2013MNRAS.429..494W

We present infrared (IR) photometry of the WC9+B0I Wolf-Rayet binary system HD 137603 (WR 70) observed with telescopes at the European Southern Observatory, the South African Astronomical Observatory and the Anglo-Australian Telescope 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 2.82yr (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 5.88yr 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 5.9x10-7M☉yr-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 per cent during our campaign, possibly as a consequence of the stars moving in an elliptical orbit.