Free Astronomy Magazine May-June 2020

osing 33 MAY-JUNE 2020 SPACE CHRONICLES A rtist’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 ma- terial. Bright spots in radio emissions from water mole- cules are distributed around the interface of the jets and the diffuse gas. [NAOJ] A LMA 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. [ALMA (ESO/NAOJ/NRAO), Tafoya et al.] published by The Astrophysical Jour- nal 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,” ex- plains 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 en- trained by the jets, which is telltale evidence that it is impacting on the surroundings. The team assumes that this entrain- ment 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 com- panion 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 re- sulting 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 mole- cules. We suppose that spots of these water emissions indicate the inter- face 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 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.” !