The Antares emission nebula and mass loss of Scorpii A^*

Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: dreimers@hs.uni-hamburg.de

Received: 16 April 2008
Accepted: 25 August 2008

Abstract

Aims. The Antares nebula is a peculiar emission nebula seen in numerous [] lines and in radio free-free emission, probably associated with the H ii region caused by α Sco B in the wind of α 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 α 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 α Sco A.

Results. We found that the Antares nebula shows, in addition to numerous [Fe ii] lines, the Balmer line recombination spectrum H_α, H up to H₁₀, and [N ii] 6583/6548 Å, H_α 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_α velocities, the H ii region is found to be located ~215 AU behind the plane of the sky of α Sco A. From the H_α/[N ii] intensity ratio and the non-visibility of the [O ii] 3726/3729 Å lines we estimate a low mean electron temperature of = 4900 K and an N abundance enhanced by a factor of ~3 due to the CNO cycle in α Sco A. The shape and size of the H ii region yield a mean mass-loss rate of (1.05 ± 0.3) 10^-6  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.