A team of astronomers led by the University of Arizona has developed a comprehensive three-dimensional picture of a fading hypergiant star. The team, coordinated by UArizona’s Ambesh Singh and Lucy Ziurys, studied the distribution, orientations, and velocities of a range of molecules in the vicinity of VY Canis Majoris, a red hypergiant star.
Their results, which scientists presented on June 13 at the American Astronomical Society’s 240th Meeting in Pasadena, California, provide new insight into the processes that surround the demise of big stars. Robert Humphreys of the University of Minnesota and Anita Richards of the University of Manchester in the United Kingdom collaborated on the project.
Hypergiant stars, which are extreme supergiant stars, are extremely rare, with just a few identified in the Milky Way. Betelgeuse, the second brightest star in 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 out after they enter the red giant phase but maintain a spherical shape, hypergiants have periodic mass loss events that result in complex, highly irregular shapes made up of arcs, clumps, and knots.
VY Canis Majoris — or VY CMa for short — is a pulsing variable star in the somewhat southern constellation of Canis Major, located around 3,009 light-years from Earth. According to Ziurys, VY CMa is perhaps the most massive star in the Milky Way, spanning 10,000 to 15,000 astronomical units (one AU being the typical distance between Earth and the sun).
“Imagine Betelgeuse on steroids,” said Ziurys, a Regents Professor with dual appointments in the University of Arizona Department of Chemistry and Biochemistry and the Steward Observatory, both in the College of Science. “It’s a lot bigger, a lot more huge, and it erupts violently every 200 years or so.”
“What hypergiant stars do at the conclusion of their life is of special interest to us,” said Singh, a fourth-year doctorate student in Ziurys’ group. “People used to believe that these enormous stars just grow into supernovae explosions, but that is no longer the case.”
“We should observe a lot more supernovae bursting across the sky if that’s the case,” Ziurys remarked. “We now believe they may silently collide into black holes, but we don’t know which ones will do so, or why or how.”
Previous spectroscopy and imaging of VY CMa with NASA’s Hubble Space Telescope revealed the presence of unique arcs, clumps, and knots, several of which extended thousands of AU from the central star. The scientists set out to identify particular chemicals surrounding the hypergiant and map them to prior photos of the dust collected by the Hubble Space Telescope in order to learn more about the processes by which hypergiant stars end their lives.
“Nobody has been able to make a comprehensive image of this star,” Ziurys said, revealing that her team set out to learn about the mechanisms by which the star sheds mass, which appear to be distinct from those seen in smaller stars approaching their red giant phase at the end of their lifetimes.
“You don’t observe this wonderful, symmetrical mass loss,” Ziurys explained, “but rather convection cells that fly through the star’s photosphere like gigantic bullets and expel material in all directions.” “These are similar to the sun’s coronal arcs, but a billion times bigger.”
The Atacama Large Millimeter Array (ALMA) in Chile was used to track a variety of chemicals in debris blasted from the star surface. Preliminary maps of sulfur oxide, sulfur dioxide, silicon oxide, phosphorus oxide, and sodium chloride have been acquired while some observations are still ongoing. The team used these data to create a map of VY CMa’s global molecular outflow structure on sizes that included all of the star’s ejected material.
“The molecules track the arcs in the envelope, indicating that the dust and molecules are properly mixed,” Singh explained. “The wonderful thing about molecule emissions at radio wavelengths is that they supply us with velocity information, whereas dust emissions are static.”
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 necessitated some heavy lifting in terms of computational power.
“We’ve processed about a terabyte of data from ALMA so far,” he added, “and we’re still getting data that we have to go through to achieve the finest resolution possible.” “Just calibrating and cleaning the data takes up to 20,000 iterations, which takes a day or two for each molecule,” says the researcher.
“We can now place these findings on maps in the sky,” Ziurys explained. “Only tiny sections of this massive structure have previously been explored, but understanding mass loss and how these massive stars die requires a comprehensive examination of the entire area. That’s why we wanted to make an all-encompassing picture.”
The team hopes to publish their findings in a series of articles thanks to support from the National Science Foundation.