“Beautiful and Dangerous Physics” -First Wolf-Rayet Star System to be Discovered in the Milky Way

 

Astronomers discovered “a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy. This is the first such Wolf-Rayet star system to be discovered in our own galaxy,” explains Joseph Callingham of the Netherlands Institute for Radio Astronomy (ASTRON), lead author of the study reporting this system. “We never expected to find such a system in our own backyard.”

Physics of How Massive Stars Die

“When I made the first image of Apep I was completely blown away,” Callingham wrote to The Daily Galaxy in an email. “It was surprising that something so bright and beautiful had not been discovered until my observations. If my career spans 50 years, I do not think I will make a more beautiful image. However, not only was the image beautiful – it told us that we do not understand the physics of how massive stars die.”

First Star in the Milky Way to End its life as a Long-duration Gamma-ray Burst 

In 2018, the international team of astronomers have found the multiple star system located 8,000 light years away in a unique phase of evolution. One of the stars is a Wolf-Rayet star, meaning it has lost its hydrogen-rich outer envelope, either through strong stellar winds and/or via interactions with its binary companion. The Wolf-Rayet star is also rapidly rotating, and thus is the first known candidate in our Milky Way to end its life as a long-duration gamma-ray burst (GRB), among the most energetic events in the universe.

The system, comprising a pair of scorching luminous stars, was nicknamed Apep by the team after the serpentine Egyptian god of chaos. The brighter Wolf-Rayet star is on the brink of a massive supernova explosion.

When asked, have any subsequent Wolf Rayet star systems been discovered that produced a long-duration gamma ray burst, Callingham told The Daily Galaxy: “None in the Milky Way yet – only suggestions of systems in other galaxies (which are much further away and much harder to study).” 

Apep Star System in a Whole New Class

The findings, published in Nature Astronomy, are controversial as no gamma-ray burst has ever been detected within our own galaxy, the Milky Way. “The rapid rotation puts Apep into a whole new class. Normal supernovae are already extreme events but adding rotation to the mix can really throw gasoline on the fire.”

The progenitors of long gamma-ray bursts are thought to be rapidly-rotating evolved massive stars. Their rapid rotation generates extreme magnetic fields that collimate bipolar relativistic jets. Long GRBs observed in other galaxies also tend to be associated with supernovae without hydrogen or helium in their spectra, consistent with Wolf-Rayet stars that have already lost their outer layers.

Recipe for a Perfect Stellar Storm

The researchers think this might be the recipe for a perfect stellar storm to produce a gamma-ray burst, which are the most extreme events in the Universe after the Big Bang itself. Fortunately, Apep appears not to be aimed at Earth, because a strike by a gamma-ray burst from this proximity could strip ozone from the atmosphere, drastically increasing our exposure to UV light from the Sun.

The VISIR instrument on ESO’s Very Large Telescope (VLT) captured the stunning image at the top of the page of the newly-discovered massive binary star system. With 2 Wolf-Rayet stars orbiting each other in the binary, the serpentine swirls surrounding Apep are formed by the collision of two sets of powerful stellar winds, which create the spectacular dust plumes seen in the image. The blue sources at the center of the image are a triple star system — which consists of a binary star system and a companion single star bound together by gravity. Though only two star-like objects are visible in the image, the lower source is in fact an unresolved binary Wolf-Rayet star. The triple star system was captured by the NACO adaptive optics instrument on the VLT.

The system, which comprises a nest of massive stars surrounded by a “pinwheel” of dust, is officially known only by unwieldy catalog references like 2XMM J160050.7-514245. However, the astronomers chose to give this fascinating object a catchier moniker — “Apep”.

The Sculpted Plume

That sculpted plume is what makes the system so important, said Peter Tuthill,.Professor and ARC Future Fellow in the School of Physics and director of the Sydney Institute for Astronomy (SIfA) at the University of Sydney, Australia. “When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel,” Professor Tuthill said.

“The curved tail is formed by the orbiting binary stars at the center, which inject dust into the expanding wind creating a pattern like a rotating lawn sprinkler. Because the wind expands so much, it inflates the tiny coils of dust revealing the physics of the stars at the heart of the system.”

“The plume encodes two things,” Calligham told The Daily Galaxy, “the orbital dynamics of the systems and the physics of the winds. That, in turn, allows us to understand what is happening at the heart of Apep (where two massive stars are near the end of their lives).

A Conundrum –l“Like finding a feather caught in a hurricane”

However, the data on the plume presented a conundrum: the stellar winds were expanding 10 times faster than the dust. “It was just astonishing,” Tuthill said. “It was like finding a feather caught in a hurricane just drifting along at walking pace.”

 Long-duration GRBs –Explosion of Wolf-Rayet Stars

GRBs are among the most powerful explosions in the Universe. Lasting between a few thousandths of a second and a few hours, they can release as much energy as the Sun will output over its entire lifetime. Long-duration GRBs — those which last for longer than 2 seconds — are believed to be caused by the supernova explosions of rapidly-rotating Wolf-Rayet stars. Meanwhile, short GRBs derive from mergers of binary neutron stars via gravitational waves, as discovered by LIGO in 2017. 

Blink of an eye in cosmological terms

Some of the most massive stars evolve into Wolf-Rayet stars towards the end of their lives. This stage is short-lived, and Wolf-Rayets survive in this state for only a few hundred thousand years — the blink of an eye in cosmological terms. In that time, they throw out huge amounts of material in the form of a powerful stellar wind, hurling matter outwards at millions of kilometers per hour; Apep’s stellar winds were measured to travel at an astonishing 12 million km/h.

 

Compared to the extraordinary speed of Apep’s winds, the dust pinwheel itself swirls outwards at a leisurely pace, “crawling” along at less than 2 million km/h. The wild discrepancy between the speed of Apep’s rapid stellar winds and that of the unhurried dust pinwheel is thought to result from one of the stars in the binary launching both a fast and a slow wind — in different directions.

This would imply that the star is undergoing near-critical rotation — that is, rotating so fast that it is nearly ripping itself apart. A Wolf-Rayet star with such rapid rotation is believed to produce a long-duration GRB when its core collapses at the end of its life.

The Last Word

“Ultimately, we can’t be certain what the future has in store for Apep,” Tuthill said. “The system might slow down enough so it explodes as a normal supernova rather than a gamma-ray burst. However, in the meantime, it is providing astronomers a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy.”

Image credit: ESO/Callingham et al.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Joseph Callingham, OUP Blog and ESO 

Image credit: ESO/Callingham et al.

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