Observing comets with the upcoming CUBES near-UV spectrograph

High-resolution spectroscopy of the near-UV regime provides access to a tremendous diversity of iron-peak and heavy elements in stellar spectra, is critical in extragalactic observations, such as studies of the circumgalacticmedium of distant galaxies, and is an extremely interesting regime for the study of small bodies of the solar system.  The Cassegrain U-Band Efficient Spectrograph (CUBES), to be installed on the Very Large Telescope in 2028, has been designed to cover this very exciting part of the electromagnetic spectrum at the edge of the atmospheric cut-off.

CUBES will open-up exciting new scientific opportunities for ESO’s Paranal Observatory, providing a world-leading capability well into the 2030s. Looking ahead to the start of ELT operations, we note that the VLT will be competitive for observations shortwards of ~400 nm (e.g. Evans et al. 2016) because of the protected silver coatings of four of the five ELT mirrors, that result in diminished performance at <450 nm compared to Al-coated mirrors. CUBES will exploit this unique part of parameter space. 

CUBES will cover the 300-400nm range at high spectral resolution (R~20,000) with an efficiency much higher than currently available high-resolution spectrographs covering the near-UV. It will also offer a lower resolution mode (R~5,000) with a larger entrance slit for case that require high sensitivity in the near-UV without the need of high spectral resolution. This mode will be particularly adapted for the observation of faint extended targets. CUBES will be significantly more efficient that any current instrument covering the near-UV, providing a S/N>20 for targets of  U=18 mag at 313 nm in 0.007nm wavelength bin. The instrument will also be equipped with an autoguider combined to a set of broad-band filters, allowing the user to obtain images of the target immediately before performing the spectroscopy. The combination of high spectral resolution and efficiency of CUBES will enable ground-breaking results in various fields, from extragalactic astronomy to stellar nucleosynthesis.

CUBES will also be particularly interesting for the observation of solar system object as the gas coma of comets contains a large number of emission features in range covered by CUBES, which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, (through the OH emission band at 310 nm), revealing how much ice may be hidden in the main asteroid belt.  CUBES will allow us to constrain key molecular abundances in cometary ices, such as the N₂/CO ratio through observation of ions and will be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects. Finally, CUBES will be used to measure the D/H ratio in comets providing an unprecedented homogeneous sample of measurements in a range of comets, that will allow us to constrain their formation temperature and the origin of water on Earth.