174 research outputs found
The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution
In this letter we examine the evolution of the radial metallicity gradient
induced by secular processes, in the disk of an -body Milky Way-like galaxy.
We assign a [Fe/H] value to each particle of the simulation according to an
initial, cosmologically motivated, radial chemical distribution and let the
disk dynamically evolve for 6 Gyr. This direct approach allows us to take into
account only the effects of dynamical evolution and to gauge how and to what
extent they affect the initial chemical conditions. The initial [Fe/H]
distribution increases with R in the inner disk up to R ~ 10 kpc and decreases
for larger R. We find that the initial chemical profile does not undergo major
transformations after 6 Gyr of dynamical evolution. The final radial chemical
gradients predicted by the model in the solar neighborhood are positive and of
the same order of those recently observed in the Milky Way thick disk.
We conclude that: 1) the spatial chemical imprint at the time of disk
formation is not washed out by secular dynamical processes, and 2) the observed
radial gradient may be the dynamical relic of a thick disk originated from a
stellar population showing a positive chemical radial gradient in the inner
regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical
Journal Letter
The Overdensity in Virgo, Sagittarius Debris, and the Asymmetric Spheroid
We investigate the relationship between several previously identified
Galactic halo stellar structures in the direction of Virgo using imaging and
spectroscopic observations of F turnoff stars and blue horizontal branch stars
from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic
Understanding and Exploration (SEGUE). We show that the Sagittarius dwarf
leading tidal tail does not pass through the solar neighborhood; it misses the
Sun by more than 15 kpc, passing through the Galactic plane outside the Solar
Circle. It also is not spatially coincident with the large stellar overdensity
S297+63-20.5 in the Virgo constellation. S297+63-20.5 has a distinct turnoff
color and kinematics. Faint (g ~ 20.3) turnoff stars in S297+63-20.5 have
line-of-sight, Galactic standard of rest velocities V(GSR)= 130 +/- 10 km/s,
opposite in sign to infalling Sgr tail stars. The path of the Sgr leading tidal
tail is also inconsistent with the positions of some of the nearer stars with
which it has been associated, and whose velocities have favored models with
prolate Milky Way potentials. We additionally show that the number densities of
brighter (g ~ 19.8) F turnoff stars are not symmetric about the Galactic
center, and that this discrepancy is not primarily due to the S297+63-20.5
moving group. Either the spheroid is asymmetric about the Galactic center, or
there are additional substructures that conspire to be on the same side of the
Galaxy as S297+63-20.5. The S297+63-20.5 overdensity in Virgo is likely
associated with two other previously identified Virgo substructures: the Virgo
Stellar Stream (VSS) and the Virgo Overdensity (VOD). However, the velocity
difference between the VSS and S297+63-20.5 and the difference in distance
estimates between the VOD and S297+63-20.5 must be reconciled.Comment: 10 figures, ApJ in pres
The chemical signature of the Galactic spiral arms revealed by Gaia DR3
Taking advantage of the recent Gaia Data Release 3 (DR3), we map chemical
inhomogeneities in the Milky Way's disc out to a distance of 4 kpc from
the Sun, using different samples of bright giant stars. The samples are
selected using effective temperatures and surface gravities from the GSP-Spec
module, and are expected to trace stellar populations of different typical age.
The cool (old) giants exhibit a relatively smooth radial metallicity gradient
with an azimuthal dependence. Binning in Galactic azimuth , the slope
gradually varies from [M/H] dex kpc at to dex kpc at . On the
other hand, the relatively hotter (and younger) stars present remarkable
inhomogeneities, apparent as three (possibly four) metal-rich elongated
features in correspondence of the spiral arms' locations in the Galactic disc.
When projected onto Galactic radius, those features manifest themselves as
statistically significant bumps on top of the observed radial metallicity
gradients with amplitudes up to dex, making the assumption of
a linear radial decrease not applicable to this sample. The strong correlation
between the spiral structure of the Galaxy and the observed chemical pattern in
the young sample indicates that the spiral arms might be at the origin for the
detected chemical inhomogeneities. In this scenario, the spiral arms would
leave in the younger stars a strong signature, which progressively disappears
when cooler (and older) giants are considered.Comment: 9 pages, 6 figures. Accepted for publication in A&A Letter
Stellar atmosphere parameters with MAx, a MAssive compression of x^2 for spectral fitting
MAx is a new tool to estimate parameters from stellar spectra. It is based on
the maximum likelihood method, with the likelihood compressed in a way that the
information stored in the spectral fluxes is conserved. The compressed data are
given by the size of the number of parameters, rather than by the number of
flux points. The optimum speed-up reached by the compression is the ratio of
the data set to the number of parameters. The method has been tested on a
sample of low-resolution spectra from the Sloan Extension for Galactic
Understanding and Exploration (SEGUE) survey for the estimate of metallicity,
effective temperature and surface gravity, with accuracies of 0.24 dex, 130K
and 0.5 dex, respectively. Our stellar parameters and those recovered by the
SEGUE Stellar Parameter Pipeline agree reasonably well. A small sample of
high-resolution VLT-UVES spectra were also used to test the method and the
results have been compared to a more classical approach. The speed and
multi-resolution capability of MAx combined with its performance compared with
other methods indicates that it will be a useful tool for the analysis of
upcoming spectral surveys.Comment: 17 pages, 10 figures, minor changes after the chief language editor.
A&A, in pres
The thick disk rotation-metallicity correlation as a fossil of an "inverse chemical gradient" in the early Galaxy
The thick disk rotation--metallicity correlation, \partial
V_\phi/\partial[Fe/H] =40\div 50 km s^{-1}dex^{-1} represents an important
signature of the formation processes of the galactic disk. We use
nondissipative numerical simulations to follow the evolution of a Milky Way
(MW)-like disk to verify if secular dynamical processes can account for this
correlation in the old thick disk stellar population. We followed the evolution
of an ancient disk population represented by 10 million particles whose
chemical abundances were assigned by assuming a cosmologically plausible radial
metallicity gradient with lower metallicity in the inner regions, as expected
for the 10-Gyr-old MW. Essentially, inner disk stars move towards the outer
regions and populate layers located at higher |z|. A rotation--metallicity
correlation appears, which well resembles the behaviour observed in our Galaxy
at a galactocentric distance between 8 kpc and 10 kpc. In particular,we measure
a correlation of \partial V_\phi/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} for
particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body
models can account for the V_\phi vs. [Fe/H] correlation observed in the thick
disk of our Galaxy, suggesting that processes internal to the disk such as
heating and radial migration play a role in the formation of this old stellar
component. In this scenario, the positive rotation-metallicity correlation of
the old thick disk population would represent the relic signature of an ancient
"inverse" chemical (radial) gradient in the inner Galaxy, which resulted from
accretion of primordial gas.Comment: Accepted for publication on Astronomy and Astrophysic
The expected performance of stellar parametrization with Gaia spectrophotometry
Gaia will obtain astrometry and spectrophotometry for essentially all sources
in the sky down to a broad band magnitude limit of G=20, an expected yield of
10^9 stars. Its main scientific objective is to reveal the formation and
evolution of our Galaxy through chemo-dynamical analysis. In addition to
inferring positions, parallaxes and proper motions from the astrometry, we must
also infer the astrophysical parameters of the stars from the
spectrophotometry, the BP/RP spectrum. Here we investigate the performance of
three different algorithms (SVM, ILIUM, Aeneas) for estimating the effective
temperature, line-of-sight interstellar extinction, metallicity and surface
gravity of A-M stars over a wide range of these parameters and over the full
magnitude range Gaia will observe (G=6-20mag). One of the algorithms, Aeneas,
infers the posterior probability density function over all parameters, and can
optionally take into account the parallax and the Hertzsprung-Russell diagram
to improve the estimates. For all algorithms the accuracy of estimation depends
on G and on the value of the parameters themselves, so a broad summary of
performance is only approximate. For stars at G=15 with less than two
magnitudes extinction, we expect to be able to estimate Teff to within 1%, logg
to 0.1-0.2dex, and [Fe/H] (for FGKM stars) to 0.1-0.2dex, just using the BP/RP
spectrum (mean absolute error statistics are quoted). Performance degrades at
larger extinctions, but not always by a large amount. Extinction can be
estimated to an accuracy of 0.05-0.2mag for stars across the full parameter
range with a priori unknown extinction between 0 and 10mag. Performance
degrades at fainter magnitudes, but even at G=19 we can estimate logg to better
than 0.2dex for all spectral types, and [Fe/H] to within 0.35dex for FGKM
stars, for extinctions below 1mag.Comment: MNRAS, in press. Minor corrections made in v
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