Absence of hot gas within the Wolf-Rayet bubble around WR 16

Abstract

We present the analysis of XMM-Newton archival observations towards the Wolf-Rayet (WR) bubble around WR 16. Despite the closed bubble morphology of this WR nebula, the XMM-Newton observations show no evidence of diffuse emission in its interior as in the similar WR bubbles NGC 6888 and S 308. We use the present observations to estimate a 3-σ upper limit to the X-ray luminosity in the 0.3-1.5 keV energy band equal to 7.4 × 10³² erg s^-1 for the diffuse emission from the WR nebula, assuming a distance of 2.37 kpc. The WR nebula around WR 16 is the fourth observed by the current generation of X-ray satellites and the second not detected. We also examine FUSE spectra to search for nebular O vi absorption lines in the stellar continuum of WR 16. The present far-UV data and the lack of measurements of the dynamics of the optical WR bubble do not allow us to confirm the existence of a conductive layer of gas at T ~ 3 × 10⁵ K between the cold nebular gas and the hot gas in its interior. The present observations result in an upper limit of n_e < 0.6 cm^-3 on the electron density of the X-ray emitting material within the nebula.