229 research outputs found
Nonlinear Higher Order Spectral Solution for a Two-Dimensional Moving Load on Ice
International audienceWe calculate the nonlinear response of an infinite ice sheet to a moving load in the time domain in two dimensions, using a higher-order spectral method. The nonlinearity is due to the moving boundary, as well as the nonlinear term in Bernoulli's equation and the elastic plate equation. We compare the nonlinear solution with the linear solution and with the nonlinear solution found by Parau & Dias (J. Fluid Mech., vol. 460, 2002, pp. 281–305). We find good agreement with both solutions (with the correction of an error in the Parau & Dias 2002 results) in the appropriate regimes. We also derive a solitary wavelike expression for the linear solution – close to but below the critical speed at which the phase speed has a minimum. Our model is carefully validated and used to investigate nonlinear effects. We focus in detail on the solution at a critical speed at which the linear response is infinite, and we show that the nonlinear solution remains bounded. We also establish that the inclusion of nonlinearities leads to significant new behaviour, which is not observed in the linear solution
Control of protein synthesis in yeast mitochondria: The concept of translational activators
AbstractMitochondria contain their own genome which codes for a small number of proteins. Most mitochondrial translation products are part of the membrane-embedded reaction centers of the respiratory chain complexes. In the yeast Saccharomyces cerevisiae, the expression of these proteins is regulated by translational activators that bind mitochondrial mRNAs, in most cases to their 5′-untranslated regions, and each mitochondrial mRNA appears to have its own translational activator(s). Recent studies showed that these translational activators can be part of feedback control loops which only permit translation if the downstream assembly of nascent translation products can occur. In several cases, the accumulation of a non-assembled protein prevents further synthesis of this protein but not translation in general. These control loops prevent the synthesis of potentially harmful assembly intermediates of the reaction centers of mitochondrial enzymes. Since such regulatory feedback loops only work if translation occurs in the compartment in which the complexes of the respiratory chain are assembled, these control mechanisms require the presence of a translation machinery in mitochondria. This might explain why eukaryotic cells maintained DNA in mitochondria during the last two billion years of evolution. This review gives an overview of the mitochondrial translation system and summarizes the current knowledge on translational activators and their role in the regulation of mitochondrial protein synthesis. This article is part of a Special Issue entitled: Protein import and quality control in mitochondria and plastids
Role of the basin boundary conditions in gravity wave turbulence
Gravity wave turbulence is studied experimentally in a large wave basin where
irregular waves are generated unidirectionally. The role of the basin boundary
conditions (absorbing or reflecting) and of the forcing properties are
investigated. To that purpose, an absorbing sloping beach opposite to the
wavemaker can be replaced by a reflecting vertical wall. We observe that the
wave field properties depend strongly on these boundary conditions. Quasi-one
dimensional field of nonlinear waves propagate before to be damped by the beach
whereas a more multidirectional wave field is observed with the wall. In both
cases, the wave spectrum scales as a frequency-power law with an exponent that
increases continuously with the forcing amplitude up to a value close to -4,
which is the value predicted by the weak turbulence theory. The physical
mechanisms involved are probably different according to the boundary condition
used, but cannot be easily discriminated with only temporal measurements. We
have also studied freely decaying gravity wave turbulence in the closed basin.
No self-similar decay of the spectrum is observed, whereas its Fourier modes
decay first as a time power law due to nonlinear mechanisms, and then
exponentially due to linear viscous damping. We estimate the linear, nonlinear
and dissipative time scales to test the time scale separation that highlights
the important role of a large scale Fourier mode. By estimation of the mean
energy flux from the initial decay of wave energy, the Kolmogorov-Zakharov
constant is evaluated and found to be compatible with a recent theoretical
value.Comment: Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015,
in press in JF
A Search for Variability in Exoplanet Analogues and Low-Gravity Brown Dwarfs
We report the results of a -band survey for photometric variability in a
sample of young, low-gravity objects using the New Technology Telescope (NTT)
and the United Kingdom InfraRed Telescope (UKIRT). Surface gravity is a key
parameter in the atmospheric properties of brown dwarfs and this is the first
large survey that aims to test the gravity dependence of variability
properties. We do a full analysis of the spectral signatures of youth and
assess the group membership probability of each target using membership tools
from the literature. This results in a 30 object sample of young low-gravity
brown dwarfs. Since we are lacking in objects with spectral types later than
L9, we focus our statistical analysis on the L0-L8.5 objects. We find that the
variability occurrence rate of L0-L8.5 low-gravity brown dwarfs in this survey
is . We reanalyse the results of Radigan 2014 and find that
the field dwarfs with spectral types L0-L8.5 have a variability occurrence rate
of . We determine a probability of that the samples are
drawn from different distributions. This is the first quantitative indication
that the low-gravity objects are more likely to be variable than the field
dwarf population. Furthermore, we present follow-up and
observations of the young, planetary-mass variable object PSO 318.5-22 over
three consecutive nights. We find no evidence of phase shifts between the
and bands and find higher amplitudes. We use the lightcurves
to measure a rotational period of hr for PSO 318.5-22.Comment: accepted for publication in MNRA
Global Climate and Atmospheric Composition of the Ultra-Hot Jupiter WASP-103b from HST and Spitzer Phase Curve Observations
We present thermal phase curve measurements for the hot Jupiter WASP-103b
observed with Hubble/WFC3 and Spitzer/IRAC. The phase curves have large
amplitudes and negligible hotspot offsets, indicative of poor heat
redistribution to the nightside. We fit the phase variation with a range of
climate maps and find that a spherical harmonics model generally provides the
best fit. The phase-resolved spectra are consistent with blackbodies in the
WFC3 bandpass, with brightness temperatures ranging from K on the
nightside to K on the dayside. The dayside spectrum has a
significantly higher brightness temperature in the Spitzer bands, likely due to
CO emission and a thermal inversion. The inversion is not present on the
nightside. We retrieved the atmospheric composition and found the composition
is moderately metal-enriched ( solar)
and the carbon-to-oxygen ratio is below 0.9 at confidence. In
contrast to cooler hot Jupiters, we do not detect spectral features from water,
which we attribute to partial HO dissociation. We compare the phase curves
to 3D general circulation models and find magnetic drag effects are needed to
match the data. We also compare the WASP-103b spectra to brown dwarfs and young
directly imaged companions and find these objects have significantly larger
water features, indicating that surface gravity and irradiation environment
play an important role in shaping the spectra of hot Jupiters. These results
highlight the 3D structure of exoplanet atmospheres and illustrate the
importance of phase curve observations for understanding their complex
chemistry and physics.Comment: 25 pages, 17 figures, 7 tables; accepted to A
High contrast imaging at the LBT: the LEECH exoplanet imaging survey
In Spring 2013, the LEECH (LBTI Exozodi Exoplanet Common Hunt) survey began
its 130-night campaign from the Large Binocular Telescope (LBT) atop Mt
Graham, Arizona. This survey benefits from the many technological achievements
of the LBT, including two 8.4-meter mirrors on a single fixed mount, dual
adaptive secondary mirrors for high Strehl performance, and a cold beam
combiner to dramatically reduce the telescope's overall background emissivity.
LEECH neatly complements other high-contrast planet imaging efforts by
observing stars at L' (3.8 m), as opposed to the shorter wavelength
near-infrared bands (1-2.4 m) of other surveys. This portion of the
spectrum offers deep mass sensitivity, especially around nearby adolescent
(0.1-1 Gyr) stars. LEECH's contrast is competitive with other extreme
adaptive optics systems, while providing an alternative survey strategy.
Additionally, LEECH is characterizing known exoplanetary systems with
observations from 3-5m in preparation for JWST.Comment: 12 pages, 5 figures. Proceedings of the SPIE, 9148-2
A Near-infrared Variability Survey of Young Planetary-mass Objects
We present a photometric variability survey of young planetary-mass objects
using the New Technology Telescope in the Js and Ks bands. Surface gravity
plays an important role in the atmospheric structure of brown dwarfs, as young
low gravity L dwarfs have a higher variability rate than field L dwarfs. In
this study, we extend variability studies to young T-type planetary-mass
objects and investigate the effects of surface gravity on the variability of L
and T dwarfs across a large sample. We conduct continuous monitoring for 18
objects with spectral types from L5 to T8 and detect four new variables and two
variable candidates. Combining with previous variability surveys of field and
young L and T objects, we find that young objects tend to be more variable than
field objects within peak-to-peak variability amplitude ranges of 0.5-10 per
cent and period ranges of 1.5-20 hr. For the first time, we constrain the
variability rate of young T dwarfs to be 56 per cent compared to 25 per cent
for field T dwarfs. Both field and young samples have higher variability rates
at the L/T transition than outside the L/T transition. The differences in the
variability rates between field and young samples are about 1 sigma and
therefore larger sample sizes are needed to confirm and refine the results.
Besides the L/T transition, young L dwarfs with strong variability tend to
assemble in a narrow spectral type range of L6-L7.5. This work supports the
critical role of surface gravity on the atmospheric structure from L to T
spectral types.Comment: Accepted for publication in MNRAS, 21 pages of main text including 6
tables and 17 figure
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