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Register a new userThe GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS
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S. Buder
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The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey & $Gaia$ provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, & sub-giant stars. [...] We investigate correlations between chemical compositions, ages, & kinematics for this sample. Stellar parameters & elemental abundances are derived from the GALAH spectra with the spectral synthesis code SME. [...] We report Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba & we note that we employ non-LTE calculations for Li, O, Al, & Fe. We show that the use of astrometric & photometric data improves the accuracy of the derived spectroscopic parameters, especially $log g$. [...] we recover the result that stars of the high-$alpha$ sequence are typically older than stars in the low-$alpha$ sequence, the latter spanning $-0.7<$[Fe/H]$<+0.5$. While these two sequences become indistinguishable in [$alpha$/Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high-$alpha$ & the low-$alpha$ sequence even in this regime. [...] we find that the old stars ($>8$ Gyr have lower angular momenta $L_z$ than the Sun, which implies that they are on eccentric orbits & originate from the inner disk. Contrary to some previous smaller scale studies we find a continuous evolution in the high-$alpha$-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate high-$alpha$ metal-rich population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low $alpha$-sequence & show a gradient in $L_z$ from low [Fe/H] ($L_z>L_{z,odot}$) towards higher [Fe/H] ($L_z<L_{z,odot}$), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.
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Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick disks near the solar neighbourhood. The data cover a small range of Galactocentric radius ($7.9 leq R_mathrm{GC} leq 9.5$ kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude $260 ^circ leq ell leq 280^circ$). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined `thick and `thin disks of the Galaxy. The thin disk (low-$alpha$ population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/d$z=-0.18 pm 0.01$ dex kpc$^{-1}$, which is broadly consistent with previous studies. In contrast, its vertical $alpha$-abundance profile is almost flat, with a gradient of d[$alpha$/M]/d$z$ = $0.008 pm 0.002$ dex kpc$^{-1}$. The steep vertical metallicity gradient of the low-$alpha$ population is in agreement with models where radial migration has a major role in the evolution of the thin disk. The thick disk (high-$alpha$ population) has a weaker vertical metallicity gradient d[M/H]/d$z = -0.058 pm 0.003$ dex kpc$^{-1}$. The $alpha$-abundance of the thick disk is nearly constant with height, d[$alpha$/M]/d$z$ = $0.007 pm 0.002$ dex kpc$^{-1}$. The negative gradient in metallicity and the small gradient in [$alpha$/M] indicate that the high-$alpha$ population experienced a settling phase, but also formed prior to the onset of major SNIa enrichment. We explore the implications of the distinct $alpha$-enrichments and narrow [$alpha$/M] range of the sub-populations in the context of thick disk formation.
The age-metallicity relation is a fundamental tool for constraining the chemical evolution of the Galactic disc. In this work we analyse the observational properties of this relation using binary stars that have not interacted consisting of a white dwarf - from which we can derive the total age of the system - and a main sequence star - from which we can derive the metallicity as traced by the [Fe/H] abundances. Our sample consists of 46 widely separated, but unresolved spectroscopic binaries identified within the Sloan Digital Sky Survey, and 189 white dwarf plus main sequence common proper motion pairs identified within the second data release of Gaia. This is currently the largest white dwarf sample for which the metallicity of their progenitors have been determined. We find a flat age-metallicity relation displaying a scatter of [Fe/H] abundances of approximately 0.5 dex around the solar metallicity at all ages. This independently confirms the lack of correlation between age and metallicity in the solar neighbourhood that is found in previous studies focused on analysing single main sequence stars and open clusters.
We analyse the kinematics of disc stars observed by the RAVE survey in and beyond the Solar neighbourhood.We detect significant overdensities in the velocity distributions using a technique based on the wavelet transform.We find that the main local kinematic groups are large scale features, surviving at least up to ~1 kpc from the Sun in the direction of anti-rotation, and also at ~700 pc below the Galactic plane.We also find that for regions located at different radii than the Sun, the known groups appear shifted in the velocity plane. For example, the Hercules group has a larger azimuthal velocity for regions inside the Solar circle and a lower value outside. We have also discovered a new group at (U, V) = (92,-22) km/s in the Solar neighbourhood and confirmed the significance of other previously found groups. Some of these trends detected for the first time are consistent with dynamical models of the effects of the bar and the spiral arms. More modelling is required to definitively characterise the non-axisymmetric components of our Galaxy using these groups.
We present a self-consistent, absolute isochronal age scale for young (< 200 Myr), nearby (< 100 pc) moving groups in the solar neighbourhood based on homogeneous fitting of semi-empirical pre-main-sequence model isochrones using the tau^2 maximum-likelihood fitting statistic of Naylor & Jeffries in the M_V, V-J colour-magnitude diagram. The final adopted ages for the groups are: 149+51-19 Myr for the AB Dor moving group, 24+/-3 Myr for the {beta} Pic moving group (BPMG), 45+11-7 Myr for the Carina association, 42+6-4 Myr for the Columba association, 11+/-3 Myr for the {eta} Cha cluster, 45+/-4 Myr for the Tucana-Horologium moving group (Tuc-Hor), 10+/-3 Myr for the TW Hya association, and 22+4-3 Myr for the 32 Ori group. At this stage we are uncomfortable assigning a final, unambiguous age to the Argus association as our membership list for the association appears to suffer from a high level of contamination, and therefore it remains unclear whether these stars represent a single population of coeval stars. Our isochronal ages for both the BPMG and Tuc-Hor are consistent with recent lithium depletion boundary (LDB) ages, which unlike isochronal ages, are relatively insensitive to the choice of low-mass evolutionary models. This consistency between the isochronal and LDB ages instills confidence that our self-consistent, absolute age scale for young, nearby moving groups is robust, and hence we suggest that these ages be adopted for future studies of these groups. Software implementing the methods described in this study is available from http: //www.astro.ex.ac.uk/people/timn/tau-squared/.
The age-metallicity relation (AMR) is a fundamental observational constraint for understanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in obtaining accurate stellar ages for individual field stars. We have started an observational campaign for providing the much needed observational input by using wide white dwarf-main sequence (WDMS) binaries. White dwarfs are natural clocks and can be used to derive accurate ages. Metallicities can be obtained from the main sequence companions. Since the progenitors of white dwarfs and the main sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this work we present the AMR derived from analysing a pilot sample of 23 WDMS binaries and provide clear observational evidence for the lack of correlation between age and metallicity at young and intermediate ages (0-7 Gyrs).
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