39 research outputs found
He abundances in disc galaxies. I. Predictions from cosmological chemodynamical simulations
Accepted for publication in A&AWe investigate how the stellar and gas-phase He abundances evolve as a function of time within simulated star-forming disc galaxies with different star formation histories. We make use of a cosmological chemodynamical simulation for galaxy formation and evolution, which includes star formation as well as energy and chemical enrichment feedback from asymptotic giant branch stars, core-collapse supernovae, and Type Ia supernovae. The predicted relations between the He mass fraction, Y, and the metallicity, Z, in the interstellar medium of our simulated disc galaxies depend on the galaxy star formation history. In particular, dY/dZ is not constant and evolves as a function of time, depending on the specific chemical element that we choose to trace Z; in particular, dY/dX O and dY/dX C increase as a function of time, whereas dY/dX N decreases. In the gas-phase, we find negative radial gradients of Y, due to the inside-out growth of our simulated galaxy discs as a function of time; this gives rise to longer chemical enrichment timescales in the outer galaxy regions, where we find lower average values for Y and Z. Finally, by means of chemical-evolution models, in the galactic bulge and inner disc, we predict steeper Y vs. age relations at high Z than in the outer galaxy regions. We conclude that for calibrating the assumed Y-Z relation in stellar models, C, N, and C+N are better proxies for the metallicity than O because they show steeper and less scattered relations.Peer reviewedFinal Published versio
A variational encoder-decoder approach to precise spectroscopic age estimation for large Galactic surveys
Constraints on the formation and evolution of the Milky Way Galaxy require
multi-dimensional measurements of kinematics, abundances, and ages for a large
population of stars. Ages for luminous giants, which can be seen to large
distances, are an essential component of studies of the Milky Way, but they are
traditionally very difficult to estimate precisely for a large dataset and
often require careful analysis on a star-by-star basis in asteroseismology.
Because spectra are easier to obtain for large samples, being able to determine
precise ages from spectra allows for large age samples to be constructed, but
spectroscopic ages are often imprecise and contaminated by abundance
correlations. Here we present an application of a variational encoder-decoder
on cross-domain astronomical data to solve these issues. The model is trained
on pairs of observations from APOGEE and Kepler of the same star in order to
reduce the dimensionality of the APOGEE spectra in a latent space while
removing abundance information. The low dimensional latent representation of
these spectra can then be trained to predict age with just 1,000 precise
seismic ages. We demonstrate that this model produces more precise
spectroscopic ages ( 22% overall, 11% for red-clump stars) than
previous data-driven spectroscopic ages while being less contaminated by
abundance information (in particular, our ages do not depend on [/M]).
We create a public age catalog for the APOGEE DR17 data set and use it to map
the age distribution and the age-[Fe/H]-[/M] distribution across the
radial range of the Galactic disk
The Galactic Interstellar Object Population: A Framework for Prediction and Inference
The Milky Way is thought to host a huge population of interstellar objects
(ISOs), numbering approximately around the Sun, which
are formed and shaped by a diverse set of processes ranging from planet
formation to galactic dynamics. We define a novel framework: firstly to predict
the properties of this Galactic ISO population by combining models of processes
across planetary and galactic scales, and secondly to make inferences about the
processes modelled, by comparing the predicted population to what is observed.
We predict the spatial and compositional distribution of the Galaxy's
population of ISOs by modelling the Galactic stellar population with data from
the APOGEE survey and combining this with a protoplanetary disk chemistry
model. Selecting ISO water mass fraction as an example observable quantity, we
evaluate its distribution both at the position of the Sun and averaged over the
Galactic disk; our prediction for the Solar neighbourhood is compatible with
the inferred water mass fraction of 2I/Borisov. We show that the well-studied
Galactic stellar metallicity gradient has a corresponding ISO compositional
gradient. We also demonstrate the inference part of the framework by using the
current observed ISO composition distribution to constrain the parent star
metallicity dependence of the ISO production rate. This constraint, and other
inferences made with this framework, will improve dramatically as the Vera C.
Rubin Observatory Legacy Survey of Space and Time (LSST) progresses and more
ISOs are observed. Finally, we explore generalisations of this framework to
other Galactic populations, such as that of exoplanets.Comment: Accepted to A
The evolution of the Milky Way's thin disc radial metallicity gradient with K2 asteroseismic ages
The radial metallicity distribution of the Milky Way's disc is an important
observational constraint for models of the formation and evolution of our
Galaxy. It informs our understanding of the chemical enrichment of the Galactic
disc and the dynamical processes therein, particularly radial migration. We
investigate how the metallicity changes with guiding radius in the thin disc
using a sample of red-giant stars with robust astrometric, spectroscopic and
asteroseismic parameters. Our sample contains stars with guiding radii
kpc < < kpc and asteroseismic ages covering the whole
history of the thin disc with precision . We use MCMC analysis to
measure the gradient and its intrinsic spread in bins of age and construct a
hierarchical Bayesian model to investigate the evolution of these parameters
independently of the bins. We find a smooth evolution of the gradient from
dex/kpc in the youngest stars to dex/kpc in
stars older than Gyr, with no break at intermediate ages. Our results are
consistent with those based on asteroseismic ages from CoRoT, with that found
in Cepheid variables for stars younger than Gyr, and with open clusters for
stars younger than Gyr. For older stars we find a significantly lower
metallicity in our sample than in the clusters, suggesting a survival bias
favouring more metal-rich clusters. We also find that the chemical evolution
model of Chiappini (2009) is too metal-poor in the early stages of disc
formation. Our results provide strong new constraints for the growth and
enrichment of the thin disc and radial migration, which will facilitate new
tests of model conditions and physics.Comment: 15 pages, 16 figures. Accepted for publication in MNRA
Chronologically dating the early assembly of the Milky Way
The standard cosmological model predicts that galaxies are built through hierarchical assembly on cosmological timescales1,2. The Milky Way, like other disk galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Owing to Gaia Data Release 23 and spectroscopic surveys4, the stellar remnants of such mergers have been identified5,6,7. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging due to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology—the study of stellar oscillations—with kinematics and chemical abundances to estimate precise stellar ages (~11%) for a sample of stars observed by the Kepler space mission8. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disk) before the infall of the satellite galaxy Gaia-Enceladus/Sausage5,6 around 10 billion years ago
The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants
The contribution of dissolved globular clusters (GCs) to the stellar content
of the Galactic halo is a key constraint on models for GC formation and
destruction, and the mass assembly history of the Milky Way. Earlier results
from APOGEE pointed to a large contribution of destroyed GCs to the stellar
content of the inner halo, by as much as 25, which is an order of magnitude
larger than previous estimates for more distant regions of the halo. We set out
to measure the ratio between N-rich and normal halo field stars, as a function
of distance, by performing density modelling of halo field populations in
APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich
stars contribute a much higher 16.8 fraction to the total
stellar halo mass budget than the 2.7 ratio contributed at
10 kpc. Under the assumption that N-rich stars are former GC members that now
reside in the stellar halo field, and assuming the ratio between first-and
second-population GC stars being 1:2, we estimate a total contribution from
disrupted GC stars of the order of 27.5 at r = 1.5 kpc
and 4.2 at r = 10 kpc. Furthermore, since our methodology
requires fitting a density model to the stellar halo, we integrate such density
within a spherical shell from 1.5-15 kpc in radius, and find a total stellar
mass arising from dissolved and/or evaporated GCs of =
9.6 10 M.Comment: Paper accepted for Publication in MNRA