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.

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 around 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 push through the surrounding gas,” explains Tafoya. “The jets carve through the surrounding material in as little as 60 years. A person could watch their progress throughout their lifetime.”

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.

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.

The team suggests how this entrainment could be the key to formimg 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. “These are old stars which show characteristic radio emission from water molecules. Our ALMA observations lead us to think that the water heated to generate the radio emission is located the interface region between the jets and the surrounding material. Perhaps all these ‘water fountain’ sources consist of a central binary system which has just launched a new, double jet, just like W43A.”

The team are already working on new ALMA observations of other, similar stars. They are hoping to gain new insight into how planetary nebulae form, and what lies in the future for stars like the Sun.

“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 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.”

The paper and research team
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), José F. Gómez (Instituto de Astrofísica de Andalucía, CSIC), Jun-ichi Nakashima (Sun Yat-sen University), Gabor Orosz (University of Tasmania/Xinjiang Astronomical Observatory), and Bosco H. K. Yung (Nicolaus Copernicus Astronomical Center)

This research was supported by:
MEXT KAKENHI (No. 16H02167), the Invitation Program for Foreign Researchers of the Japan Society for Promotion of Science (JSPS grant S14128), MINECO (Spain) Grant AYA2017-84390-C2-R (co-funded by FEDER), State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofisica de Andalucía (SEV-2017-0709), Australian Research Council Discovery project DP180101061 of the Australian government, CAS LCWR 2018-XBQNXZ-B-021, and National Key R&D Program 2018YFA0404602 of China.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.