1,160 research outputs found
Spin-transfer mechanism of ferromagnetism in polymerized fullerenes: calculations
A mechanism of the high temperature ferromagnetism in polymerized fullerenes
is suggested. It is assumed that some of the C molecules in the crystal
become magnetically active due to spin and charge transfer from the
paramagnetic impurities (atoms or groups), such as hydrogen, fluorine, hydroxyl
group OH, amino group NH, or methyl group CH, dispersed in the
fullerene matrix. The exchange interaction between the spins localized on the
magnetically active fullerenes is evaluated using \textit{ab initio}
calculations. The nearest neighbour and next nearest neighbour exchange
interaction is found to be in the range eV, that is, high enough
to account for the room temperature ferromagnetism.Comment: typos corrected, 8 pages, 3 figures, LaTe
Gradients of metallicity and age of stars in the dwarf spheroidal galaxies KKs3 and ESO269-66
We compare the properties of the stellar populations of the globular clusters
and field stars in two dwarf spheroidal galaxies (dSphs): ESO269-66, a near
neighbor of the giant S0 galaxy NGC 5128, and KKs3, one of the few extremely
isolated dSphs within 10 Mpc. The histories of star formation in these galaxies
are known from previous work on deep stellar photometry using images from the
Hubble Space Telescope (HST). The age and metal content for the nuclear
globular clusters in KKs3 and ESO269-66 are known from literature spectroscopic
studies: T=12.6 billion years, [Fe/H]=-1.5 and -1.55 dex. We use the Sersic
function to construct and analyze the profiles of the surface density of the
stars with high and low metallicities (red and blue) in KKs3 and ESO269-66, and
show that (1) the profiles of the density of red stars are steeper than those
of blue stars, which is indicative of gradients of metallicity and age in the
galaxies, and (2) the globular clusters in KKs3 and ESO269-66 contain roughly 4
and 40%, respectively, of all the old stars in the galaxies with metallicities
[Fe/H]~-1.5 to -1.6 dex and ages of 12-14 billion years. The globular clusters
are, therefore, relicts of the first, most powerful bursts of star formation in
the central regions of these objects. It is probable that, because of its
isolation, KKs3 has lost a smaller fraction of old low-metallicity stars than
ESO269-66.Comment: 7 pages, 2 figures, 3 table
Associations of Dwarf Galaxies
Hubble Space Telescope Advanced Cameras for Surveys has been used to
determine accurate distances for 20 galaxies from measurements of the
luminosity of the brightest red giant branch stars. Five associations of dwarf
galaxies that had originally been identified based on strong correlations on
the plane of the sky and in velocity are shown to be equally well correlated in
distance. Two more associations with similar properties have been discovered.
Another association is identified that is suggested to be unbound through tidal
disruption. The associations have the spatial and kinematic properties expected
of bound structures with 1 - 10 x 10^11 solar mass. However, these entities
have little light with the consequence that mass-to-light ratios are in the
range 100 - 1000 in solar units. Within a well surveyed volume extending to 3
Mpc, all but one known galaxy lies within one of the groups or associations
that have been identified.Comment: 50 pages, 2 tables, 15 encapsulated figures, 1 (3 part) jpg figure.
Submitted to Astronomical Journa
Observation of Magnetic Edge State and Dangling Bond State on Nanographene in Activated Carbon Fibers
The electronic structure of nanographene in pristine and fluorinated
activated carbon fibers (ACFs) have been investigated with near-edge x-ray
absorption fine structure (NEXAFS) and compared with magnetic properties we
reported on previously. In pristine ACFs in which magnetic properties are
governed by non-bonding edge states of the \pi-electron, a pre-peak assigned to
the edge state was observed below the conduction electron {\pi}* peak close to
the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak,
which was assigned to the \sigma-dangling bond state, was observed between the
pre-peak of the edge state and the {\pi}* peak in the NEXAFS profile. The
intensities of the extra peak correlate closely with the spin concentration
created upon fluorination. The combination of the NEXAFS and magnetic
measurement results confirms the coexistence of the magnetic edge states of
\pi-electrons and dangling bond states of \sigma-electrons on fluorinated
nanographene sheets.Comment: 4 figures, to appear in Phys. Rev.
Tidal dwarfs in the M81 group: the second generation?
We derive quantitative star formation histories of the four suspected tidal
dwarf galaxies in the M81 group, HolmbergIX, BK3N, Arp-loop (A0952+69), and
Garland, using Hubble Space Telescope/Wide Field Planetary Camera2 images in
F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the
Local Universe. We consider the spatial distribution and ages of resolved
stellar populations in these dwarf irregular galaxies. We use synthetic
color-magnitude diagrams to derive the ages of the major star formation
episodes, star formation rates, and approximate metallicity ranges. All the
galaxies show evidence of continuous star formation between about 20 and 200
Myr ago with star formation rates in the range 7.5*10^(-3)- 7.6*10^(-4)
M(sun)/yr. The metallicity of the detected stars spans a wide range, and have
lower than solar abundance. A possible scenario is that all four dwarf galaxies
were formed from material in the metal-poor outer part of the giant spiral
galaxy M81 after the tidal interaction between M81, M82, and NGC3077 about 200
Myr ago. While we do not directly detect pronounced old stellar populations,
the photometric limits of our data are such that the presence of such a
population is not entirely ruled out
Surface photometry of new nearby dwarf galaxies
We present CCD surface photometry of 16 nearby dwarf galaxies, many of which
were only recently discovered. Our sample comprises both isolated galaxies and
galaxies that are members of nearby galaxy groups. The observations were
obtained in the Johnson B and V bands (and in some cases in Kron-Cousins I). We
derive surface brightness profiles, total magnitudes, and integrated colors.
For the 11 galaxies in our sample with distance estimates the absolute B
magnitudes lie in the range of -10>Mb>-13. The central surface brightness
ranges from 22.5 to 27.0 mag/sq.arcsec. Most of the dwarf galaxies show
exponential light profiles with or without a central light depression.
Integrated radial color gradients, where present, appear to indicate a more
centrally concentrated younger population and a more extended older population.Comment: accepted by A&
Kramers-Kronig constrained variational analysis of optical spectra
A universal method of extraction of the complex dielectric function
from
experimentally accessible optical quantities is developed. The central idea is
that is parameterized independently at each node of a
properly chosen anchor frequency mesh, while is
dynamically coupled to by the Kramers-Kronig (KK)
transformation. This approach can be regarded as a limiting case of the
multi-oscillator fitting of spectra, when the number of oscillators is of the
order of the number of experimental points. In the case of the normal-incidence
reflectivity from a semi-infinite isotropic sample the new method gives
essentially the same result as the conventional KK transformation of
reflectivity. In contrast to the conventional approaches, the proposed
technique is applicable, without readaptation, to virtually all types of
linear-response optical measurements, or arbitrary combinations of
measurements, such as reflectivity, transmission, ellipsometry {\it etc.}, done
on different types of samples, including thin films and anisotropic crystals.Comment: 10 pages, 7 figure
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