Centaurs transitioning to JFCs: thermal and dynamical evolution

1- Context

Jupiter-family Comets are continuously replenished from their outer solar system reservoirs. Before they enter the inner solar system, they spend a significant amount of time as Centaurs. This dynamical cascade between populations, and the individual orbital track that these icy objects follow, can entail some extensive modifications of their internal structure and composition (Gkotsinas et al. 2022). In this context, the transient population of Centaurs is a key target for understanding progenitors of JFCs. In particular, studying the origin of cometary activity amongst Centaurs is relevant for constraining the overall evolution and stratification pattern of JFC nuclei. Recently, Sarid et al. (2019) reported the existence of a specific dynamical pathway which appears to facilitate the transition between the Centaur and JFC populations. Through forward dynamical modeling, they inferred that the majority of objects which eventually become JFCs should leave the Centaur population through this gateway. As a corollary, objects currently observed on gateway orbits should likely transition into JFCs in the near future. As such, they would represent compelling targets to investigate how dynamical and thermal evolution alters comet nuclei before they become JFCs. 

In this work, we provide an independent look at the transition from Centaurs to JFCs, with a specific emphasis on the gateway region. We use the population of JFC dynamical clones produced by Nesvorny et al. (2017), studied in detail by Gkotsinas et al. (2022), in order to constrain the thermal structure of objects as they transition from Centaurs to JFCs. Eventually, we want to assess the relevance of this region to the populations of active objects, their physical properties as they cascade from the outer solar system, and the onset and development of activity in the giant planet region.

2- Methods

We uses the simulation outcomes of the coupled thermal and dynamical evolution of JFC clones by Gkotsinas et al. (2022). We consider a sample of 350 JFC dynamical clones generated from simulations performed by Nesvorny et al. (2017). Coupling the thermal evolution of these clones to their orbital evolution is made through a 1D thermal evolution model derived from Guilbert-Leputre et al. (2011). Assumptions are made to overcome the complexity arising from the very different timescales involved in the evolution on our long timescales. 

3- Results

Among the 350 clones, we find that 191 objects reach the gateway region at least once in their lifetime (54.6%). Of those, 73 were Centaurs prior to entering the gateway (i.e. 20.9% of the overall clone population), while 102 objects (29.1%) were previously JFCs. In other words, these clones had already transitioned from Centaurs to JFCs without going through the gateway, which they entered then later during their lifetime. An additional 16 clones (4.6%) were coming from Jupiter-crossing orbits. As reported by Sarid et al. (2019), we find that multiple distinct passages through the gateway (for clones which do reach that region) are usually possible: on average, our clones enter 7 to 8 times in the gateway. Overall, when we constrain the origin of clones the first time they reach the gateway, we find that strictly speaking, our population has only 20.9% of Centaurs which actually go through the gateway prior to becoming JFCs for the first time. Since 159 clones (45.4%) never go through the gateway at any point of their lifetime, we find that most Centaurs (79.2%) transition to the JFC population outside of the gateway region. 

We find some statistically significant differences in the thermal processing of the two sub-populations, i.e. clones passing through the gateway vs. clones reaching JFC orbits without ever entering the gateway in their lifetime. We find that objects are more processed on their first entrance in the gateway than the rest of Centaurs, when they transition to JFC orbits outside of the gateway. This is mainly due to the fact that more than half of these objects (102 clones or 53.4% of objects going through the gateway) have already been close to the Sun on JFC orbits, prior to reaching the gateway. These clones are thus more processed on average than those of Centaur origin. Clones of 29P and LD2 experience a variety of timescales of residency in the giant planet region, and of evolutions in orbital elements. This inevitably entails a diversity in thermal processing, especially when clones spend significant periods on orbits with relatively small perihelion distances.  

4- Perspectives

This study suggests that the gateway as defined by Sarid et al. (2019) might not be as significant as primarily thought. The activity of gateway Centaurs reflects the composition and structure inherited from their previous stages of evolution, which includes a JFC phase for a significant fraction of them. As such, the pattern of outgassing could arise from a layer substantially altered prior to current observations. Depending on thermophysical parameters, compositions, and other poorly constrained properties, it is however entirely possible that the effects of such thermal processing could be limited to a modest near-surface layer. From a statistical point of view, there is a ~50% chance that gateway Centaurs could be significantly altered, and only ~20% chance that their activity would be representative of a "pristine" (i.e. less altered) body. Given these results, we believe that some caution ought to be applied when claiming that gateway Centaurs and their activity patterns are representative of what most JFCs experience. Moreover, since the vast majority of active Centaurs are currently found outside of the gateway, we believed that studying the onset and development of activity in the giant planet region should not be restricted in any way.  

 

Acknowledgements

This study is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802699). We gratefully acknowledge support from the PSMN (Pôle Scientifique de Modélisation Numérique) of the ENS de Lyon for the computing resources.

 

Bibliography

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Guilbert-Lepoutre et al. (2011) A&A, 529, 71

Nesvorny et al. (2017) ApJ, 845, 25

Sarid et al. (2019) ApJ Letters, 883, 25