The Fierce Winds of the Double Wolf-Rayet System: Apep

In late 2020, Australian astronomers (working as part of a larger international collaboration) used several radio telescopes located across the southern hemisphere to simultaneously observe a rare, interesting triple stellar system that features a pair of the hottest types of stars known – Wolf-Rayet stars.

The findings, reported in the Monthly Notices of the Royal Astronomical Society, have highlighted that the high-velocity winds being driven from each of the individual Wolf-Rayet (WR) stars located in the binary engine of the system, are colliding in a radio emission region that has been mapped on milliarcsecond scales to reveal a similar geometry as the beautiful, pinwheel-spiral structure previously confirmed with mid-infrared imaging.

“We knew that Apep was a system composed of two Wolf-Rayet stars, and having this elegant spiral of dust around. But the conditions at its core were still a mystery, and there were no clues to explain the extremely luminous radio emission arising from there,” said the lead author of the study, Dr. Benito Marcote.

“But now this study revealed a direct and detailed image of the region where the two stellar winds collide. Given that one of the stars has a stronger wind, this region is blended in one direction and appears as a bow-shaped structure. With this image we were able to pinpoint the positions of the two stars in the sky and the properties of their stellar winds, linking them to the basis of the spiral dust plume.”

“But the importance of the study also lies in the fact that only a very few (less than a handful) of these binary systems have been imaged to date with this resolution. This study allows us to learn how these massive stars behave when they are in couples, as it is expected that some changes happen with respect to those that are isolated,” he said.

These rare and luminously extravagant types of stars were first discovered in the mid-1860s by Charles Wolf and George Rayet at the Paris observatory, with their bright emission spectra jumping out in spectroscopic studies. Yet for all their beauty and flare, WR stars are very short-lived, burning through their fuel loads faster than other stars – which is why approximately 200 or so have only been found in our galaxy, in a sea of billions of stars.

This particular system, aptly named after the Egyptian mythological serpent-God, Apep, was discovered by Peter Tuthill from the University of Sydney some years back, who used the powerful telescopes and infrared cameras at the European Southern Observatory’s Very Large Telescope (VLT) located in the high Atacama Desert in Chile to study the spiral dust structure.

Whilst radio and x-ray astronomers knew this system was always going to be special, what the infrared imaging revealed was nothing short of spectacular – a nebula surrounding the star system like a coiled serpent rearing to strike. Apep was the perfect name for it, even more so as it has now been confirmed at a Gamma-Ray Burst progenitor from within our own galaxy.

And to really get stuck into observing the colliding wind binary details of Apep at the smallest possible resolution in radio wavelengths, astronomers needed to string together an array of telescopes that crossed entire continents.

To gather these recent findings of Apep, the research team used a particular technique in astronomy that combines the data from more than one radio telescope,  known as interferometry. This process is not limited to radio astronomy and has been used in other optical-based systems, such as the VLT, Chile.

“Imaging the system with very high resolution was critical here to understand the origin of the emission. The two stars in Apep are too close and common telescopes cannot disentangle them, or provide details on the region in between,” said Benito. 

“To do that we needed about a thousand times more resolution than the one reached by typical interferometers. And this was only possible by using the VLBI technique: combining antennas spread over continental distances.”

“These VLBI networks of antennas are the only ones capable of resolving the ‘banana’ shape structure that we found at the heart of Apep,” he said.

Having greater resolution in astronomical objects assists scientists with modelling and understanding space-based phenomena, similar to having high-resolution images here on Earth. But radio telescopes don’t collect light, instead, they collect longer wavelength radio signals – which is why radio telescopes are generally bigger.

However, numerous challenges (financial, engineering, ongoing maintenance, etc.) arise if we were to just keep building bigger telescopes. So instead, we simulate one of these using individual radio telescopes located at distant locations from each other.

This then provides the ability to build a ‘virtual’ telescope with a diameter of the greatest distance between the two furthest individual components – which is known as a baseline. It would be like having two 10m dish antennas located 100 metres apart. Each dish individually has the resolution capable of its own diameter (i.e. 10m), but when both dishes are used to simulate a larger telescope and observe the same object at the same time, then the baseline (or virtual diameter of the telescope) becomes 100 metres – effectively building a larger telescope.

When this is replicated with telescopes located across the continent, or even telescopes internationally, this process is known as Very Long Baseline Interferometry (or VLBI) because the baseline (i.e. the virtual size of the telescope) can stretch thousands of kilometres.

It was this technique that was developed and implemented to capture the first image of a black hole located 55 million light-years away, in unprecedented detail. The Event Horizon Telescope (EHT) literally linked radio telescopes together to simulate a single antenna as big as Earth.

There are numerous things that VLBIs are used for these days, but in particular, being able to provide very high-resolution radio astronomy imaging has changed our understanding of the Universe as a whole.

VLBIs are established all over the world, with Australia also having its own - known as the Australian Long Baseline Array (LBA) and consists of telescopes that stretch across the continents east-west dimension from Perth to Tidbinbilla, as well as a north-south aspect, running from Hobart to Katherine. The LBA is operated by Australia’s national science agency, the CSIRO (and the Australian Telescope National Facility (ATNF) in particular), the University of Tasmania and NASA’s Deep Space tracking stations.

This includes the CSIRO ATNF managed Compact Array (6 x 22m dishes), the Parkes radio telescope (Murriyang – 64m) and the Mopra telescope, located in Coonabarabran (22m). In addition to this, both the Hobart (26m) and Ceduna telescopes (30m), which are managed by the University of Tasmania and the 70m Dish (DSS 43) located at the CDSCC.

For this particular study of Apep, the team of astrophysicists also used the Katherine (12m), Warkworth and Yarragaudee (12m) telescopes, as well as the 12m Warkworth telescope, situated on New Zealand’s north island, and the 26m Harebeesthoek telescope located in South Africa, recording the data with a total bandwidth of 64 MHz, with most stations recording in full polarisation.

The name Apep is not from modern times. It originates from ancient Egyptian mythology, representing the giant serpent God, who was the absolute enemy of the Sun God, Ra. The nickname was given to the system by astronomer Joe Callingham from ASTRON after studies showed its windswept nebulosity resembled a coiling dragon. The official name, whilst a little more clinical, comes from the XMM-Newton space telescope catalogue and is known as 2XMM J160050.7-514245.

Located about 6,500 light-years away in the southern constellation of Norma, the Apep system features three-star – two central Wolf-Rayet (WR) stars, which orbit a centre of mass every hundred or so years, and a third supergiant to the north, which completes an orbit around the system’s core region, roughly every 10,000 years.

“This system is by far one of the brightest colliding-wind binaries at X-ray, infrared, and radio wavelengths but it was not discovered until 2012!,” said co-author Dr. Joe Callingham, who is received his Ph.D. from the University of Sydney in 2017, and is currently working as a Veni Fellow at ASTRON in the Netherlands.

“More scientifically, the system has confounded our understanding of how massive stars die. For example, the beautiful pinwheel pattern you see in the infrared should be travelling at the speed of the winds of the Wolf-Rayet stars in the system. However, we have found that is not the case! That is very odd and requires us to invent exotic solutions - such as one of the Wolf-Rayet stars at the centre is rapidly rotating.” 

“Finally, at the heart of Apep is not one but in fact two Wolf-Rayet stars. This is also unexpected and generally considered the first strong case of such a system in our Milky Way,” he said.

What gives Apep its beautiful nebulosity are the powerful stellar winds and cosmic dust that is thrown off the WR stars, as they furiously expel their mass into the surrounding region at velocities of roughly 12 million km/h – enough of a kick to circle the Earth 300 times within the same period.

WR stars are extremely intriguing objects to astronomers because they have extremely hot surface temperatures (usually ranging from 30,000 Kelvin through to 210,000 Kelvin – making them the hottest stars) and are surrounded by their own nebula of material that has been ejected from the surface. They’re also very bright to observe due to their high surface temperature and nebulosity. So bright, that if Apep was placed only 32 light-years away it would shine with half the brightness of the full Moon.

These are evolved massive stars (O and B class) which exhibit broad emission spectral lines of ionised helium, nitrogen, and carbon, and are well on their way to detonating in massive supernovae explosions. One of the most massive and most luminous stars known, R136a1, which shines with a luminosity of 6.2 million Suns, and contains 215 times the mass of our own Sun, is a WR star located in the Tarantula Nebula, nestled within the nearby Large Magellanic Cloud galaxy.

WR stars are also classified by their emission spectra, and in particular, the broad emission lines observed for ionised nitrogen (WN stars), ionised carbon (WC stars), and sometimes ionised oxygen. In Apep’s case, the two main central components feature a carbon-sequence star (WC8) and a nitrogen-sequence star (WN4-6b).

The Apep system also emits across a large portion of the electromagnetic spectrum as well – radio waves through to x-rays have been observed by a number of ground and space-based telescopes, including the Molonglo Observatory Synthesis Telescope, the Anglo-Australian Telescope, the array of radio telescopes mentioned above, in addition to space-based x-ray observatories.

The reason for all this interest is because scientists have concluded that Apep is a Milky Way progenitor for a cataclysmic and powerful event known as a Gamma-Ray Burst, which has been observed in other galaxies.

“We think there might be a Wolf-Rayet star in the system that is near critical rotation (e.g. spinning so fast that it is close to ripping itself apart).”

“Models of the progenitors of long-duration Gamma-ray bursts have long expected the only way to get the jet (and thus enough energy) is by having a fast rotating Wolf-Rayet star when it undergoes core-collapse (aka a supernova). We never expected to find such a system in our proverbial backyard!” said Joe.

There’s also a rarity to observing a double WR stellar system because the lifetimes of WR stars is much shorter than the main sequence stage of stellar evolution, and these massive stars don’t last more than roughly 10 million years – this means the WR stars we see today, are all astronomically noticeably young.

In these latest findings, the Australian Long Baseline Array was used to detect, on milliarcsecond scales, a bow-shaped radio emission source, which represents a wind collision region (mostly aligned to the north-south direction of the system), that is being generated by the two powerful WR stars, as they fiercely puff off their outer layers.

By further studying this important radio-emitting region, as well as the larger, dust pinwheel nebula surrounding the system, astrophysicists hope to further refine the orbit of all three stars and learn about the nature of these rare systems, and what it might mean for our galaxy.

“Apep is by far the most luminous radio-emitting CWB outside of the singular eta Carinae. We still don’t fully understand why that is the case, and maybe that will tell us why the dust/gas dynamics is so odd,” said Joe. 

“I have a student at Leiden University working with me to try and figure that out. The real progress next will come either from optical VLBI observations (where we can resolve the two stars) or theoretical work (such as magneto-hydrodynamic simulations) to show that our model of critically-rotating Wolf-Rayet star works.”

Video credit: University of Sydney.