164 research outputs found
On detecting the large separation in the autocorrelation of stellar oscillation times series
The observations carried out by the space missions CoRoT and Kepler provide a
large set of asteroseismic data. Their analysis requires an efficient procedure
first to determine if the star is reliably showing solar-like oscillations,
second to measure the so-called large separation, third to estimate the
asteroseismic information that can be retrieved from the Fourier spectrum. We
develop in this paper a procedure, based on the autocorrelation of the seismic
Fourier spectrum. We have searched for criteria able to predict the output that
one can expect from the analysis by autocorrelation of a seismic time series.
First, the autocorrelation is properly scaled for taking into account the
contribution of white noise. Then, we use the null hypothesis H0 test to assess
the reliability of the autocorrelation analysis. Calculations based on solar
and CoRoT times series are performed in order to quantify the performance as a
function of the amplitude of the autocorrelation signal. We propose an
automated determination of the large separation, whose reliability is
quantified by the H0 test. We apply this method to analyze a large set of red
giants observed by CoRoT. We estimate the expected performance for photometric
time series of the Kepler mission. Finally, we demonstrate that the method
makes it possible to distinguish l=0 from l=1 modes. The envelope
autocorrelation function has proven to be very powerful for the determination
of the large separation in noisy asteroseismic data, since it enables us to
quantify the precision of the performance of different measurements: mean large
separation, variation of the large separation with frequency, small separation
and degree identification.Comment: A&A, in pres
Atmospheric scintillation at Dome C, Antarctica: implications for photometry and astrometry
We present low-resolution turbulence profiles of the atmosphere above Dome C,
Antarctica, measured with the MASS instrument during 25 nights in March-May
2004. Except for the lowest layer, Dome C has significantly less turbulence
than Cerro Tololo and Cerro Pachon. In particular, the integrated turbulence at
16 km is always less than the median values at the two Chilean sites. From
these profiles we evaluate the photometric noise produced by scintillation, and
the atmospheric contribution to the error budget in narrow-angle differential
astrometry. In comparison with the two mid-latitude sites in Chile, Dome C
offers a potential gain of about 3.6 in both photometric precision (for long
integrations) and narrow-angle astrometry precision. These gain estimates are
preliminary, being computed with average wind-speed profiles, but the validity
of our approach is confirmed by independent data. Although the data from Dome C
cover a fairly limited time frame, they lend strong support to expectations
that Dome C will offer significant advantages for photometric and astrometric
studies.Comment: 12 pages, 5 figures, accepted by PASP 26th April, 200
Helioseismology, solar models and neutrino fluxes
We present our results concerning a systematical analysis of helioseismic
implications on solar structure and neutrino production. We find
Y, and
gr/cm. In the interval , the quantity is
determined with and accuracy of \permille~or better. At the solar center
still one has remarkable accuracy, . We compare the predictions
of recent solar models (standard and non-standard) with the helioseismic
results. By constructing helioseismically constrained solar models, the central
solar temperature is found to be K with a conservatively
estimated accuracy of 1.4%, so that the major unceratainty on neutrino fluxes
is due to nuclear cross section and not to solar inputs.Comment: 14 pages including 9 figures, LaTex file, espcrc2.sty is needed; to
appear in Nucl. Phys. B Proc. Suppl., Proceedings of TAUP97 conference,
Laboratori Nazionali del Gran Sasso, September 199
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
Near-Infrared interferometry of Eta Carinae with high spatial and spectral resolution using the VLTI and the AMBER instrument
We present the first NIR spectro-interferometry of the LBV Eta Carinae. The K
band observations were performed with the AMBER instrument of the ESO Very
Large Telescope Interferometer using three 8.2m Unit Telescopes with baselines
from 42 to 89m. The aim of this work is to study the wavelength dependence of
Eta Car's optically thick wind region with a high spatial resolution of 5 mas
(11 AU) and high spectral resolution. The medium spectral resolution
observations (R=1,500) were performed in the wavelength range around both the
HeI 2.059 micron and the Br gamma 2.166 micron emission lines, the high
spectral resolution observations (R=12,000) only in the Br gamma line region.
In the K-band continuum, a diameter of 4.0 +/-0.2 mas (Gaussian FWHM, fit range
28-89m) was measured for Eta Car's optically thick wind region. If we fit
Hillier et al. (2001) model visibilities to the observed AMBER visibilities, we
obtain 50 % encircled-energy diameters of 4.2, 6.5 and 9.6mas in the 2.17
micron continuum, the HeI, and the Br gamma emission lines, respectively. In
the continuum near the Br gamma line, an elongation along a position angle of
120+/-15 degrees was found, consistent with previous VLTI/VINCI measurements by
van Boekel et al. (2003). We compare the measured visibilities with predictions
of the radiative transfer model of Hillier et al. (2001), finding good
agreement. Furthermore, we discuss the detectability of the hypothetical hot
binary companion. For the interpretation of the non-zero differential and
closure phases measured within the Br gamma line, we present a simple geometric
model of an inclined, latitude-dependent wind zone. Our observations support
theoretical models of anisotropic winds from fast-rotating, luminous hot stars
with enhanced high-velocity mass loss near the polar regions.Comment: 22 pages, 14 figures, 2 tables; A&A in pres
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Aberrant Otx2 Expression Enhances Migration and Induces Ectopic Proliferation of Hindbrain Neuronal Progenitor Cells
Dysregulation of Otx2 is a hallmark of the pediatric brain tumor medulloblastoma, yet its functional significance in the establishment of these tumors is unknown. Here we have sought to determine the functional consequences of Otx2 overexpression in the mouse hindbrain to characterize its potential role in medulloblastoma tumorigenesis and identify the cell types responsive to this lineage-specific oncogene. Expression of Otx2 broadly in the mouse hindbrain resulted in the accumulation of proliferative clusters of cells in the cerebellar white matter and dorsal brainstem of postnatal mice. We found that brainstem ectopia were derived from neuronal progenitors of the rhombic lip and that cerebellar ectopia were derived from granule neuron precursors (GNPs) that had migrated inwards from the external granule layer (EGL). These hyperplasias exhibited various characteristics of medulloblastoma precursor cells identified in animal models of Shh or Wnt group tumors, including aberrant localization and altered spatiotemporal control of proliferation. However, ectopia induced by Otx2 differentiated and dispersed as the animals reached adulthood, indicating that factors restricting proliferative lifespan were a limiting factor to full transformation of these cells. These studies implicate a role for Otx2 in altering the dynamics of neuronal progenitor cell proliferation
The magnetism of the solar interior for a complete MHD solar vision
The solar magnetism is no more considered as a purely superficial phenomenon.
The SoHO community has shown that the length of the solar cycle depends on the
transition region between radiation and convection. Nevertheless, the internal
solar (stellar) magnetism stays poorly known. Starting in 2008, the American
instrument HMI/SDO and the European microsatellite PICARD will enrich our view
of the Sun-Earth relationship. Thus obtaining a complete MHD solar picture is a
clear objective for the next decades and it requires complementary observations
of the dynamics of the radiative zone. For that ambitious goal, space
prototypes are being developed to improve gravity mode detection. The Sun is
unique to progress on the topology of deep internal magnetic fields and to
understand the complex mechanisms which provoke photospheric and coronal
magnetic changes and possible longer cycles important for human life. We
propose the following roadmap in Europe to contribute to this "impressive"
revolution in Astronomy and in our Sun-Earth relationship: SoHO (1995-2007),
PICARD (2008-2010), DynaMICS (2009-2017) in parallel to SDO (2008-2017) then a
world-class mission located at the L1 orbit or above the solar poles.Comment: 10 pages, 8 figures, will appear in the proceedings of 2005 ESLAB
symposium 19-21 April 200
- âŠ