Abstract
A study of the internal variation of the electron density in Galactic H II regions has been conducted on a sample of objects of different sizes and evolutionary stages. The [S II] λ 6716/λ 6731 line ratio was adopted as electron density indicator. Long slit spectrophotometry of high signal-to-noise ratio with spectral dispersion of 0.75 Å/pixel and spatial scale of 0.9 arcsec/pixel were obtained at different slit positions and orientations. In some regions, we have found radial gradients with the electron density decreasing from the centre to the edges. The H II regions with no systematic density variations detected were found to be diffuse with mean electron densities of the order of Ne: 20-140 cm-3, indicating that these are evolved objects. The region {M 20} shows a systematic non-radial variation of electron density with maximum values occurring at its prominent dark lanes. The present work supports the common assumption that classical and evolved H II regions generally have low electron densities, so that collisional de-excitation processes play a minor role on the observed spectra.