Author/s: S. Madonna, M. Bautista, H. L. Dinerstein, N. C. Sterling, J. García-Rojas, K. F. Kaplan, M. del Mar Rubio-Díez, N. Castro-Rodríguez, F. Garzón
Reference: 2018 ApJ 861 L8 | Link
At the end of their lives, stars of moderate mass eject their outermost layers forming planetary nebulae. Through this process, they inject into the interstellar medium the chemical elements that have been synthesized inside them for billions of years. The elements heavier than iron cannot be produced in the nuclear fusion reactions that occur inside stars because that process would require more energy than they could generate. These elements are formed by a process known as neutron capture, which occurs in the final stages of a star’s life. It occurs either in violent events related to the death of very high-mass stars such as supernova explosions or neutron stars collisions (one of which was recently detected by gravitational wave observatories), which generate a huge number of free neutrons or in the final phase of the life of low- to intermediate-mass stars (between 1 and 8 masses of the Sun), where the neutron flux is much lower. In the first case, the process is called the "r-process" (r for rapid) and in the second case, the "s-process" (s for slow). In this work we have detected, for the first time, new spectral emission features of tellurium in the near-infrared spectral range of two planetary nebulae (and of bromine in one of them) thanks to the data obtained with the EMIR spectrograph, on the Gran Telescopio Canarias, and IGRINS, on the Harlan J. Smith Telescope, at the McDonald Observatory in Texas, USA. These are the clearest detections of ions belonging to these two heavy elements in one of the places where they form. The calculated abundances of Tellurium in the planetary nebulae NGC 7027 and IC 418 indicate that this element is much more abundant than expected in the solar vicinity, where the abundance pattern is distributed as expected if the r-process were responsible for the origin of these heavy elements; some of the tellurium in these planetary nebulae must have thus originated through the s-process. Investigating these elements in all their places of origin (planetary nebulae, neutron star mergers, and supernovae from massive stars) can help us to better understand the contribution of the s-process and the r-process to the formation of heavy elements, and to refine theoretical models of the chemical evolution of the Universe.