Science

James Webb captures a rare and fascinating phenomenon

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Since its inception, Webb has showered us with breathtaking images. This time he aimed at the star WR 140, located at a distance of about 5600 light years in the constellation Cygnus. No, this is not an optical defect of the device: he found that this star is surrounded by a series of concentric spheres, from which the telescope was able to capture all the details.

This star is actually a double colliding wind system, denoting a system formed by two massive stars from which powerful stellar winds (which move at almost 3000 km/s) emanate. It consists of a Wolf-Rayet star (about 20 solar masses) of type WC7 orbiting a hot and massive star (about 50 solar masses) of spectral type O4-5, both of which are extremely rare types of stars.

Wolf-Rayet stars are very hot stars, with a mass of several tens of solar masses. Most of the time they are no longer on the main sequence because they have exhausted almost all the hydrogen in their core. Their outer layers are rich in oxygen, nitrogen, silicon and carbon. They are losing mass at a very high rate in the form of extremely fast stellar winds. O-type stars are also very hot, very bright and blue in color. They are among the most massive stars known and therefore have relatively short lifespans.

The result of the interaction of two massive stars

The two stars of the WR 140 system are animated by very fast stellar winds blowing at about 3,000 kilometers per second. Therefore, they both lose mass at a rather frantic rate, on the order of 10-5 (for a Wolf-Rayet star) and 10-6 (for an O star) solar masses per year. Each of them describes an elliptical orbit and therefore periodically (about every 8 years) is close enough to each other (a point called the periapsis), close enough – at a distance of 1.3 AU. so that their winds collide.

Then a powerful shock front is formed in this place, from which several types of radiation (mainly X-ray and radio) can emanate. WR 140 is also one of the brightest non-compact stellar X-ray sources. This phenomenon also results in precise periodic dust emissions that eventually spread through space in successive layers around the binary system.

The process can be seen in the video below (Sources: NASA/ESA/J. Olmsted (STScI)). The strongest winds from the Wolf-Rayet star blow behind the O star, and dust is formed behind it as the mixed stellar matter cools. The process is periodically repeated, and the dust gradually takes the form of a characteristic windmill.

Dust (composed mostly of carbon) condenses in the stellar wind, absorbing a small fraction of the stars’ UV radiation. This radiation heats the dust to a temperature of about 1000 K, causing it to radiate in the infrared, which decreases the further away from the stars. These layers of dust glowing in the infrared, Webb’s MIRI (Mid-InfraRed Instrument) was able to capture all the details.

A new dust shell is created every eight years

Dust is blown out of the binary system by the stellar wind, causing the various layers of dust to expand. They expand and cool as they move away, losing heat and density. In the image taken by James Webb, in addition to the diffraction spikes characteristic of the instrument (straight lines starting from the center), we notice that the edge of each layer of dust is more visible – perspective induces denser material at the edges.

As mentioned above, two stars reach their periapsis approximately every 8 years (7.9 ± 0.2 years to be exact). Therefore, every 8 years a new “shell” of dust is formed (the last periapsis was observed in 2016). Like the rings of a tree, the number of which allows you to determine its age, so it is enough to count the dust shells that make up the WR 140 nebula to estimate the age of the last visible shell. The image shows about twenty, which means that we are seeing here about 160 years of dust generated by the system.

James Webb’s observation of WR 140 was requested by a team led by astrophysicist Ryan Lau of the Japan Space Agency’s Institute of Space and Astronautical Sciences (JAXA). The images were processed by two independent scientists, Melina Thevenot and Judy Schmidt – the latter responsible for several other Webb images, including recent images of Jupiter. An article on the WR 140 is in preparation; so we will soon learn a little more about the intricacies of this new observation.

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