48 research outputs found
The K2-HERMES Survey: Age and Metallicity of the Thick Disc
Asteroseismology is a promising tool to study Galactic structure and
evolution because it can probe the ages of stars. Earlier attempts comparing
seismic data from the {\it Kepler} satellite with predictions from Galaxy
models found that the models predicted more low-mass stars compared to the
observed distribution of masses. It was unclear if the mismatch was due to
inaccuracies in the Galactic models, or the unknown aspects of the selection
function of the stars. Using new data from the K2 mission, which has a
well-defined selection function, we find that an old metal-poor thick disc, as
used in previous Galactic models, is incompatible with the asteroseismic
information. We show that spectroscopic measurements of [Fe/H] and
[/Fe] elemental abundances from the GALAH survey indicate a mean
metallicity of for the thick disc. Here is the
effective solar-scaled metallicity, which is a function of [Fe/H] and
[/Fe]. With the revised disc metallicities, for the first time, the
theoretically predicted distribution of seismic masses show excellent agreement
with the observed distribution of masses. This provides an indirect
verification of the asteroseismic mass scaling relation is good to within five
percent. Using an importance-sampling framework that takes the selection
function into account, we fit a population synthesis model of the Galaxy to the
observed seismic and spectroscopic data. Assuming the asteroseismic scaling
relations are correct, we estimate the mean age of the thick disc to be about
10 Gyr, in agreement with the traditional idea of an old -enhanced
thick disc.Comment: 21 pages, submitted to MNRA
The GALAH survey: Milky Way disc metallicity and alpha-abundance trends in combined APOGEE-GALAH catalogues
GALAH and APOGEE are two high resolution multi object spectroscopic surveys
that provide fundamental stellar parameters and multiple elemental abundance
estimates for 400,000 stars in the Milky Way. They are complimentary in
both sky coverage and wavelength regime. Thus combining the two surveys will
provide us a large sample to investigate the disc metallicity and alpha
abundance trends. We use the Cannon data-driven approach selecting training
sets from among 20,000 stars in common for the two surveys to predict the
GALAH scaled stellar parameters from APOGEE spectra as well as APOGEE scaled
stellar parameters from GALAH spectra. We provide two combined catalogues with
GALAH scaled and APOGEE scaled stellar parameters each having 500,000
stars after quality cuts. With 470,000 stars that are common in both
these catalogues, we compare the GALAH scaled and APOGEE scaled metallicity
distribution functions (MDF), radial and vertical metallicity gradients as well
as the variation of [/Fe] vs [Fe/H] trends along and away from the
Galactic mid plane. We find mean metallicities of APOGEE scaled sample to be
higher compared to that for the GALAH scaled sample. We find similar
[/Fe] vs [Fe/H] trends using both samples consistent with previous
observational as well as simulation based studies. Radial and vertical
metallicity gradients derived using the two survey scaled samples are
consistent except in the inner and outer Galactocentric radius bins. Our
gradient estimates in the solar neighborhood are also consistent with previous
studies and are backed by larger sample size compared to previous works.Comment: 21 pages, 19 figures, submitted to MNRA
The GALAH Survey: Chemical Clocks
Previous studies have found that the elemental abundances of a star correlate
directly with its age and metallicity. Using this knowledge, we derive ages for
a sample of 250,000 stars taken from GALAH DR3 using only their overall
metallicity and chemical abundances. Stellar ages are estimated via the machine
learning algorithm , using main sequence turnoff stars with precise
ages as our input training set. We find that the stellar ages for the bulk of
the GALAH DR3 sample are accurate to 1-2 Gyr using this method. With these
ages, we replicate many recent results on the age-kinematic trends of the
nearby disk, including the age-velocity dispersion relationship of the solar
neighborhood and the larger global velocity dispersion relations of the disk
found using and GALAH. The fact that chemical abundances alone can be
used to determine a reliable age for a star have profound implications for the
future study of the Galaxy as well as upcoming spectroscopic surveys. These
results show that the chemical abundance variation at a given birth radius is
quite small, and imply that strong chemical tagging of stars directly to birth
clusters may prove difficult with our current elemental abundance precision.
Our results highlight the need of spectroscopic surveys to deliver precision
abundances for as many nucleosynthetic production sites as possible in order to
estimate reliable ages for stars directly from their chemical abundances.
Applying the methods outlined in this paper opens a new door into studies of
the kinematic structure and evolution of the disk, as ages may potentially be
estimated for a large fraction of stars in existing spectroscopic surveys. This
would yield a sample of millions of stars with reliable age determinations, and
allow precise constraints to be put on various kinematic processes in the disk,
such as the efficiency and timescales of radial migration.Comment: 13 pages, 15 figures, submitted to MNRA
The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and eDR3
© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab3504Since the advent of astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, -Sausage-Enceladus (GSE), appears to be an early "building block" given its virial mass at infall (). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low- abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via [Na/Fe] vs. [Mg/Mn] in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including , we can characterise an unprecedented 24 abundances of this structure with GALAH+ DR3. Our chemical selection allows us to prevent circular reasoning and characterise the dynamical properties of the GSE, for example mean . We find only of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.Peer reviewedFinal Accepted Versio
The GALAH survey: tracing the Galactic disk with Open Clusters
Open clusters are unique tracers of the history of our own Galaxy's disk.
According to our membership analysis based on \textit{Gaia} astrometry, out of
the 226 potential clusters falling in the footprint of GALAH or APOGEE, we find
that 205 have secure members that were observed by at least one of the survey.
Furthermore, members of 134 clusters have high-quality spectroscopic data that
we use to determine their chemical composition. We leverage this information to
study the chemical distribution throughout the Galactic disk of 21 elements,
from C to Eu. The radial metallicity gradient obtained from our analysis is
0.0760.009 dex kpc, which is in agreement with previous works
based on smaller samples. Furthermore, the gradient in the [Fe/H] - guiding
radius (r) plane is 0.0730.008 dex kpc. We show
consistently that open clusters trace the distribution of chemical elements
throughout the Galactic disk differently than field stars. In particular, at
given radius, open clusters show an age-metallicity relation that has less
scatter than field stars. As such scatter is often interpreted as an effect of
radial migration, we suggest that these differences are due to the physical
selection effect imposed by our Galaxy: clusters that would have migrated
significantly also had higher chances to get destroyed. Finally, our results
reveal trends in the [X/Fe]rage space, which are important
to understand production rates of different elements as a function of space and
time.Comment: 18 pages, 10 figures, accepted for publication MNRA
The GALAH+ Survey : Third Data Release
© 2021 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab1242The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2% of stars are within 75 stellar clusters. We derive stellar parameters , , [Fe/H], , & using our modified version of the spectrum synthesis code Spectroscopy Made Easy (SME) and 1D MARCS model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65% dwarfs, 34% giants, and 1% other/unclassified stars. Based on unflagged chemical composition and age, we find 62% young low-, 9% young high-, 27% old high-, and 2% stars with . Based on kinematics, 4% are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.Peer reviewe