One-dimensional and time-dependent modelling of complex organic molecules in protostars

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Bonn, Germany
³ Department of Physics and Astronomy, University College London, Gower Street, London, UK
⁴ SURF, Amsterdam, The Netherlands
⁵ Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands
⁶ Institute for Molecules and Materials, Radboud University, 6525 AJ Nijmegen, The Netherlands
⁷ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, 9 av. du Colonel Roche, 31028 Toulouse Cedex 4, France
⁸ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁹ Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea
¹⁰ Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

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

Received: 27 June 2025
Accepted: 22 December 2025

Abstract

Complex organic molecules (COMs), the building blocks of life, have been extensively detected under various physical conditions, from quiescent clouds to star-forming regions. They therefore serve as excellent tracers of the local physical and chemical properties of these environments. Proper models that are capable of grasping the formation and destruction of COMs are crucial to understanding observations. However, given that distinct COMs can be detected from different locations and at varying times, we improved UCLCHEM – a gas-grain chemical code – to a 1D, time-dependent model tailored to protostars. In this update, we examine two stages of a protostar, the prestellar and heating stages, incorporating a simple radiative mechanism for both the internal and external radiation fields of the cloud. This approach relies on the key assumption that the dust and gas temperatures are completely coupled. Ultimately, we implemented an updated version of our model to interpret observations obtained through both single-dish and interferometry under varying conditions, including a SgrB2(N1) hot core, massive Galactic clumps, and a hot core in Orion. We show that our model can reproduce these observations well. We highlight that some COMs are positioned at a higher temperature in the envelope, and others at a lower temperature, which could potentially leading to misinterpretations when using a single-point model. In the case of SgrB2(N1), the best model indicates that the cosmic-ray ionisation rate significantly exceeds the value typically used for the standard interstellar medium. Our model is as an efficient computational tool that will be particularly useful for gaining better insights into COM observations.