442 research outputs found
Optimal Masks for Low-Degree Solar Acoustic Modes
We suggest a solution to an important problem of observational
helioseismology of the separation of lines of solar acoustic (p) modes of low
angular degree in oscillation power spectra by constructing optimal masks for
Doppler images of the Sun. Accurate measurements of oscillation frequencies of
low-degree modes are essential for the determination of the structure and
rotation of the solar core. However, these measurements for a particular mode
are often affected by leakage of other p modes arising when the Doppler images
are projected on to spherical-harmonics masks. The leakage results in
overlaping peaks corresponding to different oscillation modes in the power
spectra. In this paper we present a method for calculating optimal masks for a
given (target) mode by minimizing the signals of other modes appearing in its
vicinity. We apply this method to time series of 2 years obtained from
Michelson Doppler Imager (MDI) instrument on board SOHO space mission and
demonstrate its ability to reduce efficiently the mode leakage.Comment: to be published in Astrophys.J. Letter
Characteristics of Non-Allophanic Andisols derived from Low Activity Clay Regoliths in Nilgiri Hills (Southern India)
19International audienceLow activity clay soils on old planation surfaces of the tropics are generally considered as stable end points of soil formation. It is therefore surprising to find Andosols on them. We characterised the properties of six profiles representative of these soils in the Western part of Nilgiri Hills (2000–2500 m above mean sea level), Southern India, where the present climatic conditions are cool (mean annual temperature 15°C) and humid (mean annual rainfall 2500 mm). Thick (50–80 cm) dark–reddish brown topsoil overlies strongly desilicated yellowish–red materials. This horizon has andic properties to a sufficient depth and the carbon content requirement of the melanic epipedon to place these soils in the Andisol order. Our data as well as the history of the Nilgiri Hills suggest that the formation of these non-allophanic Andisols result from the succession of two main steps. First, a ‘lateritic' weathering cycle led to the relative accumulation of secondary Al and Fe oxides. Later, the accumulation of organic matter favoured by a more recent climatic change induced complexation by organic acids of Al and Fe oxides, and the production of enough metal–humus complexes to give rise to andic properties. Such soils, in which secondary Al and Fe oxides, generally considered as indicators of an advanced weathering stage, are involved in a new cycle of soil formation, are original Andisols
Oscillation frequencies and mode lifetimes in alpha Centauri A
We analyse our recently-published velocity measurements of alpha Cen A
(Butler et al. 2004). After adjusting the weights on a night-by-night basis in
order to optimize the window function to minimize sidelobes, we extract 42
oscillation frequencies with l=0 to 3 and measure the large and small frequency
separations. We give fitted relations to these frequencies that can be compared
with theoretical models and conclude that the observed scatter about these fits
is due to the finite lifetimes of the oscillation modes. We estimate the mode
lifetimes to be 1-2 d, substantially shorter than in the Sun.Comment: Accepted by Ap
Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime
Convection in stars excites resonant acoustic waves which depend on the sound
speed inside the star, which in turn depends on properties of the stellar
interior. Therefore, asteroseismology is an unrivaled method to probe the
internal structure of a star. We made a seismic study of the metal-poor
subgiant star nu Indi with the goal of constraining its interior structure. Our
study is based on a time series of 1201 radial velocity measurements spread
over 14 nights obtained from two sites, Siding Spring Observatory in Australia
and ESO La Silla Observatory in Chile. The power spectrum of the high precision
velocity time series clearly presents several identifiable peaks between 200
and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09
uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been
identified with amplitudes in the range 53 to 173 cm/s. The mode damping time
is estimated to be about 16 days (1-sigma range between 9 and 50 days),
substantially longer than in other stars like the Sun, the alpha Cen system or
the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte
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
Seismic and spectroscopic characterization of the solar-like pulsating CoRoT target HD 49385
The star HD 49385 is the first G-type solar-like pulsator observed in the
seismology field of the space telescope CoRoT. The satellite collected 137 days
of high-precision photometric data on this star, confirming that it presents
solar-like oscillations. HD 49385 was also observed in spectroscopy with the
NARVAL spectrograph in January 2009. Our goal is to characterize HD 49385 using
both spectroscopic and seismic data. The fundamental stellar parameters of HD
49385 are derived with the semi-automatic software VWA, and the projected
rotational velocity is estimated by fitting synthetic profiles to isolated
lines in the observed spectrum. A maximum likelihood estimation is used to
determine the parameters of the observed p modes. We perform a global fit, in
which modes are fitted simultaneously over nine radial orders, with degrees
ranging from l=0 to l=3 (36 individual modes). Precise estimates of the
atmospheric parameters (Teff, [M/H], log g) and of the vsini of HD 49385 are
obtained. The seismic analysis of the star leads to a clear identification of
the modes for degrees l=0,1,2. Around the maximum of the signal (nu=1013
microHz), some peaks are found significant and compatible with the expected
characteristics of l=3 modes. Our fit yields robust estimates of the
frequencies, linewidths and amplitudes of the modes. We find amplitudes of
about 5.6 +/- 0.8 ppm for radial modes at the maximum of the signal. The
lifetimes of the modes range from one day (at high frequency) to a bit more
than two days (at low frequency). Significant peaks are found outside the
identified ridges and are fitted. They are attributed to mixed modes.Comment: 13 pages, 14 figures, accepted in A&
Selective involvement of serum response factor in pressure-induced myogenic tone in resistance arteries
OBJECTIVE: In resistance arteries, diameter adjustment in response to pressure changes depends on the vascular cytoskeleton integrity. Serum response factor (SRF) is a dispensable transcription factor for cellular growth, but its role remains unknown in resistance arteries. We hypothesized that SRF is required for appropriate microvascular contraction. METHODS AND RESULTS: We used mice in which SRF was specifically deleted in smooth muscle or endothelial cells, and their control. Myogenic tone and pharmacological contraction was determined in resistance arteries. mRNA and protein expression were assessed by quantitative real-time PCR (qRT-PCR) and Western blot. Actin polymerization was determined by confocal microscopy. Stress-activated channel activity was measured by patch clamp. Myogenic tone developing in response to pressure was dramatically decreased by SRF deletion (5.9+/-2.3%) compared with control (16.3+/-3.2%). This defect was accompanied by decreases in actin polymerization, filamin A, myosin light chain kinase and myosin light chain expression level, and stress-activated channel activity and sensitivity in response to pressure. Contractions induced by phenylephrine or U46619 were not modified, despite a higher sensitivity to p38 blockade; this highlights a compensatory pathway, allowing normal receptor-dependent contraction. CONCLUSIONS: This study shows for the first time that SRF has a major part to play in the control of local blood flow via its central role in pressure-induced myogenic tone in resistance arteries
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