules.

Aims. We study CO depletion towards four IRDCs to investigate its correlation with the H₂ number density and dust temperature, calculated from Herschel far-infrared images.

Methods. We used ¹³CO J = 1 → 0 and 2 → 1 maps to measure the CO depletion factor, f_D, across IRDCs G23.46-00.53, G24.49-00.70, G24.94-00.15, and G25.16-00.28. We also considered a normalised CO depletion factor, f′_D, which takes a value of unity, that is, no depletion, in the outer lower-density and warmer regions of the clouds. We then investigated the dependence of f_D and f′_D on the gas density, n_H, and dust temperature, T_dust.

Results. The CO depletion rises as the density increases and reaches maximum values of f′_D ∼ 10 in some regions with n_H ≳ 3 × 10⁵ cm⁻³, although with significant scatter at a given density. We find a tighter, less scattered relation of f′_D with temperature that rapidly rise for temperatures ≲18 K. We propose a functional form f′_D = exp(T₀/[T_dust − T₁]) with T₀ ≃ 4 K and T₁ ≃ 12 K to reproduce this behaviour.

Conclusions. We conclude that CO is strongly depleted from the gas phase in cold, dense regions of IRDCs. This means that if it is not accounted for, CO depletion can lead to an underestimation of the total cloud masses based on CO line fluxes by factors up to ∼5. These results indicate a dominant role for thermal desorption in setting near equilibrium abundances of gas-phase CO in IRDCs and provide important constraints for astrochemical models and the chemodynamical history of gas in the early stages of star formation.