118 research outputs found
Spectroscopic Analysis and Chemodynamic Exploration of the Milky Way with Million-Star Surveys
The Milky Way is traditionally depicted as a composite of three main stellar components: the halo, disk, and bulge. The closer we look and the more information we gather, this picture becomes more complicated. The components are in fact overlapping in stellar properties, such as positions, kinematics, ages, and chemical composition.
The most promising way to further our understanding of the formation of the Milky Way, the main aim of Galactic Archaeology, is to explore chemical information and stellar ages with large stellar surveys. In Chapter 1, we motivate this exploration in more detail and describe the challenges that come with the collection of big data from spectroscopic surveys.
In Chapter 2, we introduce the large-scale spectroscopic `Galactic Archaeology with HERMES' (GALAH) survey, whose spectra were analysed in the course of this Thesis in order to estimate stellar properties. The very large data flow of stellar surveys has been referred to as the industrial revolution of Galactic archaeology and the analysis requires new efficient and automated techniques. This Thesis describes the spectroscopic analyses of the more than 650,000 stars in the GALAH survey, which deliver up to 30 abundances with unprecedented accuracy on such scales, based on the papers by Buder et al. (2018) and Buder et al. (in prep. b).
In Chapter 3, we use the chemical information from GALAH together with dynamical information and stellar ages to analyse the Galactic disk in the solar neighborhood, which has shown to be assembled from two populations. This work is published by Buder et al. (2019) and we find that the two populations of the disk can be separated more clearly when using stellar chemical composition and age, rather than phase-space information.
In Chapter 4, we use chemodynamic information and stellar ages to analyse the transition between the disk and halo. This work will be submitted by Buder et al. (in prep. a) and we confirm that the old disk overlaps significantly with the halo. With our performed chemodynamic decompositions we are able to link the disk population that is enhanced in alpha-element abundances with the high-alpha halo population identified by Nissen & Schuster (2010). We further show that the accreted halo population of the `Sausage', identified by Belokurov et al. (2018), is strongly correlated with the low-alpha halo population identified by Nissen & Schuster (2010). Our stellar age estimates suggest that the halo components and the oldest high-alpha disk stars are coeval and show no strong age gradient, which would rule out several formation scenarios of the Milky Way.
In Chapter 5, we conclude the work of this Thesis and outline further ways to continue the research in the field of Galactic archaeology with large stellar surveys
Chemical Doppelgangers in GALAH DR3: the Distinguishing Power of Neutron-Capture Elements Among Milky Way Disk Stars
The observed chemical diversity of Milky Way stars places important
constraints on Galactic chemical evolution and the mixing processes that
operate within the interstellar medium. Recent works have found that the
chemical diversity of disk stars is low. For example, the APOGEE "chemical
doppelganger rate," or the rate at which random pairs of field stars appear as
chemically similar as stars born together, is high, and the chemical
distributions of APOGEE stars in some Galactic populations are well-described
by two-dimensional models. However, limited attention has been paid to the
heavy elements (Z > 30) in this context. In this work, we probe the potential
for neutron-capture elements to enhance the chemical diversity of stars by
determining their effect on the chemical doppelganger rate. We measure the
doppelganger rate in GALAH DR3, with abundances rederived using The Cannon, and
find that considering the neutron-capture elements decreases the doppelganger
rate from 2.2% to 0.4%, nearly a factor of 6, for stars with -0.1 < [Fe/H] <
0.1. While chemical similarity correlates with similarity in age and dynamics,
including neutron-capture elements does not appear to select stars that are
more similar in these characteristics. Our results highlight that the
neutron-capture elements contain information that is distinct from that of the
lighter elements and thus add at least one dimension to Milky Way abundance
space. This work illustrates the importance of considering the neutron-capture
elements when chemically characterizing stars and motivates ongoing work to
improve their atomic data and measurements in spectroscopic surveys.Comment: 23 pages, 16 figures, 1 table. Submitted to AAS Journals, comments
welcome. Associated catalog of high precision, Cannon-rederived abundances
for GALAH giants to be made publicly available upon acceptance and available
now upon request. See Walsen et al. 2023 for a complementary, high precision,
Cannon-rederived abundance catalog for GALAH solar twin
3D NLTE Lithium abundances for late-type stars in GALAH DR3
Lithium's susceptibility to burning in stellar interiors makes it an
invaluable tracer for delineating the evolutionary pathways of stars, offering
insights into the processes governing their development. Observationally, the
complex Li production and depletion mechanisms in stars manifest themselves as
Li plateaus, and as Li-enhanced and Li-depleted regions of the HR diagram. The
Li-dip represents a narrow range in effective temperature close to the
main-sequence turn-off, where stars have slightly super-solar masses and
strongly depleted Li. To study the modification of Li through stellar
evolution, we measure 3D non-local thermodynamic equilibrium (NLTE) Li
abundance for 581 149 stars released in GALAH DR3. We describe a novel method
that fits the observed spectra using a combination of 3D NLTE Li line profiles
with blending metal line strength that are optimized on a star-by-star basis.
Furthermore, realistic errors are determined by a Monte Carlo nested sampling
algorithm which samples the posterior distribution of the fitted spectral
parameters. The method is validated by recovering parameters from a synthetic
spectrum and comparing to 26 stars in the Hypatia catalogue. We find 228 613 Li
detections, and 352 536 Li upper limits. Our abundance measurements are
generally lower than GALAH DR3, with a mean difference of 0.23 dex. For the
first time, we trace the evolution of Li-dip stars beyond the main sequence
turn-off and up the subgiant branch. This is the first 3D NLTE analysis of Li
applied to a large spectroscopic survey, and opens up a new era of precision
analysis of abundances for large surveys.Comment: 20 pages, 17 figures, accepted for publication in MNRA
The role of carbon in red giant spectro-seismology
Although red clump stars function as reliable standard candles, their surface
characteristics (i.e. , , and [Fe/H]) overlap with those
of red giant branch stars, which are not standard candles. Recent results have
revealed that spectral features containing carbon (e.g. CN molecular bands)
carry information correlating with the "gold-standard" asteroseismic
classifiers that distinguish red clump from red giant branch stars. However,
the underlying astrophysical processes driving the correlation between these
spectroscopic and asteroseismic quantities in red giants remain inadequately
explored. This study aims to enhance our understanding of this
"spectro-seismic" effect, by refining the list of key spectral features
predicting red giant evolutionary state. In addition, we conduct further
investigation into those key spectral features to probe the astrophysical
processes driving this connection. We employ the data-driven The Cannon
algorithm to analyse high-resolution () Veloce Rosso spectra from
the Anglo-Australian Telescope for 301 red giant stars (where asteroseismic
classifications from the TESS mission are known for 123 of the stars). The
results highlight molecular spectroscopic features, particularly those
containing carbon (e.g. CN), as the primary indicators of the evolutionary
states of red giant stars. Furthermore, by investigating CN isotopic pairs
(that is, CN and CN) we find suggestions of
statistically significant differences in the reduced equivalent widths of such
lines, suggesting that physical processes that change the surface abundances
and isotopic ratios in red giant stars, such as deep mixing, are the driving
forces of the "spectro-seismic" connection of red giants.Comment: 13 pages, 9 figures, accepted to MNRA
Chemically peculiar A and F stars with enhanced s-process and iron-peak elements: stellar radiative acceleration at work
We present metal-rich (dex) A and F stars
whose surface abundances deviate strongly from Solar abundance ratios and
cannot plausibly reflect their birth material composition. These stars are
identified by their high [Ba/Fe] abundance ratios (dex) in
the LAMOST DR5 spectra analyzed by Xiang et al. (2019). They are almost
exclusively main sequence and subgiant stars with K.
Their distribution in the Kiel diagram (--) traces a sharp
border at low temperatures along a roughly fixed-mass trajectory (around
that corresponds to an upper limit in convective envelope mass
fraction of around . Most of these stars exhibit distinctly enhanced
abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg
and Ca. Rotational velocity measurements from GALAH DR2 show that the majority
of these stars rotate slower than typical stars in an equivalent temperature
range. These characteristics suggest that they are related to the so-called
Am/Fm stars. Their abundance patterns are qualitatively consistent with the
predictions of stellar evolution models that incorporate radiative
acceleration, suggesting they are a consequence of stellar internal evolution
particularly involving the competition between gravitational settling and
radiative acceleration. These peculiar stars constitute 40% of the whole
population of stars with mass above 1.5, affirming that "peculiar"
photospheric abundances due to stellar evolution effects are a ubiquitous
phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced
chemically peculiar A/F stars with individual element abundances provides the
statistics to test more stringently the mechanisms that alter the surface
abundances in stars with radiative envelopes.Comment: 21 pages, 17 figures, accepted for publication in Ap
Chasing the impact of the Gaia-Sausage-Enceladus merger on the formation of the Milky Way thick disc
We employ our Bayesian Machine Learning framework BINGO (Bayesian INference
for Galactic archaeOlogy) to obtain high-quality stellar age estimates for
68,360 red giant and red clump stars present in the 17th data release of the
Sloan Digital Sky Survey, the APOGEE-2 high-resolution spectroscopic survey. By
examining the denoised age-metallicity relationship of the Galactic disc stars,
we identify a drop in metallicity with an increase in [Mg/Fe] at an early
epoch, followed by a chemical enrichment episode with increasing [Fe/H] and
decreasing [Mg/Fe]. This result is congruent with the chemical evolution
induced by an early-epoch gas-rich merger identified in the Milky Way-like
zoom-in cosmological simulation Auriga. In the initial phase of the merger of
Auriga 18 there is a drop in metallicity due to the merger diluting the metal
content and an increase in the [Mg/Fe] of the primary galaxy. Our findings
suggest that the last massive merger of our Galaxy, the Gaia-Sausage-Enceladus,
was likely a significant gas-rich merger and induced a starburst, contributing
to the chemical enrichment and building of the metal-rich part of the thick
disc at an early epoch.Comment: 5 pages, 5 figures, submitted to MNRAS Letters (comments are welcome
Peeking beneath the precision floor -- II. Probing the chemo-dynamical histories of the potential globular cluster siblings, NGC 288 and NGC 362
The assembly history of the Milky Way (MW) is a rapidly evolving subject,
with numerous small accretion events and at least one major merger proposed in
the MW's history. Accreted alongside these dwarf galaxies are globular clusters
(GCs), which act as spatially coherent remnants of these past events. Using
high precision differential abundance measurements from our recently published
study, we investigate the likelihood that the MW clusters NGC 362 and NGC 288
are galactic siblings, accreted as part of the Gaia-Sausage-Enceladus (GSE)
merger. To do this, we compare the two GCs at the 0.01 dex level for 20+
elements for the first time. Strong similarities are found, with the two
showing chemical similarity on the same order as those seen between the three
LMC GCs, NGC 1786, NGC 2210 and NGC 2257. However, when comparing GC abundances
directly to GSE stars, marked differences are observed. NGC 362 shows good
agreement with GSE stars in the ratio of Eu to Mg and Si, as well as a clear
dominance in the r- compared to the s-process, while NGC 288 exhibits only a
slight r-process dominance. When fitting the two GC abundances with a GSE-like
galactic chemical evolution model, NGC 362 shows agreement with both the model
predictions and GSE abundance ratios (considering Si, Ni, Ba and Eu) at the
same metallicity. This is not the case for NGC 288. We propose that the two are
either not galactic siblings, or GSE was chemically inhomogeneous enough to
birth two similar, but not identical clusters with distinct chemistry relative
to constituent stars.Comment: Second paper in a series. Accepted for publication by MNRAS, 17
pages, 11 figure
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