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Register a new userVertical distribution of Galactic disk stars IV - AMR and AVR from clump giants
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C. Soubiran
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We present the parameters of 891 stars, mostly clump giants, including atmospheric parameters, distances, absolute magnitudes, spatial velocities, galactic orbits and ages. One part of this sample consists of local giants, within 100 pc, with atmospheric parameters either estimated from our spectroscopic observations at high resolution and high signal-to-noise ratio, or retrieved from the literature. The other part of the sample includes 523 distant stars, which we have estimated atmospheric parameters from high resolution but low signal-to-noise Echelle spectra. This new sample is kinematically unbiased, with well-defined boundaries in magnitude and colours. We revisit the basic properties of the Galactic thin disk as traced by clump giants. We find the metallicity distribution to be different from that of dwarfs, with less metal-rich stars. We find evidence for a vertical metallicity gradient of -0.31 dex/kpc and for a transition at 4-5 Gyr in both the metallicity and velocities. The age - metallicity relation (AMR), which exhibits a very low dispersion, increases smoothly from 10 to 4 Gyr, with a steeper increase for younger stars. The age-velocity relation (AVR) is characterized by the saturation of the V and W dispersions at 5 Gyr, and continuous heating in U.
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Nearly 400 Tycho-2 stars have been observed in a 720 square degree field in the direction of the North Galactic Pole with the high resolution echelle spectrograph ELODIE. Absolute magnitudes, effective temperatures, gravities and metallicities have been estimated, as well as distances and 3D velocities. Most of these stars are clump giants and span typical distances from 200pc to 800pc to the galactic mid-plane. This new sample, free of any kinematical and metallicity bias, is used to investigate the vertical distribution of disk stars. The old thin disk and thick disk populations are deconvolved from the velocity-metallicity distribution of the sample and their parameters are determined. The thick disk is found to have a moderate rotational lag of -51+-5 km/s with respect to the Sun with velocity ellipsoid (sigma_U, sigma_V, sigma_W)=(63+-6, 39+-4, 39+-4) km/s, mean metallicity of [Fe/H]=-0.48+-0.05 and a high local normalization of 15+-7%. Combining this NGP sample with a local sample of giant stars from the Hipparcos catalogue, the orientation of the velocity ellipsoid is investigated as a function of distance to the plane and metallicity. We find no vertex deviation for old stars, consistent with an axisymmetric Galaxy. Paper II is devoted to the dynamical analysis of the sample, puting new constraints on the vertical force perpendicular to the galactic plane and on the total mass density in the galactic plane.
High resolution spectra data of red clump stars towards the NGP have been obtained with the high resolution spectrograph Elodie at OHP for Tycho-2 selected stars. Combined with Hipparcos local analogues, we determine both the gravitational force law perpendicaular to the Galactic plane, and the total surface mass density and thickness of the Galactic disk. The surface mass density of the Galactic disk within 800 pc derived from this analysis is Sigma(|z|<800pc)=76 Msol.pc-2 and, removing the dark halo contribution, the total disk mass density is Sigma0=67 Msol.pc-2 at solar radius. The thickness of the total disk mass distribution is dynamicaly measured for the first time and is found to be 390pc in relative agreement with the old stellar disk scale height. All dynamical evidences concerning the structure of the disk (its local volume density -i.e. the Oort limit-, its surface density and its thickness) are compatible with our knowledge of the corresponding stellar disk properties.
The perturbation mechanism of the Galactic disk has puzzled us for a long time. The imprints from perturbations provide important diagnostics on the disk formation and evolution. Here we try to constrain when the vertical perturbation took place in the disk by tracking the phase mixing history. Firstly, we clearly depict the spiral structures of radial ($v_R$) and azimuthal ($v_{phi}$) velocities in the phase space of the vertical position and velocity ($z$-$v_z$) with 723,871 LAMOST-Gaia combined stars. Then, we investigate the variation of the spirals with stellar age ($tau$) by dividing the sample into seven stellar age bins. Finally, we find that the spirals explicitly exist in all the bins, even in the bin of $tau<0.5$,Gyr, except for the bin of $tau>6.0$,Gyr. This constrains the vertical perturbation probably starting no later than 0.5,Gyr ago. But we can not rule out whether the young stars ($tau<0.5$,Gyr) inherit the oscillations from the perturbed ISM where they born from. This study provides some important observational evidences to understand the disk perturbation mechanisms, even the formation and evolution of our Galaxy.
We seek to constrain the formation of the Galactic bulge by means of analysing the detailed chemical composition of a large sample of red clump stars in Baades window. We measure [Fe/H] in a sample of 219 bulge red clump stars from R=20000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT, using an automatic procedure, differentially to the metal-rich local reference star muLeo. For a subsample of 162 stars, we also derive [Mg/H] from spectral synthesis around the MgI triplet at 6319A. The Fe and Mg metallicity distributions are both asymmetric, with median values of +0.16 and +0.21 respectively. The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred around [Fe/H]=-0.30 and [Mg/H]=-0.06 with a large dispersion and a narrow metal-rich component centred around [Fe/H]=+0.32 and [Mg/H]=+0.35. The metal poor component shows high [Mg/Fe] ratios (around 0.3) whereas stars in the metal rich component are found to have near solar ratios. Babusiaux et al. (2010) also find kinematical differences between the two components: the metal poor component shows kinematics compatible with an old spheroid whereas the metal rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarios for the two populations: a rapid formation timescale as an old spheroid for the metal poor component (old bulge) and for the metal rich component, a formation over a longer time scale driven by the evolution of the bar (pseudo-bulge).
We have determined the wavelength dependence of the extinction in the near-infrared bands ($J$, $H$, $K_{mathrm S}$) toward the Galactic center from the VVV (VISTA Variables in the Via Lactea) aperture photometry of the stars in the region $|l|lesssim2^circ.0$ and $0^circ.5lesssim|b|lesssim1^circ.0$; this region consists of 12 VVV tiles. We have found significant systematic discrepancy up to $sim0.1$ mag between the stellar magnitudes of the same stars in overlapping VVV tiles. However, by carefully using the positions of red clump stars in color-magnitude diagrams as a tracer of the extinction and reddening, we are able to determine the average of the ratios of total to selective extinction to be $A(K_{mathrm S})/E(H-K_{mathrm S})=1.44pm0.04$, $A(K_{mathrm S})/E(J-K_{mathrm S})=0.423pm0.024$, $A(H)/E(J-H)=1.25pm0.04$; from these ratios, a steep power law $A(lambda)proptolambda^{-alpha}$ whose index $alpha$ is $sim2.0-2.3$ in the $J,H,K_{mathrm S}$ wavelength range is estimated. The obtained wavelength dependence is consistent with those obtained with the MKO photometric system employed in SIRIUS camera attached to the IRSF telescope in previous studies. Such a steep decline of extinction toward the longer wavelengths is also in line with recent results based on deep imaging surveys. The determined extinction law seems to be variable in the VVV tile to tile, and it is not clear how much of this is due to real sight line variations and due to observational systematic effects. Thus, there might be room for improvement of the extinction law determination from the existing VVV data, but this steep extinction law tends to locate heavily reddened objects in the Galactic plane more distant from us when their distance moduli are calculated from the observed reddening values.
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