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Resonant Trapping of the Moving Groups G18-39 and G21-22 in the Galactic Halo

The 3D Galactic orbits of stars in the two groups G18-39 and G21-22 pertaining to the Galactic halo have been computed in a Galactic potential including a Galactic bar. The orbits have been related with the orbital structure of resonant orbits on the Galactic plane created by the bar component. We find that the majority of stars in both groups are trapped mainly by two resonant families already studied in a previous analysis. We show that the observed U--V velocity field of the stars in both groups can be naturally explained as a result of their trapping by these two resonant families, taking the angular rotation speed of the bar approximately in the interval 45--60 $\mathrm{km\,s^{-1}\,kpc^{-1}}$. This analysis may help to understand the identification of other known star groups as the possible result of the interactions produced by resonances on stars close to resonant families. For the two groups G18-39 and G21-22 we conclude that the majority of their stars are members of the supergroups of stars in the Galaxy trapped by two resonant families generated by the Galactic bar.