149 research outputs found
Towards ultra-high resolution 3D reconstruction of a whole rat brain from 3D-PLI data
3D reconstruction of the fiber connectivity of the rat brain at microscopic
scale enables gaining detailed insight about the complex structural
organization of the brain. We introduce a new method for registration and 3D
reconstruction of high- and ultra-high resolution (64 m and 1.3 m
pixel size) histological images of a Wistar rat brain acquired by 3D polarized
light imaging (3D-PLI). Our method exploits multi-scale and multi-modal 3D-PLI
data up to cellular resolution. We propose a new feature transform-based
similarity measure and a weighted regularization scheme for accurate and robust
non-rigid registration. To transform the 1.3 m ultra-high resolution data
to the reference blockface images a feature-based registration method followed
by a non-rigid registration is proposed. Our approach has been successfully
applied to 278 histological sections of a rat brain and the performance has
been quantitatively evaluated using manually placed landmarks by an expert.Comment: 9 pages, Accepted at 2nd International Workshop on Connectomics in
NeuroImaging (CNI), MICCAI'201
The Revival of Galactic Cosmic Ray Nucleosynthesis?
Because of the roughly linear correlation between Be/H and Fe/H in low
metallicity halo stars, it has been argued that a ``primary'' component in the
nucleosynthesis of Be must be present in addition to the ``secondary''
component from standard Galactic cosmic ray nucleosynthesis. In this paper we
critically re-evaluate the evidence for the primary versus secondary character
of Li, Be, and B evolution, analyzing both in the observations and in Galactic
chemical evolution models. While it appears that [Be/H] versus [Fe/H] has a
logarithmic slope near 1, it is rather the Be-O trend that directly arises from
the physics of spallation production. Using new abundances for oxygen in halo
stars based on UV OH lines, we find that the Be-O slope has a large uncertainty
due to systematic effects, rendering it difficult to distinguish from the data
between the secondary slope of 2 and the primary slope of 1. The possible
difference between the Be-Fe and Be-O slopes is a consequence of the variation
in O/Fe versus Fe: recent data suggests a negative slope rather than zero
(i.e., Fe O) as is often assumed. In addition to a phenomenological
analysis of Be and B evolution, we have also examined the predicted LiBeB, O,
and Fe trends in Galactic chemical evolution models which include outflow.
Based on our results, it is possible that a good fit to the LiBeB evolution
requires only traditional the Galactic cosmic ray spallation, and the (primary)
neutrino-process contribution to B11. We thus suggest that these two processes
might be sufficient to explain Li6, Be, and B evolution in the Galaxy, without
the need for an additional primary source of Be and B.Comment: 25 pages, latex, 8 ps figures, figure 1 correcte
The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?
We review five independent techniques which are used to set the distance
scale to globular clusters, including subdwarf main sequence fitting utilizing
the recent Hipparcos parallax catalogue. These data together all indicate that
globular clusters are farther away than previously believed, implying a
reduction in age estimates. This new distance scale estimate is combined with a
detailed numerical Monte Carlo study designed to assess the uncertainty
associated with the theoretical age-turnoff luminosity relationship in order to
estimate both the absolute age and uncertainty in age of the oldest globular
clusters. Our best estimate for the mean age of the oldest globular clusters is
now Gyr, with a one-sided, 95% confidence level lower limit of
9.5 Gyr. This represents a systematic shift of over 2 compared to our
earlier estimate, due completely to the new distance scale---which we emphasize
is not just due to the Hipparcos data. This now provides a lower limit on the
age of the universe which is consistent with either an open universe, or a
flat, matter dominated universe (the latter requiring H_0 \le 67 \kmsmpc).
Our new study also explicitly quantifies how remaining uncertainties in the
distance scale and stellar evolution models translate into uncertainties in the
derived globular cluster ages. Simple formulae are provided which can be used
to update our age estimate as improved determinations for various quantities
become available.Comment: 41 pages, including 10 eps figs, uses aaspp4.sty and flushrt.sty,
submitted to Ap.J., revised to incorporate FULL Hipparcos catalogue dat
Testing Spallation Processes With Beryllium and Boron
The nucleosynthesis of Be and B by spallation processes provides unique
insight into the origin of cosmic rays. Namely, different spallation schemes
predict sharply different trends for the growth of LiBeB abundances with
respect to oxygen. ``Primary'' mechanisms predict BeB O, and are well
motivated by the data if O/Fe is constant at low metallicity. In contrast,
``secondary'' mechanisms predict BeB O and are consistent with
the data if O/Fe increases towards low metallicity as some recent data suggest.
Clearly, any primary mechanism, if operative, will dominate early in the
history of the Galaxy. In this paper, we fit the BeB data to a two-component
scheme which includes both primary and secondary trends. In this way, the data
can be used to probe the period in which primary mechanisms are effective. We
analyze the data using consistent stellar atmospheric parameters based on
Balmer line data and the continuum infrared flux. Results depend sensitively on
Pop II O abundances and, unfortunately, on the choice of stellar parameters.
When using recent results which show O/Fe increasing toward lower metallicity,
a two-component Be-O fits indicates that primary and secondary components
contribute equally at [O/H] = -1.8 for Balmer line data; and
[O/H] = -1.4 to -1.8 for IRFM. We apply these constraints to recent
models for LiBeB origin. The Balmer line data does not show any evidence for
primary production. On the other hand, the IRFM data does indicate a preference
for a two-component model, such as a combination of standard GCR and
metal-enriched particles accelerated in superbubbles. These conclusions rely on
a detailed understanding of the abundance data including systematic effects
which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
Implications of a new temperature scale for halo dwarfs on LiBeB and chemical evolution
Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic
microwave background anisotropies together predict a primordial Li7 abundance a
factor of 2--3 higher than that observed in galactic halo dwarf stars. A recent
analysis of Li7 observations in halo stars, using significantly higher surface
temperature for these stars, found a higher Li plateau abundance. These results
go a long way towards resolving the discrepancy with BBN. Here, we examine the
implications of the higher surface temperatures on the abundances of Be and B
which are thought to have been produced in galactic cosmic-ray nucleosynthesis
by spallation of CNO together with Li (produced in alpha + alpha collisions).
While the Be abundance is not overly sensitive to the surface temperature, the
derived B abundances and more importantly the derived oxygen abundances are
very temperature dependent. If the new temperature scale is correct, the
implied increased abundances of these elements poses a serious challenge to
models of galactic cosmic ray nucleosynthesis and galactic chemical evolution.Comment: 23 pages, 10 eps figure
VLT Observations of Turnoff stars in the Globular Cluster NGC 6397
VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor
globular cluster NGC 6397 have been obtained. Atmospheric parameters and
abundances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba)
were derived for program stars. The mean iron abundance is [Fe/H] = -2.02, with
no star-to-star variation. The mean abundances of the alpha-elements (Ca, Ti)
and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances
derived for field stars of similar metallicity. Magnesium is also almost solar,
consistent with the values found by Idiart & Th\'evenin (2000) when non-LTE
effects (NLTE hereafter) are taken into account. The sodium abundance derived
for five stars is essentially solar, but one object (A447) is clearly Na
deficient. These results are compatible with the expected abundance range
estimated from the stochastic evolutionary halo model by Argast et al. (2000)
when at the epoch of [Fe/H] -2 the interstellar medium is supposed to
become well-mixed.Comment: to appear in A&
TOpic: rare and special cases, the real "Strange cases"
Introduction: The bladder hernia represents approximately 1-3% of
all inguinal hernias, where patients aged more than 50 years have a
higher incidence (10%). Many factors contribute to the development of a bladder hernia,
including the presence of a urinary outlet obstruction causing chronic
bladder distention, the loss of bladder tone, pericystitis, the perivesical
bladder fat protrusion and the obesity
Distances, ages, and epoch of formation of globular clusters
We review the results on distances and absolute ages of galactic globular
clusters (GCs) obtained after the release of the Hipparcos catalogue. Several
methods for the Population II local distance scale are discussed, exploiting
NEW RESULTS for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the
so-called Short and Long Distance Scales may be reconciled whether a consistent
reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC.
Distances and ages for the 9 clusters discussed in Paper I are re-derived using
an enlarged sample of local subdwarfs, which includes about 90% of the
metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole
Hipparcos catalogue. On average, our revised distance moduli are decreased by
0.04 mag with respect to Paper I. The corresponding age of the GCs is
t=11.5+-2.6 Gyr (95% confidence range). The relation between Mv(ZAHB) and
metallicity for the nine programme clusters turns out to be
Mv(ZAHB)=(0.18+-0.09)([Fe/H]+1.5)+(0.53+-0.12).Thanks to Hipparcos the major
contribution to the total error budget associated with the subdwarf fitting
technique has been moved from parallaxes to photometric calibrations, reddening
and metallicity scale. This total uncertainty still amounts to about +-0.12
mag. Comparing the corresponding (true) LMC distance modulus 18.64+-0.12 mag
with other existing determinations, we conclude that at present the best
estimate for the distance of the LMC is: 18.54+-0.03+-0.06, suggesting that
distances from the subdwarf fitting method are 1 sigma too long. Consequently,
our best estimate for the age of the GCs is revised to: Age = 12.9+-2.9 Gyr
(95% confidence range). The best relation between Mv(ZAHB) and [Fe/H] is:
Mv(ZAHB) =(0.18+-0.09)([Fe/H]+1.5)+(0.63+-0.07).Comment: 76 pages, 6 encapsulated figures and 6 tables. Latex, uses
aasms4.sty. Revised and improved version, with new data on field RR Lyraes in
LMC. Accepted in the Astrophysical Journa
Empirically Constrained Color-Temperature Relations. II. uvby
(Abriged) A new grid of theoretical color indices for the Stromgren uvby
photometric system has been derived from MARCS model atmospheres and SSG
synthetic spectra for cool dwarf and giant stars. At warmer temperatures this
grid has been supplemented with the synthetic uvby colors from recent Kurucz
atmospheric models without overshooting. Our transformations appear to
reproduce the observed colors of extremely metal-poor turnoff and giant stars
(i.e., [Fe/H]<-2). Due to a number of assumptions made in the synthetic color
calculations, however, our color-temperature relations for cool stars fail to
provide a suitable match to the uvby photometry of both cluster and field stars
having [Fe/H]>-2. To overcome this problem, the theoretical indices at
intermediate and high metallicities have been corrected using a set of color
calibrations based on field stars having accurate IRFM temperature estimates
and spectroscopic [Fe/H] values. Encouragingly, isochrones that employ the
transformations derived in this study are able to reproduce the observed CMDs
(involving u-v, v-b, and b-y colors) for a number of open and globular clusters
(including M92, M67, the Hyades, and 47Tuc) rather well. Moreover, our
interpretations of such data are very similar, if not identical, with those
given by VandenBerg & Clem (2003, AJ, 126, 778) from a consideration of BV(RI)c
observations for the same clusters. In the present investigation, we have also
analyzed the observed Stromgren photometry for the classic Population II
subdwarfs, compared our "final" (b-y)-Teff relationship with those derived
empirically in a number of recent studies, and examined in some detail the
dependence of the m1 index on [Fe/H].Comment: 70 pages, 26 figures. Accepted for publication in AJ (Feb 2004).
Postscript version with high resolution figures and complete Table 3
available at http://astrowww.phys.uvic.ca/~jclem/uvb
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