97 research outputs found
The metal-weak Milky Way stellar disk hidden in the Gaia-Sausage-Enceladus debris: the APOGEE DR17 view
We have for the first time identified the early stellar disk in the Milky Way
by using a combination of elemental abundances and kinematics. Using data from
APOGEE DR17 and Gaia we select stars in the Mg-Mn-Al-Fe plane with elemental
abundances indicative of accreted origin and find stars with both halo-like and
disk-like kinematics. The stars with halo-like kinematics lie along a lower
sequence in [Mg/Fe], while the stars with disk-like kinematics lie along a
higher sequence. Through with asteroseismic observations, we determine the
stars with halo-like kinematics are old, 9-11 Gyr and that the more evolved
stellar disk is about 1-2 Gyr younger. We show that the in situ fraction of
stars on deeply bound orbits is not small, in fact the inner Galaxy likely
harbours a genuine in-situ population together with an accreted one. In
addition, we show that the selection of Gaia-Sausage-Enceladus in the En-Lz
plane is not very robust. In fact, radically different selection criteria give
almost identical elemental abundance signatures for the accreted stars.Comment: 32 pages, 19 figures, accepted to Ap
Determining Ages of APOGEE Giants with Known Distances
We present a sample of local red giant stars observed using the New Mexico
State University 1 m telescope with the APOGEE spectrograph, for which we
estimate stellar ages and the age distribution from the high-resolution
spectroscopic stellar parameters and accurate distance measurements from
Hipparcos. The high-resolution (R ~ 23,000), near infrared (H-band, 1.5-1.7
micron) APOGEE spectra provide measurements of the stellar atmospheric
parameters (temperature, surface gravity, [M/H], and [alpha/M]). Due to the
smaller uncertainties in surface gravity possible with high-resolution spectra
and accurate Hipparcos distance measurements, we are able to calculate the
stellar masses to within 40%. For red giants, the relatively rapid evolution of
stars up the red giant branch allows the age to be constrained based on the
mass. We examine methods of estimating age using both the mass-age relation
directly and a Bayesian isochrone matching of measured parameters, assuming a
constant star formation history (SFH). To improve the prior on the SFH, we use
a hierarchical modeling approach to constrain the parameters of a model SFH
from the age probability distribution functions of the data. The results of an
alpha dependent Gaussian SFH model shows a clear relation between age and
[alpha/M] at all ages. Using this SFH model as the prior for an empirical
Bayesian analysis, we construct a full age probability distribution function
and determine ages for individual stars. The age-metallicity relation is flat,
with a slight decrease in [M/H] at the oldest ages and a ~ 0.5 dex spread in
metallicity. For stars with ages < 1 Gyr we find a smaller spread, consistent
with radial migration having a smaller effect on these young stars than on the
older stars.Comment: 14 page, 18 figures, accepted to ApJ with minor revisions, full
electronic table of data available upon publicatio
The HR 1614 moving group is not a dissolving cluster
The HR 1614 overdensity in velocity space and has for a long time been known
as an old (~2 Gyr) and metal-rich ([Fe/H]~0.2) nearby moving group that has a
dissolving cluster origin. The existence of such old and metal-rich groups in
the solar vicinity is quite unexpected since the vast majority of nearby moving
groups are known to be young. In the light of new and significantly larger data
sets we aim to re-investigate the properties and origin of the HR 1614 moving
group.
To identify and characterise the HR 1614 moving group we use astrometric data
from Gaia DR2; distances, extinction, and reddening corrections from the
StarHorse code; elemental abundances from the GALAH and APOGEE spectroscopic
surveys; and photometric metallicities from the SkyMapper survey. Bayesian ages
were estimated for the SkyMapper stars. Since the Hercules stream is the
closest kinematical structure to the HR 1614 moving group in velocity space, we
use it for comparison purposes. Stars that are likely to be members of the two
groups were selected based on their space velocities.
The HR 1614 moving group is located mainly at negative U velocities, does not
form an arch of constant energy in the U-V space and is tilted in V. The
overdensity is not chemically homogeneous but that its stars exist at a wide
range of both metallicities, ages, and elemental abundance ratios. They are
essentially similar to what is observed in the Galactic thin and thick disks, a
younger population (~3 Gyr) that is metal-rich (-0.2<[Fe/H]<0.4) and
alpha-poor. It should therefore not be considered as a dissolving open cluster,
or an accreted population. We suggest that HR 1614 has a complex origin that
could be explained by combining several different mechanisms such as resonances
with the Galactic bar and spiral structure, phase-mixing of dissolving spiral
structure, and phase-mixing due to an external perturbation.Comment: Accepted for publication in A&
Comparative analysis of atmospheric parameters from high-resolution spectroscopic sky surveys: APOGEE, GALAH, Gaia-ESO
SDSS-IV APOGEE-2, GALAH and Gaia-ESO are high resolution, ground-based,
multi-object spectroscopic surveys providing fundamental stellar atmospheric
parameters and multiple elemental abundance ratios for hundreds of thousands of
stars of the Milky Way. We undertake a comparison between the most recent data
releases of these surveys to investigate the accuracy and precision of derived
parameters by placing the abundances on an absolute scale. We discuss the
correlations in parameter and abundance differences as a function of main
parameters. Uncovering the variants provides a basis to on-going efforts of
future sky surveys. Quality samples from the APOGEE-GALAH, APOGEE-GES and
GALAH-GES overlapping catalogs are collected. We investigate the mean variants
between the surveys, and linear trends are also investigated. We compare the
slope of correlations and mean differences with the reported uncertainties. The
average and scatter of vrad, Teff, log g, [M/H] and vmicro, along with numerous
species of elemental abundances in the combined catalogs show that in general
there is a good agreement between the surveys. We find large radial velocity
scatters ranging from 1.3 km/s to 4.4 km/s when comparing the three surveys. We
observe weak trends: e.g. in Teff vs. log g for the APOGEE-GES
stars, and a clear correlation in the vmicro-vmicro planes in the
APOGEE-GALAH common sample. For [/H], [Ti/H] (APOGEE-GALAH giants) and
[Al/H] (APOGEE-GALAH dwarfs) potential strong correlations are discovered as a
function of the differences in the main atmospheric parameters, and we find
weak trends for other elements. In general we find good agreement between the
three surveys within their respective uncertainties. However, there are certain
regimes in which strong variants exist, which we discuss. There are still
offsets larger than 0.1 dex in the absolute abundance scales.Comment: Accepted in A&A, 23 pages, 13 figures, 6 tables. A minor correction
is applied to the Gaia-ESO Survey's solar reference: Grevesse et al. (2007)
instead of Grevesse & Sauval (1998
VINTERGATAN III: how to reset the metallicity of the Milky Way
Using the cosmological zoom simulation VINTERGATAN, we present a new scenario
for the onset of star formation at the metal-poor end of the low-[/Fe]
sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by
two distinct gas flows. One is enriched by outflows from massive galaxies, but
not the other. While the former feeds the inner galactic region, the latter
fuels an outer gas disk, inclined with respect to the main galactic plane, and
with a significantly poorer chemical content. The first passage of the last
major merger galaxy triggers tidal compression in the outer disk, which
increases the gas density and eventually leads to star formation, at a
metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of
the low-[/Fe] sequence. These in situ stars have halo-like kinematics,
similarly to what is observed in the Milky Way, due to the inclination of the
outer disk which eventually aligns with the inner one via gravitational
torques. We show that this tilting disk scenario is likely to be common in
Milky-Way like galaxies. This process implies that the low-[/Fe]
sequence is populated in situ, simultaneously from two formation channels, in
the inner and the outer galaxy, with distinct metallicities. This contrasts
with purely sequential scenarios for the assembly of the Milky Way disk and
could be tested observationally.Comment: MNRAS in press. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
VINTERGATAN II: the history of the Milky Way told by its mergers
Using the VINTERGATAN cosmological zoom simulation, we explore the
contributions of the in situ and accreted material, and the effect of galaxy
interactions and mergers in the assembly of a Milky Way-like galaxy. We find
that the initial growth phase of galaxy evolution, dominated by repeated major
mergers, provides the necessary physical conditions for the assembly of a
thick, kinematically hot disk populated by high-[/Fe] stars, formed
both in situ and in accreted satellite galaxies. We find that the diversity of
evolutionary tracks followed by the simulated galaxy and its progenitors leads
to very little overlap of the in situ and accreted populations for any given
chemical composition. At a given age, the spread in [/Fe] abundance
ratio results from the diversity of physical conditions in VINTERGATAN and its
satellites, with an enhancement in [/Fe] found in stars formed during
starburst episodes. Later, the cessation of the merger activity promotes the in
situ formation of stars in the low-[/Fe] regime, in a radially
extended, thin and overall kinematically colder disk, thus establishing
chemically bimodal thin and thick disks, in line with observations. We draw
links between notable features in the [Fe/H] - [/Fe] plane with their
physical causes, and propose a comprehensive formation scenario explaining
self-consistently, in the cosmological context, the main observed properties of
the Milky Way.Comment: MNRAS in press. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
VINTERGATAN I: The origins of chemically, kinematically and structurally distinct discs in a simulated Milky Way-mass galaxy
Spectroscopic surveys of the Milky Way's stars have revealed spatial,
chemical and kinematical structures that encode its history. In this work, we
study their origins using a cosmological zoom simulation, VINTERGATAN, of a
Milky Way-mass disc galaxy. We find that in connection to the last major merger
at , cosmological accretion leads to the rapid formation of an
outer, metal-poor, low-[/Fe] gas disc around the inner, metal-rich
galaxy containing the old high-[/Fe] stars. This event leads to a
bimodality in [/Fe] over a range of [Fe/H]. A detailed analysis of how
the galaxy evolves since is presented. We demonstrate the way in
which inside-out growth shapes the radial surface density and metallicity
profile and how radial migration preferentially relocates stars from the inner
to the outer disc. Secular disc heating is found to give rise to increasing
velocity dispersions and scaleheights with stellar age, which together with
disc flaring explains several trends observed in the Milky Way, including
shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy
formation scenario imprints non-trivial mappings between structural
associations (i.e. thick and thin discs), velocity dispersions,
-enhancements, and ages of stars, e.g. the most metal-poor stars in the
low-[/Fe] sequence are found to have a scaleheight comparable to old
high-[/Fe] stars. Finally, we illustrate how at low spatial resolution,
comparable to the thickness of the galaxy, the proposed pathway to distinct
sequences in [/Fe]-[Fe/H] cannot be captured.Comment: 20 pages, MNRAS submitted, comments welcome. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
Chemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way
We present Galactic mean metallicity maps derived from the first year of the
SDSS-III APOGEE experiment. Mean abundances in different zones of
Galactocentric radius (0 < R < 15 kpc) at a range of heights above the plane (0
< |z| < 3 kpc), are derived from a sample of nearly 20,000 stars with
unprecedented coverage, including stars in the Galactic mid-plane at large
distances. We also split the sample into subsamples of stars with low and
high-[{\alpha}/M] abundance ratios. We assess possible biases in deriving the
mean abundances, and find they are likely to be small except in the inner
regions of the Galaxy. A negative radial gradient exists over much of the
Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular
near the Galactic mid-plane and for low-[{\alpha}/M] stars. At R > 6 kpc, the
gradient flattens as one moves off of the plane, and is flatter at all heights
for high-[{\alpha}/M] stars than for low-[{\alpha}/M] stars. Alternatively,
these gradients can be described as vertical gradients that flatten at larger
Galactocentric radius; these vertical gradients are similar for both low and
high-[{\alpha}/M] populations. Stars with higher [{\alpha}/M] appear to have a
flatter radial gradient than stars with lower [{\alpha}/M]. This could suggest
that the metallicity gradient has grown steeper with time or, alternatively,
that gradients are washed out over time by migration of stars.Comment: 16 pages, 12 figures, submitted to A
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