We re-examine the stellar kinematics of the solar neighbourhood in terms of the velocity υ of the Sun with respect to the local standard of rest. We show that the classical determination of its component V in the direction of Galactic rotation via Strömberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U, V, W) = (11.1 +0.69 −0.75 , 12.24 +0.47 −0.47 , 7.25 +0.37 −0.36 ) km s −1 , with additional systematic uncertainties ∼(1, 2, 0.5) km s −1 . In particular, V is 7 km s −1 larger than previously estimated. The new values of (U, V, W) are extremely insensitive to the metallicity gradient within the disc.

The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of %10 6 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg 2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux-and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R % 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

(shortened) From a kinematically unbiased subsample of the Hipparcos catalogue we have redetermined as a function of colour the kinematics of main-sequence stars. The stars' mean heliocentric velocity nicely follows the asymmetric drift relation, except for stars blueward of B-V=0.1. Extrapolating to zero dispersion yields for the velocity of the Sun w.r.t. the LSR in km/s: U_0=10.00+/-0.36 (radially inwards), V_0=5.23+/-0.62 (in direction of galactic rotation), and W_0=7.17+/-0.38 (vertically upwards). A plot of velocity dispersion vs. colour beautifully shows Parenago's discontinuity: the dispersion is constant for B-V>0.62 and decreases towards bluer colour. We determine the velocity-dispersion tensor sigma^2_ij as function of B-V. The mixed moments involving vertical motion are zero within the errors, while sigma^2_xy is non-zero at about (10km/s)^2 independent of colour. The resulting vertex deviations are about 20 deg for early-type stars and 10+/-4 deg for old-disc stars. The persistence of the vertex deviation to late-type stars implies that the Galactic potential is significantly non-axisymmetric at the solar radius. If spiral arms are responsible for this, they cannot be tightly wound. Except for stars bluer than B-V=0.1 the ratios of the principal velocity dispersions are 2.2 : 1.4 :1, while the absolute values increase with colour from sigma_1=20km/s at B-V=0.2 to sigma_1=38km/s at Parenago's discontinuity and beyond. These ratios imply significant heating of the disc by spiral structure and that R_0/R_d=3 to 3.5, where R_d is the scale length of the disc.Comment: 7 pages, LaTeX, mn.sty, 6 postscript figures, submitted to MNRA

A parameterized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scale length R d, * of the stellar disk to R 0 . The disk and bulge dominate v c (R) at R < ∼ R 0 only for R d, * /R 0 < ∼ 0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R ≪ R 0 to affect profoundly the structure of the best-fitting model at R ≫ R 0 . For example, changing the disk slightly from an exponential surface-density profile significantly changes the form of v c (R) at R ≫ R 0 , where the disk makes a negligible contribution to v c . Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct halos rather than outwards-increasing mass-to-light ratios.The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z > ∼ R and at R > R 0 are especially important.

Hydrodynamic modeling of the inner Galaxy suggests that the radius of the outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the Sun. How does this resonance a †ect the distribution function in the outer parts of a barred disk, and can we identify any e †ect of the resonance in the velocity distribution actually observed in the solar neighborhood ? To answer these questions, detailed simulations of the velocity distribution, in the outer parts of an exponential stellar disk with nearly Ñat f (¿), rotation curve and a rotating central bar have been performed. For a model resembling the old stellar disk, the OLR causes a distinct feature in over a signiÐcant fraction of the outer disk. For positions f (¿) up to 2 kpc outside the OLR radius and at bar angles of D10¡È70¡, this feature takes the form of a bimodality between the dominant mode of low-velocity stars centered on the local standard of rest (LSR) and a secondary mode of stars predominantly moving outward and rotating more slowly than the LSR. Such a bimodality is indeed present in inferred from the Hipparcos data for late-type stars in the f (¿) solar neighborhood. If one interprets this observed bimodality as induced by the OLRÈand there are hardly any viable alternativesÈthen one is forced to deduce that the OLR radius is slightly smaller than Moreover, by a quantitative comparison of the observed with the simulated distributions, one Ðnds R 0 . that the pattern speed of the bar is 1.85^0.15 times the local circular frequency, where the error is dominated by the uncertainty in bar angle and local circular speed. Also, other, less prominent but still signiÐcant, features in the observed resemble properties of the simulated velocity distributions, in f (¿) particular a ripple caused by orbits trapped in the outer 1 : 1 resonance.