Abstract
Seventeen H II regions and three areas corresponding to the interarm medium of M33 have been observed in the Xx4000 - 6700A wavelength range using the image tube spectrograph attached to the Newtonian focus of the Asiago 122 cm reflector. The emission lines [N II] x6584, Hα, [0 III] λ5007 and Hβ were detected in all the observed regions. The observed regions of the interarm medium are located in the plane of the galaxy at a distance from the nucleus 400 < R < 1300 pc. Additional evidence in favor of the ionization of the interarm medium being produced mainly by OB stars is presented. From the magnitudes and colors of 34 stars projected into a 2'.4 x 0'.7 area of the disk of M33 it is found that at least a considerable fraction of the OB stars needed to ionize the inner interarm medium are located inside it. It is found that the contribution to the emission lines of the interarm medium due to dust scattered light, or to the ionization produced by SN remnants, cosmic rays and X rays, is not appreciable.
