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 $23.8^{+1.5}_{-1.8} \rm \, days$ and $23.2^{+11.5}_{-3.2} \rm \, days$, 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 $1000\,\rm \mu Hz$. 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 ${\simeq 2500}\,\rm \mu Hz$ 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 $5300 \lesssim T_\mathrm{eff} \lesssim 5700$\,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, $\nu_\mathrm{max}$, the mean large frequency separation, $\Delta\nu$, and the maximum mode amplitude, $A_\mathrm{max}$, 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