76 research outputs found
A Revised Effective Temperature Scale for the Kepler Input Catalog
We present a catalog of revised effective temperatures for stars observed in
long-cadence mode in the Kepler Input Catalog (KIC). We use SDSS griz filters
tied to the fundamental temperature scale. Polynomials for griz
color-temperature relations are presented, along with correction terms for
surface gravity effects, metallicity, and statistical corrections for binary
companions or blending. We compare our temperature scale to the published
infrared flux method (IRFM) scale for VJKs in both open clusters and the Kepler
fields. We find good agreement overall, with some deviations between (J -
Ks)-based temperatures from the IRFM and both SDSS filter and other diagnostic
IRFM color-temperature relationships above 6000 K. For field dwarfs we find a
mean shift towards hotter temperatures relative to the KIC, of order 215 K, in
the regime where the IRFM scale is well-defined (4000 K to 6500 K). This change
is of comparable magnitude in both color systems and in spectroscopy for stars
with Teff below 6000 K. Systematic differences between temperature estimators
appear for hotter stars, and we define corrections to put the SDSS temperatures
on the IRFM scale for them. When the theoretical dependence on gravity is
accounted for we find a similar temperature scale offset between the
fundamental and KIC scales for giants. We demonstrate that statistical
corrections to color-based temperatures from binaries are significant. Typical
errors, mostly from uncertainties in extinction, are of order 100 K.
Implications for other applications of the KIC are discussed.Comment: Corrected for sign flip errors in the gravity corrections. Erratum to
this paper is attached in Appendix. Full version of revised Table 7 can be
found at http://home.ewha.ac.kr/~deokkeun/kic/sdssteff_v2.dat.g
Magnetic activity and differential rotation in the young Sun-like stars KIC 7985370 and KIC 7765135
Aims. We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, to determine their activity level, spot distribution, and differential rotation. Both stars were previously discovered by us to be young stars and were observed by the NASA Kepler mission.
Methods. The fundamental stellar parameters (vsini, spectral type, T_eff, log g, and [Fe/H]) were derived from optical spectroscopy by comparison with both standard-star and synthetic spectra. The spectra of the targets allowed us to study the chromospheric activity based on the emission in the core of hydrogen Hα and Ca ii infrared triplet (IRT) lines, which was revealed by the subtraction of inactive templates. The high-precision Kepler photometric data spanning over 229 days were then fitted with a robust spot model. Model selection and parameter estimation were performed in a Bayesian manner, using a Markov chain Monte Carlo method.
Results. We find that both stars are Sun-like (of G1.5 V spectral type) and have an age of about 100–200 Myr, based on their lithium content and kinematics. Their youth is confirmed by their high level of chromospheric activity, which is comparable to that displayed by the early G-type stars in the Pleiades cluster. The Balmer decrement and flux ratio of their Ca ii-IRT lines suggest that the formation of the core of these lines occurs mainly in optically thick regions that are analogous to solar plages. The spot model applied to the Kepler photometry requires at least seven persistent spots in the case of KIC 7985370 and nine spots in the case of KIC 7765135 to provide a satisfactory fit to the data. The assumption of the longevity of the star spots, whose area is allowed to evolve with time, is at the heart of our spot-modelling approach. On both stars, the surface differential rotation is Sun-like, with the high-latitude spots rotating slower than the low-latitude ones. We found, for both stars, a rather high value of the equator-to-pole differential rotation (dΩ ≈ 0.18 rad d^-1), which disagrees with the predictions of some mean-field models of differential rotation for rapidly rotating stars. Our results agree instead with previous works on solar-type stars and other models that predict a higher latitudinal shear, increasing with equatorial angular velocity, that can vary during the magnetic cycle
Atmospheric parameters of 82 red giants in the Kepler field
Context: Accurate fundamental parameters of stars are essential for the
asteroseismic analysis of data from the NASA Kepler mission. Aims: We aim at
determining accurate atmospheric parameters and the abundance pattern for a
sample of 82 red giants that are targets for the Kepler mission. Methods: We
have used high-resolution, high signal-to-noise spectra from three different
spectrographs. We used the iterative spectral synthesis method VWA to derive
the fundamental parameters from carefully selected high-quality iron lines.
After determination of the fundamental parameters, abundances of 13 elements
were measured using equivalent widths of the spectral lines. Results: We
identify discrepancies in log g and [Fe/H], compared to the parameters based on
photometric indices in the Kepler Input Catalogue (larger than 2.0 dex for log
g and [Fe/H] for individual stars). The Teff found from spectroscopy and
photometry shows good agreement within the uncertainties. We find good
agreement between the spectroscopic log g and the log g derived from
asteroseismology. Also, we see indications of a potential metallicity effect on
the stellar oscillations. Conclusions: We have determined the fundamental
parameters and element abundances of 82 red giants. The large discrepancies
between the spectroscopic log g and [Fe/H] and values in the Kepler Input
Catalogue emphasize the need for further detailed spectroscopic follow-up of
the Kepler targets in order to produce reliable results from the asteroseismic
analysis.Comment: 16 Pages, 12 Figures, accepted for publication in A&
Overview of the LAMOST- project
The NASA mission obtained long-term high-quality photometric
observations for a large number of stars in its original field of view from
2009 to 2013. In order to provide reliable stellar parameters in a homogeneous
way, the LAMOST telescope began to carry out low-resolution spectroscopic
observations for as many stars as possible in the field in 2012. By
September 2018, 238,386 low-resolution spectra with SNR had been
collected for 155,623 stars in the field, enabling the determination
of atmospheric parameters and radial velocities, as well as spectral
classification of the target stars. This information has been used by
astronomers to carry out research in various fields, including stellar
pulsations and asteroseismology, exoplanets, stellar magnetic activity and
flares, peculiar stars and the Milky Way, binary stars, etc. We summarize the
research progress in these fields where the usage of data from the
LAMOST- (LK) project has played a role. In addition, time-domain
medium-resolution spectroscopic observations have been carried out for about
12,000 stars in four central plates of the field since 2018. The
currently available results show that the LAMOST- medium resolution
(LK-MRS) observations provide qualified data suitable for research in
additional science projects including binaries, high-amplitude pulsating stars,
etc. As LAMOST is continuing to collect both low- and medium-resolution spectra
of stars in the field, we expect more data to be released continuously
and new scientific results to appear based on the LK project data.Comment: 15 pages, 9 figures, 1 table, RAA accepte
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
LAMOST Observations in 15 \textit{K}2 Campaigns: I. Low resolution spectra from LAMOST DR6
The LAMOST-\textit{K}2 (L\textit{K}2) project, initiated in 2015, aims to
collect low-resolution spectra of targets in the \textit{K}2 campaigns, similar
to LAMOST-\textit{Kepler} project. By the end of 2018, a total of 126
L\textit{K}2 plates had been observed by LAMOST. After cross-matching the
catalog of the LAMOST data release 6 (DR6) with that of the \textit{K}2
approved targets, we found 160,619 usable spectra of 84,012 objects, most of
which had been observed more than once. The effective temperature, surface
gravity, metallicity, and radial velocity from 129,974 spectra for 70,895
objects are derived through the LAMOST Stellar Parameter Pipeline (LASP). The
internal uncertainties were estimated to be 81 K, 0.15 dex, 0.09 dex and 5
kms, respectively, when derived from a spectrum with a signal-to-noise
ratio in the band (SNR) of 10. These estimates are based on results for
targets with multiple visits. The external accuracies were assessed by
comparing the parameters of targets in common with the APOGEE and GAIA surveys,
for which we generally found linear relationships. A final calibration is
provided, combining external and internal uncertainties for giants and dwarfs,
separately. We foresee that these spectroscopic data will be used widely in
different research fields, especially in combination with \textit{K}2
photometry.Comment: 31 pages, 9 figures, 6 tables, accepted by ApJ
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
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
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
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