# Spectroscopic and dynamical properties of comet C/2018 F4, likely a true average former member of the Oort cloud

## TL;DR

This study characterizes comet C/2018 F4's spectroscopic and dynamical properties, revealing it as a likely old Oort cloud member with typical comet activity and an asteroid-like spectrum, challenging assumptions about comet diversity.

## Contribution

It provides detailed spectroscopic and dynamical analysis of C/2018 F4, demonstrating its likely origin in the Oort cloud and its similarity to primitive asteroids, expanding understanding of comet heterogeneity.

## Key findings

- C/2018 F4 has a spectral slope of 4.0%/1000Å and an X-type asteroid spectrum.
- The comet exhibits a coma with activity levels comparable to other comets at similar distances.
- Orbital simulations suggest it is a dynamically old Oort cloud object, not of extrasolar origin.

## Abstract

The population of comets hosted by the Oort cloud is heterogeneous. Most studies in this area focused on highly active objects, those with small perihelion distances or examples of objects with peculiar physical properties and/or unusual chemical compositions. This may have produced a biased sample of Oort cloud comets in which the most common objects may be rare, particularly those with perihelia well beyond the orbit of the Earth. Within this context, the known Oort cloud comets may not be representative of the full sample. Here, we study the spectral properties in the visible region and the cometary activity of Comet C/2018 F4 (PANSTARRS). We also explore its orbital evolution with the aim of understanding its origin within the context of known minor bodies moving along nearly parabolic or hyperbolic paths. We present observations obtained with the 10.4 m Gran Telescopio Canarias (GTC), derive the spectral class and visible slope of C/2018 F4 and characterise its level of cometary activity. Direct N-body simulations are carried out to explore its orbital evolution. The absolute magnitude of C/2018 F4 is Hr=13.62+/-0.04. Assuming a pV=0.04 its diameter is D<10.4 km. The object presents a conspicuous coma, with a level of activity comparable to those of other comets observed at similar heliocentric distances. Comet C/2018 F4 has a visible spectrum consistent with that of an X-type asteroid, and has a spectral slope S'=4.0+/-1.0 %/1000\AA and no evidence of hydration. The spectrum matches those of well-studied primitive asteroids and comets. The analysis of its dynamical evolution prior to discovery suggests that C/2018 F4 is not of extrasolar origin. Although the present-day heliocentric orbit of C/2018 F4 is slightly hyperbolic, its observational properties and past orbital evolution are consistent with those of a dynamically old comet with an origin in the Oort cloud.