MARCS model atmospheres

In this review presented at the Symposium A stellar journey in Uppsala, June 2008, I give my account of the historical development of the MARCS code from the first version published in 1975 and its premises to the 2008 grid. It is shown that the primary driver for the development team is the science that can be done with the models, and that they constantly strive to include the best possible physical data. A few preliminary comparisons of M star model spectra to spectrophotometric observations are presented. Particular results related to opacity effects are discussed. The size of errors in the spectral energy distribution (SED) and model thermal stratification are estimated for different densities of the wavelength sampling. The number of points used in the MARCS 2008 grid (108000) is large enough to ensure errors of only a few K in all models of the grid, except the optically very thin layers of metal-poor stars. Errors in SEDs may reach about 10% locally in the UV. The published sampled SEDs are thus appropriate to compute synthetic broad-band photometry, but higher resolution spectra will be computed in the near future and published as well on the MARCS site (marcs.astro.uu.se). Test model calculations with TiO line opacity accounted for in scattering show an important cooling of the upper atmospheric layers of red giants. Rough estimates of radiative and collisional time scales for electronic transitions of TiO indicate that scattering may well be the dominant mechanism in these lines. However models constructed with this hypothesis are incompatible with optical observations of TiO (Arcturus) or IR observations of OH (Betelgeuse), although they may succeed in explaining H2O line observations. More work is needed in that direction. PACS numbers: 97.10.Ex , 97.20.Jg , 97.20.Li , 97.20.Pm , 97.20.Tr, 95.30.Ky) 1. MARCS : little history The MARCS model atmosphere code has been in use since the mid-70’s. It has its roots in Bengt Gustafsson’s early work on a Feautrier-type method for model atmosphere calculation including convection (Gustafsson 1971). A number of features, including detailed continuous opacities were already included in that pre-MARCS version. And in a Gustafsson way, it was directly applied to an astrophysical problem : determining the metallicity of F type stars (Gustafsson & Nissen 1972). A few years later the ar X iv :0 81 0. 23 75 v1 [ as tr oph ] 1 4 O ct 2 00 8