|Introduction to WR 140 = HD 193793|
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  and a non-thermal radio source . Variations of the X-ray and radio emission are observed as the stars move in their eccentric binary orbit . High-resolution imaging of the stars themselves has provided an astrometric orbit  , 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 .
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 .
|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.|
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).
|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 . 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
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