20,590 research outputs found
Generalized Galilean Algebras and Newtonian Gravity
The non-relativistic versions of the generalized Poincar\'{e} algebras and
generalized -Lorentz algebras are obtained. This non-relativistic algebras
are called, generalized Galilean algebras type I and type II and denoted by
and
respectively. Using a generalized In\"{o}n\"{u}--Wigner contraction procedure
we find that the generalized Galilean algebras type I can be obtained from the
generalized Galilean algebras type II. The -expansion procedure allows us to
find the algebra from the Newton--Hooke
algebra with central extension. The procedure developed in Ref. \cite{newton}
allow us to show that the non-relativistic limit of the five dimensional
Einstein--Chern--Simons gravity is given by a modified version of the Poisson
equation. The modification could be compatible with the effects of Dark Matter,
which leads us to think that Dark Matter can be interpreted as a
non-relativistic limit of Dark Energy.Comment: 16 pages, no figures in 755 (2016) 433-43
Non-adiabatic effects in long-pulse mixed-field orientation of a linear polar molecule
We present a theoretical study of the impact of an electrostatic field
combined with non-resonant linearly polarized laser pulses on the rotational
dynamics of linear molecules. Within the rigid rotor approximation, we solve
the time-dependent Schr\"odinger equation for several field configurations.
Using the OCS molecule as prototype, the field-dressed dynamics is analyzed in
detail for experimentally accessible static field strengths and laser pulses.
Results for directional cosines are presented and compared to the predictions
of the adiabatic theory. We demonstrate that for prototypical field
configuration used in current mixed-field orientation experiments, the
molecular field dynamics is, in general, non-adiabatic, being mandatory a
time-dependent description of these systems. We investigate several field
regimes identifying the sources of non-adiabatic effects, and provide the field
parameters under which the adiabatic dynamics would be achieved.Comment: 16 pages, 16 figures. Submitted to Physical Review
Strongly driven quantum pendulum of the OCS molecule
We demonstrate and analyze a strongly driven quantum pendulum in the angular
motion of stateselected and laser aligned OCS molecules. Raman-couplings during
the rising edge of a 50-picosecond laser pulse create a wave packet of pendular
states, which propagates in the confining potential formed by the
polarizability interaction between the molecule and the laser field. This
wave-packet dynamics manifests itself as pronounced oscillations in the degree
of alignment with a laser-intensity dependent period.Comment: 6 pages, 4 figure
Accretion and photodesorption of CO ice as a function of the incident angle of deposition
Non-thermal desorption of inter- and circum-stellar ice mantles on dust
grains, in particular ultraviolet photon-induced desorption, has gained
importance in recent years. These processes may account for the observed gas
phase abundances of molecules like CO toward cold interstellar clouds. Ice
mantle growth results from gas molecules impinging on the dust from all
directions and incidence angles. Nevertheless, the effect of the incident angle
for deposition on ice photo-desorption rate has not been studied. This work
explores the impact on the accretion and photodesorption rates of the incidence
angle of CO gas molecules with the cold surface during deposition of a CO ice
layer. Infrared spectroscopy monitored CO ice upon deposition at different
angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet
spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp.
Molecules ejected from the ice to the gas during irradiation or warm-up were
characterized by a quadrupole mass spectrometer. The photodesorption rate of CO
ice deposited at 11 K and different incident angles was rather stable between 0
and 45. A maximum in the CO photodesorption rate appeared around
70-incidence deposition angle. The same deposition angle leads to the
maximum surface area of water ice. Although this study of the surface area
could not be performed for CO ice, the similar angle dependence in the
photodesorption and the ice surface area suggests that they are closely
related. Further evidence for a dependence of CO ice morphology on deposition
angle is provided by thermal desorption of CO ice experiments
A comparative study of root system architecture in seedlings of Brachypodium spp. using three plant growth supports
This work reports a rapid procedure of comparing root development among different genotypes of Brachypodium spp., using three plant growth supports: gel chamber, ‘Termita’ chamber and Whatman paper. Eight variables of the root system architecture (RSA) (number of seminal roots, number of lateral roots, total length of the roots, length of the primary root, mean diameter of the roots, mean diameter of the primary root, total surface area and total volume of the roots) were studied in seedling of four genotypes each of Brachypodium distachyon, B. stacei and B. hybridum. Correlations between pairs of growth supports in terms of the eight variables examined were highly significant. In all three supports, B. stacei showed the greatest root system development while B. distachyon showed the least; B. hybridum, an allotetraploid species derived from hybridization between B. distachyon and B. stacei, showed intermediate development. ANOVA and LSD tests showed that significant differences exist between the supports, species and genotypes with respect to all the variables analysed. A cluster analysis was conducted to determine if the RSA traits could be used to differentiate the species and genotypes of Brachypodium. This analysis allowed differentiated between the three species and twelve genotypes of Brachypodium spp., although a certain overlap between species was observed. The Whatman paper support was the easiest to use, and is recommended for the characterization of large collections of genotypes
Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance
Thanks to the high multiplex and efficiency of Giraffe at the VLT we have
been able for the first time to observe the Li I doublet in the Main Sequence
(MS) stars of a Globular Cluster. At the same time we observed Li in a sample
of Sub-Giant (SG) stars of the same B-V colour. Our final sample is composed of
84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same
temperature range we find that the equivalent widths of the Li I doublet in SG
stars are systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis. We
derived the effective temperatures, from H fitting, and NLTE Li
abundances of the stars in our the sample, using 3D and 1D models. We find that
SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D
and 3D models. We also detect a positive slope of Li abundance with effective
temperature. These results provide an unambiguous evidence that the Li
abundance changes with evolutionary status. The physical mechanisms responsible
for this behaviour are not yet clear, and none of the existing models seems to
describe accurately these observations. Based on these conclusions, we believe
that the cosmological lithium problem still remains an open question.Comment: Proceedings of the contributed talk presented at the IAU Symposium
26
CNO behaviour in planet-harbouring stars. II. Carbon abundances in stars with and without planets using the CH band
Context. Carbon, oxygen and nitrogen (CNO) are key elements in stellar
formation and evolution, and their abundances should also have a significant
impact on planetary formation and evolution.
Aims. We present a detailed spectroscopic analysis of 1110 solar-type stars,
143 of which are known to have planetary companions. We have determined the
carbon abundances of these stars and investigate a possible connection between
C and the presence of planetary companions. Methods. We used the HARPS
spectrograph to obtain high-resolution optical spectra of our targets. Spectral
synthesis of the CH band at 4300\AA was performed with the spectral synthesis
codes MOOG and FITTING.
Results. We have studied carbon in several reliable spectral windows and have
obtained abundances and distributions that show that planet host stars are
carbon rich when compared to single stars, a signature caused by the known
metal-rich nature of stars with planets. We find no different behaviour when
separating the stars by the mass of the planetary companion.
Conclusions. We conclude that reliable carbon abundances can be derived for
solar-type stars from the CH band at 4300\AA. We confirm two different slope
trends for [C/Fe] with [Fe/H] because the behaviour is opposite for stars above
and below solar values. We observe a flat distribution of the [C/Fe] ratio for
all planetary masses, a finding that apparently excludes any clear connection
between the [C/Fe] abundance ratio and planetary mass.Comment: 10 pages, 10 figures. Accepted to A&
From circular paths to elliptic orbits: A geometric approach to Kepler's motion
The hodograph, i.e. the path traced by a body in velocity space, was
introduced by Hamilton in 1846 as an alternative for studying certain dynamical
problems. The hodograph of the Kepler problem was then investigated and shown
to be a circle, it was next used to investigate some other properties of the
motion. We here propose a new method for tracing the hodograph and the
corresponding configuration space orbit in Kepler's problem starting from the
initial conditions given and trying to use no more than the methods of
synthetic geometry in a sort of Newtonian approach. All of our geometric
constructions require straight edge and compass only.Comment: 9 pages, 4 figure
C/O vs Mg/Si ratios in solar type stars: The HARPS sample
Aims. We present a detailed study of the Mg/Si and C/O ratios and their
importance in determining the mineralogy of planetary companions. Methods.
Using 499 solar-like stars from the HARPS sample, we determine C/O and Mg/Si
elemental abundance ratios to study the nature of the possible planets formed.
We separated the planetary population in low-mass planets ( < 30 ) and high-mass planets ( > 30 ) to test for possible
relation with the mass. Results. We find a diversity of mineralogical ratios
that reveal the different kinds of planetary systems that can be formed, most
of them dissimilar to our solar system. The different values of the Mg/Si and
C/O ratios can determine different composition of planets formed. We found that
100\% of our planetary sample present C/O < 0.8. 86\% of stars with high-mass
companions present 0.8 > C/O > 0.4, while 14\% present C/O values lower than
0.4. Regarding Mg/Si, all stars with low-mass planetary companion showed values
between 1 and 2, while 85% of the high-mass companion sample does. The other
15\% showed Mg/Si values below 1. No stars with planets were found with Mg/Si >
2. Planet hosts with low-mass companions present C/O and Mg/Si ratios similar
to those found in the Sun, whereas stars with high-mass companions have lower
C/O.Comment: 9 pages, 12 figues. Accepted in A&
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