Initial conditions of star formation at ≲2000 au: Physical structure and NH₃ depletion of three early-stage cores

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching bei München, Germany
e-mail: ylin@mpe.mpg.de
² Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finland
³ Green Bank Observatory, PO Box 2, Green Bank, WV 24944, USA
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁵ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC

Received: 26 June 2023
Accepted: 22 August 2023

Abstract

Context. Pre-stellar cores represent a critical evolutionary phase in low-mass star formation. Characterisations of the physical conditions of pre-stellar cores provide important constraints on star and planet formation theory and are pre-requisites for establishing the dynamical evolution and the related chemical processes.

Aims. We aim to unveil the detailed thermal structure and density distribution of three early-stage cores -starless core L1517B and pre-stellar cores L694-2 and L429- with the high angular resolution observations of the NH₃ (1,1) and (2,2) inversion transitions obtained with VLA and GBT. In addition, we explored if and where NH₃ depletes in the central regions of the cores.

Methods. We calculated the physical parameter maps of gas kinetic temperature, NH₃ column density, line width, and centroid velocity of the three cores utilising the NH₃ (1,1) and (2,2) lines. We applied the mid-infrared extinction method to the Spitzer 8 μm map to obtain a high angular resolution hydrogen column density map. We examined the correlation between the derived parameters and the properties of individual cores. We derived the gas density profile from the column density maps and assessed the variation of NH₃ abundance as a function of gas volume density.

Results. The measured temperature profiles of the cores L429 and L1517B show a minor decrease towards the core centre, dropping from ~9 K to below 8 K, and ~11 K to 10 K, while L694-2 has a rather uniform temperature distribution of ~9 K. Among the three cores, L429 has the highest central gas density, close to sonic velocity line width, and the largest localised velocity gradient, all indicative of an advanced evolutionary stage. We resolve that the abundance of NH₃ becomes two times lower in the central region of L429, occurring around a (line-of-sight mass-averaged) gas density of 4.4 × 10⁴ cm⁻³. Compared to Ophiuchus/H-MM1 which shows an even stronger drop of the NH₃ abundance at 2 × 10⁵ cm⁻³, the abundance variations of the three cores plus Ophiuchus/H-MM1 suggest a progressive NH₃ depletion with increasing central density in pre-stellar cores.