| 1. |
- de Jong, R. S., et al.
(author)
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4MOST : Project overview and information for the First Call for Proposals
- 2019
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In: The Messenger. - : European Southern Observatory. - 0722-6691. ; 175, s. 3-11
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Journal article (other academic/artistic)abstract
- We introduce the 4-metre Multi-Object Spectroscopic Telescope (4MOST), a new high-multiplex, wide-field spectroscopic survey facility under development for the four-metre-class Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal. Its key specifications are: a large field of view (FoV) of 4.2 square degrees and a high multiplex capability, with 1624 fibres feeding two low-resolution spectrographs (R = λ/Δλ ~ 6500), and 812 fibres transferring light to the high-resolution spectrograph (R ~ 20 000). After a description of the instrument and its expected performance, a short overview is given of its operational scheme and planned 4MOST Consortium science; these aspects are covered in more detail in other articles in this edition of The Messenger. Finally, the processes, schedules, and policies concerning the selection of ESO Community Surveys are presented, commencing with a singular opportunity to submit Letters of Intent for Public Surveys during the first five years of 4MOST operations.
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| 2. |
- Caffau, E., et al.
(author)
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Velocity and abundance precisions for future high-resolution spectroscopic surveys: A study for 4MOST
- 2013
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In: Astronomical Notes - Astronomische Nachrichten. - : Wiley. - 0004-6337. ; 334:3, s. 197-216
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Journal article (peer-reviewed)abstract
- In preparation for future, large-scale, multi-object, high-resolution spectroscopic surveys of the Galaxy, we present a series of tests of the precision in radial velocity and chemical abundances that any such project can achieve at a 4 m class telescope. We briefly discuss a number of science cases that aim at studying the chemo-dynamical history of the major Galactic components (bulge, thin and thick disks, and halo) - either as a follow-up to the Gaia mission or on their own merits. Based on a large grid of synthetic spectra that cover the full range in stellar parameters of typical survey targets, we devise an optimal wavelength range and argue for a moderately high-resolution spectrograph. As a result, the kinematic precision is not limited by any of these factors, but will practically only suffer from systematic effects, easily reaching uncertainties <1km s(-1). Under realistic survey conditions (namely, considering stars brighter than r = 16 mag with reasonable exposure times) we prefer an ideal resolving power of R similar to 20 000 on average, for an overall wavelength range (with a common two-arm spectrograph design) of [395;456.5] nm and [587; 673] nm. We show for the first time on a general basis that it is possible to measure chemical abundance ratios to better than 0.1 dex for many species (Fe, Mg, Si, Ca, Ti, Na, Al, V, Cr, Mn, Co, Ni, Y, Ba, Nd, Eu) and to an accuracy of about 0.2 dex for other species such as Zr, La, and Sr. While our feasibility study was explicitly carried out for the 4MOST facility, the results can be readily applied to and used for any other conceptual design study for high-resolution spectrographs. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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| 3. |
- De Jong, R. S., et al.
(author)
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4MOST - 4-metre multi-object spectroscopic telescope
- 2012
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In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 9780819491473 ; , s. 84460T-
-
Conference paper (peer-reviewed)abstract
- The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on the telescope to perform a 5 year public survey yielding more than 20 million spectra at resolution R∼5000 (λ=390-1000 nm) and more than 2 million spectra at R∼20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid. Initial design and performance estimates for the wide-field corrector concepts are presented. Two fibre positioner concepts are being considered for 4MOST. The first one is a Phi-Theta system similar to ones used on existing and planned facilities. The second one is a new R-Theta concept with large patrol area. Both positioner concepts effectively address the issues of fibre focus and pupil pointing. The 4MOST spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high- and low-resolution fibres. A full facility simulator is being developed to guide trade-off decisions regarding the optimal field-of-view, number of fibres needed, and the relative fraction of high-to-low resolution fibres. The simulator takes mock catalogues with template spectra from Design Reference Surveys as starting point, calculates the output spectra based on a throughput simulator, assigns targets to fibres based on the capabilities of the fibre positioner designs, and calculates the required survey time by tiling the fields on the sky. The 4MOST consortium aims to deliver the full 4MOST facility by the end of 2018 and start delivering high-level data products for both consortium and ESO community targets a year later with yearly increments.
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| 4. |
- Tempel, E., et al.
(author)
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An optimized tiling pattern for multiobject spectroscopic surveys : Application to the 4MOST survey
- 2020
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 497:4, s. 4626-4643
-
Journal article (peer-reviewed)abstract
- Large multiobject spectroscopic surveys require automated algorithms to optimize their observing strategy. One of the most ambitious upcoming spectroscopic surveys is the 4MOST survey. The 4MOST survey facility is a fibre-fed spectroscopic instrument on the VISTA telescope with a large enough field of view to survey a large fraction of the southern sky within a few years. Several Galactic and extragalactic surveys will be carried out simultaneously, so the combined target density will strongly vary. In this paper, we describe a new tiling algorithm that can naturally deal with the large target density variations on the sky and which automatically handles the different exposure times of targets. The tiling pattern is modelled as a marked point process, which is characterized by a probability density that integrates the requirements imposed by the 4MOST survey. The optimal tilling pattern with respect to the defined model is estimated by the tiles configuration that maximizes the proposed probability density. In order to achieve this maximization a simulated annealing algorithm is implemented. The algorithm automatically finds an optimal tiling pattern and assigns a tentative sky brightness condition and exposure time for each tile, while minimizing the total execution time that is needed to observe the list of targets in the combined input catalogue of all surveys. Hence, the algorithm maximizes the long-term observing efficiency and provides an optimal tiling solution for the survey. While designed for the 4MOST survey, the algorithm is flexible and can with simple modifications be applied to any other multiobject spectroscopic survey.
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| 5. |
- Walcher, C.~J., et al.
(author)
-
4MOST Scientific Operations
- 2019
-
In: Messenger. - 0722-6691. ; 175, s. 12-16
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Journal article (other academic/artistic)abstract
- The 4MOST instrument is a multi-object spectrograph that will address Galactic and extragalactic science cases simultaneously by observing targets from a large number of different surveys within each science exposure. This parallel mode of operation and the survey nature of 4MOST require some distinct 4MOST- specific operational features within the overall operations model of ESO. The main feature is that the 4MOST Consortium will deliver, not only the instrument, but also contractual services to the user community, which is why 4MOST is also described as a facility. This white paper concentrates on information particularly useful to answering the forthcoming Call for Letters of Intent.
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| 6. |
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| 7. |
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| 8. |
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| 9. |
- Christlieb, N., et al.
(author)
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The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements
- 2004
-
In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 428:3, s. 1027-1037
-
Journal article (peer-reviewed)abstract
- We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe]>1 dex; hereafter r-IIstars), the Hamburg/ESO R-process Enhanced Star survey (HERES).Moderate-resolution (∼2 Å) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H]<-2.5 dex. For several hundred confirmed metal-poor giants brighter than B∼ 16.5 mag (most of them from theHES), ``snapshot'' spectra (R∼ 20 000; S/N ∼ 30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey,eight from the HES, and four comparison stars. We discovered two new r-II stars, CS 29497-004 ([Eu/Fe]=1.64± 0.22) and CS 29491-069([Eu/Fe]=1.08± 0.23). A first abundance analysis of CS 29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5 dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS 29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS 31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.Based in large part on observations collected at the European Southern Observatory, Paranal, Chile (proposal number 68.B-0320).}
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| 10. |
- Christlieb, N., et al.
(author)
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The stellar content of the Hamburg/ESO survey - IV. Selection of candidate metal-poor stars
- 2008
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 484:3, s. 721-732
-
Journal article (peer-reviewed)abstract
- We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, B - V colors (both measured directly from the digital HES spectra), as well as J - K colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 angstrom, and a calibration of the HES B - V colors, using CCD photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color range 0.3 < B - V < 1.4. These accuracies make it possible to reliably reject stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 deg(2) of the southern high Galactic latitude sky. The candidate sample consists of 20 271 stars in the magnitude range 10 less than or similar to B less than or similar to 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e. g., the three most heavy-element deficient stars currently known.
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| 11. |
- Duffau, S., et al.
(author)
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The Gaia-ESO Survey : Galactic evolution of sulphur and zinc
- 2017
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 604
-
Journal article (peer-reviewed)abstract
- Context. Due to their volatile nature, when sulphur and zinc are observed in external galaxies, their determined abundances represent the gas-phase abundances in the interstellar medium. This implies that they can be used as tracers of the chemical enrichment of matter in the Universe at high redshift. Comparable observations in stars are more difficult and, until recently, plagued by small number statistics. Aims. We wish to exploit the Gaia-ESO Survey (GES) data to study the behaviour of sulphur and zinc abundances of a large number of Galactic stars, in a homogeneous way. Methods. By using the UVES spectra of the GES sample, we are able to assemble a sample of 1301 Galactic stars, including stars in open and globular clusters in which both sulphur and zinc were measured. Results. We confirm the results from the literature that sulphur behaves as an α-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The lower ratios are observed in giant stars at Galactocentric distances less than 7.5 kpc. No such effect is observed among dwarf stars, since they do not extend to that radius. Conclusions. Given the sample selection, giants and dwarfs are observed at different Galactic locations, and it is plausible, and compatible with simple calculations, that Zn-poor giants trace a younger population more polluted by SN Ia yields. It is necessary to extend observations in order to observe both giants and dwarfs at the same Galactic location. Further theoretical work on the evolution of zinc is also necessary.
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| 12. |
- Feltzing, S., et al.
(author)
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Galactic Archeology with 4MOST
- 2017
-
In: Proceedings of the International Astronomical Union. - 1743-9213. ; 13:S334, s. 225-232
-
Journal article (other academic/artistic)abstract
- 4MOST is a new wide-field, high-multiplex spectroscopic survey facility for the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy 2400 fibres in a 4.1 square degree field-of-view using a positioner based on the tilting spine principle. In this contribution we give an outline of the major science goals we wish to achieve with 4MOST in the area of Galactic Archeology. The 4MOST Galactic Archeology surveys have been designed to address long-standing and far-reaching problems in Galactic science. They are focused on four major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical characterisation of the major Milky Way stellar components, 3) The Galactic Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor stars. In addition to a top-level description of the Galactic surveys we provide information about how the community will be able to join 4MOST via a call for Public Spectroscopic Surveys that ESO will launch.
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| 13. |
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| 14. |
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| 15. |
- Guiglion, G., et al.
(author)
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4MOST Survey Strategy Plan
- 2019
-
In: Messenger. - 0722-6691. ; 175, s. 17-21
-
Journal article (other academic/artistic)abstract
- The current status of and motivation for the 4MOST survey strategy, as developed by the Consortium science team, are presented here. Key elements of the strategy are described, such as sky coverage, number of visits and total exposure times in different parts of the sky, and how to deal with different observing conditions. The task of organising the strategy is not simple, with many different surveys that have vastly different target brightnesses and densities, sample completeness levels, and signal-to-noise requirements. We introduce here a number of concepts that we will use to ensure all surveys are optimised. Astronomers who are planning to submit a Participating Survey proposal are strongly encouraged to read this article and any relevant 4MOST Survey articles in this issue of The Messenger such that they can optimally complement and benefit from the planned surveys of the 4MOST Consortium.
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| 16. |
- Hansen, C. J., et al.
(author)
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Silver and palladium help unveil the nature of a second r-process
- 2012
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545
-
Journal article (peer-reviewed)abstract
- Context. The rapid neutron-capture process, which created about half of the heaviest elements in the solar system, is believed to have been unique. Many recent studies have shown that this uniqueness is not true for the formation of lighter elements, in particular those in the atomic number range 38 < Z < 48. Among these, palladium (Pd) and especially silver (Ag) are expected to be key indicators of a possible second r-process, but until recently they have been studied only in a few stars. We therefore target Pd and Ag in a large sample of stars and compare these abundances to those of Sr, Y, Zr, Ba, and Eu produced by the slow (s-) and rapid (r-) neutron-capture processes. Hereby we investigate the nature of the formation process of Ag and Pd. Aims. We study the abundances of seven elements (Sr, Y, Zr, Pd, Ag, Ba, and Eu) to gain insight into the formation process of the elements and explore in depth the nature of the second r-process. Methods. By adopting a homogeneous one-dimensional local thermodynamic equilibrium (1D LTE) analysis of 71 stars, we derive stellar abundances using the spectral synthesis code MOOG, and the MARCS model atmospheres. We calculate abundance ratio trends and compare the derived abundances to site-dependent yield predictions (low-mass O-Ne-Mg core-collapse supernovae and parametrised high-entropy winds), to extract characteristics of the second r-process. Results. The seven elements are tracers of different (neutron-capture) processes, which in turn allows us to constrain the formation process(es) of Pd and Ag. The abundance ratios of the heavy elements are found to be correlated and anti-correlated. These trends lead to clear indications that a second/weak r-process, is responsible for the formation of Pd and Ag. On the basis of the comparison to the model predictions, we find that the conditions under which this process takes place differ from those for the main r-process in needing lower neutron number densities, lower neutron-to-seed ratios, and lower entropies, and/or higher electron abundances. Conclusions. Our analysis confirms that Pd and Ag form via a rapid neutron-capture process that differs from the main r-process, the main and weak s- processes, and charged particle freeze-outs. We find that this process is efficiently working down to the lowest metallicity sampled by our analysis ([Fe/H] = -3.3). Our results may indicate that a combination of these explosive sites is needed to explain the variety in the observationally derived abundance patterns.
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| 17. |
- Hansen, C. J., et al.
(author)
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Stellar science from a blue wavelength range
- 2015
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In: Russian Review. - : Wiley. - 1467-9434. ; 74:4, s. 665-676
-
Journal article (peer-reviewed)abstract
- From stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small-scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal-poor stars, and allow for studies of heavy elements (Z = 38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal-poor stars. This means that some elements cannot be studied in the visual-redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. It is therefore important that the next generation of spectrographs are designed such that they cover a wide wavelength range and can observe a large number of stars simultaneously. Only then, we can gain the full information from stellar spectra, from both metal-poor to metal-rich ones, that will allow us to understand the aforementioned formation scenarios in greater detail. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi-object spectrograph commissioned for the ESO VISTA 4 m-telescope. While 4MOST is also intended for studies of active galactic nuclei, baryonic acoustic oscillations, weak lensing, cosmological constants, supernovae and other transients, we focus here on high-density, wide-area survey of stars and the science that can be achieved with high-resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high-resolution spectrograph, we find that a sampling of >= 2.5 (pixels per resolving element), spectral resolution of 18 000 or higher, and a wavelength range covering 393-436 nm, is the most well-balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (+/- 0.1 dex) in the blue and allow us to confront the outlined scientific questions. (C) 2015 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim
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| 18. |
- Hayek, W, et al.
(author)
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The Hamburg/ESO R-process enhanced star survey (HERES) IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491-069 and HE 1219-0312
- 2009
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 504:2, s. 511-524
-
Journal article (peer-reviewed)abstract
- We report on a detailed abundance analysis of two strongly r-process enhanced, very metal-poor stars newly discovered in the HERES project, CS 29491-069 ([Fe/H] = -2.51, [r/Fe] = +1.1) and HE 1219-0312 ([Fe/H] = -2.96, [r/Fe] = +1.5). The analysis is based on high-quality VLT/UVES spectra and MARCS model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS 29491-069, and 18 in HE 1219-0312; in both cases including the Th II 4019 angstrom line. The heavy-element abundance patterns of these two stars are mostly well-matched to scaled solar residual abundances not formed by the s-process. We also compare the observed pattern with recent high-entropy wind (HEW) calculations, which assume core-collapse supernovae of massive stars as the astrophysical environment for the r-process, and find good agreement for most lanthanides. The abundance ratios of the lighter elements strontium, yttrium, and zirconium, which are presumably not formed by the main r-process, are reproduced well by the model. Radioactive dating for CS 29491-069 with the observed thorium and rare-earth element abundance pairs results in an average age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when using HEW model predictions. Chronometry seems to fail in the case of HE 1219-0312, resulting in a negative age due to its high thorium abundance. HE 1219-0312 could therefore exhibit an overabundance of the heaviest elements, which is sometimes called an "actinide boost".
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| 19. |
- Hill, V., et al.
(author)
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The Hamburg/ESO R-process Enhanced Star survey (HERES)
- 2017
-
In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 607
-
Journal article (peer-reviewed)abstract
- We report an abundance analysis for the highly r-process-enhanced (r-II) star CS 29497-004, a very metal-poor giant with solar system T-eff = 5013 K and [Fe/H] = -2 : 85, whose nature was initially discovered in the course of the HERES project. Our analysis is based on high signal-to-noise ratio, high-resolution (R similar to 75 000) VLT/UVES spectra and MARCS model atmospheres under the assumption of local thermodynamic equilibrium, and obtains abundance measurements for a total of 46 elements, 31 of which are neutron-capture elements. As is the case for the other similar to 25 r-II stars currently known, the heavy-element abundance pattern of CS 29497-004 well-matches a scaled solar system second peak r-process-element abundance pattern. We confirm our previous detection of Th, and demonstrate that this star does not exhibit an "actinide boost". Uranium is also detected (log is an element of (U) = -2 : 20 +/- 0 : 30), albeit with a large measurement error that hampers its use as a precision cosmo-chronometer. Combining the various elemental chronometer pairs that are available for this star, we derive a mean age of 12 : 2 +/- 3 : 7 Gyr using the theoretical production ratios from published waitingpoint approximation models. We further explore the high-entropy wind model (Farouqi et al. 2010, ApJ, 712, 1359) production ratios arising from different neutron richness of the ejecta (Y-e), and derive an age of 13 : 7 +/- 4 : 4 Gyr for a best-fitting Y-e = 0 : 447. The U/Th nuclei-chronometer is confirmed to be the most resilient to theoretical production ratios and yields an age of 16 : 5 +/- 6 : 6 Gyr. Lead (Pb) is also tentatively detected in CS 29497-004, at a level compatible with a scaled solar r-process, or with the theoretical expectations for a pure r-process in this star.
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| 20. |
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| 21. |
- Mashonkina, L., et al.
(author)
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The Hamburg/ESO R-process enhanced star survey (HERES) V. Detailed abundance analysis of the r-process enhanced star HE 2327-5642
- 2010
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 516, s. A46-
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Journal article (peer-reviewed)abstract
- Aims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327-5642, for which [Fe/H] = -2.78, [r/Fe] = +0.99. Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between Fe I and Fe II, and Ca I and Ca II. Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327-5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327-5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutron-capture peak elements, Sr to Pd, in HE 2327-5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = -0.92 +/- 0.13, while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327-5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327-5642 is a radial-velocity variable based on our high-resolution spectra covering similar to 4.3 years.
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| 22. |
- Mashonkina, L., et al.
(author)
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The Hamburg/ESO R-process Enhanced Star survey (HERES) X. HE 2252-4225, one more r-process enhanced and actinide-boost halo star
- 2014
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 569, s. A43-
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Journal article (peer-reviewed)abstract
- Context. Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better. Aims. We present a detailed abundance analysis of a very metal-poor giant star discovered in the HERES project, HE 2252-4225, which exhibits overabundances of the r-process elements with [r/Fe] = +0.80. Methods. We determined the stellar atmosphere parameters, T-eff = 4710 K, log g = 1.65, and [Fe/H] = -2.63, and chemical abundances by analysing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionisation balance between Fe i and Fe ii. Results. Accurate abundances for a total of 38 elements, including 22 neutron-capture elements beyond Sr and up to Th, were determined in HE 2252-4225. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.12 dex. This object is deficient in carbon, as expected for a giant star with T-eff < 4800 K. The stellar Na-Zn abundances are well fitted by the yields of a single supernova of 14.4 M-circle dot. For the neutron-capture elements in the Sr-Ru, Ba-Yb, and Os-Ir regions, the abundance pattern of HE 2252-4225 is in excellent agreement with the average abundance pattern of the strongly r-process enhanced stars CS 22892-052, CS 31082-001, HE 1219-0312, and HE 1523-091. This suggests a common origin of the first, second, and third r-process peak elements in HE 2252-4225 in the classical r-process. We tested the solar r-process pattern based on the most recent s-process calculations of Bisterzo, Travaglio, Gallino, Wiescher, and Kappeler and found that elements in the range from Ba to Ir match it very well. No firm conclusion can be drawn about the relationship between the first neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar r-process, owing to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive dating results in a stellar age of tau = 1.5 +/- 1.5 Gyr that is not expected for a very metal-poor halo star.
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| 23. |
- Norris, J. E., et al.
(author)
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The most metal-poor stars. I. Discovery, data, and atmospheric parameters
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 25-
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Journal article (peer-reviewed)abstract
- We report the discovery of 34 stars in the Hamburg/ESO Survey for metal-poor stars and the Sloan Digital Sky Survey that have [Fe/H] ≲ -3.0. Their median and minimum abundances are [Fe/H] = -3.1 and -4.1, respectively, while 10 stars have [Fe/H] < -3.5. High-resolution, high signal-to-noise spectroscopic data - equivalent widths and radial velocities - are presented for these stars, together with an additional four objects previously reported or currently being investigated elsewhere. We have determined the atmospheric parameters, effective temperature (T eff), and surface gravity (log g), which are critical in the determination of the chemical abundances and the evolutionary status of these stars. Three techniques were used to derive these parameters. Spectrophotometric fits to model atmosphere fluxes were used to derive T eff, log g, and an estimate of E(B - V); Hα, Hβ, and Hγ profile fitting to model atmosphere results provided the second determination of T eff and log g; and finally, we used an empirical T eff-calibrated Hδ index, for the third, independent T eff determination. The three values of T eff are in good agreement, although the profile fitting may yield systematically cooler T eff values, by ∼100 K. This collective data set will be analyzed in future papers in the present series to utilize the most metal-poor stars as probes of conditions in the early universe.
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| 24. |
- Norris, J. E., et al.
(author)
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The most metal-poor stars. IV. the two populations with [Fe/H] ≲ -3.0
- 2013
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 28-
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Journal article (peer-reviewed)abstract
- We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] ≲ -3.0, utilizing chemical abundances from high-resolution, high signal-to-noise model-atmosphere analyses. The C-rich population represents ∼28% of stars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no stars, and two others with [Fe/H] ∼ -5.5 and uncertain classification. The population is O-rich ([O/Fe] ≳ +1.5); the light elements Na, Mg, and Al are enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little evidence for enhancements relative to solar values. These results are best explained in terms of the admixing and processing of material from H-burning and He-burning regions as achieved by nucleosynthesis in zero-heavy-element models in the literature of "mixing and fallback" supernovae (SNe); of rotating, massive, and intermediate-mass stars; and of Type II SNe with relativistic jets. The available (limited) radial velocities offer little support for the C-rich stars with [Fe/H] < -3.1 being binary. More data are required before one could conclude that binarity is key to an understanding of this population. We suggest that the C-rich and C-normal populations result from two different gas cooling channels in the very early universe of material that formed the progenitors of the two populations. The first was cooling by fine-structure line transitions of C II and O I (to form the C-rich population); the second, while not well defined (perhaps dust-induced cooling?), led to the C-normal group. In this scenario, the C-rich population contains the oldest stars currently observed.
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| 25. |
- Norris, John E., et al.
(author)
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The oxygen abundance of the ultra-metal-poor star HE 0557-4840
- 2012
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In: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 753:2, s. 150-
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Journal article (peer-reviewed)abstract
- We present a high-resolution ultraviolet (UV) spectrum of the ultra-metal-poor (UMP) carbon-enhanced red giant HE 0557-4840 (T-eff/log g/[Fe/H] = 4900/2.2/-4.8). Combining these data with earlier observations, the radial velocity is 212.0 +/- 0.4 km s(-1), with no evidence of variability during 2006 February to 2007 December. One-dimensional (1D) LTE model-atmosphere analysis of UV Fe and CH lines confirms the iron and carbon abundances obtained previously ([Fe/H] = -4.8 and [C/Fe](1D) = +1.7), and places a more stringent limit on nitrogen abundance of [N/Fe](1D) < +1.0. Analysis of the UV OH lines yields [O/Fe](1D) = +2.3 +/- 0.4. When corrections are made for three-dimensional (3D) effects we obtain [C/Fe](3D) = +1.1, [N/Fe](3D) < +0.1, and [O/Fe](3D) = +1.4. Comparison of the abundances of HE 0557-4840 with those of supernova models of Nomoto et al. and Joggerst et al. suggests that none is able to explain fully the observed abundance pattern. For HE 0557-4840, the Frebel et al. transition discriminant D-trans(= log(10([C/H]) + 0.3 x 10([O/H])) = -3.4 +/- 0.2, consistent with fine-structure transitions of C II and O I being a major cooling mechanism of star-forming regions at the earliest times. Of the four stars known to have [Fe/H] less than or similar to -4.3, three are strongly carbon and oxygen enhanced. If the suggestion by Caffau et al. that SDSS J102915+172927 ([Fe/H] = -4.7) does not belong to the class of C-rich, O-rich, UMP stars is supported by future similar discoveries, one will need to consider multiple channels for the production of stars having [Fe/H] less than or similar to -4.3.
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| 26. |
- Perez, A. E. Garcia, et al.
(author)
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A new sample of extremely/ultra metal-poor stars
- 2008
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In: Physica Scripta. - 0031-8949 .- 1402-4896. ; T133, s. 014036-
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Journal article (peer-reviewed)abstract
- A sample of 30 very metal-poor stars from the Hamburg-European Southern Observatory (ESO) objective-prism survey have been observed at high spectral resolution at the Very Large Telescope (VLT) using the Ultraviolet and Visual Echelle Spectrograph (UVES). Two of the observed stars are very interesting not only because of their very low iron content, approximately four orders of magnitude lower than the solar value, but also because we detected the neutral lithium resonance line at 670.8 nm. Hydrogen lines suggest that the two observed stars have effective temperatures around 6000-6250K and according to isochrones, they are either on the main-sequence or on the subgiant branch, in which case they would probably be the most metal-poor dwarfs or warm subgiants with lithium detections known. These detections would allow to determine more accurately the slope of the trend of the lithium abundance with [Fe/H] than was possible with samples of unevolved stars restricted to higher metallicities.
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| 27. |
- Ren, J., et al.
(author)
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The Hamburg/ESO R-process Enhanced Star survey (HERES) : VII. Thorium abundances in metal-poor stars
- 2012
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537
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Journal article (peer-reviewed)abstract
- We report thorium abundances for 77 metal-poor stars in the metallicity range of -3.5 < [Fe/H] < -1.0, based on "snapshot" spectra obtained with VLT-UT2/UVES during the HERES Survey. We were able to determine the thorium abundances with better than 1 sigma confidence for 17 stars, while for 60 stars we derived upper limits. For five stars common with previous studies, our results were in good agreement with the literature results. The thorium abundances span a wide range of about 4.0 dex, and scatter exists in the distribution of log (Th/Eu) ratios for lower metallicity stars, supporting previous studies suggesting the r-process is not universal. We derived ages from the log (Th/Eu) ratios for 12 stars, resulting in large scattered ages, and two stars with significant enhancement of Th relative to Eu are found, indicating the "actinide boost" does not seem to be a rare phenomenon and thus highlighting the risk in using log (Th/Eu) to derive stellar ages.
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| 28. |
- Schoerck, T, et al.
(author)
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The stellar content of the Hamburg/ESO survey V. The metallicity distribution function of the Galactic halo
- 2009
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 507:2, s. 817-832
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Journal article (peer-reviewed)abstract
- We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R similar to 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z(cr) = 10(-3.4) Z(circle dot) reproduces the sharp drop at [Fe/H] similar to -3.6 present in the HES MDF. Although currently about ten stars at [Fe/H] < -3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] similar to -5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < -3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.
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| 29. |
- Tempel, E., et al.
(author)
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Probabilistic fibre-to-target assignment algorithm for multi-object spectroscopic surveys
- 2020
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Journal article (peer-reviewed)abstract
- Context. Several new multi-object spectrographs are currently planned or under construction that are capable of observing thousands of Galactic and extragalactic objects simultaneously. Aims. In this paper we present a probabilistic fibre-to-target assignment algorithm that takes spectrograph targeting constraints into account and is capable of dealing with multiple concurrent surveys. We present this algorithm using the 4-m Multi-Object Spectroscopic Telescope (4MOST) as an example. Methods. The key idea of the proposed algorithm is to assign probabilities to fibre-target pairs. The assignment of probabilities takes the fibre positioner's capabilities and constraints into account. Additionally, these probabilities include requirements from surveys and take the required exposure time, number density variation, and angular clustering of targets across each survey into account. The main advantage of a probabilistic approach is that it allows for accurate and easy computation of the target selection function for the different surveys, which involves determining the probability of observing a target, given an input catalogue. Results. The probabilistic fibre-to-target assignment allows us to achieve maximally uniform completeness within a single field of view. The proposed algorithm maximises the fraction of successfully observed targets whilst minimising the selection bias as a function of exposure time. In the case of several concurrent surveys, the algorithm maximally satisfies the scientific requirements of each survey and no specific survey is penalised or prioritised. Conclusions. The algorithm presented is a proposed solution for the 4MOST project that allows for an unbiased targeting of many simultaneous surveys. With some modifications, the algorithm may also be applied to other multi-object spectroscopic surveys.
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| 30. |
- Xylakis-Dornbusch, T., et al.
(author)
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A method for identifying metal-poor stars with Gaia BP/RP spectra
- 2022
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 666
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Journal article (peer-reviewed)abstract
- Context. The study of the oldest and most metal-poor stars in our Galaxy promotes our understanding of the Galactic chemical evolution and the beginning of Galaxy and star formation. However, they are notoriously difficult to find, with only seven stars at [Fe/H] < −5.0 having been detected to date. Thus, the spectrophotometric data of 219 million sources which became available in the third Gaia Data Release comprise a very promising dataset for the identification of metal-poor stars.Aims. We want to use the low-resolution Gaia Blue Photometer/Red Photometer (BP/RP) spectra to identify metal-poor stars. Our primary aspiration is to help populate the poorly constrained tail of the metallicity distribution function of the stellar halo of the Galaxy.Methods. We developed a metal-poor candidate selection method based on flux ratios from the BP/RP Gaia spectra, using simulated synthetic spectra.Results. We found a relation between the relative iron abundance and the flux ratio of the Ca H & K region to that of the Hβ line. This relation is temperature and surface gravity dependent, and it holds for stars with 4800 K ≤ Teff ≤ 6300 K. We applied it to noisy simulated synthetic spectra and inferred [Fe/H] with an uncertainty of σ[Fe/H] ≲ 0.65 dex for −3 ≤ [Fe/H] ≤ 0.5 and G = 15–17 mag, which is sufficient to identify stars at [Fe/H] < −2.0 reliably. We predict that by selecting stars with inferred [Fe/H] ≤ −2.5 dex, we can retrieve 80% of the stars with [Fe/H] ≤ −3 and have a success rate of about 50%, that is one in two stars we select would have [Fe/H] ≤ −3. We do not take into account the effect of reddening, so our method should only be applied to stars which are located in regions of low extinction.
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| 31. |
- Yong, D., et al.
(author)
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The most metal-poor stars. II. Chemical abundances of 190 metal-poor stars including 10 new stars with [Fe/H] ≤ -3.5
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 26-
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Journal article (peer-reviewed)abstract
- We present a homogeneous chemical abundance analysis of 16 elements in 190 metal-poor Galactic halo stars (38 program and 152 literature objects). The sample includes 171 stars with [Fe/H] ≤ -2.5, of which 86 are extremely metal poor, [Fe/H] ≤ -3.0. Our program stars include 10 new objects with [Fe/H] ≤ -3.5. We identify a sample of "normal" metal-poor stars and measure the trends between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for this sample. Using this mean trend, we identify objects that are chemically peculiar relative to "normal" stars at the same metallicity. These chemically unusual stars include CEMP-no objects, one star with high [Si/Fe], another with high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic processes at lowest metallicity that are distinct from the main r-process. Finally, for many elements, we find a significant trend between [X/Fe] versus T eff, which likely reflects non-LTE and/or three-dimensional effects. Such trends demonstrate that care must be exercised when using abundance measurements in metal-poor stars to constrain chemical evolution and/or nucleosynthesis predictions.
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| 32. |
- Yong, David, et al.
(author)
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The most metal-poor stars. III. The metallicity distribution function and carbon-enhanced metal-poor fraction
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 27-
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Journal article (peer-reviewed)abstract
- We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with [Fe/H] <= -3.0, based on high-resolution, high signal-to-noise spectroscopy, of which some 32 objects lie below [Fe/H] = -3.5. After accounting for the completeness function, the "corrected" MDF does not exhibit the sudden drop at [Fe/H] = -3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to [Fe/H] = -4.1. Similar results are obtained from the "raw" MDF. We find that the fraction of CEMP objects below [Fe/H] = -3.0 is 23% +/- 6% and 32% +/- 8% when adopting the Beers & Christlieb and Aoki et al. CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers & Christlieb criterion.
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| 33. |
- Zhang, L., et al.
(author)
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HERES VI. Galactic chemical evolution of Si and C (Zhang+, 2011)
- 2011
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In: VizieR Online Data Catalog. ; 352
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Journal article (peer-reviewed)abstract
- The present work is based on the spectra of 253 HERES stars. The sample selection and observations are described in Christlieb et al. (2004A\ampA...428.1027C). For the convenience of the reader, we repeat here that the spectra were obtained with the Ultraviolet-Visual Echelle Spectrograph (UVES) mounted on the 8m Unit Telescope 2 (Kueyen) of the Very Large Telescope (VLT). (1 data file).
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| 34. |
- Zhang, L., et al.
(author)
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The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic chemical evolution of silicon
- 2011
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 528, s. A92-
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Journal article (peer-reviewed)abstract
- Aims. To obtain detailed silicon abundances of metal-poor stars, we aim to explore the correlation between the abundance ratios and the stellar parameters and the chemical enrichment of the interstellar medium (ISM). Methods. We determined the silicon abundances of 253 metal-poor stars in the metallicity range -4 < [Fe/H] < -1.5, based on non-local thermodynamic equilibrium (NLTE) line formation calculations of neutral silicon and high-resolution spectra obtained with VLT-UT2/UVES. Results. The T-eff dependence of [Si/Fe] noticed in previous investigations is diminished in our abundance analysis owing to the inclusion of NLTE effects. An increasing slope of [Si/Fe] towards decreasing metallicity is present in our results, in agreement with Galactic chemical evolution models. Intrinsic scatter of [Si/Fe] in our sample is small. We identified two dwarfs with [Si/Fe] similar to + 1.0: HE 0131-3953, and HE 1430-1123. These main-sequence turnoff stars are also carbon-enhanced. They may have been pre-enriched by sub-luminous supernovae. Conclusions. The small intrinsic scatter of [Si/Fe] in our sample may imply that these stars formed in a region where the yields of type II supernovae were mixed into a large volume, or that the formation of these stars was strongly clustered, even if the ISM was enriched by single SNa II in a small mixing volume.
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