88 research outputs found
A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solar
neighbourhood. This distribution is centred on solar metallicity. We show that
previous metallicity distributions, selected on the basis of spectral type, are
biased against stars with solar metallicity or higher. A selection of G-dwarf
stars is inherently biased against metal rich stars and is not representative
of the solar neighbourhood metallicity distribution. Using a sample selected on
colour, we obtain a distribution where approximately half the stars in the
solar neighbourhood has a metallicity higher than [Fe/H]=0. The percentage of
mid-metal-poor stars ([Fe/H]-0.5) is approximately 4 per cent, in agreement
with present estimates of the thick disc. In order to have a metallicity
distribution comparable to chemical evolution model predictions, we convert the
star fraction to mass fraction, and show that another bias against metal-rich
stars affects dwarf metallicity distributions, due to the colour (or spectral
type) limits of the samples. Reconsidering the corrections due to the
increasing thickness of the stellar disc with age, we show that the Simple
Closed-Box model with no instantaneous recycling approximation gives a
reasonable fit to the observed distribution. Comparisons with the
age-metallicity relation and abundance ratios suggest that the Simple
Closed-Box model may be a viable model of the chemical evolution of the Galaxy
at solar radius.Comment: 21 pages, 15 figures, accepted for publication in MNRA
Clumpy streams in a smooth dark halo: the case of Palomar 5
By means of direct N-body simulations and simplified numerical models, we
study the formation and characteristics of the tidal tails around Palomar 5,
along its orbit in the Milky Way potential. Unlike previous findings, we are
able to reproduce the substructures observed in the stellar streams of this
cluster, without including any lumpiness in the dark matter halo. We show that
overdensities similar to those observed in Palomar 5 can be reproduced by the
epicyclic motion of stars along its tails, i.e. a simple local accumulation of
orbits of stars that escaped from the cluster with very similar positions and
velocities. This process is able to form stellar clumps at distances of several
kiloparsecs from the cluster, so it is not a phenomenon confined to the inner
part of Palomar 5's tails, as previously suggested. Our models can reproduce
the density contrast between the clumps and the surrounding tails found in the
observed streams, without including any lumpiness in the dark halo, suggesting
new upper limits on its granularity.Comment: 6 pages, 7 figures. A&A Letters, accepted. Top panel of Fig. A1
replaced, minor typos corrected. High resolution version available at
http://mygepi.obspm.fr/~paola/Pal5
Radial mixing in the outer Milky Way disk caused by an orbiting satellite
Using test particle simulations we examine the structure of the outer
Galactic disk as it is perturbed by a satellite in a tight eccentric orbit
about the Galaxy. A satellite of mass a few times 10^9 Msol can heat the outer
Galactic disk, excite spiral structure and a warp and induce streams in the
velocity distribution. We examine particle eccentricity versus the change in
mean radius between initial and current orbits. Correlations between these
quantities are reduced after a few satellite pericenter passages. Stars born in
the outer galaxy can be moved in radius from their birth positions and be
placed in low eccentricity orbits inside their birth radii. We propose that
mergers and perturbations from satellite galaxies and subhalos can induce
radial mixing in the stellar metallicity distribution.Comment: minor revisio
The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies
We compare the star-formation history and dynamics of the Milky Way (MW) with
the properties of distant disk galaxies. During the first ~4 Gyr of its
evolution, the MW formed stars with a high star-formation intensity (SFI),
Sigma_SFR~0.6 Msun/yr/kpc2 and as a result, generated outflows and high
turbulence in its interstellar medium. This intense phase of star formation
corresponds to the formation of the thick disk. The formation of the thick disk
is a crucial phase which enables the MW to have formed approximately half of
its total stellar mass by z~1 which is similar to "MW progenitor galaxies"
selected by abundance matching. This agreement suggests that the formation of
the thick disk may be a generic evolutionary phase in disk galaxies. Using a
simple energy injection-kinetic energy relationship between the 1-D velocity
dispersion and SFI, we can reproduce the average perpendicular dispersion in
stellar velocities of the MW with age. This relationship, its inferred
evolution, and required efficiency are consistent with observations of galaxies
from z~0-3. The high turbulence generated by intense star formation naturally
resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism
that links the evolution of MW to the observed characteristics of distant disk
galaxies.Comment: 5 pages, 4 figures; accepted to ApJ Letter
Stragglers of the thick disc
Young alpha-rich (YAR) stars have been detected in the past as outliers to
the local age [/Fe] relation. These objects are enhanced in
-elements but apparently younger than typical thick disc stars. We
study the global kinematics and chemical properties of YAR giant stars in
APOGEE DR17 survey and show that they have properties similar to those of the
standard thick disc stellar population. This leads us to conclude that YAR are
rejuvenated thick disc objects, most probably evolved blue stragglers. This is
confirmed by their position in the Hertzsprung-Russel diagram (HRD). Extending
our selection to dwarfs allows us to obtain the first general straggler
distribution in an HRD of field stars. We also compare the elemental abundances
of our sample with those of standard thick disc stars, and find that our YAR
stars are shifted in oxygen, magnesium, sodium, and the slow neutron-capture
element cerium. Although we detect no sign of binarity for most objects, the
enhancement in cerium may be the signature of a mass transfer from an
asymptotic giant branch companion. The most massive YAR stars suggest that mass
transfer from an evolved star may not be the only formation pathway, and that
other scenarios, such as collision or coalescence should be considered.Comment: 18 Pages, 20 Figures, 1 Table; accepted for publication in Astronomy
& Astrophysic
The distribution of globular clusters in kinematic spaces does not trace the accretion history of the host galaxy
Reconstructing how all the stellar components of the Galaxy formed and
assembled over time, by studying the properties of the stars which make it, is
the aim of Galactic archeology. In these last years, thanks to the launch of
the ESA Gaia astrometric mission, and the development of many spectroscopic
surveys, we are for the first time in the position to delve into the layers of
the past of our galaxy. Globular clusters (GCs) play a fundamental role in this
research field since they are among the oldest stellar systems in the Milky Way
(MW) and so bear witness of its entire past. In the recent years, there have
been several attempts to constrain the nature of clusters (accreted or formed
in the MW itself) through the analysis of kinematic spaces and to reconstruct
from this the properties of the accretions events experienced by the MW through
time. This work aims to test a widely-used assumption about the clustering of
the accreted populations of GCs in the integrals of motions space. We analyze a
set of dissipation-less N-body simulations that reproduce the accretion of one
or two satellites with their GC population on a MW-type galaxy. Our results
demonstrate that a significant overlap between accreted and
"kinematically-heated" in-situ GCs is expected in kinematic spaces, for mergers
with mass ratios of 1:10. In contrast with standard assumptions made in the
literature so far, we find that accreted GCs do not show dynamical coherence,
that is they do not cluster in kinematic spaces. In addition, GCs can also be
found in regions dominated by stars which have a different origin (i.e.
different progenitor). This casts doubt on the association between GCs and
field stars that is generally made in the literature to assign them to a common
origin. Our findings severely question the recovered accretion history of the
MW based on the phase-space clustering of the GC population.Comment: Submitted to A&A, 23 pages, 13 figure
The thick disk of the galaxy: sequel of a merging event
Accurate characterization of thick disc properties from recent kinematic and photometric surveys provides converging evidences that this intermediate population is a sequel of the violent heating of early disc populations by a merging satellite galaxy. The thick disc population is revisited under the light of new data in a number of galactic sample fields. Various thick disc hypotheses are fitted to observational data through a maximum likelihood technique. The resulting characteristics of the thick disc are the following: a scale height of 760 ± 50pc, with a local density of 5.6 ± 1% of the thin disc. The scale length is constrained to be 2.8 ± 0.8kpc, well in agreement with the disc scale length (2.5 ± 0.3kpc). The mean metallicity of the thick disc is found to be -0.7 ±0.2 dex, with no significant metallicity gradients. These photometric constraints in combination with kinematic data give new constraints on the thick disc formation. We show that thick disc characteristics are hardly compatible with a top-down formation scenario but fully compatible with a violent merging event arising at the early thin disc life time as described by Quinn, Hernquist & Fullagar (1993)
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