D. Katz

Paris Diderot University, Lutetia Parisorum, Île-de-France, France

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Publications (77)94.36 Total impact

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    ABSTRACT: By analyzing a N-body simulation of a bulge formed simply via a bar instability mechanism operating on a kinematically cold stellar disk, and by comparing the results of this analysis with the structural and kinematic properties of the main stellar populations of the Milky Way bulge, we conclude that the bulge of our Galaxy is not a pure stellar bar formed from a pre-existing thin stellar disk, as some studies have recently suggested. On the basis of several arguments emphasized in this paper, we propose that the bulge population which, in the Milky Way, is observed not to be part of the peanut structure corresponds to the old galactic thick disk, thus implying that the Milky Way is a pure thin+thick disk galaxy, with only a possible limited contribution of a classical bulge.
    11/2014;
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    ABSTRACT: We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation history of the Milky Way in the first Gyrs, in both the inner (R<7-8kpc) and outer (R>9-10kpc) discs as sampled in the solar vicinity. We show that, in the inner disc, half of the stellar mass formed during the thick disc phase, in the first 4-5 Gyr. This phase was followed by a significant dip in the star formation activity (at 8-9 Gyr) and a period of roughly constant lower level star formation for the remaining 8 Gyr. The thick disc phase has produced as many metals in 4 Gyr as the thin disc in the remaining 8 Gyr. Our results suggest that a closed box model is able to fit all the available constraints in the inner disc. A closed box system is qualitatively equivalent to a regime where the accretion rate, at high redshift, maintains a high gas fraction in the inner disc. In such conditions, the SFR is mainly governed by the high turbulence of the ISM. By z~1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas not consumed during the thick disc phase, and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z~2, about 2 Gyr before the onset of the thin disc formation in the inner disc.
    10/2014;
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    ABSTRACT: The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of $\mu$as (prad, micropixel), which is the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented
    07/2014;
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    ABSTRACT: By means of idealized, dissipationless N-body simulations which follow the formation and subsequent buckling of a stellar bar, we study the characteristics of boxy/peanut-shaped bulges and compare them with the properties of the stellar populations in the Milky Way bulge. The main results of our modeling, valid for the general family of boxy/peanut shaped bulges, are the following: (i) because of the redistribution of stars in the disk initiated at the epoch of bar formation, it is the whole stellar disk, from the innermost regions to the outer Lindblad resonance of the stellar bar, that is mapped into a boxy bulge; (ii) the contribution of stars to the local bulge density depends on their birth radius: stars born in the innermost disk tend to dominate the innermost regions of the boxy bulge, while stars originating closer to the OLR dominate the outer regions of the boxy/peanut structure; (iii) stellar birth radii are imprinted in the bulge kinematics, the larger the birth radii of stars ending up in the bulge, the greater their rotational support and the higher their line-of-sight velocity dispersions (but note that this last trend depends on the bar viewing angle); (iv) the higher the classical bulge- over-disk ratio, the larger its fractional contribution of stars at large vertical distance from the galaxy mid-plane. (ABRIDGED) On the basis of their chemical and kinematic characteristics, the results of our modeling suggests that the populations A, B and C, as defined by the ARGOS survey, can be associated, respectively, with the inner thin disk, to the young thick and to the old thick disk, following the nomenclature recently suggested for stars in the solar neighborhood by Haywood et al. (2013).
    04/2014;
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    ABSTRACT: Constraints on the Galactic bulge/bar structure and formation history from stellar kinematics and metallicities mainly come from relatively high-latitude fields (|b|>4) where a complex mix of stellar population is seen. We aim here to constrain the formation history of the Galactic bar by studying the radial velocity and metallicity distributions of stars in-situ (|b|<1). We observed red clump stars in four fields along the bar's major axis (l=10,6,-6 and b=0 plus a field at l=0,b=1) with low-resolution spectroscopy from VLT/FLAMES, observing around the CaII triplet. We developed robust methods for extracting radial velocity and metallicity estimates from these low signal-to-noise spectra. We derived distance probability distributions using Bayesian methods rigorously handling the extinction law. We present radial velocities and metallicity distributions, as well as radial velocity trends with distance. We observe an increase in the radial velocity dispersion near the Galactic plane. We detect the streaming motion of the stars induced by the bar in fields at l=+/-6, the highest velocity components of this bar stream being metal-rich ([Fe/H]~0.2 dex). Our data is consistent with a bar inclined at 26+/-3 from the Sun-Galactic centre line. We observe a significant fraction of metal-poor stars, in particular in the field at l=0,b=1. We confirm the flattening of the metallicity gradient along the minor axis when getting closer to the plane, with a hint that it could actually be inverted. Our stellar kinematics corresponds to the expected behaviour of a bar issued from the secular evolution of the Galactic disc. The mix of several populations, seen further away from the plane, is also seen in the bar in-situ since our metallicity distributions highlight a different spatial distribution between metal-poor and metal-rich stars, the more metal-poor stars being more centrally concentrated.
    01/2014;
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    ABSTRACT: We report the first robust measurement of the Milky Way star formation history using the imprint left on chemical abundances of long-lived stars. The formation of the Galactic thick disc occurs during an intense star formation phase between 9.0 (z~1.5) and 12.5 Gyr (z~4.5) ago and is followed by a dip (at z~1.1) lasting about 1 Gyr. Our results imply that the thick disc is as massive as the Milky Way's thin disc, suggesting a fundamental role of this component in the genesis of our Galaxy, something that had been largely unrecognized. This new picture implies that huge quantities of gas necessary to feed the building of the thick disc must have been present at these epochs, in contradiction with the long-term infall assumed by chemical evolution models in the last two decades. These results allow us to fit the Milky Way within the emerging features of the evolution of disc galaxies in the early Universe.
    The Astrophysical Journal 01/2014; 781(2). · 6.73 Impact Factor
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    ABSTRACT: Context. The derivation of radial velocities from large numbers of spectra that typically result from survey work, requires automation. However, except for the classical cases of slowly rotating late-type spectra, existing methods of measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to the idiosyncrasies of individual spectra. The radial velocity spectrometer (RVS) on the Gaia mission, which will start operating very soon, prompted a new attempt at creating a measurement pipeline to handle a wide variety of spectral types. Aims: The present paper describes the theoretical background on which this software is based. However, apart from the assumption that only synthetic templates are used, we do not rely on any of the characteristics of this instrument, so our results should be relevant for most telescope-detector combinations. Methods: We propose an approach based on the simultaneous use of several alternative measurement methods, each having its own merits and drawbacks, and conveying the spectral information in a different way, leading to different values for the measurement. A comparison or a combination of the various results either leads to a "best estimate" or indicates to the user that the observed spectrum is problematic and should be analysed manually. Results: We selected three methods and analysed the relationships and differences between them from a unified point of view; with each method an appropriate estimator for the individual random error is chosen. We also develop a procedure for tackling the problem of template mismatch in a systematic way. Furthermore, we propose several tests for studying and comparing the performance of the various methods as a function of the atmospheric parameters of the observed objects. Finally, we describe a procedure for obtaining a knowledge-based combination of the various Doppler-shift measurements.
    Astronomy and Astrophysics 01/2014; 562:97. · 5.08 Impact Factor
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    ABSTRACT: We observe red clump stars in four fields along the Galactic bar major axis (l=10°, -6°, 6° and b=0°, plus a field at l=0°, b=1°) with low-resolution spectroscopy from FLAMES/GIRAFFE (setup LR08) at the VLT, observing around the CaII triplet. We developed robust methods to extract radial velocity and metallicity estimates from these low signal-to-noise spectra. Results have been derived by fixing atmospheric parameters typical of a red clump star (Teff=4750K, logg=2.5). For some targets, the metallicity could not be derived.(1 data file).
    12/2013;
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    ABSTRACT: The Gaia spectrometer (RVS) has no built-in calibration device and the RVS will rely on its own observations to carry out the wavelength calibration. A small sample (˜1420) of bright RVS F-G-K standard stars will be used for the Radial Velocity Zero Point (RVZP) of the RVS. These standard stars have been observed on the ground with the Sophie, Coralie and Narval spectrometers, and their Spectroscopic Radial Velocities (SRVs) have been published by Soubiran et al. (2013). These SRVs are not Kinematic Radial Velocities (KRVs) which are what we need for studies of galatic dynamics. However in this paper we show that these SRVs are well suited to establish the Radial Velocity Zero Point of the Gaia RVS, provided that CU6 pipelines of the DPAC such as Calibration of the RVS (DU630) and Determination of RVs by cross-correlation with templates (DU650 STA) really use wavelength lines and spectra computed with 3D hydrodynamical model atmospheres, which conveniently treat the convective shifts. Final SRVs given by the Gaia CU6 pipelines and later published in the Gaia final catalogue would then only be corrected from the gravitational shift by the DPAC CU8 in order to become true KRVs.
    11/2013;
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    ABSTRACT: We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.
    Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor
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    ABSTRACT: The Radial Velocity Spectrograph (RVS) on board of Gaia needs to be calibrated using stable reference stars known in advance. The catalogue presented here has being built for that purpose. It includes 1420 radial velocity standard star candidates selected on strict criteria in order to fulfill the Gaia-RVS requirements. A large programme of ground based observations is underway since 2006 to monitor these stars and verify their stability which has to be better than 300 m/s over several years. The observations have been made on the echelle spectrographs ELODIE and SOPHIE on the 1.93-m telescope at Observatoire de Haute-Provence (OHP), NARVAL on the Telescope Bernard Lyot at Observatoire du Pic du Midi and CORALIE on the Euler-Swiss Telescope at La Silla. Data from the OHP and Geneva Observatory archives have also been retrieved as well as HARPS spectra from the ESO archive. We provide for each star a mean radial velocity in the SOPHIE scale, derived from the combination of velocities measured with those instruments, after having carefully estimated their differences of zero-points. In total 10214 radial velocity measurements have been obtained for the 1420 stars. With a mean time baseline of 6.35 years, 92.9% of the candidates fulfill a target stability criterion of 300 m/s. 343 stars are found to be constant at the level of 100 m/s over 10 years. Comparisons with the catalogues of Nidever et al.(2002) and Chubak et al.(2012) show an excellent agreement for FGK stars, with zero-point differences lower than 100 m/s and a remarkably low RMS scatter of 33 m/s in the case of Nidever et al.(2002), suggesting that the precision of the catalogue presented here is better than this value. This catalog will likely be useful for other large-scale spectroscopic surveys such as APOGEE, Gaia-ESO, HERMES and LAMOST.
    Astronomy and Astrophysics 02/2013; · 5.08 Impact Factor
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    ABSTRACT: At the beginning of the Gaia mission, it will be necessary to calibrate the 3 instruments, and to have data for performance check. A particular scanning mode will be used, allowing quick multiple observations over a small number of sources. This mode and the selection of corresponding sources are described.
    12/2012;
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    ABSTRACT: The formation of the Milky Way disk is still an open question. Many scenarios are proposed. Different formation scenarios predict different disk chemical trends. This work aims to chemically characterize the Milky Way disk inside and outside the solar neighbourhood, to better constrain its formation scenario. This is possible thanks to high resolution spectra of 200 disk stars observed using the Giraffe spectrograph on the Very Large Telescope (VLT). They were selected to have galactic altitudes |Z| that cover both the thin and thick disk (|Z| up to 2 kpc). The new automatic spectra analysis software SPADES (Stellar PArameters DEtermination Software, Posbic et al. 2012) was used to determine the stellar parameters, and most importantly, the elemental abundances of these stars. The distances of these stars were also determined. The metallicity distribution function of the disk using this sample was calculated. It showed a large contribution of the thick disk stars and a smooth transition at the metallicity of the thick disk/halo interface. The vertical behaviour of the metallicity distribution function was also studied. A vertical metallicity gradient in the disk of partial [Fe/H] / partial |Z| = -0.19 ± 0.14 dex/kpc was marginally detected at the 1.4 sigma level. The [Ti/Fe] and [Ca/Fe] vs [Fe/H] trends for the stars are determined. The main result of the analysis is that the trends of [Ca/Fe] vs [Fe/H] and [Ti/Fe] vs [Fe/H] show no significant difference close (i.e. |Z| leq 1 kpc) and farther away (1 < |Z| < 2.5 kpc) from the Galactic plane. This suggests that thick disk gas and stars have been enriched by the same proportion of type II and type I super-novae from the galactic plane up to at least 2.5 kpc. These results support thick disk formation scenarios like collapse or gas-rich accretion and disfavour a thick disk formed of stars captured during a merger event.
    12/2012;
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    ABSTRACT: The intermediate-to-old population of the inner galactic disk remains a relatively uncharted territory. We present the results of a high resolution spectroscopic survey of over 200 red clump stars in the inner galactic disc (3-5 kpc from the galactic centre, Z ≤ 300pc from the plane). The metallicity distribution of the sample is clearly more metal-rich than the solar neighborhood, implying a significant radial gradient in the galactic disc at the intermediate ages probed by red clump stars. We discuss the implications of these results on the existence of gradients and their flattening with age. Finally, for the first time, this result brings support to the idea that the metal rich part of the bulge MDF may originate from the inner disk through dynamical instabilities.
    02/2012;
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    ABSTRACT: The Radial Velocity Spectrometer (RVS) on board of Gaia will perform a large spectroscopic survey to determine the radial velocities of some 1.5 × 10^8 stars. We present the status of ground-based observations of a sample of 1420 candidate standard stars designed to calibrate the RVS. Each candidate star has to be observed several times before Gaia launch (and at least once during the mission) to ensure that its radial velocity remains stable during the whole mission. Observations are performed with the high-resolution spectrographs SOPHIE, NARVAL and CORALIE, completed with archival data of the ELODIE and HARPS instruments. The analysis shows that about 7% of the current catalogue exhibits variations larger than the adopted threshold of 300 m s^{-1}. Consequently, those stars should be rejected as reference targets, due to the expected accuracy of the Gaia RVS. Emphasis is also put here on our observations of bright asteroids to calibrate the ground-based velocities by a direct comparison with celestial mechanics. It is shown that the radial velocity zero points of SOPHIE, NARVAL and CORALIE are consistent with each other, within the uncertainties. Despite some scatter, their temporal variations remain small with respect to our adopted stability criterion.
    12/2011;
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    ABSTRACT: 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).
    Astronomy and Astrophysics 11/2011; · 5.08 Impact Factor
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    ABSTRACT: The Radial Velocity Spectrometer (RVS) on board of Gaia will perform a large spectroscopic survey to determine the radial velocities of some 1.5x10^8 stars. We present the status of ground-based observations of a sample of 1420 candidate standard stars designed to calibrate the RVS. Each candidate star has to be observed several times before Gaia launch (and at least once during the mission) to ensure that its radial velocity remains stable during the whole mission. Observations are performed with the high-resolution spectrographs SOPHIE, NARVAL and CORALIE, completed with archival data of the ELODIE and HARPS instruments. The analysis shows that about 7% of the current catalogue exhibits variations larger than the adopted threshold of 300 m/s. Consequently, those stars should be rejected as reference targets, due to the expected accuracy of the Gaia RVS. Emphasis is also put here on our observations of bright asteroids to calibrate the ground-based velocities by a direct comparison with celestial mechanics. It is shown that the radial velocity zero points of SOPHIE, NARVAL and CORALIE are consistent with each other, within the uncertainties. Despite some scatter, their temporal variations remain small with respect to our adopted stability criterion.
    10/2011;
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    ABSTRACT: Aims: This work investigates the properties (metallicity and kinematics) and interfaces of the Galactic thick disc as a function of height above the Galactic plane. The main aim is to study the thick disc in a place where it is the main component of the sample. Methods: We take advantage of former astrometric work in two fields of several square degrees in which accurate proper motions were measured down to V-magnitudes of 18.5 in two directions, one near the north galactic pole and the other at a galactic latitude of 46° and galactic longitude near 0°. Spectroscopic observations have been acquired in these two fields for a total of about 400 stars down to magnitude 18.0, at spectral resolutions of 3.5 to 6.25 Å. The spectra have been analysed with the code ETOILE, comparing the target stellar spectra with a grid of 1400 reference stellar spectra. This comparison allowed us to derive the parameters effective temperature, gravity, [Fe/H] and absolute magnitude for each target star. Results: The Metallicity Distribution Function (MDF) of the thin-thick-disc-halo system is derived for several height intervals between 0 and 5 kpc above the Galactic plane. The MDFs show a decrease of the ratio of the thin to thick disc stars between the first and second kilo-parsec. This is consistent with the classical modelling of the vertical density profile of the disc with 2 populations with different scale heights. A vertical metallicity gradient, ∂[Fe/H]/∂z = -0.068 ± 0.009 dex kpc-1, is observed in the thick disc. It is discussed in terms of scenarios of formation of the thick disc. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii and at the T193cm telescope, Observatoire de Haute-Provence, France.Full Tables A.1-A.3, B.1-B.3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/525/A90
    Astronomy and Astrophysics 01/2011; 525. · 5.08 Impact Factor
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    ABSTRACT: The calibration of the Radial Velocity Zero-Point (RVZP) of the Radial Velocity Spectrometer (RVS) will be performed with the help of Radial Velocity (RV) standard stars and asteroids. A full-sky list of RV standard stars candidates has been built for this purpose within the Development Unit (DU) 640 of the Gaia DPAC Consortium. A ground-based campaign of RV observations has been initiated to eliminate unsuitable candidates. Simulations have also been performed in order to estimate in advance the number of RV standard stars and asteroids crossing the RVS field of view. These standard stars and asteroids will also be very helpful for controlling any harmful effect (especially the CCD radiation damage) on calibration and RVZP during the lifetime of the Gaia project. Kinematic RV are expected to be published at the end of the project for the brightest stars, taking into account gravitational redshift and convective shifts for the lines present in the RVS spectral wavelength range.
    EAS Publications Series 01/2011;
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    ABSTRACT: During the five years of the mission, the Gaia spectrograph, the Radial Velocity Spectrometer (RVS) will repeatedly survey the celestial sphere down to magnitude V ~ 17-18. This talk presents: (i) the system which is currently developed within the Gaia Data Processing and Analysis Consortium (DPAC) to reduce and calibrate the spectra and to derive the radial and rotational velocities, (ii) the RVS expected performances and (iii) scientific returns.
    EAS Publications Series 01/2011;

Publication Stats

351 Citations
94.36 Total Impact Points

Institutions

  • 2008–2014
    • Paris Diderot University
      Lutetia Parisorum, Île-de-France, France
  • 2007
    • French National Centre for Scientific Research
      • Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA)
      Paris, Ile-de-France, France
  • 1998–2007
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 2004–2005
    • Liverpool John Moores University
      Liverpool, England, United Kingdom