Modeling molecular hyperfine line emission

Abstract

In this paper we discuss two approximate methods previously suggested for modeling hyperfine spectral line emission for molecules whose collisional transitions rates between hyperfine levels are unknown. Hyperfine structure is seen in the rotational spectra of many commonly observed molecules such as HCN, HNC, NH3, N2H+, and C17O. The intensities of these spectral lines can be modeled by numerical techniques such as Lambda-iteration that alternately solve the equations of statistical equilibrium and the equation of radiative transfer. However, these calculations require knowledge of both the radiative and collisional rates for all transitions. For most commonly observed radio frequency spectral lines, only the net collisional rates between rotational levels are known. For such cases, two approximate methods have been suggested. The first method, hyperfine statistical equilibrium (HSE), distributes the hyperfine level populations according to their statistical weight, but allows the population of the rotational states to depart from local thermodynamic equilibrium (LTE). The second method, the proportional method approximates the collision rates between the hyperfine levels as fractions of the net rotational rate apportioned according to the statistical degeneracy of the final hyperfine levels. The second method is able to model non-LTE hyperfine emission. We compare simulations of N2H+ hyperfine lines made with approximate and more exact rates and find that satisfactory results are obtained.

34 pages. Pages 22-34 are data tables. For ApJ.

References in corpus (4)

• Systematic Molecular Differentiation in Starless Cores
• The Different Structures of the Two Classes of Starless Cores
• The Excitation of N${}_2$H${}^{+}$ in Interstellar Molecular Clouds. I - Models
• Radiative Transfer and Starless Cores

Cited by in corpus (45)

• Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes
• First detection of water vapor in a pre-stellar core
• Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping
• Deuterated methanol in the pre-stellar core L1544
• The dynamics of collapsing cores and star formation
• The Galactic Center Cloud G0.253+0.016: A Massive Dense Cloud with low Star Formation Potential
• Detection of (15)NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate (14)N/(15)N values
• Radiation-Hydrodynamic Simulations of Protostellar Outflows: Synthetic Observations and Data Comparisons
• Chemistry and Radiative Transfer of Water in Cold, Dense Clouds
• Resolved Depletion Zones and Spatial Differentiation of N2H+ and N2D+
• Line Profiles of Cores within Clusters: II Signatures of Dynamical Collapse during High Mass Star Formation
• Why does ammonia not freeze out in the center of pre-stellar cores?
• A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds
• Mapping deuterated methanol toward L1544: I. Deuterium fraction and comparison with modeling
• Radiative transfer modelling of W33A MM1: 3-D structure and dynamics of a complex massive star forming region
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Observation of HCN hyperfine line anomalies towards low- and high-mass star-forming cores
• NH_3(1_0-0_0) in the pre-stellar core L1544
• Seeds of Life in Space (SOLIS). III. Zooming into the methanol peak of the pre-stellar core L1544
• Probing Episodic Accretion in Very Low Luminosity Objects
• LOC program for line radiative transfer
• Physical properties and chemical composition of the cores in the California molecular cloud
• Anomalous HCN emission from warm giant molecular clouds
• Self-consistent dust and non-LTE line radiative transfer with SKIRT
• A multi-wavelength observation and investigation of six infrared dark clouds
• HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
• A large (~1 pc) contracting envelope around the prestellar core L1544
• ART${}^2$: A 3D Parallel Multi-wavelength Radiative Transfer Code for Continuum and Atomic and Molecular Lines
• Radiative Transfer of HCN: Interpreting observations of hyperfine anomalies
• Chemical evolution of Red MSX Sources in the southern sky
• Large-Scale Spectroscopic Mapping of the $\rho$ Ophiuchi Molecular Cloud Complex I. The C${}_2$H to N${}_2$H${}^{+}$ Ratio as a Signpost of Cloud Characteristics
• From Filaments to Oscillating Starless Cores
• Hyperfine resolved rate coefficients of HC17O+ with H2 (j = 0)
• APEX-SEPIA660 Early Science: Gas at densities above $10^7$ cm${}^{-3}$ towards OMC-1
• Radiative transfer modeling of the observed line profiles in G31.41+0.31
• Multi-wavelength modelling of the circumstellar environment of the massive proto-star AFGL 2591 VLA 3
• Chemical evolution of N2H+ in six massive star-forming regions
• Distribution of Water Vapor in Molecular Clouds. II
• A Survey of Deuterated Ammonia in the Cepheus Star-Forming Region L1251
• Tracing the contraction of the pre-stellar core L1544 with HC${}^{17}$O${}^{+}$ $J$ = 1-0 emission
• Detectability of deuterated water in prestellar cores
• Tracing the large-scale magnetic field morphology in protoplanetary disks using molecular line polarization
• A statistical study towards the high-mass BGPS clumps with the MALT90 survey
• Physical and chemical properties of Red MSX Sources in the southern sky: HII regions
• Insights into the first and second hydrostatic core stages from numerical simulations