Дисертації з теми "Stellar chemical abundances"

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has built the largest moderately high-resolution (R approximate to 22,500) spectroscopic map of the stars across the Milky Way, and including dust-obscured areas. The APOGEE Stellar Parameter and Chemical Abundances Pipeline (ASPCAP) is the software developed for the automated analysis of these spectra. ASPCAP determines atmospheric parameters and chemical abundances from observed spectra by comparing observed spectra to libraries of theoretical spectra, using. 2 minimization in a multidimensional parameter space. The package consists of a FORTRAN90 code that does the actual minimization and a wrapper IDL code for book-keeping and data handling. This paper explains in detail the ASPCAP components and functionality, and presents results from a number of tests designed to check its performance. ASPCAP provides stellar effective temperatures, surface gravities, and metallicities precise to 2%, 0.1 dex, and 0.05 dex, respectively, for most APOGEE stars, which are predominantly giants. It also provides abundances for up to 15 chemical elements with various levels of precision, typically under 0.1 dex. The final data release (DR12) of the Sloan Digital Sky Survey III contains an APOGEE database of more than 150,000 stars. ASPCAP development continues in the SDSS-IV APOGEE-2 survey.

Globular cluster (GC) stars have occupied a fundamental role in our understanding of stellar evolution thanks to the assumption that their stars can be idealized as a simple stellar population, i.e. coeval and chemically homogeneous. However, there is a growing body of observational facts which challenge this traditional view, revealing the presence of multiple stellar populations in GCs. In this thesis I present my contribution to the study of multiple populations in GCs mainly from the spectroscopic point of view. By taking advantage of high and mid-resolution spectra collected at various telescopes, I studied the chemical abundances for large sample of stars in the GCs, M4, M22, and Omega Centauri. All the three studied cases show evidence of multiple populations, but they reveal themselves in different ways, indicating that these GCs have experienced different enrichment histories. Thanks to the analysis of chemical content of stars belonging to distinct stellar populations, important informations have been obtained on the nature of the progenitors that enriched the intracluster medium from which the second generation formed.
Il ruolo fondamentale che gli ammassi globulari (AG) hanno storicamente occupato negli studi di evoluzione stellare si deve all'assunzione per cui tutte le stelle di un ammasso costituiscano una popolazione stellare semplice, cioe' siano coeve e chimicamente omogenee. Tuttavia, una crescente quantita' di dati osservativi sta mettendo in crisi questa visione tradizionale, rivelando la presenza di popolazioni stellari multiple all'interno di uno stesso AG. In questa tesi discutero' il mio contributo, principalmente spettroscopico, allo studio delle popolazioni multiple in AG. Dall'analisi di spettri ad alta e media risoluzione ottenuti in diversi telescopi, ho determinato le abbondanze chimiche per vasti campioni di stelle negli AG M4, M22 e Omega Centauri. Tutti e tre i casi di AG studiati mostrano evidenze di popolazioni multiple, che si manifestano pero' in maniera differente, indicando che questi oggetti hanno subito una diversa evoluzione di arricchimento chimico. Grazie all'analisi del contenuto chimico di stelle appartenenti a popolazioni distinte, sono state ottenute importanti informazioni sulla natura dei progenitori che hanno arricchito il mezzo dal quale si e' formata la seconda generazione di stelle.

The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.

We are made of stardust in the sense that most atomic nuclei around us have been formed by stars. Stars synthesise new elements and expel them to the interstellar medium, from which later new generations of stars are born. We can map this chemical evolution by analysing the atmospheric contents of old Galactic halo stars. I have done two such investigations. A vigourous debate is going on whether the oxygen-to-iron ratio varies strongly with the general metal-content of halo stars. In my first study, I made an abundance analysis of 43 halo stars, and found no support for such a variation. I have also found that there probably is a cosmic spread in the abundances of oxygen, magnesium, silicon, and calcium relative to iron for halo stars. This may be an indication that the halo was built up by subsystems with differences in the star formation rate. In my second study, I performed a thorough abundance analysis of the star HE0338-3945, which is strangely overabundant in both r- and s-elements. Several other stars have been found with abundance patterns curiously similar to this star, and I define new criteria for the class r+s stars. The abundance similarities among the r+s stars suggest a common formation scenario. However, as the s-elements usually are considered to be produced in binary systems of low mass, and r-elements in supernovae of Type II, this scenario is not obvious. In the article I discuss seven hypotheses, and several of them are dismissed.

Stellar chemical abundances provide astronomers with vital information about the production of chemical elements. Some stars preserve the composition of the environment in which they were born on their surfaces. By analysing the light from a star, the abundances of elements, its age and its path in space can be derived, and translated into the language of galactic history. The spallative history of boron in the early Galaxy was reinvestigated by observations of an ultraviolet spectral line in the old star HD 140283 with the Hubble Space Telescope. The line was barely detected and the upper limit abundance derived was lower than expected, which calls for further observations of this line in halo stars. Stars evolved into subgiants were observed with the ESO CAT, La Silla, and NOT, La Palma, to deduce their usefulness for galactic evolution studies. The high resolution spectroscopy study of the 26 objects showed that these stars are indeed useful for such studies. They are more luminous than dwarf stars and their ages can be accurately derived. They do not seem to have changed their surface abundances due to their evolution into giants. Subgiants can successfully be used to observationally reach regions further from the Earth, which can remove local biases that may appear when only observing nearby dwarf stars. A NLTE investigation of neutral Ca showed that cool metal rich dwarf stars did not deviate significantly from LTE, as had earlier been suggested. By an LTE analysis of a sample of 17 such dwarfs, using recent MARCS atmospheres, synthetic spectroscopy and modern atomic line data, cool metal rich dwarfs were shown not to deviate significantly from the expected abundance patterns in a number of elements. This increases the number of potential targets for studies of galactic chemical evolution in the metal rich regime since most stars are cool.

A number of stellar streams, moving and kinematic groups were identified in the Milky Way galaxy. Some of them are suspected to originate from accreted satellites. Can we also find such traces of ancient merger events in the solar neighbourhood? Helmi et al. (2006) identified three new coherent groups of stars in the Geneva-Copenhagen survey (Nordström et al. 2004) and suggested that those might correspond to remains of disrupted satellites. With the detailed chemical composition analysis of the newly identified kinematic group we aim to contribute to the Galactic substructure studies. The main aim of the study is to perform a high-resolution spectroscopic elemental abundance analysis in stars belonging to one of the newly identified kinematic groups and to compare the results with other stars in the Galactic disc. We performed the detailed chemical analysis of 21 stars attributed to Group 3 of the Geneva-Copenhagen survey and six comparison Galactic thin disc stars. The main atmospheric parameters and abundances of 22 chemical elements were determined. All programme stars are overabundant in oxygen and α-elements compared with the Galactic thin-disc. This abundance pattern has similar characteristics to those of the Galactic thick disc. The abundances of chemical elements produced predominantly by the r-process are overabundant in comparison with Galactic thin-disc dwarfs of the same metallicity. The abundances of iron-group elements and chemical elements produced mainly... [to full text]
Paukščių Tako galaktikoje yra identifikuota žvaigždžių srautų, judančių bei kinematinių grupių, kurių kilmė siejama su įkritusiomis galaktikomis. Aktualu ištirti, ar yra tokių senųjų substruktūrų pėdsakų mūsų Saulės aplinkoje? Helmi ir kt. (2006), panaudoję Nordström ir kt. (2004) Ženevos–Kopenhagos apžvalgos (ŽKA) katalogą, identifikavo tris naujas koherentines žvaigždžių grupes, kurios pasižymi išskirtiniais kinematiniais parametrais ir gali būti užgalaktinės kilmės. Šiuo disertacijos darbu siekiama prisidėti prie Galaktikos substruktūrų tyrimų, nustatant detalią cheminę vienos iš Helmi ir kt. identifikuotų žvaigždžių grupių sudėtį. Svarbu išsiaiškinant, ar ŽKA kinematinės grupės žvaigždžių atmosferų cheminė sudėtis skiriasi nuo Galaktikos disko žvaigždžių. Nustatėme 21 3–osios ŽKA kinematinės grupės bei 6 palyginamųjų plonojo disko žvaigždžių atmosferų pagrindinius parametrus bei 22 cheminių elementų gausas. Kinematinės žvaigždžių grupės deguonies ir α–elementų gausos yra padidėjusios lyginant su plonuoju disku ir yra panašios į storojo disko. Cheminių elementų, daugiausia pagaminamų s–procese, gausos ir geležies grupės elementų gausos yra panašios į to paties metalingumo plonojo disko nykštukių cheminių elementų gausas, o cheminių elementų, daugiausia pagaminamų r–procese, gausos yra padidėjusios lyginant su plonuoju disku. Panaši cheminė tirtos kinematinės grupės bei storojo Galaktikos disko sudėtis rodo, kad kinematinės žvaigždžių grupės ir storojo disko žvaigždžių... [toliau žr. visą tekstą]

Presentation des resultats et difficultes rencontres en abordant les problemes d'evolution des galaxies et plus precisement de la galaxie. Un exemple d'application sur les etoiles naines f montre qu'elles ne subissent aucune anomalie d'abondances en elements lourds par la physique particuliere de leurs atmospheres. La spectrometrie a basse resolution ( equiv. A 2a) est abordee. Une application a la structure galactique et a la photometrie rgu de bale est presentee. Le spectrometre multifentes essefem qui repond au besoin de spectrographie statistique est presente et discut

The Milky Way has undergone several mergers with other galaxies during its lifetime. The mergers have been identified via stellar debris in the Halo of the Milky Way. The practice of mapping these mergers is called galactic archaeology. To perform this archaeologic inspection, three stellar features must be mapped: chemistry, kinematics and age. Historically, the latter has been difficult to determine, but can today to high degree be determined through asteroseismology. Red giants are well fit for these analyses. In this thesis, the red giant HE1405-0822 is completely characterized, using spectroscopy, asteroseismology and orbit integration, to map its origin. HE1405-0822 is a CEMP-r/s enhanced star in a binary system. Spectroscopy and asteroseismology are used in concert, iteratively to get precise stellar parameters, abundances and age. Its kinematics are analyzed, e.g. in action and velocity space, to see if it belongs to any known kinematical substructures in the Halo. It is shown that the mass accretion that HE1405-0822 has undergone has given it a seemingly younger age than probable. The binary probably transfered C- and s-process rich matter, but how it gained its r-process enhancement is still unknown. It also does not seem like the star comes from a known merger event based on its kinematics, and could possibly be a heated thick disk star.
Vintergatan har genomgått flera sammanslagningar med andra galaxer under sin livstid. Dessa sammanslagningar har identifierats genom rester av stjärnor i Vintergatans Halo. Arbetssättet för att kartlägga dessa sammanslagningar kallas galaktisk arkeologi. För att kunna göra en arkeologisk undersökning krävs tre egenskaper hos de undersökta stjärnorna: kemi, kinematik och ålder. Historiskt sett har den sistnämnda varit svår att bestämma, men kan idag bestämmas med hög precision m.h.a. asteroseismologi. Röda jättar lämpar sig väl för dessa analyser. I denna uppsats undersöks den röda jätten HE1405-0822. Den kartläggs helt m.h.a. spektroskopi, asteroseismologi och bananalys. HE1405-0822 är en CEMP-r/s-förhöjd stjärna i ett binärt system. Spektroskopi och asteroseismologi används tillsammans, iterativt, för att få precisa stjärnparametrar, kemiskt innehåll och ålder. Dess kinematik analyseras, t.ex. i verkan- och hastighetsrummet, för att se om den tillhör någon känd kinematisk substruktur i Halon. Det visas att massöverföringen som HE1405-0822 genomgått har gett den en skenbart yngre ålder än vad som är troligt. Denna binära kompanjon har troligtvis övertfört C- och s-process-rikt material, men hur den fick sin mängd r-processämnen är fortfarande okänt. Det verkar inte som att stjärnan kommer från någon tidigare kartlagd sammanslagning baserat på dess kinematik, och skulle kunna vara en stjärna med upphettad kinematik från Vintergatans tjocka disk.

The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) project aims to characterize the nature of the early universe through the study of metal-poor stars in the stellar halo of the galaxy. Once completed, this will be the largest set of abundances determined for metal-poor stars from high resolution spectra. In this paper, we present chemical abundances and trends of eleven elements for the first ~80 stars of the ~500 star study. These 80 stars serve as a pilot sample to test the automated stellar parameter and abundance determination pipeline newly developed for the CASH project called CASHCODE. Among the pilot sample, two stars with [Fe/H]<-3.5 were discovered and their abundance analysis is discussed.
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Damped Lyman α systems (DLAs) are useful probes of the chemical enrichment of the universe as they provide accurate abundance measurements of many chemical species. Using a sample of 30 DLAs (with large metal column densities) observed with the High Resolution Echelle Spectrometer on the Keck I telescope, the abundances of several elements (i.e. iron, zinc, chromium, silicon, sulphur, phosphorus, manganese, and boron) are derived and presented. A comparison is drawn between the abundances from these metal-rich DLAs with literature samples encompassing the largest compilation of high resolution observations of other DLAs, and stars from the Milky Way and its satellite galaxies to understand the astrophysical nature of DLAs. Furthermore, the first ever extragalactic study of boron is presented. Using the sample of 30 metal-rich DLAs, two 3σ detections and one near detection 2.97σ) were found. From the comparison of [B/O] and, for the first time, [B/S], with studies in the Milky Way, there appears to be an excess of boron relative to its parent nucleus (oxygen) in these three DLA systems, suggesting that there may be a higher cosmic ray flux in DLAs than in the Milky Way.
Graduate
0606

In the era of large spectroscopic surveys, high-quality spectra can contribute to our understanding of the Galactic chemical evolution, not only providing high-precision abundances of elements belonging to the different nucleosynthesis channels, but also giving constraints to one of the most elusive quantity in astrophysics, i.e. stellar age. Some abundance ratios, called chemical clocks, have proved to be excellent indicators of the stellar age. We can distinguish them in two families: ratios composed by elements whose surface abundances vary during the stellar evolution, such as [C/N], or ratios that are modified by the Galactic chemical evolution, such as [Y/Mg], [Y/Al] etc. In this PhD thesis, I described a method to calibrate empirical relations between abundance ratios, metallicity and stellar ages, using open clusters and solar-like stars as calibrators.

The nature of the first generations of stars that formed after the Big Bang is one of the major topics in contemporary astrophysics and cosmology. Such stars formed out of gas that was relatively free of elements heavier than helium, during a time before generations of supernovae had polluted the environment. Previous hunts have found many of these ‘old’ stars in the Galactic halo. However, not all parts of the Milky Way evolved at the same rate. Cosmological models of galaxy formation have drawn the conclusion that, due to the "inside-out" formation of galaxies like the Milky Way, the oldest stars should today preferentially reside close to the centre of the Galaxy – in or near the Galactic bulge. This thesis documents the initial results of the EMBLA Survey, the first successful search for ancient and metal-poor stars in the bulge of the Milky Way. By utilising the metallicity indicator provided by SkyMapper photometry, we were able to search through more than five million bulge stars, looking for those rare objects with extremely low iron abundances. The AAOmega spectrograph on the AAT gave us the opportunity to obtain intermediate- spectroscopy of approximately 350 stars simultaneously; during 24 nights of observing spread over three years, we observed more than 14,000 candidate metal-poor stars to spectroscopically confirm the photometric metallicity estimates. We found that 49% of the stars observed with the AAT had metallicities below [Fe/H]= 1.0, and around 1,000 stars with [Fe/H]< 2.0. This is a remarkable achievement given that previous searches had only found a total of 21 stars with metallicities that low. Thirty-seven of the most metal-poor stars were then observed with high-resolution spectro- scopy using 8 m telescopes like Magellan and the Very Large Telescope (VLT), to determine their detailed chemical compositions. As part of the Gaia-ESO Survey, four stars were observed in 2012 on the VLT, with metallicities of 2.72[Fe/H] 2.48. We then observed a further ten stars with the MIKE spectrograph at Magellan in 2012, and in 2014 we observed a final 23 targets. We have found the first EMP stars in the bulge; nine of our stars have [Fe/H]< 3.0, and one has [Fe/H]= 4.0. We compared the abundances found in our sample with stars of the same metallicities found in the Galactic halo, and found in general similar trends. Unexpectedly, however, we only found one carbon-enhanced metal-poor (CEMP) star (3%) while 20% of halo stars with [Fe/H]< 2.0 are CEMP stars. In order to verify the predicted old ages of our stars, we also investigated their kinematics. We found that half the stars examined have tightly bound orbits; remaining within the inner Galaxy rather than being merely halo stars passing through the bulge region. This is crucial, as the oldest stars are predicted to have the lowest binding energies. The two most metal-poor stars in our sample have binding energies low enough that there is a 50% chance they formed at redshifts of z > 12, which would make them the oldest known objects in the Universe.

The efficiency of different spectroscopic techniques are examined through four different approaches: detailed analysis of IR spectra from the APOGEE database and examination of persistence, observing extremely metal-poor stars using the Plaskett telescope at the DAO, three analyses of various applications of machine learning in astronomy, and efficient transmission of light through optical fibres. Through the first study, the technical effects of persistence in the APOGEE's IR spectra are examined, and a new technique for removing the persistence is introduced. Most of the globular cluster Pal 1's spectra in the APOGEE database are affected by persistence. Therefore, the Pal 1 spectra are corrected for the persistence and their stellar abundances are determined independently from the APOGEE's pipeline, ASPCAP. Our results for the known members of Pal 1 were in a close agreement with the results from Sakari et al. (2011). Comparison between the results from the corrected and the original spectra suggest that the persistence could have a critical effect on the results. The second study of this thesis focused on observations of extremely metal-poor (EMP) stars from the Pristine survey. Through the DAO-Pristine project, we narrowed down the initial list of the Pristine survey by observing over 50 targets during 25 observing nights. The Ca II triplet absorption lines of the observed targets were examined and used for estimating the metallicity of the objects. Twelve candidate EMP stars with weak Ca II triplet lines are chosen from the observed targets. These candidate EMP stars will be observed with larger telescopes for more accurate determination of their metallicity. This thesis also presents the result of a threefold analysis for using machine learning techniques in astronomy. The supervised machine learning methods are used for determination of the stellar parameters of stars using their raw spectra, and unsupervised machine learning methods are used for classification of supernovae Type Ia from their calibrated spectra. The supervised analysis of the IR and optical spectra suggested that the StarNet neural network (Fabbro et al. 2017) can predict the stellar parameters of the APOGEE database and synthetic spectra, efficiently and accurately. The effect of persistence in the StarNet's results are examined, and we showed that the persistence does not have a critical effect on the overall performance of the StarNet. In addition, multiple unsupervised machine learning techniques such as K-mean and Self Organizing Maps (SOMs) are used for classification of the supernovae Type Ia spectra. The preliminary results suggest that a minimum of three subclasses of supernovae Type Ia can be found from our data, which are consistent with the previous studies. Finally, this thesis presents our final results for an optical system we designed for the MSE project. At UVic, we have used the standard collimated beam method, or "ring test," to measure the Focal Ratio Degradation (FRD) of MSE-like fibres. The FRD of the system is determined from the ratio of the Full Width Half Maximum (FWHM) to the radius of the ring. Early ring test results from a sample of MSE-like fibres show an FRD of 3.7%, which meets the MSE science requirement (i.e. FRD < 5% at f/2). Also, we have automated the ring test for fast, repeatable, and efficient measurements of an individual fibre in multi-fibre bundles. Our future tests will include automated non-static fibres in preparation for the MSE build phases.
Graduate

Tesis (Doctor en Astronomía)--Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física, 2008.
Estudiamos diferentes aspectos de estrellas con exoplanetas (EH) y de estrellas de tipo Vega. Detectamos excesos IR en 20% de las estrellas EH, el cual se atribuye a la presencia de polvo circunestelar.

Since stars retain signatures of their galactic origins in their chemical compositions, we can exploit the chemical abundance distributions that we observe in stellar systems to put constraints on the nature of their progenitors. In this thesis, I present results from three projects aimed at understanding how high resolution spectroscopic observations of nearby stellar systems might be interpreted. The first project presents one possible explanation for the origin of peculiar abundance distributions observed in ultra-faint dwarf satellites of the Milky Way. The second project explores to what extent the distribution of chemical elements in the stellar halo can be used to trace Galactic accretion history from the birth of the Galaxy to the present day. Finally, a third project focuses on developing an input optimization algorithm for the second project to produce better estimates of halo accretion histories. In conclusion, I propose some other new ways to use statistical models and techniques along with chemical abundance distribution data to uncover galactic histories.

Galactic formation models have long sought to reproduce the observed chemical and kinematical properties of the Milky Way's stellar halo and disk. Recently it is the so-called "intermediate population", the stellar thick disk, that is driving advances in our understanding of the formation of spiral galaxies. The thick disk is kinematically more like the thin disk than the halo, for all the thick disk has a velocity dispersion twice that of the thin disk and rotates ~40 km/s more slowly. It is generally accepted that the thick disk's metallicity distribution function peaks at a lower metallicity than the thin disk but at higher metallicity than the halo. The lower bound of the thick disk is still uncertain, as many observational studies have found only a few thick disk candidate stars or clusters that are more metal poor than (Fe/H)=1. Beers et al. (2002) have so far proposed the largest sample of metal poor thick disk candidates, presenting 9 stars at (Fe/H)= -1.2 or lower and 46 more stars at (Fe/H)= -1 or lower, all of which are believed to belong to the thick disk. Beers et al. (2002) present possible thick disk stars as metal poor as (Fe/H)~ -2.5, roughly 1 dex lower than is suggested by current Galactic formation models (Brook et al., 2005). This study is a high-resolution spectroscopic follow-up of 29 of the stars Beers et al. (2002) and Chiba & Beers (2000) identify as potential metal poor members of the thick disk and an additional 40 stars from the cannonical thick disk, halo, and thin disk. None of the very metal-poor stars identified by Beers et al. (2002) can be confirmed as members of the thick disk and many are not metal poor at all. Only two stars more metal poor than (Fe/H)= 1.2 retain their thick disk membership. These two stars exhibit some of the chemical characteristics of the cannonical thick disk: high α-element abundances and a relatively low s--/r-- process element ratio. Also of interest are six stars with thin disk kinematic signatures but thick disk α-element abundances. That only a small number of metal poor thick disk stars could be confirmed in this study indicates that the thick disk is neither as populous nor as metal poor as has been proposed by Beers et al. (2002).
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