VLTI/AMBER unveils a dusty pinwheel nebula in WR 118

Millour, F., Driebe, T., Chesneau, O., Groh, J. H., Hofmann, K.-H., Murakawa, K., Ohnaka, K., Schertl, D., Weigelt, G.

A&A, Vol. 506, Issue 3, pp. L49-L52 (2009)


Context: Most Wolf-Rayet stars (WR) of the WC9 subtype exhibit a dusty circumstellar envelope, but it is still a matter of debate how dust can form in their harsh environment. In a few cases, a pinwheel-like structure of the dusty envelope has been detected; therefore, it has been suggested that dust formation in all dusty WR stars might be linked to colliding winds in a binary system.

Aims: We probed the innermost region of the circumstellar dust shell of the deeply embedded WR star WR 118.

Methods: We carried out spectro-interferometric observations using the AMBER instrument of ESO's Very Large Telescope Interferometer in low-spectral resolution mode (R=35). The K-band observations were obtained with three 1.8 m telescopes spanning projected baselines between 9.2 and 40.1 m.

Results: At high spatial frequencies, the AMBER visibilities exhibit a prominent lobe, indicating that the envelope contains one or several zones with a large local intensity gradient. The strong closure phase signal clearly shows that the circumstellar envelope of WR 118 can only be described by an asymmetric intensity distribution. We show that a pinwheel nebula seen at low inclination is consistent with the AMBER data. Its size was determined to be 13.9 ± 1.1 mas.

Conclusions: WR 118 possibly harbors a pinwheel nebula, which suggests a binary nature of the system. According to our best model, the period of the system would be ≈60 days (for d=3 kpc), making WR 118 the shortest-period pinwheel nebula known so far.

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