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

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.