Deciphering chemical heritage: Isotopic study of methanol and other complex organic molecules in a solar-type protostar NGC 1333 IRAS 4A2

Abstract

Hot corinos are early-stage low-mass protostars where many complex organic molecules (COMs) are detected. Understanding these organics is important to reveal their heritage in later evolutionary stages. Isotopic studies are an effective way to uncover the chemical heritage and gain insights into the astrophysical processes involved. In this study, we present a comprehensive analysis of COMs and their isotopologues in the NGC 1333 IRAS 4A2 hot corino, focusing on their excitation temperatures, column densities, and formation pathways. Using high-resolution ALMA observations and LTE spectral modeling, we identified deuterated isotopologues of several COMs, and for methanol and some other species we also detected ¹³C- and ¹⁸O-substituted forms. The derived excitation temperatures ($T_{\text{ex}}$) range from 80 to 200 K, indicating diverse thermal conditions within the hot corino. Although the limited spatial resolution and spectral width prevented resolving individual emission regions, our findings suggest a chemical stratification consistent with steep temperature gradients observed in similar environments. Our analysis reveals that molecular binding energies of the species are primarily responsible for this chemical stratification and align well with the temperature gradient observed in similar regions. Additionally, the isotopic and deuterium fractionation observed in IRAS 4A2 is lower than that reported for IRAS 16293-2422B, possibly reflecting hotter inner regions or faster formation timescales. These results provide new insights into the physical and chemical processes governing hot corinos during star formation.