145 research outputs found
Stellar turbulence and mode physics
An overview of selected topical problems on modelling oscillation properties
in solar-like stars is presented. High-quality oscillation data from both
space-borne intensity observations and ground-based spectroscopic measurements
provide first tests of the still-ill-understood, superficial layers in distant
stars. Emphasis will be given to modelling the pulsation dynamics of the
stellar surface layers, the stochastic excitation processes and the associated
dynamics of the turbulent fluxes of heat and momentum.Comment: Proc. HELAS Workshop on 'Synergies between solar and stellar
modelling', eds M. Marconi, D. Cardini, M. P. Di Mauro, Astrophys. Space
Sci., in the pres
Uncertainties in models of stellar structure and evolution
Numerous physical aspects of stellar physics have been presented in Ses- sion
2 and the underlying uncertainties have been tentatively assessed. We try here
to highlight some specific points raised after the talks and during the general
discus- sion at the end of the session and eventually at the end of the
workshop. A table of model uncertainties is then drawn with the help of the
participants in order to give the state of the art in stellar modeling
uncertainties as of July 2013.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
Amplitudes and lifetimes of solar-like oscillations observed by CoRoT* Red-giant versus main-sequence stars
Context. The advent of space-borne missions such as CoRoT or Kepler providing
photometric data has brought new possibilities for asteroseismology across the
H-R diagram. Solar-like oscillations are now observed in many stars, including
red giants and main- sequence stars. Aims. Based on several hundred identified
pulsating red giants, we aim to characterize their oscillation amplitudes and
widths. These observables are compared with those of main-sequence stars in
order to test trends and scaling laws for these parameters for both
main-sequence stars and red giants. Methods. An automated fitting procedure is
used to analyze several hundred Fourier spectra. For each star, a modeled
spectrum is fitted to the observed oscillation spectrum, and mode parameters
are derived. Results. Amplitudes and widths of red-giant solar-like
oscillations are estimated for several hundred modes of oscillation. Amplitudes
are relatively high (several hundred ppm) and widths relatively small (very few
tenths of a {\mu}Hz). Conclusions. Widths measured in main-sequence stars show
a different variation with the effective temperature than red giants. A single
scaling law is derived for mode amplitudes of both red giants and main-sequence
stars versus their luminosity to mass ratio. However, our results suggest that
two regimes may also be compatible with the observations.Comment: Accepted in A&A on 2011 February 8th, now includes corrections
(results now more precise on \Gamma and A_max in Section 4.3 and 4.4, fig. 7
corrected consequently
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
Strong Dynamo Action in Rapidly Rotating Suns
Stellar dynamos are driven by complex couplings between rotation and
turbulent convection, which drive global-scale flows and build and rebuild
stellar magnetic fields. When stars like our sun are young, they rotate much
more rapidly than the current solar rate. Observations generally indicate that
more rapid rotation is correlated with stronger magnetic activity and perhaps
more effective dynamo action. Here we examine the effects of more rapid
rotation on dynamo action in a star like our sun. We find that vigorous dynamo
action is realized, with magnetic field generated throughout the bulk of the
convection zone. These simulations do not possess a penetrative tachocline of
shear where global-scale fields are thought to be organized in our sun, but
despite this we find strikingly ordered fields, much like sea-snakes of
toroidal field, which are organized on global scales. We believe this to be a
novel finding.Comment: 8 pages, 4 figs. Published in conference proceedings "Unsolved
Problems in Stellar Physics", held July 2-6 2007 Cambridge, Englan
Modeling Kepler Observations of Solar-like Oscillations in the Red-giant Star HD 186355
We have analysed oscillations of the red giant star HD 186355 observed by the
NASA Kepler satellite. The data consist of the first five quarters of science
operations of Kepler, which cover about 13 months. The high-precision
time-series data allow us to accurately extract the oscillation frequencies
from the power spectrum. We find the frequency of the maximum oscillation
power, {\nu}_max, and the mean large frequency separation, {\Delta}{\nu}, are
around 106 and 9.4 {\mu}Hz respectively. A regular pattern of radial and
non-radial oscillation modes is identified by stacking the power spectra in an
echelle diagram. We use the scaling relations of {\Delta}{\nu} and {\nu}_max to
estimate the preliminary asteroseismic mass, which is confirmed with the
modelling result (M = 1.45 \pm 0.05 M_sun) using the Yale Rotating stellar
Evolution Code (YREC7). In addition, we constrain the effective temperature,
luminosity and radius from comparisons between observational constraints and
models. A number of mixed l = 1 modes are also detected and taken into account
in our model comparisons. We find a mean observational period spacing for these
mixed modes of about 58 s, suggesting that this red giant branch star is in the
shell hydrogen-burning phase.Comment: 26 pages, 5 figures and 2 table
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
Preparation of Kepler lightcurves for asteroseismic analyses
The Kepler mission is providing photometric data of exquisite quality for the
asteroseismic study of different classes of pulsating stars. These analyses
place particular demands on the pre-processing of the data, over a range of
timescales from minutes to months. Here, we describe processing procedures
developed by the Kepler Asteroseismic Science Consortium (KASC) to prepare
light curves that are optimized for the asteroseismic study of solar-like
oscillating stars in which outliers, jumps and drifts are corrected.Comment: Accepted for publication in MNRAS. 5 pages, 2 figure
CoRoT photometry and high-resolution spectroscopy of the interacting eclipsing binary AU Mon
Analyses of very accurate CoRoT space photometry, past Johnson V
photoelectric photometry and high-resolution \'echelle spectra led to the
determination of improved and consistent fundamental stellar properties of both
components of AU Mon. We derived new, accurate ephemerides for both the orbital
motion (with a period of 11.113d) and the long-term, overall brightness
variation (with a period of 416.9d) of this strongly interacting Be + G
semi-detached binary. It is shown that this long-term variation must be due to
attenuation of the total light by some variable circumbinary material. We
derived the binary mass ratio = 0.17\p0.03 based on the
assumption that the G-type secondary fills its Roche lobe and rotates
synchronously. Using this value of the mass ratio as well as the radial
velocities of the G-star, we obtained a consistent light curve model and
improved estimates of the stellar masses, radii, luminosities and effective
temperatures. We demonstrate that the observed lines of the B-type primary may
not be of photospheric origin. We also discover rapid and periodic light
changes visible in the high-quality residual CoRoT light curves. AU Mon is put
into perspective by a comparison with known binaries exhibiting long-term
cyclic light changes.Comment: Accepted for publication in MNRA
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