189 research outputs found
Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4
We study the magnetic susceptibility of mixed-valent La2-xSrxCoO4 single
crystals in the doping range of 0.5<= x <= 0.8 for temperatures up to 1000 K.
The magnetism below room temperature is described by paramagnetic Co2+ in the
high-spin state and by Co3+ in the non-magnetic low-spin state. Above room
temperature, an increase in susceptibility compared to the behavior expected
from Co2+ is seen, which we attribute to a spin-state transition of Co3+. The
susceptibility is analyzed by comparison to full-multiplet calculations for the
thermal population of the high- and intermediate-spin states of Co3+
Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the
charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and
low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade
phenomenon as a source for the extremely insulating nature of the
La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation
accounts for the large contrast in the Co-O bond lengths, and in turn, the high
charge ordering temperature. Yet, the low magnetic ordering temperature is
naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From
the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow
band material.Comment: 5 pages, 3 figure
Anisotropic Susceptibility of La_2-xSr_xCoO_4 related to the Spin States of Cobalt
We present a study of the magnetic susceptibility of La_2-xSr_xCoO_4 single
crystals in a doping range 0.3<=x<=0.8. Our data shows a pronounced magnetic
anisotropy for all compounds. This anisotropy is in agreement with a low-spin
ground state (S=0) of Co^3+ for x>=0.4 and a high-spin ground state (S=3/2) of
Co^2+. We compare our data with a crystal-field model calculation assuming
local moments and find a good description of the magnetic behavior for x>=0.5.
This includes the pronounced kinks observed in the inverse magnetic
susceptibility, which result from the anisotropy and low-energy excited states
of Co^2+ and are not related to magnetic ordering or temperature-dependent
spin-state transitions
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
Asteroseismic inference on rotation, gyrochronology and planetary system dynamics of 16 Cygni
The solar analogs 16 Cyg A and 16 Cyg B are excellent asteroseismic targets
in the \Kepler field of view and together with a red dwarf and a Jovian planet
form an interesting system. For these more evolved Sun-like stars we cannot
detect surface rotation with the current \Kepler data but instead use the
technique of asteroseimology to determine rotational properties of both 16 Cyg
A and B. We find the rotation periods to be
and , and the angles of inclination to be
56^{+6}_{-5} \, ^{\circ} and 36^{+17}_{-7} \, ^{\circ}, for A and B
respectively. Together with these results we use the published mass and age to
suggest that, under the assumption of a solar-like rotation profile, 16 Cyg A
could be used when calibrating gyrochronology relations. In addition, we
discuss the known 16 Cyg B star-planet eccentricity and measured low obliquity
which is consistent with Kozai cycling and tidal theory.Comment: 10 pages accepted for publication in the MNRA
Asteroseismology of solar-type stars with K2
We present the first detections by the NASA K2 Mission of oscillations in
solar-type stars, using short-cadence data collected during K2 Campaign\,1
(C1). We understand the asteroseismic detection thresholds for C1-like levels
of photometric performance, and we can detect oscillations in subgiants having
dominant oscillation frequencies around . Changes to the
operation of the fine-guidance sensors are expected to give significant
improvements in the high-frequency performance from C3 onwards. A reduction in
the excess high-frequency noise by a factor of two-and-a-half in amplitude
would bring main-sequence stars with dominant oscillation frequencies as high
as into play as potential asteroseismic targets for
K2.Comment: Accepted for publication in PASP; 16 pages, 2 figure
The diameter of the CoRoT target HD 49933. Combining the 3D limb darkening, asteroseismology, and interferometry
Context. The interpretation of stellar pulsations in terms of internal
structure depends on the knowledge of the fundamental stellar parameters.
Long-base interferometers permit us to determine very accurate stellar radii,
which are independent constraints for stellar models that help us to locate the
star in the HR diagram. Aims: Using a direct interferometric determination of
the angular diameter and advanced three-dimensional (3D) modeling, we derive
the radius of the CoRoT target HD 49933 and reduce the global stellar parameter
space compatible with seismic data. Methods: The VEGA/CHARA
spectro-interferometer is used to measure the angular diameter of the star. A
3D radiative hydrodynamical simulation of the surface is performed to compute
the limb darkening and derive a reliable diameter from visibility curves. The
other fundamental stellar parameters (mass, age, and Teff) are found by fitting
the large and small p-mode frequency separations using a stellar evolution
model that includes microscopic diffusion. Results: We obtain a limb-darkened
angular diameter of {\theta}LD = 0.445 \pm 0.012 mas. With the Hipparcos
parallax, we obtain a radius of R = 1.42 \pm 0.04 Rsun. The corresponding
stellar evolution model that fits both large and small frequency separations
has a mass of 1.20 \pm 0.08 Msun and an age of 2.7 Gy. The atmospheric
parameters are Teff = 6640 \pm 100 K, log g = 4.21 \pm 0.14, and [Fe/H] =
-0.38.Comment: 4 pages, 4 figure
Global asteroseismic properties of solar-like oscillations observed by Kepler : A comparison of complementary analysis methods
We present the asteroseismic analysis of 1948 F-, G- and K-type main-sequence
and subgiant stars observed by the NASA {\em Kepler Mission}. We detect and
characterise solar-like oscillations in 642 of these stars. This represents the
largest cohort of main-sequence and subgiant solar-like oscillators observed to
date. The photometric observations are analysed using the methods developed by
nine independent research teams. The results are combined to validate the
determined global asteroseismic parameters and calculate the relative precision
by which the parameters can be obtained. We correlate the relative number of
detected solar-like oscillators with stellar parameters from the {\em Kepler
Input Catalog} and find a deficiency for stars with effective temperatures in
the range \,K and a drop-off in
detected oscillations in stars approaching the red edge of the classical
instability strip. We compare the power-law relationships between the frequency
of peak power, , the mean large frequency separation,
, and the maximum mode amplitude, , and show that
there are significant method-dependent differences in the results obtained.
This illustrates the need for multiple complementary analysis methods to be
used to assess the robustness and reproducibility of results derived from
global asteroseismic parameters.Comment: 14 pages, 9 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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