Visible and near-IR spectroscopic characterization of different dynamical classes of comets

Linking the taxonomic classification of comets with their dynamic classification is somehow still debated. Comets are historically distinguished in: typical and carbon-depleted based on A’Hearn et al. (1985) work. Fink (2009) has further distinguished three other types of comets: Tempel 1 type, Giacobini-Zimmer type and the unusual Yanaka object.

However, recent Solar System formation models (Brasser and Morbidelli 2013) indicate that comets are derived from the same parent population, i.e. the primordial trans-neptunian disc, before being scattered in the two main reservoirs of Oort Cloud and Scattered disk. This might suggest that the observed differences are evolutionary rather than primordial. 

Cometary outgassing is dependent on the activity, that is, the abundance of various species of volatiles subliming from the nucleus varies with heliocentric distances and this significantly influences our classification and deserves better understanding. 

Long Period Comets (LPCs) have been found to be already active very far away from the Sun, as far as 26 AU (Jewitt et al. 2019; Hui et al. 2018; Hui et al. 2019) and data analysis suggests that their activity must have started as far as 35 AU (Jewitt et al. 2021). At those heliocentric distances, the temperatures are too cold to allow water ice to sublimate. Despite other mechanisms are at study, the sublimation of supervolatile ices like CO2 and CO, which are among the most abundant ices in comets after water (e.g. Bockelée-Morvan et al. 2017), is believed to be responsible for this distant activity (e.g. Fulle et al. 2020A; Fulle et al. 2020B; Bodewits et al. 2015; Ootsubo et al. 2012). 

However a full understanding of the processes driving the cometary activity from far away from the Sun to the perihelion distance is still to be fully understood and the subtle differences among individual comets often seem to prevail with respect to a common pathway. Therefore, given the small statistics available, each individual addition is going to be very helpful and valuable to build a comprehensive insight of these processes.

Here we present visible and near-IR spectroscopic characterization of a sample of more than 10 comets of different dynamical classes (JFC, Halley, Oort cloud) observed at various heliocentric distances (from 1 to 6 AU) using TNG with Dolores and NICS instruments covering the spectral ranges from 300 to 2500 nm.

Preliminary reduction indicates a large variety of behaviors from very faint and inactive objects to typically CN-dominated objects to peculiar objects such as C/2016 R2.

C/2016 R2 has been shown by several authors (e.g. Cochran & MacKay 2018; Opitom et al. 2019) to be a quite unique water-poor object rich in CO+/N2+. However, only recently an updated fluorescence emission model of CO+ has been published (Bromley et al., 2024).

Additionally, modern calculations seem to suggest that such N2-rich comets might nevertheless be more common than we expected (Anderson et al. 2023) and could also be the key to understand the long-dated problem of the N2-depletion of comets with respect to the protosolar nebula.

We present additional observations of this peculiar comet and other more typical comets that will be used to further constrain the models of the emission mechanisms and to obtain a better understanding of cometary composition and evolution.

 

References

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