200 research outputs found
Improving 1D Stellar Models with 3D Atmospheres
Stellar evolution codes play a major role in present-day astrophysics, yet
they share common issues. In this work we seek to remedy some of those by the
use of results from realistic and highly detailed 3D hydrodynamical simulations
of stellar atmospheres. We have implemented a new temperature stratification
extracted directly from the 3D simulations into the Garching Stellar Evolution
Code to replace the simplified atmosphere normally used. Secondly, we have
implemented the use of a variable mixing-length parameter, which changes as a
function of the stellar surface gravity and temperature -- also derived from
the 3D simulations. Furthermore, to make our models consistent, we have
calculated new opacity tables to match the atmospheric simulations. Here, we
present the modified code and initial results on stellar evolution using it.Comment: 4 pages, 5 figures; submitted to the conference proceedings:
Seismology of the Sun and the Distant Stars 201
The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
We present the first chemical evolution model for Enceladus, alias the Gaia
Sausage, to investigate the star formation history of one of the most massive
satellites accreted by the Milky Way during a major merger event. Our best
chemical evolution model for Enceladus nicely fits the observed stellar
[/Fe]-[Fe/H] chemical abundance trends, and reproduces the observed
stellar metallicity distribution function, by assuming low star formation
efficiency, fast infall time scale, and mild outflow intensity. We predict a
median age for Enceladus stars Gyr, and - at the time
of the merger with our Galaxy ( Gyr ago from Helmi et al.) - we
predict for Enceladus a total stellar mass . By looking at the predictions of our best model, we
discuss that merger events between the Galaxy and systems like Enceladus may
have inhibited the gas accretion onto the Galaxy disc at high redshifts,
heating up the gas in the halo. This scenario could explain the extended period
of quenching in the star formation activity of our Galaxy about 10 Gyr ago,
which is predicted by Milky Way chemical evolution models, in order to
reproduce the observed bimodality in [/Fe]-[Fe/H] between thick- and
thin-disc stars.Comment: Accepted for publication in MNRAS Letter
Systematic Differences in Spectroscopic Analysis of Red Giants
A spectroscopic analysis of stellar spectra can be carried out using
different approaches; different methods, line lists, atomic parameters, solar
abundances etc. The resulting atmospheric parameters from these choices can
vary beyond quoted uncertainties in the literature. Here we characterize these
differences by systematically comparing some of the commonly adopted
ingredients; line lists, equivalent width measurements and atomic parameters.
High resolution and high signal-to-noise spectroscopic data of one
helium-core-burning red giant star in each of the three open clusters, NGC6819,
M67 and NGC188 have been obtained with the FIES spectrograph at the Nordic
Optical Telescope. The M67 target has been used to benchmark the analysis, as
it is a well studied cluster with asteroseismic data from the K2 mission. For
the other two clusters we have obtained higher quality data than previously
analyzed, which allows us to establish their chemical composition more
securely. Using a line by line analysis, we tested several different
combinations of line lists and programs to measure equivalent widths of
absorption lines to characterize systematic differences within the same
spectroscopic method. The obtained parameters for the benchmark star in M67
vary up to ~170 K in T_eff, ~0.4 dex in log g and ~0.25 dex in [Fe/H] between
the tested setups. Using the combination of equivalent width measurement
program and line list that best reproduce the inferred log g from
asteroseismology, we determined the atmospheric parameters for the three stars
and securely established the chemical composition of NGC6819 to be ~solar,
[Fe/H]=-0.02+-0.01 dex. We have highlighted the significantly different results
obtained with different combinations of line lists, programs and atomic
parameters, which emphasize the importance of benchmark stars studied with
several different methods to anchor spectroscopic analyses.Comment: Accepted for publication in Astronomy and Astrophysic
What asteroseismology can do for exoplanets
We describe three useful applications of asteroseismology in the context of
exoplanet science: (1) the detailed characterisation of exoplanet host stars;
(2) the measurement of stellar inclinations; and (3) the determination of
orbital eccentricity from transit duration making use of asteroseismic stellar
densities. We do so using the example system Kepler-410 (Van Eylen et al.
2014). This is one of the brightest (V = 9.4) Kepler exoplanet host stars,
containing a small (2.8 Rearth) transiting planet in a long orbit (17.8 days),
and one or more additional non-transiting planets as indicated by transit
timing variations. The validation of Kepler-410 (KOI-42) was complicated due to
the presence of a companion star, and the planetary nature of the system was
confirmed after analyzing a Spitzer transit observation as well as ground-based
follow-up observations.Comment: 4 pages, Proceedings of the CoRoT Symposium 3 / Kepler KASC-7 joint
meeting, Toulouse, 7-11 July 2014. To be published by EPJ Web of Conference
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
Revised Stellar Properties of Kepler Targets for the Quarter 1-16 Transit Detection Run
We present revised properties for 196,468 stars observed by the NASA Kepler
Mission and used in the analysis of Quarter 1-16 (Q1-Q16) data to detect and
characterize transiting exoplanets. The catalog is based on a compilation of
literature values for atmospheric properties (temperature, surface gravity, and
metallicity) derived from different observational techniques (photometry,
spectroscopy, asteroseismology, and exoplanet transits), which were then
homogeneously fitted to a grid of Dartmouth stellar isochrones. We use
broadband photometry and asteroseismology to characterize 11,532 Kepler targets
which were previously unclassified in the Kepler Input Catalog (KIC). We report
the detection of oscillations in 2,762 of these targets, classifying them as
giant stars and increasing the number of known oscillating giant stars observed
by Kepler by ~20% to a total of ~15,500 stars. Typical uncertainties in derived
radii and masses are ~40% and ~20%, respectively, for stars with photometric
constraints only, and 5-15% and ~10% for stars based on spectroscopy and/or
asteroseismology, although these uncertainties vary strongly with spectral type
and luminosity class. A comparison with the Q1-Q12 catalog shows a systematic
decrease in radii for M dwarfs, while radii for K dwarfs decrease or increase
depending on the Q1-Q12 provenance (KIC or Yonsei-Yale isochrones). Radii of
F-G dwarfs are on average unchanged, with the exception of newly identified
giants. The Q1-Q16 star properties catalog is a first step towards an improved
characterization of all Kepler targets to support planet occurrence studies.Comment: 20 pages, 14 figures, 5 tables; accepted for publication in ApJS;
electronic versions of Tables 4 and 5 are available as ancillary files (see
sidebar on the right), and an interactive version of Table 5 is available at
the NASA Exoplanet Archive (http://exoplanetarchive.ipac.caltech.edu/
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