Observed H_α profile (black dots) in the selected program stars and the best-fit empirical model from Eq. (2) (bold blue line, 6562.0 to 6562.8 Å). The red and black lines correspond to the best-fit theoretical models computed in 1D LTE and 1D NLTE, respectively. The part of the H_α line used in the fit is the same for all stars in the sample. The thin blue line is only shown to help guide the eye; the red wing of H_α was not used in the fit. Line bisectors are indicated in the inset. The spectra are arranged in order of increasing metallicity.