# Lightcurves and Rotational Properties of the Pristine Cold Classical Kuiper Belt Objects

## TL;DR

This study investigates the rotational and physical properties of Cold Classical Kuiper Belt Objects, revealing they rotate slower, are more elongated, and have fewer contact binaries than other trans-Neptunian populations, providing insights into early Solar System formation.

## Contribution

It provides the first comprehensive lightcurve survey of Cold Classical Kuiper Belt Objects, highlighting their unique rotational characteristics and binary fraction compared to other trans-Neptunian objects.

## Key findings

- Cold Classicals have slower rotations than other TNOs.
- Approximately 36% of CCs are small amplitude lightcurve objects.
- Estimated 10-25% of CCs could be contact binaries.

## Abstract

We present a survey on the rotational and physical properties of the dynamically low inclination Cold Classical trans-Neptunian objects. The Cold Classicals (CCs) are primordial planetesimals and contain relevant information about the early phase of our Solar System and planet formation over the first 100 million years after the formation of the Sun. Our project makes use of the Magellan and the Lowell's Discovery Channel Telescopes for photometric purposes. We obtained partial/complete lightcurves for 42 CCs. We use statistical tests to derive general properties about the shape and rotational frequency distributions of the CC population, and infer that the CCs have slower rotations and are more elongated/deformed than the other trans-Neptunian objects. Based on the available full lightcurves, the mean rotational period of the CC population is 9.48$\pm$1.53h whereas the mean period of the rest of the trans-Neptunian objects is 8.45$\pm$0.58h. About 65% of the trans-Neptunian objects (excluding the CCs) have a lightcurve amplitude below 0.2mag compared to the 36% of CCs with small amplitude. We present the full lightcurve of one new likely contact binary: 2004 VC131 with a potential density of 1gcc for a mass ratio of 0.4. We also have hints that 2004 MU8 and 2004 VU75 are maybe potential contact binaries based on their sparse lightcurves but more data are needed to confirm such a find. Assuming equal-sized binaries, we find that only ~10-25% of the Cold Classicals could be contact binaries, suggesting that there is a deficit of contact binaries in this population compared to previous estimates and compared to the abundant (~40-50%) possible contact binaries in the 3:2 resonant population. This estimate is a lower limit and will increase if non equal-sized contact binaries are also considered. Finally, we put in context the early results of the New Horizons flyby of (486958) 2014 MU69.