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Register a new userThe GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood
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L. Duong
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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.
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Open clusters are unique tracers of the history of our own Galaxys disk. According to our membership analysis based on textit{Gaia} astrometry, out of the 226 potential clusters falling in the footprint of GALAH or APOGEE, we find that 205 have secure members that were observed by at least one of the survey. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disk of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is $-$0.076$pm$0.009 dex kpc$^{-1}$, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H] - guiding radius (r$_{rm guid}$) plane is $-$0.073$pm$0.008 dex kpc$^{-1}$. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disk differently than field stars. In particular, at given radius, open clusters show an age-metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]$-$r$_{rm guid}$$-$age space, which are important to understand production rates of different elements as a function of space and time.
We infer the gravitational potential of the Galactic disk by analysing the phase-space densities of 120 stellar samples in 40 spatially separate sub-regions of the solar neighbourhood, using Gaias second data release (DR2), in order to quantify spatially dependent systematic effects that bias this type of measurement. The gravitational potential was inferred under the assumption of a steady state in the framework of a Bayesian hierarchical model. We performed a joint fit of our stellar tracers three-dimensional velocity distribution, while fully accounting for the astrometric uncertainties of all stars. The inferred gravitational potential is compared, post-inference, to a model for the baryonic matter and halo dark matter components. We see an unexpected but clear trend for all 40 spatially separate sub-regions: Compared to the potential derived from the baryonic model, the inferred gravitational potential is significantly steeper close to the Galactic mid-plane (<60 pc), but flattens such that the two agree well at greater distances (~400 pc). The inferred potential implies a total matter density distribution that is highly concentrated to the Galactic mid-plane and decays quickly with height. Apart from this, there are discrepancies between stellar samples, implying spatially dependent systematic effects which are, at least in part, explained by substructures in the phase-space distributions. In terms of the inferred matter density distribution, the very low matter density that is inferred at greater heights is inconsistent with the observed scale height and matter distribution of the stellar disk, which cannot be explained by a misunderstood density of cold gas or other hidden mass. Our interpretation is that these results must be biased by a time-varying phase-space structure, possibly a breathing mode, that is large enough to affect all stellar samples in the same manner.
We present isochrone ages and initial bulk metallicities ($rm [Fe/H]_{bulk}$, by accounting for diffusion) of 163,722 stars from the GALAH Data Release 2, mainly composed of main sequence turn-off stars and subgiants ($rm 7000 K>T_{eff}>4000 K$ and $rm log g>3$ dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appear to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick disks in the solar neighborhood. We also present abundance ratio trends for 16 elements as a function of age, across different $rm [Fe/H]_{bulk}$ bins. In general, we find the trends in terms of [X/Fe] vs age from our far larger sample to be compatible with studies based on small ($sim$ 100 stars) samples of solar twins but we now extend it to both sub- and super-solar metallicities. The $alpha$-elements show differing behaviour: the hydrostatic $alpha$-elements O and Mg show a steady decline with time for all metallicities while the explosive $alpha$-elements Si, Ca and Ti are nearly constant during the thin disk epoch (ages $lessapprox $ 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu have the opposite trend, thus favouring a primary production from sources with a short time-delay such as core-collapse supernovae over long-delay events such as neutron star mergers.
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.
We analysed the chemodynamical evolution of the Galactic disc using precise [Mg/Fe] abundances from a previous study and accurate Gaia data. For this purpose, we estimated ages and dynamical properties for 366 MSTO solar neighbourhood stars from the AMBRE Project using PARSEC isochrones together with astrometric and photometric values from Gaia DR2. We find a radial gradient of -0.099 ${pm}$ 0.031 dex kpc$^{-1}$ for [M/H] and +0.023 ${pm}$ 0.009 dex kpc for the [Mg/Fe] abundance. The steeper [Mg/Fe] gradient than that found in the literature is a result of the improvement of the AMBRE [Mg/Fe] estimates in the metal-rich regime. In addition, we find a significant spread of stellar age at any given [Mg/Fe] value, and observe a clear correlated dispersion of the [Mg/Fe] abundance with metallicity at a given age. While for [M/H] < -0.2, a clear age-[Mg/Fe] trend is observed, more metal-rich stars display ages from 3 up to 12 Gyr, describing an almost flat trend in the [Mg/Fe]-age relation. Moreover, we report the presence of radially migrated stars for a wide range of stellar ages, although we note the large uncertainties of the amplitude of the inferred change in orbital guiding radii. Finally, we observe the appearance of a second chemical sequence in the outer disc, 10-12 Gyr ago, populating the metal-poor, low-[Mg/Fe] tail. These stars are more metal-poor than the coexisting stellar population in the inner parts of the disc, and show lower [Mg/Fe] abundances than prior disc stars of the same metallicity, leading to a chemical discontinuity. Our data favour the rapid formation of an early disc that settled in the inner regions, followed by the accretion of external metal-poor gas -- probably related to a major accretion event such as the Gaia-Enceladus/Sausage one -- that may have triggered the formation of the thin disc population and steepened the abundance gradient in the early disc.
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