ALMA Investigates Nature of Water-Fountain Stellar Systems

One of the most poorly understood stellar evolutionary paths is that of binary stellar systems undergoing common-envelope evolution, when the enormous atmosphere of a giant star engulfs the orbit of a smaller companion. Although this interaction leads to a great variety of astrophysical systems, direct empirical studies are difficult because few objects experiencing common-envelope evolution are known. Now, astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have observed 15 stellar systems which are most likely experiencing common-envelope evolution phase or have experienced it in the last 200 years. Dubbed water fountains, these systems are enshrouded in thick dusty envelopes currently being excavated by nascent jets.

“We were extra curious about these stars because they seem to be blowing out quantities of dust and gas into space, some in the form of jets with speeds up to 1.8 million kmh (1.1 million mph),” said Dr. Theo Khouri, an astronomer with the Onsala Space Observatory and the Department of Space, Earth and Environment at the Chalmers University of Technology.

“We thought we might find out clues to how the jets were being created, but instead, we found much more than that.”

Dr. Khouri and colleagues s used ALMA to measure signatures of carbon monoxide (CO) molecules in the light from the stars and compared signals from different isotopes of carbon and oxygen.

Unlike its sister molecule, carbon dioxide (CO₂), carbon monoxide is relatively easy to discover in space and is a favorite tool for astronomers.

“Thanks to ALMA’s exquisite sensitivity, we were able to detect the very faint signals from several different molecules in the gas ejected by these stars,” Dr. Khouri noted.

“When we looked closely at the data, we saw details that we weren’t expecting to see.”

The new ALMA observations confirmed that the stars were all blowing off their outer layers.

“But the proportions of the different oxygen atoms in the molecules indicated that the stars were in another respect not as extreme as they had seemed,” said Dr. Wouter Vlemmings, an astronomer with the Onsala Space Observatory and the Department of Space, Earth and Environment at the Chalmers University of Technology.

“We realized that these stars started their lives with the same mass as the Sun or only a few times more.”

“Now our measurements showed that they have ejected up to 50% of their total mass just in the last few hundred years. Something flamboyant must have happened to them.”

“Actually, we think the known ‘water fountains’ could be almost all the systems of their kind in the whole of our Galaxy,” Dr. Khouri said.

“If that’s true, then these stars are the key to understanding the strangest, most wonderful, and most important process that two stars can experience in their lives together.”

The team’s results were published in the journal Nature Astronomy.

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T. Khouri et al. Observational identification of a sample of likely recent common-envelope events. Nat Astron, published online December 16, 2021; doi: 10.1038/s41550-021-01528-4

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