Galactic Science Cases

 WST can play a transformational role in the area of star and planet formation. Specifically, the IFS will allow the study with unprecedented detail of the star formation processes in massive and dense environments; the low-resolution MOS is ideal to investigate the properties of dispersed populations, while the high-resolution MOS would be essential to measure abundances and infer stellar activity in large samples of planet-hosting stars, thus unveiling the role of host-star chemical composition and magnetic fields in shaping planetary systems.

Sky map with circles comparing the regions up to where a certain stellar type can be observed in high-resolution mode with WST (red and yellow solid lines) and 4MOST (red and yellow dashed lines). The region where Gaia DR3 spectroscopy is available is shown in cyan. Image credit: Laura Magrini (INAF).

We are living in a golden era for Galactic, stellar, and exoplanet astrophysics, thanks to the ESA Gaia mission, along with the numerous stellar surveys undertaken in the last decade. WST will allow us to further push the boundaries of our understanding of the Milky Way (MW) and its component stars and populations. Precise abundances of several chemical elements, covering the different nucleosynthesis channels, for a few million stars will allow a more comprehensive understanding of the origin of the elements which is essential for a variety of other astrophysical issues. WST will also greatly enhance our ability to measure abundance ratios that are sensitive to age (the so-called “chemical clocks”) for statistically significant samples of stars and to produce extensive age maps of the Galactic disk. The combination of chemistry, kinematics and ages will maximize our ability to accurately identify related groups of stars and reconstruct the star-formation history of the Milky Way disk in extremely fine detail. Low and high resolution MOS observations, as well as IFS spectra in the densest parts, will allow to disentangle co-spatial stellar populations in the MW Bulge and promise to be transformative; they will indeed open a window into this critical component of our MW that is seen in the formation stages in other galaxies at high redshift with James Webb Space Telescope (JWST). The complementarity will be extremely powerful. WST will give a substantial contribution to identify and characterize the assembly and accretion history of the MW, in particular by the chemodynamical identifications of past accretions, through large samples that extend to fainter magnitudes to probe further out in the halo (> 10 kpc). Reconstruction of the full assembly/accretion history of the MW, discriminating between stars formed in situ and those formed in progenitor galaxies of different masses and star formation efficiencies, will in turn deepen our understanding of the Milky Way’s role in the broader cosmological context and how our Galaxy can help us to understand the properties of other galaxies across cosmic time.

The line-detectability for a few cherry-picked elements (listed on the y-axis). High-spectral resolution is essential to dramatically improve the number of suitable spectral lines: more than a factor of 5 comparing R=40K (black points) with R=5K (yellow points). Adapted from Kordopatis et al. (2023).