Velocity distribution of stars in the solar neighbourhood

A two-dimensional velocity distribution in the UV plane has been obtained for stars in the solar neighbourhood, using Hipparcos astrometry for over 4000 'survey' stars with parallaxes greater than 10 mas and radial velocities found in the Hipparcos Input Catalogue. In addition to the already known grouping characteristics (field stars plus young moving groups), the velocity distribution seems to exhibit a more complex structure characterized by several longer branches running almost parallel to each other across the UV plane. By using the wavelet transform technique to analyse the distribution, the branches are visible at relatively high significance levels of 90 per cent or higher. They are roughly equidistant with a separation of about 15 km s⁻¹ for early-type stars and about 20 km s⁻¹ for late-type stars, creating an overall quasi-periodic structure which can also be detected by means of a two-dimensional Fourier transform. This branch-like velocity distribution might be caused by the Galactic spiral structure.     

A revision of the solar neighbourhood metallicity distribution

We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centred on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal-rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood have metallicities higher than [Fe/H]=0 . The percentage of mid-metal-poor stars ([Fe/H]<−0.5) is approximately 4 per cent, in agreement with present estimates of the thick disc.
In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections resulting from the increasing thickness of the stellar disc with age, we show that the simple closed-box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age–metallicity relation and abundance ratios suggest that the simple closed-box model may be a viable model of the chemical evolution of the Galaxy at solar radius.     

A cosmological study of the star formation history in the solar neighbourhood

We use a cosmological galactic evolutionary approach to model the Milky Way. A detailed treatment of the mass aggregation and dynamical history of the growing dark halo is included, together with a self-consistent physical treatment for the star formation processes within the growing galactic disc. This allows us to calculate the temporal evolution of star and gas surface densities at all galactic radii, in particular, the star formation history (SFH) at the solar radius. A large range of cosmological mass aggregation histories (MAHs) is capable of producing a galaxy with the present-day properties of the Milky Way. The resulting SFHs for the solar neighbourhood bracket the available observational data for this feature, the most probable MAH yielding the optimal comparison with these observations. We also find that the rotation curve for our Galaxy implies the presence of a constant density core in its dark-matter halo.     

Kinematics of SDSS subdwarfs: structure and substructure of the Milky Way halo

We construct a new sample of ∼1700 solar neighbourhood halo subdwarfs from the Sloan Digital Sky Survey (SDSS), selected using a reduced proper-motion diagram. Radial velocities come from the SDSS spectra and proper motions from the light-motion curve catalogue of Bramich et al. Using a photometric parallax relation to estimate distances gives us the full phase-space coordinates. Typical velocity errors are in the range  30–50 km s⁻¹  . This halo sample is one of the largest constructed to date and the disc contamination is at a level of ≲1 per cent. This enables us to calculate the halo velocity dispersion to excellent accuracy. We find that the velocity dispersion tensor is aligned in spherical polar coordinates and that  (σ _r , σ_φ, σ_θ) = (143 ± 2, 82 ± 2, 77 ± 2) km s⁻¹  . The stellar halo exhibits no net rotation, although the distribution of   v _φ  shows tentative evidence for asymmetry. The kinematics are consistent with a mildly flattened stellar density falling with distance like   r ^−3.75  .
Using the full phase-space coordinates, we look for signs of kinematic substructure in the stellar halo. We find evidence for four discrete overdensities localized in angular momentum and suggest that they may be possible accretion remnants. The most prominent is the solar neighbourhood stream previously identified by Helmi et al., but the remaining three are new. One of these overdensities is potentially associated with a group of four globular clusters (NGC 5466, NGC 6934, M2 and M13) and raises the possibility that these could have been accreted as part of a much larger progenitor.     

Determination of the IMF on the basis of a recently derived SFR history in the solar neighbourhood

The initial mass function (IMF) in the solar neighbourhood is determined on the basis of a recently derived history of the star formation rate (SFR) which shows the presence of a star formation burst about 8 Gyr ago. The observed present-day mass function (PDMF) is considered, and the average past distribution of stars of a given mass is estimated. Two cases are considered, namely (i) constant SFR, and (ii) variable SFR as derived from the new metallicity distribution of G dwarfs. The resulting IMF is compared with previous determinations by Scalo and Kroupa et al., and the variation with stellar mass of the slope of the IMF is compared with reference determinations in the literature.     

The recent star formation history of the Hipparcos solar neighbourhood

We use data from the Hipparcos catalogue to construct colour–magnitude diagrams for the solar neighbourhood, which are then treated using advanced Bayesian analysis techniques to derive the star formation rate history, SFR ( t ), of this region over the last 3 Gyr. The method we use allows the recovery of the underlying SFR ( t ) without the need of assuming any a priori structure or condition on SFR ( t ), and hence yields a highly objective result. The remarkable accuracy of the data permits the reconstruction of the local SFR ( t ) with an unprecedented time resolution of ≈50 Myr. An SFR ( t ) that has an oscillatory component of period ≈0.5 Gyr is found, superimposed on a small level of constant star formation activity. Problems arising from the non-uniform selection function of the Hipparcos satellite are discussed and treated. Detailed statistical tests are then performed on the results, which confirm the inferred SFR ( t ) to be compatible with the observed distribution of stars.     

The local axisymmetric instability criterion in a thin, rotating, multicomponent disc

Purely gravitational perturbations are considered in a thin rotating disc composed of gas and several stellar components. The dispersion relation for the axisymmetric density waves propagating through the disc is found and the criterion for the local axisymmetric stability of the whole system is formulated. In the appropriate limit of two-component gas we confirm the findings of Jog & Solomon and extend consideration to the case when one component is collisionless. Gravitational stability of the Galactic disc in the solar neighbourhood based on the multicomponent instability condition is explored using recent measurements of the stellar composition and kinematics in the local Galactic disc obtained by the Hipparcos satellite.     

Information from the Kinematics of F and G Stars in the Solar Neighborhood

We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp     

Semiregular variables in the solar neighbourhood

Period–luminosity sequences have been shown to exist among the semiregular variables (SRVs) in the Magellanic Clouds, the bulge of the Milky Way galaxy and elsewhere. Using modern-period and revised Hipparcos parallax data, this paper demonstrates that they also appear among the M giant SRVs of the solar neighbourhood. Their distribution in the   K , log  P   diagram resembles that of Bulge stars more closely than those in the Magellanic Clouds. The prevalence of mass-loss among local M-type SRVs and its dependence on period and spectral subtype are also discussed.   K −[12]  , a measure of circumstellar dust emission, increases clearly with V amplitude, M giant subtype and  log  P   .     

Kinematics of stellar populations with RAVE data

We study the kinematics of the Galactic thin and thick disk populations using stars from the RAVE survey’s second data release together with distance estimates from Breddels et al. (2010). The velocity distribution exhibits the expected moving groups present in the solar neighborhood. We separate thick and thin disk stars by applying the X (stellar-population) criterion of Schuster et al. (1993), which takes into account both kinematic and metallicity information. For 1906 thin disk and 110 thick disk stars classified in this way, we find a vertical velocity dispersion, mean rotational velocity and mean orbital eccentricity of (σ_W, 〈V_Φ〉, 〈e〉)_thin = (18 ± 0.3 km s⁻¹, 223 ± 0.4 km s⁻¹, 0.07 ± 0.07) and (σ_W, 〈V_Φ〉, 〈e〉)_thick = (35 ± 2 km s⁻¹, 163 ± 3 km s⁻¹, 0.31 ± 0.16), respectively. From the radial Jeans equation, we derive a thick disk scale length in the range 1.5-2.2 kpc, whose greatest uncertainty lies in the adopted form of the underlying potential. The shape of the orbital eccentricity distribution indicates that the thick disk stars in our sample most likely formed in situ with minor gas-rich mergers and/or radial migration being the most likely cause for their orbits. We further obtain mean metal abundances of 〈[M/H]〉_thin = +0.03 ± 0.17, and 〈[M/H]〉_thick = −0.51 ± 0.23, in good agreement with previous estimates. We estimate a radial metallicity gradient in the thin disk of −0.07 dex kpc⁻¹, which is larger than predicted by chemical evolution models where the disk grows inside-out from infalling gas. It is, however, consistent with models where significant migration of stars shapes the chemical signature of the disk, implying that radial migration might play at least part of a role in the thick disk’s formation.     

Kinematics of the Galactic halo from horizontal branch stars in the Hamburg/ESO survey

Large samples of field horizontal branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ∼7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially dominated orbits to tangentially dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic Centre might help to differentiate them.     

Kinematics and metallicity analysis for nearby F,G and K stars

A sample containing 1 026 stars of spectral types F, G, and K, mainly dwarfs, from the solar neighbourhood with available space velocities and metallicities is treated. The treatment comprises a statistical analysis of the metallicity and velocity data and calculation of galactocentric orbits. Sample stars identified as members of the galactic halo are detached from the rest of the sample based on the values of their metallicities, velocity components and galactocentric orbits. In identifying halo stars a new, kinematical, criterion is proposed. Except one, these halo stars are the metal‐poorest ones in the sample. Besides, they have very high velocities with respect to LSR. On the other hand, the separation between the thin disc and thick one is done statistically based on LSR space velocities, membership probability (Schwarzschild distribution with assumed parameters) and galactocentric orbits. In the metallicity these two groups are not much different. For each of the three subsamples the mean motion and velocity ellipsoid are calculated. The elements of the velocity ellipsoids agree well with the values found in the literature, especially for the thin disc. The fractions of the subsystems found for the present sample are: thin disc 93%, thick disc 6%, halo 1%. The sample stars established to be members of the thin disc are examined for existence of star streams. Traces of both, known and unknown, star streams are not found (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)