Spatially resolved nebulae around the Ofpe/WN9 stars S 61 and BE 381

We present new high-resolution coronographic imaging and medium-resolution spectroscopy of the circumstellar region around S 61 and BE 381, two Ofpe/W N9 stars in the Large Magellanic Cloud. The observations were carried out a t the ESO/NTT (La Silla) in January 1996 and July 1998. The excellent seein g conditions allowed the circumstellar nebulae associated with both S 61 an d BE 381 to be spatially resolved for the first time. The nebula surroundin g S 61 has the appearance of a shell with a mild central axisymmetry. The s urface brightness is not uniform, and the northern region of the nebula is the brightest. The nebula exhibits a bipolar structure with an overall morp hology very similar to nebulae around other LBVs or Opfe/WN9 stars, especia lly S119. The diameter of the shell is 7.3 ", corresponding to a linear siz e of 1.8 pc. From the profile of nebular emission lines we clearly detect a n expansion motion with a velocity of 28 km s(-1), which indicates a dynami cal age of similar to 30000 yrs. We find an electron density of 400 cm(-3) and an electron temperature of 6120 K. The nebula is similar to other LBV n ebulae in that it is nitrogen enriched. The observed chemical and dynamical properties confirm that the nebula is associated with the central star and is of stellar origin. This result implies that S 61 is likely to have unde rgone a LBV-type ouburst and, therefore, strenghtens the suggestion that Of pe/WN9 stars are quiescent LBVs. The situation is different for BE 381. The H alpha images of BE 381 also reveal the presence of a faint nebulosity ar ound the star: most of the nebular flux appears to be emitted by an are of gas located to the east of BE 381, while a much dimmer are is detectable on the western side. The arcs delineate a shell of 13 " in diameter. correspo nding to a linear size of 3.2 pc. which appears to be expanding with a velo city of 14 km s(-1). From the nebular emission lines we derive an electron density ranging between 30 cm(-3) and 120 cm(-3) (assuming T-e = 10000 K), and a N+/S+ ratio between 1.5 and 2.3, which are typical of HII regions. We therefore conclude that the shell detected around BE 381 is not of stellar origin and probably represents the relic of the interstellar bubble blown by BE 381 during its O main-sequence phase.