American scientists have discovered organic molecules in planetary nebulae located on the farthest fringes of the Milky Way. Previously, it was thought that these parts of the Galaxy were too cold to support complex chemical processes. The results of the study are presented at the 238th meeting of the American Astronomical Society.
Using the ALMA radio telescope complex in Chile’s Atacama desert, researchers at the University of Arizona, led by Lucy Ziurys, observed radio emissions of hydrogen cyanide (HCN), formyl ion (HCO+), and carbon monoxide (CO) in five planetary nebulae formed at the site of dying stars.
It is estimated that planetary nebulae supply up to 90 percent of the interstellar medium, and only 10 percent produce supernovae. Previously, it was assumed that the matter of planetary nebulae is represented by atoms, but now it turned out that planetary nebulae serve as sources of molecules for the formation of new stars and planets. When a star loses most of its mass at the end of its life and becomes a white dwarf, it usually emits strong ultraviolet radiation. For a long time, it was thought that this radiation destroys any molecules in the interstellar medium, turning them into atoms. The results of observations showed that this is not the case.
Previously, planetary nebulae were observed only in visible light and were considered spherical, since stars end their lives in the form of a sphere. But high-resolution radio frequency images revealed their complex geometry. Some regions of dense material ejected by a dying star have bipolar or even four-pole structures. The authors believe that the change in the shape of nebulae can be caused by the processes of nucleosynthesis — the formation of new elements inside the star.
“This tells us that in a dying star, spherical to its last phase, after passing through the planetary nebula stage, a very interesting dynamic occurs that changes this spherical shape,” Professor Ziouris said in a university press release.
Scientists also reported the discovery of molecular clouds of methanol in the distant center of the Galaxy. Using the University of Arizona’s 12-meter ARO telescope, researchers studied 20 molecular clouds in the Cygnus arms and found methanol in all twenty. The study site is located at a distance of 13 to 23.5 kiloparsecs from the center of the Galaxy, which is much further than the outer boundary of the habitable zone — the region with the most favorable conditions for the origin and safe evolution of life, located within 10 kiloparsecs from the center of the Milky Way.
According to scientists, their discovery calls into question the existing idea of the galactic habitable zone, according to which for the existence of conditions in which life can develop, a planetary system can not be too close to the galactic center with its extremely high density of stars and intense radiation, as well as too far from it, since the edge of the Galaxy lacks critical elements for life, such as oxygen, carbon, and nitrogen. The results of the study show that processes that lead to the formation of organic substances also occur on the outskirts of the Galaxy.
Currently, the authors continue to search for other organic compounds in the interstellar medium, such as methyl cyanide and other molecules with a ring structure and functional groups that serve as key building blocks for biomolecules. According to the researchers, organic molecules present in the forming planetary systems can condense on the surface of asteroids, which then deliver them to the nascent planets, where these molecules can potentially give rise to the development of life.