In 2018, astronomers discovered “a ringside seat into beautiful and dangerous physics. This is the first such system to be discovered in our own galaxy,” explains Joseph Callingham of the Netherlands Institute for Radio Astronomy (ASTRON) about a star system like none seen before in our galaxy. “We never expected to find such a system in our own backyard.”
The international team of astronomers found the star system about 8,000 light years from Earth. One of the stars in the system is the first known candidate in the Milky Way to produce a dangerous gamma-ray burst, among the most energetic events in the universe.
The Dangerous Physics
The system, comprising a pair of scorchingly luminous stars, was nicknamed Apep by the team after the serpentine Egyptian god of chaos. One star is on the brink of a massive supernova explosion.
The findings, published today 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 fast rotation of the massive evolved star may provide the necessary angular momentum to sustain a large-scale magnetic field that launches a collimated jet of gamma-rays when the star eventually explodes as a supernova.
The researchers think this might be the recipe for a perfect stellar storm to produce a long-duration gamma-ray burst (GRB), which are the most extreme events in the Universe after the Big Bang itself. Fortunately, the rotation axis of Apep appears not to be aimed at Earth. A direct strike by a GRB within a few hundred light years 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 two 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.
A Triple Star System
The reddish pinwheel in this image is data from the VISIR instrument on ESO’s Very Large Telescope (VLT), and shows the spectacular plumes of dust surrounding Apep. 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”.
Sculpted Plume is What Makes the System Important
That sculpted plume is what makes the system so important, said Peter Tuthill, research group leader at the University of Sydney.
“When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel,” 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.”
A ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy”
However, the data on the plume presented a conundrum: the stellar winds were expanding 10 times faster than the dust. In single massive stars, the outer gaseous envelope absorbs the intense radiation, expanding outward and carrying the dust along with it. But in this massive evolved binary system, the dust and gas winds appear to be decoupled.
“It was just astonishing,”Tuthill said. “It was like finding a feather caught in a hurricane just drifting along at walking pace.”
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.
Apep’s stellar winds measured at an astonishing 12 million km/h.
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.
These stellar winds have created the elaborate plumes surrounding the 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. This binary is responsible for sculpting the serpentine swirls surrounding Apep, which are formed in the wake of the colliding stellar winds from the two Wolf-Rayet stars.
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.
Reaching near-critical rotation
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. From Earth, we will eventually see the more massive Wolf-Rayet star explode as a supernova and possibly the off-axis afterglow of a long GRB, even if the rotation axis is not perfectly aligned with our line-of-sight.
“This system remains deeply enigmatic and intriguing,”Tuthill told The Daily Galaxy. “It does not fit the mould … that the sequence of future events – rapid spin to a GRB is certainly plausible and justified in the research papers. But this science in particular is complicated and the Apep system, frankly, hard to fathom.
“The critical rotation rate has not been calculated exactly,” Tuthill added,”because we don’t have full details about what is going on. But it would be something approaching the critical speed (beyond which the star starts flying apart at the equator) – maybe this would be several hundred km/sec If it went GRB I suspect we would see something from Earth. It would appear as some kind of nova to us – but not exceptional or damaging as we are not looking (we think) along the beam axis.”
“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.