On the contrary, for the r-process the situation is more complex; we are sure that merging neutron stars play a role, but it is unclear whether they are the only actors or whether other types of star explosions can play a significant role.
The measurement of the abundances of these elements in the stars of our Galaxy, and in particular in those located in the halo, notoriously older, therefore plays an essential role as it can settle the different roles in this complex situation. It also allows us to study the chemical enrichment of the Galaxy from a different point of view than that of the other chemical elements and therefore, ultimately, to better understand its evolution.
The aim of the MINCE (Measuring at Intermediate metallicity Neutron-Capture Elements) project is exactly to measure the abundances of neutron capture elements in the stars of the galactic halo characterized by a metal content (for astronomers "metals" are all the elements after helium in the periodic table) between one tenth and one hundredth of the solar metallicity (more precisely in the range, -2.5<[Fe/H]< -1.5).
The reason for the choice is not accidental: currently, the efforts of astronomers are mainly concentrated in studies of stars in the galactic halo with even lower metallicity, less than one thousandth of the solar metallicity. This effort towards increasingly metal-poor has led over the years to an observational gap for intermediate metallicity halo stars due to the limited data collected, and this is where MINCE comes into play. The idea is exactly to fill this gap, using telescopes (not necessary 8 meter class) of various observatories in the world and focusing on sufficiently bright stars (magnitude G < 11), always however with the aim of obtaining observations of excellent quality (for high signal/noise ratio), an indispensable prerequisite for a correct measurement of the abundances of neutron capture elements.
The MINCE survey started in early 2019 with the aim of obtaining spectra of over 1000 objects. Over 400 stars were actually observed in the first two years following the submission of observational proposals (later accepted) at various observatories scattered in the northern and southern hemisphere. The observatories/instruments involved are HARPS-N at the Galileo National Telescope (TNG), FIES at the Nordic Optical Telescope (NOT), Sophie at the Observatoire de Haute Provence, ESPaDOnS at the Canada-France-Hawaii telescope (CFHT), UVES at the Very Large Telescope (ESO at Paranal), FEROS at 2.2 meters (ESO at La Silla) and VUES at the Vilnius Observatory.
In the recent article published in the journal Astronomy & Astrophysics, Vol. 668, id.A168, we present the description of this survey and the detailed study of the first sample of MINCE, composed of 59 spectra of 46 stars, for which we proceeded to measure their respective radial velocities, Galactic orbits and of course, of the abundances of the elements up to zinc.
The final product of MINCE will be an open database where all the measurements of the abundances of the chemical elements will be collected.
DOI: https://doi.org/10.1051/0004-6361/202244515
ArXiv: https://arxiv.org/abs/2212.00040
Examples of spectra obtained for 4 stars centered around the Zn I line, 481.0 nm. The zinc content is estimated by measuring the area subtended by the spectral line under examination (Zn I, in our case), and thanks to the quality of the spectra obtained, it is already possible to visually highlight the different abundances in the 4 objects. The shifting is an artifact to allow them to be better distinguished.
The zinc, as a function of the metallicity of the stars. This trend should be then compared to the theoretical models (represented by lines in the figure) to try to replicate, and therefore understand, the evolution of the chemical enrichment of our Galaxy and beyond. Stars belonging to Enceladus and Sequoia (in the figure shown respectively in red and green) are compared to the Galactic stars in blue.
