34 research outputs found
The GALAH survey: Multiple stars and our Galaxy. I. A comprehensive method for deriving properties of FGK binary stars
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 (, , [Fe/H], ,
, , , and ), 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 10 AU) with mass ratios . The derived metallicity of these binary stars is statistically
lower than that of single dwarf stars from the same magnitude-limited sample.Comment: Accepted for publication in A&
The GALAH survey: Co-orbiting stars and chemical tagging
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 2017. They identified these pairs as potentially co-moving pairs using
proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1.7
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.Comment: 14 pages, 9 figures, submitted to MNRAS. Updated for Gaia DR2 value
The GALAH survey: Co-orbiting stars and chemical tagging
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 2017. They identified these pairs as potentially co-moving pairs using
proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1.7
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.Comment: 14 pages, 9 figures, submitted to MNRAS. Updated for Gaia DR2 value
The GALAH Survey : Non-LTE departure coefficients for large spectroscopic surveys
19 pages, 25 figures, 2 tables, arXiv abstract abridged; accepted for publication in A&AMassive 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 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 1D MARCS model atmospheres that spans , , and . 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 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between and for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the versus plane by up to , and it can remove spurious differences between the dwarfs and giants by up to . The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy.Peer reviewe
Topographic Correction Module at Storm (TC@Storm)
Different solar position in combination with terrain slope and aspect result in different illumination of inclined surfaces. Therefore,
the retrieved satellite data cannot be accurately transformed to the spectral reflectance, which depends only on the land cover. The
topographic correction should remove this effect and enable further automatic processing of higher level products. The topographic
correction TC@STORM was developed as a module within the SPACE-SI automatic near-real-time image processing chain
STORM. It combines physical approach with the standard Minnaert method. The total irradiance is modelled as a three-component
irradiance: direct (dependent on incidence angle, sun zenith angle and slope), diffuse from the sky (dependent mainly on sky-view
factor), and diffuse reflected from the terrain (dependent on sky-view factor and albedo). For computation of diffuse irradiation from
the sky we assume an anisotropic brightness of the sky. We iteratively estimate a linear combination from 10 different models, to
provide the best results. Dependent on the data resolution, we mask shades based on radiometric (image) or geometric properties.
The method was tested on RapidEye, Landsat 8, and PROBA-V data. Final results of the correction were evaluated and statistically
validated based on various topography settings and land cover classes. Images show great improvements in shaded areas
Automatic Near-Real-Time Image Processing Chain for Very High Resolution Optical Satellite Data
In response to the increasing need for automatic and fast satellite image processing SPACE-SI has developed and implemented a
fully automatic image processing chain STORM that performs all processing steps from sensor-corrected optical images (level 1) to
web-delivered map-ready images and products without operator's intervention.
Initial development was tailored to high resolution RapidEye images, and all crucial and most challenging parts of the planned full
processing chain were developed: module for automatic image orthorectification based on a physical sensor model and supported by
the algorithm for automatic detection of ground control points (GCPs); atmospheric correction module, topographic corrections
module that combines physical approach with Minnaert method and utilizing anisotropic illumination model; and modules for high
level products generation. Various parts of the chain were implemented also for WorldView-2, THEOS, Pleiades, SPOT 6, Landsat
5-8, and PROBA-V. Support of full-frame sensor currently in development by SPACE-SI is in plan.
The proposed paper focuses on the adaptation of the STORM processing chain to very high resolution multispectral images. The
development concentrated on the sub-module for automatic detection of GCPs. The initially implemented two-step algorithm that
worked only with rasterized vector roads and delivered GCPs with sub-pixel accuracy for the RapidEye images, was improved with
the introduction of a third step: super-fine positioning of each GCP based on a reference raster chip. The added step exploits the high
spatial resolution of the reference raster to improve the final matching results and to achieve pixel accuracy also on very high
resolution optical satellite data
Machine learning techniques meet binaries
We briefly review the various machine learning methods and discuss how they can be used in efficient identification and analysis of spectroscopic binary stars. They can be treated as complementary to conventional methods, and we argue that some amount of human oversight is always needed and in fact highly beneficial when employing machine learning. We propose that a general dimensionality reduction technique can serve to diagnose and classify a given data set, and in case of GALAH spectra, our method quite effectively reveals a population of SB2 and SB3 systems. Once identified, the binary spectra can be analysed with the help of generative models, which can be constructed using machine learning techniques such as The Cannon and The Payne. Furthermore, in the case of spectroscopically unresolved multiple stars, we can recover the multiple contributions to an observed spectrum by reversing the process and proceeding from analysis to identification
The GALAH Survey: lithium-strong KM dwarfs
Identifying and characterizing young stars in the Solar neighbourhood is essential to find and describe planets in the early stages of their evolution. This work seeks to identify nearby young stars showing a lithium 6708 Å absorption line in the GALAH survey. A robust, data- driven approach is used to search for corresponding templates in the pool of 434 215 measured dwarf spectra in the survey. It enables a model-free search for best-matching spectral templates for all stars, including M dwarfs with strong molecular absorption bands. 3147 stars have been found to have measurable lithium: 1408 G and 892 K0-K5 dwarfs (EW(Li) > 0.1 Å), 335 K5-K9 (>0.07 Å) and 512 M0-M4 dwarfs (>0.05 Å). Stars with such lithium features are used to investigate the possibility of searching for young stars above the main sequence based merely on their parallaxes and broad-band photometry. The selection of young stars above the main sequence is highly effective for M dwarfs, moderately effective for K dwarfs and ineffective for G dwarfs. Using a combination of the lithium information and the complete 6D kinematics from Gaia and GALAH, 305 new candidate moving group members have been found, 123 of which belong to the Scorpius-Centaurus association, 36 to the Pleiades and 25 to the Hyades clusters
The GALAH Survey: lithium-strong KM dwarfs
Identifying and characterizing young stars in the Solar neighbourhood is essential to find and describe planets in the early stages of their evolution. This work seeks to identify nearby young stars showing a lithium 6708 Å absorption line in the GALAH survey. A robust, data- driven approach is used to search for corresponding templates in the pool of 434 215 measured dwarf spectra in the survey. It enables a model-free search for best-matching spectral templates for all stars, including M dwarfs with strong molecular absorption bands. 3147 stars have been found to have measurable lithium: 1408 G and 892 K0-K5 dwarfs (EW(Li) > 0.1 Å), 335 K5-K9 (>0.07 Å) and 512 M0-M4 dwarfs (>0.05 Å). Stars with such lithium features are used to investigate the possibility of searching for young stars above the main sequence based merely on their parallaxes and broad-band photometry. The selection of young stars above the main sequence is highly effective for M dwarfs, moderately effective for K dwarfs and ineffective for G dwarfs. Using a combination of the lithium information and the complete 6D kinematics from Gaia and GALAH, 305 new candidate moving group members have been found, 123 of which belong to the Scorpius-Centaurus association, 36 to the Pleiades and 25 to the Hyades clusters