P/2019 LD2 in Transition: Dynamical and Thermal Context for an Evolving Active Centaur

The Centaur-Comet Gateway (Sarid et al. 2019) is a recently discovered region of small body orbital evolution, between the outer and inner solar system, where Centaurs transfer into the active comet population. This region may also be associated with a transition in the thermal environment a small body experiences. The best known and most prominent resident of the Gateway is centaur 29P/Schwassmann-Wachmann (hereafter SW1), which is one of the most active small bodies in the solar system, both in quiescent and outburst behavior. SW1 is likely to migrate out of the Gateway in the near-future, with subsequent long-term evolution pushing it to become one of the biggest Jupiter-family comets (JFC) ever seen (Sarid et al. 2019).

Recently discovered P/2019 LD2 (ATLAS), hereafter LD2, was quickly identified as an active member of the Centaur population, due to dynamical studies (Kareta et al. 2020) and apparent continuous cometary activity. However, the dynamical history, thermal environment, and impact of such environments on the activity of LD2 are poorly understood. We performed dynamical simulations to constrain LD2's orbital history and resulting thermal environment over the past 3000 years, in order to better understand its past, present and future behavior (Steckloff et al. 2020). Forward modeling shows LD2 becoming a JFC in ~40 years, representing the first known opportunity to observe the evolution of an active nucleus as it experiences this population-defining transition.

We find that LD2 is skirting the Gateway region, rapidly transitioning from a wholly outer solar system orbit to a JFC-like one. Our calculations show that it is unlikely to have spent significant past time in the inner solar system, suggesting that its nucleus is pristine in terms of physical and chemical processing (Steckloff et al. 2020). This could explain LD2's relatively high level of distant activity as a recently activated primordial body. Since its current heliocentric distance is too far for efficient water-ice sublimation to drive activity, other volatile species or triggering mechanisms must be involved. LD2's coma could be supported by outbursts or continuous low-level outgassing. Some of this may be fueled by the rapid amorphous-to-crystalline phase transition of water ice and subsequent release of ice-trapped volatile species, which is predicted to proceed at the relevant equivalent blackbody temperatures. Together with SW1, LD2 may be one of the best examples of active small bodies driven by amorphous-crystalline ice transition and sublimation flow of highly volatile species.