69 research outputs found
A low upper-limit on the lithium isotope ratio in HD140283
We have obtained a high-S/N (900-1100), high-resolving-power (R=95000)
spectrum of the metal-poor subgiant HD 140283 in an effort to measure its
6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value
consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026.
This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by
one of the authors. HD 140283 provides no support for the suggestion that
Population II stars may preserve their 6Li on the portion of the subgiant
branch where 7Li is preserved. However, this star does not defeat the
suggestion either; being at the cool end of subgiant branch of the Spite
plateau, it may be sufficiently cool that 6Li depletion has already set in, or
the star may be sufficiently metal poor that little Galactic production of 6Li
had occurred. Continued investigation of other subgiants is necessary to test
the idea. We also consider the implications of the HD 140283 upper limit in
conjunction with other measurements for models of 6Li production by cosmic rays
from supernovae and structure formation shocks.Comment: 8 pages, 4 figures; accepted for publication in Astronomy and
Astrophysic
Granulation in Red Giants: observations by the Kepler mission and 3D convection simulations
The granulation pattern that we observe on the surface of the Sun is due to
hot plasma from the interior rising to the photosphere where it cools down, and
descends back into the interior at the edges of granules. This is the visible
manifestation of convection taking place in the outer part of the solar
convection zone. Because red giants have deeper convection zones and more
extended atmospheres than the Sun, we cannot a priori assume that granulation
in red giants is a scaled version of solar granulation. Until now, neither
observations nor 1D analytical convection models could put constraints on
granulation in red giants. However, thanks to asteroseismology, this study can
now be performed. The resulting parameters yield physical information about the
granulation. We analyze \sim1000 red giants that have been observed by Kepler
during 13 months. We fit the power spectra with Harvey-like profiles to
retrieve the characteristics of the granulation (time scale tau_gran and power
P_gran). We also introduce a new time scale, tau_eff, which takes into account
that different slopes are used in the Harvey functions. We search for a
correlation between these parameters and the global acoustic-mode parameter
(the position of maximum power, nu_max) as well as with stellar parameters
(mass, radius, surface gravity (log g) and effective temperature (T_eff)). We
show that tau_eff nu_max^{-0.89} and P_gran nu_max^{-1.90}, which is consistent
with the theoretical predictions. We find that the granulation time scales of
stars that belong to the red clump have similar values while the time scales of
stars in the red-giant branch are spread in a wider range. Finally, we show
that realistic 3D simulations of the surface convection in stars, spanning the
(T_eff, log g)-range of our sample of red giants, match the Kepler observations
well in terms of trends.Comment: 43 pages, 13 figures. Accepted for publication in Ap
Asteroseismology of Solar-type Stars with Kepler I: Data Analysis
We report on the first asteroseismic analysis of solar-type stars observed by
Kepler. Observations of three G-type stars, made at one-minute cadence during
the first 33.5d of science operations, reveal high signal-to-noise solar-like
oscillation spectra in all three stars: About 20 modes of oscillation can
clearly be distinguished in each star. We discuss the appearance of the
oscillation spectra, including the presence of a possible signature of faculae,
and the presence of mixed modes in one of the three stars.Comment: 5 pages, 4 figure, submitted to Astronomische Nachrichte
A uniform asteroseismic analysis of 22 solar-type stars observed by Kepler
Asteroseismology with the Kepler space telescope is providing not only an
improved characterization of exoplanets and their host stars, but also a new
window on stellar structure and evolution for the large sample of solar-type
stars in the field. We perform a uniform analysis of 22 of the brightest
asteroseismic targets with the highest signal-to-noise ratio observed for 1
month each during the first year of the mission, and we quantify the precision
and relative accuracy of asteroseismic determinations of the stellar radius,
mass, and age that are possible using various methods. We present the
properties of each star in the sample derived from an automated analysis of the
individual oscillation frequencies and other observational constraints using
the Asteroseismic Modeling Portal (AMP), and we compare them to the results of
model-grid-based methods that fit the global oscillation properties. We find
that fitting the individual frequencies typically yields asteroseismic radii
and masses to \sim1% precision, and ages to \sim2.5% precision (respectively 2,
5, and 8 times better than fitting the global oscillation properties). The
absolute level of agreement between the results from different approaches is
also encouraging, with model-grid-based methods yielding slightly smaller
estimates of the radius and mass and slightly older values for the stellar age
relative to AMP, which computes a large number of dedicated models for each
star. The sample of targets for which this type of analysis is possible will
grow as longer data sets are obtained during the remainder of the mission.Comment: 13 pages, 5 figures in the main text, 22 figures in Appendix.
Accepted for publication in Ap
Properties of 42 Solar-type Kepler Targets from the Asteroseismic Modeling Portal
Recently the number of main-sequence and subgiant stars exhibiting solar-like
oscillations that are resolved into individual mode frequencies has increased
dramatically. While only a few such data sets were available for detailed
modeling just a decade ago, the Kepler mission has produced suitable
observations for hundreds of new targets. This rapid expansion in observational
capacity has been accompanied by a shift in analysis and modeling strategies to
yield uniform sets of derived stellar properties more quickly and easily. We
use previously published asteroseismic and spectroscopic data sets to provide a
uniform analysis of 42 solar-type Kepler targets from the Asteroseismic
Modeling Portal (AMP). We find that fitting the individual frequencies
typically doubles the precision of the asteroseismic radius, mass and age
compared to grid-based modeling of the global oscillation properties, and
improves the precision of the radius and mass by about a factor of three over
empirical scaling relations. We demonstrate the utility of the derived
properties with several applications.Comment: 12 emulateapj pages, 9 figures, 1 online-only extended figure, 1
table, ApJS accepted (typo corrected in Eq.8
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Asteroseismology of red giants & galactic archaeology
Red-giant stars are low- to intermediate-mass (~M)
stars that have exhausted hydrogen in the core. These extended, cool and hence
red stars are key targets for stellar evolution studies as well as galactic
studies for several reasons: a) many stars go through a red-giant phase; b) red
giants are intrinsically bright; c) large stellar internal structure changes as
well as changes in surface chemical abundances take place over relatively short
time; d) red-giant stars exhibit global intrinsic oscillations. Due to their
large number and intrinsic brightness it is possible to observe many of these
stars up to large distances. Furthermore, the global intrinsic oscillations
provide a means to discern red-giant stars in the pre-helium core burning from
the ones in the helium core burning phase and provide an estimate of stellar
ages, a key ingredient for galactic studies. In this lecture I will first
discuss some physical phenomena that play a role in red-giant stars and several
phases of red-giant evolution. Then, I will provide some details about
asteroseismology -- the study of the internal structure of stars through their
intrinsic oscillations -- of red-giant stars. I will conclude by discussing
galactic archaeology -- the study of the formation and evolution of the Milky
Way by reconstructing its past from its current constituents -- and the role
red-giant stars can play in that.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1
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