Temperature and dissolved oxygen in lakes of the lower Oricono river floodplain (Venezuela)

Des mesures mensuelles ont été faites sur cinq lacs pendant deux ans. Des cycles nycthéméraux ont également été observés. Le gradient thermique vertical était généralement faible (de l'ordre de 1° C de la surface au fond) mais des différences jusqu'à 3° C ont pu être observées durant les hautes eaux dans des lacs. D'une façon générale, ces lacs peuvent être classées comme polymictiques chauds. Dans tous les lacs, l'oxygène dissous était corrélé négativement avec le niveau de l'eau, les lacs étant en général en dessous de la saturation. Une hypoxie s'est développée durant les hautes eaux dans les cinq lacs, persistant durant la décrue. En basses eaux, il n'y a pas de stratification d'oxygène, ce qui indique une circulation verticale de la masse d'eau. Celle-ci résulte de la faible profondeur du milieu qui favorise l'influence du vent et du refroidissement nocturne. En période de hautes eaux, lorsque le niveau monte, l'arrivée de l'eau dans les lacs réduit la probabilité de développement d'une stratification stable et durable. Cependant le faible gradient thermique peut s'opposer à une circulation pendant plusieurs semaines. La morphométrie des différents lacs de l'Orénoque apparait comme facteur déterminant dans la distribution de la température et de l'oxygène dissous. (Résumé d'auteur

Molecular gas associated with IRAS 10361-5830

We analyze the distribution of the molecular gas and the dust in the molecular clump linked to IRAS 10361-5830, located in the environs of the bubble-shaped HII region Gum 31 in the Carina region, with the aim of determining the main parameters of the associated material and investigating the evolutionary state of the young stellar objects identified there. Using the APEX telescope, we mapped the molecular emission in the J=3-2 transition of three CO isotopologues, 12CO, 13CO and C18O, over a 1.5' x 1.5' region around the IRAS position. We also observed the high density tracers CS and HCO+ toward the source. The cold dust distribution was analyzed using submillimeter continuum data at 870 \mu\ obtained with the APEX telescope. Complementary IR and radio data at different wavelengths were used to complete the study of the ISM. The molecular gas distribution reveals a cavity and a shell-like structure of ~ 0.32 pc in radius centered at the position of the IRAS source, with some young stellar objects (YSOs) projected onto the cavity. The total molecular mass in the shell and the mean H$_2$ volume density are ~ 40 solar masses and ~(1-2) x 10$^3$ cm$^{-3}$, respectively. The cold dust counterpart of the molecular shell has been detected in the far-IR at 870 \mu\ and in Herschel data at 350 \mu. Weak extended emission at 24 \mu\ from warm dust is projected onto the cavity, as well as weak radio continuum emission. A comparison of the distribution of cold and warm dust, and molecular and ionized gas allows us to conclude that a compact HII region has developed in the molecular clump, indicating that this is an area of recent massive star formation. Probable exciting sources capable of creating the compact HII region are investigated. The 2MASS source 10380461-5846233 (MSX G286.3773-00.2563) seems to be responsible for the formation of the HII region.Comment: Accepted in A&A. 11 pages, 10 Postscript figure

Hyperelastic cloaking theory: Transformation elasticity with pre-stressed solids

Transformation elasticity, by analogy with transformation acoustics and optics, converts material domains without altering wave properties, thereby enabling cloaking and related effects. By noting the similarity between transformation elasticity and the theory of incremental motion superimposed on finite pre-strain it is shown that the constitutive parameters of transformation elasticity correspond to the density and moduli of small-on-large theory. The formal equivalence indicates that transformation elasticity can be achieved by selecting a particular finite (hyperelastic) strain energy function, which for isotropic elasticity is semilinear strain energy. The associated elastic transformation is restricted by the requirement of statically equilibrated pre-stress. This constraint can be cast as \tr {\mathbf F} = constant, where $\mathbf{F}$ is the deformation gradient, subject to symmetry constraints, and its consequences are explored both analytically and through numerical examples of cloaking of anti-plane and in-plane wave motion.Comment: 20 pages, 5 figure

Helical structures from an isotropic homopolymer model

We present Monte Carlo simulation results for square-well homopolymers at a series of bond lengths. Although the model contains only isotropic pairwise interactions, under appropriate conditions this system shows spontaneous chiral symmetry breaking, where the chain exists in either a left- or a right-handed helical structure. We investigate how this behavior depends upon the ratio between bond length and monomer radius.Comment: 10 pages, 3 figures, accepted for publication by Physical Review Letter

Followup Observations of SDSS and CRTS Candidate Cataclysmic Variables

We present photometry of 11 and spectroscopy of 35 potential cataclysmic variables from the Sloan Digital Sky Survey, the Catalina Real-Time Transient Survey and vsnet-alerts. The photometry results include quasi-periodic oscillations during the decline of V1363 Cyg, nightly accretion changes in the likely Polar (AM Herculis binary) SDSS J1344+20, eclipses in SDSS J2141+05 with an orbital period of 76+/-2 min, and possible eclipses in SDSS J2158+09 at an orbital period near 100 min. Time-resolved spectra reveal short orbital periods near 80 min for SDSS J0206+20, 85 min for SDSS J1502+33, and near 100 min for CSS J0015+26, RXS J0150+37, SDSS J1132+62, SDSS J2154+15 and SDSS J2158+09. The prominent HeII line and velocity amplitude of SDSS J2154+15 are consistent with a Polar nature for this object, while the lack of this line and a low velocity amplitude argue against this classification for RXS J0150+37. Single spectra of 10 objects were obtained near outburst and the rest near quiescence, confirming the dwarf novae nature of these objects.Comment: 34 pages, 14 figures, in press at A

Variaciones morfologicas del rotifero Keratella americana (Carlin, 1943) de una laguna de inundacion del rio Orinoco, Venezuela

L'analyse dy rapport de la longueur de l'épine postérieure à celle de la lorica a montré que l'épine postérieure est significativement plus courte pendantles basses eaux que pendant la crue. Compte-tenu des facteurs habituellement associé au polymorphisme de #Brachionus et #Keratella, on émet l'hypothèse que la variation observée est liée à la disponibilité de la nourriture. Une longue épine postérieure serait un avantage en période de moindre nourriture (période de hautes eaux) tandis qu'à l'étiage une courte épine serait profitable quand la disponibilité des proies augmente. Il a aussi été observé que de faibles variations d'abondance d' #Asplanchna, prédateur du rotifère, affectent la densité de #K. americana, mais sans effet notable sur la longueur de l'épine. En se fondant sur la nomenclature existante existante pour #K. cochlearis, nous proposons la désignation de #K. americana Carlin, 1943 (f. #hispida nov. f.) comme super-espèce incluant lees formes #typica (longue épine postérieure), f. #tecta (sans épine postérieure) et, intermédiaires entre ces extrêmes, et en ordre de longueur d'épine décroissante, les formes #micracantha et #tuberculata$. (Résumé d'auteur