459,378 research outputs found
Stability and mode analysis of solar coronal loops using thermodynamic irreversible energy principles
We study the modes and stability of non - isothermal coronal loop models with
different intensity values of the equilibrium magnetic field. We use an energy
principle obtained via non - equilibrium thermodynamic arguments. The principle
is expressed in terms of Hermitian operators and allow to consider together the
coupled system of equations: the balance of energy equation and the equation of
motion. We determine modes characterized as long - wavelength disturbances that
are present in inhomogeneous media. This character of the system introduces
additional difficulties for the stability analysis because the inhomogeneous
nature of the medium determines the structure of the disturbance, which is no
longer sinusoidal. Moreover, another complication is that we obtain a
continuous spectrum of stable modes in addition to the discrete one. We obtain
a unique unstable mode with a characteristic time that is comparable with the
characteristic life-time observed for loops. The feasibility of wave-based and
flow-based models is examined.Comment: 29 pages 10 figure
Hypocotyl adventitious root organogenesis differs from lateral root development
Wound-induced adventitious root (AR) formation is a requirement for plant survival upon root damage inflicted by pathogen attack, but also during the regeneration of plant stem cuttings for clonal propagation of elite plant varieties. Yet, adventitious rooting also takes place without wounding. This happens for example in etiolated Arabidopsis thaliana hypocotyls, in which AR initiate upon de-etiolation or in tomato seedlings, in which AR initiate upon flooding or high water availability. In the hypocotyl AR originate from a cell layer reminiscent to the pericycle in the primary root (PR) and the initiated AR share histological and developmental characteristics with lateral roots (LRs). In contrast to the PR however, the hypocotyl is a determinate structure with an established final number of cells. This points to differences between the induction of hypocotyl AR and LR on the PR, as the latter grows indeterminately. The induction of AR on the hypocotyl takes place in environmental conditions that differ from those that control LR formation. Hence, AR formation depends on differentially regulated gene products. Similarly to AR induction in stem cuttings, the capacity to induce hypocotyl AR is genotype-dependent and the plant growth regulator auxin is a key regulator controlling the rooting response. The hormones cytokinins, ethylene, jasmonic acid, and strigolactones in general reduce the root-inducing capacity. The involvement of this many regulators indicates that a tight control and fine-tuning of the initiation and emergence of AR exists. Recently, several genetic factors, specific to hypocotyl adventitious rooting in A. thaliana, have been uncovered. These factors reveal a dedicated signaling network that drives AR formation in the Arabidopsis hypocotyl. Here we provide an overview of the environmental and genetic factors controlling hypocotyl-born AR and we summarize how AR formation and the regulating factors of this organogenesis are distinct from LR induction
How Strong is the Case for Dollarization in Costa Rica? A Note on the Business Cycle Comovements with the United States
We evaluate the proposal for official dollarization in Costa Rica by applying a new approach to measure the business cycle comovements with the United States. While the literature often focuses on the correlation of shocks, we point out that the response of each country to the shocks is also an important aspect of stabilization policy. We analyze whether Costa Rica and the United States share a common synchronized response to shocks, i.e. a common business cycle, using the Engle and Kozicki (1993) and Cubadda (1999, 2007) serial correlation common features tests, in a quarterly GDP data set from 1991 to 2008. Although we find some tendency towards common AR(p) structures and common long run trends, we reject the hypothesis that the two countries share a common business cycle. Based on this evidence, we conclude that official dollarization in Costa Rica would impede the efforts of its stabilization policy, despite the relatively high contemporaneous correlation of shocks.dollarization, business cycle comovement, serial correlation common feature, Central America, Costa Rica
How Strong is the Case for Dollarization in Costa Rica? A Note on the Business Cycle Comovements with the United States
We evaluate the proposal for official dollarization in Costa Rica by applying a new approach to measure the business cycle comovements with the United States. While the literature often focuses on the correlation of shocks, we point out that the response of each country to the shocks is also an important aspect of stabilization policy. We analyze whether Costa Rica and the United States share a common synchronized response to shocks, i.e. a common business cycle, using the Engle and Kozicki (1993) and Cubadda (1999, 2007) serial correlation common features tests, in a quarterly GDP data set from 1991 to 2008. Although we find some tendency towards common AR(p) structures and common long run trends, we reject the hypothesis that the two countries share a common business cycle. Based on this evidence, we conclude that official dollarization in Costa Rica would impede the efforts of its stabilization policy, despite the relatively high contemporaneous correlation of shocks.Dollarization, Business Cycle Comovement, Serial Correlation Common Feature, Central America, Costa Rica
Nebular abundances of southern symbiotic stars
We have calculated relative elemental abundances for a sample of 43 symbiotic
stars. Helium abundances and the relative elemental abundances N/O, Ne/O, Ar/O
were derived from new spectra collected in the optical range through low
dispersion spectroscopy. The He ionic abundances were derived taking into
account self-absorption effects in Balmer lines. We found that the symbiotic
stars in the galactic bulge have heavy element abundances showing the same wide
distribution as other bulge objects. In the galactic disk, the symbiotic stars
follow the abundance gradient as derived from different kinds of objects.Comment: 12 pages, 6 figures, A&A - accepte
Planetary nebulae in the inner Milky Way: new abundances
The study of planetary nebulae in the inner-disk and bulge gives important
information on the chemical abundances of elements such as He, N, O, Ar, Ne,
and on the evolution of these abundances, which is associated with the
evolution of intermediate-mass stars and the chemical evolution of the Galaxy.
We present accurate abundances of the elements He, N, S, O, Ar, and Ne for a
sample of 54 planetary nebulae located towards the bulge of the Galaxy, for
which 33 have the abundances derived for the first time. The abundances are
derived based on observations in the optical domain made at the National
Laboratory for Astrophysics (LNA, Brazil). The data show a good agreement with
other results in the literature, in the sense that the distribution of the
abundances is similar to those works.Comment: Accepted for publication in RevMexAA (29 pages, 15 figures, 7 tables,
uses rmaa.cls
Tax systems and tax reforms in Latin America: country studies
This paper is part of a wider research concerning taxation systems an reforms carried on ar the Departemnt of Pubic economics of the Uiversity of Pavia -I taly. These country studies refer to Argentina, Brazil, Chile, Costa Rica, Colombia, Mexico, Paraguay and Uruguay and are due to eminent native esperts. The papers depict and discuss the tax systems ad reforms in the aforementioned countries since the early 1990 to 2006.Tax Systems; Tas reofoerms; Latin American countries
Planetary nebulae in the inner Milky Way
New abundances of planetary nebulae located towards the bulge of the Galaxy
are derived based on observations made at LNA (Brazil). We present accurate
abundances of the elements He, N, S, O, Ar, and Ne for 56 PNe located towards
the galactic bulge. The data shows a good agreement with other results in the
literature, in the sense that the distribution of the abundances is similar to
those works. From the statistical analysis performed, we can suggest a
bulge-disk interface at 2.2 kpc for the intermediate mass population, marking
therefore the outer border of the bulge and inner border of the disk.Comment: 2 pages, 1 figure, uses iaus.cls, in press, IAU Symp. 265, Chemical
abundances in the Universe: Connecting the first Stars to Planets, Ed. K.
Cunha, M. Spite, B. Barbu
Readout of GEM Detectors Using the Medipix2 CMOS Pixel Chip
We have operated a Medipix2 CMOS readout chip, with amplifying, shaping and
charge discriminating front-end electronics integrated on the pixel-level, as a
highly segmented direct charge collecting anode in a three-stage gas electron
multiplier (Triple-GEM) to detect the ionization from Fe X-rays and
electrons from Ru. The device allows to perform moderate energy
spectroscopy measurements (20 % FWHM at 5.9 keV -rays) using only digital
readout and two discriminator thresholds. Being a truly 2D-detector, it allows
to observe individual clusters of minimum ionizing charged particles in
(70:30) and (70:30) mixtures and to achieve excellent
spatial resolution for position reconstruction of primary clusters down to
, based on the binary centroid determination method.Comment: 18 pages, 14 pictures. submitted to Nuclear Instruments and Methods
in Physics Research
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