Using ground-based IFUs to constrain cometary activity drivers

Despite over a century of cometary observations, we are still struggling to understand the activity of comets. Questions such as ‘what drives the activity of comets at different distances from the Sun?’ or ‘Does the activity of all comets evolve in a similar way?’ remain  unanswered. This is in part due to the difficulty to observe the drivers of cometary activity: H₂O, CO, and CO₂ can only be detected simultaneously using space observatories. JWST has proven an incredible resource to answer these questions [1,2], but only a limited number of comets have been observed so far.

The last decade has seen the advent of large field of view Integral Field Spectrographs (IFUs) for observations at optical wavelengths. These instruments, such as MUSE on the 8-m Very Large Telescope [3], present an interesting opportunity to characterize the distant activity of comets, due to the combination of spatial and spectral information they provide. Large scale IFUs on ground-based telescopes can be particularly useful to probe cometary activity drivers that are typically difficult to measure from the ground, including H₂O, CO, CO₂- and potentially O₂ - through the study of some of their dissociation products such as oxygen [4,5]. IFUs are generally very sensitive to faint extended emission and are thus ideal tools for the observations of active small bodies in the solar system.

We have observed a number of comets over a wide range of distances from the Sun with the MUSE instrument of the VLT over the last 7 years. We use these data to measure the ratio between the green (557 nm) and the two red line (630 and 637 nm) forbidden oxygen lines at optical wavelength, usually referred to as G/R ratio. We focus on observations of 29P//Schwassmann–Wachmann 1, the most distant comet observed with MUSE, for which we were able to detect forbidden oxygen lines. Comparing the very high measured G/R ratio, consistent with an activity driven by CO, to that measured for other comets at smaller heliocentric distances, and to direct observations of the activity drivers with JWST or ground-based radio observatories, we demonstrate that MUSE constitutes a powerful tool to study the activity evolution of comets that were previously thought too distant or too faint for ground-based optical spectroscopy.

References: [1] Snodgrass C. et al., 2025, MNRAS in press (arXiv 2503.14071) [2] Faggi S.  et al., 2024, Nature Astronomy, Volume 8, Issue 10, pp. 1237-1245 [3] Bacon R., et al., 2010, Proceedings of the SPIE, Volume 7735, id. 773508 [4] Kwon Y., Opitom C., Lippi M.,   Astronomy & Astrophysics, Volume 674, id.A206, 11 pp. [5] Opitom C., et al., 2020, Astronomy & Astrophysics, Volume 644, id.A143, 7 pp.