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

The study of complex nitrogen-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 Atacama Large Millimeter/submillimeter Array 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 modeling, 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 ≥ 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.