8,845 research outputs found
Characterization of the size and position of electron-hole puddles at a graphene p-n junction
The effect of an electron-hole puddle on the electrical transport when
governed by snake states in a bipolar graphene structure is investigated. Using
numerical simulations we show that information on the size and position of the
electron-hole puddle can be obtained using the dependence of the conductance on
magnetic field and electron density of the gated region. The presence of the
scatterer disrupts snake state transport which alters the conduction pattern.
We obtain a simple analytical formula that connects the position of the
electron-hole puddle with features observed in the conductance. Size of the
electron-hole puddle is estimated from the magnetic field and gate potential
that maximizes the effect of the puddle on the electrical transport.Comment: This is an author-created, un-copyedited version of an article
published in Nanotechnology. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from it. The Version of Record is available online at
doi:10.1088/0957-4484/27/10/10520
Impending carotid blowout stabilization using an LT-D tube
Adequate stabilization of a patient presenting with a carotid blowout is one of the most challenging issues an on-call ENT surgeon can be confronted with. Reducing the bleeding and securing the airway are essential before more definitive management. We present the case of a 72-year-old patient with head and neck cancer who arrived at the emergency room with a carotid blowout and who was successfully stabilized using a King LT-D ventilation tube
The chemical evolution of Barium and Europium in the Milky Way
We compute the evolution of the abundances of barium and europium in the
Milky Way and we compare our results with the observed abundances from the
recent UVES Large Program "First Stars". We use a chemical evolution model
which already reproduces the majority of observational constraints. We confirm
that barium is a neutron capture element mainly produced in the low mass AGB
stars during the thermal-pulsing phase by the 13C neutron source, in a slow
neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H]
as well as the Ba solar abundance, we suggest that Ba should be also produced
as an r-process element by massive stars in the range 10-30 solar masses. On
the other hand, europium should be only an r-process element produced in the
same range of masses (10-30 solar masses), at variance with previous
suggestions indicating a smaller mass range for the Eu producers. As it is well
known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low
metallicities, although smaller in the newest data. With our model we estimate
for both elements (Ba and Eu) the ranges for the r-process yields from massive
stars which better reproduce the trend of the data. We find that with the same
yields which are able to explain the observed trends, the large spread in the
[Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an
inhomogeneous models for the chemical evolution of our Galaxy. We therefore
derive the amount by which the yields should be modified to fully account for
the observed spread. We then discuss several possibilities to explain the size
of the spread. We finally suggest that the production ratio of [Ba/Eu] could be
almost constant in the massive stars.Comment: 14 pages, 17 figures, accepted for pubblication in A&
Grid of theoretical NLTE equivalent widths of four Ba II lines and barium abundance in cool stars
We present a grid of computed non-local thermodynamic equilibrium (NLTE)
equivalent widths (EW) and NLTE abundance corrections for four Ba II lines:
4554, 5853, 6141, and 6496 A. The grid can be useful in deriving the NLTE
barium abundance in stars having parameters in the following ranges: effective
temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5,
microturbulent velocity 0 km s^-1 to 3 km s^-1, metallicity [Fe/H] from -2 to
+0.5, and [Ba/Fe] from -0.4 to +0.6. The NLTE abundance can be either derived
by EW interpolation (using the observed Ba II line EW) or by using the NLTE
correction applied to a previously determined LTE abundance. Ba II line
equivalent widths and the NLTE corrections were calculated using the updated
MULTI code and the Ba II atomic model that was previously applied to determine
the NLTE barium abundance in different types of stars. The grid is available
on-line through the web, and we find that the grid Ba NLTE corrections are
almost as accurate as direct NLTE profile fitting (to within 0.05-0.08 dex).
For the weakest Ba II line (5853 A) the LTE abundances almost agree with the
NLTE abundances, whereas the other three Ba II lines, 4554, 6141, and 6496 A,
need NLTE corrections even at the highest metallicities tested here. The 4554 A
line is extremely strong and should not be used for abundance analysis above
[Fe/H]= -1. Furthermore, we tested the impact of different model atmospheres
and spectrum synthesis codes and found average differences of 0.06 dex and 0.09
dex, respectively, for all four lines. At these metallicities we find an
average Delta NLTE of +/-0.1 dex for the three useful Ba lines for subsolar
cool dwarfs.Comment: 9 pages 8 figures submitted to A&
The Earliest Phases of Galaxy Evolution
In this paper we study the very early phases of the evolution of our Galaxy
by means of a chemical evolution model which reproduces most of the
observational constraints in the solar vicinity and in the disk. We have
restricted our analysis to the solar neighborhood and present the predicted
abundances of several elements (C, N, O, Mg, Si, S, Ca, Fe) over an extended
range of metallicities to compared to previous
models. We adopted the most recent yield calculations for massive stars taken
from different authors (Woosley & Weaver 1995 and Thielemann et al. 1996) and
compared the results with a very large sample of data, one of the largest ever
used to this purpose. These data have been analysed with a new and powerful
statistical method which allows us to quantify the observational spread in
measured elemental abundances and obtain a more meaningful comparison with the
predictions from our chemical evolution model. Our analysis shows that the
``plateau'' observed for the [/Fe] ratios at low metallicities () is not perfectly constant but it shows a slope, especially for
oxygen. This slope is very well reproduced by our model with both sets of
yields. This is not surprising since realistic chemical evolution models,
taking into account in detail stellar lifetimes, never predicted a completely
flat plateau. This is due either to the fact that massive stars of different
mass produce a slightly different O/Fe ratio or to the often forgotten fact
that supernovae of type Ia, originating from white dwarfs, start appearing
already at a galactic age of 30 million years and reach their maximum at 1 Gyr.Comment: 32 pages, 9 figures, to be published in Ap
An ab initio study of the C3(+) cation using multireference methods
The energy difference between the linear 2 sigma(sup +, sub u) and cyclic 2B(sub 2) structures of C3(+) has been investigated using large (5s3p2d1f) basis sets and multireference electron correlation treatments, including complete active space self consistent fields (CASSCF), multireference configuration interaction (MRCI), and averaged coupled-pair functional (ACPF) methods, as well as the single-reference quadratic configuration interaction (QCISD(T)) method. Our best estimate, including a correction for basis set incompleteness, is that the linear form lies above the cyclic from by 5.2(+1.5 to -1.0) kcal/mol. The 2 sigma(sup +, sub u) state is probably not a transition state, but a local minimum. Reliable computation of the cyclic/linear energy difference in C3(+) is extremely demanding of the electron correlation treatment used: of the single-reference methods previously considered, CCSD(T) and QCISD(T) perform best. The MRCI + Q(0.01)/(4s2p1d) energy separation of 1.68 kcal/mol should provide a comparison standard for other electron correlation methods applied to this system
Spectra of binaries classified as lambda Boo stars
High angular resolution observations have shown that some stars classified as
lambda Boo are binaries with low values of angular separation and magnitude
difference of the components; therefore the observed spectrum of these objects
is a combination of those of the two components. These composite spectra have
been used to define spectroscopic criteria able to detect other binaries among
stars classified as lambda Boo. The application of this method to HD 111786 is
presented: the contribution of 5 components to the observed spectrum is
demonstrated by the shape of the O I 7774 Angstrom feature. This result makes
unreliable any attempt to perform an abundance analysis of this object which
therefore must be definitely rejected from the class of the peculiar lambda Boo
stars. This approach allowed us also to recognize that the SB2 star HD 153808
is in reality a triple system.Comment: Accepted for publication by A&
Galactic abundance gradients from Cepheids : On the iron abundance gradient around 10-12 kpc
Context: Classical Cepheids can be adopted to trace the chemical evolution of
the Galactic disk since their distances can be estimated with very high
accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic
Cepheids located in the outer disk together with accurate distance
determinations based on near-infrared photometry are adopted to constrain the
Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined
using high resolution Cepheid spectra collected with three different
observational instruments: ESPaDOnS@CFHT, Narval@TBL and [email protected] ESO/MPG
telescope. Cepheid distances were estimated using near-infrared (J,H,K-band)
period-luminosity relations and data from SAAO and the 2MASS catalog. Results:
The least squares solution over the entire data set indicates that the iron
gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the
5-17 kpc range. However, the change of the iron abundance across the disk seems
to be better described by a linear regime inside the solar circle and a
flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter
region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014
dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an
increase in the spread in iron abundance. Current evidence indicates that the
spread in metallicity depends on the Galactocentric longitude. Finally, current
data do not support the hypothesis of a discontinuity in the iron gradient at
Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread
in iron abundance as a function of the Galactocentric longitude indicates that
linear radial gradients should be cautiously treated to constrain the chemical
evolution across the disk.Comment: 5 tables, 8 figures, Accepted in A&
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