Summary

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 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. We wish to underline that 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. Complementary observations are still needed. One of the goals of SPONGE is precisely to secure such observations and develop our theoretical knowledge in order to interpret these data. Three subprojects have already started. One yearly meeting of the whole collaboration in hybrid mode, seems a good way to keep the information circulating within the collaboration.

Scientific context

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 program

D. Romano - Carbon isotopes in M dwarfs and G/K subgiants, results of the first year
L. Lombardo - C isotopes observed in metal-poor stars in the MINCE and CERES surveys
D. Katz - Nitrogen abundances from Gaia RVS spectra
E. Caffau – Carbon isotopic ratios in stars of extremely low metallicity
F. Lucertini – Sulphur abundances in Local Group dwarf galaxies
M. Limongi – Nucleosynthesis of CNOPS elements and isotopes in low-metallicity supernovae
P. Bonifacio – Archival search of IR spectra suitable for the determination of P abundances
D. Aguado – Oxygen abundances in low metallicity stars
M. Steffen – Formation of the UV NH bands in 3D CO5BOLD models
H.-G. Ludwig – Role of magnetic fields on line formation in 3D CO5BOLD models
A. Mucciarelli – Carbon abundances in the Magellanic Clouds and in the Sgr dSph using MOONS GTO spectra
P. Panuzzo – Masses in metal-poor binary systems
B. Barbuy – The CUBES instrument for the ESO VLT
L. Colzi – Astrobiology perspective from observations of molecules in the ISM
P. Molaro – The role of binaries among extremely metal poor stars
G. Cescutti – The MINCE survey, status and perspectives
A. Gallagher – 1.5D NLTE computations using 3D CO5BOLD models
P. François – Carbon and oxygen in Local Group dwarf galaxies