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Watch the death of a rare giant star

A team of astronomers led by the University of Arizona has created a detailed, three-dimensional image of a dying hypergiant star. The team, led by UArizona researchers Ambesh Singh and Lucy Ziurys, tracked the distribution, directions and velocities of a variety of molecules surrounding a red hypergiant star known as VY Canis Majoris.

Their results, which they presented on June 13 at the American Astronomical Society’s 240th meeting in Pasadena, California, provide insights, on an unprecedented scale, into the processes that accompany the death of giant stars. The work was carried out with partners Robert Humphreys from the University of Minnesota and Anita Richards from the University of Manchester in the UK.

Extreme supergiants known as hypergiants are very rare, with only a few known to exist in the Milky Way. Examples include Betelgeuse, the second brightest star in the constellation Orion, and NML Cygni, also known as V1489 Cygni, in the constellation Cygnus. Unlike lower mass stars – which are more likely to inflate as they enter the red giant phase but generally retain a spherical shape – hypergiants tend to experience significant, sporadic mass loss events that form complex, highly irregular structures consisting of arcs. , lumps and knots.

VY Canis Majoris, or VY CMa, abbreviated, is located approximately 3,009 light-years from Earth and is a pulsating variable star in the slightly southern constellation Canis Major. VY CMa ranges from 10,000 to 15,000 astronomical units (with 1 AU being the average distance between Earth and the Sun) VY CMa is possibly the most massive star in the Milky Way, according to Ziurys.

“Think of it like Betelgeuse on steroids,” said Ziurys, a Regent’s Professor with joint appointments in the Arizona Department of Chemistry and Biochemistry and the Steward Observatory, both part of the College of Science. “It is much larger, much more massive and undergoes violent mass eruptions every 200 years or so.”

The team chose to study VY CMa because it is one of the best examples of these types of stars.

“We’re particularly interested in what hyper-giant stars do at the end of their lives,” said Singh, a fourth-year doctoral student in Ziury’s lab. “People used to think that these massive stars were simply evolving into supernova explosions, but we are no longer sure.”

“If that were the case, we should see many more supernova explosions across the sky,” Ziurys added. “We now believe that they can quietly collapse into black holes, but we do not know who ends their lives like that, or why it happens and how.”

Earlier imagery of VY CMa with NASA’s Hubble Space Telescope and spectroscopy showed the presence of distinct arcs and other lumps and nodules, many extending thousands of AU from the central star. To reveal more details about the processes by which hypergiant stars end their lives, the team set about tracking down certain molecules around the hypergiant and mapping them into pre-existing images of the dust, taken by the Hubble Space Telescope.

“No one has been able to get a complete picture of this star,” said Ziurys, explaining that her team was trying to understand the mechanisms by which the star ejects mass, which appear to be different from the smaller stars entering its red giant phase. at the end of their lives.

“You do not see this fine, symmetrical mass loss, but rather convection cells that blow through the star’s photosphere like giant spheres and shoot out mass in different directions,” said Ziurys. “These are analogous to the coronal arcs seen in the sun, but a billion times larger.”

The team used the Atacama Large Millimeter Array, or ALMA, in Chile to track a variety of molecules in material ejected from the star surface. While some observations are still ongoing, preliminary maps of sulfur oxide, sulfur dioxide, silica, phosphorus oxide and sodium chloride were obtained. From these data, the group constructed an image of the global molecular outflow structure of VY CMa on scales that included all ejected material from the star.

“The molecules trace the arcs in the casing, which tells us that molecules and dust are well mixed,” Singh said. “The great thing about emissions of molecules at radio wavelengths is that they provide us with velocity information, as opposed to dust emissions, which are static.”

By moving ALMA’s 48 radio bowls to different configurations, the researchers were able to obtain information about the directions and velocities of the molecules and map them across the different regions of the hypergiant’s envelope in considerable detail, even correlating them with different mass ejection events over time. .

Processing data required some heavy lifting in terms of computing power, Singh said.

“So far we have processed almost one terabyte from ALMA, and we are still getting data that we need to go through to get the best possible resolution,” he said. “Calibration and cleaning of data alone requires up to 20,000 iterations, which takes a day or two for each molecule.”

“With these observations, we can now put these on maps in the sky,” Ziurys said. “Until now, only small parts of this huge structure had been studied, but you can not understand the mass loss and how these big stars die if you do not look at the whole region. That is why we wanted to create a complete picture.”

With funding from the National Science Foundation, the team plans to publish its findings in a series of articles.

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