arXiv subjects

Elisabetta Caffau

Publications and source records attributed to Elisabetta Caffau.

At least 55 records · Page 3Linked to original sources

arXiv2017

The Pristine survey I: Mining the Galaxy for the most metal-poor stars

We present the Pristine survey, a new narrow-band photometric survey focused on the metallicity-sensitive Ca H & K lines and conducted in the northern hemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope (CFHT). This paper reviews our overall survey strategy and discusses the data processing and metallicity calibration. Additionally we review the application of these data to the main aims of the survey, which are to gather a large sample of the most metal-poor stars in the Galaxy, to further characterise the faintest Milky Way satellites, and to map the (metal-poor) substructure in the Galactic halo. The current Pristine footprint comprises over 1,000 deg2 in the Galactic halo ranging from b~30 to 78 and covers many known stellar substructures. We demonstrate that, for SDSS stellar objects, we can calibrate the photometry at the 0.02-magnitude level. The comparison with existing spectroscopic metallicities from SDSS/SEGUE and LAMOST shows that, when combined with SDSS broad-band g and i photometry, we can use the CaHK photometry to infer photometric metallicities with an accuracy of ~0.2 dex from [Fe/H]=-0.5 down to the extremely metal-poor regime ([Fe/H]<-3.0). After the removal of various contaminants, we can efficiently select metal-poor stars and build a very complete sample with high purity. The success rate of uncovering [Fe/H]SEGUE<-3.0 stars among [Fe/H]Pristine<-3.0 selected stars is 24% and 85% of the remaining candidates are still very metal poor ([Fe/H]<-2.0). We further demonstrate that Pristine is well suited to identify the very rare and pristine Galactic stars with [Fe/H]<-4.0, which can teach us valuable lessons about the early Universe.

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arXiv2016

Abundances in a sample of turnoff and subgiant stars in NGC 6121 (M4)

{The aim of this work is to investigate the relations between the abundances of different elements in the globular cluster M4, selected for its uniform deficiency of iron, to explore the best models explaining the pattern of these observed abundances. Moreover, in turnoff stars, the abundances of the elements are not suspected to be affected by internal mixing. In M4, using low and moderate resolution spectra obtained for 91 turnoff (and subgiant) stars with the ESO FLAMES-Giraffe spectrograph, we have extended previous measurements of abundances (of Li, C and Na) to other elements (C, Si, Ca, Sr and Ba), using model atmosphere analysis. We have also studied the influence of the choice of the microturbulent velocity. Firstly, the peculiar turnoff star found to be very Li-rich in a previous paper does not show any other abundance anomalies relative to the other turnoff stars in M4. ~Secondly, an anti-correlation between C and Na has been detected, the slope being significative at more than $3σ$. This relation between C and Na is in perfect agreement with the relation found in giant stars selected below the RGB bump. ~Thirdly, the strong enrichment of Si and of the neutron-capture elements Sr and Ba, already observed in the giants in M4, is confirmed. ~Finally, the relations between Li, C, Na, Sr and Ba constrain the enrichment processes of the observed stars. The abundances of the elements in the turnoff stars appear to be compatible with production processes by massive AGBs, but are also compatible with the production of second generation elements (like Na) and low Li produced by, for example, fast rotating massive stars.

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arXiv2016

The Detailed Science Case for the Maunakea Spectroscopic Explorer: the Composition and Dynamics of the Faint Universe

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged)

astro-ph.IM↗

arXiv2016

A concise overview of the Maunakea Spectroscopic Explorer

This short document is intended as a companion and introduction to the Detailed Science Case (DSC) for the Maunakea Spectroscopic Explorer. It provides a concise summary of the essential characteristics of MSE from the perspective of the international astronomical community. MSE is a wide field telescope (1.5 square degree field of view) with an aperture of 11.25m. It is dedicated to multi-object spectroscopy at several different spectral resolutions in the range R ~ 2500 - 40000 over a broad wavelength range (0.36 - 1.8μm). MSE will enable transformational science in areas as diverse as exoplanetary host characterization; stellar monitoring campaigns; tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of the distant Galaxy; connecting galaxies to the large scale structure of the Universe; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars. MSE is the largest ground based optical and near infrared telescope in its class, and it will occupy a unique and critical role in the emerging network of astronomical facilities active in the 2020s. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, eROSITA, SKA, and WFIRST, and is an ideal feeder facility for E-ELT, TMT and GMT.

astro-ph.IM↗

arXiv2015

Lithium abundance in a turnoff halo star on an extreme orbit

The lithium abundance in turnoff stars of the old population of our Galaxy is remarkably constant in the metallicity interval -2.8\textless{}[Fe/H] \textless{}-2.0, defining a plateau. The Li abundance of these turnoff stars is clearly lower than the abundance predicted by the primordial nucleosynthesis in the frame of the standard Big Bang nucleosynthesis. Different scenarios have been proposed for explaining this discrepancy, along with the very low scatter of the lithium abundance around the plateau. The recently identified very high velocity star, WISE J072543.88-235119.7 appears to belong to the old Galactic population, and appears to be an extreme halo star on a bound, retrograde Galactic orbit. In this paper, we study the abundance ratios and, in particular the lithium abundance, in this star. The available spectra (ESO-Very Large Telescope) are analyzed and the abundances of Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba are determined.The abundance ratios in WISE J072543.88-235119.7 are those typical of old turnoff stars. The lithium abundance in this star ~is in close agreement with the lithium abundance found in the metal-poor turnoff stars located at moderate distance from the Sun. This high velocity star confirms, in an extreme case, that the very small scatter of the lithium plateau persists independent of the dynamic and kinematic properties of the stars.

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arXiv2015

The Science Case for Multi-Object Spectroscopy on the European ELT

This White Paper presents the scientific motivations for a multi-object spectrograph (MOS) on the European Extremely Large Telescope (E-ELT). The MOS case draws on all fields of contemporary astronomy, from extra-solar planets, to the study of the halo of the Milky Way and its satellites, and from resolved stellar populations in nearby galaxies out to observations of the earliest 'first-light' structures in the partially-reionised Universe. The material presented here results from thorough discussions within the community over the past four years, building on the past competitive studies to agree a common strategy toward realising a MOS capability on the E-ELT. The cases have been distilled to a set of common requirements which will be used to define the MOSAIC instrument, entailing two observational modes ('high multiplex' and 'high definition'). When combined with the unprecedented sensitivity of the E-ELT, MOSAIC will be the world's leading MOS facility. In analysing the requirements we also identify a high-multiplex MOS for the longer-term plans for the E-ELT, with an even greater multiplex (>1000 targets) to enable studies of large-scale structures in the high-redshift Universe. Following the green light for the construction of the E-ELT the MOS community, structured through the MOSAIC consortium, is eager to realise a MOS on the E-ELT as soon as possible. We argue that several of the most compelling cases for ELT science, in highly competitive areas of modern astronomy, demand such a capability. For example, MOS observations in the early stages of E-ELT operations will be essential for follow-up of sources identified by the James Webb Space Telescope (JWST). In particular, multi-object adaptive optics and accurate sky subtraction with fibres have both recently been demonstrated on sky, making fast-track development of MOSAIC feasible.

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arXiv2015

The Galactic evolution of sulphur as traced by globular clusters

Sulphur is an important, volatile alpha element but its role in the Galactic chemical evolution is still uncertain. We derive the S abundances in RGB stars in three Galactic globular clusters (GC) that cover a wide metallicity range (-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent with canonical chemical evolution models. To date, very few measurements of [S/Fe] exist for stars in GCs, which are good tracers of the chemical enrichment of their environment. However, some light and alpha elements show star-to-star variations within individual GCs and it is yet unclear whether sulphur also varies between GC stars. We used the the infrared spectrograph CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in M30). Multiplet 3 is better suited for S abundance derivation than the more commonly used lines of multiplet 1 at 920 nm, since its lines are not blended by telluric absorption or other stellar features at low metallicity. We used spectral synthesis to derive the [S/Fe] ratio of the stars assuming local thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20 dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE corrections are estimated to be in the order of the systematic uncertainties. With the tentative exception of two stars with measured high S abundances, we conclude that sulphur behaves like a typical alpha element in the studied Galactic GCs, showing enhanced abundances with respect to the solar value at metallicities below [Fe/H] = -1.0 dex without a considerable spread.

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arXiv2013

Molecular bands in extremely metal-poor stars. Granulation effects

The bands of diatomic molecules are important abundance indicators, especially in metal-poor stars, where they are still measurable in metallicity regimes where the atomic lines of their constituting metallic elements have become vanishingly small. In order to use them for abundance determinations it is imperative to understand the formation of these bands. In this contribution we report on our results obtained using CO5BOLD hydrodynamical simulations. Some effects that are qualitatively different from what found in 1D computations are highlighted. Due to the large number of lines that form the bands, their spectrum synthesis is computationally challenging. We discuss some of the computational strategies we employed to parallelise the computation and possible future developments.

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arXiv2013

The photospheric solar oxygen project: II. Non-concordance of the oxygen abundance derived from two forbidden lines

In the Sun, the two forbidden [OI] lines at 630 and 636 nm were previously found to provide discrepant oxygen abundances. aims: We investigate whether this discrepancy is peculiar to the Sun or whether it is also observed in other stars. method: We make use of high-resolution, high signal-to-noise ratio spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant star observed with THEMIS, HARPS, and UVES to investigate the coherence of the two lines. results: The two lines provide oxygen abundances that are consistent, within observational errors, in all the giant stars examined by us. On the other hand, for the two dwarf stars for which a measurement was possible, for Procyon, and for the sub-giant star Capella, the 636 nm line provides systematically higher oxygen abundances, as already seen for the Sun. conclusions: The only two possible reasons for the discrepancy are a serious error in the oscillator strength of the NiI line blending the 630 nm line or the presence of an unknown blend in the 636 nm line, which makes the feature stronger. The CN lines blending the 636 nm line cannot be responsible for the discrepancy. The CaI autoionisation line, on the red wing of which the 636 nm line is formed, is not well modelled by our synthetic spectra. However, a better reproduction of this line would result in even higher abundances from the 636 nm, thus increasing the discrepancy.

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arXiv2013

ELT-MOS White Paper: Science Overview & Requirements

The workhorse instruments of the 8-10m class observatories have become their multi-object spectrographs (MOS), providing comprehensive follow-up to both ground-based and space-borne imaging. With the advent of deeper imaging surveys from, e.g., the HST and VISTA, there are a plethora of spectroscopic targets which are already beyond the sensitivity limits of current facilities. This wealth of targets will grow even more rapidly in the coming years, e.g., after the completion of ALMA, the launch of the JWST and Euclid, and the advent of the LSST. Thus, one of the key requirements underlying plans for the next generation of ground-based telescopes, the Extremely Large Telescopes (ELTs), is for even greater sensitivity for optical and infrared spectroscopy. Here we revisit the scientific motivation for a MOS capability on the European ELT, combining updated elements of science cases advanced from the Phase A instrument studies with new science cases which draw on the latest results and discoveries. These science cases address key questions related to galaxy evolution over cosmic time, from studies of resolved stellar populations in nearby galaxies out to observations of the most distant galaxies, and are used to identify the top-level requirements on an 'E-ELT/MOS'. We argue that several of the most compelling ELT science cases demand MOS observations, in highly competitive areas of modern astronomy. Recent technical studies have demonstrated that important issues related to e.g. sky subtraction and multi-object AO can be solved, making fast- track development of a MOS instrument feasible. To ensure that ESO retains world leadership in exploring the most distant objects in the Universe, galaxy evolution and stellar populations, we are convinced that a MOS should have high priority in the instrumentation plan for the E-ELT.

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arXiv2012

4MOST - 4-metre Multi-Object Spectroscopic Telescope

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. We consider two fibre positioner concepts, a well-known Phi-Theta system and a new R-Theta concept with a large patrol area. The spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high- and low-resolution. 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. Mock catalogues with template spectra from seven Design Reference Surveys are simulated to verify the science requirements of 4MOST. 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.

astro-ph.IM↗

arXiv2012

NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars

(Abridged) Extremely metal-poor stars contain the fossil records of the chemical composition of the early Galaxy. The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI. With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. -We find that [Ca/Fe] = 0.50 $\pm$ 0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. -The scatter of the ratios [X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where X is a "light metal" (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or even smaller than, the scatter around the mean value of [X/Ca]. -We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. -The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity, than the abundance deduced from the subordinate lines.

astro-ph.SR↗

arXiv2012

Detailed abundances in extremely metal poor dwarf stars extracted from SDSS

We report on the result of an ongoing campaign to determine chemical abundances in extremely metal poor (EMP) turn-off (TO) stars selected from the Sloan Digital Sky Survey (SDSS) low resolution spectra. This contribution focuses principally on the largest part of the sample (18 stars out of 29), observed with UVES@VLT and analyzed by means of the automatic abundance analysis code MyGIsFOS to derive atmosphere parameters and detailed compositions. The most significant findings include i) the detection of a C-rich, strongly Mg-enhanced star ([Mg/Fe]=1.45); ii) a group of Mn-rich stars ([Mn/Fe]>-0.4); iii) a group of Ni-rich stars ([Ni/Fe]>0.2). Li is measured in twelve stars, while for three upper limits are derived.

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arXiv2011

SPADES: a Stellar PArameters DEtermination Software

With the large amounts of spectroscopic data available today and the very large surveys to come (e.g. Gaia), the need for automatic data analysis software is unquestionable. We thus developed an automatic spectra analysis program for the determination of stellar parameters: radial velocity, effective temperature, surface gravity, micro-turbulence, metallicity and the elemental abundances of the elements present in the spectral range. Target stars for this software should include all types of stars. The analysis method relies on a line by line comparison of the spectrum of a target star to a library of synthetic spectra. The idea is built on the experience acquired in developing the TGMET (Katz et al. 1998 and Soubiran et al. 2003) ETOILE (Katz 2001) and Abbo (Bonifacio & Caffau 2003) softwares. The method is presented and the performances are illustrated with GIRAFFE-like simulated spectra with high resolution (R = 25000), with high and low signal to noise ratios (down to SNR= 30). These spectra should be close to what could be targeted by the Gaia-ESO Survey (GCDS).

astro-ph.GA↗

arXiv2011

LTE model atmopsheres MARCS, ATLAS and CO5BOLD

In this talk we review the basic assumptions and physics covered by classical 1D LTE model atmospheres. We will focus on ATLAS and MARCS models of F-G-K stars and describe what resources are available through the web, both in terms of codes and model-atmosphere grids. We describe the advances made in hydrodynamical simulations of convective stellar atmospheres with the CO5BOLD code and what grids and resources are available, with a prospect of what will be available in the near future.

astro-ph.SR↗

arXiv2011

Sulphur in the metal poor globular cluster NGC 6397

Sulphur (S) is a non-refractory alpha-element that is not locked into dust grains in the interstellar medium. Thus no correction to the measured, interstellar sulphur abundance is needed and it can be readily compared to the S content in stellar photospheres. Here we present the first measurement of sulphur in the metal poor globular cluster (GC) NGC 6397, as detected in a MIKE/Magellan high signal-to-noise, high-resolution spectrum of one red giant star. While abundance ratios of sulphur are available for a larger number of Galactic stars down to an [Fe/H] of ~ -3.5 dex, no measurements in globular clusters more metal poor than -1.5 dex have been reported so far. We find a NLTE, 3-D abundance ratio of [S/Fe] = +0.52 +/- 0.20 (stat.) +/- 0.08 (sys.), based on the S I, Multiplet 1 line at 9212.8A. This value is consistent with a Galactic halo plateau as typical of other alpha-elements in GCs and field stars, but we cannot rule out its membership with a second branch of increasing [S/Fe] with decreasing [Fe/H], claimed in the literature, which leads to a large scatter at metallicities around -2 dex. The [S/Mg] and [S/Ca] ratios in this star are compatible with a Solar value to within the (large) uncertainties. Despite the very large scatter in these ratios across Galactic stars between literature samples, this indicates that sulphur traces the chemical imprints of the other alpha-elements in metal poor GCs. Combined with its moderate sodium abundance ([S/Na]_NLTE=0.48), the [S/Fe] ratio in this GC extends a global, positive S-Na correlation that is not seen in field stars and might indicate that proton-capture reactions contributed to the production of sulphur in the (metal poor) early GC environments.

astro-ph.GA↗

arXiv2011

Extremely metal-poor stars in SDSS fields

Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal-poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metalllicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1000 the solar metallicity and 4 stars with about 1/10000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X-Shooter has the unique capability of performing the necessary follow-up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr,...) for EMP candidates. We here report on the results for the first two stars observed in the course of our franco-italian X-Shooter GTO. The two stars were targeted to be of metallicity around -3.0, the analysis of the X-Shooter spectra showed them to be of metallicity around -2.0, but with a low alpha to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X-Shooter allows an in situ study of the outer Halo, for the two stars studied here we estimate distances of 3.9 and 9.1 Kpc, these are likely the most distant dwarf stars studied in detail to date.

astro-ph.SR↗

arXiv2011

Local stars formed at z>10: a sample extracted from the SDSS

As the Universe emerged from its initial hot and dense phase, its chemical composition was extremely simple, being limited to stable H and He isotopes, and traces of Li. The first stars that formed had such initial composition. However, they quickly began to produce a whole array of heavier nuclei, polluting the interstellar medium. While none among these first stars has been detected to date, an increasing sample exists of their direct descendant, stars with heavy elements content of the order of 1/1000 of the solar value, or less. In most cases, such stars should have formed at redshift of about 10 or beyond, and their chemical composition can provide crucial constraints to the nature of the very first stars. Extremely metal poor (EMP) stars are exceedingly rare. We used the low resolution spectra obtained by the Sloan Digital Sky Survey (SDSS) to search for EMP candidates: results of VLT-UVES high resolution follow-up for 16 of them is presented here. A newly developed automatic abundance analysis and parameter determination code, MyGIsFOS, has been employed to analyze the detailed chemical abundances of such stars.

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