880 research outputs found
Stellar Dynamics at the Galactic Center with an Extremely Large Telescope
We discuss experiments achievable via monitoring of stellar dynamics near the
massive black hole at the Galactic center with a next generation, extremely
large telescope (ELT). Given the likely observational capabilities of an ELT
and current knowledge of the stellar environment at the Galactic center, we
synthesize plausible samples of stellar orbits around the black hole. We use
the Markov Chain Monte Carlo method to evaluate the constraints that orbital
monitoring places on the matter content near the black hole. Results are
expressed as functions of the number N of stars with detectable orbital motions
and the astrometric precision dtheta and spectroscopic precision dv at which
stellar proper motions and radial velocities are monitored. For N = 100, dtheta
= 0.5 mas, and dv = 10 km/s -- a conservative estimate of the capabilities of a
30 meter telescope -- the extended matter distribution enclosed by the orbits
will produce measurable deviations from Keplerian motion if >1000 Msun is
enclosed within 0.01 pc. The black hole mass and distance to the Galactic
center will be measured to better than ~0.1%. Lowest-order relativistic
effects, such as the prograde precession, will be detectable if dtheta < 0.5
mas. Higher-order effects, including frame dragging due to black hole spin,
requires dtheta < 0.05 mas, or the favorable discovery of a compact, highly
eccentric orbit. Finally, we calculate the rate at which monitored stars
undergo detectable nearby encounters with background stars. Such encounters
probe the mass function of stellar remnants that accumulate near the black
hole. We find that ~30 encounters will be detected over a 10 yr baseline for
dtheta = 0.5 mas.Comment: 14 pages, 5 figures; discussion no longer aperture-specific (TMT ->
ELT), matches ApJ versio
Characterization of solar cells for space applications. Volume 11: Electrical characteristics of 2 ohm-cm, 228 micron wraparound solar cells as a function of intensity, temperature, and irradiation
Parametric characterization data on Spectrolab 2 by 4 cm, 2 ohm/cm, 228 micron thick wraparound cell, a candidate for the Solar Electric Propulsion Mission, are presented. These data consist of the electrical characteristics of the solar cell under a wide range of temperature and illumination intensity combinations of the type encountered in space applications
Geotecnologias como apoio à remedição de parcelas permanentes em Floresta Ombrófila Mista.
Resumo
Estimativa de parâmetros de crescimento, produção e dinâmica de um fragmento de Floresta com Araucária usando dados de parcelas permanentes.
Editores técnicos: Marcílio José Thomazini, Elenice Fritzsons, Patrícia Raquel Silva, Guilherme Schnell e Schuhli, Denise Jeton Cardoso, Luziane Franciscon. EVINCI. Resumos
Characterization of solar cells for space applications. Volume 8: Electrical characteristics of Spectrolab BSF, BSR, textured 290-micron solar cells (K7) as a function of intensity, temperature and irradiation
A set of parametric data is presented on the Spectrolab textured, back-surface-field, back-surface-reflector solar cell which is a commercially available product
Characterization of solar cells for space applications. Volume 12: Electrical characteristics of Solarex BSF, 2-ohm-cm, 50-micron solar cells (1978 pilot line) as a function of intensity, temperature, and irradiation
Electrical characteristics of Solarex back-surface-field, 2-ohm-cm, 50-micron N/P silicon solar cells are presented in graphical and tabular format as a function of solar illumination intensity, temperature, and irradiation
Characterization of solar cells for space applications. Volume 13: Electrical characteristics of Hughes LPE gallium arsenide solar cells as a function of intensity and temperature
Electrical characteristics of Hughes Liquid phase epitaxy, P/N gallium aluminum arsenide solar cells are presented in graphical and tabular format as a function of solar illumination intensity and temperature
High-Spatial Resolution SED of NGC 1068 from Near-IR to Radio. Disentangling the thermal and non-thermal contributions
We investigate the ideas that a sizable fraction of the interferometrically
unresolved infrared emission of the nucleus of NGC 1068 might originate from
other processes than thermal dust emission from the torus. We examine the
contribution of free-free or synchrotron emissions to the central mid- and
near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained
with parsec scale radio data available for NGC 1068 in the 10^9 - 10^11 Hz
regime, and compared to the highest-resolution interferometric data available
in the mid-infrared. It is shown that the unresolved emission in the
interferometric observation (<~1pc) is still dominatedd by dust emission and
not by contributions from synchrotron or free-free emission. As recent studies
suggest, the interferometric observations prefer a clumpy structure of the dust
distribution. Extrapolation of the radio free-free or synchrotron emission to
the IR indicates that their contribution is <20% even for the unresolved
fraction of the interferometric flux. The slope of the available radio data is
consistent with a power law exponent alpha = 0.29 +/- 0.07 which we interprete
in terms of either free-free emission or synchrotron radiation from
quasi-monochromatic electrons. We apply emission models for both mechanisms in
order to obtain physical parameters. (abridged)Comment: 8 pages, 3 figures; accepted by A&
The innermost region of AGN tori: implications from the HST/NICMOS Type 1 point sources and near-IR reverberation
Spatially resolving the innermost torus in AGN is one of the main goals of
its high-spatial-resolution studies. This could be done in the near-IR
observations of Type 1 AGNs where we see directly the hottest dust grains in
the torus. We discuss two critical issues in such studies. Firstly, we examine
the nuclear point sources in the HST/NICMOS images of nearby Type 1 AGNs, to
evaluate the possible contribution from the central putative accretion disk.
After a careful subtraction of host bulge flux, we show that near-IR colors of
the point sources appear quite interpretable simply as a composite of a
black-body-like spectrum and a relatively blue distinct component as expected
for a torus and an accretion disk, respectively. Our radiative transfer models
for clumpy tori also support this simple two-component interpretation. The
observed near-IR colors suggest a fractional accretion disk contribution of
~25% or less at 2.2 micron. Secondly, we show that the innermost torus radii as
indicated by the recent near-IR reverberation measurements are systematically
smaller by a factor of ~3 than the predicted dust sublimation radius with a
reasonable assumption for graphite grains of sublimation temperature 1500 K and
size 0.05 micron in radius. The discrepancy might indicate a much higher
sublimation temperature or a typical grain size being much larger in the
innermost tori, though the former case appears to be disfavored by the observed
colors of the HST point sources studied above. The near-IR interferometry with
a baseline of ~100 m should be able to provide the important, independent size
measurements, based on the low accretion disk contribution obtained above.Comment: accepted for publication in A&
Yeast translation elongation factor eEF3 promotes late stages of tRNA translocation
In addition to the conserved translation elongation factors eEF1A and eEF2, fungi require a third essential elongation factor, eEF3. While eEF3 has been implicated in tRNA binding and release at the ribosomal A and E sites, its exact mechanism of action is unclear. Here, we show that eEF3 acts at the mRNA–tRNA translocation step by promoting the dissociation of the tRNA from the E site, but independent of aminoacyl-tRNA recruitment to the A site. Depletion of eEF3 in vivo leads to a general slowdown in translation elongation due to accumulation of ribosomes with an occupied A site. Cryo-EM analysis of native eEF3-ribosome complexes shows that eEF3 facilitates late steps of translocation by favoring non-rotated ribosomal states, as well as by opening the L1 stalk to release the E-site tRNA. Additionally, our analysis provides structural insights into novel translation elongation states, enabling presentation of a revised yeast translation elongation cycle
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