Ionization structure and chemical abundances of the Wolf-Rayet nebula NGC 6888 with integral field spectroscopy^⋆,⋆⋆

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía S/N, 18008 Granada, Spain
e-mail: alba@iaa.es
² Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politécnica de Catalunya, C. Comte Urgell 187, 08036 Barcelona, Spain
³ Centro Astronómico Hispano Alemán, Calar Alto, CSIC-MPG, C/Jess Durbán Remn 2-2, 04004 Almería, Spain

Received: 9 November 2011
Accepted: 22 February 2012

Abstract

Context. The study of nebulae around Wolf-Rayet (WR) stars gives us clues about the mass-loss history of massive stars, as well as about the chemical enrichment of the interstellar medium (ISM).

Aims. This work aims to search for the observational footprints of the interactions between the ISM and stellar winds in the WR nebula NGC 6888 in order to understand its ionization structure, chemical composition, and kinematics.

Methods. We have collected a set of integral field spectroscopy observations across NGC 6888, obtained with PPAK in the optical range performing both 2D and 1D analyses. Attending to the 2D analysis in the northeast part of NGC 6888, we have generated maps of the extinction structure and electron density. We produced statistical frequency distributions of the radial velocity and diagnostic diagrams. Furthermore, we performed a thorough study of integrated spectra in nine regions over the whole nebula.

Results. The 2D study has revealed two main behaviours. We have found that the spectra of a localized region to the southwest of this pointing can be represented well by shock models assuming n = 1000 cm^-3, twice solar abundances, and shock velocities from 250 to 400 km s^-1. With the 1D analysis we derived electron densities ranging from  <100 to 360 cm^-3. The electron temperature varies from  ~7700 K to  ~10 200 K. A strong variation of up to a factor 10 between different regions in the nitrogen abundance has been found: N/H appears lower than the solar abundance in those positions observed at the edges and very enhanced in the observed inner parts. Oxygen appears slightly underabundant with respect to solar value, whereas the helium abundance is found to be above it. We propose a scenario for the evolution of NGC 6888 to explain the features observed. This scheme consists of a structure of multiple shells: i) an inner and broken shell with material from the interaction between the supergiant and WR shells, presenting an overabundance in N/H and a slight underabundance in O/H; ii) an outer shell very intense in [OIII]λ5007 Å corresponding to the main sequence bubble broken up as a consequence of the collision between supergiant and WR shells. Nitrogen and oxygen do not appear enhanced here, but helium appears enriched; iii) and finally it includes an external and faint shell that surrounds the whole nebula like a thin skin representing the early interaction between the winds from the main sequence star with the ISM for which typical circumstellar abundances are derived.