- R. Sahai, P-S. Huang, S. Scibelli, M. R. Morris, K. Hinkle, C-F. Lee. The Rapidly Evolving AGB Star, V Hya: ALMA finds a Multi-Ring Circus with High-Velocity Outflows. arXiv.org (Accepted for ApJ), 2022; DOI: https://doi.org/10.48550/arXiv.2202.09335
Researchers from UCLA and NASA’s Jet Propulsion Laboratory discovered that the carbon-rich star has expelled six slowly expanding molecular rings and an hourglass-shaped structure ejecting matter out into space at high speeds, signaling that the star is undergoing rapid evolution as it ends its life in a blaze of glory before shutting down its energy production.
“This is the first and only time that a series of expanding rings has been seen around a star that is in its death throes — a series of expanding ‘smoke rings’ that we have calculated are being blown every few hundred years,” said Mark Morris, a UCLA professor of physics and astronomy and a co-author of the study.
The results of the study, which was conducted using the Atacama Large Millimeter/submillimeter Array, known as ALMA, and data from the Hubble Space Telescope, are published March 28 in the Astrophysical Journal.
More than 90% of stars with a mass equal to or greater than the sun’s mass evolve into what scientists call asymptotic giant branch stars, or AGB stars, of which V Hya is an example. The star is located approximately 1,300 light-years from Earth in the constellation Hydra.
Among these millions of stars, V Hya has been of particular interest to astronomers due to its unique behaviors and features, including extremely large eruptions of plasma that occur approximately every eight years and the presence of a nearly invisible companion star that contributes to V Hya’s explosive behavior.
“We have caught this dying star in the process of shedding its atmosphere — ultimately most of its mass — which is something that most late-stage red giant stars do,” Morris said. “But much to our surprise, we have found that the matter in this case is being expelled as a series of rings.”
Morris said the team also observed high-speed blasts of gas, perpendicular to those rings, that were expelled into two opposite directions. He added that the mechanism that produces the rings is unknown and will require further investigation.
“We suspect that it might be related to the presence of orbiting companion stars, but it is difficult to explain that given the few-hundred-year interval between ring ejections,” Morris said. “This star is providing a new and fascinating wrinkle to our understanding of how stars end their lives.”
Raghvendra Sahai, an astronomer at JPL and the study’s lead author, said the research indicates that previous assumptions about star deaths may be wrong.
“Our study dramatically reveals that the traditional model of how AGB stars die — through the mass ejection of fuel via a slow, relatively steady, spherical wind over 100,000 years or more — is at best, incomplete, or at worst, incorrect,” he said.
The six rings that have expanded outward from V Hya over the course of roughly 2,100 years have formed a warped, disk-like structure, creating a dust-rich region around the star, the researchers report. The team dubbed the structure the DUDE, for Disk Undergoing Dynamical Expansion.
“V Hya is in the brief but critical transition phase that dying stars go through at the end of their lives,” Sahai said. “It’s the phase when they lose most of their mass. It’s likely that this phase does not last very long, so it is difficult to catch them in the act. We got lucky with V Hya, and were able to image all of the different activities going on in and around this star to better understand how dying stars lose mass at the end of their lives.”
V Hya’s final act also has produced an hourglass-shaped structure centered on the star and oriented perpendicularly to the disk. The two lobes of the hourglass have been shaped by a directed, fast wind that is blowing in two opposite directions at speeds up to 500,000 miles per hour.
Due to the large quantities of dust surrounding the star, studying V Hya required a unique instrument with the power to clearly see cold matter that is impossible to detect with optical telescopes. ALMA’s receivers are exceptionally sensitive to very short radio wavelengths of approximately 1 millimeter, which revealed the star’s multiple rings and outflows of molecular gas in stark clarity.
The researchers used additional infrared, optical and ultraviolet data to produce a remarkable picture of a spectacular show in our galaxy, much of which was unexpected, Morris said.
“Each time we observe V Hya, it becomes more and more like a circus, with each new evolutionary stage characterized by an even bigger variety of impressive feats,” Sahai said. “V Hydrae has impressed us with its multiple rings and acts, and because our own sun may one day experience a similar fate, it has us at rapt attention.”
The research was funded in part by the National Science Foundation and NASA.
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Scientists observe mysterious death of a star emitting six rings