V Hya is a carbon-rich asymptotic giant branch (AGB) star. It is located approximately 1,300 light-years from Earth in the constellation Hydra. V Hya has been of particular interest to scientists among millions of stars due to its so-far unique behaviors and features.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) and data from the Hubble Space Telescope (HST), scientists- from UCLA and NASA’s Jet Propulsion Laboratory- studying V Hya witnessed the mysterious death in unprecedented detail. They found six slowly-expanding rings and two hourglass-shaped structures caused by the high-speed ejection of matter. These rings expanded outward from V Hya over roughly 2,100 years.
More than 90-percent of stars with a mass equal to or greater than the Sun evolve into AGB stars as the fuel required to power nuclear processes is stripped away. As asymptotic giant branch (AGB) stars age, they eject over half or more of their mass in slow winds over 100,000 years or more.
Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory and the principal researcher on the study, said, “Our study dramatically confirms that the traditional model of how AGB stars die is incomplete, or at worst, incorrect.”
“A close stellar or substellar companion likely plays a significant role in their deaths. Understanding the physics of binary interactions is important across astrophysics and one of its greatest challenges. In the case of V Hya, the combination of a nearby and a hypothetical distant companion star is responsible, at least to some degree, for the presence of its six rings and the high-speed outflows that are causing the star’s miraculous death.”
Scientists caught V Hydra in shedding its atmosphere, i.e., most of its mass. The surprising fact is, the matter, in this case, is being expelled as a series of outflowing rings.
Mark Morris, an astronomer at UCLA and a co-author of the research, added, “This is the first and only time that anybody has seen that the gas being ejected from an AGB star can be flowing out in the form of a series of expanding ‘smoke rings.’”
Having matter alongside, these rings drives the growth of a high-density flared and warped disk-like structure around the star. Scientists dubbed this structure the DUDE, or Disk Undergoing Dynamical Expansion.
Morris said, “The end state of stellar evolution—when stars undergo the transition from being red giants to ending up as white dwarf stellar remnants—is a complex process that is not well understood. The discovery that this process can involve the ejections of rings of gas, simultaneous with the production of high-speed, intermittent jets of material, brings a new and fascinating wrinkle to our exploration of how stars die.”
Sahai added, “V Hya is in the brief but critical transition phase that does not last very long, and it is difficult to find stars in this phase, or rather ‘catch them in the act. We got lucky and were able to image all of the different mass-loss phenomena in V Hya to understand better how dying stars lose mass at the end of their lives.”
Along with rings and warped disks, scientists noticed that V Hya’s final act features two hourglass-shaped structures—and an additional jet-like structure—expanding at high speeds of more than half a million miles per hour (240 km/s).
Sahai said, “We first observed the presence of very fast outflows in 1981. Then, in 2022, we found a jet-like flow consisting of compact plasma blobs ejected at high speeds from V Hya. And now, our discovery of wide-angle outflows in V Hya connects the dots, revealing how all these structures can be created during the evolutionary phase that this extra-luminous red giant star is now in.”
An astronomer and NSF program officer for NRAO/ALMA, Joe Pesce, said, “The processes taking place at the end stages of low mass stars, and during the AGB phase, in particular, have long fascinated astronomers and have been challenging to understand. The capabilities and resolution of ALMA are finally allowing us to witness these events with the extraordinary detail necessary to provide some answers and enhance our understanding of an event that happens to most of the stars in the Universe.”
Sahai said, “The incorporation of infrared, optical, and ultraviolet data into the study created a complete multi-wavelength picture of what might be one of the greatest shows in the Milky Way, at least for astronomers. Each time we observe V Hya with new observational capabilities, it becomes more and more like a circus, characterized by an even bigger variety of impressive feats. 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.”
Journal Reference:
- 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: 10.48550/arXiv.2202.09335