344 research outputs found
Solar-like oscillations in cluster stars
We present a brief overview of the history of attempts to obtain a clear
detection of solar-like oscillations in cluster stars, and discuss the results
on the first clear detection, which was made by the Kepler Asteroseismic
Science Consortium (KASC) Working Group 2.Comment: 4 pages, 7 figures, accepted by Astronomische Nachrichte
Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants
Context : We still do not know which mechanisms are responsible for the
transport of angular momentum inside stars. The recent detection of mixed modes
that contain the signature of rotation in the spectra of Kepler subgiants and
red giants gives us the opportunity to make progress on this issue.
Aims: Our aim is to probe the radial dependance of the rotation profiles for
a sample of Kepler targets. For this purpose, subgiants and early red giants
are particularly interesting targets because their rotational splittings are
more sensitive to the rotation outside the deeper core than is the case for
their more evolved counterparts.
Methods: We first extract the rotational splittings and frequencies of the
modes for six young Kepler red giants. We then perform a seismic modeling of
these stars using the evolutionary codes CESAM2k and ASTEC. By using the
observed splittings and the rotational kernels of the optimal models, we
perform inversions of the internal rotation profiles of the six stars.
Results: We obtain estimates of the mean rotation rate in the core and in the
convective envelope of these stars. We show that the rotation contrast between
the core and the envelope increases during the subgiant branch. Our results
also suggest that the core of subgiants spins up with time, contrary to the RGB
stars whose core has been shown to spin down. For two of the stars, we show
that a discontinuous rotation profile with a deep discontinuity reproduces the
observed splittings significantly better than a smooth rotation profile.
Interestingly, the depths that are found most probable for the discontinuities
roughly coincide with the location of the H-burning shell, which separates the
layers that contract from those that expand. These results will bring
observational constraints to the scenarios of angular momentum transport in
stars.Comment: Accepted in A&A, 27 pages, 18 figure
Lamost observations in the kepler field. I. Database of low-resolution spectra*
The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionizing our view of pulsating stars. They have revealed a vast sea of low-amplitude pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows for the accurate determination of the frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they cannot be derived from the Kepler photometry itself. The Kepler Input Catalog provides values for the effective temperature, surface gravity, and metallicity, but not always with sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper, we describe those observations and provide a useful database for the whole astronomical communit
UVSat: a concept of an ultraviolet/optical photometric satellite
Time-series photometry from space in the ultraviolet can be presently done
with only a few platforms, none of which is able to provide wide-field
long-term high-cadence photometry. We present a concept of UVSat, a twin space
telescope which will be capable to perform this kind of photometry, filling an
observational niche. The satellite will host two telescopes, one for
observations in the ultraviolet, the other for observations in the optical
band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of
the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck
The 2003-4 multisite photometric campaign for the Beta Cephei and eclipsing star 16 (EN) Lacertae with an Appendix on 2 Andromedae, the variable comparison star
A multisite photometric campaign for the Beta Cephei and eclipsing variable
16 Lacertae is reported. 749 h of high-quality differential photoelectric
Stromgren, Johnson and Geneva time-series photometry were obtained with ten
telescopes during 185 nights. After removing the pulsation contribution, an
attempt was made to solve the resulting eclipse light curve by means of the
computer program EBOP. Although a unique solution was not obtained, the range
of solutions could be constrained by comparing computed positions of the
secondary component in the Hertzsprung-Russell diagram with evolutionary
tracks.
For three high-amplitude pulsation modes, the uvy and the Geneva UBG
amplitude ratios are derived and compared with the theoretical ones for
spherical-harmonic degrees l <= 4. The highest degree, l = 4, is shown to be
incompatible with the observations. One mode is found to be radial, one is l =
1, while in the remaining case l = 2 or 3.
The present multisite observations are combined with the archival photometry
in order to investigate the long-term variation of the amplitudes and phases of
the three high-amplitude pulsation modes. The radial mode shows a
non-sinusoidal variation on a time-scale of 73 yr. The l = 1 mode is a triplet
with unequal frequency spacing, giving rise to two beat-periods, 720.7 d and
29.1 yr. The amplitude and phase of the l = 2 or 3 mode vary on time-scales of
380.5 d and 43 yr.
The light variation of 2 And, one of the comparison stars, is discussed in
the Appendix.Comment: 18 pages, 19 figures, accepted for publication in MNRA
Characterizing two solar-type Kepler subgiants with asteroseismology: KIC10920273 and KIC11395018
Determining fundamental properties of stars through stellar modeling has
improved substantially due to recent advances in asteroseismology. Thanks to
the unprecedented data quality obtained by space missions, particularly CoRoT
and Kepler, invaluable information is extracted from the high-precision stellar
oscillation frequencies, which provide very strong constraints on possible
stellar models for a given set of classical observations. In this work, we have
characterized two relatively faint stars, KIC10920273 and KIC11395018, using
oscillation data from Kepler photometry and atmospheric constraints from
ground-based spectroscopy. Both stars have very similar atmospheric properties;
however, using the individual frequencies extracted from the Kepler data, we
have determined quite distinct global properties, with increased precision
compared to that of earlier results. We found that both stars have left the
main sequence and characterized them as follows: KIC10920273 is a
one-solar-mass star (M=1.00 +/- 0.04 M_sun), but much older than our Sun
(t=7.12 +/- 0.47 Gyr), while KIC11395018 is significantly more massive than the
Sun (M=1.27 +/- 0.04 M_sun) with an age close to that of the Sun (t=4.57 +/-
0.23 Gyr). We confirm that the high lithium abundance reported for these stars
should not be considered to represent young ages, as we precisely determined
them to be evolved subgiants. We discuss the use of surface lithium abundance,
rotation and activity relations as potential age diagnostics.Comment: 12 pages, 3 figures, 5 tables. Accepted by Ap
Kepler observations of variability in B-type stars
The analysis of the light curves of 48 B-type stars observed by Kepler is
presented. Among these are 15 pulsating stars, all of which show low
frequencies characteristic of SPB stars. Seven of these stars also show a few
weak, isolated high frequencies and they could be considered as SPB/beta Cep
hybrids. In all cases the frequency spectra are quite different from what is
seen from ground-based observations. We suggest that this is because most of
the low frequencies are modes of high degree which are predicted to be unstable
in models of mid-B stars. We find that there are non-pulsating stars within the
beta Cep and SPB instability strips. Apart from the pulsating stars, we can
identify stars with frequency groupings similar to what is seen in Be stars but
which are not Be stars. The origin of the groupings is not clear, but may be
related to rotation. We find periodic variations in other stars which we
attribute to proximity effects in binary systems or possibly rotational
modulation. We find no evidence for pulsating stars between the cool edge of
the SPB and the hot edge of the delta Sct instability strips. None of the stars
show the broad features which can be attributed to stochastically-excited modes
as recently proposed. Among our sample of B stars are two chemically peculiar
stars, one of which is a HgMn star showing rotational modulation in the light
curve.Comment: 19 pages, 11 figures, 4 table
Asteroseismic fundamental properties of solar-type stars observed by the NASA Kepler Mission
We use asteroseismic data obtained by the NASA Kepler Mission to estimate the
fundamental properties of more than 500 main-sequence and sub-giant stars. Data
obtained during the first 10 months of Kepler science operations were used for
this work, when these solar-type targets were observed for one month each in a
survey mode. Stellar properties have been estimated using two global
asteroseismic parameters and complementary photometric and spectroscopic data.
Homogeneous sets of effective temperatures were available for the entire
ensemble from complementary photometry; spectroscopic estimates of T_eff and
[Fe/H] were available from a homogeneous analysis of ground-based data on a
subset of 87 stars. [Abbreviated version... see paper for full abstract.]Comment: Accepted for publication in ApJS; 90 pages, 22 figures, 6 tables.
Units on rho in tables now listed correctly as rho(Sun
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