Model photospheres for late-type stars from the inversion of high-resolution spectroscopic observations: Groombridge 1830 and epsilon Eridani

An inversion technique to recover LTE one-dimensional model photospheres fo r late-type stars, which was previously applied to the Sun by Allende Priet o et al. in 1998, is now employed to reconstruct, semiempirically, the phot ospheres of cooler dwarfs: the metal-poor Groombridge 1830 and the active s tar of solar metallicity epsilon Eridani. The model atmospheres we find rep roduce satisfactorily all the considered weak-to-moderate neutral lines of metals, satisfying in detail the excitation equilibrium of iron, the wings of strong lines, and the slope of the optical continuum. The retrieved mode ls show a slightly steeper temperature gradient than flux-constant model at mospheres in the layers where log tau less than or equal to -0.5. We argue that these differences should reflect missing ingredients in the flux-const ant models and point to granular-like inhomogeneities as the best candidate . The iron ionization equilibrium is well satisfied by the model for Gmb 18 30, but not for epsilon Eri, for which a discrepancy of 0.2 dex between the logarithmic iron abundance derived from neutral and singly ionized lines m ay signal departures from LTE. The chemical abundances of calcium, titanium , chromium, and iron derived with the empirical models from neutral lines d o not differ much from previous analyses based on flux-constant atmospheric structures.