Resolution Multi Element Spectrograph Research Articles

Survey for Distant Solar Twins (SDST) – III. Identification of new solar twin and solar analogue stars

ABSTRACT The Survey for Distant Solar Twins aims to find stars very similar to the Sun at distances of 1–$4\, {\rm kpc}$, several times more distant than any currently known solar twins and analogues. The goal is to identify the best stars with which to test whether the fine-structure constant, α, varies with dark matter density in our Galaxy. Here, we use epic, our line-by-line differential technique, to measure the stellar parameters – effective temperature Teff, surface gravity log g, and metallicity [Fe/H] – from moderate-resolution (R ≲ 32 000) spectra of 877 solar twin and analogue candidates (547 at 1–$4\, {\rm kpc}$) observed with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) on the Anglo-Australian Telescope. These are consistent with expectations for Teff and log g from photometry, and for [Fe/H] from the Besançon stellar population model. epic provides small enough uncertainties ($\sim 90\, {\rm K}$, $0.08\, {\rm dex}$, and $0.05\, {\rm dex}$, respectively), even at the low signal-to-noise ratios available (${\rm S/N}\gtrsim$ 25 per pixel), to identify 299 new solar analogues ($\ge 90~{{\ \rm per\ cent}}$ confidence) and 20 solar twins (≥50 per cent confidence), 206 and 12 of which are at 1–$4\, {\rm kpc}$. By extending epic to measure line broadening and lithium abundance from HERMES spectra, and with ages derived from isochrone fitting with our stellar parameters, we identify 174 solar analogues at 1–$4\, {\rm kpc}$ that are relatively inactive, slowly rotating, and with no evidence of spectroscopic binarity. These are the preferred targets for follow-up spectroscopy to measure α.

J-PLUS: Stellar parameters, C, N, Mg, Ca, and [α/Fe] abundances for two million stars from DR1

Context. The Javalambre Photometric Local Universe Survey (J-PLUS) has obtained precise photometry in 12 specially designed filters for large numbers of Galactic stars. Deriving their precise stellar atmospheric parameters and individual elemental abundances is crucial for studies of Galactic structure and the assembly history and chemical evolution of our Galaxy. Aims. Our goal is to estimate not only stellar parameters (effective temperature, Teff, surface gravity, log g, and metallicity, [Fe/H]), but also [α/Fe] and four elemental abundances ([C/Fe], [N/Fe], [Mg/Fe], and [Ca/Fe]) using data from the first data release (DR1) of J-PLUS. Methods. By combining recalibrated photometric data from J-PLUS DR1, Gaia DR2, and spectroscopic labels from the Large sky Area Multi-Object fiber Spectroscopic Telescope, we designed and trained a set of cost-sensitive neural networks, the CSNet, to learn the nonlinear mapping from stellar colours to their labels. Special attention was paid to the poorly populated regions of the label space by giving different weights according to their density distribution. Results. We achieved precisions of δ Teff ∼ 55 K, δlog g ∼ 0.15 dex, and δ [Fe/H] ∼ 0.07 dex, respectively, over a wide range of temperatures, surface gravities, and metallicities. The uncertainties of the abundance estimates for [α/Fe] and the four individual elements are in the 0.04–0.08 dex range. We compare our parameter and abundance estimates with those from other spectroscopic catalogs such as the Apache Point Observatory for Galactic Evolution Experiment and the Galactic Archaeology with High Efficiency and Resolution Multi-Element Spectrograph and find an overall good agreement. Conclusions. Our results demonstrate the potential of well-designed, high-quality photometric data for determinations of stellar parameters as well as individual elemental abundances. Applying the method to J-PLUS DR1, we obtained the aforementioned parameters for about two million stars, providing an outstanding dataset for chemo-dynamic analyses of the Milky Way. The catalog of the estimated parameters is publicly accessible.

Estimating [α/Fe] from Gaia low-resolution BP/RP spectra using the ExtraTrees algorithm

Context. Gaia Data Release 3 will contain more than a billion sources with positions, parallaxes, and proper motions. In addition, for hundreds of millions of stars, it will include low-resolution blue photometer (BP) and red photometer (RP) spectra. Obtained by dispersing light with prisms, these spectra have resolutions that are too low to allow us to measure individual spectral lines and bands. However, the combined BP/RP spectra can be used to estimate some stellar properties such as Teff, log g, and [M/H]. Aims. We investigate the feasibility of using the ExtraTrees algorithm to estimate the alpha element to iron abundance ratio [α/Fe] from low-resolution BP/RP spectra. Methods. To infer [α/Fe] from the spectra, we created regression models using the ExtraTrees algorithm trained on two samples: a set of synthetic spectra and a set of observed spectra from stars that have known [α/Fe] since they have also been observed using the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) as part of the Galactic Archaeology with HERMES survey. We applied each model to the other sample and to a larger observed sample to assess the performance of the models. In addition, we used our models to analyse stars from the Gaia-Enceladus structure. Results. We find that the model trained on synthetic data has some ability to reconstruct [α/Fe] from synthetic spectra, but little to none when used on observed spectra. The model trained on observed data reconstructs realistic [α/Fe] from observed spectra, but only for cool stars (⪅5000 K) that have the same correlations as in the training sample between [α/Fe] and other properties such as [Fe/H]. Conclusions. Models using the ExtraTrees algorithm can be used to estimate [α/Fe] from low-resolution BP/RP spectra of cool stars. However, they do this by exploiting correlations between [α/Fe] and other parameters, rather than the causal effect of [α/Fe] on the spectrum. Hence, they are unlikely to be useful in studies that attempt to distinguish stars that only differ in [α/Fe].

HERBS I: Metallicity and alpha enhancement along the Galactic bulge minor axis

ABSTRACT To better understand the origin and evolution of the Milky Way bulge, we have conducted a survey of bulge red giant branch and clump stars using the High Efficiency and Resolution Multi-Element Spectrograph on the Anglo–Australian Telescope. We targeted ARGOS survey stars with predetermined bulge memberships, covering the full metallicity distribution function. The spectra have signal-to-noise ratios comparable to, and were analysed using the same methods as the GALAH survey. In this work, we present the survey design, stellar parameters, distribution of metallicity, and alpha-element abundances along the minor bulge axis at latitudes b = −10°, − 7.5°, and −5°. Our analysis of ARGOS stars indicates that the centroids of ARGOS metallicity components should be located ≈0.09 dex closer together. The vertical distribution of α-element abundances is consistent with the varying contributions of the different metallicity components. Closer to the plane, alpha abundance ratios are lower as the metal-rich population dominates. At higher latitudes, the alpha abundance ratios increase as the number of metal-poor stars increases. However, we find that the trend of alpha-enrichment with respect to metallicity is independent of latitude. Comparison of our results with those of GALAH DR2 revealed that for [Fe/H] ≈ −0.8, the bulge shares the same abundance trend as the high-α disc population. However, the metal-poor bulge population ([Fe/H] ≲ −0.8) show enhanced alpha abundance ratios compared to the disc/halo. These observations point to fairly rapid chemical evolution in the bulge, and that the metal-poor bulge population does not share the same similarity with the disc as the more metal-rich populations.

The GALAH survey: observational overview andGaiaDR1 companion

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$\simeq$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 3 kpc and giants at 1 to 10 kpc. 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.

First light results from the High Efficiency and Resolution Multi-Element Spectrograph at the Anglo-Australian Telescope

The High Efficiency and Resolution Multi Element Spectrograph, HERMES, is a facility-class optical spectrograph for the Anglo-Australian Telescope (AAT). It is designed primarily for Galactic Archaeology, the first major attempt to create a detailed understanding of galaxy formation and evolution by studying the history of our own galaxy, the Milky Way. The goal of the GALAH survey is to reconstruct the mass assembly history of the Milky Way through a detailed chemical abundance study of one million stars. The spectrograph is based at the AAT and is fed by the existing 2dF robotic fiber positioning system. The spectrograph uses volume phase holographic gratings to achieve a spectral resolving power of 28,000 in standard mode and also provides a high-resolution mode ranging between 40,000 and 50,000 using a slit mask. The GALAH survey requires an SNR greater than 100 for a star brightness of V=14 in an exposure time of one hour. The total spectral coverage of the four channels is about 100 nm between 370 and 1000 nm for up to 392 simultaneous targets within the 2-degree field of view. HERMES has been commissioned over three runs, during bright time in October, November, and December 2013, in parallel with the beginning of the GALAH pilot survey, which started in November 2013. We present the first-light results from the commissioning run and the beginning of the GALAH survey, including performance results such as throughput and resolution, as well as instrument reliability.