Long-term semi-regular dust formation by the WC9+B0I system WR 70

Peredur M. Williams (1), Karel A. van der Hucht (2,3), Francois van Wyk (4), Fred Marang (4), Patricia A. Whitelock (4,5), Patrice Bouchet (6) and Diah Y. A. Setia Gunawan (7)

1 - Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ;
2 - Space Research Organization Netherlands, Sorbonnelaan 2,
NL-3584 CA Utrecht, The Netherlands;
3 - Astronomical Institute `Anton Pannekoek', University of
Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands;
4 - South African Astronomical Observatory, P.O. Box 9, 7935 Observatory,
South Africa;
5 - Astronomy, Cosmology and Gravitation Centre, Astronomy Department,
University of Cape Town, 7700 Rondebosch, South Africa;
6 - Service d'Astrophysique DSM/IRFU/SAp CEA - Saclay, L'Orme des Merisiers, b^atiment 709, F-91191 Gif-sur-Yvette France;
7 - Atacama Large Millimetre/submillimetre Array (ALMA), Alonso de Cordova 3107, Vitacura, Santiago 763 0355, Chile

We present infrared photometry of the WC9+B0I Wolf-Rayet binary system WR 70 (HD 137603) observed with telescopes at ESO, the SAAO and the AAT between 1983 and 2010 which shows persistent but variable circumstellar dust emission. Optical spectroscopy confirms the classification of the companion as a B0 supergiant and suggests that the Balmer lines in its spectrum suffer in-fill from wind emission. Re-examination of the reddening suggests a revised distance of 3.5~kpc. In the near-IR, the amplitude of variation increases with wavelength, with no significant variation in J (nor is there in the visible), implying that it is the amount of dust in the wind of WR 70 that is varying. Period searches show a period near 2.82~y. (1030~d.) but the variations are not strictly regular and there are other factors affecting the dust formation and repeatability of the light curves. There may be a secondary period near 5.88 yr. but there is no evidence for periods less than a year. A model of the spectral energy distribution in 1991 gives a dust formation rate of 5.9 x 10^7 M_Sun y^-1, around one-third of the available carbon from the WC9 wind going into the wind-collision region, estimating its size from average WC9 and B0Ia wind properties. The fraction of carbon going into dust varied between ~ 11 and 46 percent during our campaign, possibly as a consequence of the stars moving in an elliptical orbit.

Reference: MNRAS in Press
Status: Manuscript has been accepted

Weblink: ftp://ftp.roe.ac.uk:/pub/pmw/WR70rv.psf

Comments: ...a system crying out for an orbit

Email: pmw@roe.ac.uk