1,401 research outputs found
Sounding stellar cycles with Kepler - preliminary results from ground-based chromospheric activity measurements
Due to its unique long-term coverage and high photometric precision,
observations from the Kepler asteroseismic investigation will provide us with
the possibility to sound stellar cycles in a number of solar-type stars with
asteroseismology. By comparing these measurements with conventional
ground-based chromospheric activity measurements we might be able to increase
our understanding of the relation between the chromospheric changes and the
changes in the eigenmodes.
In parallel with the Kepler observations we have therefore started a
programme at the Nordic Optical Telescope to observe and monitor chromospheric
activity in the stars that are most likely to be selected for observations for
the whole satellite mission. The ground-based observations presented here can
be used both to guide the selection of the special Kepler targets and as the
first step in a monitoring programme for stellar cycles. Also, the
chromospheric activity measurements obtained from the ground-based observations
can be compared with stellar parameters such as ages and rotation in order to
improve stellar evolution models.Comment: submitted to the proceedings of the IAU symposium No. 264, 200
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
Characteristics of solar-like oscillations in red giants observed in the CoRoT exoplanet field
Observations during the first long run (~150 days) in the exo-planet field of
CoRoT increase the number of G-K giant stars for which solar-like oscillations
are observed by a factor of 100. This opens the possibility to study the
characteristics of their oscillations in a statistical sense. We aim to
understand the statistical distribution of the frequencies of maximum
oscillation power (nu_max) in red giants and to search for a possible
correlation between nu_max and the large separation (delta_nu). The nu_max
distribution shows a pronounced peak between 20 - 40 microHz. For about half of
the stars we obtain delta_nu with at least two methods. The correlation between
nu_max and delta_nu follows the same scaling relation as inferred for
solar-like stars. The shape of the nu_max distribution can partly be explained
by granulation at low frequencies and by white noise at high frequencies, but
the population density of the observed stars turns out to be also an important
factor. From the fact that the correlation between delta_nu and nu_max for red
giants follows the same scaling relation as obtained for sun-like stars, we
conclude that the sound travel time over the pressure scale height of the
atmosphere scales with the sound travel time through the whole star
irrespective of evolution.Comment: Accepted for publication in Astronomy and Astrophysics (CoRoT special
issue), 5 pages, 7 figures and 1 tabl
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
Evolutionary influences on the structure of red-giant acoustic oscillation spectra from 600d of Kepler observations
Context: The Kepler space mission is reaching continuous observing times long
enough to start studying the fine structure of the observed p-mode spectra.
Aims: In this paper, we aim to study the signature of stellar evolution on the
radial and p-dominated l=2 modes in an ensemble of red giants that show
solar-type oscillations. Results: We find that the phase shift of the central
radial mode (eps_c) is significantly different for red giants at a given large
frequency separation (Dnu_c) but which burn only H in a shell (RGB) than those
that have already ignited core He burning. Even though not directly probing the
stellar core the pair of local seismic observables (Dnu_c, eps_c) can be used
as an evolutionary stage discriminator that turned out to be as reliable as the
period spacing of the mixed dipole modes. We find a tight correlation between
eps_c and Dnu_c for RGB stars and no indication that eps_c depends on other
properties of these stars. It appears that the difference in eps_c between the
two populations becomes if we use an average of several radial orders, instead
of a local, i.e. only around the central radial mode, Dnu to determine the
phase shift. This indicates that the information on the evolutionary stage is
encoded locally, in the shape of the radial mode sequence. This shape turns out
to be approximately symmetric around the central radial mode for RGB stars but
asymmetric for core He burning stars. We computed radial modes for a sequence
of RG models and find them to qualitatively confirm our findings. We also find
that, at least in our models, the local Dnu is an at least as good and mostly
better proxy for both the asymptotic spacing and the large separation scaled
from the model density than the average Dnu. Finally, we investigate the
signature of the evolutionary stage on the small frequency separation and
quantify the mass dependency of this seismic parameter.Comment: 12 pages, 9 figures, accepted for publication in A&
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
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
Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime
Convection in stars excites resonant acoustic waves which depend on the sound
speed inside the star, which in turn depends on properties of the stellar
interior. Therefore, asteroseismology is an unrivaled method to probe the
internal structure of a star. We made a seismic study of the metal-poor
subgiant star nu Indi with the goal of constraining its interior structure. Our
study is based on a time series of 1201 radial velocity measurements spread
over 14 nights obtained from two sites, Siding Spring Observatory in Australia
and ESO La Silla Observatory in Chile. The power spectrum of the high precision
velocity time series clearly presents several identifiable peaks between 200
and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09
uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been
identified with amplitudes in the range 53 to 173 cm/s. The mode damping time
is estimated to be about 16 days (1-sigma range between 9 and 50 days),
substantially longer than in other stars like the Sun, the alpha Cen system or
the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte
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