2017A&A...608A..73K

Context. The velocity distribution of stars in the solar neighbourhood is inhomogeneous and rich with stellar streams and kinematic structures. These may retain important clues regarding the formation and dynamical history of the Milky Way. However, the nature and origin of many of the streams and structures is unclear, hindering our understanding of how the Milky Way formed and evolved.
Aims. We aim to study the velocity distribution of stars of the solar neighbourhood and investigate the properties of individual kinematic structures in order to improve our understanding of their origins.
Methods. Using the astrometric data provided by Gaia DR1/TGAS and radial velocities from RAVE DR5 we perform a wavelet analysis with the a trous algorithm of 55 831 stars that have U and V velocity uncertainties less than 4km/s. An auto-convolution histogram method is used to filter the output data, and we then run Monte Carlo simulations to verify that the detected structures are real and are not caused by noise due to velocity uncertainties. Additionally we analysed our stellar sample by splitting all stars into a nearby sample (<300pc) and a distant sample (>300pc), and two chemically defined samples that to a first degree represent the thin and the thick disks.
Results. We detect 19 kinematic structures in the solar neighbourhood in the range of scales 3-16km/s at the 3σ confidence level. Among them we identified well-known groups (such as Hercules, Sirius, Coma Berenices, Pleiades, and Wolf 630), confirmed recently detected groups (such as Antoja12 and Bobylev16), and detected a new structure at (U,V)~=(37,8)km/s. Another three new groups are tentatively detected, but require further confirmation. Some of the detected groups show clear dependence on distance in the sense that they are only present in the nearby sample (<300pc), and others appear to be correlated with chemistry as they are only present in one of the chemically defined thin and thick disk samples.
Conclusions. With the much enlarged stellar sample and much increased precision in distances and proper motions, provided by Gaia DR1/TGAS we have shown that the velocity distribution of stars in the solar neighbourhood contains more structures than previously known. A new feature is discovered and three recently detected groups are confirmed at high confidence level. Dividing the sample based on distance and/or metallicity shows that there are variety of structures which form large-scale and small-scale groups; some of them have clear trends on metallicities, others are a mixture of both disk stars. Based on these findings we discuss possible origins of each group.