22 research outputs found
Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
Our Galaxy is thought to have undergone an active evolutionary history
dominated by star formation, the accretion of cold gas, and, in particular,
mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of
these interactions in the form of stellar streams, substructures, and
chemically distinct stellar components. The impact of dwarf galaxy mergers on
the content and morphology of the Galactic disk is still being explored. Recent
studies have identified kinematically distinct stellar substructures and moving
groups, which may have extragalactic origin. However, there is mounting
evidence that stellar overdensities at the outer disk/halo interface could have
been caused by the interaction of a dwarf galaxy with the disk. Here we report
detailed spectroscopic analysis of 14 stars drawn from two stellar
overdensities, each lying about 5 kiloparsecs above and below the Galactic
plane - locations suggestive of association with the stellar halo. However, we
find that the chemical compositions of these stars are almost identical, both
within and between these groups, and closely match the abundance patterns of
the Milky Way disk stars. This study hence provides compelling evidence that
these stars originate from the disk and the overdensities they are part of were
created by tidal interactions of the disk with passing or merging dwarf
galaxies.Comment: accepted for publication in Natur
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: mapping the shape and evolution of the radial abundance gradients with open clusters
The spatial distribution of elemental abundances and their time evolution are
among the major constraints to disentangle the scenarios of formation and
evolution of the Galaxy. We used the sample of open clusters available in the
final release of the Gaia-ESO survey to trace the Galactic radial abundance and
abundance to iron ratio gradients, and their time evolution. We selected member
stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the
Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed
the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and
[El/Fe] combining elements belonging to four different nucleosynthesis
channels, and their individual abundance and abundance ratio gradients. We also
investigated the time evolution of the gradients dividing open clusters in
three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw
different behaviours for elements belonging to different channels. We found
that the youngest clusters in the inner disc have lower metallicity than their
older counterpart and they outline a flatter gradient. We considered some
possible explanations, including the effects of gas inflow and migration. We
suggested that it might be a bias introduced by the standard spectroscopic
analysis producing lower metallicities in low gravity stars. To delineate the
shape of the `true' gradient, we should limit our analysis to stars with low
surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we
can conclude that the gradient has minimally evolved over the time-frame
outlined by the open clusters, indicating a slow and stationary formation of
the thin disc in the latest Gyr. We found a secondary role of clusters'
migration in shaping the gradient, with a more prominent role of migration for
the oldest clusters.Comment: 25 pages, 14 figures and 4 tables in the main text, 3 figures and 7
tables in the Appendix. Accepted for publication in A&
The Gaia-ESO Survey: homogenisation of stellar parameters and elemental abundances
The Gaia-ESO Survey is a public spectroscopic survey that has targeted
stars covering all major components of the Milky Way from the end
of 2011 to 2018, delivering its public final release in May 2022. Unlike other
spectroscopic surveys, Gaia-ESO is the only survey that observed stars across
all spectral types with dedicated, specialised analyses: from O
(~K) all the way to K-M (3,500~K).
The physics throughout these stellar regimes varies significantly, which has
previously prohibited any detailed comparisons between stars of significantly
different type. In the final data release (internal data release 6) of the
Gaia-ESO Survey, we provide the final database containing a large number of
products such as radial velocities, stellar parameters and elemental
abundances, rotational velocity, and also, e.g., activity and accretion
indicators in young stars and membership probability in star clusters for more
than 114,000 stars. The spectral analysis is coordinated by a number of Working
Groups (WGs) within the Survey, which specialise in the various stellar
samples. Common targets are analysed across WGs to allow for comparisons (and
calibrations) amongst instrumental setups and spectral types. Here we describe
the procedures employed to ensure all Survey results are placed on a common
scale to arrive at a single set of recommended results for all Survey
collaborators to use. We also present some general quality and consistency
checks performed over all Survey results.Comment: A&A accepted, minor revision, 36 pages, 38 figure
The Role of Attention in Ambiguous Reversals of Structure-From-Motion
Multiple dots moving independently back and forth on a flat screen induce a compelling illusion of a sphere rotating in depth (structure-from-motion). If all dots simultaneously reverse their direction of motion, two perceptual outcomes are possible: either the illusory rotation reverses as well (and the illusory depth of each dot is maintained), or the illusory rotation is maintained (but the illusory depth of each dot reverses). We investigated the role of attention in these ambiguous reversals. Greater availability of attention – as manipulated with a concurrent task or inferred from eye movement statistics – shifted the balance in favor of reversing illusory rotation (rather than depth). On the other hand, volitional control over illusory reversals was limited and did not depend on tracking individual dots during the direction reversal. Finally, display properties strongly influenced ambiguous reversals. Any asymmetries between ‘front’ and ‘back’ surfaces – created either on purpose by coloring or accidentally by random dot placement – also shifted the balance in favor of reversing illusory rotation (rather than depth). We conclude that the outcome of ambiguous reversals depends on attention, specifically on attention to the illusory sphere and its surface irregularities, but not on attentive tracking of individual surface dots