6,651 research outputs found
Sulphur abundances in metal-poor stars
We investigate the debated "sulphur discrepancy" found among metal-poor stars
of the Galactic halo with [Fe/H] < -2. This discrepancy stems in part from the
use of two different sets of sulphur lines, the very weak triplet at 8694-95 A
and the stronger triplet lines at 9212 - 9237 A. For three representative cases
of metal-poor dwarf, turnoff and subgiant stars, we argue that the abundances
from the 8694-95 lines have been overestimated which has led to a continually
rising trend of [S/Fe] as metallicity decreases. Given that the near-IR region
is subject to CCD fringing, these weak lines become excessively difficult to
measure accurately in the metallicity regime of [Fe/H] < -2. Based on
homogeneously determined spectroscopic stellar parameters, we also present
updated [S/Fe] ratios from the 9212-9237 lines which suggest a plateau-like
behaviour similar to that seen for other alpha elements.Comment: accepted by A&A, 4 pages, 3 tables, 1 figure; v2: Table2 updated with
metallicities from other work
The Case of HCO Isomers, Revisited: Solving the Mystery of the Missing Propadienone
To date, two isomers of HCO have been detected, namely, propynal
(HCCCHO) and cylclopropenone (c-HCO). A third, propadienone
(CHCCO), has thus far eluded observers despite the fact that it is the
lowest in energy of the three. This previously noted result is in contradiction
of the minimum energy principle, which posits that the abundances of isomers in
interstellar environments can be predicted based on their relative stabilities
- and suggests, rather, the importance of kinetic over thermodynamic effects in
explaining the role of such species.
Here, we report results of \textit{ab initio} quantum chemical calculations
of the reaction between H and (a) HCO, (b) HCO (both propynal and
propadienone), and (c) CHCHCO. We have found that, among all possible
reactions between atomic hydrogen and either propadienone or propynal, only the
destruction of propadienone is barrierless and exothermic. That this
destruction pathway is indeed behind the non-detection of CHCCO is further
suggested by our finding that the product of this process, the radical
CHCHCO, can subsequently react barrierlessly with H to form propenal
(CHCHCHO) which has, in fact, been detected in regions where the other two
HCO isomers are observed. Thus, these results not only shed light on a
previously unresolved astrochemical mystery, but also further highlight the
importance of kinetics in understanding the abundances of interstellar
molecules.Comment: ApJ, accepted: 14 pages, 2 figure
Core compressor exit stage study, 2
A total of two three-stage compressors were designed and tested to determine the effects of aspect ratio on compressor performance. The first compressor was designed with an aspect ratio of 0.81; the other, with an aspect ratio of 1.22. Both compressors had a hub-tip ratio of 0.915, representative of the rear stages of a core compressor, and both were designed to achieve a 15.0% surge margin at design pressure ratios of 1.357 and 1.324, respectively, at a mean wheel speed of 167 m/sec. At design speed the 0.81 aspect ratio compressor achieved a pressure ratio of 1.346 at a corrected flow of 4.28 kg/sec and an adiabatic efficiency of 86.1%. The 1.22 aspect ratio design achieved a pressure ratio of 1.314 at 4.35 kg/sec flow and 87.0% adiabatic efficiency. Surge margin to peak efficiency was 24.0% with the lower aspect ratio blading, compared with 12.4% with the higher aspect ratio blading
Solvable Examples of Drift and Diffusion of Ions in Non-uniform Electric Fields
The drift and diffusion of a cloud of ions in a fluid are distorted by an
inhomogeneous electric field. If the electric field carries the center of the
distribution in a straight line and the field configuration is suitably
symmetric, the distortion can be calculated analytically. We examine the
specific examples of fields with cylindrical and spherical symmetry in detail
assuming the ion distributions to be of a generally Gaussian form. The effects
of differing diffusion coefficients in the transverse and longitudinal
directions are included
A Group Theoretical Identification of Integrable Equations in the Li\'enard Type Equation : Part II: Equations having Maximal Lie Point Symmetries
In this second of the set of two papers on Lie symmetry analysis of a class
of Li\'enard type equation of the form ,
where over dot denotes differentiation with respect to time and and
are smooth functions of their variables, we isolate the equations which
possess maximal Lie point symmetries. It is well known that any second order
nonlinear ordinary differential equation which admits eight parameter Lie point
symmetries is linearizable to free particle equation through point
transformation. As a consequence all the identified equations turn out to be
linearizable. We also show that one can get maximal Lie point symmetries for
the above Li\'enard equation only when (subscript denotes
differentiation). In addition, we discuss the linearising transformations and
solutions for all the nonlinear equations identified in this paper.Comment: Accepted for publication in Journal of Mathematical Physic
New Abundances for Old Stars - Atomic Diffusion at Work in NGC 6397
A homogeneous spectroscopic analysis of unevolved and evolved stars in the
metal-poor globular cluster NGC 6397 with FLAMES-UVES reveals systematic trends
of stellar surface abundances that are likely caused by atomic diffusion. This
finding helps to understand, among other issues, why the lithium abundances of
old halo stars are significantly lower than the abundance found to be produced
shortly after the Big Bang.Comment: 8 pages, 7 colour figures, 1 table; can also be downloaded via
http://www.eso.org/messenger
Bichiral structure of feroelectric domain wall driven by flexoelectricity
The influence of flexoelectric coupling on the internal structure of neutral
domain walls in tetragonal phase of perovskite ferroelectrics is studied. The
effect is shown to lower the symmetry of 180-degree walls which are oblique
with respect to the cubic crystallographic axes, while {100} and {110} walls
stay "untouched". Being of the Ising type in the absence of the flexoelectric
interaction, the oblique domain walls acquire a new polarization component with
a structure qualitatively different from the classical Bloch-wall structure. In
contrast to the Bloch-type walls, where the polarization vector draws a helix
on passing from one domain to the other, in the flexoeffect-affected wall, the
polarization rotates in opposite directions on the two sides of the wall and
passes through zero in its center. Since the resulting polarization profile is
invariant upon inversion with respect to the wall center it does not brake the
wall symmetry in contrast to the classical Bloch-type walls. The flexoelectric
coupling lower the domain wall energy and gives rise to its additional
anisotropy that is comparable to that conditioned by the elastic anisotropy.
The atomic orderof- magnitude estimates shows that the new polarization
component P2 may be comparable with spontaneous polarization Ps, thus
suggesting that, in general, the flexoelectric coupling should be mandatory
included in domain wall simulations in ferroelectrics. Calculations performed
for barium titanate yields the maximal value of the P2, which is much smaller
than that of the spontaneous polarization. This smallness is attributed to an
anomalously small value of a component of the "strain-polarization"
elecrostictive tensor in this material
Mean first passage times for bond formation for a Brownian particle in linear shear flow above a wall
Motivated by cell adhesion in hydrodynamic flow, here we study bond formation
between a spherical Brownian particle in linear shear flow carrying receptors
for ligands covering the boundary wall. We derive the appropriate Langevin
equation which includes multiplicative noise due to position-dependent mobility
functions resulting from the Stokes equation. We present a numerical scheme
which allows to simulate it with high accuracy for all model parameters,
including shear rate and three parameters describing receptor geometry
(distance, size and height of the receptor patches). In the case of homogeneous
coating, the mean first passage time problem can be solved exactly. In the case
of position-resolved receptor-ligand binding, we identify different scaling
regimes and discuss their biological relevance.Comment: final version after minor revision
A non-LTE study of neutral and singly-ionized calcium in late-type stars
Non-local thermodynamical equilibrium (NLTE) line formation for neutral and singly-ionized calcium is considered through a range of spectral types when the Ca abundance varies from the solar value down to [Ca/H] = -5. Departures from LTE significantly affect the profiles of Ca I lines over the whole range of stellar parameters considered. However, at [Ca/H] >= -2, NLTE abundance correction of individual lines may be small in absolute value due to the different influence of NLTE effects on line wings and the line core. At lower Ca abundances, NLTE leads to systematically depleted total absorption in the line and positive abundance corrections, exceeding +0.5 dex for Ca I 4226 at [Ca/H] = -4.9. In contrast, NLTE effects strengthen the Ca II lines and lead to negative abundance corrections. NLTE corrections are small, <= 0.02 dex, for the Ca II resonance lines. For the IR lines of multiplet 3d - 4p, they grow in absolute value with decreasing Ca abundance exceeding 0.4 dex in metal-poor stars with [Fe/H] <= -3. Ca abundances are determined for the Sun, Procyon, and seven metal-poor stars, using high S/N and high-resolution spectra at visual and near-IR wavelengths. Lines of Ca I and Ca II give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from Ca I and Ca II lines) is \eps{Ca,\odot} = 6.38+-0.01. For Procyon, the mean Ca abundance from Ca I lines is markedly subsolar, [Ca/H] = -0.14+-0.03. The W(Ca I 4226)/W(Ca II 8498) equivalent width ratio is predicted to be sensitive to surface gravity for extremely metal-poor stars, while this is not the case for the ratio involving the Ca II resonance line(s)
On the long-range gravity in warped backgrounds
In this paper the Randall-Sundrum model with brane-localized curvature terms
is considered. Within some range of parameters a compact extra dimension in
this model can be astronomically large. In this case the model predicts small
deviation from Newton's law at astronomical scales, caused by the massive
modes. The existence of this deviation can result in a slight affection on the
planetary motion trajectories.Comment: LaTeX, 9 pages, typos corrected, reference adde
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