Abstract

Our knowledge on the evolution of the elements C, N, O, S and P and of their isotopes is still limited. These elements are the key to form the “bricks” necessary to build lifeforms. The project “Sulphur, Phosphorus, Oxygen, Nitrogen, and (C)arbon Galactic Evolution” (SPONGE) was started in November 2024 at a kick-off meeting held at the Centre de Conferences Jules Janssen and had its first annual meeting in November 2025 at the Centre de Conferences Jules Janssen. The colaboration aims at improving the observational knowledge on these elements and their evolution, as well as improving our theoretical models for the nucleosynthesis and for the chemical evolution of these elements.

The large spectroscopic surveys like Gaia, WEAVE, 4MOST and MOONS GTO will not suffice to provide us with a complete picture of the abundances of these elements. The PLATO mission, planned for launch early in 2027, will provide us with a set of precise asteroseismic ages. Complementary observations to all these surveys are needed. One of the goals of SPONGE is precisely to secure such observations and develop our theoretical knowledge in order to interpret these data.

Scientific justification

Understanding how galaxies form and evolve remains one of the most formidable tasks of modern astrophysics. To address this issue, galactic archaeologists use the abundances and abundance ratios of chemical elements measured in gas and stars. In fact, different elements are restored to the ISM on different time scales by stars of different initial masses, which makes their abundance ratios excellent probes of the assembly histories and evolutive time scales of the host galaxies. The CNOPS elements are especially suitable for this kind of studies: their abundances are determined with different techniques in stellar atmospheres as well as in dense and diffuse gas, both in the local universe and in high-redshift systems, using as diagnostics absorption/emission lines at different wavelengths, ranging from the ultraviolet to the radio.

In the last decade, Gaia has measured stellar positions and motions with unprecedented accuracy, and its synergy with high-resolution spectrographs on Earth has provided new insights to theoretical models of the evolution of different Galactic components (including the remnants of former Milky Way satellites) and nearby galaxies. The CNOPS elements and their rare isotopes are of particular interest, in that they also enter the formation of the complex organic molecules (COMs) that are thought to be the precursors of life. COMs have been observed extensively by radio astronomers, however, measurements of CNOPS isotopic ratios in molecular clouds of the outer Galactic disc (i.e., objects probing the low-metallicity regime) are still rare. Determinations of the abundances of CNOPS elements and their isotopes in stars allow, in principle, to probe the full metallicity range (from very metal-poor to super-solar regimes), but are affected by several problems. In particular, C and N abundances in bright stars above the RGB bump (i.e., stars easily accessible also in the satellites of the Milky Way) are affected by mixing processes that alter the abundances inherited by the stars at birth: cooler/fainter stars must be observed to allow a useful comparison with the predictions of chemical evolution models. We note that N is one of the most difficult elements to measure in stars. It can be accessed through near-UV and IR CN bands, however, the analysis is complicated (the band strength depends on the abundances of both C and N) and hampered by granulation effects and deviations from local thermodynamic equilibrium (LTE) that are difficult to deal with. The analysis of the NH UV band at 336 nm is simpler, but high-quality spectra in this region can be obtained only with a few telescope/spectrograph combinations at present, which greatly reduces data availability. While this band is outside the range of any of the existing or upcoming large spectroscopic surveys (e.g., APOGEE, GALAH, WEAVE, 4MOST, MOONS GTO), CUBES, the forthcoming ESO VLT spectrograph, will cover the UV region between 300 and 400 nm with high efficiency and a resolution (R 20,000) well suited to NH band analyses. Moving to the rare isotopes, we emphasise that the determination of the 12C/13C and 16O/18O abundance ratios in M dwarf stars constitutes one of the main science requirements behind the development of a K-spectrograph for ANDES, the future high-resolution (R 100,000) spectrograph for the ELT. ANDES will also allow access to S and P lines in IR bands for stars beyond the boundaries of the solar neighbourhood that are difficult to observe with current instrumentation.

Preliminary scientific programme

L. Lombardo - Nitrogen abundances observed in metal-poor stars in the MINCE surveys
L. Sgatti - Artificial intelligence tools developed for abundance determination
A. Al-azzawi – CNO abundances and C isotopic ratios from GIARPS spectra of sub-giant stars
E. Caffau – Extremely low carbon abundance in an actinide boost stars
F. Lucertini – Phosphorous abundances in the Magellanic Clouds
M. Limongi – Theoretical chemical yields
P. Bonifacio – CNO abundances in apparently young metal-poor stars in the Galactic Halo
Zou Yipeng – Radio observations of sulfur monoxide to determine sulphur isotopic ratios
D. Aguado – TBD
M. Steffen – 3D NLTE abundance corrections for oxygen in metal-poor stars
C.J. Hansen – CNO abundances and isotopic ratios in the CERES stars
H.-G. Ludwig – Granulation signatures in 3D hydrodynamical simulations
A. Mucciarelli – ATLAS opacities for the computation of 3D CO5BOLD models
B. Barbuy – P abundanaces in Globular Clusters
P. Molaro – The role of binaries among extremely metal poor stars
A. Gallagher – 1.5D NLTE computations for C I and O I IR atomic lines

The talks shall be complemented by three to four discussions on specific topics, like was done in November 2025