10,875 research outputs found
An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on the strong
emission lines of [SII] and [SIII] in the red part of the spectrum through the
definition of a sulphur abundance parameter S23. This calibration presents two
important advantages against the commonly used one based on the optical oxygen
lines: it remains single-valued up to abundances close to solar and is rather
independent of the degree of ionization of the nebula.Comment: 10 pages, 8 figures. Accepted for publication in MNRA
Numerical study of relaxation in electron glasses
We perform a numerical simulation of energy relaxation in three-dimensional
electron glasses in the strongly localized regime at finite temperatures. We
consider systems with no interactions, with long-range Coulomb interactions and
with short-range interactions, obtaining a power law relaxation with an
exponent of 0.15, which is independent of the parameters of the problem and of
the type of interaction. At very long times, we always find an exponential
regime whose characteristic time strongly depends on temperature, system size,
interaction type and localization radius. We extrapolate the longest relaxation
time to macroscopic sizes and, for interacting samples, obtain values much
larger than the measuring time. We finally study the number of electrons
participating in the relaxation processes of very low energy configurations.Comment: 6 eps figures. To be published in Phys. Rev.
Length-weight relationships of coral reef fishes from the Alacran Reef, Yucatan, Mexico
Length-weight relationships were computed for 42 species of coral reef fishes from 14 families from the Alacran Reef (Yucatan, Mexico). A total of 1 892 individuals was used for this purpose. The fish species were caught by different fishing techniques such as fishhooks, harpoons, gill and trawl nets. The sampling period was from March 1998 to January 2000
Mesohabitat Associations Of The Devil Tryonia, Tryonia Diaboli (Gastropoda: Truncatelloidea: Cochliopidae)
The Cochliopidae of Texas include both stygobitic species, those that occupy only underground habitats, and epigean species, those living only in aboveground habitats. The devil tryonia, Tryonia diaboli, was described from the Devils River of Texas from river wrack, without additional habitat information. This species has been largely ignored since its description, so details of its habitat and ecology are obscure. In Dolan Springs and Finegan Springs, flowing into the Devils River, we sampled macroinvertebrates at five sites that form a gradient from the aquifer to the mainstem Devils River. We found the highest abundances of T. diaboli in aquifer samples, decreasing sharply downstream from the spring orifice. Our findings indicate that T. diaboli is stygophilic, occupying a transitional area including the aquifer as well as aboveground portions of springs
Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association
We present the discovery of a companion near the deuterium burning mass limit
located at a very wide distance, at an angular separation of 4.6+/-0.1 arcsec
(projected distance of ~ 670 AU) from UScoCTIO108, a brown dwarf of the very
young Upper Scorpius association. Optical and near-infrared photometry and
spectroscopy confirm the cool nature of both objects, with spectral types of M7
and M9.5, respectively, and that they are bona fide members of the association,
showing low gravity and features of youth. Their masses, estimated from the
comparison of their bolometric luminosities and theoretical models for the age
range of the association, are 60+/-20 and 14^{+2}_{-8} MJup, respectively. The
existence of this object around a brown dwarf at this wide orbit suggests that
the companion is unlikely to have formed in a disk based on current planet
formation models. Because this system is rather weakly bound, they did not
probably form through dynamical ejection of stellar embryos.Comment: 10 pages, including 4 figures and 2 table
On the ongoing multiple blowout in NGC 604
Several facts regarding the structure of NGC 604 are examined here. The three
main cavities, produced by the mechanical energy from massive stars which in
NGC 604 are spread over a volume of 10 pc, are shown here to be
undergoing blowout into the halo of M33. High resolution long slit spectroscopy
is used to track the impact from massive stars while HST archive data is used
to display the asymmetry of the nebula.
NGC 604 is found to be a collection of photoionized filaments and sections of
shells in direct contact with the thermalized matter ejected by massive stars.
The multiple blowout events presently drain the energy injected by massive
stars and thus the densest photoionized gas is found almost at rest and is
expected to suffer a slow evolution.Comment: 15 pages (11 text), 4 figures. To be published in Ap
Ultra Low-Power Analog Median Filters
The design and implementation of three analog median filter topologies, whose transistors operate in the deep weak-inversion region, is described. The first topology is a differential pairs array, in which drain currents are driven into two nodes in a differential fashion, while the second topology is based on a wide range OTA, which is used to maximize the dynamic range. Finally, the third topology uses three range-extended OTAs. The proposed weak-inversion filters were designed and fabricated in ON Semiconductor 0.5 micrometer technology through MOSIS. Experimental results of three-input fabricated prototypes for all three topologies are show, where power consumptions of 90nW in the first case, and 270nW in the other two cases can be noticed. A dual power supply +/-1.5 Volts were used
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