Kinematic structures of the solar neighbourhood revealed byGaiaDR1/TGAS and RAVE

Abstract

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 of 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 à trous algorithm to 55 831 stars that have U and V velocity uncertainties less than 4 km s −1 . 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 due to velocity uncertainties. Additionally we analysed our stellar sample by splitting all stars into a nearby sample (< 300 pc) and a distant sample (> 300 pc), 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 between scales 3 − 16 km s −1 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 −1 . 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 (< 300 pc), and others appear to be correlated with chemistry as they are only present in either of the chemically defined thin and thick disk samples. Conclusions. With the much enlarged stellar sample and much increased precision in distances, 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 are as large-scale and small-scale groups, some of them have clear trends on metallicities, others are a mixture of both disk stars and based on that we discuss possible origin of each group.

Disentangling the Arcturus stream

“…The list of the centres of the peaks and the corresponding uncertainties are given in Table A.2 for scale J = 2 and in Table A.3 for scale J = 3. The fact that well-known groups like Sirius, Coma Berenices, Hyades, Pleiades and Hercules were identified at the expected positions shows that our method is sound (see Tables 3, 5, 6 and 7 in Kushniruk et al 2017 that summarise literature values for the U and V velocities of Sirius, Coma Berenices, Hyades, Pleiades and Hercules). These groups are detected in all four planes.…”

“…To search for kinematic structures the methodology described in Kushniruk et al (2017) was used with some additions. To detect over-densities a wavelet transform was applied to the stellar sample in the U − V, V − √ U 2 + 2V 2 , L z − L 2 x + L 2 y , and L z − √ J r planes.…”

“…The wavelet transform part as well as noise filtering from the output wavelet maps were performed in MR software 4 developed by CEA (Saclay, France) and Nice Observatory. More details on the algorithm itself can be found in Starck & Murtagh (2002), and more details on the methods used to search for overdensities and structures can be found in Kushniruk et al (2017).…”

“…Many studies have shown that the velocity distribution of stars in the Milky Way disk is clumpy (e.g. Dehnen 1998;Skuljan et al 1999;Famaey et al 2005;Antoja et al 2008Antoja et al , 2012Kushniruk et al 2017;Ramos et al 2018). The kinematic and chemical properties of such structures can be used to constrain the properties and the formation history of the Milky Way.…”

High-drag Interstellar Objects and Galactic Dynamical Streams

“…The dynamical LSR, defined as the circular orbit velocity at the Sun's location, would be the mean motion near the Sun for an axisymmetric galaxy. The Milky Way's velocity fields however are non-uniform, with a substantial fraction of the stars in the solar neighborhood being concentrated in unbound collections of stars called in this paper dynamical streams (but also known as associations or moving groups) (see, e.g., Famaey et al 2005;Kushniruk et al 2017;Gaia Collaboration et al 2018).…”