8,142 research outputs found
Photospheric activity, rotation, and star-planet interaction of the planet-hosting star CoRoT-6
The CoRoT satellite has recently discovered a hot Jupiter that transits
across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We
model the photospheric activity of the star and use the maps of the active
regions to study stellar differential rotation and the star-planet interaction.
We apply a maximum entropy spot model to fit the optical modulation as observed
by CoRoT during a uninterrupted interval of about 140 days. Photospheric active
regions are assumed to consist of spots and faculae in a fixed proportion with
solar-like contrasts. Individual active regions have lifetimes up to 30-40
days. Most of them form and decay within five active longitudes whose different
migration rates are attributed to the stellar differential rotation for which a
lower limit of \Delta \Omega / \Omega = 0.12 \pm 0.02 is obtained. Several
active regions show a maximum of activity at a longitude lagging the
subplanetary point by about 200 degrees with the probability of a chance
occurrence being smaller than 1 percent. Our spot modelling indicates that the
photospheric activity of CoRoT-6 could be partially modulated by some kind of
star-planet magnetic interaction, while an interaction related to tides is
highly unlikely because of the weakness of the tidal force.Comment: 9 pages, 7 figures, accepted to Astronomy & Astrophysic
Excitation of the GDR and the Compressional Isoscalar Dipole State by alpha scattering
The excitation of the isovector giant dipole resonance (GDR) by alpha
scattering is investigated as a method of probing the neutron excess in exotic
nuclei. DWBA calculations are presented for 28O and 70Ca and the interplay of
Coulomb and nuclear excitation is discussed. Since the magnitude of the Coulomb
excitation amplitude is strongly influenced by the Q-value, the neutron excess
plays an important role, as it tends to lower the energy of the GDR. The
excitation of the compressional isoscalar dipole state in 70Ca by alpha
scattering is also investigated. It is shown that the population of this latter
state may be an even more sensitive probe of the neutron skin than the
isovector GDR.Comment: 7 pages, 5 figures, Latex2
Excitation of Pygmy Dipole Resonance in neutron-rich nuclei via Coulomb and nuclear fields
We study the nature of the low-lying dipole strength in neutron-rich nuclei,
often associated to the Pygmy Dipole Resonance. The states are described within
the Hartree-Fock plus RPA formalism, using different parametrizations of the
Skyrme interaction. We show how the information from combined reactions
processes involving the Coulomb and different mixtures of isoscalar and
isovector nuclear interactions can provide a clue to reveal the characteristic
features of these states.Comment: 9 Pages, 8 figures, contribution to International Symposium On
Nuclear Physics, December 8-12, 2009,Bhabha Atomic Research Centre, Mumbai,
Indi
A condition-specific codon optimization approach for improved heterologous gene expression in Saccharomyces cerevisiae
All authors are with the Department of Chemical Engineering, The University of Texas at Austin, 200 E Dean Keeton St. Stop C0400, Austin, TX 78712, USA -- Hal S. Alper is with the Institute for Cellular and Molecular Biology, The University of Texas at Austin, 2500 Speedway Avenue, Austin, TX 78712, USA
-- Amanda M. Lanza Current Address: Bristol-Myers Squibb, Biologics Development, 35 South Street, Hopkinton, MA 01748, USABackground: Heterologous gene expression is an important tool for synthetic biology that enables metabolic engineering and the production of non-natural biologics in a variety of host organisms. The translational efficiency of heterologous genes can often be improved by optimizing synonymous codon usage to better match the host organism. However, traditional approaches for optimization neglect to take into account many factors known to influence synonymous codon distributions. Results: Here we define an alternative approach for codon optimization that utilizes systems level information and codon context for the condition under which heterologous genes are being expressed. Furthermore, we utilize a probabilistic algorithm to generate multiple variants of a given gene. We demonstrate improved translational efficiency using this condition-specific codon optimization approach with two heterologous genes, the fluorescent protein-encoding eGFP and the catechol 1,2-dioxygenase gene CatA, expressed in S. cerevisiae. For the latter case, optimization for stationary phase production resulted in nearly 2.9-fold improvements over commercial gene optimization algorithms. Conclusions: Codon optimization is now often a standard tool for protein expression, and while a variety of tools and approaches have been developed, they do not guarantee improved performance for all hosts of applications. Here, we suggest an alternative method for condition-specific codon optimization and demonstrate its utility in Saccharomyces cerevisiae as a proof of concept. However, this technique should be applicable to any organism for which gene expression data can be generated and is thus of potential interest for a variety of applications in metabolic and cellular engineering.Chemical EngineeringInstitute for Cellular and Molecular [email protected]
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
Daily variability of Ceres' Albedo detected by means of radial velocities changes of the reflected sunlight
Bright features have been recently discovered by Dawn on Ceres, which extend
previous photometric and Space Telescope observations. These features should
produce distortions of the line profiles of the reflected solar spectrum and
therefore an apparent radial velocity variation modulated by the rotation of
the dwarf planet. Here we report on two sequences of observations of Ceres
performed in the nights of 31 July, 26-27 August 2015 by means of the
high-precision HARPS spectrograph at the 3.6-m La Silla ESO telescope. The
observations revealed a quite complex behaviour which likely combines a radial
velocity modulation due to the rotation with an amplitude of approx +/- 6 m/s
and an unexpected diurnal effect. The latter changes imply changes in the
albedo of Occator's bright features due to the blaze produced by the exposure
to solar radiation. The short-term variability of Ceres' albedo is on
timescales ranging from hours to months and can both be confirmed and followed
by means of dedicated radial velocity observations.Comment: 5 pag, 1fig, two tables, MNRAS Letters 201
Isoscalar and isovector dipole excitations: Nuclear properties from low-lying states and from the isovector giant dipole resonance
Abstract This review paper concerns the research devoted to the study of the properties of dipole excitations in nuclei. The main focus is on questions related to isospin effects in these types of excitations. Particular attention is given to the experimental and theoretical efforts made to understand the nature and the specific structure of the low-lying dipole states known as the Pygmy Dipole Resonance (PDR). The main experimental methods employed in the study of the PDR are reviewed as well as the most interesting theoretical aspects. The main features of the experiments and of theoretical models are reported with special emphasis on the reaction cross sections populating the dipole states. Results are organized for nuclei according to different mass regions. The knowledge of the isovector dipole response as well as its low energy part is important in order to deduce the nuclear polarizability as accurate as possible. This issue is discussed in this paper together with the connection with the neutron skin and the nuclear equation of state. The important role played by the dipole response to deduce other physical quantities of general interest is discussed in the last two chapters. One concerns the level density and the other the isospin mixing in nuclei at finite temperature and its relation with beta decay
Excitations of pygmy dipole resonances in exotic and stable nuclei via Coulomb and nuclear fields
We study heavy-ion inelastic scattering processes in neutron-rich nuclei including the full response to the different multipolarities. Among these we focus in particular on the excitation of low-lying dipole states commonly associated to the pygmy dipole resonance. The multipole response is described within the Hartree-Fock plus RPA formalism with Skyrme interaction. We show how the combined information from reactions processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states. We have performed calculation for the excitation of 132Sn generated in the reactions with 4He, 40Ca, and 48Ca at several incident energies, as well as for the system 17O +208Pb. Our results suggest that the investigation of the PDR states can be better carried out at low incident energies (below 50 MeV/nucleon). In fact, at these energies the PDR peak is relatively stronger than the giant dipole resonance (GDR) one and the narrow width of the low-lying quadrupole and octupole states should not blur its presence.Ministerio de Ciencia e Innovación (España) y FEDER FPA2009-07653 FIS2008-04189Programa Consolider-Ingenio 2010 (España) CSD2007-00042Junta de Andalucía P07-FQM-02894 FQM16
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