Studying isotopic ratios in coets
Proposer(s): Cyrielle Opitom, Alessandra Migliorini, Monica Lazzarin, FIorangela La Forgia, Yuna Kwon
Description:
Comets are among the most pristine relics of the protoplanetary disc, where planets formed and evolved. When a comet approaches the Sun, the ices contained in its nucleus sublimate to form an atmosphere of gas and dust around the nucleus called the coma. By observing the composition of the coma, we can probe cometary ices retaining precious clues about the conditions prevailing in the early stages of our solar system.
Isotopic rations are particularly sensitive to the physico-chemical conditions prevailing at the time of comet formation. Several isotopic ratios can be measured in the coma of comets at optical wavelengths (14N/15N and 12C/13C in particular), providing precious clues about nitrogen reservoirs in the early solar system for example (Biver et al., 2022). However, measuring isotopic ratios requires very high signal to noise ratio observations, which can only be obtained for bright comets. These measurements are typically done with high resolution spectrographs, which are usually fibre-fed or have a relatively small entrance slit (of the order of a few arcseconds) while comets are extended targets that can be several arcminutes to several degrees big for the brightest and most active objects. This means that a large part of the light from the object is just waisted. The WST, with a large number of fibres over a large FoV would enable us to collect light from a large part of the comet, to co-add it, and to be able to measure isotopic ratio for a much larger sample of comets. This would open an era of statistical analysis of variations of isotopic ratios among comets, that could potentially be linked to their formation conditions in the early solar system. The WST would also offer the opportunity to measure isotopic ratio using the N2+ and CO+ ions, which has never been done before but would allow us to probe two very important molecules in terms of volatile reservoirs in the proto-planetary disk.
A multi-fibres instrument with a high spectral resolution would also enable measurements of these isotopic ratios simultaneously at different spatial positions in the coma for the brightest targets to search for variations, which has never been done before and is impossible with current facilities. This would allow us to determine if some processes in the coma influence isotope fractionation and could explain differences between isotopic ratio measurements made with different techniques at different spatial scales.
Transverse interests: This is related to other solar system science cases
Target properties
Target type: Comets: extended targets, needing non-sidereal tracking, ToO-type
Target density: /
Magnitude range: mag(V) in the range 0-10
Max Nobject in a 5 years survey: Difficult to estimate as the number of new objects varies, but of the order of 1/year.
Requirements
Instruments: MOS
Field of view: Several arcminutes
Wavelength range: Ideal 300-930 nm
Spectral resolution: >=40,000
Typical exposure time: Of the order of 1h
WST’s transformative nature
Main competitor(s): UVES + other optical high resolution spectrographs
Limits of competitor(s): Very small entrance slits -> huge flux loss for extended objects
Unique elements of WST for this science case: MOS over a large FOV (comets can extend > 1degree)