Dynamics of tidal tails of open clusters: I. effects of bar, spiral arms and giant molecular clouds

Abstract

Open clusters gradually dissolve, and their stars disperse into the Galactic field. Lost stars form tidal tails-elongated streams that trace the cluster orbit ahead of and behind its core. From the shape and orientation of the tidal tails, it is possible to infer the shape of the gravitational potential governing the cluster's motion. The orbits of open clusters, including those in the Solar neighbourhood, are sensitive to the gravitational potential of the inner Galaxy, which is dominated by the Galactic bar. Using n-body simulations of synthetic and real open clusters, we investigate how sensitive the shapes and orientations of tidal tails are to variations of the gravitational potential of the Milky Way. We consider the effects of the bar as well as spiral arms, giant molecular clouds (GMCs) and satellite galaxies. We analyse the stellar distributions within tidal tails using statistical metrics that quantify the differences between tail morphologies. Such non-parametric approach enables us to efficiently explore tidal tails across a large parameter space of gravitational potential models. We find that the Galactic bar-particularly its pattern speed-has a strong influence on the orbits of open clusters and the shape of their tails. Spiral arms have a limited effect, and satellite galaxies do not disturb the tidal tails of nearby open clusters. Perturbations by GMCs affect most clusters, with distortions stronger than those by the bar observed in old and in-plane clusters. We identify nearby open clusters that are most sensitive to the pattern speed of the bar. By observing the tidal tails of a handful of well-selected nearby clusters, we should be able to measure the pattern speed of the bar with a precision in the order of $1\ \mathrm{km\,s^{-1}\,kpc^{-1}}$ independently from length and orientation of the bar. We will present the observability of tidal tails in paper II.