Characterization of star-forming regions using non-LTE modeling: the importance of collisional rates - C. T. Bop, and F. Lique

HCN, HNC, and their isotopologues are ubiquitous molecules that can serve as chemical thermometers and evolutionary tracers to characterize star-forming regions. Despite their importance in conveying vital information for studies on the chemistry and evolution of these regions, the collision rates of some of these molecules have not been available for rigorous investigations in the past. Therefore, we calculated new rate coefficients for HCN, HNC, and their isotopologues induced by collisions with H₂. With this updated set of collisional rate coefficients and millimeter observations, we derived column densities for the target species, as well as the D/H, ¹³C/C, and ¹⁵N/N isotopic fractions, and the isomeric ratios. The continuum data at 3 mm and 850 µm allowed us to compute the emissivity spectral index and examine grain growth as an evolutionary tracer. We found that the H¹³CN/HN¹³C ratio is anticorrelated with the deuterium fraction of HCN, making it a useful proxy for temperature. The spectral index shows a tentative anticorrelation with the H¹³CN/HN¹³C ratio, suggesting grain growth in the evolved, hotter, and less deuterated sources. Unlike TMC 1-C, the south-to-north gradient in dust temperature and spectral index observed in NGC 1333-C7 suggests feedback from the main NGC 1333 cloud. Therefore, our findings indicate that dust temperature, deuterium fraction, and spectral index are complementary evolutionary tracers