548 research outputs found
17 new very low-mass members in Taurus. The brown dwarf deficit revisited
Recent studies of the substellar population in the Taurus cloud have revealed
a deficit of brown dwarfs (BD) compared to the Trapezium cluster population
(Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However,
these works have concentrated on the highest stellar density regions of the
Taurus cloud. We have performed a large scale optical survey of this region,
covering a total area of 30 deg^2, and encompassing the densest part of the
cloud as well as their surroundings, down to a mass detection limits of 15
Jupiter Masses (MJ). In this paper, we present the optical spectroscopic
follow-up observations of 97 photometrically selected potential new low-mass
Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These
observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs.
Combining our observations with previously published results, we derive an
updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio
now appears consistent with the value previously derived in the Trapezium
cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication
that the relative numbers of BDs with respect to stars is decreased by a factor
2 in the central regions of the aggregates with respect to the more distributed
population. Our findings are best explained in the context of the
embryo-ejection model where brown dwarfs originate from dynamical interactions
in small N unstable multiple systems.Comment: 20 pages, 15 figure
A Serravallian (Middle Miocene) shark fauna from Southeastern Spain and its palaeoenvironment significance
The study of a new Serravallian (Middle Miocene) locality from the Southeastern Spain has yielded a shark assemblage characterized by microremains of at least seven taxa (Deania calcea, ÂżIsistius triangulus, ÂżSqualiolus cf. S. schaubi, ÂżParaetmopterus sp., Pristiophorus sp., Scyliorhinus sp. and a cf. Squaliformes indet) of three different orders (Squaliformes, Pristiophoriformes and Carcharhiniformes). In addition, associated macroremains have also been found, including teeth of ÂżCosmopolitodus hastalis, Isurus sp., Hemipristis serra, Odontaspis sp., Carcharhinus spp. and ÂżOtodus (Megaselachus) megalodon. The assemblage contains taxa with disparate environmental preferences including not only neritic and epipelagic sharks but also an important number of meso and bathypelagic representatives. The migration of deep water taxa to shallower waters through submarine canyons/coastal upwelling is proposed as the most plausible cause for explaining the origin of such assemblage. Interestingly, the composition of the deep-water taxa here reported contrast with the chondrichthyans assemblages from the Pliocene and extant Mediterranean communities. This entails a complex biogeographic history, where the Messinian salinity crisis strongly affected the posterior evolution of the Mediterranean ecosystems but some other factors, such us the existence of anoxic events during the Quaternary, could have also played an important role
Library Design in Combinatorial Chemistry by Monte Carlo Methods
Strategies for searching the space of variables in combinatorial chemistry
experiments are presented, and a random energy model of combinatorial chemistry
experiments is introduced. The search strategies, derived by analogy with the
computer modeling technique of Monte Carlo, effectively search the variable
space even in combinatorial chemistry experiments of modest size. Efficient
implementations of the library design and redesign strategies are feasible with
current experimental capabilities.Comment: 5 pages, 3 figure
Vortex shear effects in layered superconductors
Motivated by recent transport and magnetization measurements in BSCCO samples
[B. Khaykovich et. al., Phys. Rev. B 61, R9261 (2000)], we present a simple
macroscopic model describing effects of inhomogeneous current distribution and
shear in a layered superconductor. Parameters of the model are deduced from a
microscopic calculation. Our model accounts for the strong current
non-linearities and the re-entrant temperature dependence observed in the
experiment.Comment: 11 pages, 7 figures, submitted to Phys. Rev.
A Possible Detection of Occultation by a Proto-planetary Clump in GM Cephei
GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an
abrupt variable and to have a circumstellar disk with very active accretion.
Our monitoring observations in 2009-2011 revealed the star to show sporadic
flare events, each with brightening of < 0.5 mag lasting for days. These
brightening events, associated with a color change toward the blue, should
originate from an increased accretion activity. Moreover, the star also
underwent a brightness drop of ~1 mag lasting for about a month, during which
the star became bluer when fainter. Such brightness drops seem to have a
recurrence time scale of a year, as evidenced in our data and the photometric
behavior of GM Cep over a century. Between consecutive drops, the star
brightened gradually by about 1 mag and became blue at peak luminosity. We
propose that the drop is caused by obscuration of the central star by an
orbiting dust concentration. The UX Orionis type of activity in GM Cep
therefore exemplifies the disk inhomogeneity process in transition between
grain coagulation and planetesimal formation in a young circumstellar disk.Comment: In submission to the Astrophysical Journal, 4 figure
X-ray emission from the young brown dwarfs of the Taurus Molecular Cloud
The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed
to systematically investigate the X-ray properties of young stellar/substellar
objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings
(of which three are archival observations), supplemented with one archival
Chandra observation, allows us to study 17 BDs with M spectral types. Half of
this sample (9 out of 17 BDs) is detected; 7 BDs are detected here for the
first time in X-rays. We observed a flare from one BD. We confirm several
previous findings on BD X-ray activity: a log-log relation between X-ray and
bolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; a
shallow log-log relation between X-ray fractional luminosity and mass; a
log-log relation between X-ray fractional luminosity and effective temperature;
a log-log relation between X-ray surface flux and effective temperature. We
find no significant log-log correlation between the X-ray fractional luminosity
and EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractional
luminosity. The median X-ray fractional luminosity of nonaccreting BDs is about
4 times lower than the mean saturation value for rapidly rotating low-mass
field stars. Our TMC BDs have higher X-ray fractional luminosity than BDs in
the Chandra Orion Ultradeep Project. The X-ray fractional luminosity declines
from low-mass stars to M-type BDs, and as a sample, the BDs are less efficient
X-ray emitters than low-mass stars. We thus conclude that while the BD
atmospheres observed here are mostly warm enough to sustain coronal activity, a
trend is seen that may indicate its gradual decline due to the drop in
photospheric ionization degree (abridged).Comment: 20 pages and 19 Figures. Accepted by A&A, to appear in a special
section/issue dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST). Preprint with higher resolution figures is available
at http://hal.ccsd.cnrs.fr/ccsd-0009049
Urinary Excretion of Mimosine Derivatives by Cows with and without Experience in Consumption of \u3cem\u3eLeucaena leucocephala\u3c/em\u3e
Leucaena leucocephala is a leguminous tree widely distributed in the tropical regions of the world. In Mexico, it has been incorporated into silvopastoral systems and is highly regarded, owing to its high content of crude protein. Nonetheless, L. leucocephala contains secondary metab-olites, such as mimosine, a non-protein free amino acid, which may induce toxic effects in unadapted ruminants that consume the forage (Hammond 1995). Although Synergistes jonesii, an anaerobic bacterium, has the ability to degrade 3,4-DHP and 2,3-DHP to non-toxic compounds (Allison et al. 1992), in Mexico its presence has not yet been confirmed. Recent work has suggested the occurrence of sub-clinical toxicity to 3,4-DHP and 2,3-DHP in cattle grazing L. leucocephala in Australia and Thailand (Graham 2007; Dalzell et al. 2012; Phaikaew et al. 2012). Several options such as the transfer of rumen liquor and the adaptation of ruminants to the intake of L. leucocephala have been studied in an attempt to reduce the excretion of mimosine and its metabolites (Palmer et al. 2010).
The aim of the present work was to evaluate the effect of the experience of consumption of L. leucocephala on excretion of mimosine derivatives (3,4-DHP and 2,3-DHP) in the urine of cattle
rac-1-Acetyl-5-benzyl-2-thioxoimidazolidin-4-one
In the title compound, C12H12N2O2S, the molÂecules have a wing-like conformation, with a distance of 3.797â
(2)â
Ă
between the centroids of the five- and six-membered rings. In the crystal structure, molÂecules are linked by NâHâŻO hydrogen bonds, forming infinite one-dimensional zigzag chains, running along [001], with a C(4) graph-set motif
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