Hyperfine structure of HCN J = 1-0 and implied physical information concerning NGC 7538 IRS1

Abstract

The so-called intensity anomalies of hyperfine components of HCN(v = 0, J = 1-0) in the NGC 7538 molecular cloud are studied with a high resolution spectrometer. The measured intensity ratio R02 = T(a)*(F = 0-1)/T(a)*(F = 2-1) is clearly larger than the value 0.2, which is expected under the optically thin local thermodynamic equilibrium (LTE) condition, and the ratio R12= T(a)*(F = 1-1)/T(a)*(F = 2-1) smaller than the LTE value 0.6 at most of the observed positions. Radiative transfer calculations were performed using the large velocity gradient (LVG) model with a spherically symmetric velocity field. The line overlaps have also been taken into account. The analysis of the observational data shows that the three hyperfine components of the J = 1-0 transition are optically thick. While the optical depths of F = 2-1 and F = 1-1 are much larger than that of F = 0-1 hyperfine component, the scattering effect must be taken into account. The stronger two hyperfine components, F = 2-1, and F = 1-1, are formed in the envelope with the average hydrogen density 1.6 X 10(4) cm-3, but the thinner F = 0-1 hyperfine component in the core and in the envelope. The excitation temperatures of the three hyperfine components are different and the LTE assumption for the HCN hyperfine intensity ratio is unrealistic.