Understanding Cometary Activity Beyond the Water-Sublimation Line: Characterizing Comets in the Centaur-to-Jupiter-Family Transition
Charles Schambeau
Florida Space Institute, University of Central Florida

Centaurs are icy solar system small bodies in orbits between Neptune and Jupiter that represent intermediaries in the dynamical link between the outer solar system's Trans- Neptunian Objects (TNOs; more specifically the scattered disk objects) to the inner solar system's Jupiter-Family comets (JFCs). In situ observations of spacecraft-visited JFCs nuclei have revealed highly evolved bodies which have undergone complex thermal processing [1] and the New Horizons Kuiper Belt Extended Mission's flyby of the TNO 2014 MU 69 [2] has given us an example of the ``pristine'' materials we can expect to populate the cold storage environment of the outer solar system. Although 2014 MU 69 is in the classical cold Kuiper Belt, it represents an example of the types of cryogenically stored materials potentially contained in the scattered disk of progenitor small icy bodies that supply the JFCs population. To fully understand the evolutionary link between the preserved outer Solar System materials and the more easily observationally accessible but also more thermally processed JFCs, we must understand the material evolution ongoing in the Centaur region.

We present results from an ongoing observing campaign to characterize activity patterns, nucleus properties, and activity drivers of Jupiter Family comets (JFCs) with perihelion distances q > 4.5 au and of active Centaurs by acquiring long-baseline visible and near-IR imaging. The active lifetimes of JFCs often start when they are still in the Centaur region [3, 4, 5], yet the transition from Centaur to JFC happens in a region where water sublimation cannot be the driver of the activity [6]. The presence of cometary activity beyond the historically established distance for strong water sublimation near 3 au has been well documented [5, 7], but the observational dataset for most distantly active comets has been sparse, limiting our ability to fully understand the transition region from the outer to inner solar system. Dedicated observations of the comets' behavior can let us use thermal modeling to probe the compositional interior structure of the comet and investigate how (for example) CO/CO2 sublimation or H2O crystallization might drive the activity [8]. We have acquired high-cadence broadband imaging for a strategically selected population of objects in the Centaur-to-Jupiter- family transition region since early 2016 and present results of our campaign's nuclei characterization and activity behavior monitoring efforts.

[1] Sunshine, J. M., et al. 2016, JGR, 121, 2194. [2] Stern, S. A., et al. 2018, SSRv, 214, 77. [3] Mazzotta Epifani et al. 2007, MNRAS, 381, 713-722. [4] Jewitt, D.: 2009, ApJ, 137:4296-4312. [5] Meech, K.J., et al.: 2009, Icarus, 201, 719-739. [6] Meech, K.J. & Svoren, J.: 2004, Comets II, Univ. AZ Press, 317-335. [7] Kelley, M., et al.: 2013, Icarus, 225, 475-494. [8] Meech, K.J., et al.: 2017, AJ, 153:206.