Interaction of Hot Stars and of the Interstellar Medium. V. The Compact Optical Regions Sharpless 255, 257 and 269

Summary. H photographs obtained using a Lallemand electronic camera, low dispersion spectra in the range A and high dispersion spectra near Hoc have been obtained for three galactic nebulae. The excitation of these nebulae is low, the electron densities and reddening are moderate. The densest regions (up to 1000 or 3000 electrons per cubic centimeter) generally are associated with abrupt brightness variations. The Hoc radial velocity field of Sh 2-255 and the identifica tion of the exciting star of Sh 2-269 are discussed. All three nebulae, as inferred from their relatively high density and from their association with OH and H2O sources might represent an early stage of evolution of H II regions. Key words: young H II regions - exciting stars - distribution of electron density - interstellar molecules