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Astron. Astrophys. 343, 536-544 (1999)


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

A. Pasquali 1, A. Nota  **  2 and M. Clampin 2

1 ST-ECF/ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany (apasqual@eso.org)
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA (nota@stsci.edu; clampin@stsci.edu)

Received 21 August 1998 / Accepted 27 October 1998

Abstract

We present new high-resolution coronographic imaging and medium-resolution spectroscopy of the circumstellar region around S 61 and BE 381, two Ofpe/WN9 stars in the Large Magellanic Cloud. The observations were carried out at the ESO/NTT (La Silla) in January 1996 and July 1998. The excellent seeing conditions allowed the circumstellar nebulae associated with both S 61 and BE 381 to be spatially resolved for the first time. The nebula surrounding S 61 has the appearance of a shell with a mild central axisymmetry. The surface brightness is not uniform, and the northern region of the nebula is the brightest. The nebula exhibits a bipolar structure with an overall morphology very similar to nebulae around other LBVs or Opfe/WN9 stars, especially S119. The diameter of the shell is [FORMULA], corresponding to a linear size of 1.8 pc. From the profile of nebular emission lines we clearly detect an expansion motion with a velocity of 28 km s-1, which indicates a dynamical age of [FORMULA] 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 nebulae 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 undergone a LBV-type ouburst and, therefore, strenghtens the suggestion that Ofpe/WN9 stars are quiescent LBVs. The situation is different for BE 381. The H[FORMULA] images of BE 381 also reveal the presence of a faint nebulosity around the star; most of the nebular flux appears to be emitted by an arc of gas located to the east of BE 381, while a much dimmer arc is detectable on the western side. The arcs delineate a shell of [FORMULA] in diameter, corresponding to a linear size of 3.2 pc, which appears to be expanding with a velocity 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 Te = 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.

Key words: stars: circumstellar matter – stars: emission-line, Be – stars: individual: BE 381 LMC – stars: individual: S 61 LMC – stars: supergiants

* Based on observations obtained at the European Southern Observatory, La Silla, Chile
** Affiliated with the Astrophysics Division, Space Science Department of the European Space Agency

Send offprint requests to: A. Pasquali

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999

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