A grid of MARCS model atmospheres for late-type stars I. Methods and general properties

Context. In analyses of stellar spectra and colours, and for the analysis of integrated light from galaxies, a homogeneous grid of model atmospheres of late-type stars and corresponding flux spectra is needed. Aims. To construct an extensive grid of spherically symmetric models (supplemented with plane-parallel ones for the highest surface gravities) , built on up-to-date atomic and molecular data, and make it available for public use. Methods. The most recent version of the MARCS program is used. Results. We present a grid of about 104 model atmospheres for stars with 2500 K ≤ Teff ≤ 8000 K, −1 ≤ logg = log(GM/R2) ≤ 5 (cgs) with various masses and radii, −5 ≤[Me/H]≤ +1, with [α/Fe] = 0.0 and 0.4 and different choices of C and N abundances, including ”CN-cycled” models with C/N = 4.07 (solar), 1.5 and 0.5, C/O ranging from 0.09 to (normally) 5.0 to also represent stars of spectral types R, S and N, and with 1.0 ≤ ξt ≤ 5 km/s. We also list thermodynamic quantities (T , Pg, Pe, ρ, partial pressures of molecules, etc) and provide them on the World Wide Web, as well as calculated fluxes in approximately 108,000 wavelength points. Underlying assumptions in addition to 1D stratification (spherical or plane-parallel) include hydrostatic equilibrium, mixing-length convection and LTE. A number of general properties of the models are discussed, in particular in relation to the effects of changing abundances, of blanketing and of sphericity. We have found and illustrate positive and negative feed-backs between sphericity and molecular blanketing. Models are compared with other available grids and excellent agreement is found with plane-parallel models of Castelli and Kurucz (if convection is treated consistently) within the overlapping parameter range. Although there are considerable departures from the spherically symmetric NextGen models, the agreement with more recent PHOENIX models is gratifying. Conclusions. The models of the grid show considerable regularities, but some interesting departures from general patterns occur for the coolest models due to the molecular opacities. We have tested a number of approximate ”rules of thumb” concerning effects of blanketing and sphericity and found them to often be astonishingly accurate. Some interesting new phenomena have been discovered and explored, such as the intricate coupling between blanketing and sphericity, and the strong effects of carbon enhancement on metal-poor models. We give further details of line absorption data for molecules, as well as details of models and comparisons with observations in subsequent papers.