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Episodic (variable) dust formation by WR stars

Links
WR140 Introduction
Colliding-wind binaries
WR140 orbital motion
WR48a - the system with the longest period
WR104 pinwheel
Massive Stars Web site
Galactic Wolf Rayet Catalogue
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 rotating pinwheel 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.

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 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 WR98a (IRAS 17380-3031), 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 WR140 since 2001 has tracked another cycle, including the maximum in 2009, and of WR48a its slow return to another maximum in about 2011, making it the longest period episodic dust maker known; more information is given in the table below. IR light curves of WRs
Episodic and variable dust-making WR stars, with dates of infrared maxima and dust-formation periods
star spectrum (inferred) observed infrared maxima P(y) refs next max
WR 140 WC7 + O5 (1970), 1977, 1985, 1993, 2001, 2009 7.94 [1], [2], [13] 2017
WR 137 WC7 + O9 (1971), 1984, 1997, (2010 caught by WISE) + mini episodes 13.05 [3], [4] 2023
WR 125 WC7 + O9 1992-3 >22 [5] 2013+?
WR 19 WC5 + O9 (1987), 1997-8, 2007 10.1 [6], [12] 2017
WR 48a WC8 + Oe 1979, 2011 + mini episodes - persistent dust formation ~ 32.5 [7], [9], [14] 2044.2
WR 98a WC8-9 1991.1, 1992.7 ... 2000.4, periodic dust formation 1.54 [8], [9] 2017.4
HD 36402 WC4(+O) +O8I: ~1996.9, 2011 4.7 [15], [16] 2015.7
GC IRS 29N WC9 (1995), 2005, variable >6 [11] ?
WR 65 WC9 + OB 1979-80 ~ 4.8 [17] ~2017.8
WR 112 WC8 + OB 1997.6 ~ 12.3 [17] ~2022.2
rev. 2 November 2015
Peredur Williams
References (also orbital information, if known):
[1] Williams et al. MNRAS 243, 662, 1990
[2] Fahed et al. MNRAS 418, 2, 2011 (RV orbit: P = 2896.5 ± 0.7 d, e = 0.8962)
[3] Williams et al. MNRAS 324, 156, 2001
[4] Lefèvre et al. MNRAS 360, 141, 2005 (RV orbit: P = 13.05 ± 0.18 y, e = 0.18)
[5] Williams et al. MNRAS 266, 247, 1994
[6] Veen et al. A&A 339, L45, 1998
[7] Danks et al. A&A 118, 301, 1983
[8] Monnier et al. ApJ 525, L97, 1999 (pinwheel rotation P = 565 ± 50 days)
[9] Williams et al. IAU Symposium 212, 115, 2003
[10] Tuthill et al. ApJ 675, 698, 2008
[11] Rafelski et al. ApJ 659, 1241, 2007.
[12] Williams, Rauw & van der Hucht MNRAS 395, 2221, 2009 (RV orbit: e = 0.8)
[13] Taranova & Shenavrin Astronomy Letters, 37, 30, 2011
[14] Williams et al. MNRAS 420, 2526, 2012
[15] Williams et al. MNRAS 431, 1160, 2013
[16] Moffat, Niemela & Marraco ApJ 348, 232, 1990
[17] Williams & van der Hucht arXiv.1508.00724