521 research outputs found
Self consistent modelling of the projection factor for interferometric distance determination
Astronomy and Astrophysics, v. 428, p. 131-137, 2004. http://dx.doi.org/10.1051/0004-6361:20041419International audienc
Architecture for Integrated Mems Resonators Quality Factor Measurement
In this paper, an architecture designed for electrical measurement of the
quality factor of MEMS resonators is proposed. An estimation of the measurement
performance is made using PSPICE simulations taking into account the
component's non-idealities. An error on the measured Q value of only several
percent is achievable, at a small integration cost, for sufficiently high
quality factor values (Q > 100).Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
High resolution spectroscopy for Cepheids distance determination. II. A period- projection factor relation
The projection factor is a key quantity for the interferometric
Baade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB) methods
of determining the distance of Cepheids. Indeed, it allows a consistent
combination of angular and linear diameters of the star. We aim to determine
consistent projection factors that include the dynamical structure of the
Cepheids' atmosphere. Methods. Hydrodynamical models of delta Cep and l Car
have been used to validate a spectroscopic method of determining the projection
factor. This method, based on the amplitude of the radial velocity curve, is
applied to eight stars observed with the HARPS spectrometer. The projection
factor is divided into three sub-concepts : (1) a geometrical effect, (2) the
velocity gradient within the atmosphere, and (3) the relative motion of the
"optical" pulsating photosphere compared to the corresponding mass elements
(hereafter fo-g). Both, (1) and (3) are deduced from geometrical and
hydrodynamical models, respectively, while (2) is derived directly from
observations. The Fe I 4896.439 A line is found to be the best one to use in
the context of IBW and SB methods. A coherent and consistent period-projection
factor relation (hereafter Pp relation) is derived for this specific spectral
line: p = [0.064 +- 0.020] log P + [1.376 +- 0.023]. This procedure is then
extended to derive dynamic projection factors for any spectral line of any
Cepheid. This Pp relation is an important tool for removing bias in the
calibration of the period-luminosity relation of Cepheids. Moreover, it reveals
a new physical quantity fo-g to investigate in the near future
Both systemic and local application of Granulocyte-colony stimulating factor (G-CSF) is neuroprotective after retinal ganglion cell axotomy
<p>Abstract</p> <p>Background</p> <p>The hematopoietic Granulocyte-Colony Stimulating Factor (G-CSF) plays a crucial role in controlling the number of neutrophil progenitor cells. Its function is mediated via the G-CSF receptor, which was recently found to be expressed also in the central nervous system. In addition, G-CSF provided neuroprotection in models of neuronal cell death. Here we used the retinal ganglion cell (RGC) axotomy model to compare effects of local and systemic application of neuroprotective molecules.</p> <p>Results</p> <p>We found that the <it>G-CSF receptor </it>is robustly expressed by RGCs <it>in vivo </it>and <it>in vitro</it>. We thus evaluated G-CSF as a neuroprotectant for RGCs and found a dose-dependent neuroprotective effect of G-CSF on axotomized RGCs when given subcutaneously. As stem stell mobilization had previously been discussed as a possible contributor to the neuroprotective effects of G-CSF, we compared the local treatment of RGCs by injection of G-CSF into the vitreous body with systemic delivery by subcutaneous application. Both routes of application reduced retinal ganglion cell death to a comparable extent. Moreover, G-CSF enhanced the survival of immunopurified RGCs <it>in vitro</it>.</p> <p>Conclusion</p> <p>We thus show that G-CSF neuroprotection is at least partially independent of potential systemic effects and provide further evidence that the clinically applicable G-CSF could become a treatment option for both neurodegenerative diseases and glaucoma.</p
HD 173977: An ellipsoidal d Scuti star variable
Astronomy and Astrophysics, v. 426, p. 247-252, 2004. http://dx.doi.org/10.1051/0004-6361:20034068International audienc
First AMBER/VLTI observations of hot massive stars
AMBER is the first near infrared focal instrument of the VLTI. It combines
three telescopes and produces spectrally resolved interferometric measures.
This paper discusses some preliminary results of the first scientific
observations of AMBER with three Unit Telescopes at medium (1500) and high
(12000) spectral resolution. We derive a first set of constraints on the
structure of the circumstellar material around the Wolf Rayet Gamma2 Velorum
and the LBV Eta Carinae
High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry
The ratio of pulsation to radial velocity (the projection factor) is
currently limiting the accuracy of the interferometric Baade-Wesselink method.
This work aims at establishing a link between the line asymmetry evolution over
the Cepheids' pulsation cycles and their projection factor, with the final
objective to improve the accuracy of the Baade-Wesselink method for distance
determinations. We present HARPS high spectral resolution observations of nine
galactic Cepheids having a good period sampling. We fit spectral line profiles
by an asymmetric bi-Gaussian to derive radial velocity, Full-Width at
Half-Maximum in the line (FWHM) and line asymmetry for all stars. We then
extract correlations curves between radial velocity and asymmetry. A geometric
model providing synthetic spectral lines, including limb-darkening, a constant
FWHM (hereafter sigma_c) and the rotation velocity is used to interpret these
correlations curves. For all stars, comparison between observations and
modelling is satisfactory, and we were able to determine the projected rotation
velocities and sigma_c for all stars. We also find a correlation between the
rotation velocity (Vrot sin i) and the period of the star: Vrot sin i = (11.5
+- 0.9) log(P) + (19.8 +- 1.0) [km/s]. Moreover, we observe a systematic shift
in observational asymmetry curves (noted gamma_O), related to the period of the
star, which is not explained by our static model: gamma_O = (10.7+-0.1) log(P)
+ (9.7+-0.2) [in %] . For long-period Cepheids, in which velocity gradients,
compression or shock waves seem to be large compared to short- or medium period
Cepheids we observe indeed a greater systematic shift in asymmetry curves.
(abridged
Evolution of predator dispersal in relation to spatio-temporal prey dynamics : how not to get stuck in the wrong place!
Peer reviewedPublisher PD
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