Image of W43A, an aging star in full metamorphosis. You can see the bipolar jet (in blue) emerging from the central star, a low-speed flow of material (green) and the dust clouds carried by the jets (orange). Source: ALMA (ESO / NAOJ / NRAO), Tafoya et al.

An international group of astronomers has observed an old star, the red giant W43A, in the process of metamorphosis into a planetary nebula, a key moment in the life of low-mass stars.

The observatory SOUL (Atacama Large Millimeter / submillimeter Array) has shown that W43A shows a bipolar jet of material at high speed – between 175 and 130 kilometers per second – which is colliding with the surrounding material and carving a bipolar shell into it. SOUL it has thus captured the moment when spherical symmetry is lost and complex morphology develops.

“The process of formation of a bipolar or multipolar nebula is, today, a burning problem in studies of stellar evolution,” he says. José Francisco Gómez, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participates in the work. We are increasingly convinced that the cavities that the jets generate in the envelope material expelled by the star in previous stages are more dense and slower than that of the jets, at the origin of these forms. ”

Both the jets and the shell are estimated to be about sixty years old, so they were created simultaneously and in a short space of time. “Considering the youth of the jets in relation to the life span of a star, we can say that we are witnessing the ‘exact moment’ in which the jets have begun to sculpt the surrounding gas,” he explains. Daniel Tafoya, researcher of the Chalmers University (Sweden) who heads the study.

Furthermore, and unlike what we usually see in astrophysical phenomena, we are facing a process that a human being can follow throughout his life ”. In fact, the image of SOUL Clearly trace the distribution of gas and dust clouds carried by the jets. The team assumes that this drag is key to the formation of a bipolar planetary nebula, and proposes the following scenario: the aged star expels, in a first phase, the gas from its outer layers spherically, so that the nucleus is left bare.

If the star has a companion, the companion’s gas is poured onto the core of the dying star, and a percentage of this new material forms the jets, which will alter the morphology of the material around the star.

Thus, whether or not the star has a companion is a key factor in determining the structure of the resulting planetary nebula. It is a previously proposed scenario to explain the rupture of spherical symmetry in planetary planets, something that must occur in the phases immediately prior to their formation. However, this beginning is short, and is obscured by the gas and dust expelled as the material is released from the star’s envelope, so that its observation is very complicated.

“W43A is a peculiar object classified as a ‘water source’, an aging star that shows the characteristic radio emission of water vapor molecules. This radio signal reveals to us the region in which the jets interact with the surrounding material, ”he points out. Hiroshi Imai, researcher at the University of Kagoshima (Japan) who participates in the work. The jet shows lumps, or denser regions, and that points to the existence of a companion star with an eccentric orbit: when the stars get closer, the companion loses more material and the jet changes intensity.

“Among the one hundred billion stars in the Milky WayTo date, we have only identified fifteen water sources. This is probably because the life time of the jets is quite short, so we are very lucky to see such rare objects, “he concludes. José Francisco Gómez (IAA-CSIC).

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