Science Gazette

The mysterious death of a carbon star unfolds like a six-ring circus

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V Hydrae (V Hya) scientists have watched the star’s enigmatic death throes in unparalleled detail. The researchers observed six slowly-expanding rings and two hourglass-shaped formations generated by the high-speed ejection of matter into space using the Atacama Large Millimeter/submillimeter Array (ALMA) and data from the Hubble Space Telescope (HST). The study’s findings have been published in The Astrophysical Journal.

V Hya is a carbon-rich asymptotic giant branch (AGB) star found in the constellation Hydra around 1,300 light-years from Earth. As the fuel necessary to power nuclear processes is taken away, more than 90% of stars with masses equal to or larger than the Sun develop into AGB stars. Among these millions of stars, V Hya has piqued scientists’ curiosity owing to its so far unusual behaviors and traits, such as extreme-scale plasma explosions that occur every 8.5 years and the existence of an almost undetectable partner star that contributes to V Hya’s explosive nature.

“Our study dramatically confirms 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,” said Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory and the study’s principal investigator. “A near stellar or substellar partner is quite likely to play a large role in their deaths, and understanding the physics of binary interactions is both crucial in astronomy and one of its biggest problems. In the case of V Hya, the existence of its six rings and the high-speed outflows that are causing the star’s spectacular death are caused, at least in part, by the interaction of a close and a hypothetical distant partner star.”

Mark Morris, a UCLA astronomer and co-author on the study, stated, “V Hydra has been caught in the process of losing its atmosphere, and hence most of its mass, as most late-stage red giant stars do. Surprisingly, we discovered that the substance in this situation is being evacuated as a series of outflowing circles. This is the first and only time anybody has seen gas expelled from an AGB star streaming out in the shape of a series of expanding’smoke rings.'”

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Over the span of around 2,100 years, the six rings have extended outward from V Hya, contributing matter to and driving the creation of a high-density flared and twisted disk-like structure encircling the star. This structure has been termed the DUDE, or Disk Undergoing Dynamical Expansion, by the researchers.

“The final stage of stellar evolution — when stars change from being red giants to become white dwarf stellar remnants — is a complicated and poorly understood process,” Morris added. “The revelation that this process may entail the ejection of rings of gas at the same time as the formation of high-speed, intermittent jets of material adds a new and exciting twist to our understanding of how stars die.”

Sahai said, “V Hya is in a short but crucial transition phase, which does not endure long, and it is difficult to detect stars in this phase, or rather ‘catch them in the act.’ We were fortunate to be able to photograph all of the distinct mass-loss processes in V Hya, allowing us to better grasp how dying stars lose mass at the conclusion of their lifetimes.”

In addition to a complete set of expanding rings and a warped disk, V Hya’s last act includes two hourglass-shaped structures – as well as an extra jet-like structure – that grow at rates of over 500,000 miles per hour (240 km/s). Large hourglass structures have previously been observed in planetary nebulae, including MyCn 18 (also known as the Engraved Hourglass Nebula), a young emission nebula located approximately 8,000 light-years from Earth in the southern constellation of Musca, and the more well-known Southern Crab Nebula, an emission nebula located approximately 7,000 light-years from Earth in the southern constellation Centaurus.

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Sahai said, “In 1981, we discovered the existence of very rapid outflows. In 2022, we discovered a jet-like flow made up of compact plasma blobs expelled at high speeds from V Hya. And now, our finding of wide-angle outflows in V Hya ties everything together, demonstrating how all of these structures may be formed during the evolutionary period that this extra-luminous red giant star is now in.”

Because of the star’s distance and the density of the dust around it, investigating V Hya requires a one-of-a-kind device capable of clearly seeing stuff that is both extremely far away and difficult or impossible to detect with normal optical telescopes. The scientists used ALMA’s Band 6 (1.23mm) and Band 7 (.85mm) receivers, which showed the star’s many rings and outflows in stunning detail.

“The mechanisms occurring at the final stages of low mass stars, particularly during the AGB phase, have long captivated and perplexed astronomers,” stated Joe Pesce, an astronomer and NSF program officer for NRAO/ALMA. “The capabilities and resolution of ALMA are now enabling us to see these occurrences with the amazing precision required to offer some answers and expand our comprehension of an event that occurs to the vast majority of stars in the Universe.”

The combination of infrared, optical, and ultraviolet data into the research generated a full multi-wavelength image of what may be one of the best displays in the Milky Way, at least for astronomers, according to Sahai. “Each time we see V Hya with fresh eyes, it becomes more and more like a circus, with an ever-increasing range of astonishing performances. V Hydrae has amazed us with its many rings and actions, and since our own Sun may one day suffer a same fate, we are paying close watch.”

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