Digging into the Massive Protostar S255IR NIRS3: A Study of Nitrogen-Bearing Molecules and Their Prebiotic Chemistry

Abstract

The study of complex nitrogen (N)-bearing molecules is essential for probing the physical and chemical evolution of star-forming regions. In this paper, we present the identification of rotational emission lines from several complex N-bearing species such as methyl cyanide (CH$_{3}$CN), ethyl cyanide (C$_{2}$H$_{5}$CN), vinyl cyanide (C$_{2}$H$_{3}$CN), cyanamide (NH$_{2}$CN), and formamide (NH$_{2}$CHO) toward the high-mass protostar S255IR NIRS3 using ALMA band 4 observations. In addition, the vibrationally excited transitions of cyanoacetylene (HC$_{3}$N, $ν_{7}$ = 2) were detected. The column densities and excitation temperatures of these molecules were derived through LTE spectral modelling, yielding excitation temperatures in the range of 175$-$220 K. The high excitation temperatures (175$-$220 K) indicate that the identified N-bearing molecules arise from the warm inner regions ($T \geq 100$ K) of the source. The fractional abundances were further estimated relative to H$_{2}$, CH$_{3}$OH, and CH$_{3}$CN. A Pearson correlation heat map of the abundances reveals a strong positive correlation ($r > 0.7$) among three molecules in the cyanide family, such as CH$_{3}$CN, C$_{2}$H$_{3}$CN, and C$_{2}$H$_{5}$CN, suggesting that these N-bearing molecules may be chemically linked. Comparison with three-phase warm-up chemical models shows that the observed abundances of CH$_{3}$CN, C$_{2}$H$_{5}$CN, C$_{2}$H$_{3}$CN, NH$_{2}$CN, NH$_{2}$CHO, and HC$_{3}$N ($ν_{7}$ = 2) relative to H$_{2}$ are consistent with model predictions within factors of 1.04, 0.67, 1.28, 0.76, 0.72, and 0.96, respectively. Finally, we discuss the potential formation pathways of the identified N-bearing molecules in the context of gas-grain chemistry within S255IR NIRS3.