The spectroscopic properties of the nebulae around seven Galactic and LMC Wolf-Rayet stars

TL;DR

This study spectroscopically analyzed nebulae around seven Wolf-Rayet stars in the Milky Way and LMC to assess chemical enrichment and detect neutral carbon lines, revealing nitrogen enrichment in some nebulae and confirming origins of [C I] lines.

Contribution

First detailed spectroscopic comparison of nebulae around Galactic and LMC Wolf-Rayet stars, including detection of [C I] lines and analysis of chemical abundances.

Findings

Galactic WR nebulae around WR 8, 16, 40 are nitrogen-enriched.

NGC 3199 shows abundances similar to H II regions.

[C I] lines in NGC 3199 originate mainly from ionized regions.

Abstract

We have obtained spectroscopic observations of the nebulae around seven Wolf-Rayet (WR) stars, four in the Milky Way (WR 8, 16, 18 and 40) and three in the LMC (BAT99-2, -11 and -38). They were observed using the ESO NTT with the EFOSC-2 instrument, except for one nebula, NGC 3199, which was observed using the UVES echelle spectrometer on the VLT. The aims of these observations were to (a) quantify the degree of chemical enrichment in WR nebulae which had previously been suggested to have abundances reflecting nucleosynthetic processing; and (b) to attempt to detect the far-red lines of neutral carbon (e.g. [C I] 9850A) in the nebulae around WC stars. Nebular densities, temperatures and elemental abundances were derived using standard emission line diagnostics. Our high spectral resolution UVES dataset for NGC 3199 allowed the determination of line broadening temperatures using lines from several different species. These showed consistent patterns and fair agreement with the nebular temperatures derived from diagnostic line ratios. Amongst the Galactic WR nebulae, the nebulae around WR 8, 16 and 40 were found to be strongly nitrogen-enriched while NGC 3199 displayed abundances which were similar to those of Galactic H II regions. Amongst the LMC WR nebulae, none displayed an N/O ratio that significantly exceeded LMC H II region values. The far-red [C I] lines were detected from NGC 3199, with the strength of the 8727A line indicating that at least part of it was produced by C+ recombination rather than collisional excitation. The line widths of the [C I] 9824 and 9850A lines were however the same as those of collisionally excited nebular lines, indicating that those lines originated largely from ionized regions within the nebula.