Search for Past Stellar Encounters and the Origin of 3I/ATLAS

TL;DR

The study addresses the origin of the interstellar object 3I/ATLAS by tracing its past trajectory through the Galaxy and searching for close stellar encounters using Gaia DR3 data. It propagates the orbits of 3I/ATLAS and ~30 million Gaia stars within a Galactic potential and uses a Monte Carlo framework to account for uncertainties, identifying 25 close encounters with median distances $d^{\mathrm{med}}_{\mathrm{enc}}<1$ pc, though their speeds $v^{\mathrm{enc}}$ exceed $20$ km s$^{-1}$, making them unlikely hosts under typical ejection scenarios. Among these, the strongest perturber is a wide M-dwarf binary that passed within $d_{\mathrm{enc}}=0.242$ pc at $v_{\mathrm{enc}}=28.39$ km s$^{-1}$ around $1.64$ Myr ago, but the overall cumulative effect on 3I/ATLAS’s speed and direction is weak. The present kinematics favor a thin-disk origin, since 3I/ATLAS aligns with the thin-disk velocity dispersion and local stellar density dominates near the Sun.

Abstract

3I/ATLAS, the third discovered interstellar object, has a heliocentric speed of 58 km/s and exhibits cometary activity. To constrain the origin of 3I/ATLAS and its past dynamical evolution, we propagate the orbits of 3I/ATLAS and nearby stars to search for stellar encounters. Integrating orbits in the Galactic potential and propagating the astrometric and radial-velocity uncertainties of 30 million Gaia stars, we identify 25 encounters with median encounter distances less than 1 pc. However, because the encounter speeds between 3I/ATLAS and each encounter exceed 20 km/s, none is a plausible host under common ejection mechanisms. We infer stellar masses for most stars and quantify the gravitational perturbations exerted by each individual star or each binary system on 3I/ATLAS. The strongest gravitational scattering perturber is a wide M-dwarf binary. Among all past encounters, the binary's barycenter and 3I/ATLAS reach the small encounter distance of 0.242 pc and the encounter speed of 28.39 km/s,1.64 Myr ago. We further demonstrate that the cumulative influence of the stellar encounters on both the speed and direction of 3I/ATLAS is weak. Based on the present kinematics of 3I/ATLAS to assess its origin, we find that a thin-disk origin is strongly favored, because the thin disk both exhibits a velocity distribution closely matching that of 3I/ATLAS and provides the dominant local number density of stars.

Figures (1)

• Figure 1: Distributions of nominal and median encounter distances $d^{\mathrm{}}_{\mathrm{enc}}$ and encounter speeds $v^{\mathrm{}}_{\mathrm{enc}}$ for encounters with each 95% quantile of $d^{\mathrm{}}_{\mathrm{enc}}$ below 1.5 pc. The left panel represents the nominal values of $d^{\mathrm{nom}}_{\mathrm{enc}}$ and $v^{\mathrm{nom}}_{\mathrm{enc}}$. The right panel represents median values of $d^{\mathrm{med}}_{\mathrm{enc}}$ and $v^{\mathrm{med}}_{\mathrm{enc}}$. Error bars indicate 5% and 95% quantiles of $d^{\mathrm{}}_{\mathrm{enc}}$ and $v^{\mathrm{}}_{\mathrm{enc}}$ in MC results. The color bar is based on Equation \ref{['Eq:g']} using the median of $g^{\mathrm{}}$.