11,849 research outputs found
No evidence of dark matter in the solar neighborhood
We measured the surface mass density of the Galactic disk at the solar
position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick
disk stars. The results match the expectations for the visible mass only, and
no dark matter is detected in the volume under analysis. The current models of
dark matter halo are excluded with a significance higher than 5sigma, unless a
highly prolate halo is assumed, very atypical in cold dark matter simulations.
The resulting lack of dark matter at the solar position challenges the current
models.Comment: Proceeding of the first binational Sochias-AAA meeting, held in San
Juan, Argentin
The structures underlying soliton solutions in integrable hierarchies
We point out that a common feature of integrable hierarchies presenting
soliton solutions is the existence of some special ``vacuum solutions'' such
that the Lax operators evaluated on them, lie in some abelian subalgebra of the
associated Kac-Moody algebra. The soliton solutions are constructed out of
those ``vacuum solitons'' by the dressing transformation procedure.Comment: Talk given at the I Latin American Symposium on High Energy Physics,
I SILAFAE, Merida, Mexico, November/96, 5 pages, LaTeX, needs aipproc.tex,
aipproc.sty, aipproc.cls, available from
ftp://ftp.aip.org/ems/tex/macros/proceedings/6x9
No evidence for a dark matter disk within 4 kpc from the Galactic plane
We estimated the dynamical surface mass density (Sigma) at the solar
Galactocentric distance between 2 and 4 kpc from the Galactic plane, as
inferred from the observed kinematics of the thick disk. We find Sigma(z=2
kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range
considered by our investigation. We compared our results with the expectations
for the visible mass, adopting the most recent estimates in the literature for
contributions of the Galactic stellar disk and interstellar medium, and
proposed models of the dark matter distribution. Our results match the
expectation for the visible mass alone, never differing from it by more than
0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark
component. We assume that the dark halo could be undetectable with our method,
but the dark disk, recently proposed as a natural expectation of the LambdaCDM
models, should be detected. Given the good agreement with the visible mass
alone, models including a dark disk are less likely, but within errors its
existence cannot be excluded. In any case, these results put constraints on its
properties: thinner models (scale height lower than 4 kpc) reconcile better
with our results and, for any scale height, the lower-density models are
preferred. We believe that successfully predicting the stellar thick disk
properties and a dark disk in agreement with our observations could be a
challenging theoretical task.Comment: Accepted for publication in ApJ Letter
Infrared Emission from Clusters in the Starforming Disk of He2-10
We have made subarcsecond-resolution images of the central 10" of the
Wolf-Rayet dwarf galaxy He 2-10 at 11.7 microns, using the Long Wavelength
Spectrometer on the Keck Telescope. The spatial distribution of the infrared
emission roughly agrees with that of the rising spectrum radio sources seen by
Kobulnicky & Johnson (1999) and confirms that those sources are compact HII
regions rather than SNR or other objects. The infrared sources are more
extended than the subarcsecond rising spectrum radio sources, although the
entire complex is still less than 5" in extent. On sizescales of 1" the
infrared and radio emission are in excellent agreement, with each source
requiring several hundred to a thousand O stars for excitation. The nebulae lie
in a flattened disk-like distribution about 240 by 100 pc and provide all of
the flux measured by IRAS for the entire galaxy in the 12 micron band; 30% of
the total IRAS flux from the galaxy emanates from one 15-30 pc source. In this
galaxy, intense star formation, probably triggered by an accretion event, is
confined to a central disk which breaks up into distinct nebulae which
presumably mark the sites of young super star clusters.Comment: Accepted for Publication in the Astronomical Journa
Functional near-infrared spectroscopy in the neuropsychological assessment of spatial memory: A systematic review; 35123299
Functional near-infrared spectroscopy (fNIRS) is a non-invasive optical imaging technique that employs near-infrared light to measure cortical brain oxygenation. The use of fNIRS has increased exponentially in recent years. Spatial memory is defined as the ability to learn and use spatial information. This neuropsychological process is constantly used in our daily lives and can be measured by fNIRS but no research has reviewed whether this technique can be useful in the neuropsychological assessment of spatial memory. This study aimed to review empirical work on the use of fNIRS in the neuropsychological assessment of human spatial memory. We used four databases: PubMed, PsycINFO, Scopus and Web of Science, and a total of 18 articles were found to be eligible. Most of the articles assessed spatial or visuospatial working memory with a predominance in computer-based tasks, used fNIRS equipment of 16 channels and mainly measured the prefrontal cortex (PFC). The studies analysed found linear or quadratic relationships between working memory load and PFC activity, greater activation of PFC activity and worse behavioural results in healthy older people in comparison with healthy adults, and hyperactivation of PFC as a form of compensation in clinical samples. We conclude that fNIRS is compatible with the standard neuropsychological assessment of spatial memory, making it possible to complement behavioural results with data of cortical functional activity. © 202
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
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