Thick Disk Population Research Articles

The nature of the KFR08 stellar stream

The origin of a new kinematically identified metal-poor stellar stream, the KFR08 stream, has not been established. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Ni, Zn, Sr, Y, Zr, Ba, La, and Eu for 16 KFR08 stream members based on analysis of high resolution spectra. Based on the abundance ratios of 14 elements, we use the chemical tagging method to identify the stars which have the same chemical composition, and thus, might have a common birthplace, such as a cluster. Although three stars were tagged with similar elemental abundances ratios, we find that, statistically, it is not certain that they originate from a dissolved star cluster. This conclusion is consistent with the large dispersion of [Fe/H] ($\sigma_{\rm{[Fe/H]}} = 0.29$) among the 16 stream members. We find that our stars are $\alpha$ enhanced and that the abundance patterns of the stream members are well matched to the thick disk. In addition, most of the stream stars have estimated stellar ages larger than 11 Gyr. These results, together with the hot kinematics of the stream stars, suggest that the KFR08 stream is originated from the thick disk population which was perturbed by a massive merger in the early universe.

THE SULPHUR ABUNDANCE BEHAVIOUR IN THE GALACTIC DISC STARS

The sulphur abundances in the atmospheres of the F-G-K-type dwarf stars that belong to the thin and thick disc populations (in the metallicity range – 1.0 < [Fe/H] < 0.3) were determined. The observations were conducted using the 1.93 m telescope at Observatoire de Haute-Provence (OHP, France) equipped with the echelle-type spectrograph ELODIE (R = 40 000). The abundances were derived under the LTE approximation; the synthetic spectrum for the sulphur lines in the region of 6743-6762 AA was computed accounting for the hyperfine structure. The correlation between the abundances of some elements and metallicity [Fe/H] in the Galactic thin and thick disc stars was analyzed. To discuss the sources of sulphur production, the comparison between the sulphur abundance trend and current theoretical computations of the Galactic chemical evolution was made.

Ultracool white dwarfs and the age of the Galactic disc★

We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\rm eff}$ < 4000 K) white dwarfs (WDs) in the solar neighbourhood. For the first time, a large number of cool and ultracool WDs have distance and tangential velocities measurements available. Our targets have distances ranging from 21 pc to >100 pc, and include five stars within 30 pc. Contrary to expectations, all but two of them have tangential velocities smaller than 150 km s$^{-1}$ thus suggesting Galactic disc membership. The oldest WDs in this sample have WD cooling ages of 10 Gyr, providing a firm lower limit to the age of the thick disc population. Many of our targets have uncharacteristically large radii, indicating that they are low mass WDs. It appears that we have detected the brighter population of cool and ultracool WDs near the Sun. The fainter population of ultracool CO-core WDs remain to be discovered in large numbers. The Large Synoptic Survey Telescope should find these elusive, more massive ultracool WDs in the solar neighbourhood.

Oxygen abundances in G- and F-type stars from HARPS

We present a detailed and uniform study of oxygen abundance from two different oxygen lines at 6158$\AA$ and 6300$\AA$ in a large sample of solar-type stars. The results are used to check the behaviour of these spectral lines as oxygen abundance indicators and to study the evolution of oxygen in thick and thin disk populations of the Galaxy. Equivalent width measurements were carried out for the [OI]~6158$\AA$ and OI~6300$\AA$ lines. LTE abundances were obtained from these two lines in 610 and 535 stars, respectively. We were able to measure oxygen abundance from both indicators in 447 stars, enabling us, for the first time, to compare them in a uniform way. Careful error analysis has been performed. We found that oxygen abundances derived from the 6158$\AA$ and 6300$\AA$ lines agree to within 0.1dex in 58\% of the stars in our sample, and this result improves for higher signal-to-noise values. We confirm an oxygen enhancement in stars of the thick disk, as has also been seen for other $\alpha$-elements. The new oxygen abundances confirm previous findings for a progressive linear rise in the oxygen-to-iron ratio with a slope equal to 0.78 from solar metallicity to [Fe/H]$\sim$-1. However, the slope we measured is steeper than the one found in previous studies based on the oxygen triplet. Below [Fe/H]=$-$0.6 our stars show [O/Fe] ratios as high as $\sim$0.8, which can be interpreted as evidence for oxygen overproduction in the Galactic thick disk. These high oxygen abundances do not pose a problem to chemodynamical models since there is a range of parameters that can accommodate our results.

POPULATION STUDIES. XIII. A NEW ANALYSIS OF THE BIDELMAN-MACCONNELL “WEAK-METAL” STARS—CONFIRMATION OF METAL-POOR STARS IN THE THICK DISK OF THE GALAXY

A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R~3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the SEGUE Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and [Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities -1.8 < [Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy vs. angular momentum diagram. A comparison is made with recent results for a similar-size sample of RAVE stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.

In the thick of it: metal-poor disc stars in RAVE

By selecting in the RAVE-DR4 survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest three-component model (thin disc, thick disc, halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to [M/H]=-2 dex, contributing roughly at 3% of the entire thick disc population and having a shorter scale-length compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity and the orbital velocity, which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find dVphi/d[M/H] ~ 50 km/s/dex, which is in a very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc, and suggest that given the above mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely.

Interacting planetary nebulae

In the present paper, we discuss the classification of planetary nebulae that show interaction with the interstellar medium in terms of Galactic population. Furthermore, we investigate the case of interaction of each object in the framework of the isothermal and adiabatic shock models and we derive the mean critical, leading, and stopping densities of the nebulae located in the Galactic thin and thick disks. A sample of 34 objects with available proper motions, radial velocities, and reliable distances in the literature was used for these purposes. The results show that 16 and 10 objects are classified as Galactic thin-disk and thick-disk populations, respectively. Seven objects are classified as probable thin-disk populations, and one as a probable thick-disk population. The flow and cooling time of the post-shock nebular gas are derived and the cooling/flow time ratio shows a linear correlation versus the Galactic height of the interacting planetary nebulae. The results reveal that there is a tendency for the interacting planetary nebulae belonging to the thin disk to follow an isothermal shock model, whilst those belonging to the thick disk to follow an adiabatic shock model.

Kinematics and chemical properties of the Galactic stellar populations

(Abridged) We analyze chemical and kinematical properties of about 850 FGK solar neighborhood long-lived dwarfs observed with the HARPS high-resolution spectrograph. The stars in the sample have logg > 4 dex, 5000 < Teff < 6500 K, and -1.39 < [Fe/H] < 0.55 dex. We apply a purely chemical analysis approach based on the [alpha/Fe] vs. [Fe/H] plot to separate Galactic stellar populations into the thin disk, thick disk and high-alpha metal-rich (hamr). Our analysis shows a negative gradient of the rotational velocity of the thin disk stars with [Fe/H] (-17 km s^-1 dex^-1), and a steep positive gradient for both the thick disk and hamr stars with the same magnitude of about +42 km s^-1 dex^-1. For the thin disk stars we observed no correlation between orbital eccentricities and metallicity, but observed a steep negative gradient for the thick disk and hamr stars with practically the same magnitude (about -0.18 dex^-1). Our results suggest that radial migration played an important role in the formation and evolution of the thin disk. For the thick disk stars it is not possible to reach a firm conclusion about their origin. Based on the eccentricity distribution of the thick disk stars only their accretion origin can be ruled out, and the heating and migration scenario could explain the positive steep gradient of V_phi with [Fe/H]. Analyzing the hamr stellar population we found that they share properties of both the thin and thick disk population. A comparison of the properties of the hamr stars with that of the subsample of stars from the N-body/SPH simulation using radial migration suggest that they may have originated from the inner Galaxy. Further detailed investigations would help to clarify their exact nature and origin.

TYC 1031 1262 1: an anomalous Cepheid in a double-lined eclipsing binary★

Multi-color light curves and radial velocities for TYC\,1031\,1262\,1 have been obtained and analyzed. TYC\,1031\,1262\,1 includes a Cepheid with a period of 4.15270$\pm$0.00061 days. The orbital period of the system is about 51.2857$\pm$0.0174 days. The pulsation period indicates a secular period increase with an amount of 2.46$\pm$0.54 min/yr. The observed B, V, and R magnitudes were cleaned for the intrinsic variations of the primary star. The remaining light curves, consisting of eclipses and proximity effects, are obtained and analyzed for orbital parameters. The system consists of two evolved stars, F8II+G6II, with masses of M$_1$=1.640$\pm$0.151 {\Msun} and M$_2$=0.934$\pm$0.109 {\Msun} and radii of R$_1$=26.9$\pm$0.9 {\Rsun} and R$_2$=15.0$\pm$0.7 {\Rsun}. The pulsating star is almost filling its corresponding Roche lobe which indicates the possibility of mass loss or transfer having taken place. We find an average distance of d=5070$\pm$250\,pc using the BVR and JHK magnitudes and also the V-band extinction. Kinematic properties and the distance to the galactic plane with an amount of 970 pc indicate that it belongs to the thick-disk population. Most of the observed and calculated parameters of the TYC\,1031\,1262\,1 lead to a classification of an Anomalous Cepheid.

IDENTIFYING CONTRIBUTIONS TO THE STELLAR HALO FROM ACCRETED, KICKED-OUT, AND IN SITU POPULATIONS

[Abridged] We present a medium-resolution spectroscopic survey of late-type giant stars at mid-Galactic latitudes of (30$^{\circ}<|b|<60^{\circ}$), designed to probe the properties of this population to distances of $\sim$9 kpc. Because M giants are generally metal-rich and we have limited contamination from thin disk stars by the latitude selection, most of the stars in the survey are expected to be members of the thick disk ($<$[Fe/H]$>\sim$-0.6) with some contribution from the metal-rich component of the nearby halo. Here we report first results for 1799 stars. The distribution of radial velocity (RV) as a function of l for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, that in some cases form distinct sequences in RV-$l$ space. High-resolution echelle spectra taken for 34 of these "RV outliers" reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: seventeen of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way; eight have abundances coincident with those of the Galactic disk and more metal-rich halo; and nine of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that this sample consists predominantly of stars accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV outlier sample may have been formed in the inner Galaxy and subsequently kicked to higher eccentricity orbits, but the sample is not large enough to distinguish conclusively between this interpretation and the alternative that these stars represent the tail of the velocity distribution of the thick disk. Our data do not rule out the possibility that a minority of the sample could have formed from gas {\it in situ} on their current orbits.

Thin disc, thick disc and halo in a simulated galaxy

Within a cosmological hydrodynamical simulation, we form a disc galaxy with sub- components which can be assigned to a thin stellar disc, thick disk, and a low mass stellar halo via a chemical decomposition. The thin and thick disc populations so selected are distinct in their ages, kinematics, and metallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity dispersion ({\sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The thick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by \sim21 km/s, possess higher velocity dispersion ({\sigma}U,V,W = 49, 44, 35 km/s), relatively low [Fe/H] and high [O/Fe]. The halo component comprises less than 4% of stars in the "solar annulus" of the simulation, has low metallicity, a velocity ellipsoid defined by ({\sigma}U,V,W = 62, 46, 45 km/s) and is formed primarily in-situ during an early merger epoch. Gas-rich mergers during this epoch play a major role in fuelling the formation of the old disc stars (the thick disc). This is consistent with studies which show that cold accretion is the main source of a disc galaxy's baryons. Our simulation initially forms a relatively short (scalelength \sim1.7 kpc at z=1) and kinematically hot disc, primarily from gas accreted during the galaxy's merger epoch. Far from being a competing formation scenario, migration is crucial for reconciling the short, hot, discs which form at high redshift in {\Lambda}CDM, with the properties of the thick disc at z=0. The thick disc, as defined by its abundances maintains its relatively short scale-length at z = 0 (2.31 kpc) compared with the total disc scale-length of 2.73 kpc. The inside-out nature of disc growth is imprinted the evolution of abundances such that the metal poor {\alpha}-young population has a larger scale-length (4.07 kpc) than the more chemically evolved metal rich {\alpha}-young population (2.74 kpc).

The thick disk rotation-metallicity correlation as a fossil of an “inverse chemical gradient” in the early Galaxy

The thick disk rotation--metallicity correlation, \partial V_\phi/\partial[Fe/H] =40\div 50 km s^{-1}dex^{-1} represents an important signature of the formation processes of the galactic disk. We use nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. A rotation--metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular,we measure a correlation of \partial V_\phi/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body models can account for the V_\phi vs. [Fe/H] correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient "inverse" chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.

Characteristics of the closest known G-type exoplanet host 82 Eri

We present a consistent model atmosphere analysis and evolutionary tracks for the very nearby G-type star 82 Eri that was most recently announced by Pepe et al. to harbour two or three super-Earth companions with orbital periods of 18, 40 and 90 d. As a bright and high-velocity object, 82 Eri was already realized by Jan Hendrik Oort in his 1926 treatise to be of ‘probably different origin’ and we confirm its being a member of the ancient thick-disc population on account of its outstanding iron-to-magnesium deficiency. Thus, in our immediate neighbourhood – only 6 pc away – there exists a system of extremely old and probably rocky super-Earths that were already formed when the Milky Way was at its infancy, 12 or even 13 billion years in the past.

Overabundance ofα-elements in exoplanet-hosting stars

We present the results for a chemical abundance analysis between planet-hosting and stars without planets for 12 refractory elements for a total of 1111 nearby FGK dwarf stars observed within the context of the HARPS GTO programs. Of these stars, 109 are known to harbour high-mass planetary companions and 26 stars are hosting exclusively Neptunians and super-Earths. We found that the [X/Fe] ratios for Mg, Al, Si, Sc, and Ti both for giant and low-mass planet hosts are systematically higher than those of comparison stars at low metallicities ([Fe/H] < from -0.2 to 0.1 dex depending on the element). The most evident discrepancy between planet-hosting and stars without planets is observed for Mg. Our data suggest that the planet incidence is greater among the thick disk population than among the thin disk for mettallicities bellow -0.3 dex. After examining the [alpha/Fe] trends of the planet host and non-host samples we conclude that a certain chemical composition, and not the Galactic birth place of the stars, is the determinating factor for that. The inspection of the Galactic orbital parameters and kinematics of the planet-hosting stars shows that Neptunian hosts tend to belong to the "thicker" disk compared to their high-mass planet-hosting counterparts.We also found that Neptunian hosts follow the distribution of high-alpha stars in the UW vs V velocities space, but they are more enhanced in Mg than high-alpha stars without planetary companions. Our results indicate that some metals other than iron may also have an important contribution to planet formation if the amount of iron is low. These results may provide strong constraints for the models of planet formation, especially for planets with low mass.

A SHORT SCALE LENGTH FOR THE α-ENHANCED THICK DISK OF THE MILKY WAY: EVIDENCE FROM LOW-LATITUDE SEGUE DATA

We examine the \alpha-element abundance ratio, [\alpha/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc < R < 16 kpc, 0.15 kpc < |Z| < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z|. Our results show that the high-\alpha\ thick disk population has a short scale length (L_thick ~ 1.8 kpc) compared to the low-\alpha population, which is typically associated with the thin disk. We find that the fraction of high-\alpha\ stars in the inner disk increases at large |Z|, and that high-\alpha\ stars lag in rotation compared to low-\alpha\ stars. In contrast, the fraction of high-\alpha\ stars in the outer disk is low at all |Z|, and high- and low-\alpha\ stars have similar rotational velocities up to 1.5 kpc from the plane. We interpret these results to indicate that different processes were responsible for the high-\alpha\ populations in the inner and outer disk. The high-\alpha\ population in the inner disk has a short scale length and large scale height, consistent with a scenario in which the thick disk forms during an early gas-rich accretion phase. Stars far from the plane in the outer disk may have reached their current locations through heating by minor mergers. The lack of high-\alpha\ stars at large R and |Z| also places strict constraints on the strength of radial migration via transient spiral structure.

Spitzerobservations of NGC 2264: the nature of the disk population

NGC2264 is a young cluster with a rich circumstellar disk population which makes it an ideal target for studying the evolution of stellar clusters. Our goal is to study its star formation history and to analyse the primordial disk evolution of its members. The study presented is based on data obtained with Spitzer IRAC and MIPS, combined with deep NIR ground-based FLAMINGOS imaging and previously published optical data. We build NIR dust extinction maps of the molecular cloud associated with the cluster, and determine it to have a mass of 2.1x10^3Msun above an Av of 7mag. Using a differential K_s-band luminosity function of the cluster, we estimate the size of its population to be 1436$\pm$242 members. The star formation efficiency is ~25%. We identify the disk population: (i) optically thick inner disks, (ii) anaemic inner disks, and (iii) disks with inner holes, or transition disks. We analyse the spatial distribution of these sources and find that sources with thick disks segregate into sub-clusterings, whereas sources with anaemic disks do not. Furthermore, sources with anaemic disks are found to be unembedded (Av<3mag), whereas the clustered sources with thick disks are still embedded within the parental cloud. NGC2264 has undergone more than one star-forming event, where the anaemic and extincted thick disk population appear to have formed in separate episodes. We also find tentative evidence of triggered star-formation in the Fox Fur Nebula. In terms of disk evolution, our findings support the emerging disk evolution paradigm of two distinct evolutionary paths for primordial optically thick disks: a homologous one where the disk emission decreases uniformly at NIR and MIR wavelengths, and a radially differential one where the emission from the inner region of the disk decreases more rapidly than from the outer region (forming transition disks).

Analysis of old very metal rich stars in the solar neighbourhood

A sample of mostly old metal-rich dwarf and turn-off stars with high eccentricity and low maximum height above the Galactic plane has been identified. From their kinematics, it was suggested that the inner disk is their most probable birthplace. Their chemical imprints may therefore reveal important information about the formation and evolution of the still poorly understood inner disk. To probe the formation history of these stellar populations, a detailed analysis of a sample of very metal-rich stars is carried out. We derive the metallicities, abundances of \alpha\ elements, ages, and Galactic orbits. The analysis of 71 metal-rich stars is based on optical high-resolution \'echelle spectra obtained with the FEROS spectrograph at the ESO 1.52-m Telescope at La Silla, Chile. The metallicities and abundances of C, O, Mg, Si, Ca, and Ti were derived based on LTE detailed analysis, employing the MARCS model atmospheres. We confirm the high metallicity of these stars reaching up to [Fe I/H]~0.58, and the sample of metal-rich dwarfs can be kinematically subclassified in samples of thick disk, thin disk, and intermediate stellar populations. All sample stars show solar \alpha-Fe ratios, and most of them are old and still quite metal rich. The orbits suggest that the thin disk, thick disk and intermediate populations were formed at Galactocentric distances of ~8 kpc, ~6 kpc, and ~7 kpc, respectively. The mean maximum height of the thick disk subsample of Z_max~380 pc, is lower than for typical thick disk stars. A comparison of \alpha-element abundances of the sample stars with bulge stars shows that the oxygen is compatible with a bulge or inner thick disk origin. Our results suggest that models of radial mixing and dynamical effects of the bar and bar/spiral arms might explain the presence of these old metal-rich dwarf stars in the solar neighbourhood.

THE METALLICITY GRADIENT OF THE THICK DISK BASED ON RED HORIZONTAL-BRANCH STARS FROM SDSS DR8

Based on SDSS DR8, we have selected a sample of 1728 red horizontal branch stars with |Z|<3 kpc by using a color-metallicity relation and stellar parameters. The sample stars clearly trace a typical thick disk population with peaks at |Z|=1.26 kpc and [Fe/H]=-0.54. The vertical metallicity gradient of the thick disk is estimated in two ways. One is a fit to the Gaussian peaks of the metallicity histograms of the thick disk by subtracting minor contributions from the thin disk and the inner halo based on the Besancon Galaxy model. The resulting gradient is -0.12+-0.01 dex/kpc for 0.5 <|Z| <3 kpc. The other method is to linearly fit the data based on stars with 1 <|Z| <3 kpc being the main component of the thick disk. Five subgroups are then selected in different directions in the X-|Z| plane to investigate the difference in the vertical metallicity gradient between the Galactocenter and anti-Galactocenter directions. We found that a vertical gradient o-0.22+-0.07 dex/kpc is detected for five directions except for one involving the pollution of stars from the bulge. The results indicate that the vertical gradient is dominant, but a radial gradient has a minor contribution for the thick disk population represented by RHB stars with 1 <|Z| <3 kpc. The present work strongly suggests the existence of a metallicity gradient in the thick disk, which is thought to be negligible in most previous works in the literature.

White dwarfs in the SuperCOSMOS Sky Survey: the thin disc, thick disc and spheroid luminosity functions

We present a magnitude and proper motion limited catalogue of ∼10 000 white dwarf candidates, obtained from the SuperCOSMOS Sky Survey by means of reduced proper motion selection. This catalogue extends to magnitudes R ∼ 19.75 and proper motions as low as μ ∼ 0.05 arcsec yr −1 , and covers nearly three quarters of the sky. Photometric parallaxes provide distance estimates accurate to ∼50 per cent. This catalogue is used to measure the luminosity functions for disc and spheroid white dwarfs, using strict velocity cuts to isolate subsamples belonging to each population. Disc luminosity functions measured in this manner are really a conglomerate of thin and thick disc objects, due to the significant velocity overlap between these populations. We introduce a new statistical approach to the stellar luminosity function for nearby objects that successfully untangles the contributions from the different kinematic populations, without the need for stringent velocity cuts. This improves the statistical power by allowing all stars to contribute to the luminosity function, even at tangential velocities where the populations are indistinguishable. This method is particularly suited to white dwarfs, for which population discrimination by chemical tagging is not possible. We use this technique to obtain the first measurement of the thick disc white dwarf luminosity function, while also improving constraint on both the thin disc and spheroid luminosity functions. We find that the thin disc, thick disc and spheroid populations contribute to the local white dwarf density in roughly 79 per cent/16 per cent/5 per cent proportions.

FORMATION AND EVOLUTION OF THE DISK SYSTEM OF THE MILKY WAY: [α/Fe] RATIOS AND KINEMATICS OF THE SEGUE G-DWARF SAMPLE

We employ measurements of the [alpha/Fe] ratio derived from low-resolution (R~2000) spectra of 17,277 G-type dwarfs from the SEGUE survey to separate them into likely thin- and thick-disk subsamples. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, of opposite signs, -20 to -30 km/s/dex for the thin-disk and +40 to +50 km/s/dex for the thick-disk population. The rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample, and exhibits a small trend for the thick-disk subsample. The rotational velocity decreases with distance from the plane for both disk components, with similar slopes (-9.0 {\pm} 1.0 km/s/kpc). Thick-disk stars exhibit a strong trend of orbital eccentricity with metallicity (about -0.2/dex), while the eccentricity does not change with metallicity for the thin-disk subsample. The eccentricity is almost independent of Galactocentric radius for the thin-disk population, while a marginal gradient of the eccentricity with radius exists for the thick-disk population. Both subsamples possess similar positive gradients of eccentricity with distance from the Galactic plane. The shapes of the eccentricity distributions for the thin- and thick-disk populations are independent of distance from the plane, and include no significant numbers of stars with eccentricity above 0.6. Among several contemporary models of disk evolution we consider, radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable.