WR 140

WR 140 is a spectroscopic binary containing an O-type star and a Wolf–Rayet star. It is located in the constellation of Cygnus, lying in the sky at the centre of the triangle formed by Deneb, γ Cygni and δ Cygni.

Significance WR 140 is thought to be a prototypical example of cosmic dust production. In this mode of cosmic dust production, detritus enriched in silicon and carbon is periodically blown into the wider universe by certain stars toward the end of their lives. Such stars are termed Wolf–Rayets. The outermost layers of a Wolf–Rayet star are enriched in oxygen, nitrogen, silicon and carbon. Indeed, the spectrographic presence of these elements, along with a notable absence of hydrogen, were one of the original diagnostic criteria for classifying a star as Wolf–Rayet. It is these enriched layers of the photosphere that are lost in repeating pulses. Once distant from the surface, the carbon fraction of this ejected material begins to glow at approximately 1,000 K. The heating is due to the star's UV radiation, the wavelength of its greatest luminosity. This has the effect of rebroadcasting the star's UV radiation in the infrared, which can be detected by suitable telescopes, such as the James Webb Space Telescope. The rebroadcast of the star's UV radiation by carbon and other metals traveling away from its surface creates the Doppler signature of a Wolf–Rayet—broad emission lines—rather than the far more common absorption-line spectra.

Binary system characteristics

WR 140 has been described as the brightest Wolf–Rayet star in the northern hemisphere, although WR 133 also in Cygnus is comparably bright. Being less massive, less luminous, and probably less visually bright than its primary, the Wolf–Rayet component is often identified as the secondary star, despite the fact that it dominates the spectrum with its broad emission lines. Or the components may be identified only as WR and O for clarity. The primary star is an O4–5 star, most likely a giant or supergiant. Fahed et al. deduced a spectral type of O5.5fc, with a luminousity class between III and I. This classification is commonly used for this star. The currently-accepted spectroscopic orbit is highly eccentric and has a period of 7.9±0.2 years, which has been determined from the velocity variations observed with the component's spectral lines, mostly from the Balmer absorption lines of the O4–5 primary and C IV emission lines at 465.0 nm for WR 140. Separation between the two stars varies from 1.3 AU at periastron to 23.9 AU at apastron. WR 140 is listed as a Wolf–Rayet variable star whose visual brightness varies very slightly, and it has been given the variable star designation V1687 Cyg in the General Catalogue of Variable Stars. Infrared light fluctuations during the orbit of WR 140 are studied because of its episodic dust formation. It is now regarded as the prototype colliding-wind binary. Shortly after periastron passage every eight years, the infrared brightness increases dramatically and then slowly drops again over a period of months. Here stellar winds collide with the dust formation created by the Wolf–Rayet star, causing the unusual bulges and angles in the concentric shells of dust. The dust typically emitted by Wolf–Rayet systems is not so coherent or concentric as those of WR 140. The dust lanes around Wolf–Rayet stars are most commonly observed as some variety of spiral. This is thought to be the result of the duelling stellar winds in binary systems, which compress clouds of dust into distinct shock fronts. The concentric nature of WR 140's dust shells is not well understood, although it may be related to nuclear processes in the Wolf–Rayet star's core.

Dust shells

The dust shells around WR 140 were first observed in 1999/2001 with the Keck Observatory. Ground-based infrared observations only resolved one to two discrete shells around the binary. Over 17 shells were observed with JWST MIRI, reaching out to about 45 arcseconds, or 70,000 astronomical units (AU). These represent more than 130 years of episodic dust production. A bright dust feature known as C1 has been used to analyse the mid-infrared spectrum of the shells; these shells show emission, probably due to polycyclic aromatic hydrocarbons (PAH), which is known to be highly stable. These features indicate hydrogen-poor and carbon-rich dust particles. Multi-epoch observations with Keck showed that the dust shells are accelerating under radiation pressure. When the grains first form at a distance of approximately 50 AU from the star they have a speed of 1,810+140−170 km/s. They then experience an acceleration of 900+700−400 km/s per year until they reach around 220 AU, the dust becomes optically thin and the acceleration decreases. With the help of Subaru and Keck, the shells were detected in the mid- and near-infrared. The mid-infrared detection corresponds to colder (500 K) larger dust grains (30 to 50 nm) and the near-infrared detection corresponds to hotter (1,000 K) nano-sized dust (1 nm). These nano-sized dust grains exist in excess and are either produced by grain-grain collision or by radiative torque disruption (RATD).

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