53,423 research outputs found
On the asymptotic acoustic-mode phase in red-giant stars and its dependence on evolutionary state
Asteroseismic investigations based on the wealth of data now available,in
particular from the CoRoT and Kepler missions, require a good understanding of
the relation between the observed quantities and the properties of the
underlying stellar structure. Kallinger et al. 2012 found a relation between
their determination of the asymptotic phase of radial oscillations in evolved
stars and the evolutionary state, separating ascending-branch red giants from
helium-burning stars in the `red clump'. Here we provide a detailed analysis of
this relation, which is found to derive from differences between these two
classes of stars in the thermodynamic state of the convective envelope. There
is potential for distinguishing red giants and clump stars based on the phase
determined from observations that are too short to allow distinction based on
determination of the period spacing for mixed modes. The analysis of the phase
may also point to a better understanding of the potential for using the
helium-ionization-induced acoustic glitch to determine the helium abundance in
the envelopes of these stars.Comment: MNRAS, in the pres
Tests of the asymptotic large frequency separation of acoustic oscillations in solar-type and red giant stars
Asteroseismology, i.e. the study of the internal structures of stars via
their global oscillations, is a valuable tool to obtain stellar parameters such
as mass, radius, surface gravity and mean density. These parameters can be
obtained using certain scaling relations which are based on an asymptotic
approximation. Usually the observed oscillation parameters are assumed to
follow these scaling relations. Recently, it has been questioned whether this
is a valid approach, i.e., whether the order of the observed oscillation modes
are high enough to be approximated with an asymptotic theory. In this work we
use stellar models to investigate whether the differences between observable
oscillation parameters and their asymptotic estimates are indeed significant.
We compute the asymptotic values directly from the stellar models and derive
the observable values from adiabatic pulsation calculations of the same models.
We find that the extent to which the atmosphere is included in the models is a
key parameter. Considering a larger extension of the atmosphere beyond the
photosphere reduces the difference between the asymptotic and observable values
of the large frequency separation. Therefore, we conclude that the currently
suggested discrepancies in the scaling relations might have been overestimated.
Hence, based on the results presented here we believe that the suggestions of
Mosser et al. (2013) should not be followed without careful consideration.Comment: 6 pages, 4 figures, 1 table, accepted for publication by MNRAS as a
Letter to the Edito
Oxygen adsorption effect on magnetic properties of graphite
Both experimental and theoretical studies of the magnetic properties of
micrographite and nanographite indicate a crucial role of the partial oxidation
of graphitic zigzag edges in ferromagnetism. In contrast to total and partial
hydrogenation, the oxidation of half of the carbon atoms on the graphite edges
transforms the antiferromagnetic exchange interaction between graphite planes
and over graphite ribbons to the ferromagnetic interaction. The stability of
the ferromagnetism is discussed.Comment: 14 pages, 6 figure
Universal Magnetic-Field-Driven Metal-Insulator-Metal Transformations in Graphite and Bismuth
Applied magnetic field induces metal - insulator and re-entrant
insulator-metal transitions in both graphite and rhombohedral bismuth. The
corresponding transition boundaries plotted on the magnetic field - temperature
(B - T) plane nearly coincide for these semimetals and can be best described by
power laws T ~ (B - B_c)^k, where B_c is a critical field at T = 0 and k = 0.45
+/- 0.05. We show that insulator-metal-insulator (I-M-I) transformations take
place in the Landau level quantization regime and illustrate how the IMT in
quasi-3D graphite transforms into a cascade of I-M-I transitions, related to
the quantum Hall effect in quasi-2D graphite samples. We discuss the possible
coupling of superconducting and excitonic correlations with the observed
phenomena, as well as the signatures of quantum phase transitions associated
with the M-I and I-M transformations.Comment: 23 pages including 14 figure
Measuring the extent of convective cores in low-mass stars using Kepler data: towards a calibration of core overshooting
Our poor understanding of the boundaries of convective cores generates large
uncertainties on the extent of these cores and thus on stellar ages. Our aim is
to use asteroseismology to consistently measure the extent of convective cores
in a sample of main-sequence stars whose masses lie around the mass-limit for
having a convective core. We first test and validate a seismic diagnostic that
was proposed to probe in a model-dependent way the extent of convective cores
using the so-called ratios, which are built with and
modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets
to optimize the efficiency of this diagnostic. For this purpose, we compute
grids of stellar models with both the CESAM2k and MESA evolution codes, where
the extensions of convective cores are modeled either by an instantaneous
mixing or as a diffusion process. Among the selected targets, we are able to
unambiguously detect convective cores in eight stars and we obtain seismic
measurements of the extent of the mixed core in these targets with a good
agreement between the CESAM2k and MESA codes. By performing optimizations using
the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of
extra-mixing beyond the core that is required in CESAM2k to reproduce seismic
observations for these eight stars and we show that this can be used to propose
a calibration of this quantity. This calibration depends on the prescription
chosen for the extra-mixing, but we find that it should be valid also for the
code MESA, provided the same prescription is used. This study constitutes a
first step towards the calibration of the extension of convective cores in
low-mass stars, which will help reduce the uncertainties on the ages of these
stars.Comment: 27 pages, 15 figures, accepted in A&
Reentrant Metallic Behavior of Graphite in the Quantum Limit
Magnetotransport measurements performed on several well-characterized highly
oriented pyrolitic graphite and single crystalline Kish graphite samples reveal
a reentrant metallic behavior in the basal-plane resistance at high magnetic
fields, when only the lowest Landau levels are occupied. The results suggest
that the quantum Hall effect and Landau-level-quantization-induced
superconducting correlations are relevant to understand the metallic-like
state(s) in graphite in the quantum limit.Comment: 4 pages, 5 figure
Calculation of pure dephasing for excitons in quantum dots
Pure dephasing of an exciton in a small quantum dot by optical and acoustic
phonons is calculated using the ``independent boson model''. Considering the
case of zero temperature the dephasing is shown to be only partial which
manifests itself in the polarization decaying to a finite value. Typical
dephasing times can be assigned even though the spectra exhibits strongly
non-Lorentzian line shapes. We show that the dephasing from LO phonon
scattering, occurs on a much larger time scale than that of dephasing due to
acoustic phonons which for low temperatures are also a more efficient dephasing
mechanism. The typical dephasing time is shown to strongly depend on the
quantum dot size whereas the electron phonon ``coupling strength'' and external
electric fields tend mostly to effect the residual coherence. The relevance of
the dephasing times for current quantum information processing implementation
schemes in quantum dots is discussed
Vortex Lattice in Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} Well Above the First-Order Phase-Transition Boundary
Measurements of non-local in-plane resistance originating from transverse
vortex-vortex correlations have been performed on a
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} high-T_c superconductor in a magnetic field up
to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid
vortex lattice does exist over a broad portion of the magnetic field --
temperature (H-T) phase diagram, well above the first-order transition boundary
H_{FOT}(T). The results also provide evidence for the vortex lattice melting
and vortex liquid decoupling phase transitions, occurring above the H_{FOT}(T).Comment: 14 pages, 10 figure
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