54 research outputs found
On the Electromagnetic Properties of Matter in Collapse Models
We discuss the electromagnetic properties of both a charged free particle,
and a charged particle bounded by an harmonic potential, within collapse
models. By choosing a particularly simple, yet physically relevant, collapse
model, and under only the dipole approximation, we are able to solve the
equation of motion exactly. In this way, both the finite time and large time
behavior can be analyzed accurately. We discovered new features, which did not
appear in previous works on the same subject. Since, so far, the spontaneous
photon emission process places the strongest upper bounds on the collapse
parameters, our results call for a further analysis of this process for those
atomic systems which can be employed in experimental tests of collapse models,
as well as of quantum mechanics.Comment: 17 pages, LaTeX, updated version with minor change
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
Calculation of atomic spontaneous emission rate in 1D finite photonic crystal with defects
We derive the expression for spontaneous emission rate in finite
one-dimensional photonic crystal with arbitrary defects using the effective
resonator model to describe electromagnetic field distributions in the
structure. We obtain explicit formulas for contributions of different types of
modes, i.e. radiation, substrate and guided modes. Formal calculations are
illustrated with a few numerical examples, which demonstrate that the
application of effective resonator model simplifies interpretation of results.Comment: Cent. Eur. J. Phys, in pres
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
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
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
Fundamental Properties of Stars using Asteroseismology from Kepler & CoRoT and Interferometry from the CHARA Array
We present results of a long-baseline interferometry campaign using the PAVO
beam combiner at the CHARA Array to measure the angular sizes of five
main-sequence stars, one subgiant and four red giant stars for which solar-like
oscillations have been detected by either Kepler or CoRoT. By combining
interferometric angular diameters, Hipparcos parallaxes, asteroseismic
densities, bolometric fluxes and high-resolution spectroscopy we derive a full
set of near model-independent fundamental properties for the sample. We first
use these properties to test asteroseismic scaling relations for the frequency
of maximum power (nu_max) and the large frequency separation (Delta_nu). We
find excellent agreement within the observational uncertainties, and
empirically show that simple estimates of asteroseismic radii for main-sequence
stars are accurate to <~4%. We furthermore find good agreement of our measured
effective temperatures with spectroscopic and photometric estimates with mean
deviations for stars between T_eff = 4600-6200 K of -22+/-32 K (with a scatter
of 97K) and -58+/-31 K (with a scatter of 93 K), respectively. Finally we
present a first comparison with evolutionary models, and find differences
between observed and theoretical properties for the metal-rich main-sequence
star HD173701. We conclude that the constraints presented in this study will
have strong potential for testing stellar model physics, in particular when
combined with detailed modelling of individual oscillation frequencies.Comment: 18 pages, 12 figures, 7 tables; accepted for publication in Ap
Kepler-21b: A 1.6REarth Planet Transiting the Bright Oscillating F Subgiant Star HD 179070
We present Kepler observations of the bright (V=8.3), oscillating star HD
179070. The observations show transit-like events which reveal that the star is
orbited every 2.8 days by a small, 1.6 R_Earth object. Seismic studies of HD
179070 using short cadence Kepler observations show that HD 179070 has a
frequencypower spectrum consistent with solar-like oscillations that are
acoustic p-modes. Asteroseismic analysis provides robust values for the mass
and radius of HD 179070, 1.34{\pm}0.06 M{\circ} and 1.86{\pm}0.04 R{\circ}
respectively, as well as yielding an age of 2.84{\pm}0.34 Gyr for this F5
subgiant. Together with ground-based follow-up observations, analysis of the
Kepler light curves and image data, and blend scenario models, we
conservatively show at the >99.7% confidence level (3{\sigma}) that the transit
event is caused by a 1.64{\pm}0.04 R_Earth exoplanet in a 2.785755{\pm}0.000032
day orbit. The exoplanet is only 0.04 AU away from the star and our
spectroscopic observations provide an upper limit to its mass of ~10 M_Earth
(2-{\sigma}). HD 179070 is the brightest exoplanet host star yet discovered by
Kepler.Comment: Accepted to Ap
- …