NH₃ (1,1) hyperfine intensity anomalies in the Orion A molecular cloud

Abstract

Ammonia (NH3) inversion lines, with their numerous hyperfine components, are a common tracer used in studies of molecular clouds (MCs). In local thermodynamical equilibrium, the two inner satellite lines (ISLs) and the two outer satellite lines (OSLs) of the NH3(J, K) = (1,1) transition are each predicted to have equal intensities. However, hyperfine intensity anomalies (HIAs) are observed to be omnipresent in star formation regions, a characteristic which is still not fully understood. In addressing this issue, we find that the computation method of the HIA by the ratio of the peak intensities may have defects, especially when used to process the spectra with low-velocity dispersions. Therefore, we defined the integrated HIAs of the ISLs (HIAIS) and OSLs (HIAOS) by the ratio of their redshifted to blueshifted integrated intensities (unity implies no anomaly) and developed a procedure to calculate them. Based on this procedure, we present a systematic study of the integrated HIAs in the northern part of the Orion A MC. We find that integrated HIAIS and HIAOS are commonly present in the Orion A MC and no clear distinction is found at different locations of the MC. The medians of the integrated HIAIS and HIAOS are 0.921 ± 0.003 and 1.422 ± 0.009, respectively, which is consistent with the HIA core model and inconsistent with the collapse or expansion (CE) model. In the selection of those 170 positions, where both integrated HIAs deviate by more than 3σ from unity, most (166) are characterized by HIAIS < 1 and HIAOS > 1, which suggests that the HIA core model plays a more significant role than the CE model. The remaining four positions are consistent with the CE model. We compare the integrated HIAs with the para-NH3 column density (N(para-NH3)), kinetic temperature (TK), total velocity dispersion (σv), non-thermal velocity dispersion (σNT), and the total opacity of the NH3(J, K) = (1,1) line (τ0). The integrated HIAIS and HIAOS are almost independent of N(para-NH3). The integrated HIAIS decreases slightly from unity (no anomaly) to about 0.7 with increasing TK, σv, and σNT. The integrated HIAOS is independent of TK and reaches values close to unity with increasing σv and σNT. The integrated HIAIS is almost independent of τ0, while the integrated HIAOS rises with τ0, thus showing higher anomalies. These correlations cannot be fully explained by either the HIA core nor the CE model.

Hyperfine collisional excitation of ammonia by molecular hydrogen

“…This indicates different excitation temperatures, meaning that the satellite components do not obey the local thermodynamic equilibrium (LTE) assumption in the interstellar medium. Such anomalies were first observed by Matsakis et al (1977) in source DR21 and subsequently by various authors (see e.g., Matsakis et al (1980); Gaume et al (1996); Rathborne et al (2008); Rosolowsky et al (2008); Camarata et al (2015); Caselli et al (2017); Zhou et al (2020)) in diverse sources. If the hyperfine levels are not populated thermally, the interpretation of observations requires radiative transfer models to account for non-LTE effects.…”

Star formation timescale in the molecular filament WB 673

“…Under LTE conditions and without a line-of-sight velocity gradient, the two inner satellite lines and two outer satellite lines of the NH 3 (1,1) transition have the same pairwise intensities. However, sometimes hyperfine intensity anomalies are observed in molecular clouds (see e. g. Stutzki & Winnewisser 1985;Park 2001;Zhou et al 2020), in the sense that the intensities of red-shifted components are not equal to the intensities of their blue-shifted counterparts. We quantify these anomalies calculating the ratios of the satellite's intensities after fitting them by independent Gaussian functions.…”

Interstellar Nitrogen Isotope Ratios: New NH3 Data from the Galactic Center out to the Perseus Arm

“…where T mb is the peak value of the main beam brightness temperature; m and s refer to the main and satellite group components, respectively; τ (J, K, m) is the optical depth of the main group of hyperfine components, a represents the expected intensity ratios of the satellite to the main group of hyperfine components, 0.278 and 0.222 for the inner and outer groups of HF features under conditions of LTE and optically thin line emission (see Ho & Townes 1983;Mangum et al 2015;Zhou et al 2020). For our sample, we used formula (2) to calculate the 14 NH 3 (1, 1) optical depth, according to the T mb peak values of the main component and inner and outer satellite components.…”