Spin vectors in the Koronis family: IV. Completing the sample of its largest members after 35 years of study

TL;DR

This study completes the spin vector sample of the largest Koronis family members, providing new rotation data and analyzing their spin properties to understand their evolution influenced by thermal YORP torques.

Contribution

It significantly expands the known spin vectors for Koronis family members, enabling better modeling of their spin evolution and distribution.

Findings

Distribution of retrograde and prograde rotation rates is consistent with YORP effects.

Prograde rotators have a narrower spin rate distribution, with no periods longer than 20 hours.

No newly added prograde objects are in the s6 spin-orbit resonance.

Abstract

An observational study of Koronis family members' spin properties was undertaken with two primary objectives: to reduce selection biases for object rotation period and lightcurve amplitude in the sample of members' known spin vectors, and to better constrain future modeling of spin properties evolution. Here we report rotation lightcurves of nineteen Koronis family members, and derived results that increase the sample of determined spin vectors in the Koronis family to include 34 of the largest 36 family members, completing it to $H \approx 11.3$ ($D \sim 16$ km) for the largest 32 members. The program observations were made during a total of 72 apparitions between 2005-2021, and are reported here along with several earlier unpublished lightcurves. All of the reported data were analyzed together with previously published lightcurves to determine the objects' sidereal rotation periods, spin vector orientations, and convex model shape solutions. The derived distributions of retrograde rotation rates and pole obliquities appear to be qualitatively consistent with outcomes of modification by thermal YORP torques. The distribution of spin rates for the prograde rotators remains narrower than that for the retrograde rotators; in particular, the absence of prograde rotators having periods longer than about 20 h is real, while among the retrograde rotators are several objects having longer periods up to about 65 h. None of the prograde objects newly added to the sample appear to be trapped in an $s_6$ spin-orbit resonance that is characteristic of most of the largest prograde objects; these smaller objects either could have been trapped previously and have already evolved out, or have experienced spin evolution tracks that did not include the resonance.