2016/04/11 by A. M. Mullins, R. M. Loughnane, Robert M. Loughnane +5 · 15 citations
Chemistry · Earth and Planetary Sciences · Physics and Astronomy · #Angular momentum #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Atomic physics #Emission spectrum #Hyperfine structure #Line (geometry) #Molecular Spectroscopy and Structure #Optics #Physics #Radiative transfer #Rotational transition #Spectral line #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stw835
published in Monthly Notices of the Royal Astronomical Society 459(3), 2882-2892 (Oxford University Press) · 12 pages, 7 figures. Accepted for publication in MNRAS
arxiv created 2016/04/11 · openalex publication_date 2016/04/15 · arxiv updated 2016/04/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
Molecules with hyperfine splitting of their rotational line spectra are useful probes of optical depth, via the relative line strengths of their hyperfine components. The hyperfine splitting is particularly advantageous in interpreting the physical conditions of the emitting gas because with a second rotational transition, both gas density and temperature can be derived. For HCN however, the relative strengths of the hyperfine lines are anomalous. They appear in ratios which can vary significantly from source to source, and are inconsistent with local thermodynamic equilibrium (LTE). This is the HCN hyperfine anomaly, and it prevents the use of simple LTE models of HCN emission to derive reliable optical depths. In this paper, we demonstrate how to model HCN hyperfine line emission, and derive accurate line ratios, spectral line shapes and optical depths. We show that by carrying out radiative transfer calculations over each hyperfine level individually, as opposed to summing them over each rotational level, the anomalous hyperfine emission emerges naturally. To do this requires not only accurate radiative rates between hyperfine states, but also accurate collisional rates. We investigate the effects of different sets of hyperfine collisional rates, derived via the proportional method and through direct recoupling calculations. Through an extensive parameter sweep over typical low-mass star-forming conditions, we show the HCN line ratios to be highly variable to optical depth. We also reproduce an observed effect whereby the red-blue asymmetry of the hyperfine lines (an infall signature) switches sense within a single rotational transition.