633 research outputs found
The angular sizes of dwarf stars and subgiants - Non-linear surface brightness relations in BVRcIc from interferometry
Context: The prediction of stellar angular diameters from broadband
photometry plays an important role for different applications. In particular,
long-baseline interferometry, gravitational microlensing, extrasolar planet
transits, and many other observing techniques require accurate predictions of
the angular size of stars. These predictions are based on the surface
brightness-colour (SBC) relations. Aims: Our goal is to calibrate
general-purpose SBC relations using visible colours, the most commonly
available data for most stars. Methods: We compiled the existing long-baseline
interferometric observations of nearby dwarf and subgiant stars and the
corresponding broadband photometry in the Johnson B V and Cousins Rc Ic bands.
We then adjusted polynomial SBC models to these data. Results: Due to the
presence of spectral features that depend on the effective temperature, the SBC
relations are usually not linear for visible colours. We present polynomial
fits that can be employed with BVRcIc based colours to predict the
limb-darkened angular diameters (i.e. photospheric) of dwarf and subgiant stars
with a typical accuracy of 5%. Conclusions: The derived polynomial relations
provide a satisfactory approximation to the observed surface brightness of
nearby dwarfs and subgiants. For distant stars, the interstellar reddening
should be taken into account, and will usually introduce an additional
uncertainty to the predicted angular diameters.Comment: 5 pages, accepted for publication in Astronomy & Astrophysics.
Astronomy and Astrophysics (2008) Accepte
Large magnetic circular dichroism in resonant inelastic x-ray scattering at the Mn L-edge of Mn-Zn ferrite
We report resonant inelastic x-ray scattering (RIXS) excited by circularly
polarized x-rays on Mn-Zn ferrite at the Mn L2,3-resonances. We demonstrate
that crystal field excitations, as expected for localized systems, dominate the
RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms
of spin-resolved partial density of states, which has been the standard
approach for RIXS dichroism. We observe large dichroic RIXS at the L2-resonance
which we attribute to the absence of metallic core hole screening in the
insulating Mn-ferrite. On the other hand, reduced L3-RIXS dichroism is
interpreted as an effect of longer scattering time that enables spin-lattice
core hole relaxation via magnons and phonons occurring on a femtosecond time
scale.Comment: 7 pages, 2 figures,
http://link.aps.org/doi/10.1103/PhysRevB.74.17240
Competition between decay and dissociation of core-excited OCS studied by X-ray scattering
We show the first evidence of dissociation during resonant inelastic soft
X-ray scattering. Carbon and oxygen K-shell and sulfur L-shell resonant and
non-resonant X-ray emission spectra were measured using monochromatic
synchrotron radiation for excitation and ionization. After sulfur, L2,3 ->
{\pi}*, {\sigma}* excitation, atomic lines are observed in the emission spectra
as a consequence of competition between de-excitation and dissociation. In
contrast the carbon and oxygen spectra show weaker line shape variations and no
atomic lines. The spectra are compared to results from ab initio calculations
and the discussion of the dissociation paths is based on calculated potential
energy surfaces and atomic transition energies.Comment: 12 pages, 6 pictures, 2 tables,
http://link.aps.org/doi/10.1103/PhysRevA.59.428
Electronic structure investigation of CoO by means of soft X-ray scattering
The electronic structure of CoO is studied by resonant inelastic soft X-ray
scattering spectroscopy using photon energies across the Co 2p absorption
edges. The different spectral contributions from the energy-loss structures are
identified as Raman scattering due to d-d and charge-transfer excitations. For
excitation energies close to the L3 resonance, the spectral features are
dominated by quartet-quartet and quartet-doublet transitions of the 3d7
configuration. At excitation energies corresponding to the satellites in the Co
2p X-ray absorption spectrum of CoO, the emission features are instead
dominated by charge-transfer transitions to the 3d8L-1 final state. The spectra
are interpreted and discussed with the support of simulations within the single
impurity Anderson model with full multiplet effects which are found to yield
consistent spectral functions to the experimental data.Comment: 8 pages, 2 figures, 2 tables,
http://link.aps.org/doi/10.1103/PhysRevB.65.20510
Electronic structure and chemical bonding in Ti4SiC3 investigated by soft x-ray emission spectroscopy and first principle theory
The electronic structure in the new transition metal carbide Ti4SiC3 has been
investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to
the well-studied Ti3SiC2 and TiC systems. The measured high-resolution Ti L, C
K and Si L x-ray emission spectra are discussed with ab initio calculations
based on density-functional theory including core-to-valence dipole matrix
elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds
provide increased understanding of the physical properties of these
nanolaminates. A strongly modified spectral shape is detected for the buried Si
monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of
relaxation of the crystal structure and the charge-transfer from Ti (and Si) to
C, the strength of the Ti-C covalent bond is increased. The differences between
the electronic and crystal structures of Ti4SiC3 and Ti3SiC2 are discussed in
relation to the number of Si layers per Ti layer in the two systems and the
corresponding change of materials properties.Comment: 12 pages, 7 figures, 1 tabl
Intramolecular vibronic dynamics in molecular solids: C60
Vibronic coupling in solid C60 has been investigated with a combination of resonant photoemission spectroscopy (RPES) and resonant inelastic x-ray scattering (RIXS). Excitation as a function of energy within the lowest unoccupied molecular orbital resonance yielded strong oscillations in intensity and dispersion in RPES, and a strong inelastic component in RIXS. Reconciling these two observations establishes that vibronic coupling in this core hole excitation leads to predominantly inelastic scattering and localization of the excited vibrations on the molecule on a femtosecond time scale. The coupling extends throughout the widths of the frontier valence bands.
Resonant soft X-ray Raman scattering of NiO
Resonant soft X-ray Raman scattering measurements on NiO have been made at
photon energies across the Ni 2p absorption edges. The details of the spectral
features are identified as Raman scattering due to d-d and charge-transfer
excitations. The spectra are interpreted within the single impurity Anderson
model, including multiplets, crystal-field and charge-transfer effects. At
threshold excitation, the spectral features consists of triplet-triplet and
triplet-singlet transitions of the 3d8 configuration. For excitation energies
corresponding to the charge-transfer region in the Ni 2p X-ray absorption
spectrum of NiO, the emission spectra are instead dominated by charge-transfer
transitions to the 3d9L-1 final state. Comparisons of the final states with
other spectroscopical techniques are also made.Comment: 9 pages, 2 figures, 2 tables,
http://iopscience.iop.org/0953-8984/14/13/32
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