54 research outputs found
Galactic Archaeology with CoRoT and APOGEE: Creating mock observations from a chemodynamical model
In a companion paper, we have presented the combined
asteroseismic-spectroscopic dataset obtained from CoRoT lightcurves and APOGEE
infra-red spectra for 678 solar-like oscillating red giants in two fields of
the Galactic disc (CoRoGEE). We have measured chemical abundance patterns,
distances, and ages of these field stars which are spread over a large radial
range of the Milky Way's disc. Here we show how to simulate this dataset using
a chemodynamical Galaxy model. We also demonstrate how the observation
procedure influences the accuracy of our estimated ages.Comment: 5 pages, 6 figures. To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalb\'an, and M. Steffe
Spectroscopic signatures of extratidal stars around the globular clusters NGC 6656 (M 22), NGC 3201, and NGC 1851 from RAVE
Context. Stellar population studies of globular clusters have suggested that the brightest clusters in the Galaxy might actually be the remnant nuclei of dwarf spheroidal galaxies. If the present Galactic globular clusters formed within larger stellar systems, they are likely to be surrounded by extratidal halos and/or tails made up of stars that were tidally stripped from their parent systems. Aims. The stellar surroundings around globular clusters are therefore one of the best places to look for the remnants of an ancient dwarf galaxy. Here an attempt is made to search for tidal debris around the supernovae enriched globular clusters M? 22 and NGC 1851, as well as the kinematically unique cluster NGC 3201. Methods. The stellar parameters from the RAdial Velocity Experiment (RAVE) are used to identify stars with the RAVE metallicities, radial velocities, and elemental abundances that are consistent with the abundance patterns and properties of the stars in M? 22, NGC 1851, and NGC 3201. Results. Discovery of RAVE stars that may be associated with M? 22 and NGC 1851 are reported, some of which are at projected distances ∼10 degrees away from the core of these clusters. Numerous RAVE stars associated with NGC 3201 suggest that either the tidal radius of this cluster is underestimated or that there are some unbound stars extending a few arc minutes from the edge of the cluster's radius. No other extratidal stars associated with NGC 3201 could be identified. The bright magnitudes of the RAVE stars make them easy targets for high-resolution follow-up observations, eventually allowing further chemical tagging to solidify (or exclude) stars outside the tidal radius of the cluster as tidal debris. In both our radial velocity histograms of the regions surrounding NGC 1851 and NGC 3201, a peak of stars at ∼230 km? s-1 is seen, consistent with extended tidal debris from ω Centauri
Chemical gradients in the Milky Way from the RAVE data: II. Giant stars
Aims. We provide new constraints on the chemo-dynamical models of the Milky Way by measuring the radial and vertical chemical gradients for the elements Mg, Al, Si, Ti, and Fe in the Galactic disc and the gradient variations as a function of the distanc
Constraining the Galaxy's dark halo with RAVE stars
We use the kinematics of giant stars that lie within kpc of the plane to measure the vertical profile of mass density near the
Sun. We find that the dark mass contained within the isodensity surface of the
dark halo that passes through the Sun
(), and the surface density within
kpc of the plane () are almost
independent of the (oblate) halo's axis ratio . If the halo is spherical, 46
per cent of the radial force on the Sun is provided by baryons, and only 4.3
per cent of the Galaxy's mass is baryonic. If the halo is flattened, the
baryons contribute even less strongly to the local radial force and to the
Galaxy's mass. The dark-matter density at the location of the Sun is
.
When combined with other literature results we find hints for a mildly oblate
dark halo with . Our value for the dark mass within the solar
radius is larger than that predicted by cosmological dark-matter-only
simulations but in good agreement with simulations once the effects of baryonic
infall are taken into account. Our mass models consist of three
double-exponential discs, an oblate bulge and a Navarro-Frenk-White dark-matter
halo, and we model the dynamics of the RAVE stars in the corresponding
gravitational fields by finding distribution functions that
depend on three action integrals. Statistical errors are completely swamped by
systematic uncertainties, the most important of which are the distance to the
stars in the photometric and spectroscopic samples and the solar distance to
the Galactic centre. Systematics other than the flattening of the dark halo
yield overall uncertainties per cent.Comment: 20 pages, 17 figures, accepted for publication in MNRA
Chemical gradients in the Milky Way from the RAVE data. II. Giant stars
We provide new constraints on the chemo-dynamical models of the Milky Way by
measuring the radial and vertical chemical gradients for the elements Mg, Al,
Si, Ti, and Fe in the Galactic disc and the gradient variations as a function
of the distance from the Galactic plane (). We selected a sample of giant
stars from the RAVE database using the gravity criterium 1.7log g2.8. We
created a RAVE mock sample with the Galaxia code based on the Besan\c con model
and selected a corresponding mock sample to compare the model with the observed
data. We measured the radial gradients and the vertical gradients as a function
of the distance from the Galactic plane to study their variation across the
Galactic disc. The RAVE sample exhibits a negative radial gradient of
dex kpc close to the Galactic plane ( kpc)
that becomes flatter for larger . Other elements follow the same trend
although with some variations from element to element. The mock sample has
radial gradients in fair agreement with the observed data. The variation of the
gradients with shows that the Fe radial gradient of the RAVE sample has
little change in the range kpc and then flattens. The iron
vertical gradient of the RAVE sample is slightly negative close to the Galactic
plane and steepens with . The mock sample exhibits an iron vertical
gradient that is always steeper than the RAVE sample. The mock sample also
shows an excess of metal-poor stars in the [Fe/H] distributions with respect to
the observed data. These discrepancies can be reduced by decreasing the number
of thick disc stars and increasing their average metallicity in the Besan\c con
model.Comment: 13 pages, 9 figures, 5 tables, A&A accepte
The RAVE survey: the Galactic escape speed and the mass of the Milky Way
We construct new estimates on the Galactic escape speed at various
Galactocentric radii using the latest data release of the Radial Velocity
Experiment (RAVE DR4). Compared to previous studies we have a database larger
by a factor of 10 as well as reliable distance estimates for almost all stars.
Our analysis is based on the statistical analysis of a rigorously selected
sample of 90 high-velocity halo stars from RAVE and a previously published data
set. We calibrate and extensively test our method using a suite of cosmological
simulations of the formation of Milky Way-sized galaxies. Our best estimate of
the local Galactic escape speed, which we define as the minimum speed required
to reach three virial radii , is km/s (90%
confidence) with an additional 5% systematic uncertainty, where is
the Galactocentric radius encompassing a mean over-density of 340 times the
critical density for closure in the Universe. From the escape speed we further
derive estimates of the mass of the Galaxy using a simple mass model with two
options for the mass profile of the dark matter halo: an unaltered and an
adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the
local circular velocity the latter profile yields a significantly higher mass
than the un-contracted halo, but if we instead use the statistics on halo
concentration parameters in large cosmological simulations as a constraint we
find very similar masses for both models. Our best estimate for , the
mass interior to (dark matter and baryons), is M (corresponding to M). This estimate is in good agreement with recently published
independent mass estimates based on the kinematics of more distant halo stars
and the satellite galaxy Leo I.Comment: 16 pages, 15 figures; accepted for publication in Astronomy &
Astrophysic
Chemical gradients in the Milky Way from the RAVE data II. Giant stars
Aims: We provide new constraints on the chemo-dynamical models of the Milky Way by measuring the radial and vertical chemical
gradients for the elements Mg, Al, Si, Ti, and Fe in the Galactic disc and the gradient variations as a function of the distance from the
Galactic plane (Z).
Methods: We selected a sample of giant stars from the RAVE database using the gravity criterium 1.7 < log g < 2.8. We created a
RAVE mock sample with the Galaxia code based on the Besançon model and selected a corresponding mock sample to compare
the model with the observed data. We measured the radial gradients and the vertical gradients as a function of the distance from the
Galactic plane Z to study their variation across the Galactic disc.
Results: The RAVE sample exhibits a negative radial gradient of d[Fe/H]/dR = −0.054 dex kpc−1 close to the Galactic plane
(|Z| < 0.4 kpc) that becomes flatter for larger |Z|. Other elements follow the same trend although with some variations from element to element. The mock sample has radial gradients in fair agreement with the observed data. The variation of the gradients with
Z shows that the Fe radial gradient of the RAVE sample has little change in the range |Z| 0.6 kpc and then flattens. The iron vertical
gradient of the RAVE sample is slightly negative close to the Galactic plane and steepens with |Z|. The mock sample exhibits an
iron vertical gradient that is always steeper than the RAVE sample. The mock sample also shows an excess of metal-poor stars in
the [Fe/H] distributions with respect to the observed data. These discrepancies can be reduced by decreasing the number of thick disc
stars and increasing their average metallicity in the Besançon model
Chemical gradients in the Milky Way from the RAVE data
Aims. We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besancon model. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane.
Methods. We analysed three different samples selected from three independent datasets: a sample of 19 962 dwarf stars selected from the RAVE database, a sample of 10 616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besancon model. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (R-g) and the maximum distances from the Galactic plane reached by the stars along their orbits (Z(max)). We measured the chemical gradients as functions of R-g at different Z(max).
Results. We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Z(max) < 0.4 kpc and 4.5 < R-g(kpc) < 9.5, the iron gradient for the RAVE sample is d[Fe/H]/dR(g) = -0.065 dex kpc(-1), whereas for the GCS sample it is d[Fe/H]/dR(g) = -0.043 dex kpc(-1) with internal errors of +/-0.002 and +/-0.004 dex kpc(-1), respectively. The gradients of the RAVE and GCS samples become flatter at larger Z(max). Conversely, the mock sample has a positive iron gradient of d[Fe/H]/dR(g) = +0.053 +/- 0.003 dex kpc(-1) at Z(max) < 0.4 kpc and remains positive at any Z(max). These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besancon model. In addition, the low metallicity and asymmetric drift of the thick disc causes a shift of the stars towards lower R-g and metallicity which, together with the thin-disc stars with a higher metallicity and R-g, generates a fictitious positive gradient of the full sample. The flatter gradient at larger Z(max) found in the RAVE and the GCS samples may therefore be due to the superposition of thin-and thick-disc stars, which mimicks a flatter or positive gradient. This does not exclude the possibility that the thick disc has no chemical gradient. The discrepancies between the observational samples and the mock sample can be reduced by i) decreasing the density; ii) decreasing the vertical velocity; and iii) increasing the metallicity of the thick disc in the Besancon model
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