Stellar and exoplanetary physics with the U-band of the ELT-ANDES spectrograph

ArmazoNes high Dispersion Echelle Spectrograph (ANDES) for ELT will provide wavelength coverage of 0.35-2.4 µm. The scientifically important U-band (0.35-0.41 µm) was approved recently since multiple science cases envisioned for ANDES at ELT will either benefit from or completely relied on having it. Covering the Ca II H&K region was the main motivation for many stellar and  exoplanet science cases, but some science cases require having the whole U-band. Especially achieving the top-priority science case ‘Exoplanet Atmospheres in Transmission Spectra’ critically depends on being able to characterize the impact of stellar activity during the observations. That can only be achieved by the simultaneous coverage given by the U-band of the Ca II H&K region, which has been proven to be the most sensitive activity indicator in cool stars. Lesson learned from JWST observations is that strong stellar activity contamination hampers the detection of any possible secondary atmosphere. In relation to cool star science, ANDES U-band will  expand our horizon in many ways. It enables exploring chemical compositions of the most primitive stars known - about 40% of the key observational signatures are within the spectral range of the U-band. Further, it covers the most sensitive tracers of stellar activity, the Ca II H&K lines. Several studies have shown that the sensitivity of Ca II H&K to even small activity changes cannot be matched by other chromospherically sensitive lines such as Hα, Ca IRT, or the Na D lines. Furthermore, the nucleosynthesis of the heaviest elements have key absorption lines in late-type stars throughout  the U, B, and V bands.