Retrograde orbits excess among observable interstellar objects

TL;DR

This study predicts a bias towards retrograde orbits among observable interstellar objects, influenced by size distribution and perihelion distances, which can help infer properties of the true ISO population.

Contribution

It introduces a simulation-based analysis of the orbital distribution of observable ISOs, highlighting the bias towards retrograde orbits and its dependence on size and perihelion distance.

Findings

Observable ISOs are biased towards retrograde orbits.

Steeper size-frequency distributions increase retrograde orbit fraction.

Larger perihelion distances lead to more symmetric inclination distributions.

Abstract

In this work we investigate the orbital distribution of interstellar objects (ISOs), observable by the future wide-field National Science Foundation Vera C. Rubin Observatory (VRO). We generate synthetic population of ISOs and simulate their ephemerides over a period of 10 years, in order to select those which may be observed by the VRO, based on the nominal characteristics of this survey. We find that the population of the observable ISOs should be significantly biased in favor of retrograde objects. The intensity of this bias is correlated with the slope of the size-frequency distribution (SFD) of the population, as well as with the perihelion distances. Steeper SFD slopes lead to an increased fraction of the retrograde orbits, and also of the median orbital inclination. On the other hand, larger perihelion distances result in more symmetric distribution of orbital inclinations. We believe that this is a result of the Holetschek effects, which is already suggested to cause observational bias in orbital distribution of long period comets. The most important implication of our findings is that an excess of retrograde orbits depends on the sizes and the perihelion distances. Therefore, the prograde/retrograde orbits ratio and the median inclination of the discovered population could, in turn, be used to estimate the SFD of the underlying true population of ISOs.