RED SUPERGIANTS IN THE LMC - II - SPECTROPHOTOMETRY AND MODEL ATMOSPHERES

Spectrophotometric observations for 88 red supergiant candidates in th e Large Magellanic Cloud are presented. The spectra range from 4800 to 7700 Angstrom with a resolution of 10 Angstrom. The error in the abso lute fluxes is 0.04 to 0.05 mag. The molecular bands of the member sta rs are often rather weak, i.e. many of these are not M- but K-type sup ergiants. The data are available on the Strasbourg stellar data base ( CDS). Most of the red (super)giant model atmospheres available up to n ow do not reproduce the observations well. The models of Kurucz and Le jeune, Cuisinier & Buser - often applied especially to population synt hesis - correctly describe the strengths of atomic lines and the overa ll increase of the flux towards the red, but strongly underestimate th e strengths of molecular bands. The models presented by Plez, however, tend to reproduce the observed spectra well, except for the blue, as they include a more complete list of opacity sources. Concerning physi cal properties, only the Plez models give reliable results. Considerin g the relation between effective temperature and the strengths of mole cular bands, both the Kurucz and Lejeune models predict much higher te mperatures than derived from the interferometric radius measurements d iscussed by Schmidt-Kaler and Dyck et al. The temperatures given by th e Plez models show a much better agreement with these observations. Fu rthermore, the relation between T-eff and molecular absorption is much more clearly defined. When considering metallicities, however, the Pl ez models also fail, as they predict a [Fe/H] distribution that is muc h too broad, and furthermore an increase of T-eff with increasing [Fe/ H] which clearly contradicts models of stellar evolution. The effectiv e temperatures based on the Plez models range mostly from 3500 to 4100 K. The surface gravities derived on the basis of the Geneva evolution ary models range from log g = -0.3 to 0.3, while the bolometric lumino sities based on BVRIJHK observations range from -6.6 to -9.0 mag. Exce pt for the most luminous objects, the stars form a well-defined strip in the Hertzsprung-Russell diagram extending to the Hayashi limit at 3 500 K. At present, reliable metallicities cannot be derived because of inadequacies in the model atmospheres.