NH${}_3$ (1,1) hyperfine intensity anomalies in the Orion A molecular cloud

Abstract

In LTE, the two inner satellite lines (ISLs) and the two outer satellite lines (OSLs) of the NH${}_3$ (1,1) transition are each predicted to have equal intensities. However, hyperfine intensity anomalies (HIAs) are observed to be omnipresent in star formation regions, 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 being used to process the spectra with low velocity dispersions. Therefore we define the integrated HIAs of the ISLs (HIA${}_{\mathrm{IS}}$) and OSLs (HIA${}_{\mathrm{OS}}$) by the ratio of their redshifted to blueshifted integrated intensities and develop 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${}_{\mathrm{IS}}$ and HIA${}_{\mathrm{OS}}$ 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${}_{\mathrm{IS}}$ and HIA${}_{\mathrm{OS}}$ are 0.921$\pm$0.003 and 1.422$\pm$0.009, respectively, which is consistent with the HIA core model and inconsistent with the CE model. Selecting those 170 positions where both integrated HIAs deviate by more than 3-$\sigma$ from unity, most (166) are characterized by HIA${}_{\mathrm{IS}}$<1 and HIA${}_{\mathrm{OS}}$>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-NH${}_3$ column density ($N$(para-NH${}_3$)), kinetic temperature ($T_{\mathrm{K}}$), total velocity dispersion (${\sigma }_{\mathrm{v}}$), non-thermal velocity dispersion (${\sigma }_{\mathrm{NT}}$), and the total opacity of the NH${}_3$ (1,1) line (${\tau }_0$). Their correlations can not be fully explained by neither the HIA core nor the CE model.

12 pages, 10 figures, accepted for publication in A&A

References in corpus (7)

• The distance to the Orion Nebula
• An Ammonia Spectral Atlas of Dense Cores in Perseus
• Evolution of column density distributions within Orion~A
• Multi-wavelength observations of Southern Hot Molecular Cores traced by methanol masers - I. Ammonia and 24 GHz Continuum Data
• Large Area Mapping at 850 Microns. V. Analysis of the Clump Distribution in the Orion A South Molecular Cloud
• VLA observations of ammonia in high-mass star formation regions
• HfS, Hyperfine Structure Fitting Tool

Cited by in corpus (5)

• Evidence for dense gas heated by the explosion in Orion KL
• Interstellar Nitrogen Isotope Ratios: New NH3 Data from the Galactic Center out to the Perseus Arm
• Star formation timescale in the molecular filament WB 673
• KAgoshima Galactic Object survey with Nobeyama 45-metre telescope by Mapping in Ammonia lines (KAGONMA): Discovery of parsec-scale CO depletion in the Canis Major star-forming region
• NH3 (1,1) hyperfine intensity anomalies in infall sources