Using the Atacama Large Millimeter/submillimeter Wave Array (ALMA), astronomers have observed the carbon-rich star V Hydrae that is apparently transitioning from an asymptotic giant branch (AGB) star to a bipolar planetary nebula, and discovered a remarkable set of six slowly-expanding rings within a flared, warped disk around the star.
AGB stars age over 10,000-100,000 years, they eject over half or more of their mass in slow winds, and then, in a short 100-1,000 year period, are transformed into planetary nebulae with a dazzling variety of morphologies and widespread presence of point-symmetry.
Recent studies with the NASA/ESA Hubble Space Telescope support the idea that directed, high-speed outflows initiated during the very late AGB phase play a crucial role in the transformation to planetary nebulae.
However, evidence for such jets and disks in AGB stars is rare; this phase is presumably so short that few nearby stars are likely to be caught in the act.
V Hydrae, a carbon-rich AGB star located some 1,300 light-years away in the constellation of Hydra, is the nearest and best example of this phase, and a key object for understanding the formation of bipolar and multipolar planetary nebulae.
Also known as V Hya, the star is believed to have an unseen secondary companion surrounded by an accretion disk and undergo partial eclipses by the disk every 16.9 years.
“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 Dr. Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory.
“It is very likely that a close stellar or substellar companion plays a significant role in their deaths, and understanding the physics of binary interactions is both 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.”
“V Hya has been caught in the process of shedding its atmosphere — ultimately most of its mass — which is something that most late-stage red giant stars do,” said Professor Mark Morris, an astronomer at the University of California, Los Angeles.
“Much to our surprise, we have found that the matter, in this case, is being expelled as a series of outflowing rings.”
“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.”
The six rings have expanded outward from V Hya over the course of roughly 2,100 years, adding matter to and driving the growth of a high-density flared and warped disk-like structure around the star.
The team has dubbed this structure the disk undergoing dynamical expansion (DUDE).
“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,” Professor Morris said.
“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.”
“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,” Dr. Sahai said.
“We got lucky and were able to image all of the different mass-loss phenomena in V Hya to better understand how dying stars lose mass at the end of their lives.”
In addition to a full set of expanding rings and a warped disk, V Hya’s final act features two hourglass-shaped structures — and an additional jet-like structure — that are expanding at high speeds of 240 km/s.
“We first observed the presence of very fast outflows in 1981,” Dr. Sahai said.
“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.”
The team’s paper will be published in the Astrophysical Journal.
R. Sahai et al. 2022. The Rapidly Evolving AGB Star, V Hya: ALMA finds a Multi-Ring Circus with High-Velocity Outflows. ApJ, in press; arXiv: 2202.09335
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