63 research outputs found
[alpha/Fe] in the thin and the thick disk towards an automatic parametrization of stellar spectra
We test an automatic procedure to measure [Fe/H] and [alpha/Fe] on high
resolution spectra. The test sample is the intersection of the ELODIE library
and a catalogue of 830 stars having well determined abundances.Comment: Three-dimensional Universe with Gaia, 4-7 October 2004, Observatoire
de Paris-Meudon, France (ESA SP-576), eds M. Perryman & C. Turo
The incorrect rotation curve of the Milky Way
In the fundamental quest of the rotation curve of the Milky Way, the
tangent-point (TP) method has long been the simplest way to infer velocities
for the inner, low latitude regions of the Galactic disk from observations of
the gas component. We test the validity of the method on realistic gas
distribution and kinematics of the Milky Way, using a numerical simulation of
the Galaxy. We show that the resulting velocity profile strongly deviates from
the true rotation curve of the simulation, as it overstimates it in the central
regions, and underestimates it around the bar corotation. Also, its shape
strongly depends on the orientation of the stellar bar. The discrepancies are
caused by highly non-uniform azimuthal velocities, and the systematic selection
by the TP method of high-velocity gas along the bar and spiral arms, or
low-velocity gas in less dense regions. The velocity profile is in good
agreement with the rotation curve only beyond corotation, far from massive
asymmetric structures. Therefore the observed velocity profile of the Milky Way
inferred by the TP method is expected to be very close to the true Galactic
rotation curve for 4.5<R<8 kpc. Another consequence is that the Galactic
velocity profile for R<4-4.5 kpc is very likely flawed by the non-uniform
azimuthal velocities, and does not represent the true Galactic rotation curve,
but instead local motions. The real shape of the innermost rotation curve is
probably shallower than previously thought. Using a wrong rotation curve has a
dramatic impact on the modelling of the mass distribution, in particular for
the bulge component of which derived enclosed mass within the central kpc and
scale radius are, respectively, twice and half of the actual values. We thus
strongly argue against using terminal velocities or the velocity curve from the
TP method for modelling the mass distribution of the Milky Way. (abridged)Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 10
figures, revised version after A&A language editin
Elemental abundances as a function of kinematics in the Milky Way's disk
We present the results of our investigation of three samples kinematically
representative of the thin and thick disks and the Hercules stream using the
catalogue of Soubiran & Girard (2005). We have observed abundance trends and
age distribution of each component. Our results show that the two disks are
chemically well separated, they overlap greatly in metallicity and both show
parallel decreasing trends of alpha elements with increasing metallicity, in
the interval -0.80 < [Fe/H] < -0.30. The thick disk is clearly older than the
thin disk with a tentative evidence of an Age-Metallicity Relation over 2-3 Gyr
and a hiatus in star formation before the formation of the thin disk. In order
to improve the statistics on the disk's abundance trends, we have developed an
automatic code, TGMET{\Large\alpha}; to determine (Teff, logg, [Fe/H],
[\alpha/Fe]) for thousands of stellar spectra available in spectroscopic
archives. We have assessed the performances of the algorithm for 350 spectra of
stars being part of the abundance catalogue
Gaia FGK Benchmark Stars and their reference parameters
In this article we summarise on-going work on the so-called Gaia FGK
Benchmark Stars. This work consists of the determination of their atmospheric
parameters and of the construction of a high-resolution spectral library. The
definition of such a set of reference stars has become crucial in the current
era of large spectroscopic surveys. Only with homogeneous and well documented
stellar parameters can one exploit these surveys consistently and understand
the structure and history of the Milky Way and therefore other of galaxies in
the Universe.Comment: to appear in ASI Conference Series, 2014, Vol. 10 for the Workshop of
Spectral Libraries held in Lyon, Oct. 201
Assessment of [Fe/H] determinations for FGK stars in spectroscopic surveys
The number of stars with a measured value of [Fe/H] is considerably
increasing thanks to spectroscopic surveys. However different methodologies,
inputs and assumptions used in spectral analyses lead to different precisions
in [Fe/H] and possibly to systematic differences that need to be evaluated. It
is essential to understand the characteristics of each survey to fully exploit
their potential, in particular if the surveys are combined. The purpose of this
study is to compare [Fe/H] determinations from the largest spectroscopic
surveys (APOGEE, GALAH, the Gaia ESO survey, RAVE, LAMOST, SEGUE ) to other
catalogues taken as reference. Offsets and dispersions of the residuals are
examined as well as their trends with other parameters. We use reference
samples providing independent determinations of [Fe/H] which are compared to
those from the surveys for common stars. The distribution of the residuals is
assessed through simple statistics that measures the offset between two
catalogues and the dispersion representative of the precision of both
catalogues. When relevant, linear fits are performed. A large sample of
FGK-type stars with [Fe/H] based on high-resolution, high signal to noise
spectroscopy was built from the PASTEL catalogue to provide a reference sample.
We also use FGK members in open and globular clusters to assess the internal
consistency of [Fe/H] of each survey. The agreement of median [Fe/H] values for
clusters observed by different surveys is discussed. All the surveys
overestimate the low metallicities, and some of them also underestimate the
high metallicities. They perform well in the most populated intermediate
metallicity range, whatever the resolution. In most cases the typical precision
that we deduce from the comparisons is in good agreement with the uncertainties
quoted in the catalogues. Some exceptions to this general behaviour are
discussed.Comment: accepted in A&
Analysis of stellar populations with large empirical libraries at high spectral resolution
The stellar population models dramatically progressed with the arrival of
large and complete libraries, ELODIE, CFLIB (=Indo-US) and MILES at a
relatively high resolution. We show that the quality of the fits is not anymore
limited by the size of the stellar libraries in a large range of ages (0.1 to
10 Gyrs) and metallicities (-2 to +0.4 dex). The main limitations of the
empirical stellar libraries are (i) the coverage of the parameters space (lack
of hot stars of low metallicity), (ii) the precision and homogeneity of the
atmospheric parameters and (iii) the non-resolution of individual element
abundances (in particular [/Fe]). Detailed abundance measurements in
the large libraries, and usage of theoretical libraries are probably the next
steps, and we show that a combination between an empirical (ELODIE) and a
theoretical library (Coelho et al. 2005) immediately improves the modeling of
(-enhanced) globular clusters.Comment: 4 pages; proceedings of IAU Symposium No. 241, "Stellar Populations
as Building Blocks of Galaxies", editors A. Vazdekis and R. Peletie
Modelling and interpreting optical spectra of galaxies at R=10000
One way to extract more information from the integrated light of galaxies is
to improve the spectral resolution at which observations and analysis are
carried out. The population synthesis code currently providing the highest
spectral resolution is Pegase-HR, which was made available by D. Le Borgne et
al. in 2004. Based on an empirical stellar library, it provides synthetic
spectra between 4000 and 6800 A at lambda/d(lambda)=10000 for any star
formation history, with or without chemical evolution. Such a resolution is
particularly useful for the study of low mass galaxies, massive star clusters,
or other galaxy regions with low internal velocity dispersions.
After a summary of the main features of Pegase-HR and comparisons with other
population synthesis codes, this paper focuses on the inversion of optical
galaxy spectra. We explore the limits of what information can or can not be
recovered, based on theoretical principles and extensive simulations. First
applications to extragalactic objects are shown.Comment: Inv. talk in "The Spectral Energy Distribution of Gas-Rich Galaxies:
Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu
and R.J. Tuffs, AIP Conf. Ser., in pres
Gaia FGK Benchmark Stars: fundamental Teff and log g of the third version
Context. Large spectroscopic surveys devoted to the study of the Milky Way,
including Gaia, use automated pipelines to massively determine the atmospheric
parameters of millions of stars. The Gaia FGK Benchmark Stars are reference
stars with Teff and log g derived through fundamental relations, independently
of spectroscopy, to be used as anchors for the parameter scale. The first and
second versions of the sample have been extensively used for that purpose, and
more generally to help constrain stellar models. Aims. We provide the third
version of the Gaia FGK Benchmark Stars, an extended set intended to improve
the calibration of spectroscopic surveys, and their interconnection. Methods.
We have compiled about 200 candidates which have precise measurements of
angular diameters and parallaxes. We determined their bolometric fluxes by
fitting their spectral energy distribution. Masses were determined using two
sets of stellar evolution models. In a companion paper we describe the
determination of metallicities and detailed abundances. Results. We provide a
new set of 192 Gaia FGK Benchmark Stars with their fundamental Teff and logg,
and with uncertainties lower than 2% for most stars. Compared to the previous
versions, the homogeneity and accuracy of the fundamental parameters are
significantly improved thanks to the high quality of the Gaia data reflecting
on distances and bolometric fluxes.Comment: accepted in A&
The wide-field, multiplexed, spectroscopic facility WEAVE : survey design, overview, and simulated implementation
Funding for the WEAVE facility has been provided by UKRI STFC, the University of Oxford, NOVA, NWO, Instituto de Astrofísica de Canarias (IAC), the Isaac Newton Group partners (STFC, NWO, and Spain, led by the IAC), INAF, CNRS-INSU, the Observatoire de Paris, Région Île-de-France, CONCYT through INAOE, Konkoly Observatory (CSFK), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Lund University, the Leibniz Institute for Astrophysics Potsdam (AIP), the Swedish Research Council, the European Commission, and the University of Pennsylvania.WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366-959 nm at R ∼ 5000, or two shorter ranges at R ∼ 20,000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for ∼ 3 million stars and detailed abundances for ∼ 1.5 million brighter field and open-cluster stars; (ii) survey ∼ 0.4 million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ∼ 400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in z 1 million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.PostprintPeer reviewe
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