Artist's impression of star W43A and its surroundings
Artist’s impression of W43A based on the ALMA observation results. ​​​
​NAOJ

ALMA reveals an aged star’s metamorphosis

An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (Alma) has captured the very moment when an old star first starts to alter its environment. The star has ejected high-speed, bipolar gas jets which are now colliding with the surrounding material; the age of the observed jet is estimated to be less than 60 years. These features help scientists understand how the complex shapes of planetary nebulae are formed.
Sun-like stars evolve to puffed-up red giants in the final stage of their lives. Then, the star expels gas to form a remnant called a planetary nebula. There is a wide variety in the shapes of planetary nebulae; some are spherical, but others are bipolar or show complicated structures. Astronomers are interested in the origins of this variety, but the thick dust and gas expelled by an old star obscure the system and make it difficult to investigate the inner-workings of the process.

To tackle this problem, a team of astronomers led by Daniel Tafoya at Chalmers University of Technology, Sweden, pointed Alma at W43A, an old star system about 7000 light years from Earth in the constellation Aquila, the Eagle.

Thanks to Alma’s high resolution, the team obtained a very detailed view of the space around W43A. “The most notable structures are its small bipolar jets,” says Tafoya, the lead author of the research paper published by the Astrophysical Journal Letters. The team found that the velocity of the jets is as high as 175 km per second, which is much higher than previous estimations. Based on this speed and the size of the jets, the team calculated the age of the jets to be less than a human life-span.

“Considering the youth of the jets compared to the overall lifetime of a star, it is safe to say we are witnessing the 'exact moment' that the jets have just started to shove through the surrounding gas,” explains Tafoya. “When the jets carve through the surrounding material in some 60 years, a single person can watch the progress in their life.”

In fact, the Alma image clearly maps the distribution of dusty clouds entrained by the jets, which is telltale evidence that it is impacting on the surroundings.

The team assumes that this entrainment is the key to form a bipolar-shaped planetary nebula. In their scenario, the aged star originally ejects gas spherically and the core of the star loses its envelope. If the star has a companion, gas from the companion pours onto the core of the dying star, and a portion of this new gas forms the jets. Therefore, whether or not the old star has a companion is an important factor to determine the structure of the resulting planetary nebula.

“W43A is one of the peculiar so called ‘water fountain’ objects,” says Hiroshi Imai at Kagoshima University, Japan, a member of the team. “Some old stars show characteristic radio emissions from water molecules. We suppose that spots of these water emissions indicate the interface region between the jets and the surrounding material. We named them ‘water fountains,’ and it could be a sign that the central source is a binarity system launching a new jet.”

“There are only 15 ‘water fountain’ objects identified to date, despite the fact that more than 100 billion stars are included in our Milky Way galaxy,” explains José Francisco Gómez, astronomer at Instituto de Astrofísica de Andalucía, Spain. “This is probably because the lifetime of the jets is quite short, so we are very lucky to see such rare objects.”

Daniel Tafoya is looking forward to new insights on these remarkable stars, which are also similiar to our Sun.

– We believe that these stars have a lot to tell us about what happens when stars like the Sun die. They give us new knowledge about why the sky's most beautiful objects, the planetary nebulae, look the way they do. They are also telling us about how stars like the Sun return material to the galaxy that can be part of the next generation of new stars, he says.

More about the research

These observation results were presented in D. Tafoya et al. “Shaping the envelope of the asymptotic giant branch star W43A with a collimated fast jet” published by the Astrophysical Journal Letters on February 13, 2020.

The research team members are: Daniel Tafoya (Chalmers University of Technology), Hiroshi Imai (Kagoshima University, Japan), José F. Gómez (Instituto de Astrofísica de Andalucía, CSIC), Jun-ichi Nakashima (Sun Yat-sen University, China), Gabor Orosz (University of Tasmania, Australia/Xinjiang Astronomical Observatory, China), and Bosco H. K. Yung (Nicolaus Copernicus Astronomical Center, Poland).



Images

For high-resolution images, see the press release from NAOJ: https://www.nao.ac.jp/en/news/science/2020/20200305-alma.html

A (top) - Artist’s impression of W43A based on the Alma observation results. Diffuse spherical gas was emitted from the star in the past. W43A has just started ejecting bipolar jets which entrain the surrounding material. Bright spots in radio emissions from water molecules are distributed around the interface of the jets and the diffuse gas.
Credit: NAOJ.

B - Alma image of the old star system W43A. The high velocity bipolar jets ejected from the central aged star are seen in blue, low velocity outflow is shown in green, and dusty clouds entrained by the jets are shown in orange.
Credit: ALMA (ESO/NAOJ/NRAO), Tafoya et al.

Contacts:
 
Robert Cumming, communicator, Onsala Space Observatory, Chalmers, 031-772 5500, 070-493 31 14, robert.cumming@chalmers.se

Daniel Tafoya, astronomer, Onsala Space Observatory, Chalmers, 031 772 5519, daniel.tafoya@chalmers.se


Published: Fri 06 Mar 2020.