2,145 research outputs found
Observations on an Isolated Population of Sagitta hispida Conant (Chaetognatha) in a Tropical Lagoon System of Northeast Yucatan (Mexico)
Monthly zooplankton collections were carried out from January to December 1991 at two sampling sites, Cuenca Norte and Bojórquez lagoon, in the Nichupté lagoon system, a partially enclosed network of interconnected waterways located in the northeastern region of the Yucatan Peninsula (Mexico) adjacent to the Caribbean Sea. Only one species of Chaetognatha, Sagitta hispida Conant, was present and was more abundant at Cuenca Norte (total density 450.6 organisms/m3) than at Bojórquez (138.6 organisms/m3). The latter site is smaller, more physically isolated, and more environmentally stressed than the former. From monthly gonadal and length-frequency analyses of 1390 specimens, it was found that (1) total length significantly differed among four successive maturity stages, (2) juvenile and immature specimens occurred in greater numbers at Bojórquez. while more mature specimens comprised a greater percentage of individuals found at Cuenca Norte, and (3) individuals collected at Bojórquez, where slightly higher temperatures were recorded, were significantly smaller than those from Cuenca Norte. The latter two findings indicate that Sagitta hispida spawns at a higher frequency at Bojórquez, possibly due to the cumulative effect of higher temperature
Observations on an Isolated Population of Sagitta hispida Conant (Chaetognatha) in a Tropical Lagoon System of Northeast Yucatan (Mexico)
Monthly zooplankton collections were carried out from January to December 1991 at two sampling sites, Cuenca Norte and Bojórquez lagoon, in the Nichupté lagoon system, a partially enclosed network of interconnected waterways located in the northeastern region of the Yucatan Peninsula (Mexico) adjacent to the Caribbean Sea. Only one species of Chaetognatha, Sagitta hispida Conant, was present and was more abundant at Cuenca Norte (total density 450.6 organisms/m3) than at Bojórquez (138.6 organisms/m3). The latter site is smaller, more physically isolated, and more environmentally stressed than the former. From monthly gonadal and length-frequency analyses of 1390 specimens, it was found that (1) total length significantly differed among four successive maturity stages, (2) juvenile and immature specimens occurred in greater numbers at Bojórquez. while more mature specimens comprised a greater percentage of individuals found at Cuenca Norte, and (3) individuals collected at Bojórquez, where slightly higher temperatures were recorded, were significantly smaller than those from Cuenca Norte. The latter two findings indicate that Sagitta hispida spawns at a higher frequency at Bojórquez, possibly due to the cumulative effect of higher temperature
Possible ring material around centaur (2060) Chiron
We propose that several short duration events observed in past stellar
occultations by Chiron were produced by rings material. From a reanalysis of
the stellar occultation data in the literature we determined two possible
orientations of the pole of Chiron's rings, with ecliptic coordinates
l=(352+/-10) deg, b=(37+/-10) deg or l=(144+/-10) deg, b=(24+/-10) deg . The
mean radius of the rings is (324 +/- 10) km. One can use the rotational
lightcurve amplitude of Chiron at different epochs to distinguish between the
two solutions for the pole. Both imply lower lightcurve amplitude in 2013 than
in 1988, when the rotational lightcurve was first determined. We derived
Chiron's rotational lightcurve in 2013 from observations at the 1.23-m CAHA
telescope and indeed its amplitude is smaller than in 1988. We also present a
rotational lightcurve in 2000 from images taken at CASLEO 2.15-m telescope that
is consistent with our predictions. Out of the two poles the l=(144+/-10) deg,
b=(24+/-10) deg solution provides a better match to a compilation of rotational
lightcurve amplitudes from the literature and those presented here. We also
show that using this preferred pole, Chiron's long term brightness variations
are compatible with a simple model that incorporates the changing brightness of
the rings as the tilt angle with respect to the Earth changes with time. Also,
the variability of the water ice band in Chiron's spectra in the literature can
be explained to a large degree by an icy ring system whose tilt angle changes
with time and whose composition includes water ice, analogously to the case of
Chariklo. We present several possible formation scenarios for the rings from
qualitative points of view and speculate on the reasons why rings might be
common in centaurs. We speculate on whether the known bimodal color
distribution of centaurs could be due to presence of rings and lack of them
Controlled nucleation of topological defects in the stripe domain patterns of Lateral multilayers with Perpendicular Magnetic Anisotropy: competition between magnetostatic, exchange and misfit interactions
Magnetic lateral multilayers have been fabricated on weak perpendicular
magnetic anisotropy amorphous Nd-Co films in order to perform a systematic
study on the conditions for controlled nucleation of topological defects within
their magnetic stripe domain pattern. A lateral thickness modulation of period
is defined on the nanostructured samples that, in turn, induces a lateral
modulation of both magnetic stripe domain periods and average
in-plane magnetization component . Depending on lateral multilayer
period and in-plane applied field, thin and thick regions switch independently
during in-plane magnetization reversal and domain walls are created within the
in-plane magnetization configuration coupled to variable angle grain boundaries
and disclinations within the magnetic stripe domain patterns. This process is
mainly driven by the competition between rotatable anisotropy (that couples the
magnetic stripe pattern to in-plane magnetization) and in-plane shape
anisotropy induced by the periodic thickness modulation. However, as the
structural period becomes comparable to magnetic stripe period ,
the nucleation of topological defects at the interfaces between thin and thick
regions is hindered by a size effect and stripe domains in the different
thickness regions become strongly coupled.Comment: 10 pages, 7 figures, submitted to Physical Review
Anomalous couplings for D-branes and O-planes
We study anomalous Wess-Zumino couplings of D-branes and O-planes in a
general background and derive them from a direct string computation by
factorizing in the RR channel various one-loop amplitudes. In particular, we
find that Op-planes present gravitational anomalous couplings involving the
Hirzebruch polynomial L, similarly to the roof genus A encoding Dp-brane
anomalous couplings. We determine, in each case, the precise dependence of
these couplings on the curvature of the tangent and normal bundles.Comment: 24 pages, LaTex, 5 figure
MacroH2A subtypes contribute antagonistically to the transcriptional regulation of the ribosomal cistron during seasonal acclimatization of the carp fish
<p>Abstract</p> <p>Background</p> <p>Incorporation of histone variants into chromatin is one of the epigenetic mechanisms used for regulation of gene expression. Macro (m)H2A is a histone variant that has two different subtypes in vertebrates: mH2A1 and mH2A2. It is known that mH2A is associated with gene silencing, but recent studies indicate that these mH2A subtypes could contribute more widely to transcriptional regulation. We have previously demonstrated that the gene-reprogramming response mediates adaptation of the carp fish to its environment, and that ribosomal gene transcription is seasonally regulated in carp. However, there have been few studies investigating how epigenetic mechanisms contribute to environmental adaptation and, in particular, to ribosomal cistron regulation.</p> <p>Results</p> <p>In this paper, we report the occurrence of differential incorporation of mH2A subtypes into chromatin during seasonal adaptation in the carp, an event that concurs with opposing transcriptional states. Moreover, we observed that enrichment of mH2A1 in the ribosomal cistron during winter, and conversely, enrichment of mH2A2 during summer. mH2A1 consistently colocalizes with a heterochromatin marker (H3K27me2; histone H3 trimethylated at lysine 27) and mH2A2 with a euchromatin marker (H3K4me3; histone H3 trimethylated at lysine 4). Similar results were found for the L41gene, with enrichment of mH2A in the promoter region.</p> <p>Conclusions</p> <p>We have characterized both mH2A subtypes from carp fish, and evaluated their participation in the regulation of the ribosomal cistron. Our findings indicate that differential incorporation of mH2A subtypes into the ribosome could regulate gene expression during the acclimatization process in carp. Our results reveal differential chromatin incorporation of the mH2A subtypes during the environmental adaptation process, correlating wtih antagonistic transcriptional states in the carp ribosomal cistron.</p
Visible and near-infrared observations of asteroid 2012 DA14 during its closest approach of February 15, 2013
Near-Earth asteroid 2012 DA14 made its closest approach on February 15, 2013,
when it passed at a distance of 27,700 km from the Earth's surface. It was the
first time an asteroid of moderate size was predicted to approach that close to
the Earth, becoming bright enough to permit a detailed study from ground-based
telescopes. Asteroid 2012 DA14 was poorly characterized before its closest
approach. We acquired data using several telescopes on four Spanish
observatories: the 10.4m Gran Telescopio Canarias (GTC) and the 3.6m Telescopio
Nazionale Galileo (TNG), both in the El Roque de los Muchachos Observatory
(ORM, La Palma); the 2.2m CAHA telescope, in the Calar Alto Observatory
(Almeria); the f/3 0.77m telescope in the La Hita Observatory (Toledo); and the
f/8 1.5m telescope in the Sierra Nevada Observatory (OSN, Granada). We obtained
visible and near-infrared color photometry, visible spectra and time-series
photometry. Visible spectra together with color photometry of 2012 DA14 show
that it can be classified as an L-type asteroid, a rare spectral type with a
composition similar to that of carbonaceous chondrites. The time-series
photometry provides a rotational period of 8.95 +- 0.08 hours after the closest
approach, and there are indications that the object suffered a spin-up during
this event. The large amplitude of the light curve suggests that the object is
very elongated and irregular, with an equivalent diameter of around 18m. We
obtain an absolute magnitude of H_R = 24.5 +- 0.2, corresponding to H_V = 25.0
+- 0.2. The GTC photometry also gives H_V = 25.29 +- 0.14. Both values agree
with the value listed at the Minor Planet Center shortly after discovery. From
the absolute photometry, together with some constraints on size and shape, we
compute a geometric albedo of p_V = 0.44 +- 0.20, which is slightly above the
range of albedos known for L-type asteroids (0.082 - 0.405).Comment: 7 pages, 4 figures, 1 table. Accepted in A&A (June 17 2013
Four tops for LHC
We design a search strategy for the Standard Model t¯tt¯tproduction at the LHC in the same-sign dilepton and trilepton channels. We study different signal features and, given the small expected number of signal events, we scrutinize in detail all reducible and irreducible backgrounds. Our analysis shows that by im-posing a basic set of jet and lepton selection criteria, the SM pp→t¯tt¯tprocess could be evidenced in the near future, within Run-II, when combining both multi-lepton search channels. We argue that this search strategy should also be used as a guideline to test New Physics coupling predominantly to top-quarks. In particular, we show that a non-resonant New Physics enhancement in the four-top final state would be de-tectable through this search strategy. We study two top-philicsimplified models of this kind, a neutral scalar boson and a Z0, and present current and future exclusion limits on their mass and couplings.Instituto de Física La Plat
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