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

¹ Xinjiang Astronomical Observatory, CAS 150, Science 1-Street Urumqi, Xinjiang 830011, PR China
e-mail: wug@xao.ac.cn
² Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Urumqi 830011, PR China
³ University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049, PR China
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
⁶ School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China

Received: 10 September 2019
Accepted: 16 June 2020

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 (HIA_IS) and OSLs (HIA_OS) 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 HIA_IS and HIA_OS 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 HIA_IS and HIA_OS 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 HIA_IS < 1 and HIA_OS > 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 (T_K), total velocity dispersion (σ_v), non-thermal velocity dispersion (σ_NT), and the total opacity of the NH₃(J, K) = (1,1) line (τ₀). The integrated HIA_IS and HIA_OS are almost independent of N(para-NH₃). The integrated HIA_IS decreases slightly from unity (no anomaly) to about 0.7 with increasing T_K, σ_v, and σ_NT. The integrated HIA_OS is independent of T_K and reaches values close to unity with increasing σ_v and σ_NT. The integrated HIA_IS is almost independent of τ₀, while the integrated HIA_OS rises with τ₀, thus showing higher anomalies. These correlations cannot be fully explained by either the HIA core nor the CE model.