Hypergiant stars, which are extreme supergiant stars, are extremely rare, with only a few known to exist in the Milky Way. Astronomers obtained the first detailed map of the envelope of the red hypergiant star VY Canis Majoris by tracing molecular emissions in outflows around the star, which sheds light on the mechanisms involved in the final stages of extreme supergiant 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 in the vicinity of VY Canis Majoris, a red hypergiant star.
Their findings, which they presented on June 13 at the 240th Meeting of the American Astronomical Society in Pasadena, California, provide unprecedented insight into the processes that accompany the death of giant stars. The research was carried out in collaboration with Robert Humphreys of the University of Minnesota and Anita Richards of the University of Manchester in the United Kingdom.
Hypergiant stars, which are extreme supergiant stars, are extremely rare, with only a few known to exist in the Milky Way. Betelgeuse, the second brightest star in the constellation Orion, and NML Cygni, also known as V1489 Cygni in the constellation Cygnus, are two examples. Unlike stars with lower masses, which tend to puff up once they enter the red giant phase but generally retain a spherical shape, hypergiants experience significant, sporadic mass loss events, resulting in complex, highly irregular structures composed of arcs, clumps, and knots.
We are particularly interested in what hypergiant stars do at end of their lives. People used to think these massive stars simply evolve into supernovae explosions, but we are no longer sure about that.
Ambesh Singh
Located about 3,009 light-years from Earth, VY Canis Majoris — or VY CMa, for short — is a pulsating variable star in the slightly southern constellation of Canis Major. Spanning anywhere 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 as Betelgeuse on steroids,” said Ziurys, a Regents Professor with joint appointments in UArizona Department of Chemistry and Biochemistry and 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 are particularly interested in what hypergiant stars do at end of their lives,” said Singh, a fourth-year doctoral student in Ziurys’ lab. “People used to think these massive stars simply evolve into supernovae explosions, but we are no longer sure about that.”
“If that were the case, we should see many more supernovae explosions across the sky,” Ziurys added. “We now think they might quietly collapse into black holes, but we don’t know which ones end their lives like that, or why that happens and how.”
Previous Hubble Space Telescope imaging and spectroscopy of VY CMa revealed the presence of distinct arcs and other clumps and knots, many of which extended thousands of AU from the central star. To learn more about the processes that cause hypergiant stars to die, the researchers set out to trace specific molecules around the hypergiant and map them to Hubble Space Telescope images of dust.
“Nobody has been able to make a complete image of this star,” Ziurys said, explaining that her team set out to understand the mechanisms by which the star sheds mass, which appear to be distinct from those of smaller stars nearing the end of their lives.
“You don’t see this nice, symmetrical mass loss, but rather convection cells that blow through the star’s photosphere like giant bullets and eject mass in different directions,” Ziurys explained. “These are similar to coronal arcs seen in the sun, but a billion times larger.”
The team used the Atacama Large Millimeter Array, or ALMA, in Chile to trace a variety of molecules in material ejected from the stellar surface. While some observations are still ongoing, preliminary maps of sulfur oxide, sulfur dioxide, silicon oxide, phosphorus oxide, and sodium chloride have been obtained. The group used these data to create an image of VY CMa’s global molecular outflow structure on scales that included all ejected material from the star.
“The molecules trace the arcs in the envelope, indicating that molecules and dust are well-mixed,” Singh explained. “The nice thing about radio wavelength molecule emissions is that they provide us with velocity information, as opposed to static dust emission.”
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 by moving ALMA’s 48 radio dishes into different configurations, even correlating them to different mass ejection events over time.
According to Singh, processing the data required some heavy lifting in terms of computing power.
“We’ve processed nearly a terabyte from ALMA so far, and we’re still receiving data that we need to go through to get the best resolution possible,” he said. “Simply calibrating and cleaning the data necessitates up to 20,000 iterations, which takes a day or two for each molecule.”
“We can now put these observations on maps in the sky,” Ziurys said. “Until now, only small sections of this massive structure had been studied, but understanding mass loss and how these massive stars die requires a comprehensive examination of the entire region. That’s why we wanted to create an all-encompassing image.”
