Astronomy: unraveling a spiral stream of dusty embers

Using nearly two decades of mid-infrared imaging from the world’s largest observatories, an international team of astronomers has captured the spiral motion of dust streaming from the binary star system Wolf-Rayet (WR) 112.

Analysis of this dusty outflow identifies WR 112 as a highly efficient dust factory that produces an entire Earth mass of dust every year, the astronomers reveal in a study published today in the Astrophysical Journal.

Dust formation in space is typically seen in the gentle outflows from cool stars with a Sun-like mass. It is somewhat unusual in the extreme environment around massive stars and their violent winds.

However, interesting things happen when the fast winds of two massive stars in a binary interact.

Animated model of the spiral dust nebula around WR 112 (left) and the actual corresponding observations (right).  The φ symbol on the model animation indicates the orbital phase of the central binary, where φ = 0 is at the beginning of its 20-year orbit, and φ = 1 is at the end of its orbit. The animation pauses at each phase that is displayed by the real observations. 

Indeed, the team’s model images showed a remarkable likeness to the real images of WR 112. The models and the series of imaging observations revealed that the rotation period of this dusty "edge-on" spiral (and the orbital period of the central binary system itself) is 20 years. The looked drastically different when viewed face-on versus edge-on, reflecting the importance of the viewing angle of WR 112.

With the revised picture of WR 112, the research team was able to deduce how much dust this binary system is forming.

“Spirals are repetitive patterns, so since we understand how much time it takes to form one full dusty spiral turn – around 20 years – we can actually trace the age of dust produced by the binary stars at the center of its spiral,” said Lau. “There is freshly formed dust at the very central core of the spiral, while the dust we see that’s four spiral turns away is about 80 years old.

“Therefore,” he continued, “we can essentially trace out an entire human lifetime along the dusty spiral stream revealed in our observations. So I could actually pinpoint on the images where the dust is that formed when I was born: right now, it’s somewhere in between the first and second spiral turn.”

To their surprise, the astronomers found that WR 112 is a highly efficient dust factory that outputs dust at a rate equivalent to producing an entire Earth mass of dust every year. This was unusual given WR 112's 20-year orbital period; the most efficient dust producers in this type of WR binary star system tend to have shorter orbital periods of less than a year, such as WR 104 with its 220-day period.

WR 112 therefore demonstrates the diversity of WR binary systems that are capable of efficiently forming dust and highlights their potential role as significant sources of dust not only in our galaxy but in galaxies beyond, the researchers believe.

Lastly, they said, these results demonstrate the potential for new discoveries using multi-epoch mid-IR imaging via the MIMIZUKU instrument on the University of Tokyo Atacama Observatory set to begin operations in Chile.

“Mid-IR results from our study notably utilize the largest observatories in the world and set the stage for the next decade of astronomical discoveries with 30-metre telescopes and NASA’s pcoming James Webb Space Telescope,” concluded Lau.