Chemical differences among collapsing low-mass protostellar cores

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi 830011, PR China
e-mail: sunjingfei@xao.ac.cn; xiaohu.li@xao.ac.cn
² Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Urumqi 830011, PR China
³ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
⁴ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁵ University of Chinese Academy of Sciences, Beijing 101408, PR China

Received: 6 October 2023
Accepted: 8 December 2023

Abstract

Context. Organic features lead to two distinct types of Class 0/I low-mass protostars: hot corino sources exhibiting abundant saturated complex organic molecules (COMs) and warm carbon-chain chemistry (WCCC) sources exhibiting abundant unsaturated carbon-chain molecules. Some observations suggest that the chemical variations between WCCC sources and hot corino sources are associated with local environments and the luminosity of protostars.

Aims. We aim to investigate the physical conditions that significantly affect WCCC and hot corino chemistry, as well as to reproduce the chemical characteristics of prototypical WCCC sources and hybrid sources, where both carbon-chain molecules and COMs are abundant.

Methods. We conducted a gas-grain chemical simulation in collapsing protostellar cores, adopting a selection of typical physical parameters for the fiducial model. By adjusting the values of certain physical parameters, such as the visual extinction of ambient clouds (A_V^amb), cosmic-ray ionization rate (ζ), maximum temperature during the warm-up phase (T_max), and contraction timescale of protostars (t_cont), we studied the dependence of WCCC and hot corino chemistry on these physical parameters. Subsequently, we ran a model with different physical parameters to reproduce scarce COMs in prototypical WCCC sources.

Results. The fiducial model predicts abundant carbon-chain molecules and COMs. It also reproduces WCCC and hot corino chemistry in the hybrid source L483. This suggests that WCCC and hot corino chemistry can coexist in some hybrid sources. Ultraviolet (UV) photons and cosmic rays can boost WCCC features by accelerating the dissociation of CO and CH₄ molecules. On the other hand, UV photons can weaken the hot corino chemistry by photodissociation reactions, while the dependence of hot corino chemistry on cosmic rays is relatively complex. The value of T_max does not affect any WCCC features, while it can influence hot corino chemistry by changing the effective duration of two-body surface reactions for most COMs. The long t_cont can boost WCCC and hot corino chemistry by prolonging the effective duration of WCCC reactions in the gas phase and surface formation reactions for COMs, respectively. The scarcity of COMs in prototypical WCCC sources can be explained by insufficient dust temperatures in the inner envelopes that are typically required to activate hot corino chemistry. Meanwhile, the high ζ and the long t_cont favors the explanation for scarce COMs in these sources.

Conclusions. The chemical differences between WCCC sources and hot corino sources can be attributed to the variations in local environments, such as A_V^amb and ζ, as well as the protostellar property, t_cont.