High-inclination Centaur reservoirs beyond Neptune

TL;DR

The study identifies and validates long-lived TNO reservoirs in the Solar System's polar corridor beyond Neptune, capable of delivering high-inclination Centaurs over gigayear timescales. By performing time-forward simulations with the four giant planets, Galactic tide, and passing stars using the REBOUND IAS15 integrator, it shows reservoirs in $[50:140]$ au persist for 4.5 Gyr and exhibit peak populations at $T=0.5$ and $T=-1.5$, with Saturn inducing secondary pathways that shape the polar corridor. Centaur injection scales linearly with the Tisserand parameter, and the Centaur inclination at minimum semimajor axis follows a near-linear $I$–$T$ relation largely independent of initial semi-major axis, implying high-inclination Centaurs mainly originate from polar corridor reservoirs rather than from low-inclination disks. The work links these reservoirs to the early Solar System’s stellar environment and provides linear relations that can help distinguish origins, with LSST data expected to constrain reservoir extent and population size.

Abstract

(Abridged) Numerical simulations of the past evolution of high-inclination Centaurs showed they originated from orbits beyond Neptune that were perpendicular to the Solar System's invariable plane in a region called the polar corridor. Recently, a study of Centaur injection in the three-body problem showed that Neptune-crossing TNOs in the polar corridor in the range [40:160] au have dynamical times that exceed the Solar System's age suggesting the presence of long-lived Centaur-producing reservoirs. We demonstrate the existence of such reservoirs in the Solar System by simulating the TNOs' time-forward evolution in the presence of the giant planets, the Galactic tide and passing stars using the IAS15 integrator of the REBOUND and REBOUNDx packages. We also assess the efficiency of Centaur injection as a function of the initial inclination and determine if high-inclination Centaurs may be produced by low inclination reservoirs. We find that TNO reservoirs in the semi-major axis range [50:140] au are long-lived and their populations peak at the Tisserand parameters T=0.5 and T=-1.5. Saturn is found to induce secondary structures in the polar corridor by holding the perihelia of a fraction of high-inclination reservoir material. We find that the Centaur inclination at minimum semi-major axis depends linearly on the Tisserand parameter regardless of the initial semi-major axis. Its amplitude shows that low inclination reservoirs such as the early protoplanetary disk are unlikely to produce high-inclination Centaurs in contrast to reservoirs in the polar corridor. We identified the likely location of the closest reservoirs to Neptune populated by TNOs captured in the early Solar System that produce high-inclination Centaurs. The Legacy Survey of Space and Time will be able to constrain the reservoirs' extent and population size

Figures (13)

• Figure 1: Initial state of the TNO reservoirs for different Tisserand parameters, $T$, as indicated in the top-right corner of each panel. Hot TNOs are shown in red and cold TNOs in blue. The solid orange and blue curves in the inclination panels are, respectively, the Tisserand inclination pathway (\ref{['TissIncP']}) and the initial inclination of circular orbits (\ref{['Icirc']}). In the eccentricity panels, the curves indicate perihelia that match the four giant planets' semimajor axes.
• Figure 2: TNO reservoirs at $4.5\,$Gyr for different Tisserand parameters $(T$). Hot TNOs are shown in red and cold TNOs in blue. Unstable TNOs are shown in black at the last sampling epoch before instability. The first and third columns display the orbital distributions in the $(a,I)$ and $(a,e)$ planes. The second and fourth columns show $I$, $e,$ and $a$ at minimum semimajor axis, where a vertical black line denotes Neptune's semimajor axis. The solid orange and blue curves in the inclination panels are, respectively, the Tisserand inclination pathway $I_{\rm Tiss}(a,T,a_{\rm N})$ (\ref{['TissIncP']}) and the circular orbits' initial inclination, $I_{\rm circ}(a,T,a_{\rm N})$ (\ref{['Icirc']}), with respect to Neptune. The solid green curves for retrograde and prograde inclinations respectively are the Tisserand inclinations pathways with respect to Saturn $I_{\rm Tiss}(a,-2,a_{\rm S})$ ($135^\circ$ inclination) and $I_{\rm Tiss}(a,2,a_{\rm S})$ ($45^\circ$ inclination). In the eccentricity panels, the curves indicate perihelia that match the four giant planets' semimajor axes. The Tisserand inclination pathway with respect to Neptune is not shown for $T=2.8$ because expression (\ref{['TissIncP']}) is not valid for $T>2.7$ (see Paper III for details).
• Figure 3: Fraction of hot TNOs that survived $4.5\,$Gyr as a function of $T$ (and $I_\infty$) for semimajor axes $50\leq a$(au)$\leq 100$ (top panel), $110\leq a$(au)$\leq 500$, and $a=40$ au (bottom panel). The number next to each curve is the initial semimajor axis.
• Figure 4: Average fraction of cold TNOs (in blue) and hot TNOs (in red) that survived $4.5\,$Gyr as a function of $T$ (and $I_\infty$).
• Figure 5: Fraction of hot TNOs that became Centaurs as a function of $T$ (and $I_\infty$). In the top panel, $f_{\rm cent}$, corresponds to the initial semimajor axis given next to each curve. In the bottom panel, the cold and hot TNO fractions averaged over initial semi-major axes, $\langle f_{\rm cent}\rangle$, are shown in blue and red, respectively. The black line is $\langle f_{\rm cent}\rangle(\%)=17+4.3T$.