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.

Context. The Arcturus stream is an over-density of stars in velocity space and its origin has been much debated recently without any clear conclusion. The (classical) dissolved open cluster origin is essentially refuted, instead the discussions try to distinguish between an accretion, a resonant, or an external-perturbation origin for the stream. As kinematic structures are observational footprints of ongoing and past dynamical processes in disk galaxies, resolving the nature of the Arcturus stream can provide clues to the formation history of the Milky Way and its stellar populations. Aims. We aim to characterise the kinematical and chemical properties of the Arcturus stream in order to resolve its origin. Methods. The space velocities, angular momenta and actions for a sample of more than 5.8 million stars, composed from Gaia DR2, are analysed with a wavelet transform method to characterise kinematic over-densities in the Galactic disk. The kinematic characteristics of each identified group is used to select possible members of the groups from the GALAH and APOGEE spectroscopic surveys to further study and constrain their chemical properties. Results. In the velocity and angular momentum spaces the already known Sirius, Pleiades, Hyades, Hercules, AF06, Arcturus and KFR08 streams are clearly identified. The Hercules stream appears to be a mixture of thin and thick disk stars. The Arcturus stream, as well as the AF06 and KFR08 streams, are low-velocity and low-angular momentum structures with chemical compositions similar to the thick disk. These three groups extend further from the Galactic plane compared to the Hercules stream. The detections of all the groups were spaced by approximately 20 − 30 km s −1 in azimuthal velocity. Conclusions. A wide spread of chemical abundances within the Arcturus stream indicates that the group is not a dissolved open cluster. Instead the Arcturus stream, together with the AF06 and KFR08 streams, are more likely to be part of a phase-space wave, that could have been caused by an ancient merger event. This conclusion is based on that the different structures are detected in steps of 20 − 30 km s −1 in azimuthal velocity, that the kinematic and chemical features are different from what is expected for bar-originated structures, and that the lower-velocity streams extend further from the disk than bar-originated structures.

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Context. We present a spectral analysis of the binary G 224-58 AB, which consists of the coolest M extreme subdwarf (esdM5.5) and a brighter primary (esdK5). This binary may serve as a benchmark for metallicity measurement calibrations and as a test bed for atmospheric and evolutionary models for esdM objects. Aims. We perform the analysis of optical and infrared spectra of both components to determine their parameters. Methods. We determine abundances primarily using high-resolution optical spectra of the primary. Other parameters were determined from the fits of synthetic spectra computed with these abundances to the observed spectra from 0.4 to 2.5 microns for both components. Results. We determine T eff = 4625 ± 100 K, log g = 4.5 ± 0.5 for the A component and T eff = 3200 ± 100 K, log g = 5.0 ± 0. We find consistent abundances with fits to the secondary albeit at lower signal to noise. Conclusions. Abundances of different elements in G 224-58 A and G 224-58 B atmospheres cannot be described by one metallicity parameter. The offset of ∼0.4 dex between the abundances derived from alpha element and iron group elements corresponds with our expectation for metal-deficient stars. We thus clarify that some indices used to date to measure metallicities for establishing esdM stars, based on CaH, MgH, and TiO band system strength ratios in the optical and H 2 O in the infrared, relate to abundances of alpha-element group rather than to iron peak elements. For metal deficient M dwarfs with [Fe/H] < −1.0, this provides a ready explanation for apparently inconsistent metallicities derived with different methods.

Context. The HR 1614 is an overdensity in velocity space and has for a long time been known as an old (∼2 Gyr) and metal-rich ([Fe/H] ≈ +0.2) nearby moving group that has a dissolving open cluster origin. The existence of such old and metal-rich groups in the solar vicinity is quite unexpected since the vast majority of nearby moving groups are known to be young. Aims. In the light of new and significantly larger data sets than ever before (astrometric, photometric, and spectroscopic), we aim to re-investigate the properties and origin of the HR 1614 moving group. If the HR 1614 overdensity is a dissolving cluster, its stars should represent a single-age and single-elemental abundance population. Methods. To identify and characterise the HR 1614 moving group we use astrometric data from Gaia DR2; distances, extinction, and reddening corrections from the StarHorse code; elemental abundances from the GALAH and APOGEE spectroscopic surveys; and photometric metallicities from the SkyMapper survey. Bayesian ages were estimated for the SkyMapper stars. Since the Hercules stream is the closest kinematical structure to the HR 1614 moving group in velocity space and as its origin is believed to be well-understood, we use the Hercules stream for comparison purposes. Stars that are likely to be members of the two groups were selected based on their space velocities. Results. The HR 1614 moving group is located mainly at negative U velocities, does not form an arch of constant energy in the U − V space, and is tilted in V. We find that the HR 1614 overdensity is not chemically homogeneous, but that its stars exist at a wide range of metallicities, ages, and elemental abundance ratios. They are essentially similar to what is observed in the Galactic thin and thick discs, a younger population (around 3 Gyr) that is metal-rich (−0.2  ≤  [Fe/H]  ≤  0.4) and alpha-poor. These findings are very similar to what is seen for the Hercules stream, which is believed to have a dynamical origin and consists of regular stars from the Galactic discs. Conclusions. The HR 1614 overdensity has a wide spread in metallicity, [Mg/Fe], and age distributions resembling the general properties of the Galactic disc. It should therefore not be considered a dissolving open cluster, or an accreted population. Based on the kinematic and chemical properties of the HR 1614 overdensity we suggest that it has a complex origin that could be explained by combining several different mechanisms such as resonances with the Galactic bar and spiral structure, phase mixing of dissolving spiral structure, and phase mixing due to an external perturbation.