SPECTROSCOPY OF RING NEBULAE AROUND WOLF-RAYET STARS IN M33

We present high and low resolution spectroscopic observations for a sample of seven ring nebulae around Wolf-Rayet (WR) stars in M33: MC 27, MC 44, MC 46, MC 5 1, MC 57, MC 65, and MC 70. An analysis of the ionization structure of the nebulae indicates that photoionization is the main source of excitation. Physical conditions and abundances have been derived for the nebulae, and values consistent with the abundance gradient of M33 are found. Moreover, an analysis of the spectra of three WR central stars has been performed in order to re-assess previous classifications. Two-dimensional Gaussian fits to the most intense emission lines for each object (e.g., Halpha, [N II], [O III]) have been performed to establish the dynamical classification of the objects using the scheme proposed by Chu [ApJ, 249, 195 (1981)]. The nebulae around MC 27 and MC 44 do not present any evidence for expansion and are thus considered as pure ''stationary'' shells or R(s) objects. The remaining objects observed at high resolution present more complex structures. The nebula surrounding MC 51 consists of an expanding bubble immersed in a stationary amorphous H II region; the nebulae associated with MC 46 and MC 70 can be considered as a combination of an external stationary shell and an internal bubble. If it is demonstrated that their central stars are truly isolated, then these nebulae do not have equivalent counterparts in the Galaxy. The physical properties of the three bubbles discovered have been derived and compared thoroughly with those of typical galactic wind-blown bubbles. On average, the R(s) shells seem to be larger than their galactic counterparts. This difference in size and the presence of the shell+bubble structures could reflect different physical conditions between the ambient ISM in the Galaxy and in M33.