3,897 research outputs found
The Physical Parameters of the Retired A Star HD185351
We report here an analysis of the physical stellar parameters of the giant
star HD185351 using Kepler short-cadence photometry, optical and near infrared
interferometry from CHARA, and high-resolution spectroscopy. Asteroseismic
oscillations detected in the Kepler short-cadence photometry combined with an
effective temperature calculated from the interferometric angular diameter and
bolometric flux yield a mean density, rho_star = 0.0130 +- 0.0003 rho_sun and
surface gravity, logg = 3.280 +- 0.011. Combining the gravity and density we
find Rstar = 5.35 +- 0.20 Rsun and Mstar = 1.99 +- 0.23 Msun. The trigonometric
parallax and CHARA angular diameter give a radius Rstar = 4.97 +- 0.07 Rsun.
This smaller radius,when combined with the mean stellar density, corresponds to
a stellar mass Mstar = 1.60 +- 0.08 Msun, which is smaller than the
asteroseismic mass by 1.6-sigma. We find that a larger mass is supported by the
observation of mixed modes in our high-precision photometry, the spacing of
which is consistent only for Mstar =~ 1.8 Msun. Our various and independent
mass measurements can be compared to the mass measured from interpolating the
spectroscopic parameters onto stellar evolution models, which yields a
model-based mass M_star = 1.87 +- 0.07 Msun. This mass agrees well with the
asteroseismic value,but is 2.6-sigma higher than the mass from the combination
of asteroseismology and interferometry. The discrepancy motivates future
studies with a larger sample of giant stars. However, all of our mass
measurements are consistent with HD185351 having a mass in excess of 1.5 Msun.Comment: ApJ accepte
Asteroseismic classification of stellar populations among 13000 red giants observed by Kepler
Of the more than 150000 targets followed by the Kepler Mission, about 10%
were selected as red giants. Due to their high scientific value, in particular
for Galaxy population studies and stellar structure and evolution, their Kepler
light curves were made public in late 2011. More than 13000 (over 85%) of these
stars show intrinsic flux variability caused by solar-like oscillations making
them ideal for large scale asteroseismic investigations. We automatically
extracted individual frequencies and measured the period spacings of the dipole
modes in nearly every red giant. These measurements naturally classify the
stars into various populations, such as the red giant branch, the low-mass
(M/Msol
1.8) secondary clump. The period spacings also reveal that a large fraction of
the stars show rotationally induced frequency splittings. This sample of stars
will undoubtedly provide an extremely valuable source for studying the stellar
population in the direction of the Kepler field, in particular when combined
with complementary spectroscopic surveys.Comment: 6 page, 5 figures, accepted by ApJ
Localized exciton states in pi-conjugated polymers with finite torsion
We investigate localized exciton states in pi-conjugated polymers with finite
torsion. The localized states are associated with a perturbed transfer integral
for which the magnitude of cosine of the torsion angle exceeds the magnitude of
the corresponding cosine of the unperturbed system. The localized state energy
is calculated as a function of the ratio of the perturbed to unperturbed
transfer integrals. Particular attention is paid to the optically active
symmetric localized states, and the effective oscillator strength, or square of
the absolute magnitude of the transition dipole moment, is calculated as a
function of the energy. The relation of the theory to recent optical studies of
poly(di-noctylfluorene)(PF8) is discussed.Comment: 11 pages pdf documen
Readers\u27 Speakout
Dear Florence Howe:
Devoting an entire issue to the First Annual Convention of the National Women\u27s Studies Association was enormously helpful to those of us who could not attend. The reports on the Convention indicated that it was simply first-rate. Nothing in the issue, however, helped me to understand why some delegates trashed the U.S. Agency for International Development (AID) and scapegoated its representative, Kathleen Staudt. I had hoped to learn why because a number of colleagues returned from the Convention with the story that everyone agreed that AID had no business being there. But no one could explain why. All I have heard are vague stories, most of them quite silly
Optical Interferometry of early-type stars with PAVO@CHARA. I. Fundamental stellar properties
We present interferometric observations of 7 main-sequence and 3 giant stars
with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA
array. We have directly determined the angular diameters for these objects with
an average precision of 2.3%. We have also computed bolometric fluxes using
available photometry in the visible and infrared wavelengths, as well as
space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos}
parallaxes, we have derived a set of fundamental stellar properties including
linear radius, luminosity and effective temperature. Fitting the latter to
computed isochrone models, we have inferred masses and ages of the stars. The
effective temperatures obtained are in good agreement (at a 3% level) with
nearly-independent temperature estimations from spectroscopy. They validate
recent sixth-order polynomial (B-V)- empirical relations
\citep{Boyajian2012a}, but suggest that a more conservative third-order
solution \citep{vanBelle2009} could adequately describe the
(V-K)- relation for main-sequence stars of spectral type A0 and
later. Finally, we have compared mass values obtained combining surface gravity
with inferred stellar radius (\textit{gravity mass}) and as a result of the
comparison of computed luminosity and temperature values with stellar
evolutionary models (\textit{isochrone mass}). The strong discrepancy between
isochrone and gravity mass obtained for one of the observed stars,
\,Lyr, suggests that determination of the stellar atmosphere parameters
should be revised.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Asteroseismology of the open clusters NGC 6791, NGC 6811, and NGC 6819 from nineteen months of Kepler photometry
We studied solar-like oscillations in 115 red giants in the three open
clusters NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering
more than 19 months with NASA's Kepler space telescope. We present the
asteroseismic diagrams of the asymptotic parameters \delta\nu_02, \delta\nu_01
and \epsilon, which show clear correlation with fundamental stellar parameters
such as mass and radius. When the stellar populations from the clusters are
compared, we see evidence for a difference in mass of the red giant branch
stars, and possibly a difference in structure of the red clump stars, from our
measurements of the small separations \delta\nu_02 and \delta\nu_01. Ensemble
\'{e}chelle diagrams and upper limits to the linewidths of l = 0 modes as a
function of \Delta\nu of the clusters NGC 6791 and NGC 6819 are also shown,
together with the correlation between the l = 0 ridge width and the T_eff of
the stars. Lastly, we distinguish between red giant branch and red clump stars
through the measurement of the period spacing of mixed dipole modes in 53 stars
among all the three clusters to verify the stellar classification from the
color-magnitude diagram. These seismic results also allow us to identify a
number of special cases, including evolved blue stragglers and binaries, as
well as stars in late He-core burning phases, which can be potentially
interesting targets for detailed theoretical modeling.Comment: 30 pages, 15 figures, 1 table, accepted to Ap
Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars
Benchmark stars are crucial as validating standards for current as well as
future large stellar surveys of the Milky Way. However, the number of suitable
metal-poor benchmarks is currently limited. We aim to construct a new set of
metal-poor benchmarks, based on reliable interferometric effective temperature
() determinations and a homogeneous analysis with a desired
precision of in . We observed ten late-type metal-poor
dwarf and giants: HD2665, HD6755, HD6833, HD103095, HD122563, HD127243,
HD140283, HD175305, HD221170, and HD224930. Only three of the ten stars
(HD103095, HD122563, and HD140283) have previously been used as benchmarks. For
the observations, we used the high angular resolution optical interferometric
instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb
darkening models and determined directly from the
Stefan-Boltzmann relation, with an iterative procedure to interpolate over
tables of bolometric corrections. Surface gravities () were estimated
from comparisons to Dartmouth stellar evolution model tracks. We collected
spectroscopic observations from the ELODIE and FIES spectrographs and estimated
metallicities () from a 1D non-LTE abundance analysis of
unblended lines of neutral and singly ionized iron. We inferred
to better than for five of the stars (HD103095, HD122563, HD127243,
HD140283, and HD224930). The of the other five stars are
reliable to between ; the higher uncertainty on the for
those stars is mainly due to their having a larger uncertainty in the
bolometric fluxes. We also determined and with
median uncertainties of and ,
respectively. These ten stars can, therefore, be adopted as a new, reliable set
of metal-poor benchmarks.Comment: 13 pages, 7 figures, 8 tables + 10 online tables, abstract shortened
to meet arXiv requirements, accepted in A&
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