Wolf-Rayet 104 ‘pinwheel’ star reveals a surprise (and some relief)

A recent study reveals that the famous Wolf-Rayet 104 “pinwheel star” holds more mystery but is even less likely to be the potential “death star” it was once thought to be.

Research by W. M. Keck Observatory Instrument Scientist and astronomer Grant Hill finally confirms what has been suspected for years: WR 104 has at its heart a pair of massive stars orbiting each other with a period of about 8 months. The collision between their powerful winds gives rise to its rotating pinwheel of dust that glows in the infrared, and spins with the same period.

The pinwheel structure of WR 104 was discovered at Keck Observatory in 1999 and the remarkable images of it turning in the sky astonished astronomers. One of the two stars that were suspected to orbit each other—a Wolf-Rayet star—is a massive, evolved star that produces a powerful wind highly enriched with carbon. The second star—a less evolved but even more massive OB star—has a strong wind that is still mostly hydrogen.

Collisions between winds like these are thought to allow hydrocarbons to form, often referred to as “dust” by astronomers. When discovered, WR 104 also made headlines as a potential gamma-ray burst (GRB) that could be aimed right at us. Models of the pinwheel images indicated that it was rotating in the plane of the sky, as if we were looking directly down on someone spinning a streaming garden hose over their head.

That could have meant that the rotational poles of the two stars might be pointed in our direction as well. When one of the stars would end its life as a supernova, the explosion might be energetic enough to create a GRB that would beam in the polar directions. Since it is located right here in our own galaxy, and seemed to be aimed right at us, at the time, WR 104 gained a second nickname—the “Death Star.”

Hill’s research, published in the Monthly Notices of the Royal Astronomical Society, is based on spectroscopy using three of Keck Observatory’s instruments—the Low Resolution Imaging Spectrometer (LRIS), the Echellette Spectrograph and Imager (ESI), and the Near-Infrared Spectrograph (NIRSPEC). With these spectra, he was able to measure velocities for the two stars, calculate their orbit and identify features in the spectra arising from the colliding winds. There turned out to be a very big surprise in store though.

“Our view of the pinwheel dust spiral from Earth absolutely looks face-on (spinning in the plane of the sky), and it seemed like a pretty safe assumption that the two stars are orbiting the same way,” says Hill. “When I started this project, I thought the main focus would be the colliding winds and a face-on orbit was a given. Instead, I found something very unexpected. The orbit is tilted at least 30 or 40 degrees out of the plane of the sky.”

While a relief for those worried about a nearby GRB pointed right at us, this represents a real curveball. How can the dust spiral and the orbit be tilted so much to each other? Is there more physics that needs to be considered when modeling the formation of the dust plume?

“This is such a great example of how with astronomy we often begin a study and the universe surprises us with mysteries we didn’t expect,” muses Hill.

“We may answer some questions but create more. In the end, that is sometimes how we learn more about physics and the universe we live in. In this case, WR 104 is not done surprising us yet.”