The distribution of nearby stars in phase space mapped by Hipparcos III. Clustering and streaming among A-F type stars

This paper presents the detailed results obtained in the search of density- velocity inhomogeneities in a volume limited and absolute magnitude limited sample of A-F type dwarfs within 125 parsecs of the Sun. A 3-D wavelet analysis is used to extract inhomogeneities, both in the density and velocity distributions. Having established a real picture of the phase space without assumption we come back to previously known observational facts regarding clusters and associations, superclusters. In the 3-D position space, well known open clusters (Hyades, Coma Berenices and Ursa Major), associations (parts of the Scorpio-Centaurus association) as well as the Hyades evaporation track are retrieved. Three new probably loose clusters are identified (Bootes, Pegasus 1 and 2). The sample is relatively well mixed in the position space since less than 7 per cent of the stars belong to structures with coherent kinematics, most likely gravitationally bound. In the velocity space, the majority of large scale velocity structures ($\sigma$ ~ 6.3 $km s-1$) are Eggen's superclusters (Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association. A new supercluster-like structure is found with a mean velocity between the Sun and Sirius SCl velocities. These structures are all characterized by a large age range which reflects the overall sample age distribution. Moreover, a few old streams of ~ 2 Gyr are also extracted at this scale with high U components. We show that all these large velocity dispersion structures represent 46% of the sample. Smaller scales (\sigma ~ 3.8 and 2.4 $km s-1$) reveal that superclusters are always substructured by 2 or more streams which generally exhibit a coherent age distribution. Percentages of stars in these streams are 38% and 18% respectively.Comment: 25 pages, Latex, 29 figures, 4 tables to be published in A&A Supplements Serie

Galactic Spiral Structure

We describe the structure and composition of six major stellar streams in a population of 20 574 local stars in the New Hipparcos Reduction with known radial velocities. We find that, once fast moving stars are excluded, almost all stars belong to one of these streams. The results of our investigation have lead us to re-examine the hydrogen maps of the Milky Way, from which we identify the possibility of a symmetric two-armed spiral with half the conventionally accepted pitch angle. We describe a model of spiral arm motions which matches the observed velocities and composition of the six major streams, as well as the observed velocities of the Hyades and Praesepe clusters at the extreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned at a focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiral arm just before apocentre, follow the arm for more than half an orbit, and leave the arm soon after pericentre. Spiral pattern speed equals the mean rate of precession of apocentre. Spiral arms are shown to be stable configurations of stellar orbits, up to the formation of a bar and/or ring. Pitch angle is directly related to the distribution of orbital eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve to form bars and rings. We show that orbits of gas clouds are stable only in bisymmetric spirals. We conclude that spiral galaxies evolve toward grand design two-armed spirals. We infer from the velocity distributions that the Milky Way evolved into this form about 9 Gyrs ago.Comment: Published in Proc Roy Soc A. A high resolution version of this file can be downloaded from http://papers.rqgravity.net/SpiralStructure.pdf. A simplified account with animations begins at http://rqgravity.net/SpiralStructur

Reconstructing the Star Formation History of the Galaxy

The evolution of the star formation rate in the Galaxy is one of the key ingredients quantifying the formation and determining the chemical and luminosity evolution of galaxies. Many complementary methods exist to infer the star formation history of the components of the Galaxy, from indirect methods for analysis of low-precision data, to new exact analytic methods for analysis of sufficiently high quality data. We summarise available general constraints on star formation histories, showing that derived star formation rates are in general comparable to those seen today. We then show how colour-magnitude diagrams of volume- and absolute magnitude-limited samples of the solar neighbourhood observed by Hipparcos may be analysed, using variational calculus techniques, to reconstruct the local star formation history. The remarkable accuracy of the data coupled to our maximum-likelihood variational method allows objective quantification of the local star formation history with a time resolution of ~ 50 Myr. Over the past 3Gyr, the solar neighbourhood star formation rate has varied by a factor of ~ 4, with characteristic timescale about 0.5Gyr, possibly triggered by interactions with spiral arms.Comment: 12 pages, Proc. of the Sept. 20-24, 1999 Vulcano Workshop ``The chemical evolution of the Milky Way: stars vs. clusters'', eds. F. Matteucci & F. Giovanell

The distribution of nearby stars in phase space mapped by Hipparcos: I. The potential well and local dynamical mass

Hipparcos data provide the first, volume limited and absolute magnitude limited homogeneous tracer of stellar density and velocity distributions in the solar neighbourhood. The density of A-type stars more luminous than $M_v=2.5$ can be accurately mapped within a sphere of 125 pc radius, while proper motions in galactic latitude provide the vertical velocity distribution near the galactic plane. The potential well across the galactic plane is traced practically hypothesis-free and model-free. The local dynamical density comes out as \rho_{0}=0.076 \pm0.015~M_{\sun}~{pc}^{-3} a value well below all previous determinations leaving no room for any disk shaped component of dark matter.Comment: 24 pages, 13 figures, latex. To appear in A&A (main journal

On the age heterogeneity of the Pleiades, Hyades and Sirius moving groups

We investigate the nature of the classical low-velocity structures in the local velocity field, i.e. the Pleiades, Hyades and Sirius moving groups. After using a wavelet transform to locate them in velocity space, we study their relation with the open clusters kinematically associated with them. By directly comparing the location of moving group stars in parallax space to the isochrones of the embedded clusters, we check whether, within the observational errors on the parallax, all moving group stars could originate from the on-going evaporation of the associated cluster. We conclude that, in each moving group, the fraction of stars making up the velocity-space overdensity superimposed on the background is higher than the fraction of stars compatible with the isochrone of the associated cluster. These observations thus favour a dynamical (resonant) origin for the Pleiades, Hyades and Sirius moving groups.Comment: 8 pages, 8 figures, accepted for publication in A&

Asteroseismology of exoplanets host stars: the special case of ι\iota Horologii (HD17051)

{This paper presents detailed analysis and modelisation of the star HD17051 (alias $\iota$ Hor), which appears as a specially interesting case among exoplanet host stars. As most of these stars, $\iota$ Hor presents a metallicity excess which has been measured by various observers who give different results, ranging from [Fe/H] = 0.11 to 0.26, associated with different atmospheric parameters. Meanwhile the luminosity of the star may be determined owing to Hipparcos parallax. Although in the southern hemisphere, this star belongs to the Hyades stream and its external parameters show that it could even be one of the Hyades stars ejected during cluster formation. The aim of this work was to gather and analyse our present knowledge on this star and to prepare seismic tests for future observations with the HARPS spectrometer (planned for November 2006).} {We have computed evolutionary tracks with various metallicities, in the two frameworks of primordial overmetallicity and accretion. We have concentrated on models inside the error boxes given by the various observers in the log g - log T$_{eff}$ diagram. We then computed the adiabatic oscillation frequencies of these models to prepare future observations.} {The detailed analysis of $\iota$ Hor presented in this paper already allowed to constrain its external parameters, mass and age. Some values given in the literature could be rejected as inconsistent with the overall analysis. We found that a model computed with the Hyades parameters (age, metallicity) was clearly acceptable, but other ones were possible too. We are confident that observations with HARPS will allow for a clear conclusion about this star and that it will bring important new light on the physics of exoplanet host stars.}Comment: to be published in Astronomy and Astrophysic

The Pattern Speed of the Galactic Bar

Most late-type stars in the solar neighborhood have velocities similar to the local standard of rest (LSR), but there is a clearly separated secondary component corresponding to a slower rotation and a mean outward motion. Detailed simulations of the response of a stellar disk to a central bar show that such a bi-modality is expected from outer-Lindblad resonant scattering. When constraining the run of the rotation curve by the proper motion of Sgr A* and the terminal gas velocities, the value observed for the rotation velocity separating the two components results in a value of (53+/-3)km/s/kpc for the pattern speed of the bar, only weakly dependent on the precise values for Ro and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter

Identification of Moving Groups and Member Selection using Hipparcos Data

A new method to identify coherent structures in velocity space --- moving groups --- in astrometric catalogues is presented: the Spaghetti method. It relies on positions, parallaxes, and proper motions and is ideally suited to search for moving groups in the Hipparcos Catalogue. No radial velocity information is required. The method has been tested extensively on synthetic data, and applied to the Hipparcos measurements for the Hyades and IC2602 open clusters. The resulting lists of members agree very well with those of Perryman et al. for the Hyades and of Whiteoak and Braes for IC2602.Comment: 14 pages, 9 encapsulated postscript figures, LaTeX using mn.sty; accepted for publication in the MNRA