The 2:1 mean motion resonance with Neptune is located at about 47.8 AU and contains more than 85 known trans-Neptunian objects (TNOs). During Neptune’s migration outward in the Solar System, TNOs from various locations were pushed and scattered into the 2:1 resonance giving this population of TNOs a large range of formation locations and surface colors. The 2:1 resonance with Neptune likely contains some TNOs of the dynamically Cold Classical population that show a very-red/ultra-red surface. Previously, we studied the rotational properties of the 3:2 resonance and dynamically Cold Classical TNOs and estimated the fraction of contact binaries in these two different dynamical populations (Thirouin and Sheppard, 2018, 2019). As the 3:2 and 2:1 resonances delimit the reservoir of dynamically Cold Classical TNOs in the main Kuiper belt, we obtain the lightcurves of a number of 2:1 TNOs in order to estimate the number of contact binaries and identify rotational differences between the various different highly evolved resonant populations and the more primitive Cold Classicals. For consistency and to limit observational biases, we designed a survey dedicated to the 2:1 resonant TNOs in the same way we did for the 3:2 and Cold Classical populations, using the Lowell Discovery Telescope and the Magellan telescope for lightcurves and color photometry. Here, we report some preliminary results on our observations of the 2:1 resonant TNOs which correspond to 58 percent of the 2:1 TNOs with a visual magnitude V less than 23.5mag. Sparse and complete lightcurves obtained are used to constrain the shape and rotational frequency distributions. We also compare the properties of the Cold Classicals to the various Neptune resonant populations. Using our results and the literature, we estimate that the 2:1 resonance could likely have fewer contact binaries than the 3:2 resonant population and are more consistent with our estimate for the dynamically Cold Classicals contact binary population. This low estimate for the 2:1 resonance contact binary percentage is not expected based on some binary survival modeling (Nesvorny and Vorouhlicky, 2019).
This work is supported by the National Science Foundation (NSF), grant number 1734484.