Shape models and spin states of Jupiter Trojans: Testing the streaming instability formation

The origin of Jupiter Trojans (JTs) remains an open problem. The currently leading theory assumes that JTs were captured to their orbits near the Lagrangian points during the early reconfiguration of the giant planets (Morbidelli et al. 2005; Nesvorný et al. 2013). The natural source region for the majority of JTs would then be the population of planetesimals born in a massive trans-Neptunian disk. As a result, JTs should share the physical properties of currently observed trans-Neptunian objects (TNOs), as well as comets and irregular satellites of giant planets. Other theories avoid involving the planetary reconfiguration event and postulate that JTs formed at their current location together with Jupiter (see reviews in Marzari et al. 2002; Emery et al. 2015), or were born in the Jupiter coorbital zone and accompanied its early inward migration (Pirani et al. 2019). In this work, we adopt the capture model for JTs as a baseline hypothesis, because several other pieces of evidence support the view that giant planets underwent a violent instability at some early moment of the Solar System evolution (see, e.g., Nesvorný 2018).

In our contribution, we compile photometric datasets for about 900 JTs and apply the convex inversion technique in order to derive their shapes and spin states. We obtained full solutions for 71 JTs, and partial solutions (of a little less statistical significance) for additional 15 JTs. This represents an increase in the sample of known spin state and shape solutions for JTs by a factor of ∼3. JTs librating about L4 and L5 points contribute roughly equally to our solutions. We found a factor of ~1.5 between the number of prograde and retrograde populations, or ~60% abundance of prograde rotators in the overall JT population.

We found evidence that the observed distribution of the rotation pole obliquities/latitudes of JTs qualitatively resembles expectations from a numerical simulation of the streaming instability, the leading mechanism for the formation of planetesimals in the trans-Neptunian disk. JTs pole distribution has a slightly smaller north/south asymmetry, but this can be plausibly reconciled by the effects of a brief period of post-formation collisional activity. Our numerical simulations of the post-capture spin evolution indicate the JTs pole distribution is not significantly affected by dynamical processes.

Acknowledgments
The work of JH and JD has been supported by the Czech Science Foundation through grant 20-08218S. The work of DV has been supported by the Czech Science Foundation through grant 21-11058S. The work of OP has been supported by INTER-EXCELLENCE grant LTAUSA18093 from the Ministry of Education, Youth, and Sports.

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