The volatile composition of C/2021 A1 (Leonard): Comparison between infrared and UV-optical measurements

Aims: We report on the composition of comet C/2021 A1 (Leonard) as measured by CRIRES+<, the upgraded high resolution infrared echelle spectrometer of VLT/ESO. We compare our results with those obtained in the near-UV-visible using TRAPPIST-South. Methods: C/2021 A1 (Leonard) was observed with CRIRES+ on December 29, 2021 and January 1-3, 2022, and monitored almost daily using TRAPPIST-South from about mid-December 2021 to the end of January 2022. During the TRAPPIST observations the comet showed (almost) periodic outbursts, with an estimated interval of about 4.4 days. One of these outbursts occurred close in time with our CRIRES+observations on January 1, allowing us to investigate fresh material released from the nucleus. Both data sets were reduced using established algorithms tailored for each telescope and instrument. The obtained results in the two spectral ranges were searched for correlations and similarities to compare the primary and secondary species in the coma. Results: The abundance ratios (with respect to water) of CH3OH, C2H6, and CH4 in C/2021 A1, as measured in the infrared with CRIRES+, are lower than the average values measured in other comets, suggesting a depletion in organics for this comet. On the contrary, measurements in the near UV-Visible spectral region obtained close in time with TRAPPIST-South show a C2/CN ratio that falls in the typical range. The different conclusions that can be reached observing the same comet in different spectral ranges (organic depleted vs. organic typical) open a series of questions on the processes occurring in the coma. Their comprehensive interpretation requires deeper investigations in future studies.