156 research outputs found
Climbing the cosmic ladder with stellar twins
Distances to stars are key to revealing a three-dimensional view of the Milky
Way, yet their determination is a major challenge in astronomy. Whilst the
brightest nearby stars benefit from direct parallax measurements, fainter stars
are subject of indirect determinations with uncertainties exceeding 30%. We
present an alternative approach to measuring distances using
spectroscopically-identified twin stars. Given a star with known parallax, the
distance to its twin is assumed to be directly related to the difference in
their apparent magnitudes. We found 175 twin pairs from the ESO public HARPS
archives and report excellent agreement with Hipparcos parallaxes within 7.5%.
Most importantly, the accuracy of our results does not degrade with increasing
stellar distance. With the ongoing collection of high-resolution stellar
spectra, our method is well-suited to complement Gaia.Comment: published online on MNRA
Stellar twins determine the distance of the Pleiades
© 2016 ESO.Since the release of the Hipparcos catalogue in 1997, the distance to the Pleiades open cluster has been heavily debated. The distance obtained from Hipparcos and those by alternative methods differ by 10 to 15%. As accurate stellar distances are key to understanding stellar structure and evolution, this dilemma puts the validity of some stellar evolution models into question. Using our model-independent method to determine parallaxes based on twin stars, we report individual parallaxes of 15 FGK type stars in the Pleiades in anticipation of the astrometric mission Gaia. These parallaxes give a mean cluster parallax of 7.42 ± 0.09 mas,which corresponds to a mean cluster distance of 134.8 ± 1.7 pc. This value agrees with the current results obtained from stellar evolution models
The Gaia-ESO Survey: the selection function of the Milky Way field stars
The Gaia-ESO Survey was designed to target all major Galactic components
(i.e., bulge, thin and thick discs, halo and clusters), with the goal of
constraining the chemical and dynamical evolution of the Milky Way. This paper
presents the methodology and considerations that drive the selection of the
targeted, allocated and successfully observed Milky Way field stars. The
detailed understanding of the survey construction, specifically the influence
of target selection criteria on observed Milky Way field stars is required in
order to analyse and interpret the survey data correctly. We present the target
selection process for the Milky Way field stars observed with VLT/FLAMES and
provide the weights that characterise the survey target selection. The weights
can be used to account for the selection effects in the Gaia-ESO Survey data
for scientific studies. We provide a couple of simple examples to highlight the
necessity of including such information in studies of the stellar populations
in the Milky Way.Comment: 18 pages, 19 figures, Accepted for publication in MNRAS (April 25,
2016
The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
We present the first results of the EMBLA survey (Extremely Metal-poor BuLge
stars with AAOmega), aimed at finding metal-poor stars in the Milky Way bulge,
where the oldest stars should now preferentially reside. EMBLA utilises
SkyMapper photometry to pre-select metal-poor candidates, which are
subsequently confirmed using AAOmega spectroscopy. We describe the discovery
and analysis of four bulge giants with -2.72<=[Fe/H]<=-2.48, the lowest
metallicity bulge stars studied with high-resolution spectroscopy to date.
Using FLAMES/UVES spectra through the Gaia-ESO Survey we have derived
abundances of twelve elements. Given the uncertainties, we find a chemical
similarity between these bulge stars and halo stars of the same metallicity,
although the abundance scatter may be larger, with some of the stars showing
unusual [{\alpha}/Fe] ratios.Comment: 7 pages, 5 figures. Accepted for publication by MNRA
The Gaia-ESO Survey: Empirical estimates of stellar ages from lithium equivalent widths (EAGLES)
We present an empirical model of age-dependent photospheric lithium
depletion, calibrated using a large, homogeneously-analysed sample of 6200
stars in 52 open clusters, with ages from 2--6000 Myr and , observed in the Gaia-ESO spectroscopic survey. The model is used
to obtain age estimates and posterior age probability distributions from
measurements of the Li I 6708A equivalent width for individual (pre) main
sequence stars with , a domain where age
determination from the HR diagram is either insensitive or highly
model-dependent. In the best cases, precisions of 0.1 dex in log age are
achievable; even higher precision can be obtained for coeval groups and
associations where the individual age probabilities of their members can be
combined. The method is validated on a sample of exoplanet-hosting young stars,
finding agreement with claimed young ages for some, but not others. We obtain
better than 10 per cent precision in age, and excellent agreement with
published ages, for seven well-studied young moving groups. The derived ages
for young clusters ( Gyr) in our sample are also in good agreement with
their training ages, and consistent with several published, model-insensitive
lithium depletion boundary ages. For older clusters there remain systematic age
errors that could be as large as a factor of two. There is no evidence to link
these errors to any strong systematic metallicity dependence of (pre) main
sequence lithium depletion, at least in the range . Our methods and model are provided as software -- "Empirical AGes from
Lithium Equivalent widthS" (EAGLES).Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
The Gaia–ESO Survey: dynamical models of flattened, rotating globular clusters
We present a family of self-consistent axisymmetric rotating globular cluster models which are fitted to spectroscopic data for NGC 362, NGC 1851, NGC 2808, NGC 4372, NGC 5927 and NGC 6752 to provide constraints on their physical and kinematic properties, including their rotation signals. They are constructed by flattening Modified Plummer profiles, which have the same asymptotic behaviour as classical Plummer models, but can provide better fits to young clusters due to a slower turnover in the density profile. The models are in dynamical equilibrium as they depend solely on the action variables. We employ a fully Bayesian scheme to investigate the uncertainty in our model parameters (including mass-to-light ratios and inclination angles) and evaluate the Bayesian evidence ratio for rotating to non-rotating models. We find convincing levels of rotation only in NGC 2808. In the other clusters, there is just a hint of rotation (in particular, NGC 4372 and NGC 5927), as the data quality does not allow us to draw strong conclusions. Where rotation is present, we find that it is confined to the central regions, within radii of R ≤ 2rh. As part of this work, we have developed a novel q-Gaussian basis expansion of the line-of-sight velocity distributions, from which general models can be constructed via interpolation on the basis coefficients.This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Università ’ e della Ricerca (MIUR) in the form of the grant ‘Premiale VLT 2012’
The Gaia-ESO Survey: Hydrogen lines in red giants directly trace stellar mass
Red giant stars are perhaps the most important type of stars for Galactic and
extra-galactic archaeology: they are luminous, occur in all stellar
populations, and their surface temperatures allow precise abundance
determinations for many different chemical elements. Yet, the full star
formation and enrichment history of a galaxy can be traced directly only if two
key observables can be determined for large stellar samples - age and chemical
composition. While spectroscopy is a powerful method to analyse the detailed
abundances of stars, stellar ages are the "missing link in the chain", since
they are not a direct observable. However, spectroscopy should be able to
estimate stellar masses, which for red giants directly infer ages provided
their chemical composition is known.
Here we establish a new empirical relation between the shape of the hydrogen
line in the observed spectra of red giants and stellar mass determined from
asteroseismology. The relation allows to determine stellar masses and ages with
the accuracy of 10-15%. The method can be used with confidence for stars in the
following range of stellar parameters: 4000 < Teff < 5000 K, 0.5 < log g < 3.5,
-2.0 < [Fe/H] < 0.3, and luminosities log L/LSun < 2.5. Our analysis provides
observational evidence that the Halpha spectral characteristics of red giant
stars are tightly correlated with their mass and therefore their age. We also
show that the method samples well all stellar populations with ages above 1
Gyr. Targeting bright giants, the method allows to obtain simultaneous age and
chemical abundance information far deeper than would be possible with
asteroseismology, extending the possible survey volume to remote regions of the
Milky Way and even to neighbouring galaxies like Andromeda or the Magellanic
Clouds already with present instrumentation, like VLT and Keck facilities
The Gaia-ESO Survey: Churning through the Milky Way
We attempt to determine the relative fraction of stars that have undergone
significant radial migration by studying the orbital properties of metal-rich
([Fe/H]) stars within 2 kpc of the Sun using a sample of more than 3,000
stars selected from iDR4 of the Gaia-ESO Survey. We investigate the kinematic
properties, such as velocity dispersion and orbital parameters, of stellar
populations near the sun as a function of [Mg/Fe] and [Fe/H], which could show
evidence of a major merger in the past history of the Milky Way. This was done
using the stellar parameters from the Gaia-ESO Survey along with proper motions
from PPMXL to determine distances, kinematics, and orbital properties for these
stars to analyze the chemodynamic properties of stellar populations near the
Sun. Analyzing the kinematics of the most metal-rich stars ([Fe/H]), we
find that more than half have small eccentricities () or are on nearly
circular orbits. Slightly more than 20\% of the metal-rich stars have
perigalacticons kpc. We find that the highest [Mg/Fe], metal-poor
populations have lower vertical and radial velocity dispersions compared to
lower [Mg/Fe] populations of similar metallicity by km s. The
median eccentricity increases linearly with [Mg/Fe] across all metallicities,
while the perigalacticon decreases with increasing [Mg/Fe] for all
metallicities. Finally, the most [Mg/Fe]-rich stars are found to have
significant asymmetric drift and rotate more than 40 km s slower than
stars with lower [Mg/Fe] ratios. While our results cannot constrain how far
stars have migrated, we propose that migration processes are likely to have
played an important role in the evolution of the Milky Way, with metal-rich
stars migrating from the inner disk toward to solar neighborhood and past
mergers potentially driving enhanced migration of older stellar populations in
the disk
The Gaia-ESO Survey: the selection function of the Milky Way field stars
The Gaia-ESO Survey was designed to target all major Galactic components (i.e. bulge, thin and thick discs, halo and clusters), with the goal of constraining the chemical and dynamical evolution of the Milky Way. This paper presents the methodology and considerations that drive the selection of the targeted, allocated and successfully observed Milky Way field stars. The detailed understanding of the survey construction, specifically the influence of target selection criteria on observed Milky Way field stars is required in order to analyse and interpret the survey data correctly. We present the target selection process for the Milky Way field stars observed with Very Large Telescope/Fibre Large Array Multi Element Spectrograph and provide the weights that characterize the survey target selection. The weights can be used to account for the selection effects in the Gaia-ESO Survey data for scientific studies. We provide a couple of simple examples to highlight the necessity of including such information in studies of the stellar populations in the Milky Way
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