Walter Dehnen

Local kinematics and the local standard of rest

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 Vin the direction of Galactic rotation via Str¨ ombergs 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 )k m s −1 , with additional systematic uncertainties ∼(1, 2, 0.5) km s −1 . In particular, Vis 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 radial velocity experiment (RAVE): First data release

We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R 1⁄4 7500) in the Ca-triplet region (8410–8795 8) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9 < I < 12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including reobservations). Those data were obtained on 67 nights between 2003 April 11 and 2004 April 3. The total sky coverage within this data release is 4760 deg. The average signal-to-noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km s . Combining internal errors and zero-point errors, the mode is found to be 2 km s . Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km s . We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal-to-noise ratio. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2, and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS, and the TwoMicron All Sky Survey). The data release can be accessed via the RAVE Web site.

The RAVE survey: constraining the local Galactic escape speed

We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high-velocity stars from the RAVE survey and two previously published data sets. We use cosmological simulations of disc galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498 <v(esc) <608 km s(-1) (90 per cent confidence), with a median likelihood of 544 km s(-1). The fact that v(esc)(2) is significantly greater than 2v(circ)(2) (where v(circ) = 220 km s(-1) is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. We use our constraints on v(esc) to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of 1.42(-0.54)(+1.14) x 10(12) M-circle dot and virial radius of (90 per cent confidence). For this model the circular velocity at the virial radius is 142(-21)(+31) km s(-1). Although our halo masses are model dependent, we find that they are in good agreement with each other.

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar over-density, close to the Galactic plane at (l = 240 ◦ , b = 8 ◦ ), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this over-density is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disk. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits well the extant position, distance and velocity information on the “Galactic Ring”, as well as that of the M-giant overdensities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at approximately the Solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disk, as well as a vertical scale height similar to that of the thick disk. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disk, that the thick disk is continually growing, even up to the present time, and that thick disk globular clusters were accreted onto the Milky Way from dwarf galaxies in co-planar orbits.

Improving convergence in smoothed particle hydrodynamics simulations without pairing instability

The numerical convergence of smoothed particle hydrodynamics (SPH) can be severely restricted by random force errors induced by particle disorder, especially in shear flows, which are ubiquitous in astrophysics. The increase in the number NH of neighbours when switching to more extended smoothing kernels at fixed resolution (using an appropriate definition for the SPH resolution scale) is insufficient to combat these errors. Consequently, trading resolution for better convergence is necessary, but for traditional smoothing kernels this option is limited by the pairing (or clumping) instability. Therefore, we investigate the suitability of the Wendland functions as smoothing kernels and compare them with the traditional B-splines. Linear stability analysis in three dimensions and test simulations demonstrate that the Wendland kernels avoid the pairing instability for all NH, despite having vanishing derivative at the origin (disproving traditional ideas about the origin of this instability/ instead, we uncover a relation with the kernel Fourier transform and give an explanation in terms of the SPH density estimator). The Wendland kernels are computationally more convenient than the higher-order B-splines, allowing large NH and hence better numerical convergence (note that computational costs rise sub-linear with NH). Our analysis also shows that at low NH the quartic spline kernel with NH ~= 60 obtains much better convergence then the standard cubic spline.

The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution

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 affect the distribution function in the outer parts of a barred disk, and can we identify any effect of the resonance in the velocity distribution actually observed in the solar neighborhood? To answer these questions, detailed simulations of the velocity distribution, f(v), in the outer parts of an exponential stellar disk with nearly flat 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 f(v) over a significant fraction of the outer disk. For positions up to 2 kpc outside the OLR radius and at bar angles of ~10°–70°, 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 f(v) inferred from the Hipparcos data for late-type stars in the solar neighborhood. If one interprets this observed bimodality as induced by the OLR—and there are hardly any viable alternatives—then one is forced to deduce that the OLR radius is slightly smaller than R0. Moreover, by a quantitative comparison of the observed with the simulated distributions, one finds 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 significant, features in the observed f(v) resemble properties of the simulated velocity distributions, in particular a ripple caused by orbits trapped in the outer 1:1 resonance.

The Distribution of Nearby Stars in Velocity Space Inferred from Hipparcos Data

The velocity distribution f(v) of nearby stars is estimated, via a maximum likelihood algorithm, from the positions and tangential velocities of a kinematically unbiased sample of 14,369 stars observed by the Hipparcos satellite. The distribution f shows rich structure in the radial and azimuthal motions, vR and v, but not in the vertical velocity, vz: there are four prominent and many smaller maxima, many of which correspond to well-known moving groups. While samples of early-type stars are dominated by these maxima, also up to about a quarter of red main-sequence stars are associated with them. These moving groups are responsible for the vertex deviation measured even for samples of late-type stars; they appear more frequently for ever redder samples, and as a whole they follow an asymmetric drift relation, in the sense that those only present in red samples predominantly have large |vR| and lag in v with respect to the local standard of rest (LSR). The question arises, how did these old moving groups get on their eccentric orbits? A plausible mechanism known from solar system dynamics that is able to manage a shift in orbit space is sketched. This mechanism involves locking into an orbital resonance; in this respect is intriguing that Oorts constants, as derived from Hipparcos data, imply a frequency ratio between azimuthal and radial motion of exactly Ω:κ = 3:4. Apart from these moving groups, there is a smooth background distribution, akin to Schwarzschilds ellipsoidal model, with axis ratios σR:σ:σz ≈ 1:0.6:0.35. The contours are aligned with the vR-direction, but not with respect to the v- and vz-axes: the mean vz increases for stars rotating faster than the LSR. This effect can be explained by the stellar warp of the Galactic disk. If this explanation is correct, the warps inner edge must not be within the solar circle, while its pattern rotates with frequency 13 km s-1 kpc-1 retrograde with respect to the stellar orbits.

The distribution of nearby stars in velocity space inferred from Hipparcos data

The velocity distribution f(v) of nearby stars is estimated, via a maximum likelihood algorithm, from the positions and tangential velocities of a kinematically unbiased sample of 14,369 stars observed by the Hipparcos satellite. The distribution f shows rich structure in the radial and azimuthal motions, vR and v, but not in the vertical velocity, vz: there are four prominent and many smaller maxima, many of which correspond to well-known moving groups. While samples of early-type stars are dominated by these maxima, also up to about a quarter of red main-sequence stars are associated with them. These moving groups are responsible for the vertex deviation measured even for samples of late-type stars; they appear more frequently for ever redder samples, and as a whole they follow an asymmetric drift relation, in the sense that those only present in red samples predominantly have large |vR| and lag in v with respect to the local standard of rest (LSR). The question arises, how did these old moving groups get on their eccentric orbits? A plausible mechanism known from solar system dynamics that is able to manage a shift in orbit space is sketched. This mechanism involves locking into an orbital resonance; in this respect is intriguing that Oorts constants, as derived from Hipparcos data, imply a frequency ratio between azimuthal and radial motion of exactly Ω:κ = 3:4. Apart from these moving groups, there is a smooth background distribution, akin to Schwarzschilds ellipsoidal model, with axis ratios σR:σ:σz ≈ 1:0.6:0.35. The contours are aligned with the vR-direction, but not with respect to the v- and vz-axes: the mean vz increases for stars rotating faster than the LSR. This effect can be explained by the stellar warp of the Galactic disk. If this explanation is correct, the warps inner edge must not be within the solar circle, while its pattern rotates with frequency 13 km s-1 kpc-1 retrograde with respect to the stellar orbits.

A Hierarchical O(N) Force Calculation Algorithm

A novel code for the approximate computation of long-range forces between N mutually interacting bodies is presented. The code is based on a hierarchical tree of cubic cells and features mutual cell–cell interactions which are calculated via a Cartesian Taylor expansion in a symmetric way, such that total momentum is conserved. The code benefits from an improved and simple multipole acceptance criterion that reduces the force error and the computational effort. For N≳104, the computational costs are found empirically to rise sublinearly with N. For applications in stellar dynamics, this is the first competitive code with complexity O(N); it is faster than the standard tree code by a factor of 10 or more.

The Radial Velocity Experiment (RAVE)

We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity, and rotational velocity) of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). The RAVE program started in 2003, obtaining medium resolution specUniversity of Ljubljana, Faculty of Mathematics and Physics, Ljubljana, Slovenia Astrophysikalisches Institut Potsdam, Potsdam, Germany Observatoire de Strasbourg, Strasbourg, France INAF, Osservatorio Astronomico di Padova, Sede di Asiago, Italy RSAA, Australian national University, Canberra, Australia Anglo Australian Observatory, Sydney, Australia Johns Hopkins University, Baltimore MD, USA Macquarie University, Sydney, Australia Institute of Astronomy, University of Cambridge, UK e2v Centre for Electronic Imaging, School of Engineering and Design, Brunel University, Uxbridge, UK Astronomisches Rechen-Institut, Center for Astronomy of the University of Heidelberg, Heidelberg, Germany Kapteyn Astronomical Institute, University of Groningen, Groningen, the Netherlands University of Victoria, Victoria, Canada Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Australia Rudolf Pierls Center for Theoretical Physics, University of Oxford, UK Institute of Astronomy, School of Physics, University of Sydney, NSW 2006, Australia Sterrewacht Leiden, University of Leiden, Leiden, the Netherlands University of Leicester, Leicester, UK MPI fuer extraterrestrische Physik, Garching, Germany University of Central Lancashire, Preston, UK University of Rochester, Rochester NY, USA University of Edinburgh, Edinburgh, UK