Heavy Elements are Probably More Abundant in the Universe than Previously Thought

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The image shows NGC6888, a bubble of ionised gas associated with the star WR136.
The image shows NGC6888, a bubble of ionised gas associated with the star WR136. This nebula, which is generated by stellar feedback processes, has the largest temperature inhomogeneities of all the objects studied in this work. This ultimately leads to underestimating the metallicity of these objects. | © Combination of H? and [O III] images of the galactic Wolf-Rayet Ring Nebula NGC 6888, Daniel López (IAC/INT)

International team of astronomers publish new findings on the evolution of galactic structures

Heavy elements like gold, silver, and uranium are apparently more abundant in the Universe than previously assumed. This is the conclusion of an international research team including scientists from the Centre for Astronomy of Heidelberg University that investigated astrophysical processes in star formation regions and particularly the chemical composition of so-called HII regions. According to the researchers, temperature inhomogeneities in these hydrogen-rich gas clouds could be why the abundance of heavy elements is systematically underestimated. The research results offer important findings on the evolution of galactic structures.

Immediately after the Big Bang, light chemical elements like hydrogen and helium were first formed. After the Universe expanded and cooled, stars developed in which chemical elements like oxygen and nitrogen are produced to this day. In a second phase, collisions of neutron stars or supernova explosions generated heavier elements like gold, silver, and uranium. "Some of these elements remain bound in long-lived stars or their vestiges or are released by explosions and stellar winds. In interstellar space they are available as building materials to subsequent generations of stars," states Dr José Eduardo Méndez Delgado of the Institute for Astronomical Computing, which is part of the Centre for Astronomy of Heidelberg University.

The changes and spatial variations in the abundance of chemical elements reveal a lot about the evolution of galactic structures and are therefore studied intensively. The researchers from Germany, Spain, and Mexico investigated the hydrogen-rich gas clouds known at HII regions. If they are irradiated by massive stars, they emit a great deal of radiation that can be recorded even from the greatest cosmic distances. These gas clouds contain heavy elements as well. Their emission lines allow determining their chemical composition and the abundance of elements. Some of these emission lines result from collisions between heavy atoms and free electrons, whereas others are generated after the recombination of electrons with atoms.

Earlier measurements of element abundances showed that the recombination lines systematically have approximately twice as many heavy elements as their collision-induced counterparts. In 1967, it was assumed that temperature inhomogeneities in the HII regions possibly cause this discrepancy. In this type of scenario, the collision-induced and extremely bright emission lines in the hottest regions of the gas cloud are disproportionately intensified. Here, the temperature of the gas would exceed the average value. In contrast, the recombination lines are far less influenced due to their lower sensitivity to temperature variations. They would therefore provide the correct element abundances. 

The researchers on Dr Méndez Delgado’s team assumed that the as yet unproven temperature variations concentrate on areas of the HII regions that are more strongly radiated in the proximity of stars and are therefore more strongly ionised. A higher and a lower degree of ionisation should be linkable to a parameter that also quantifies the discrepancy of element abundances. "In fact, all the available observations of HII regions demonstrate such a correlation. For the first time, therefore, very strong evidence supports a general solution to this astrophysical problem," states the Heidelberg researcher. 

With their investigations, Dr Méndez Delgado and his colleagues were able to show that it is possible to derive the correct chemical abundances based on collisionally excited very luminous emission lines by analysing ions with lower ionisation like nitrogen. Dr Kathryn Kreckel, research group leader at the Institute for Astronomical Computing, believes that many conclusions on the chemical composition and the development of galactic systems could change. If they are based on collisionally excited emission lines, they underestimate the abundances of heavy elements, according to the scientist, who was also involved in the research. 

The research results were published in "Nature".

J. E. Méndez Delgado, C. Esteban, J. García-Rojas, K. Kreckel & M. Peimbert: Temperature inhomogeneities cause the abundance discrepancy in HII regions, Nature (17 May 2023).