49 research outputs found
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
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
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
A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE
We present new maps of the Milky Way disk showing the distribution of
metallicity ([Fe/H]), -element abundances ([Mg/Fe]), and stellar age,
using a sample of 66,496 red giant stars from the final data release (DR17) of
the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We
measure radial and vertical gradients, quantify the distribution functions for
age and metallicity, and explore chemical clock relations across the Milky Way
for the low- disk, high- disk, and total population
independently. The low- disk exhibits a negative radial metallicity
gradient of dex kpc, which flattens with distance from
the midplane. The high- disk shows a flat radial gradient in
metallicity and age across nearly all locations of the disk. The age and
metallicity distribution functions shift from negatively skewed in the inner
Galaxy to positively skewed at large radius. Significant bimodality in the
[Mg/Fe]-[Fe/H] plane and in the [Mg/Fe]-age relation persist across the entire
disk. The age estimates have typical uncertainties of in (age)
and may be subject to additional systematic errors, which impose limitations on
conclusions drawn from this sample. Nevertheless, these results act as critical
constraints on galactic evolution models, constraining which physical processes
played a dominant role in the formation of the Milky Way disk. We discuss how
radial migration predicts many of the observed trends near the solar
neighborhood and in the outer disk, but an additional more dramatic evolution
history, such as the multi-infall model or a merger event, is needed to explain
the chemical and age bimodality elsewhere in the Galaxy.Comment: 41 pages, 32 figures, accepted to Ap
Weighing stars from birth to death: mass determination methods across the HRD
The mass of a star is the most fundamental parameter for its structure,
evolution, and final fate. It is particularly important for any kind of stellar
archaeology and characterization of exoplanets. There exists a variety of
methods in astronomy to estimate or determine it. In this review we present a
significant number of such methods, beginning with the most direct and
model-independent approach using detached eclipsing binaries. We then move to
more indirect and model-dependent methods, such as the quite commonly used
isochrone or stellar track fitting. The arrival of quantitative
asteroseismology has opened a completely new approach to determine stellar
masses and to complement and improve the accuracy of other methods. We include
methods for different evolutionary stages, from the pre-main sequence to
evolved (super)giants and final remnants. For all methods uncertainties and
restrictions will be discussed. We provide lists of altogether more than 200
benchmark stars with relative mass accuracies between for the
covered mass range of M\in [0.1,16]\,\msun, of which are stars burning
hydrogen in their core and the other covering all other evolved stages.
We close with a recommendation how to combine various methods to arrive at a
"mass-ladder" for stars.Comment: Invited review article for The Astronomy and Astrophysics Review. 146
pages, 16 figures, 11 tables. Accepted version by the Journal. It includes
summary figure of accuracy/precision of methods for mass ranges and summary
table for individual method
Weighing stars from birth to death : mass determination methods across the HRD
Funding: C.A., J.S.G.M., and M.G.P. received funding from the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation programme (grant agreement No 670519: MAMSIE). N.B. gratefully acknowledge financial support from the Royal Society (University Research Fellowships) and from the European Research Council (ERC-CoG-646928, Multi-Pop).The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exist a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between [0.3 ,2 ]% for the covered mass range of M â[0.1 ,16 ] Mâ , 75 % of which are stars burning hydrogen in their core and the other 25 % covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.PostprintPeer reviewe