Search

Context: The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangling the scenarios of formation and evolution of the Galaxy. Aims: In this paper we used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance-to-iron ratio gradients, and their time evolution. Methods: We selected member stars in 62 open clusters, with ages from 0.1 to about 7 Gyr, located in the Galactic thin disc at galactocentric radii (R-GC) from about 6 to 21 kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins. Results: The [Fe/H] gradient has a slope of -0.054 dex kpc(-1). It can be better approximated with a two-slope shape, steeper for R-GC <= 11.2 kpc and flatter in the outer regions. We saw different behaviours for elements belonging to different channels. For the time evolution of the gradient, we found that the youngest clusters (age < 1 Gyr) in the inner disc have lower metallicity than their older counterparts and that they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggest that the most likely one may be related to a bias introduced by the standard spectroscopic analysis producing lower metallicities in the analysis of low-gravity stars. Conclusions: To delineate the shape of the 'true' gradient, we should most likely limit our analysis to stars with low surface gravity log g> 2.5 and microturbulent parameter xi< 1.8 km s(-1). Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slo

Context. The first stars marked the end of the cosmic dark ages, produced the first heavy elements, and set the stage for the formation of the first galaxies. Accurate chemical abundances of ultra metal-poor stars ([Fe/H] < −4) can be used to infer the properties of the first stars and thus the formation mechanism for low-mass second-generation stars in the early Universe. Spectroscopic studies have shown that most second-generation stars are carbon enhanced. A notable exception is SDSS J102915.14+172927.9, which is the most metal-poor star known to date, largely by virtue of the low upper limits of the carbon abundance reported in earlier studies. Aims. We re-analysed the composition of SDSS J102915.14+172927.9 with the aim of providing improved observational constraints on the lowest metallicity possible for low-mass star formation and constraining the properties of its Population III progenitor star. Methods. We developed a tailored three-dimensional model atmosphere for SDSS J102915.14+172927.9 with the Stagger code, making use of an improved surface gravity estimate based on the Gaia DR3 parallax. Snapshots from the model were used as input in the radiative transfer code Balder to compute 3D non-local thermodynamic equilibrium (non-LTE) synthetic spectra. These spectra were then used to infer abundances for Mg, Si, Ca, Fe, and Ni as well as upper limits on Li, Na, and Al. Synthetic 3D LTE spectra were computed with Scate to infer the abundance of Ti and upper limits on C and N. Results. In contrast to earlier works based on 1D non-LTE corrections applied to 3D LTE results, we are able to achieve ionisation balance for Ca I and Ca II when employing our consistent 3D non-LTE treatment. The elemental abundances are systematically higher than those found in earlier works. In particular, [Fe/H] is increased by 0.57 dex, and the upper limits of C and N are larger by 0.90 dex and 1.82 dex, respectively. Conclusions. We find that Population III progenitors with mass

The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying the slow neutron capture (s-)process nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron capture element abundances. Previous studies attribute these abundance differences to pollution from 3 − 6 M asymptotic giant branch (AGB) stars which produce significant quantities of the neutron-rich Mg isotopes 25Mg and 26Mg. We report the first-ever measurements of Mg isotopic abundance ratios at [Fe/H] ∼ −2 in a globular cluster-like system using very high-resolution and signal-to-noise spectra (R = 110 000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars; three in each s-process group. Despite the presence of star-to-star variations in 24Mg, 25Mg, and 26Mg, we find no correlation with heavy element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between 26Mg/24Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low mass (∼ 1−3 M) AGB stars are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above ∼ 3 M. This places tighter constraints on possible formation scenarios and suggests an age difference of at least ∼ 280−480 Myr between the two populations which is independent of isochrone fitting.

Context. The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangling the scenarios of formation and evolution of the Galaxy.Aims. In this paper we used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance-to-iron ratio gradients, and their time evolution.Methods. We selected member stars in 62 open clusters, with ages from 0.1 to about 7 Gyr, located in the Galactic thin disc at galactocentric radii (R-GC) from about 6 to 21 kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins.Results. The [Fe/H] gradient has a slope of -0.054 dex kpc(-1). It can be better approximated with a two-slope shape, steeper for R-GCConclusions. To delineate the shape of the 'true' gradient, we should most likely limit our analysis to stars with low surface gravity log g> 2.5 and microturbulent parameter xi< 1.8 km s(-1). Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slow and stationary formation of the thin disc over the last 3 Gyr. We found a secondary role of cluster migration in shaping the gradient, with a more prominent role of migration for the oldest clusters.

Context. It is well known that cool star atmospheres depart from local thermodynamic equilibrium (LTE). Making an accurate abundance determination requires taking those effects into account, but the necessary non-LTE (hereafter NLTE) calculations are often lacking. Aims. Our goal is to provide detailed estimates of NLTE effects for FGK type stars for all spectral lines from the ultraviolet (UV) to the near infrared (NIR) that are potentially useful as abundance diagnostics. The first paper in this series focusses on the light elements Na, Mg, and Al. Methods. The code PySME was used to compute curves of growth for 2158 MARCS model atmospheres in the parameter range 3800 < Teff < 8000 K, 0.0 < log(g) < 5.5, and −5 < [Fe/H] < +0.5. Two microturbulence values, 1 and 2 kms−1, and nine abundance points spanning −1 < [X/Fe] < 1 for element X, are used to construct individual line curves of growth by calculating the equivalent widths of 35 Na lines, 134 Mg lines, and 34 Al lines. The lines were selected in the wavelength range between 2000 Å and 3 µm. Results. We demonstrate the power of the new grids with LTE and NLTE abundance analysis by means of equivalent width measurements of five benchmark stars; the Sun, Arcturus, HD 84937, HD 140283 and HD 122563. For Na, the NLTE abundances are lower than in LTE and show markedly reduced line-to-line scatter in the metal-poor stars. For Mg, we confirm previous reports of a significant ~0.25 dex LTE ionisation imbalance in metal-poor stars that is only slightly improved in NLTE (~0.18 dex). LTE abundances based on Mg II lines agree better with models of Galactic chemical evolution. For Al, NLTE calculations strongly reduce an ~0.6 dex ionisation imbalance seen in LTE for the metal-poor stars. The abundance corrections presented in this work are in good agreement with previous studies for the subset of lines that overlap, with the exception of strongly saturated lines. Conclusions. A consensus between differ

ABSTRACT In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of $-6.5 \le \rm [Fe/H] \le -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of $\rm [Fe/H]=-3.31\, \mathrm{ and }\, -3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that ∼21 per cent of the sample have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Zmax| ≤ 3 kpc. Of particular interest is a subsample (∼11 per cent of the total) of low |Zmax| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = –4.30 and the subsample is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Zmax|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Zmax|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our sample (∼4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.

ABSTRACT In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of $-6.5 \le \rm [Fe/H] \le -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of $\rm [Fe/H]=-3.31\, \mathrm{ and }\, -3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that ∼21 per cent of the sample have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Zmax| ≤ 3 kpc. Of particular interest is a subsample (∼11 per cent of the total) of low |Zmax| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = –4.30 and the subsample is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Zmax|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Zmax|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our sample (∼4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have [Fe/H] ≤ −2.75 and only approximately seven per cent have [Fe/H] > −2.0 dex. The most metal-poor candidate in the sample has [Fe/H] < −4.75 and is notably carbon rich. Except at the lowest metallicities ([Fe/H] < −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with [C/Fe]1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus [Fe/H]1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE > 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE < 7.3 and [C/Fe]1D, LTE > +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of ∆(Log N)/∆[Fe/H] = 1.5 ± 0.1 dex per dex for −4.0 ≤ [Fe/H] ≤ −2.75, but appears to drop abruptly at [Fe/H] ≈ −4.2, in line with previous studies.

We explore the fundamental relations governing the radial and vertical velocity dispersions of stars in the Milky Way, from combined studies of complementary surveys including GALAH, LAMOST, APOGEE, the NASA Kepler and K2 missions, and Gaia DR2. We find that different stellar samples, even though they target different tracer populations and employ a variety of age estimation techniques, follow the same set of fundamental relations. We provide the clearest evidence to date that, in addition to the well-known dependence on stellar age, the velocity dispersions of stars depend on orbital angular momentum Lz, metallicity, and height above the plane |z|, and are well described by a multiplicatively separable functional form. The dispersions have a power-law dependence on age with exponents of 0.441 ± 0.007 and 0.251 ± 0.006 for σz and σR, respectively, and the power law is valid even for the oldest stars. For the solar neighbourhood stars, the apparent break in the power law for older stars, as seen in previous studies, is due to the anticorrelation of Lz with age. The dispersions decrease with increasing Lz until we reach the Sun's orbital angular momentum, after which σz increases (implying flaring in the outer disc) while σR flattens. For a given age, the dispersions increase with decreasing metallicity, suggesting that the dispersions increase with birth radius. The dispersions also increase linearly with |z|. The same set of relations that work in the solar neighbourhood also work for stars between 3 < R/kpc < 20. Finally, the high-[α/Fe] stars follow the same relations as the low-[α/Fe] stars.

In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of -6.5 <= [Fe/H] <= -2.05 dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of [Fe/H] = - 3.31 and - 3.74, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that similar to 21 per cent of the sample have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Z(max)| <= 3 kpc. Of particular interest is a subsample (similar to 11 per cent of the total) of low |Z(max)| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = -4.30 and the subsample is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Z(max)|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Z(max)|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our sample (similar to 4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.

Open clusters are unique tracers of the history of our own Galaxy's disc. According to our membership analysis based on Gala astrometry, out of the 226 potential clusters falling in the footprint of the GALactic Archaeology with HERMES (GALAH) survey or the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we find that 205 have secure members that were observed by at least one of the surveys. 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 disc of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is -0.076 +/- 0.009 dex kpc(-1), which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the (Fe/Hi-guiding radius (r(guid)) plane is -0.073 +/- 0.008 dex kpc(-1). We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disc differently than field stars. In particular, at the 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(guid)-age space, which are important to understand production rates of different elements as a function of space and time.

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have [Fe/H] ≤ −2.75 and only approximately seven per cent have [Fe/H] > −2.0 dex. The most metal-poor candidate in the sample has [Fe/H] < −4.75 and is notably carbon rich. Except at the lowest metallicities ([Fe/H] < −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with [C/Fe]1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus [Fe/H]1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE > 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE < 7.3 and [C/Fe]1D, LTE > +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of ∆(Log N)/∆[Fe/H] = 1.5 ± 0.1 dex per dex for −4.0 ≤ [Fe/H] ≤ −2.75, but appears to drop abruptly at [Fe/H] ≈ −4.2, in line with previous studies.

Using kinematics from Gaia and the large elemental abundance space of the second data release of the GALAH survey, we identify two new members of the Fimbulthul stellar stream, and chemically tag them to massive,multimetallic globular cluster ω Centauri. Recent analysis of the second data release of Gaia had revealed the Fimbulthul stellar stream in the halo of the Milky Way. It had been proposed that the stream is associated with the ω Cen, but this proposition relied exclusively upon the kinematics and metallicities of the stars to make the association. In thiswork,we find our two newmembers of the stream to be metal-poor stars that are enhanced in sodium and aluminium, typical of second population globular cluster stars, but not otherwise seen in field stars. Furthermore, the stars share the s-process abundance pattern seen in ω Cen, which is rare in field stars. Apart from one star within 1.5 deg of ω Cen, we find no other stars observed by GALAH spatially near ω Cen or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to ω Cen confirms the earlier work, and further links this tidal feature in the Milky Way halo to ω Cen.

Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D MARCS model atmospheres that spans 3000 <= T-eff/K <= 8000, - 0.5 <= log g/cm s(-2) <= 5.5, and - 5 <= [Fe/H] <= 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between - 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the [A/Fe] versus [Fe/H] plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys.

We present the serendipitous discovery of the fastest main-sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a ∼2.35M⊙A-type star located at a distance of ∼9 kpc from the Sun and has a heliocentric radial velocity of 1017 ± 2.7 kms-1 without any signature of velocity variability. The current 3D velocity of the star in the Galactic frame is 1755 ± 50 kms-1. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A+ with a velocity of ∼1800 kms-1, travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion Vy⊙ = 246.1 ± 5.3 km s-1 or position R0 = 8.12 ± 0.23 kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disc of young stars at the Galactic Centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.

Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the Kepler satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We use an importance-sampling framework, which takes the selection function into account, to fit for the metallicities of a population synthesis model using spectroscopic data. We show that spectroscopic measurements of [Fe/H] and [α/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of log (Z/Z☉) = −0.16 for the thick disc. Here Z is the effective solar-scaled metallicity, which is a function of [Fe/H] and [α/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This indirectly verifies that the asteroseismic mass scaling relation is good to within five per cent. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old α-enhanced thick disc.

Chemical tagging seeks to identify unique star formation sites from present-day stellar abundances. Previous techniques have treated each abundance dimension as being statistically independent, despite theoretical expectations that many elements can be produced by more than one nucleosynthetic process. In this work, we introduce a data-driven model of nucleosynthesis, where a set of latent factors (e.g., nucleosynthetic yields) contribute to all stars with different scores and clustering (e.g., chemical tagging) is modeled by a mixture of multivariate Gaussians in a lower-dimensional latent space. We use an exact method to simultaneously estimate the factor scores for each star, the partial assignment of each star to each cluster, and the latent factors common to all stars, even in the presence of missing data entries. We use an information-theoretic Bayesian principle to estimate the number of latent factors and clusters. Using the second Galah data release, we find that six latent factors are preferred to explain N = 2566 stars with 17 chemical abundances. We identify the rapid- and slow neutron-capture processes, as well as latent factors consistent with Fe-peak and α-element production, and another where K and Zn dominate. When we consider N ∼ 160,000 stars with missing abundances, we find another seven factors, as well as 16 components in latent space. Despite these components showing separation in chemistry, which is explained through different yield contributions, none show significant structure in their positions or motions. We argue that more data and joint priors on cluster membership that are constrained by dynamical models are necessary to realize chemical tagging at a galactic-scale. We release accompanying software that scales well with the available data, allowing for the model's parameters to be optimized in seconds given a fixed number of latent factors, components, and ∼107 abundance measurements.

The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and 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, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code SPECTROSCOPY MADE EASY. We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g. Focusing our investigation on the correlations between stellar age, iron abundance [Fe/H], and mean alpha-enhancement [α/Fe] of the magnitude-selected sample, we recover the result that stars of the high-α sequence are typically older than stars in the low-α sequence, the latter spanning iron abundances of -0.7 < [Fe/H] < +0.5. While these two sequences become indistinguishable in [α/Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high-α and the low-α sequence even in this regime. When dissecting the sample by stellar age, we find that the old stars (>8 Gyr) have lower angular momenta Lz than the Sun, which implies that they are on eccentric orbits and originate from the inner disc. Contrary to some previous smaller scale st

Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates (R, φ, z), ridge-like structures can be seen in the (R, Vφ) plane and asymmetric arch-like structures in the (VR, Vφ) plane. We show that the ridges are also clearly present when the third dimension of the (R, Vφ) plane is represented by z, Vz, VR, [Fe/H], and [α/Fe]. The maps suggest that stars along the ridges lie preferentially close to the Galactic mid-plane (|z| < 0.2 kpc), and have metallicity and α elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy − the ridges and arches are simply surfaces of constant energy. We identify eight distinct ridges in the Gaia DR2 data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in z and Vz suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.

The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and 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, and subgiant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code SPECTROSCOPY MADE EASY. We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g. Focusing our investigation on the correlations between stellar age, iron abundance [Fe/H], and mean alpha-enhancement [alpha/Fe] of the magnitude-selected sample, we recover the result that stars of the high-a sequence are typically older than stars in the low-a sequence, the latter spanning iron abundances of -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-a and the low-a sequence even in this regime. When dissecting the sample by stellar age, 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 and originate from the inner disc. Contrary to some previous

We present a study using the second data release of the GALAH survey of stellar parameters and elemental abundances of 15 pairs of stars identified by Oh et al. They identified these pairs as potentially co-moving pairs using proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1 pc) pairs of stars do in fact have similar Galactic orbits, while a further four claimed co-moving pairs are not truly co-orbiting. Eight of the 11 co-orbiting pairs have reliable stellar parameters and abundances, and we find that three of those are quite similar in their abundance patterns, while five have significant [Fe/H] differences. For the latter, this indicates that they could be co-orbiting because of the general dynamical coldness of the thin disc, or perhaps resonances induced by the Galaxy, rather than a shared formation site. Stars such as these, wide binaries, debris of past star formation episodes, and coincidental co-orbiters, are crucial for exploring the limits of chemical tagging in the Milky Way.

The latest Gaia data release enables us to accurately identify stars that are more luminous than would be expected on the basis of their spectral type and distance. During an investigation of the 329 best solar twin candidates uncovered among the spectra acquired by the GALAH survey, we identified 64 such overluminous stars. In order to investigate their exact composition, we developed a data-driven methodology that can generate a synthetic photometric signature and spectrum of a single star. By combining multiple such synthetic stars into an unresolved binary or triple system and comparing the results to the actual photometric and spectroscopic observations, we uncovered 6 definitive triple stellar system candidates and an additional 14 potential candidates whose combined spectrum mimics the solar spectrum. Considering the volume correction factor for a magnitude-limited survey, the fraction of probable unresolved triple stars with long orbital periods is ~2 per cent. Possible orbital configurations of the candidates were investigated using the selection and observational limits. To validate the discovered multiplicity fraction, the same procedure was used to evaluate the multiplicity fraction of other stellar types.

If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. Recent results using the APOGEE survey find significant fluctuations in velocity for stars in the mid-plane (|z| <0.25 kpc) out to 5 kpc, suggesting that the dynamical influence of non-axisymmetric features, i.e. the Milky Way's bar, spiral arms, and merger events extends out to the Solar neighbourhood. Their measured power spectrum has a characteristic amplitude of 11 km s−1 on a scale of 2.5 kpc. The existence of such large-scale streaming motions has important implications for determining the Sun's motion about the Galactic Centre. Using Red Clump stars from GALAH and APOGEE, we map the line-of-sight velocities around the Sun (d < 5 kpc), and |z| <1.25 kpc from the mid-plane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual fluctuations and compare our findings with mock survey generated by GALAXIA. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier APOGEE power spectrum but with 20 per cent smaller amplitude (9.3 km s−1) after taking into account a few systematics (e.g. volume completeness). Using a flexible axisymmetric model the power amplitude is further reduced to 6.3 km s−1. Additionally, our simulations show that, in the plane, distances are underestimated for high-mass Red Clump stars which can lead to spurious power amplitude of about 5.2 km s−1. Taking this into account, we estimate the amplitude of real fluctuations to be <4.6 km s−1, about a factor of three less than the APOGEE result.

Large stellar surveys of the MilkyWay require validation with reference to a set of 'benchmark' stars whose fundamental properties are well determined. For metal-poor benchmark stars, disagreement between spectroscopic and interferometric effective temperatures has called the reliability of the temperature scale into question. We present new interferometric measurements of three metal-poor benchmark stars, HD140283, HD122563, and HD103095, from which we determine their effective temperatures. The angular sizes of all the stars were determined from observations with the PAVO beam combiner at visible wavelengths at the CHARA array, with additional observations of HD103095 made with the VEGA instrument, also at the CHARA array. Together with photometrically derived bolometric fluxes, the angular diameters give a direct measurement of the effective temperature. For HD140283, we find theta(LD) = 0.324 +/- 0.005 mas, T-eff = 5787 +/- 48 K; for HD122563,theta(LD) = 0.926 +/- 0.011 mas, T-eff = 4636 +/- 37 K; and for HD103095,theta(LD) = 0.595 +/- 0.007 mas, T-eff = 5140 +/- 49 K. Our temperatures for HD140283 and HD103095 are hotter than the previous interferometric measurements by 253 and 322 K, respectively. We find good agreement between our temperatures and recent spectroscopic and photometric estimates. We conclude some previous interferometric measurements have been affected by systematic uncertainties larger than their quoted errors.

© 2018 The Author(s). Sparse open clusters can be found at high galactic latitudes where loosely populated clusters are more easily detected against the lower stellar background. Because most star formation takes place in the thin disc, the observed population of clusters far from the Galactic plane is hard to explain. We combined spectral parameters from the GALAH survey with the Gaia DR2 catalogue to study the dynamics and chemistry of five old sparse high-latitude clusters in more detail. We find that four of them (NGC 1252, NGC 6994, NGC 7772, NGC 7826) - originally classified in 1888-are not clusters but are instead chance projections on the sky. Member stars quoted in the literature for these four clusters are unrelated in our multidimensional physical parameter space; the quoted cluster properties in the literature are therefore meaningless. We confirm the existence of visually similar NGC 1901 for which we provide a probabilistic membership analysis. An overdensity in three spatial dimensions proves to be enough to reliably detect sparse clusters, but the whole six-dimensional space must be used to identify members with high confidence, as demonstrated in the case of NGC 1901.

GALAH is a large-scale magnitude-limited southern stellar spectroscopic survey. Its second data release (GALAH DR2) provides values of stellar parameters and abundances of 23 elements for 342 682 stars (Buder et al.). Here we add a description of the public release of radial velocities with a typical accuracy of 0.1 km s-1 for 336 215 of these stars, achievable due to the large wavelength coverage, high resolving power, and good signal-to-noise ratio of the observed spectra, but also because convective motions in stellar atmosphere and gravitational redshift from the star to the observer are taken into account. In the process we derive medians of observed spectra that are nearly noiseless, as they are obtained from between 100 and 1116 observed spectra belonging to the same bin with a width of 50 K in temperature, 0.2 dex in gravity, and 0.1 dex in metallicity. Publicly released 1181 median spectra have a resolving power of 28 000 and trace the well-populated stellar types with metallicities between -0.6 and +0.3. Note that radial velocities from GALAH are an excellent match to the accuracy of velocity components along the sky plane derived by Gaia for the same stars. The level of accuracy achieved here is adequate for studies of dynamics within stellar clusters, associations, and streams in the Galaxy. So it may be relevant for studies of the distribution of dark matter.

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of theMilkyWay, designed to deliver complementary chemical information to a large number of stars covered by the Gaia mission. We present the GALAH second public data release (GALAH DR2) containing 342 682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction, and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multistep approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels (Teff, logg, [Fe/H], [X/Fe], vmic, vsin i, AKS) for a representative training set of stars. This information is then propagated to the whole sample with the data-driven method of The Cannon. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D MARCS stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence to our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from Gaia will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.

Open cluster members are coeval and share the same initial bulk chemical composition. Consequently, differences in surface abundances between members of a cluster that are at different evolutionary stages can be used to study the effects of mixing and internal chemical processing. We carry out an abundance analysis of seven elements (Li, O, Na, Mg, Al, Si, and Fe) in 66 stars belonging to the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model atmospheres, and non-local thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE analysis, we find a typical star-to-star scatter in the abundance ratios of around 0.05 dex. We find trends in the abundance ratios with effective temperature, indicating systematic differences in the surface abundances between turn-off and giant stars; these trends are more pronounced when LTE is assumed. However, trends with effective temperature remain significant for Al and Si also in non-LTE. Finally, we compare the derived abundances with prediction from stellar evolution models including effects of atomic diffusion. We find overall good agreement for the abundance patterns of dwarfs and sub-giant stars, but the abundances of cool giants are lower relative to less evolved stars than predicted by the diffusion models, in particular for Mg.