CHemical Evolution in MassIve star-forming COres (CHEMICO)

I. Evolution of the temperature structure

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
² LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁵ Dipartimento di Chimica “Giacomo Ciamician”, Università di Bologna, Via P. Gobetti 85, 40129 Bologna, Italy
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ Research Laboratory for Astrochemistry, Ural Federal University, Kuibysheva St. 48, Yekaterinburg 620026, Russia

^★ Corresponding author: francesco.fontani@inaf.it

Received: 11 June 2025
Accepted: 1 July 2025

Abstract

Context. Increasing evidence shows that most stars in the Milky Way, including the Sun, are born in star-forming regions that also contain high-mass stars. However, due to both observational and theoretical challenges, our understanding of their chemical evolution is much less clear than that of their low-mass counterparts.

Aims. In this work, we present the project ‘CHemical Evolution of MassIve star-forming COres’ (CHEMICO). The project aims to investigate aspects of the chemical evolution of high-mass star-forming cores by observing representatives of the three main evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact HII (UCHII) regions.

Methods. We carried out an unbiased spectral line survey of the entire bandwidth at 3, 2, and 1.2 mm with the 30m telescope of the Insitut de Radioastronomie millimétrique towards three targets that represent the three evolutionary stages.

Results. The number of detected lines and species increases with evolution. In this first study, we derive the temperature structure of the targets through the analysis of the carbon-bearing species C₂H, c-C₃H, c-C₃H₂, C₄H, CH₃CCH, HC₃N, CH₃CN, and HC₅N. The excitation temperature, T_ex, increases with evolution in each species, although not uniformly. Hydrocarbons tend to be associated with the smallest T_ex values and enhancements with evolution, while cyanides are associated with the highest T_ex values and enhancements. In each target, the higher the number of atoms in the molecule, the higher T_ex tends to be.

Conclusions. The temperature structure evolves from a cold (~20 K), uniform envelope traced by simple hydrocarbons in the high-mass starless core stage, to a more stratified envelope in the protostellar stage made by a hot core (≥100 K), an intermediate shell with T_ex ~30–60 K, and a larger cold envelope. Finally, in the UCHII stage, a hot core surrounded only by a cold envelope remains. These results suggest a steepening of the T_ex radial profile as a function of time.