Glowing Serpent Found in the Sky

Astronomers have discovered a star generating a pinwheel of dust 8,000 lightyears away, which may shed some light on the last stages of massive star evolution.

Massive stars are dynamic lighthouses of the universe, seen from thousands of light-years away and creating the heavy elements that seed new generations of star formation. They burn through their fuel at rates millions of times higher than the Sun, using up their hydrogen stores before exploding in dramatic fashion. But the end of each massive star is not identical. Now, astronomers have discovered one particularly puzzling object producing wind patterns never seen before. The observations and analysis, led by Joe Callingham (Netherlands Institute for Radio Astronomy), appears November 19th in Nature Astronomy.

Because of the snake-like appearance of dust swirling around the star, the researchers nicknamed the system “Apep,” after the Egyptian god of chaos who often appeared as a snake.   Apep is an example of a Wolf-Rayet (WR) star, which are massive stars in the last phase of evolution before exploding as supernovae. The star itself is fusing helium and heavier elements at high temperatures. Some WR stars appear dim, but only because most of their light is emitted in the ultraviolet, due to their high temperatures.

WR stars blow off their outer layers with powerful stellar winds, much like super-charged versions of the planetary nebulae produced by stars like our Sun at the end of their lives. These stellar winds can be tracked and analyzed through spectral observations. Apep, otherwise known as 2XMM J160050.7-514245, drives wind speeds of 3,400 kilometers per second (7.6 million mph) as gas rips free from the star. In addition to the spectral observations, Callingham and his team obtained thermal imaging, which highlighted warm dust generated by the star itself. This dust contains carbon, and will eventually be incorporated into future generations of stars. The thermal images revealed a spectacular snake-like pinwheel, stretching nearly 25,000 a.u. (0.4 light-years) wide, which indicates the presence of a companion star colliding with the winds from the massive WR star at Apep’s core.

The binarity of this system makes it intriguing, but also introduces new puzzles. The team compared the speeds measured for the gas (7.6 million mph) to the observed motions of the pinwheel dust patterns and found the dust moving much slower, near 570 kilometers per second (1.3 million mph). This has not been observed in other WR systems with binary companions and suggests that the WR star may be launching two types of winds, one fast and one slow. The spiral pattern itself would be generated as the companion passed through the slow wind, carving out a snake in the sky that can be seen from Earth, nearly 8,000 light-years away.

The two winds, propagating in different directions, suggest that Apep may be on its way to forming the jets seen in some of the most enigmatic and energetic phenomena known: long-duration gamma-ray bursts. These bursts, thought to originate in WR stars like Apep, last for more than 2 seconds and stream high-energy gamma radiation towards Earth, likely in highly structured, tightly wound jets. If Apep is building up to such an event, then it could provide researchers with a rare up-close look at how long-duration gamma-ray bursts arise.

This unique object is one of only a handful of pinwheel WR stars and will certainly be visited again by other infrared telescopes such as NASA’s James Webb Space Telescope, scheduled to launch in 2021.  While we await new images, Apep’s pinwheel of gas and dust will spin for centuries to come, as the central star creates the heavy elements of future stellar generations.

Reference:

J. R. Callingham et al. "Anisotropic Winds in a Wolf–Rayet Binary Identify a Potential Gamma-Ray Burst Progenitor." Nature Astronomy. November 19, 2018.