Episodic (variable) dust formation by WR stars
|Several Wolf-Rayet stars show apparently regular
epsiodes of 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 y., which was well matched by the
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. These stars, and the persistent dust makers like WR 104, are
located within the solar circle in the Galaxy, with the persistent dust
makers more towards the Galactic Centre, suggesting a strong influence
of local metal richness. Rather surprisingly, variable dust emission with
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.
WR140, WR137 and WR19 are spectroscopic binaries, HD 36402 appears to be a triple system comprising a WC4+O binary with a period near 3.03 days in a wider orbit with an O8I supergiant, while WR104 and WR125 have spectroscopic companions suggesting binarity. There are currently no RV orbits for the last two, but the WR104 pinwheel rotates on the sky, indicating dust formation by a binary observed at low inclination. The second dust pinwheel is that formed by WR98a (IRAS 17380-3031). The infrared flux from this varies with the same period as the pinwheel rotation, indicating periodic variation in the replenishment of the dust cloud. Also, Tuthill et al. observed pinwheel dust clouds around two members (Q2 and Q3) of the Quintuplet cluster, suggesting that these were also WC late sub-type colliding-wind binaries, and Rafelski et al. have found variable IR emission from GC IRS 29N (WR 101g) (projected) very close to the Galactic centre, suggesting it too is a WC9 variable dust maker.
|Right: Infrared (3.8µm) light curves of five WR stars showing episodes of dust formation. (Those of WR137, WR125, WR19 and WR48a are shifted as for clarity.) WR48a and WR137 show secondary episodes of dust formation apparently unconnected with the major eruptions.|
|Episodic and variable dust-making WR stars, with dates of
infrared maxima and dust-formation periods
|References (most recent) with Period (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. Science 313, 935, 2006.
 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 (http://arxiv.org/abs/1111.5194)
 Williams et al. MNRAS in press, (http://arxiv.org/abs/1302.2002)