The velocity distribution of nearby stars from Hipparcos data II. The nature of the low-velocity moving groups

TL;DR

This study investigates the properties and origins of low-velocity moving groups in the local stellar velocity distribution, finding they are unlikely to be coeval clusters and may be related to transient dynamical perturbations rather than steady-state structures.

Contribution

It provides a detailed analysis of the age and metallicity of moving groups, testing their coevality and dynamical origins, and challenges the assumption that they are remnants of star formation events.

Findings

Moving groups are not coeval star clusters.

Hyades and Sirius groups show metallicity anomalies linked to spiral structure resonances.

Moving groups are similar to background stars, suggesting transient dynamical origins.

Abstract

The velocity distribution of nearby stars contains many "moving groups" that are inconsistent with the standard assumption of an axisymmetric, time-independent, and steady-state Galaxy. We study the age and metallicity properties of the low-velocity moving groups based on the reconstruction of the local velocity distribution in Paper I of this series. We perform stringent, conservative hypothesis testing to establish for each of these moving groups whether it could conceivably consist of a coeval population of stars. We conclude that they do not: the moving groups are not trivially associated with their eponymous open clusters nor with any other inhomogeneous star formation event. Concerning a possible dynamical origin of the moving groups, we test whether any of the moving groups has a higher or lower metallicity than the background population of thin disk stars, as would generically be the case if the moving groups are associated with resonances of the bar or spiral structure. We find clear evidence that the Hyades moving group has higher than average metallicity and weak evidence that the Sirius moving group has lower than average metallicity, which could indicate that these two groups are related to the inner Lindblad resonance of the spiral structure. Further we find weak evidence that the Hercules moving group has higher than average metallicity, as would be the case if it is associated with the bar's outer Lindblad resonance. The Pleiades moving group shows no clear metallicity anomaly, arguing against a common dynamical origin for the Hyades and Pleiades groups. Overall, however, the moving groups are barely distinguishable from the background population of stars, raising the likelihood that the moving groups are associated with transient perturbations. [abridged]