CWB logo Introduction to WR 140 = HD 193793
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Colliding-wind binaries
WR140 radio variation
Episodic dust-makers
WR140 orbital motion
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All about Wolf-Rayet Stars: 2015 Potsdam workshop
Stellar Winds in Interaction
2009 Campaign results
Co-ordinates J2000
R.A. 20 20 27.98
Dec. +43 51 16.3

WR 140 (= HD 193793 = BD +43° 3571 = V1687 Cyg) is a massive binary system comprising a WC7 type Wolf-Rayet star and an O5 star, probably a supergiant. Both stars have fast (3000 km/s), radiatively driven, stellar winds carrying significant mass loss, about 10-5 and 10-6 solar masses/year (WC7 and O5 stars respectively), and therefore having significant kinetic power (mechanical luminosity), equivalent to 104 and 103 solar luminosities. Between the two stars, the winds collide, releasing some of this power (about 103 solar luminosities) and producing shocks, resulting in particle acceleration and heating and compression of the plasma in the shocked winds. The consequences of these colliding-wind processes are observed at X-ray and radio wavelengths, making WR140 one of the brightest non-compact stellar X-ray sources [1] and a non-thermal radio source [2]. Variations of the X-ray and radio emission are observed as the stars move in their eccentric binary orbit [3]. High-resolution imaging of the stars themselves has provided an astrometric orbit [6] , completing the definition of the orbit in three dimensions.

WR140 was also the first Wolf-Rayet star to show a sudden brightening in its infrared flux, attributed to an episode of dust formation [4]. This dust (a form of carbon, rather like soot) condenses in the stellar wind, absorbing a small fraction of the stars' UV-optical radiation. This radiation heats the dust to a temperature of about 1000K, causing the brightening at infrared wavelengths. The newly formed dust is dispersed by the stellar wind and cools as it is carried further from the stars and experiences less heating by their radiation, causing its infrared emission to fade. A second episode of dust formation by WR140 was observed in 1985, and it was argued that these events were periodic and also linked to a binary orbit, occurring near the time of periastron passage [3].
Similar episodes were observed in 1993, 2001 and 2009 [8], having a period near 2897 days, equal to that of the binary orbit. WR140 is the prototype periodic episodic dust-maker. The dust cloud formed in the 2001 episode has been imaged at high resolution in the infrared: in mid-2001 with aperture-masking on the Keck telescope by Monnier et al. [5], then with near-IR cameras and adaptive optics systems on the Hale and William Herschel telescopes, and then at wavelengths between 3.6 and 10 µm with UKIRT and Gemini (North) as the cloud expanded and cooled [7]. Some of the 4-µm images are shown below.

Right: Spectral energy distributions of WR 140 over six decades of wavelength, from the visible to radio. Black: WR 140 at "quiescence"; red: at IR maximum; blue: at radio maximum; green: the O5 star, which dominates in the optical. IR-radio spectra

Right: K-band "light curves" near the 1977, 1985, 1993, 2001 and 2009 maxima from data in [3], [4], [7] and [8], phased to the orbit.

Far right: Infrared (4 µm) image of WR 140 observed in November 2002, (phase 0.23) showing dust emission south and east of the star (scale 4 x 4 arcseconds).

K light curves
Right: Two more 4-µm images of WR 140 at the same scale (4 x 4 arcseconds) observed in June 2003 (phase 0.29) and July 2005 (phase 0.56) showing continued expansion of the dust cloud [7]. All three images were observed with UIST on the United Kingdom Infrared Telescope

The most recent periastron passage occurred in 2009, and stimulated an intensive multi-wavelength observing campaign. Results were reported at two meetings in 2010: Stellar Winds in Interaction" and the 39th Liège International Astrophysical Colloquium "The multi-wavelength view of Hot Massive Stars" including 2009 Campaign results.

Attention has latterly concentrated on the X-rays, with results from SUZAKU reported by Suguwara et al. showing that there is still a great deal about the system and colliding wind physics that we do not understand.

rev: 26 Feb 2016
Peredur Williams
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[1] A.M.T. Pollock ApJ 320, 283, 1987
[2] R.H. Becker & R.L. White ApJ 297, 649, 1985
[3] P.M. Williams et al. MNRAS 243, 662, 1990
[4] P.M. Williams et al. MNRAS 185, 467, 1978
[5] J.D. Monnier, P.G. Tuthill & W.C. Danchi ApJ 567, L137, 2002
[6] J.D. Monnier et al. ApJL 742, 1, 2011
[7] P.M. Williams et al. MNRAS 395, 1749, 2009
[8] O.G. Taranova & V.I. Shenavrin Astronomy Letters, 37, 30, 2011
[9] Y. Suguwara et al. PASJ, 67, 121, 2015