53 research outputs found
Solar-like oscillations with low amplitude in the CoRoT target HD 181906
Context: The F8 star HD 181906 (effective temperature ~6300K) was observed
for 156 days by the CoRoT satellite during the first long run in the centre
direction. Analysis of the data reveals a spectrum of solar-like acoustic
oscillations. However, the faintness of the target (m_v=7.65) means the
signal-to-noise (S/N) in the acoustic modes is quite low, and this low S/N
leads to complications in the analysis. Aims: To extract global variables of
the star as well as key parameters of the p modes observed in the power
spectrum of the lightcurve. Methods: The power spectrum of the lightcurve, a
wavelet transform and spot fitting have been used to obtain the average
rotation rate of the star and its inclination angle. Then, the autocorrelation
of the power spectrum and the power spectrum of the power spectrum were used to
properly determine the large separation. Finally, estimations of the mode
parameters have been done by maximizing the likelihood of a global fit, where
several modes were fit simultaneously. Results: We have been able to infer the
mean surface rotation rate of the star (~4 microHz) with indications of the
presence of surface differential rotation, the large separation of the p modes
(~87 microHz), and therefore also the ridges corresponding to overtones of the
acoustic modes.Comment: Paper Accepted to be published in A&A. 10 Pages, 12 figure
Seismology of the Sun : Inference of Thermal, Dynamic and Magnetic Field Structures of the Interior
Recent overwhelming evidences show that the sun strongly influences the
Earth's climate and environment. Moreover existence of life on this Earth
mainly depends upon the sun's energy. Hence, understanding of physics of the
sun, especially the thermal, dynamic and magnetic field structures of its
interior, is very important. Recently, from the ground and space based
observations, it is discovered that sun oscillates near 5 min periodicity in
millions of modes. This discovery heralded a new era in solar physics and a
separate branch called helioseismology or seismology of the sun has started.
Before the advent of helioseismology, sun's thermal structure of the interior
was understood from the evolutionary solution of stellar structure equations
that mimicked the present age, mass and radius of the sun. Whereas solution of
MHD equations yielded internal dynamics and magnetic field structure of the
sun's interior. In this presentation, I review the thermal, dynamic and
magnetic field structures of the sun's interior as inferred by the
helioseismology.Comment: To be published in the proceedings of the meeting "3rd International
Conference on Current Developments in Atomic, Molecular, Optical and Nano
Physics with Applications", December 14-16, 2011, New Delhi, Indi
Prospects for asteroseismology
The observational basis for asteroseismology is being dramatically
strengthened, through more than two years of data from the CoRoT satellite, the
flood of data coming from the Kepler mission and, in the slightly longer term,
from dedicated ground-based facilities. Our ability to utilize these data
depends on further development of techniques for basic data analysis, as well
as on an improved understanding of the relation between the observed
frequencies and the underlying properties of the stars. Also, stellar modelling
must be further developed, to match the increasing diagnostic potential of the
data. Here we discuss some aspects of data interpretation and modelling,
focussing on the important case of stars with solar-like oscillations.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 press Revision: correcting abscissa labels on Figs 1 and
Asteroseismology from multi-month Kepler photometry: the evolved Sun-like stars KIC 10273246 and KIC 10920273
The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC
10920273 (G-type) were observed with the NASA Kepler satellite for
approximately ten months with a duty cycle in excess of 90%. Such continuous
and long observations are unprecedented for solar-type stars other than the
Sun.
We aimed mainly at extracting estimates of p-mode frequencies - as well as of
other individual mode parameters - from the power spectra of the light curves
of both stars, thus providing scope for a full seismic characterization.
The light curves were corrected for instrumental effects in a manner
independent of the Kepler Science Pipeline. Estimation of individual mode
parameters was based both on the maximization of the likelihood of a model
describing the power spectrum and on a classic prewhitening method. Finally, we
employed a procedure for selecting frequency lists to be used in stellar
modeling.
A total of 30 and 21 modes of degree l=0,1,2 - spanning at least eight radial
orders - have been identified for KIC 10273246 and KIC 10920273, respectively.
Two avoided crossings (l=1 ridge) have been identified for KIC 10273246,
whereas one avoided crossing plus another likely one have been identified for
KIC 10920273. Good agreement is found between observed and predicted mode
amplitudes for the F-type star KIC 10273246, based on a revised scaling
relation. Estimates are given of the rotational periods, the parameters
describing stellar granulation and the global asteroseismic parameters
and .Comment: 15 pages, 15 figures, to be published in Astronomy & Astrophysic
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Asteroseismology and Interferometry
Asteroseismology provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Recent developments,
including the first systematic studies of solar-like pulsators, have boosted
the impact of this field of research within Astrophysics and have led to a
significant increase in the size of the research community. In the present
paper we start by reviewing the basic observational and theoretical properties
of classical and solar-like pulsators and present results from some of the most
recent and outstanding studies of these stars. We centre our review on those
classes of pulsators for which interferometric studies are expected to provide
a significant input. We discuss current limitations to asteroseismic studies,
including difficulties in mode identification and in the accurate determination
of global parameters of pulsating stars, and, after a brief review of those
aspects of interferometry that are most relevant in this context, anticipate
how interferometric observations may contribute to overcome these limitations.
Moreover, we present results of recent pilot studies of pulsating stars
involving both asteroseismic and interferometric constraints and look into the
future, summarizing ongoing efforts concerning the development of future
instruments and satellite missions which are expected to have an impact in this
field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume
14, Issue 3-4, pp. 217-36
85Peg A: which age for a low metallicity solar like star?
We explore the possible evolutionary status of the primary component of the
binary 85 Pegasi, listed as a target for asteroseismic observations by the MOST
satellite. In spite of the assessed `subdwarf' status, and of the accurate
distance determination from the Hipparcos data, the uncertainties in the
metallicity and age, coupled with the uncertainty in the theoretical models,
lead to a range of predictions on the oscillation frequency spectrum.
Nevertheless, the determination of the ratio between the small separation in
frequency modes, and the large separation as suggested by Roxburgh (2004),
provides a very good measure of the star age, quite independent of the
metallicity in the assumed uncertainty range. In this range, the constraint on
the dynamical mass and the further constraint provided by the assumption that
the maximum age is 14 Gyr limit the mass of 85PegA to the range from 0.75 to
0.82Msun. This difference of a few hundreths of solar masses leads to well
detectable differences both in the evolutionary stage (age) and in the
asteroseismic properties. We show that the age determination which will be
possible through the asteroseismic measurements for this star is independent
either from the convection model adopted or from the microscopic metal
diffusion. The latter conclusion is strengthened by the fact that, although
metal diffusion is still described in an approximate way, recent observations
suggest that the real stars suffer a smaller metal sedimentation with respect
to the models.Comment: accepted for publication in the MNRA
Advances in Global and Local Helioseismology: an Introductory Review
Helioseismology studies the structure and dynamics of the Sun's interior by
observing oscillations on the surface. These studies provide information about
the physical processes that control the evolution and magnetic activity of the
Sun. In recent years, helioseismology has made substantial progress towards the
understanding of the physics of solar oscillations and the physical processes
inside the Sun, thanks to observational, theoretical and modeling efforts. In
addition to the global seismology of the Sun based on measurements of global
oscillation modes, a new field of local helioseismology, which studies
oscillation travel times and local frequency shifts, has been developed. It is
capable of providing 3D images of the subsurface structures and flows. The
basic principles, recent advances and perspectives of global and local
helioseismology are reviewed in this article.Comment: 86 pages, 46 figures; "Pulsation of the Sun and Stars", Lecture Notes
in Physics, Vol. 832, Rozelot, Jean-Pierre; Neiner, Coralie (Eds.), 201
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