198 research outputs found
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
SYMPA, a dedicated instrument for Jovian Seismology. II. Real performance and first results
Context. Due to its great mass and its rapid formation, Jupiter has played a
crucial role in shaping the Solar System. The knowledge of its internal
structure would strongly constrain the solar system formation mechanism.
Seismology is the most efficient way to probe directly the internal structure
of giant planets. Aims. SYMPA is the first instrument dedicated to the
observations of free oscillations of Jupiter. Principles and theoretical
performance have been presented in paper I. This second paper describes the
data processing method, the real instrumental performance and presents the
first results of a Jovian observation run, lead in 2005 at Teide Observatory.
Methods. SYMPA is a Fourier transform spectrometer which works at fixed optical
path difference. It produces Doppler shift maps of the observed object.
Velocity amplitude of Jupiter's oscillations is expected below 60 cm/s. Results
Despite light technical defects, the instrument demonstrated to work correctly,
being limited only by photon noise, after a careful analysis. A noise level of
about 12 cm/s has been reached on a 10-night observation run, with 21 % duty
cycle, which is 5 time better than previous similar observations. However, no
signal from Jupiter is clearly highlighted.Comment: 13 pages, 26 figure
Fourier Analysis of Gapped Time Series: Improved Estimates of Solar and Stellar Oscillation Parameters
Quantitative helio- and asteroseismology require very precise measurements of
the frequencies, amplitudes, and lifetimes of the global modes of stellar
oscillation. It is common knowledge that the precision of these measurements
depends on the total length (T), quality, and completeness of the observations.
Except in a few simple cases, the effect of gaps in the data on measurement
precision is poorly understood, in particular in Fourier space where the
convolution of the observable with the observation window introduces
correlations between different frequencies. Here we describe and implement a
rather general method to retrieve maximum likelihood estimates of the
oscillation parameters, taking into account the proper statistics of the
observations. Our fitting method applies in complex Fourier space and exploits
the phase information. We consider both solar-like stochastic oscillations and
long-lived harmonic oscillations, plus random noise. Using numerical
simulations, we demonstrate the existence of cases for which our improved
fitting method is less biased and has a greater precision than when the
frequency correlations are ignored. This is especially true of low
signal-to-noise solar-like oscillations. For example, we discuss a case where
the precision on the mode frequency estimate is increased by a factor of five,
for a duty cycle of 15%. In the case of long-lived sinusoidal oscillations, a
proper treatment of the frequency correlations does not provide any significant
improvement; nevertheless we confirm that the mode frequency can be measured
from gapped data at a much better precision than the 1/T Rayleigh resolution.Comment: Accepted for publication in Solar Physics Topical Issue
"Helioseismology, Asteroseismology, and MHD Connections
Analysis of the solar cycle and core rotation using 15 years of Mark-I observations:1984-1999. I. The solar cycle
High quality observations of the low-degree acoustic modes (p-modes) exist
for almost two complete solar cycles using the solar spectrophotometer Mark-I,
located at the Observatorio del Teide (Tenerife, Spain) and operating now as
part of the Birmingham Solar Oscillations Network (BiSON). We have performed a
Fourier analysis of 30 calibrated time-series of one year duration covering a
total period of 15 years between 1984 and 1999. Applying different techniques
to the resulting power spectra, we study the signature of the solar activity
changes on the low-degree p-modes. We show that the variation of the central
frequencies and the total velocity power (TVP) changes. A new method of
simultaneous fit is developed and a special effort has been made to study the
frequency-dependence of the frequency shift. The results confirm a variation of
the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on
average for low degree modes between 2.5 and 3.7 mHz. The TVP is
anti-correlated with the common activity indices with a decrease of about 20%
between the minimum and the maximum of solar cycle 22. The results are compared
with those obtained for intermediate degrees, using the LOWL data. The
frequency shift is found to increase with the degree with a weak l-dependence
similar to that of the inverse mode mass. This verifies earlier suggestions
that near surface effects are predominant.Comment: Accepted by A&A October 3 200
Otx2 Gene Deletion in Adult Mouse Retina Induces Rapid RPE Dystrophy and Slow Photoreceptor Degeneration
International audienceBACKGROUND: Many developmental genes are still active in specific tissues after development is completed. This is the case for the homeobox gene Otx2, an essential actor of forebrain and head development. In adult mouse, Otx2 is strongly expressed in the retina. Mutations of this gene in humans have been linked to severe ocular malformation and retinal diseases. It is, therefore, important to explore its post-developmental functions. In the mature retina, Otx2 is expressed in three cell types: bipolar and photoreceptor cells that belong to the neural retina and retinal pigment epithelium (RPE), a neighbour structure that forms a tightly interdependent functional unit together with photoreceptor cells. METHODOLOGY/PRINCIPAL FINDINGS: Conditional self-knockout was used to address the late functions of Otx2 gene in adult mice. This strategy is based on the combination of a knock-in CreERT2 allele and a floxed allele at the Otx2 locus. Time-controlled injection of tamoxifen activates the recombinase only in Otx2 expressing cells, resulting in selective ablation of the gene in its entire domain of expression. In the adult retina, loss of Otx2 protein causes slow degeneration of photoreceptor cells. By contrast, dramatic changes of RPE activity rapidly occur, which may represent a primary cause of photoreceptor disease. CONCLUSIONS: Our novel mouse model uncovers new Otx2 functions in adult retina. We show that this transcription factor is necessary for long-term maintenance of photoreceptors, likely through the control of specific activities of the RPE
Analysis of alpha Centauri AB including seismic constraints
Detailed models of alpha Cen A and B based on new seismological data for
alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva
evolution code including atomic diffusion. Taking into account the numerous
observational constraints now available for the alpha Cen system, we find a
stellar model which is in good agreement with the astrometric, photometric,
spectroscopic and asteroseismic data. The global parameters of the alpha Cen
system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial
helium mass fraction Y_i=0.275+-0.010 and an initial metallicity
(Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we
confirm that the mixing-length parameter alpha of the B component is larger
than the one of the A component, as already suggested by many authors (Noels et
al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is
about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10).
Moreover, we show that asteroseismic measurements enable to determine the radii
of both stars with a very high precision (errors smaller than 0.3%). The radii
deduced from seismological data are compatible with the new interferometric
results of Kervella et al. (2003) even if they are slightly larger than the
interferometric radii (differences smaller than 1%).Comment: 13 pages, 9 figures, accepted for publication in A&
Glial D-Serine Gates NMDA Receptors at Excitatory Synapses in Prefrontal Cortex.
N-methyl-D-aspartate receptors (NMDARs) subserve numerous neurophysiological and neuropathological processes in the cerebral cortex. Their activation requires the binding of glutamate and also of a coagonist. Whereas glycine and D-serine (D-ser) are candidates for such a role at central synapses, the nature of the coagonist in cerebral cortex remains unknown. We first show that the glycine-binding site of NMDARs is not saturated in acute slices preparations of medial prefrontal cortex (mPFC). Using enzymes that selectively degrade either D-ser or glycine, we demonstrate that under the present conditions, D-ser is the principle endogenous coagonist of synaptic NMDARs at mature excitatory synapses in layers V/VI of mPFC where it is essential for long-term potentiation (LTP) induction. Furthermore, blocking the activity of glia with the metabolic inhibitor, fluoroacetate, impairs NMDAR-mediated synaptic transmission and prevents LTP induction by reducing the extracellular levels of D-serine. Such deficits can be restored by exogenous D-ser, indicating that the D-amino acid mainly originates from glia in the mPFC, as further confirmed by double-immunostaining studies for D-ser and anti-glial fibrillary acidic protein. Our findings suggest that D-ser modulates neuronal networks in the cerebral cortex by gating the activity of NMDARs and that altering its levels is relevant to the induction and potentially treatment of psychiatric and neurological disorders
Site testing for submillimetre astronomy at Dome C, Antarctica
Over the past few years a major effort has been put into the exploration of
potential sites for the deployment of submillimetre astronomical facilities.
Amongst the most important sites are Dome C and Dome A on the Antarctic
Plateau, and the Chajnantor area in Chile. In this context, we report on
measurements of the sky opacity at 200 um over a period of three years at the
French-Italian station, Concordia, at Dome C, Antarctica. We also present some
solutions to the challenges of operating in the harsh polar environ- ment. Dome
C offers exceptional conditions in terms of absolute atmospheric transmission
and stability for submillimetre astron- omy. Over the austral winter the PWV
exhibits long periods during which it is stable and at a very low level (0.1 to
0.3 mm). Higher values (0.2 to 0.8 mm) of PWV are observed during the short
summer period. Based on observations over three years, a transmission of around
50% at 350 um is achieved for 75% of the time. The 200-um window opens with a
typical transmission of 10% to 15% for 25% of the time. Dome C is one of the
best accessible sites on Earth for submillimetre astronomy. Observations at 350
or 450 {\mu}m are possible all year round, and the 200-um window opens long
enough and with a sufficient transparency to be useful. Although the polar
environment severely constrains hardware design, a permanent observatory with
appropriate technical capabilities is feasible. Because of the very good
astronomical conditions, high angular resolution and time series (multi-year)
observations at Dome C with a medium size single dish telescope would enable
unique studies to be conducted, some of which are not otherwise feasible even
from space
Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations
In this work, we present the first AMBER observations, of the Wolf-Rayet and
O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the
telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered
spectrally dispersed visibilities, as well as differential and closure phases,
with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret
these data in the context of a binary system with unresolved components,
neglecting in a first approximation the wind-wind collision zone flux
contribution. We show that the AMBER observables result primarily from the
contribution of the individual components of the WR+O binary system. We discuss
several interpretations of the residuals, and speculate on the detection of an
additional continuum component, originating from the free-free emission
associated with the wind-wind collision zone (WWCZ), and contributing at most
to the observed K-band flux at the 5% level. The expected absolute separation
and position angle at the time of observations were 5.1±0.9mas and
66±15° respectively. However, we infer a separation of
3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus
implies that the binary system lies at a distance of 368+38-13 pc, in agreement
with recent spectrophotometric estimates, but significantly larger than the
Hipparcos value of 258+41-31 pc
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