Interstellar reddening towards six small areas in Puppis-Vela

We investigate the distribution of the interstellar dust towards six small volumes of the sky in the region of the Gum nebula. New high-quality four-colour uvby and H\beta\ Str\"omgren photometry obtained for 352 stars in six selected areas of Kapteyn, complemented with data obtained in a previous investigation for two of these areas, were used to estimate the colour excess and distance to these objects. The obtained colour excess versus distance diagrams, complemented with other information, when available, were analysed in order to infer the properties of the interstellar medium permeating the observed volumes. On the basis of the overall standard deviation in the photometric measurements, we estimate that colour excesses and distances are determined with an accuracy of 0.010 mag and better than 30%, respectively, for a sample of 520 stars. A comparison with 37 stars in common with the new Hipparcos catalogue attests to the high quality of the photometric distance determination. The obtained colour excess versus distance diagrams testify to the low density volume towards the observed lines-of-sight. Very few stars out to distances of 1 kpc from the Sun have colour excesses larger than E(b-y) = 0.1 mag. In spite of the low density character of the interstellar medium towards the Puppis-Vela direction, the obtained reddening as a function of the distance indicates that two or more interstellar structures are crossed towards the observed lines-of-sight. One of these structures may be associated with the very low density wall of the Local Cavity, which has a distance of 100-150 pc from the Sun. Another structure might be related to the Gum nebula, and if so, its front face would be located at about 350 pc from the Sun.Comment: Accepted by Astronomy and Astrophysic

The Bar and Spiral Structure Legacy (BeSSeL) Survey: Mapping the Milky Way with VLBI Astrometry

Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our Galaxy and to determine fundamental parameters such as the rotation velocity ($\Theta_0$) and curve and the distance to the Galactic center (R$_0$). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R$_0$ and $\Theta_0$ need a substantial revision. In particular the combination of 8.5 kpc and 220 \kms\, can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr\,A* gives best values of R$_0$ = 8.3 $\pm$ 0.23 kpc and $\Theta_0$ = 239 or 246 $\pm$ 7 \kms, for Solar motions of V$_ \odot$ = 12.23 and 5.25 \kms, respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) Survey.Comment: 6 pages, 3 figures. 'Highlight talk' held at the Meeting of the Astronomische Gesellschaft (2010). To be published in Reviews in Modern Astronomy, Volume 2

Thick disk kinematics from RAVE and the solar motion

Radial velocity surveys such as the Radial Velocity Experiment (RAVE) provide us with measurements of hundreds of thousands of nearby stars most of which belong to the Galactic thin, thick disk or halo. Ideally, to study the Galactic disks (both thin and thick) one should make use of the multi-dimensional phase-space and the whole pattern of chemical abundances of their stellar populations. In this paper, with the aid of the RAVE Survey, we study the thin and thick disks of the Milky Way, focusing on the latter. We present a technique to disentangle the stellar content of the two disks based on the kinematics and other stellar parameters such as the surface gravity of the stars. Using the Padova Galaxy Model, we checked the ability of our method to correctly isolate the thick disk component from the Galaxy mixture of stellar populations. We introduce selection criteria in order to clean the observed radial velocities from the Galactic differential rotation and to take into account the partial sky coverage of RAVE. We developed a numerical technique to statistically disentangle thin and thick disks from their mixture. We deduce the components of the solar motion relative to the Local Standard of Rest (LSR) in the radial and vertical direction, the rotational lag of the thick disk component relative to the LSR, and the square root of the absolute value of the velocity dispersion tensor for the thick disk alone. The analysis of the thin disk is presented in another paper. We find good agreement with previous independent parameter determinations. In our analysis we used photometrically determined distances. In the Appendix we show that similar values can be found for the thick disk alone as derived in the main sections of our paper even without the knowledge of photometric distances.Comment: accepted on A&A, please see companion paper "THIN disk kinem...

The origin and orbit of the old, metal-rich, open cluster NGC 6791: Insights from kinematics

NGC 6791 is a unique stellar system among Galactic open clusters being at the same time one of the oldest open clusters and the most metal rich. Combination of its properties is puzzling and poses question of its origin. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outwards to its current location. In this work we study the cluster's orbit and investigate the possible migration processes which might have displaced NGC 6791 to its present-day position, under the assumption that it actually formed in the inner disk. To this aim we performed integrations of NGC 6791's orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and disk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state-of-the art initial conditions for NGC 6791, we calculate 1000 orbits back in time for about 1 Gyr turning on and off different non-axisymmetric components of the global potential. We then compare statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 10^4 test-particles originating close to the Galactic Center (having initial galocentric radii in the range of 3-5 kpc) and undergoing radial migration during 8 Gyr of forward integration. We find that a model which incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC 6791 from the inner disk (galocentric radii of 3-5 kpc) to its present-day location. Such a model can provide orbital parameters which are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low.Comment: 11 pages, 9 figures, 7 tables, accepted for publication in A&A || v2: minor changes to match the published versio

Chemical gradients in the Milky Way from the RAVE data

Aims. 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. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane. 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. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (R-g) and the maximum distances from the Galactic plane reached by the stars along their orbits (Z(max)). We measured the chemical gradients as functions of R-g at different Z(max). Results. We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Z(max) < 0.4 kpc and 4.5 < R-g(kpc) < 9.5, the iron gradient for the RAVE sample is d[Fe/H]/dR(g) = -0.065 dex kpc(-1), whereas for the GCS sample it is d[Fe/H]/dR(g) = -0.043 dex kpc(-1) with internal errors of +/-0.002 and +/-0.004 dex kpc(-1), respectively. The gradients of the RAVE and GCS samples become flatter at larger Z(max). Conversely, the mock sample has a positive iron gradient of d[Fe/H]/dR(g) = +0.053 +/- 0.003 dex kpc(-1) at Z(max) < 0.4 kpc and remains positive at any Z(max). These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besancon model. In addition, the low metallicity and asymmetric drift of the thick disc causes a shift of the stars towards lower R-g and metallicity which, together with the thin-disc stars with a higher metallicity and R-g, generates a fictitious positive gradient of the full sample. The flatter gradient at larger Z(max) found in the RAVE and the GCS samples may therefore be due to the superposition of thin-and thick-disc stars, which mimicks a flatter or positive gradient. This does not exclude the possibility that the thick disc has no chemical gradient. 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

A deep WISE search for very late type objects and the discovery of two halo/thick-disc T dwarfs: WISE 0013+0634 and WISE 0833+0052

A method is defined for identifying late-T and Y dwarfs in WISE down to low values of signal-to-noise. This requires a WISE detection only in the W2-band and uses the statistical properties of the WISE multiframe measurements and profile fit photometry to reject contamination resulting from non-point-like objects, variables and moving sources. To trace our desired parameter space, we use a control sample of isolated non-moving non-variable point sources from the Sloan Digital Sky Survey (SDSS), and identify a sample of 158 WISEW2-only candidates down to a signal-to-noise limit of eight. For signal-to-noise ranges >10 and 8–10, respectively, ∼45 and ∼90 per cent of our sample fall outside the selection criteria published by the WISE team, mainly due to the type of constraints placed on the number of individual W2 detections. We present follow-up of eight candidates and identify WISE 0013+0634 and WISE 0833+0052, T8 and T9 dwarfs with high proper motion (∼1.3 and ∼1.8 arcsec yr−1). Both objects show a mid-infrared/near-infrared excess of ∼1–1.5 mag and are K band suppressed. Distance estimates lead to space motion constraints that suggest halo (or at least thick disc) kinematics. We then assess the reduced proper motion diagram of WISE ultracool dwarfs, which suggests that late-T and Y dwarfs may have a higher thick-disc/halo population fraction than earlier objects