The Antares emission nebula and mass loss of a Scorpii A

Aims. The Antares nebula is a peculiar emission nebula seen in numerous [Fe II] lines and in radio free-free emission, probably associated with the H II region caused by alpha Sco B in the wind of alpha Sco A. High-resolution spectra with spatial resolution were used to study the emission line spectrum, the physical nature of the nebula and to determine the mass-loss rate of the M supergiant alpha Sco A. Methods. The Antares nebula was mapped with long-slit (10) and high-resolution (R = 80 000) spectra using UVES at the VLT. The resulting 2-D images were used to reconstruct a 3-D picture of the H II region and its absolute location in space relative to alpha Sco A. Results. We found that the Antares nebula shows, in addition to numerous [Fe II] lines, the Balmer line recombination spectrum H(alpha), H(beta) up to H(10), and [N II] 6583/6548 angstrom, H(alpha) and [N II] with the same extent as seen in cm radio free-free emission. Combining velocity information from optical and GHRS/HST spectra with H(alpha) velocities, the H II region is found to be located similar to 215 AU behind the plane of the sky of alpha Sco A. From the H(alpha)/[N II] intensity ratio and the non-visibility of the [O II] 3726/3729 angstrom lines we estimate a low mean electron temperature of (T) over bar (e) = 4900 K and an N abundance enhanced by a factor of similar to 3 due to the CNO cycle in alpha Sco A. The shape and size of the H II region yield a mean mass-loss rate of (1.05 +/- 0.3) x 10(-6) M(circle dot) yr(-1). The [Fe II] lines originate predominantly at the edges (rear and front) of the H II region. UV continuum pumping as well as collisional excitation seem to be responsible for the observed iron lines.