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| CLOUDY | |
|---|---|
| Name | CLOUDY |
| Developer | Gary Ferland |
| Initial release | 1978 |
| Latest release | 17.03 |
| Programming language | Fortran |
| Operating system | Unix-like, Windows, macOS |
| License | GPL |
| Genre | Astrophysical plasma simulation |
CLOUDY
CLOUDY is a spectral synthesis code for modeling the microphysical and radiative processes in astrophysical plasmas, used to predict emission lines, continua, and ionization structures in nebulae, active galactic nuclei, stellar atmospheres, and interstellar medium environments. The code integrates atomic physics, radiative transfer, and thermal balance to simulate environments from H II regions to supernova remnants, enabling comparison with observations from instruments on Hubble Space Telescope, Chandra X-ray Observatory, and ground-based observatories like Keck Observatory and Very Large Telescope. Developed over decades by a team led by Gary Ferland, CLOUDY interfaces with atomic databases and is cited across literature from Astrophysical Journal articles to reviews in Annual Review of Astronomy and Astrophysics.
The name CLOUDY derives from the metaphor of a cloudy, irradiated gas where emission is produced, selected by its creator Gary Ferland in early publications and conference proceedings; it appears in documentation associated with Institute of Physics and laboratory reports. In literature the term refers specifically to the software package used to calculate level populations, radiative cooling, and line emissivities in plasmas subject to incident spectra such as those of O-type stars, quasars, and novae. Users and reviewers in journals like Monthly Notices of the Royal Astronomical Society and Publications of the Astronomical Society of the Pacific define CLOUDY operationally by its input commands, atomic datasets, and output spectra, distinguishing it from other codes such as XSTAR, MAPPINGS, and PHOENIX.
CLOUDY is applied to diverse astrophysical "clouds" classified in observational and theoretical frameworks from H II regions and planetary nebulae to molecular clouds and broad-line regions. Studies in Astrophysical Journal Letters and conference proceedings categorize models by ionization parameter, density, and incident continuum shape, referencing objects like Orion Nebula, Ring Nebula, NGC 1068, and 3C 273 as archetypes. Classifications used in CLOUDY-related literature often cite spectral diagnostics from surveys with Sloan Digital Sky Survey and instruments aboard James Webb Space Telescope and Spitzer Space Telescope, linking model outputs to emission-line ratio diagrams developed in works by Baldwin, Phillips & Terlevich and others.
CLOUDY simulates formation processes and physical properties by solving coupled equations for ionization balance, statistical equilibrium, and energy conservation incorporating atomic and molecular processes documented by databases maintained by National Institute of Standards and Technology, CHIANTI, and the International Atomic Energy Agency. The code models photoionization by continua from sources such as O-type star atmospheres, accretion disk spectra around black holes, and shock-excited plasmas from supernova remnants, integrating collisional excitation, recombination, charge exchange, and dust grain physics as studied by researchers at NASA Ames Research Center and in monographs by Draine. Thermal structure, pressure, and chemical abundances predicted by CLOUDY are compared against measurements from campaigns led by teams using Atacama Large Millimeter/submillimeter Array and Herschel Space Observatory.
Although CLOUDY models astrophysical plasmas rather than terrestrial atmosphere weather, its treatment of radiative transfer, scattering, and absorption informs analogous methods used in planetary atmosphere modeling for objects observed by Cassini–Huygens, Voyager 2, and missions to Mars such as Mars Reconnaissance Orbiter. Researchers adapt radiative processes computed in CLOUDY for exoplanetary atmosphere retrievals in studies involving Kepler and TESS data, connecting line opacities and continuum sources to signatures sought by teams using European Southern Observatory facilities. The code’s algorithms for line cooling and heating contribute to theoretical frameworks used to interpret climate-related radiative balance in planetary science literature appearing in Icarus and proceedings of American Geophysical Union meetings.
CLOUDY outputs are routinely compared with spectroscopic and imaging observations from facilities such as Hubble Space Telescope, Chandra X-ray Observatory, ALMA, VLA, and ground-based telescopes including Subaru Telescope and Gemini Observatory. Observational techniques employed alongside CLOUDY modeling include long-slit spectroscopy, integral field spectroscopy with instruments like MUSE on Very Large Telescope, and high-resolution echelle spectroscopy from Keck Observatory instruments; data reduction pipelines from projects like Sloan Digital Sky Survey feed measured line ratios to CLOUDY model grids. The code is incorporated into analysis workflows used by teams publishing in Astrophysical Journal and Monthly Notices of the Royal Astronomical Society, and is linked to fitting tools developed in languages like Python and packages such as Astropy and NumPy.
CLOUDY’s long development history, beginning in the late 1970s, is chronicled in conference papers and oral histories involving figures from institutions such as University of Kentucky and Yale University, reflecting the evolution of computational astrophysics through eras marked by machines from IBM and projects funded by National Science Foundation. The code has shaped pedagogical materials in curricula at universities including Harvard University, University of Cambridge, and University of California, Berkeley, appearing in theses and lectures cited in Proceedings of the International Astronomical Union. Its name and outputs have entered community discourse at meetings of societies like American Astronomical Society and in review articles in Annual Review of Astronomy and Astrophysics, cementing CLOUDY’s role in the culture of astrophysical spectroscopy.
Category:Astrophysics software