Geneva-Copenhagen Survey Research Articles

Survey of Surveys

Context. In the present-day panorama of large spectroscopic surveys, the amount, diversity, and complexity of the available data continuously increase. The overarching goal of studying the formation and evolution of our Galaxy is hampered by the heterogeneity of instruments, selection functions, analysis methods, and measured quantities. Aims. We present a comprehensive catalogue, the Survey of Surveys (SoS), built by homogeneously merging the radial velocity (RV) determinations of the largest ground-based spectroscopic surveys to date, such as APOGEE, GALAH, Gaia-ESO, RAVE, and LAMOST, using Gaia as a reference. This pilot study serves to prove the concept and to test the methodology that we plan to apply in the future to the stellar parameters and abundance ratios as well. Methods. We have devised a multi-staged procedure that includes: (i) the cross match between Gaia and the spectroscopic surveys using the official Gaia cross-match algorithm, (ii) the normalisation of uncertainties using repeated measurements or the three-cornered hat method, (iii) the cross calibration of the RVs as a function of the main parameters on which depend (magnitude, effective temperature, surface gravity, metallicity, and signal-to-noise ratio) to remove trends and zero point offsets, and (iv) the comparison with external high-resolution samples, such as the Gaia RV standards and the Geneva-Copenhagen survey, to validate the homogenisation procedure and to calibrate the RV zero-point of the SoS catalogue. Results. We provide the largest homogenised RV catalogue to date, containing almost 11 million stars, of which about half come exclusively from Gaia and half in combination with the ground-based surveys. We estimate the accuracy of the RV zero-point to be about 0.16−0.31 km s−1 and the RV precision to be in the range 0.05−1.50 km s−1 depending on the type of star and on its survey provenance. We validate the SoS RVs with open clusters from a high resolution homogeneous samples and provide the systemic velocity of 55 individual open clusters. Additionally, we provide median RVs for 532 clusters recently discovered by Gaia data. Conclusions. The SoS is publicly available and ready to be applied to various research projects, such as the study of star clusters, Galactic archaeology, stellar streams, or the characterisation of planet-hosting stars, to name a few. We also plan to include survey updates and more data sources in future versions of the SoS.

Estimating the age–metallicity distribution of a stellar sample from the probability distributions of the individual stars

ABSTRACT Estimating age distributions, or star formation histories, of stellar populations in the Milky Way is important in order to study the evolution of trends in elemental abundances and kinematics. We build on previous work to develop an algorithm for estimating the age–metallicity distribution which uses the full age–metallicity probability density functions (PDFs) of individual stars. No assumptions are made about the shape of the underlying distribution, and the only free parameter of the algorithm is used to ensure a smooth solution. In this work, we use individual age–metallicity PDFs from isochrone fitting of stars with known metallicities. The method is tested with synthetic samples and is found to recover the input age–metallicity distribution more accurately than the distribution of individually estimated ages and metallicities. The recovered sample age distribution (SAD) is always more accurate than the distribution of individual ages, also when restricted to the most precise individual ages. By applying the method to the stars in the Geneva–Copenhagen survey, we detect a possible minimum in the star formation history of the Solar neighbourhood at an age of 10 Gyr which is not seen in the distribution of individual ages. Although we apply the method only to age–metallicity distributions, the algorithm is described more generally and can in principle be applied in other parameter spaces. It is also not restricted to individual parameter distributions from isochrone fitting, meaning that an SAD can be estimated based on individual age PDFs from other methods such as asteroseismology or gyrochronology.

A kinematical age for the interstellar object 1I/’Oumuamua

1I/'Oumuamua is the first interstellar object observed passing through the Solar System. Understanding the nature of these objects will provide crucial information about the formation and evolution of planetary systems, and the chemodynamical evolution of the Galaxy as a whole. We obtained the galactic orbital parameters of this object, considering 8 different models for the Galaxy, and compared it to those of stars of different ages from the Geneva-Copenhagen Survey (GCS). Assuming that the galactic orbital evolution of this object is similar to that of stars, we applied a Bayesian analyses and used the distribution of stellar velocities, as a function of age, to obtain a probability density function for the age of 'Oumuamua. We considered two models for the age-velocity dispersion relation (AVR): the traditional power law, fitted using data from the GCS; and a model that implements a second power law for younger ages, which we fitted using a sample of 153 Open Clusters (OCs). We find that the slope of the AVR is smaller for OCs than it is for field stars. Using these AVRs, we constrained an age range of 0.01-1.87 Gyr for 'Oumuamua and characterized a most likely age ranging between 0.20-0.45 Gyr, depending on the model used for the AVR. We also estimated the intrinsic uncertainties of the method due to not knowing the exact value of the Solar motion and the particularities of 1I/'Oumuamua's ejection.

Spectroscopy of Dwarf Stars Around the North Celestial Pole**Based on observations collected with the 1.65 m telescope and VUES spectrograph at the Molėtai Astronomical Observatory of Institute of Theoretical Physics and Astronomy, Vilnius University, for the SPFOT survey.

New space missions (e.g., NASA-TESS and ESA-PLATO) will perform an in-depth analysis of bright stars in large fields of the celestial sphere searching for extraterrestrial planets and investigating their host-stars. Asteroseismic observations will search for exoplanet-hosting stars with solar-like oscillations. In order to achieve all the goals, a full characterization of the stellar objects is important. However, accurate atmospheric parameters are available for less than 30% of bright dwarf stars of the solar neighborhood. In this study we observed high-resolution (R = 60,000) spectra for all bright (V < 8 mag) and cooler than F5 spectral class dwarf stars in the northern-most field of the celestial sphere with radius of 20° from the α(2000) = 161.°03 and δ(2000) = 86.°60 that is a center of one of the preliminary ESO-PLATO fields. Spectroscopic atmospheric parameters were determined for 140 slowly rotating stars, for 73% of them for the first time. The majority (83%) of the investigated stars are in the TESS object lists and all of them are in the preliminary PLATO field. Our results have no systematic differences when compared with other recent studies. We have 119 stars in common with the Geneva–Copenhagen Survey, where stellar parameters were determined photometrically, and find a 14 ± 125 K difference in effective temperatures, 0.01 ± 0.16 in log g, and −0.02 ± 0.09 dex in metallicities. Comparing our results for 39 stars with previous high-resolution spectral determinations, we find only a 7 ± 73 K difference in effective temperatures, 0.02 ± 0.09 in log g, and −0.02 ± 0.09 dex in metallicities. We also determined basic kinematic and orbital parameters for this sample of stars. From the kinematical point of view, almost all our stars belong to the thin disk substructure of the Milky Way. The derived galactocentric metallicity gradient is −0.066 ± 0.024 dex kpc−1 (2.5σ significance) and the vertical metallicity gradient is −0.102 ± 0.099 dex kpc−1 (1σ significance) that comply with the latest inside-out thin disk formation models, including those with stellar migration taken into account.

Stellar ages and masses in the solar neighbourhood: Bayesian analysis using spectroscopy and Gaia DR1 parallaxes

We present a Bayesian implementation of isochrone fitting in deriving stellar ages and masses, incorporating absolute K magnitude ($\rm M_K$) derived from 2MASS photometry and Gaia DR1 parallax and differentiation between initial bulk metallicity and present day surface metallicity, with allowance for incorporating further constraints (e.g., asteroseismology) when available. As a test, we re-computed stellar ages and masses of $\sim 4000$ stars in the solar neighbourhood from six well-studied literature samples using both Hipparcos and TGAS parallaxes. Our ages are found to be compatible with literature values but with reduced uncertainties in general. The inclusion of parallax-based $\rm M_K$ serves as an additional constraint on the derived quantities, especially when systematic errors in stellar parameters are underestimated. We reconstructed the age-metallicity relationship in the solar neighbourhood by re-analysing the Geneva-Copenhagen Survey with the inclusion of TGAS-parallaxes and initial bulk metallicity sampling. We found a flat trend for disk stars with ages $<$11\,Gyr but with smaller scatter at all ages compared to literature.

A method to estimate stellar ages from kinematical data

We present a method to build a probability density function (pdf) for the age of a star based on its peculiar velocities $U$, $V$ and $W$ and its orbital eccentricity. The sample used in this work comes from the Geneva-Copenhagen Survey (GCS) which contains both the spatial velocities, orbital eccentricities and isochronal ages for about $14\,000$ stars. Using the GCS stars, we fitted the parameters that describe the relations between the distributions of kinematical properties and age. This parametrization allows us to obtain an age probability from the kinematical data. From this age pdf, we estimate an individual average age for the star using the most likely age and the expected age. We have obtained the stellar age pdf for the age of $9\,102$ stars from the GCS and have shown that the distribution of individual ages derived from our method is in good agreement with the distribution of isochronal ages. We also observe a decline in the mean metallicity with our ages for stars younger than 7 Gyr, similar to the one observed for isochronal ages. This method can be useful for the estimation of rough stellar ages for those stars that fall in areas of the HR diagram where isochrones are tightly crowded. As an example of this method, we estimate the age of Trappist-1, which is a M8V star, obtaining the age of $t(UVW) = 12.50(+0.29-6.23)$ Gyr.

Asymmetric metallicity patterns in the stellar velocity space with RAVE

We explore the correlations between velocity and metallicity and the possible distinct chemical signatures of the velocity over-densities of the local Galactic neighbourhood. We use the large spectroscopic survey RAVE and the Geneva Copenhagen Survey. We compare the metallicity distribution of regions in the velocity plane ($v_R,v_\phi$) with that of their symmetric counterparts ($-v_R,v_\phi$). We expect similar metallicity distributions if there are no tracers of a sub-population (e.g., a dispersed cluster, accreted stars), if the disk of the Galaxy is axisymmetric, and if the orbital effects of the spiral arms and the bar are weak. We find that the metallicity-velocity space of the solar neighbourhood is highly patterned. A large fraction of the velocity plane shows differences in the metallicity distribution when comparing symmetric $v_R$ regions. The typical differences in the median metallicity are of $0.05$ dex with a statistical significance of at least $95\%$, and with values up to $0.6$ dex. For low azimuthal velocity $v_\phi$, stars moving outwards in the Galaxy have on average higher metallicity than those moving inwards. These include stars in the Hercules and Hyades moving groups and other velocity branch-like structures. For higher $v_\phi$, the stars moving inwards have higher metallicity than those moving outwards. The most likely interpretation of the metallicity asymmetry is that it is due to the orbital effects of the bar and the radial metallicity gradient of the disk. We present a simulation that supports this idea. We have also discovered a positive gradient in $v_\phi$ with respect to metallicity at high metallicities, apart from the two known positive and negative gradients for the thick and thin disks, respectively.

Search for streams in thick disk and halo of the Milky Way

Study on the substructures of Milky Way is very important because it can be used as the stage indicator in the formation process of the Galaxy. Substructures are made by clumps of stars with similar velocity, called as stellar streams. We searched for the stellar streams in thick disk and stellar halo of the Milky Way using data from Geneva-Copenhagen survey. We built velocity curves from data and applied the wavelet transform to find dense area in the velocity distribution. For the significace test, peaks on the wavelet transform of the data are reduced using wavelet transform based on Monte Carlo simulation. We found five streams on thick disk and two in stellar halo. Based on the metallicity distribution, it is shown that both thick disk and halo streams have similar range of metallicity, thus we classified them as dynamical stream.

Extended distribution functions for our Galaxy

We extend models of our Galaxy based on distribution functions (DFs) that are analytic functions of the action integrals to extended distribution functions (EDFs), which have an analytic dependence on metallicity as well. We use a simple, but physically-motivated, functional forms for the metallicity of the interstellar medium as a function of radius and time and for the star-formation rate, and a model for the diffusion of stars through phase space to suggest the required functional form of an EDF. We introduce a simple prescription for radial migration that preserves the overall profile of the disc while allowing individual stars to migrate throughout the disc. Our models explicitly consider the thin and thick discs as two distinct components separated in age. We show how an EDF can be used to incorporate realistic selection functions in models, and to construct mock catalogues of observed samples. We show that the selection function of the Geneva-Copenhagen Survey (GCS) biases in favour of young stars, which have atypically small random velocities. With the selection function taken into account our models produce good fits of the GCS data in chemo-dynamical space and the Gilmore and Reid (1983) density data. From our EDF, we predict the structure of the SEGUE G-dwarf sample. The kinematics are successfully predicted. The predicted metallicity distribution has too few stars with [Fe/H]$\simeq-0.5$ dex and too many metal-rich stars. A significant problem may be the lack of any chemical-kinematic correlations in our thick disc. We argue that EDFs will prove essential tools for the analysis of both observational data and sophisticated models of Galaxy formation and evolution.

Stellar substructures in the solar neighbourhood

We determine detailed elemental abundances in stars belonging to the so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the chemical composition with the Galactic thin- and thick-disc stars, with the GCS Group 2 and Group 3 stars, as well as with several kinematic streams of similar metallicities. The aim is to search for chemical signatures that might give information about the formation history of this kinematic group of stars. High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph (FIES) spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. The average value of [Fe/H] for the 37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be separated into three age subgroups. Along with the main 8- and 12-Gyr-old populations, a subgroup of stars younger than 5 Gyr can be separated as well. Abundances of oxygen, alpha-elements, and r-process dominated elements are higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has similar characteristics to that of the Galactic thick disc and differs slightly from those in Hercules, Arcturus, and AF06 stellar streams. The similar chemical composition of stars in Group 1, as well as in Group 2 and 3, with that in stars of the thick disc might suggest that their formation histories are linked. The chemical composition pattern together with the kinematic properties and ages of stars in the investigated GCS groups provide evidence of their common origin and possible relation to an ancient merging event. A gas-rich satellite merger scenario is proposed as the most likely origin.

Determination of the local standard of rest using the LSS-GAC DR1

We re-estimate the peculiar velocity of the Sun with respect to the local standard of rest (LSR) using a sample of local stars within 600 pc of the Sun, selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also named the Guoshoujing Telescope) Spectroscopic Survey of the Galactic Anti-centre (LSS-GAC). The sample consists of 94 332 FGK main-sequence stars with well-determined radial velocities and atmospheric parameters. To derive the LSR, two independent analyses are applied to the data. First, we determine the solar motion by comparing the observed velocity distribution to that generated with the analytic formulism of Schönrich & Binney that has been demonstrated to show excellent agreement with rigorous torus-based dynamics modelling by Binney & McMillan. Secondly, we propose that cold populations of thin disc stars, selected by applying an orbital eccentricity cut, can be directly used to determine the LSR without the need of asymmetric drift corrections. Both approaches yield consistent results of solar motion in the direction of Galactic rotation, V⊙, that are much higher than the standard value adopted hitherto, derived from Strömgren's equation. The newly deduced values of V⊙ are 1–2 km s−1 smaller than the more recent estimates derived from the Geneva–Copenhagen Survey (GCS) sample of stars in the solar neighbourhood (within 100 pc). We attribute the small difference to the presence of several well-known moving groups in the GCS sample that, fortunately, hardly affect the LSS-GAC sample. The newly derived radial (U⊙) and vertical (W⊙) components of the solar motion agree well with the previous studies. In addition, for all components of the solar motion, the values yielded by stars of different spectral types in the LSS-GAC sample are consistent with each other, suggesting that the local disc is well relaxed and that the LSR reported in the current work is robust. Our final recommended LSR is, (U⊙, V⊙, W⊙) = (7.01 ± 0.20, 10.13 ± 0.12, 4.95 ± 0.09) km s−1.

Fractal Properties of the Solar Neighborhood Based on the Geneva-Copenhagen Survey

Abstract This study analyzes the fractal properties of the space distribution of stars in the solar neighborhood, based on the Geneva-Copenhagen Survey data. We demonstrate the existence of dynamically weakly coupled stochastic fractal structures in the circumsolar stellar medium. The number of such structures in the real stellar medium on 3 to 20 pc scales significantly exceeds the number of similar structures in the corresponding simulated random distributions. The total number of fractal structures depends on their characteristic size – the scale factor (Mf). We demonstrate that the structuredness of the stellar medium increases with the scale factor reaching its maximum around Mf ~ 18 pc and has a minimum around Mf ~ 5 pc. For Mf = 5 pc, the number of fractal structures in the survey is equal to 688, while for Mf = 18−20 pc the corresponding number exceeds 4000, which differs by more than three standard deviations from the mean fractal dimension D for random distributions. Practically, no structures are observed in random distributions in this domain of fractal dimensions. The proportion of fractal structures with [Fe/H]&gt; 0 increases slowly with increasing scale factor and the number of such structures remains in the interval from 170 to 900. On the whole, the number of structures with [Fe/H]&lt; 0 varies with scale factor in the same way as the total number of structures, and lies in the interval from 530 (Mf = 5 pc) to 3200 (Mf = 20 pc). Such significant differences between the fractal properties of the real stellar medium and its random simulations are due to peculiar “gravitational viscosity” of the gravitating medium. The results reported may play an important part in the study of structural properties and dynamic processes in the Galaxy and in the investigation of relaxation processes in particular.

THE POWER SPECTRUM OF THE MILKY WAY: VELOCITY FLUCTUATIONS IN THE GALACTIC DISK

We investigate the kinematics of stars in the mid-plane of the Milky Way on scales between 25 pc and 10 kpc with data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), the Radial Velocity Experiment (RAVE), and the Geneva-Copenhagen Survey (GCS). Using red-clump stars in APOGEE, we determine the large-scale line-of-sight velocity field out to 5 kpc from the Sun in (0.75 kpc)^2 bins. The solar motion V_{sun-c} with respect to the circular velocity V_c is the largest contribution to the power on large scales after subtracting an axisymmetric rotation field; we determine the solar motion by minimizing the large-scale power to be V_{sun-c} = 24+/-1 (ran.)+/-2 (syst [V_c])+/-5 (syst. [large-scale]) km/s, where the systematic uncertainty is due to (a) a conservative 20 km/s uncertainty in V_c and (b) the estimated power on unobserved larger scales. Combining the APOGEE peculiar-velocity field with red-clump stars in RAVE out to 2 kpc from the Sun and with local GCS stars, we determine the power spectrum of residual velocity fluctuations in the Milky Way's disk on scales between 0.2/kpc < k < 40/kpc. Most of the power is contained in a broad peak between 0.2/kpc < k < 0.9/kpc. We investigate the expected power spectrum for various non-axisymmetric perturbations and demonstrate that the central bar with commonly used parameters but of relatively high mass can explain the bulk of velocity fluctuations in the plane of the Galactic disk near the Sun. Streaming motions ~10 km/s on >~3 kpc scales in the Milky Way are in good agreement with observations of external galaxies and directly explain why local determinations of the solar motion are inconsistent with global measurements.

Spectroscopic study of solar twins and analogues

Context. Many large stellar surveys have been and are still being carried out, providing huge amounts of data, for which stellar physical parameters will be derived. Solar twins and analogues provide a means to test the calibration of these stellar catalogues because the Sun is the best-studied star and provides precise fundamental parameters. Solar twins should be centred on the solar values. Aims. This spectroscopic study of solar analogues selected from the Geneva-Copenhagen Survey (GCS) at a resolution of 48,000 provides effective temperatures and metallicities for these stars. We test whether our spectroscopic parameters, as well as the previous photometric calibrations, are properly centred on the Sun. In addition, we search for more solar twins in our sample. Methods. The methods used in this work are based on literature methods for solar twin searches and on methods we developed in previous work to distinguish the metallicity-temperature degeneracies in the differential comparison of spectra of solar analogues versus a reference solar reflection spectrum. Results. We derive spectroscopic parameters for 148 solar analogues (about 70 are new entries to the literature) and verify with a-posteriori differential tests that our values are well-centred on the solar values. We use our dataset to assess the two alternative calibrations of the GCS parameters; our methods favour the latest revision. We show that the choice of spectral line list or the choice of asteroid or time of observation does not affect the results. We also identify seven solar twins in our sample, three of which are published here for the first time. Conclusions. Our methods provide an independent means to differentially test the calibration of stellar catalogues around the values of a well-known benchmark star, which makes our work interesting for calibration tests of upcoming Galactic surveys.

Dissecting simulated disc galaxies – II. The age–velocity relation

We study the relation between stellar ages and vertical velocity dispersion (the age-velocity relation, or AVR) in a sample of seven simulated disc galaxies. In our simulations, the shape of the AVR for stars younger than 9 Gyr depends strongly on the merger history at low redshift, with even 1:10 - 1:15 mergers being able to create jumps in the AVR (although these jumps might not be detectable if the errors on stellar ages are on the order of 30%). For galaxies with a quiescent history at low redshift, we find that the vertical velocity dispersion rises smoothly for ages up to 8-9 Gyr, following a power law with a slope of ~0.5, similar to what is observed in the solar neighbourhood by the Geneva-Copenhagen Survey. For these galaxies, we show that the slope of the AVR is not imprinted at birth, but is the result of subsequent heating. By contrast, in all our simulations, the oldest stars form a significantly different population, with a high velocity dispersion. These stars are usually born kinematically hot in a turbulent phase of intense mergers at high redshift, and also include some stars accreted from satellites. This maximum in velocity dispersion is strongly decreased when age errors are included, suggesting that observations can easily miss such a jump with the current accuracy of age measurements.

Stellar substructures in the solar neighbourhood

From correlations between orbital parameters, several new coherent groups of stars were recently identified in the Galactic disc and suggested to correspond to remnants of disrupted satellites. To reconstruct their origin at least three main observational parameters - kinematics, chemical composition and age - must be known. We determine detailed elemental abundances in stars belonging to the so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical composition with Galactic thin- and thick-disc stars, as well as with the Arcturus and AF06 streams. The aim is to search for chemical signatures that might give information about the formation history of this kinematic group of stars. High-resolution spectra were obtained with the FIES spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. The average value of [Fe/H] for the 32 stars of Group 2 is -0.42 +- 0.10 dex. The investigated group consists mainly of two 8- and 12-Gyr-old stellar populations. Abundances of oxygen, alpha-elements, and r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has similar characteristics as that of the Galactic thick-disc. The similarity in chemical composition of stars in Group 2 with that in stars of the thick-disc might suggest that their formation histories are linked. The chemical composition together with the kinematic properties and ages of stars in the investigated stars provides evidence of their common origin and possible relation to an ancient merging event. A gas-rich satellite merger scenario is proposed as the most likely origin. Groups 2 and 3 of the Geneva-Copenhagen Survey might have originated in the same merging event.

Solar analogues and solar twins in the HARPS archive★

We present 63 solar analogues and twins for which high signal-to-noise ratio (S/N) archival data are available for the High Accuracy Radial velocity Planet Searcher (HARPS) high-resolution spectrograph at the European Southern Observatory (ESO) 3.6-m telescope. We perform a differential analysis of these stellar spectra relative to the solar spectrum, similar to previous work using ESO 2.2-m/fiber-fed extended range optical spectrograph (FEROS) data, and expand our analysis by introducing a new method to test the temperature and metallicity calibration of Sun-like stars in the Geneva–Copenhagen Survey (GCS). The HARPS data are significantly better than the FEROS data, with improvements in S/N, spectral resolution and number of lines we can analyse. We confirm the offsets to the photometric scale found in our FEROS study. We confirm three solar twins found in the FEROS data as solar twins in the HARPS data, as well as identify six new twins.

3D maps of the local ISM from inversion of individual color excess measurements

Three-dimensional (3D) maps of the Galactic interstellar matter (ISM) are a potential tool of wide use, however accurate and detailed maps are still lacking. One of the ways to construct the maps is to invert individual distance-limited ISM measurements, a method we have here applied to measurements of stellar color excess in the optical. We have assembled color excess data together with the associated parallax or photometric distances to constitute a catalog of ~ 23,000 sightlines for stars within 2.5 kpc. The photometric data are taken from Stromgren catalogs, the Geneva photometric database, and the Geneva-Copenhagen survey. We also included extinctions derived towards open clusters. We applied, to this color excess dataset, an inversion method based on a regularized Bayesian approach, previously used for mapping at closer distances. We show the dust spatial distribution resulting from the inversion by means of planar cuts through the differential opacity 3D distribution, and by means of 2D maps of the integrated opacity from the Sun up to various distances. The mapping assigns locations to the nearby dense clouds and represents their distribution at the spatial resolution that is allowed by the dataset properties, i.e. of the order of ~10 pc close to the Sun and increasing to ~100 pc beyond 1 kpc. Biases towards nearby and/or weakly extincted stars make this dataset particularly appropriate to map the local and neighboring cavities, and to locate faint, extended nearby clouds, both goals that are difficult or impossible with other mapping methods. The new maps reveal a ~1 kpc wide empty region in the third quadrant in the continuation of the so-called CMa tunnel of the Local Cavity, a cavity that we identify as the Superbubble GSH238+00+09 detected in radio emission maps and that is found to be bounded by the Orion and Vela clouds.

Chemical gradients in the Milky Way from the RAVE data

Aim: We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besancon model. Methods: We analysed three different samples selected from three independent datasets: a sample of 19,962 dwarf stars selected from the RAVE database, a sample of 10,616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besancon model. We measured the chemical gradients as functions of the guiding radius (Rg) at different distances from the Galactic plane reached by the stars along their orbit (Zmax). Results: The chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Zmax<0.4 kpc and 4.5<Rg(kpc)<9.5, the iron gradient for the RAVE sample is d[Fe/H]/dRg=-0.065 dex kpc^{-1}, whereas for the GCS sample it is d[Fe/H]/dRg=-0.043 dex kpc^{-1} with internal errors +-0.002 and +-0.004 dex kpc^{-1}, respectively. The gradients of the RAVE and GCS samples become flatter at larger Zmax. Conversely, the mock sample has a positive iron gradient of d[Fe/H]/dRg=+0.053+-0.003 dex kpc^{-1} at Zmax<0.4 kpc and remains positive at any Zmax. These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besancon model. The discrepancies between the observational samples and the mock sample can be reduced by i) decreasing the density, ii) decreasing the vertical velocity, and iii) increasing the metallicity of the thick disc in the Besancon model.

Galactic orbits of Hipparcos stars: Classification of stars

The Galactic orbits of 27 440 stars of all classes with accurate coordinates and parallaxes of more than 3 mas from the Hipparcos catalogue, proper motions from the Tycho-2 catalogue, and radial velocities from the Pulkovo Compilation of Radial Velocities (PCRV) are analyzed. The sample obtained is much more representative than the Geneva-Copenhagen survey and other studies of Galactic orbits in the solar neighborhood. An estimation of the influence of systematic errors in the velocities on orbital parameters shows that the errors of the proper motions due to the duplicity of stars are tangible only in the statistics of orbital parameters for very small samples, while the errors of the radial velocities are noticeable in the statistics of orbital parameters for halo stars. Therefore, previous studies of halo orbits may be erroneous. The distribution of stars in selection-free regions of the multidimensional space of orbital parameters, dereddened colors, and absolute magnitudes is considered. Owing to the large number of stars and the high accuracy of PCRV radial velocities, nonuniformities of this distribution (apart from the well-known dynamical streams) have been found. Stars with their peri- and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the halo, and perigalacticons in the disk and apogalacticons in the halo have been identified. Thus, the bulge and the halo are inhomogeneous structures, each consisting of at least two populations. The radius of the bulge has been determined as 2 kpc.