Episodic (variable) dust formation by WR stars
|Several Wolf-Rayet stars show apparently regular epsiodes of carbon
dust formation, as demonstrated by massive increases in their infrared emission,
like those of WR140. Two such episodes and the fading from an earlier one have been
observed from WR137, giving a period of 13.05 years, which is well
matched by its orbital period. Three dust formation episodes,
separated by 10.1 y., have been observed from WR19 and one from WR125.
The spectral subtypes of these four stars (WC5-7) are "earlier"
than those (WC8-9) of the Wolf-Rayet stars making dust persistently,
such as the prototype WR104 (Ve 2-45), whose newly formed dust traces a
on the sky. Numerous WC8-9 stars make dust persistently like WR104, and they are
located within the solar circle in the Galaxy, where the ambient metal abundance
is higher, suggesting a strong influence of local metal richness on the propensity
of WR stars to make dust. Rather surprisingly then, variable dust emission having a
period near 4.7 years has recently been observed from the Wolf-Rayet system
HD 36402 in the Large Magellanic Cloud, where the overall
metallicity is significantly less than that in the Galaxy.
WR 140, WR 137 and WR 19 are spectroscopic
binaries, HD 36402 appears to be a triple system comprising 
a WC4+O binary having a period near 3.03 days in a wider orbit with an O8I supergiant,
while WR 104 and WR 125 have spectroscopic companions suggesting binarity.
There are currently no RV orbits for the last two, but the WR 104 pinwheel
rotates on the sky with a period of 241.5 days
(Tuthill et al. 2008), indicating
dust formation by a binary observed at low inclination. The intensity of the
dust emission is constant, indicating formation of dust at a constant rate
exactly replenishing that flowing away from the system.
Another dust pinwheel is that formed by WR 98a,
whose infrared flux varies with the same period as its pinwheel
rotation, indicating periodic variation in the feeding of its dust cloud.
The variable WR dust makers are listed below.
|Right: Infrared (3.8µm) light curves of five WR stars showing episodes of dust formation. (The curves of WR137, WR125, WR19 and WR48a are shifted for clarity.) IR photometry of WR 140 since 2001 has tracked another cycle, including the maximum in 2009, and of WR 48a its slow return to another maximum in 2011, making it the longest period episodic dust maker known; more information is given in the table below.|
|Episodic and variable dust-making WR stars,
with dates of infrared maxima and dust-formation periods
|References (also orbital information, if known):
 Williams et al. MNRAS 243, 662, 1990
 Fahed et al. MNRAS 418, 2, 2011 (RV orbit: P = 2896.5 ± 0.7 d, e = 0.8962)
 Williams et al. MNRAS 324, 156, 2001
 Lefèvre et al. MNRAS 360, 141, 2005 (RV orbit: P = 13.05 ± 0.18 y, e = 0.18)
 Williams et al. MNRAS 266, 247, 1994
 Veen et al. A&A 339, L45, 1998
 Danks et al. A&A 118, 301, 1983
 Monnier et al. ApJ 525, L97, 1999 (pinwheel rotation P = 565 ± 50 days)
 Williams et al. IAU Symposium 212, 115, 2003
 Tuthill et al. ApJ 675, 698, 2008
 Rafelski et al. ApJ 659, 1241, 2007.
 Williams, Rauw & van der Hucht MNRAS 395, 2221, 2009 (RV orbit: e = 0.8)
 Taranova & Shenavrin Astronomy Letters, 37, 30, 2011
 Williams et al. MNRAS 420, 2526, 2012
 Williams et al. MNRAS 431, 1160, 2013
 Moffat, Niemela & Marraco ApJ 348, 232, 1990
 Williams & van der Hucht arXiv.1508.00724