Turbulent outflows from [WC]-type nuclei of planetary nebulae

Abstract

Using spectroscopic observations taken at the Observatoire de Haute-Provence (France) and the Observatoire du mont Megantic (Canada), we describe wind fluctuations in the [WC 8]-type central star of the planetary nebula NGC 40, HD 826, which was observed intensively during 22 nights. Moving features seen on the top of the Ciii λ 5696 and Civ $\lambda\lambda$5801/12 (+Ciii λ 5826) emission lines are interpreted as outflowing blobswhich are radially accelerated outwards in the Wolf-Rayet wind. The amplitudes of the variations range up to 25-30% of the adjacent continuum flux, over timescales of hours. The variabilities of both lines are quite well correlated, although they are somewhat weaker for the Civ complex. Subpeaks (or gaps) on the top of the Ciii line generally move towards the nearest line edge in a symmetric fashion in the blue and the red. Kinematic parameters of the blobs were derived and compared to those observed for massive and other low-mass Wolf-Rayet stars. Especially impressive are the significantly larger observed maximum radial acceleration values of the blobs, compared to those already reported for massive WC 5-9, or low-mass [WC 9] stars. This is attributed to the very small stellar radius of HD 826. In addition the β velocity field is found to possibly underestimate the true gradient within the stellar wind flow. On the whole, the wind of HD 826 is highly stochastically variable on a very short time-scale. This supports a turbulent origin.