MARCS

MARCS is a family of stellar atmosphere models and related synthetic spectra developed for the interpretation of spectra of stars, especially late-type stars including dwarfs and giants. Originating in the 1970s at Uppsala University under the direction of Bengt Gustafsson and collaborators, MARCS models are 1D hydrostatic, assume local thermodynamic equilibrium (LTE), and come in both plane-parallel and spherical geometries to accommodate different stellar radii and gravities. They incorporate extensive opacities, including atomic lines and molecular species such as TiO, CN, CO, and H2O, which are important for cool stars.

The model grids span broad ranges in effective temperature (roughly 2500–8000 K, depending on geometry), surface gravity (log g approximately −0.5 to 5.5), and metallicity ([Fe/H] from about −5 to +0.5). They are used to compute atmosphere structures and synthetic spectra for abundance analyses, line-formation studies, and photometric calibrations. MARCS atmospheres form the basis for many spectroscopic analyses and are commonly used to derive stellar parameters and chemical compositions of late-type stars. Publicly released grids include atmosphere structures and, in some cases, synthetic spectra or bolometric corrections, enabling comparisons with observations from large surveys and individual stars.

MARCS models are among several competing atmosphere codes; limitations include the 1D LTE assumption and the neglect of three-dimensional and nonlocal thermodynamic equilibrium effects in the most basic grids. Ongoing updates extend opacity data, physics, and parameter coverage. See also ATLAS, PHOENIX, and other stellar atmosphere model families.