7,418 research outputs found
High-resolution abundance analysis of HD 140283
HD 140283 is a reference subgiant that is metal poor and confirmed to be a
very old star. The abundances of this type of old star can constrain the nature
and nucleosynthesis processes that occurred in its (even older) progenitors.
The present study may shed light on nucleosynthesis processes yielding heavy
elements early in the Galaxy. A detailed abundance analysis of a high-quality
spectrum is carried out, with the intent of providing a reference on stellar
lines and abundances of a very old, metal-poor subgiant. We aim to derive
abundances from most available and measurable spectral lines. The analysis is
carried out using high-resolution (R = 81 000) and high signal-to-noise ratio
(800 < S/N/pixel < 3400) spectrum, in the wavelength range 3700 - 10475,
obtained with a seven-hour exposure time, using the ESPaDOnS at the CFHT. The
calculations in LTE were performed with the OSMARCS 1D atmospheric model and
the spectrum synthesis code Turbospectrum, while the analysis in NLTE is based
on the MULTI code. We present LTE abundances for 26 elements, and NLTE
calculations for the species C I, O I, Na I, Mg I, Al I, K I, Ca I, Sr II, and
Ba II lines. The abundance analysis provided an extensive line list suitable
for metal-poor subgiant stars. The results for Li, CNO, alpha-, and iron peak
elements are in good agreement with literature. The newly NLTE Ba abundance,
along with a NLTE Eu correction and a 3D Ba correction from literature, leads
to [Eu/Ba] = +0.59 +/- 0.18. This result confirms a dominant r-process
contribution, possibly together with a very small contribution from the main
s-process, to the neutron-capture elements in HD 140283. Overabundances of the
lighter heavy elements and the high abundances derived for Ba, La, and Ce
favour the operation of the weak r-process in HD 140283.Comment: 34 pages, 27 figure
Task planning and control synthesis for robotic manipulation in space applications
Space-based robotic systems for diagnosis, repair and assembly of systems will require new techniques of planning and manipulation to accomplish these complex tasks. Results of work in assembly task representation, discrete task planning, and control synthesis which provide a design environment for flexible assembly systems in manufacturing applications, and which extend to planning of manipulatiuon operations in unstructured environments are summarized. Assembly planning is carried out using the AND/OR graph representation which encompasses all possible partial orders of operations and may be used to plan assembly sequences. Discrete task planning uses the configuration map which facilitates search over a space of discrete operations parameters in sequential operations in order to achieve required goals in the space of bounded configuration sets
Vacuum Polarization for a Massless Spin-1/2 Field in the Global Monopole Spacetime at Nonzero Temperature
In this paper we present the effects produced by the temperature in the
renormalized vacuum expectation value of the zero-zero component of the
energy-momentum tensor associated with massless left-handed spinor field in the
pointlike global monopole spacetime. In order to develop this calculation we
had to obtain the Euclidean thermal Green function in this background. Because
the expression obtained for the thermal energy density cannot be expressed in a
closed form, its explicit dependence on the temperature is not completely
evident. So, in order to obtain concrete information about its thermal
behavior, we develop a numerical analysis of our result in the high-temperature
limit for specific values of the parameter which codify the presence
of the monopole.Comment: 22 pages, LaTex format, 5 figure
On planetary mass determination in the case of super-Earths orbiting active stars. The case of the CoRoT-7 system
This investigation uses the excellent HARPS radial velocity measurements of
CoRoT-7 to re-determine the planet masses and to explore techniques able to
determine mass and elements of planets discovered around active stars when the
relative variation of the radial velocity due to the star activity cannot be
considered as just noise and can exceed the variation due to the planets. The
main technique used here is a self-consistent version of the high-pass filter
used by Queloz et al. (2009) in the first mass determination of CoRoT-7b and
CoRoT-7c. The results are compared to those given by two alternative
techniques: (1) The approach proposed by Hatzes et al. (2010) using only those
nights in which 2 or 3 observations were done; (2) A pure Fourier analysis. In
all cases, the eccentricities are taken equal to zero as indicated by the study
of the tidal evolution of the system; the periods are also kept fixed at the
values given by Queloz et al. Only the observations done in the time interval
BJD 2,454,847 - 873 are used because they include many nights with multiple
observations; otherwise it is not possible to separate the effects of the
rotation fourth harmonic (5.91d = Prot/4) from the alias of the orbital period
of CoRoT-7b (0.853585 d). The results of the various approaches are combined to
give for the planet masses the values 8.0 \pm 1.2 MEarth for CoRoT-7b and 13.6
\pm 1.4 MEarth for CoRoT 7c. An estimation of the variation of the radial
velocity of the star due to its activity is also given.The results obtained
with 3 different approaches agree to give masses larger than those in previous
determinations. From the existing internal structure models they indicate that
CoRoT-7b is a much denser super-Earth. The bulk density is 11 \pm 3.5 g.cm-3 .
CoRoT-7b may be rocky with a large iron core.Comment: 12 pages, 11 figure
Conductance peaks in open quantum dots
We present a simple measure of the conductance fluctuations in open ballistic
chaotic quantum dots, extending the number of maxima method originally proposed
for the statistical analysis of compound nuclear reactions. The average number
of extreme points (maxima and minima) in the dimensionless conductance, , as
a function of an arbitrary external parameter , is directly related to the
autocorrelation function of . The parameter can be associated to an
applied gate voltage causing shape deformation in quantum dot, an external
magnetic field, the Fermi energy, etc.. The average density of maxima is found
to be , where is a universal constant
and is the conductance autocorrelation length, which is system specific.
The analysis of does not require large statistic samples,
providing a quite amenable way to access information about parametric
correlations, such as .Comment: 5 pages, 5 figures, accepted to be published - Physical Review
Letter
Intensity correlations in electronic wave propagation in a disordered medium: the influence of spin-orbit scattering
We obtain explicit expressions for the correlation functions of transmission
and reflection coefficients of coherent electronic waves propagating through a
disordered quasi-one-dimensional medium with purely elastic diffusive
scattering in the presence of spin-orbit interactions. We find in the metallic
regime both large local intensity fluctuations and long-range correlations
which ultimately lead to universal conductance fluctuations. We show that the
main effect of spin-orbit scattering is to suppress both local and long-range
intensity fluctuations by a universal symmetry factor 4. We use a scattering
approach based on random transfer matrices.Comment: 15 pages, written in plain TeX, Preprint OUTP-93-42S (University of
Oxford), to appear in Phys. Rev.
Electrostatic self-force in (2+1)-dimensional cosmological gravity
Point sources in (2+1)-dimensional gravity are conical singularities that
modify the global curvature of the space giving rise to self-interaction
effects on classical fields. In this work we study the electrostatic
self-interaction of a point charge in the presence of point masses in
(2+1)-dimensional gravity with a cosmological constant.Comment: 9 pages, Late
Path Integral Approach to the Scattering Theory of Quantum Transport
The scattering theory of quantum transport relates transport properties of
disordered mesoscopic conductors to their transfer matrix \bbox{T}. We
introduce a novel approach to the statistics of transport quantities which
expresses the probability distribution of \bbox{T} as a path integral. The
path integal is derived for a model of conductors with broken time reversal
invariance in arbitrary dimensions. It is applied to the
Dorokhov-Mello-Pereyra-Kumar (DMPK) equation which describes
quasi-one-dimensional wires. We use the equivalent channel model whose
probability distribution for the eigenvalues of \bbox{TT}^{\dagger} is
equivalent to the DMPK equation independent of the values of the forward
scattering mean free paths. We find that infinitely strong forward scattering
corresponds to diffusion on the coset space of the transfer matrix group. It is
shown that the saddle point of the path integral corresponds to ballistic
conductors with large conductances. We solve the saddle point equation and
recover random matrix theory from the saddle point approximation to the path
integral.Comment: REVTEX, 9 pages, no figure
Avaliação de protocolos para extração de DNA de Larvas individuais de helmintos.
A genotipagem de helmintos, parasitas do trato gastrintestinal de ruminantes, permite o conhecimento de polimorfismos moleculares relacionados a características adaptativas, dentre as quais se destaca a resistência a produtos anti-helmínticos. Uma vez que a determinação da freqüência gênica e da freqüência aléIica para os polimorfismos de interesse requer a avaliação individual dos helmintos, dificuldades técnicas têm sido encontradas para a recuperação de quantidade suficiente de DNA a partir de larvas para a aplicação de técnicas moleculares. Dessa maneira, o objetivo deste trabalho foi avaliar diferentes protocolos para a extração de DNA de larvas individuais de helmintos Haemonchus contortus e Trichostrongylus colubriformis no estádio L3. Após cultura de fezes de ovinos, por sete dias, as larvas de helmintos foram recuperadas e destinadas á extração de DNA
- …