Parsec scales of carbon chain and complex organic molecules in AFGL 2591 and IRAS 20126

¹ University of Calgary, Calgary, Canada
e-mail: pamela.freeman@ucalgary.ca
² IRAP, Université de Toulouse, CNRS, UPS, CNES, 31000 Toulouse, France
³ Tata Institute of Fundamental Research, Mumbai, India

Received: 22 June 2023
Accepted: 8 August 2023

Abstract

Context. There is a diverse chemical inventory in protostellar regions leading to the classification of extreme types of systems. Warm carbon chain chemistry sources, for one, are the warm and dense regions near a protostar containing unsaturated carbon chain molecules. Since the presentation of this definition in 2008, there is a growing field to detect and characterise these sources. The details are lesser known in relation to hot cores and in high-mass star-forming regions - regions of great importance in galactic evolution.

Aims. To investigate the prevalence of carbon chain species and their environment in high-mass star-forming regions, we have conducted targeted spectral surveys of two sources in the direction of Cygnus X - AFGL 2591 and IRAS 20126+4104.

Methods. We observed these sources in frequency ranges around 85, 96, and 290 GHz with the Green Bank Telescope and the IRAM 30m Telescope. We have constructed a local thermodynamic equilibrium (LTE) model using the observed molecular spectra to determine the physical environment in which these molecules originate. We map both the observed spatial distribution and the physical parameters found from the LTE model. We also determine the formation routes of these molecules in each source using the three-phase NAUTILUS chemical evolution code.

Results. We detect several lines of propyne, CH₃CCH, and cyclopropenylidene, c-C₃H₂ as tracers of carbon chain chemistry, as well as several lines of formaldehyde, H₂CO, and methanol, CH₃OH, as a precursor and a tracer of complex organic molecule chemistry, respectively. We find excitation temperatures of 20−30 K for the carbon chains and 8−85 K for the complex organics. The observed abundances, used as input for the chemical evolution code, are 10⁻⁹ to 10⁻¹⁰ for both CH₃CCH and CH₃OH. The CH₃CCH abundances are reproduced by a warm-up model, consistent with warm carbon chain chemistry, while the observed CH₃OH abundances require a shock mechanism sputtering the molecules into the gas phase.

Conclusions. Single-dish observations are useful for studying the envelope-scale chemistry of star-forming regions, including mechanisms such as warm carbon chain chemistry. As well, LTE models lend well to the wide-band maps obtained from these telescopes. The physical and chemical environment determined for complex hydrocarbons and complex organics lends understanding to high-mass star formation.