27 research outputs found
A Unified Exploration of the Chronology of the Galaxy
The Milky Way has distinct structural stellar components linked to its
formation and subsequent evolution, but disentangling them is nontrivial. With
the recent availability of high-quality data for a large numbers of stars in
the Milky Way, it is a natural next step for research in the evolution of the
Galaxy to perform automated explorations with unsupervised methods of the
structures hidden in the combination of large-scale spectroscopic, astrometric,
and asteroseismic data sets. We determine precise stellar properties for 21,076
red giants, mainly spanning 2-15 kpc in Galactocentric radii, making it the
largest sample of red giants with measured asteroseismic ages available to
date. We explore the nature of different stellar structures in the Galactic
disc by using Gaussian mixture models as an unsupervised clustering method to
find substructure in the combined chemical, kinematic, and age subspace. The
best-fit mixture model yields four distinct physical Galactic components in the
stellar disc: the thin disc, the kinematically heated thin disc, the thick
disc, and the stellar halo. We find hints of an age asymmetry between the
Northern and Southern hemisphere and we measure the vertical and radial age
gradient of the Galactic disc using the asteroseismic ages extended to further
distances than previous studies.Comment: 18 pages, 12 figures, accepted for publication in MNRA
Advanced asteroseismic modelling: breaking the degeneracy between stellar mass and initial helium abundance
Current stellar model predictions of adiabatic oscillation frequencies differ
significantly from the corresponding observed frequencies due to the
non-adiabatic and poorly understood near-surface layers of stars. However,
certain combinations of frequencies -- known as frequency ratios -- are largely
unaffected by the uncertain physical processes as they are mostly sensitive to
the stellar core. Furthermore, the seismic signature of helium ionization
provides envelope properties while being almost independent of the outermost
layers. We have developed an advanced stellar modelling approach in which we
complement frequency ratios with parameters of the helium ionization zone while
taking into account all possible correlations to put the most stringent
constraints on the stellar internal structure. We have tested the method using
the Kepler benchmark star 16 Cyg A and have investigated the potential of the
helium glitch parameters to constrain the basic stellar properties in detail.
It has been explicitly shown that the initial helium abundance and
mixing-length parameters are well constrained within our framework, reducing
systematic uncertainties on stellar mass and age arising for instance from the
well-known anti-correlation between the mass and initial helium abundance. The
modelling of six additional Kepler stars including 16 Cyg B reinforces the
above findings and also confirms that our approach is mostly independent from
model uncertainties associated with the near-surface layers. Our method is
relatively computationally expensive, however, it provides stellar masses,
radii and ages precisely in an automated manner, paving the way for analysing
numerous stars observed in the future during the ESA PLATO mission.Comment: 18 pages, 14 figures (including 5 in the appendix), 3 tables, MNRAS
in pres
Red Horizontal Branch stars: an asteroseismic perspective
Robust age estimates of red giant stars are now possible thanks to the
precise inference of their mass based on asteroseismic constraints. However,
there are cases where such age estimates can be highly precise yet very
inaccurate. An example is giants that have undergone mass loss or mass transfer
events that have significantly altered their mass. In this context, stars with
"apparent" ages significantly higher than the age of the Universe are
candidates as stripped stars, or stars that have lost more mass than expected,
most likely via interaction with a companion star, or because of the poorly
understood mass-loss mechanism along the red-giant branch. In this work we
identify examples of such objects among red giants observed by
, both at low ([Fe/H] ) and solar metallicity.
By modelling their structure and pulsation spectra, we find a consistent
picture confirming that these are indeed low-mass objects consisting of a He
core of and an envelope of . Moreover, we find that these stars are characterised by a rather
extreme coupling () between the pressure-mode and gravity-mode
cavities, i.e. much higher than the typical value for red clump stars,
providing thus a direct seismic signature of their peculiar structure. The
complex pulsation spectra of these objects, if observed with sufficient
frequency resolution, hold detailed information about the structural properties
of likely products of mass stripping, hence can potentially shed light on their
formation mechanism. On the other hand, our tests highlight the difficulties
associated with measuring reliably the large frequency separation, especially
in shorter datasets, with impact on the reliability of the inferred masses and
ages of low-mass Red Clump stars with e.g. K2 or TESS data.Comment: Accepted for publication in A&A Letter
The Kepler Smear Campaign: Light curves for 102 Very Bright Stars
We present the first data release of the Kepler Smear Campaign, using
collateral 'smear' data obtained in the Kepler four-year mission to reconstruct
light curves of 102 stars too bright to have been otherwise targeted. We
describe the pipeline developed to extract and calibrate these light curves,
and show that we attain photometric precision comparable to stars analyzed by
the standard pipeline in the nominal Kepler mission. In this paper, aside from
publishing the light curves of these stars, we focus on 66 red giants for which
we detect solar-like oscillations, characterizing 33 of these in detail with
spectroscopic chemical abundances and asteroseismic masses as benchmark stars.
We also classify the whole sample, finding nearly all to be variable, with
classical pulsations and binary effects. All source code, light curves, TRES
spectra, and asteroseismic and stellar parameters are publicly available as a
Kepler legacy sample.Comment: 35 pages, accepted ApJ
Age determination of galaxy merger remnant stars using asteroseismology
The Milky Way was shaped by the mergers with several galaxies in the past. We search for remnant stars that were born in these foreign galaxies and assess their ages in an effort to put upper limits on the merger times and thereby better understand the evolutionary history of our Galaxy. Using 5D-phase space information from Gaia eDR3, radial velocities from Gaia DR2 and chemical information from apogee DR16, we kinematically and chemically select 21 red giant stars belonging to former dwarf galaxies that merged with the Milky Way. With added asteroseismology from Kepler and K2, we determine the ages of the 21 ex situ stars and 49 in situ stars with an average σage/age of ∼31 per cent. We find that all the ex situ stars are consistent with being older than 8 Gyr. While it is not possible to associate all the stars with a specific dwarf galaxy, we classify eight of them as Gaia-Enceladus/Sausage stars, which is one of the most massive mergers in our Galaxy's history. We determine their mean age to be 9.5 ± 1.3 Gyr consistent with a merger time of 8-10 Gyr ago. The rest of the stars are possibly associated with Kraken, Thamnos, Sequoia, or another extragalactic progenitor. The age determination of ex situ stars paves the way to more accurately pinning down when the merger events occurred and hence provide tight constraints useful for simulating how these events unfolded.Funding for the Stellar Astrophysics Centre was provided by The Danish National Research Foundation (grant agreement no. DNRF106). AH acknowledges support from a Spinoza prize from the Netherlands Research Council (NWO). HHK gratefully acknowledges financial support from a Fellowship at the Institute for Advanced Study. AS acknowledges support from the European Research Council Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, G.A. n. 772293, http://www.asterochronometry.eu). JMDK gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through an Emmy Noether Research Group (grant number KR4801/1-1), as well as from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). CL acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 852839). JY acknowledges partial support from ERC Synergy Grant WHOLE SUN 810218
Asteroseismology and Spectropolarimetry of the Exoplanet Host Star λ Serpentis
The bright star lambda Ser hosts a hot Neptune with a minimum mass of 13.6 M & OPLUS; and a 15.5 day orbit. It also appears to be a solar analog, with a mean rotation period of 25.8 days and surface differential rotation very similar to the Sun. We aim to characterize the fundamental properties of this system and constrain the evolutionary pathway that led to its present configuration. We detect solar-like oscillations in time series photometry from the Transiting Exoplanet Survey Satellite, and we derive precise asteroseismic properties from detailed modeling. We obtain new spectropolarimetric data, and we use them to reconstruct the large-scale magnetic field morphology. We reanalyze the complete time series of chromospheric activity measurements from the Mount Wilson Observatory, and we present new X-ray and ultraviolet observations from the Chandra and Hubble space telescopes. Finally, we use the updated observational constraints to assess the rotational history of the star and estimate the wind braking torque. We conclude that the remaining uncertainty on the stellar age currently prevents an unambiguous interpretation of the properties of lambda Ser, and that the rate of angular momentum loss appears to be higher than for other stars with a similar Rossby number. Future asteroseismic observations may help to improve the precision of the stellar age
Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi
Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies1. Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called Gaia–Enceladus1, leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be 11.0±0.7 (stat) ±0.8 (sys) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia–Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits
Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite
Since the onset of the "space revolution" of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archeology investigations. The launch of the NASA Transiting Exoplanet Survey Satellite (TESS) mission has enabled seismic-based inferences to go full sky—providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5%-10%, and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data