52 research outputs found
On the role of conformal three-geometries in the dynamics of General Relativity
It is shown that the Chern-Simons functional, built in the spinor
representation from the initial data on spacelike hypersurfaces, is invariant
with respect to infinitesimal conformal rescalings if and only if the vacuum
Einstein equations are satisfied. As a consequence, we show that in the phase
space the Hamiltonian constraint of vacuum general relativity is the Poisson
bracket of the imaginary part of this Chern-Simons functional and Misner's time
(essentially the 3-volume). Hence the vacuum Hamiltonian constraint is the
condition on the canonical variables that the imaginary part of the Chern-
Simons functional be constant along the volume flow. The vacuum momentum
constraint can also be reformulated in a similar way as a (more complicated)
condition on the change of the imaginary part of the Chern-Simons functional
along the flow of York's time.Comment: 15 pages, plain Te
Cultivo de protoplastos de yuca y Stylosanthes
Methods of isolation and culture of cassava and Stylosanthes protoplasts are described. The application of these methods requires the routine regeneration of plants from isolated protoplasts. In cassava, protoplast isolation from leaves and shoot tips of plants cultured in vitro and from somatic embryos, differentiated from leaf segments, was possible with an enzyme mixture consisting of Onozuka cellulase, hemicellulase, and Pectolyase. Isolated protoplasts were purified by filtration and cultured in liquid medium; they regenerated cell wall after 1-2 days of culture and presented cell division after 3-4 days. Division frequency observed was 10- 20 percent in protoplast cultures from leaf mesophyll and shoot tips. In cultures of somatic embryos, this frequency was less than 5 percent. Within 3-4 wk., dividing protoplasts formed colonies. The plated colonies formed callus in several culture media. Protoplasts from 10 cassava var. have been isolated and cultured. (CIAT
Observational calibration of the projection factor of Cepheids. II. Application to nine Cepheids with HST/FGS parallax measurements
The distance to pulsating stars is classically estimated using the
parallax-of-pulsation (PoP) method, which combines spectroscopic radial
velocity measurements and angular diameter estimates to derive the distance of
the star. An important application of this method is the determination of
Cepheid distances, in view of the calibration of their distance scale. However,
the conversion of radial to pulsational velocities in the PoP method relies on
a poorly calibrated parameter, the projection factor (p-factor). We aim to
measure empirically the value of the p-factors of a homogeneous sample of nine
Galactic Cepheids for which trigonometric parallaxes were measured with the
Hubble Space Telescope Fine Guidance Sensor. We use the SPIPS algorithm, a
robust implementation of the PoP method that combines photometry,
interferometry, and radial velocity measurements in a global modeling of the
pulsation. We obtained new interferometric angular diameters using the PIONIER
instrument at the Very Large Telescope Interferometer, completed by data from
the literature. Using the known distance as an input, we derive the value of
the p-factor and study its dependence with the pulsation period. We find the
following p-factors: 1.20 0.12 for RT Aur, 1.48 0.18 for T Vul,
1.14 0.10 for FF Aql, 1.31 0.19 for Y Sgr, 1.39 0.09 for X
Sgr, 1.35 0.13 for W Sgr, 1.36 0.08 for Dor, 1.41
0.10 for Gem, and 1.23 0.12 for Car. These values are
consistently close to p = 1.324 0.024. We observe some dispersion around
this average value, but the observed distribution is statistically consistent
with a constant value of the p-factor as a function of the pulsation period.
The error budget of our determination of the p-factor values is presently
dominated by the uncertainty on the parallax, a limitation that will soon be
waived by Gaia.Comment: 18 pages, 13 figure
Observational calibration of the projection factor of Cepheids I. The Type II Cepheid kappa Pavonis
The distances of pulsating stars, in particular Cepheids, are commonly
measured using the parallax of pulsation technique. The different versions of
this technique combine measurements of the linear diameter variation (from
spectroscopy) and the angular diameter variation (from photometry or
interferometry) amplitudes, to retrieve the distance in a quasi-geometrical
way. However, the linear diameter amplitude is directly proportional to the
projection factor (hereafter p-factor), which is used to convert spectroscopic
radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric)
velocities. The value of the p-factor and its possible dependence on the
pulsation period are still widely debated. Our goal is to measure an
observational value of the p-factor of the type-II Cepheid kappa Pavonis, whose
parallax was measured with an accuracy of 5% using HST/FGS. We used this
parallax as a starting point to derive the p-factor of kappa Pav, using the
SPIPS technique, which is a robust version of the parallax-of-pulsation method
that employs radial velocity, interferometric and photometric data. We applied
this technique to a combination of new VLTI/PIONIER optical interferometric
angular diameters, new CORALIE and HARPS radial velocities, as well as
multi-colour photometry and radial velocities from the literature. We obtain a
value of p = 1.26 +/- 0.07 for the p-factor of kappa Pav. This result agrees
with several of the recently derived Period-p-factor relationships from the
literature, as well as previous observational determinations for Cepheids.
Individual estimates of the p-factor are fundamental to calibrating the
parallax of pulsation distances of Cepheids. Together with previous
observational estimates, the projection factor we obtain points to a weak
dependence of the p-factor on period.Comment: 8 pages, 6 figures, accepted in A&
Toward a renewed Galactic Cepheid distance scale from Gaia and optical interferometry
Through an innovative combination of multiple observing techniques and mod-
eling, we are assembling a comprehensive understanding of the pulsation and
close environment of Cepheids. We developed the SPIPS modeling tool that
combines all observables (radial velocimetry, photometry, angular diameters
from interferometry) to derive the relevant physical parameters of the star
(effective temperature, infrared ex- cess, reddening,...) and the ratio of the
distance and the projection factor d/p. We present the application of SPIPS to
the long-period Cepheid RS Pup, for which we derive p = 1.25 +/- 0.06. The
addition of this massive Cepheid consolidates the existing sample of p-factor
measurements towards long-period pulsators. This allows us to conclude that p
is constant or mildly variable around p = 1.29 +/- 0.04 (+/-3%) as a function
of the pulsation period. The forthcoming Gaia DR2 will provide a considerable
improvement in quantity and accuracy of the trigonometric parallaxes of
Cepheids. From this sample, the SPIPS modeling tool will enable a robust
calibration of the Cepheid distance scale.Comment: 5 pages, 4 figures, proceedings of the 22nd Los Alamos Stellar
Pulsation Conference "Wide-field variability surveys: a 21st-century
perspective" held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 201
Gaia's Cepheids and RR Lyrae Stars and Luminosity Calibrations Based on Tycho-Gaia Astrometric Solution
Gaia Data Release 1 contains parallaxes for more than 700 Galactic Cepheids
and RR Lyrae stars, computed as part of the Tycho-Gaia Astrometric Solution
(TGAS). We have used TGAS parallaxes, along with literature () photometry and spectroscopy, to calibrate the zero point
of the Period-Luminosity and Period-Wesenheit relations of classical and type
II Cepheids, and the near-infrared Period-Luminosity,
Period-Luminosity-Metallicity and optical Luminosity-Metallicity relations of
RR Lyrae stars. In this contribution we briefly summarise results obtained by
fitting these basic relations adopting different techniques that operate either
in parallax or distance (absolute magnitude) space.Comment: 5 pages, 4 figures, proceedings for the 22nd Los Alamos Stellar
Pulsation Conference Series Meeting "Wide field variability surveys: a
21st-century perspective", held in San Pedro de Atacama, Chile, Nov. 28 -
Dec. 2, 201
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