Detecting weak beryllium lines with CUBES

Abstract

Beryllium is a light element with one single stable isotope, 9 Be, which is a pure product of cosmic-ray spallation in the interstellar medium. Beryllium abundances in latetype stars can be used in studies about evolutionary mixing, Galactic chemical evolution, planet engulfment, and the formation of globular clusters. Some of these uses of Be abundances figure among the science cases of the Cassegrain U-Band Efficient Spectrograph (CUBES), a new near-UV low-and medium-resolution spectrograph under development for the Very Large Telescope. Here, we report on a study about beryllium abundances in extremely metal-poor stars in the context of the phase A of CUBES. Our motivation is to understand the limits for the detection of weak lines in extremely metal-poor stars of low Be abundances. We analyze simulated CUBES observations, performed in medium-resolution mode, based on synthetic spectra for four mock stars with [Fe/H] ≤ −3.0. We find that detecting the Be lines is possible in certain cases, but is very challenging and requires high signal-to-noise ratio. Depending on the atmospheric parameters of the target stars, and if signal-to-noise per pixel of about 400 can be achieved, it should be possible to detect Be abundances between log(Be/H) −13.1 and −13.6, with a typical uncertainty of ± 0.15 dex. Using CUBES, the required data for such studies can be obtained for stars that are fainter by two magnitudes with respect to what is possible with current instrumentation.

Design of the VLT-CUBES image slicers:

“…The percentage of vignetting per slitlet was evaluated in Zemax using Geometric Image Analysis and considering the contribution of the telescope, foreoptics and image slicer. The smallest vignetting is expected for the central configurations (1,6) in both resolution cases and the largest values for the external configurations (3,4) and particularly for HR mode in which the slitlets are thinner. This is what is observed in the results.…”

“…Be) and ironpeak, heavy elements [2] and CNO abundances (e.g. [3][4][5]). The proposed solar system and planetary science cases focus on the search for water in the asteroid belt and measurements of the N 2 /CO ratio in comets.…”