High-Precision Measurements of Brightness Variation of Nereid

TL;DR

This study presents high-precision brightness measurements of Nereid, confirming its stable 11.5-hour rotation, and suggests it was likely captured from a heliocentric orbit rather than formed in Neptune's circumplanetary disk.

Contribution

The paper provides precise photometric data confirming Nereid's stable rotation and proposes a new origin hypothesis as a captured object from a heliocentric orbit.

Findings

Nereid's rotation period is confirmed at 11.5 hours.

Optical colors of Nereid are similar to trans-Neptunian objects.

Nereid likely originated from a heliocentric orbit 4-5 AU away.

Abstract

Nereid, a satellite of Neptune, has a highly eccentric prograde orbit with a semi-major axis larger than 200 Neptune radius and is classified as an irregular satellite. Although the capture origin of irregular satellites has been widely accepted, several previous studies suggest that Nereid was formed in the circumplanetary disk of Neptune and was ejected outward to the present location by Triton. Our time-series photometric observations confirm that the spin is stable and non-chaotic with a period of 11.5 hr as indicated by Grav et al. (2003). The optical colors of Nereid are indistinguishable from those of trans-Neptunian objects and Centaurs, especially those with neutral colors. We also find the consistency of Nereid's rotation with the size-rotation distribution of small outer bodies. It is more likely that Nereid originates in an immigrant body captured from a heliocentric orbit which was 4-5 AU away from Neptune's orbit.