Letter to the Editor

VLTI/AMBER unveils a possible dusty pinwheel nebula in WR118 ^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: fmillour@mpifr-bonn.mpg.de
² Observatoire de la Côte d'Azur/CNRS, UMR 6525 H. Fizeau, Univ. Nice Sophia Antipolis, Avenue Copernic, 06130 Grasse, France

Received: 27 July 2009
Accepted: 23 September 2009

Abstract

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 (). 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 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 kpc), making WR 118 the shortest-period pinwheel nebula known so far.