Episodic dust formation by HD 192641 (WR 137) – II

• Published in: Monthly notices of the Royal Astronomical Society 2001-06, Vol.324 (1), p.156-166
• Main Authors: Williams, P. M., Kidger, M. R., van der Hucht, K. A., Morris, P. W., Tapia, M., Perinotto, M., Morbidelli, L., Fitzsimmons, A., Anthony, D. M., Caldwell, J. J., Alonso, A., Wild, V.
• Format: Article
• Language: English
• Subjects: circumstellar matter; infrared: stars; stars: individual: WR 137; stars: Wolf–Rayet
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.2001.04284.x

We present new infrared photometry of the WC7-type Wolf–Rayet star HD 192641 (WR 137) from 1985 to 1999. These data track the cooling of the dust cloud formed in the 1982–84 dust-formation episode from 1985 to 1991, the increase of the infrared flux from 1994.5 to a new dust-formation maximum in 1997 and its subsequent fading. From these and earlier data we derive a period of 4765±50 d(13.05±0.15 yr) for the dust-formation episodes. Between dust-emission episodes, the infrared spectral energy distribution has the form of a power law, λF∝λ−1.86. The rising branch of the infrared light curve (1994–97) differs in form from that of the episodic dust-maker WR 125. Time-dependent modelling shows that this difference can be attributed to a different time dependence of dust formation in WR 137, which occurred approximately ∝t2 until maximum, whereas that of WR 125 could be described by a step function, akin to a threshold effect. For an adopted distance of 1.6 kpc, the rate of dust formation was found to be 5.0×10−8 M⊙ yr−1 at maximum, accounting for a fraction fC≈1.5×10−3 of the carbon flowing in the stellar wind. The fading branches of the light curves show evidence for secondary ‘mini-eruptions’ in 1987, 1988 and 1990, behaviour very different from that of the prototypical episodic dust-maker HD 193793 (WR 140), and suggesting the presence in the WR 137 stellar wind of large-scale structures that are crossed by the wind–wind collision region.