- ID:
- ivo://CDS.VizieR/J/MNRAS/483/3196
- Title:
- GALAH carbon-enhanced stars & CEMP candidates
- Short Name:
- J/MNRAS/483/3196
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Swan bands - characteristic molecular absorption features of the C2 molecule - are a spectroscopic signature of carbon-enhanced stars. They can also be used to identify carbon-enhanced metal-poor (CEMP) stars. The GALAH (GALactic Archaeology with Hermes) is a magnitude-limited survey of stars producing high-resolution, high-signal-to-noise spectra. We used 627708 GALAH spectra to search for carbon-enhanced stars with a supervised and unsupervised classification algorithm, relying on the imprint of the Swan bands. We identified 918 carbon-enhanced stars, including 12 already described in the literature. An unbiased selection function of the GALAH survey allows us to perform a population study of carbon-enhanced stars. Most of them are giants, out of which we find 28 CEMP candidates. A large fraction of our carbon-enhanced stars with repeated observations show variation in radial velocity, hinting that there is a large fraction of variables among them. 32 of the detected stars also show strong Lithium enhancement in their spectra.
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Search Results
- ID:
- ivo://CDS.VizieR/J/MNRAS/500/4849
- Title:
- GALAH emission-line stars
- Short Name:
- J/MNRAS/500/4849
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a neural network autoencoder structure that is able to extract essential latent spectral features from observed spectra and then reconstruct a spectrum from those features. Because of the training with a set of unpeculiar spectra, the network is able to reproduce a spectrum of high signal-to-noise ratio that does not show any spectral peculiarities, even if they are present in an observed spectrum. Spectra generated in this manner were used to identify various emission features among spectra acquired by multiple surveys using the HERMES spectrograph at the Anglo-Australian telescope. Emission features were identified by a direct comparison of the observed and generated spectra. Using the described comparison procedure, we discovered 10364 candidate spectra with varying intensities (from partially filled-in to well above the continuum) of the H{alpha}/H{beta} emission component, produced by different physical mechanisms. A fraction of these spectra belong to the repeated observation that shows temporal variability in their emission profile. Among the emission spectra, we find objects that feature contributions from a nearby rarefied gas (identified through the emission of [NII] and [SII] lines) that was identified in 4004 spectra, which were not all identified as having H{alpha} emission. The positions of identified emission-line objects coincide with multiple known regions that harbour young stars. Similarly, detected nebular emission spectra coincide with visually prominent nebular clouds observable in the red all-sky photographic composites.
- ID:
- ivo://CDS.VizieR/J/MNRAS/465/3203
- Title:
- GALAH observational overview
- Short Name:
- J/MNRAS/465/3203
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Galactic Archaeology with HERMES (GALAH) Survey is a massive observational project to trace the Milky Way's history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R~28,000) spectra taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo-Australian Telescope (AAT), GALAH will determine stellar parameters and abundances of up to 29 elements for up to one million stars. Selecting targets from a colour-unbiased catalogue built from 2MASS, APASS and UCAC4 data, we expect to observe dwarfs at 0.3 to 3kpc and giants at 1 to 10kpc. This enables a thorough local chemical inventory of the Galactic thin and thick disks, and also captures smaller samples of the bulge and halo. In this paper we present the plan, process and progress as of early 2016 for GALAH survey observations. In our first two years of survey observing we have accumulated the largest high-quality spectroscopic data set at this resolution, over 200,000 stars. We also present the first public GALAH data catalogue: stellar parameters (Teff, log(g), [Fe/H], [alpha/Fe]), radial velocity, distance modulus and reddening for 10680 observations of 9860 Tycho-2 stars that may be included in the first Gaia data release.
- ID:
- ivo://CDS.VizieR/J/ApJS/228/24
- Title:
- GALAH semi-automated classification scheme
- Short Name:
- J/ApJS/228/24
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Galah is an ongoing high-resolution spectroscopic survey with the goal of disentangling the formation history of the Milky Way using the fossil remnants of disrupted star formation sites that are now dispersed around the Galaxy. It is targeting a randomly selected magnitude-limited (V<=14) sample of stars, with the goal of observing one million objects. To date, 300000 spectra have been obtained. Not all of them are correctly processed by parameter estimation pipelines, and we need to know about them. We present a semi-automated classification scheme that identifies different types of peculiar spectral morphologies in an effort to discover and flag potentially problematic spectra and thus help to preserve the integrity of the survey results. To this end, we employ the recently developed dimensionality reduction technique t-SNE (t-distributed stochastic neighbor embedding), which enables us to represent the complex spectral morphology in a two-dimensional projection map while still preserving the properties of the local neighborhoods of spectra. We find that the majority (178483) of the 209533 Galah spectra considered in this study represents normal single stars, whereas 31050 peculiar and problematic spectra with very diverse spectral features pertaining to 28579 stars are distributed into 10 classification categories: hot stars, cool metal-poor giants, molecular absorption bands, binary stars, H{alpha}/H{beta} emission, H{alpha}/H{beta} emission superimposed on absorption, H{alpha}/H{beta} P-Cygni, H{alpha}/H{beta} inverted P-Cygni, lithium absorption, and problematic. Classified spectra with supplementary information are presented in the catalog, indicating candidates for follow-up observations and population studies of the short-lived phases of stellar evolution.
- ID:
- ivo://CDS.VizieR/J/A+A/624/A19
- Title:
- GALAH survey, chemodynamical analyse with TGAS
- Short Name:
- J/A+A/624/A19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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 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 [{alpha}/Fe] of the magnitude-selected sample, we recover the result that stars of the high-{alpha} sequence are typically older than stars in the low-{alpha} 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-alpha and the low-alpha sequence even in this regime. When dissecting the sample by stellar age, we find that the old stars (>8Gyr) 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 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 10Gyr, are mainly found on the low alpha-sequence and show a gradient in L_z_ from low [Fe/H] (L_z_>L_z,{sun}_) towards higher [Fe/H] (L_z_<L_z,{sun}_), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.
6806. GALAH Survey DR2
- ID:
- ivo://CDS.VizieR/J/MNRAS/478/4513
- Title:
- GALAH Survey DR2
- Short Name:
- J/MNRAS/478/4513
- Date:
- 04 Feb 2022 00:12:04
- Publisher:
- CDS
- Description:
- The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way, 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, vsini, A_KS_) 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.
- ID:
- ivo://CDS.VizieR/J/A+A/638/A145
- Title:
- GALAH survey. FGK binary stars
- Short Name:
- J/A+A/638/A145
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Binary stellar systems form a large fraction of the Galaxy's stars. They are useful as laboratories for studying the physical processes taking place within stars, and must be correctly taken into account when observations of stars are used to study the structure and evolution of the Galaxy. We present a sample of 12760 well-characterised double-lined spectroscopic binaries that are appropriate for statistical studies of the binary populations. They were detected as SB2s using a t-distributed stochastic neighbour embedding (t-SNE) classification and a cross-correlation analysis of GALAH spectra. This sample consists mostly of dwarfs, with a significant fraction of evolved stars and several dozen members of the giant branch. To compute parameters of the primary and secondary star (Teff[1,2], logg[1,2], [Fe/H], Vr[1,2], vmic[1,2], vbroad[1,2], R[1,2], and E(B-V)), we used a Bayesian approach that includes a parallax prior from Gaia DR2, spectra from GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. The derived stellar properties and their distributions show trends that are expected for a population of close binaries (a<10AU) with mass ratios 0.5<=q<=1. The derived metallicity of these binary stars is statistically lower than that of single dwarf stars from the same magnitude-limited sample.
- ID:
- ivo://CDS.VizieR/J/A+A/639/A145
- Title:
- GALAH survey. FGK binary stars (Traven+, 2020)
- Short Name:
- J/A+A/639/A145
- Date:
- 30 Jun 2020 06:55:32
- Publisher:
- CDS
- Description:
- We here analyse a specific data-set: the extended GALAH dataset. This consists of stellar spectra from the GALAH survey (reduced as explained in Kos et al., 2017MNRAS.464.1259K), apparent magnitudes from a variety of photometric catalogues (AAVSO Photometric All Sky Survey - APASS; Henden et al. 2016, Cat. II/336, Gaia DR2; Gaia Collaboration et al. 2018, Cat. I/345. Two Micron All Sky Survey - 2MASS; Skrutskie et al. 2006, Cat. VII/233, Wide-field Infrared Survey Explorer - WISE; Wright et al. 2010, Cat. II/311), and the parallax measurements from Gaia DR2. The data provided in this catalogue are described in Table A.1 of the paper.
- ID:
- ivo://CDS.VizieR/J/MNRAS/503/3279
- Title:
- GALAH survey. Galactic disc with open clusters
- Short Name:
- J/MNRAS/503/3279
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Open clusters are unique tracers of the history of our own Galaxy's disc. According to our membership analysis based on Gaia 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+/-0.009dex/kpc, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H]-guiding radius (r_guid_) plane is -0.073+/-0.008dex/kpc. 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.
- ID:
- ivo://CDS.VizieR/J/MNRAS/487/2474
- Title:
- GALAH unresolved triple Sun-like star
- Short Name:
- J/MNRAS/487/2474
- Date:
- 02 Mar 2022 00:37:36
- Publisher:
- CDS
- Description:
- 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 candidate 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.