26 research outputs found
Theoretical Limits on Extrasolar Terrestrial Planet Detection with Coronagraphs
Many high contrast coronagraph designs have recently been proposed. In this
paper, their suitability for direct imaging of extrasolar terrestrial planets
is reviewed. We also develop a linear-algebra based model of coronagraphy that
can both explain the behavior of existing coronagraphs and quantify the
coronagraphic performance limit imposed by fundamental physics. We find that
the maximum theoretical throughput of a coronagraph is equal to one minus the
non-aberrated non-coronagraphic PSF of the telescope. We describe how a
coronagraph reaching this fundamental limit may be designed, and how much
improvement over the best existing coronagraph design is still possible. Both
the analytical model and numerical simulations of existing designs also show
that this theoretical limit rapidly degrades as the source size is increased:
the ``highest performance'' coronagraphs, those with the highest throughput and
smallest Inner Working Angle (IWA), are the most sensitive to stellar angular
diameter. This unfortunately rules out the possibility of using a small IWA
(lambda/d) coronagraph for a terrestrial planet imaging mission.
Finally, a detailed numerical simulation which accurately accounts for
stellar angular size, zodiacal and exozodiacal light is used to quantify the
efficiency of coronagraph designs for direct imaging of extrasolar terrestrial
planets in a possible real observing program. We find that in the photon noise
limited regime, a 4m telescope with a theoretically optimal coronagraph is able
to detect Earth-like planets around 50 stars with 1hr exposure time per target
(assuming 25% throughput and exozodi levels similar to our solar system). We
also show that at least 2 existing coronagraph design can approach this level
of performance in the ideal monochromatic case considered in this study.Comment: Accepted for publication to ApJ Sup
The Pulsation of Chi Cygni Imaged by Optical Interferometry; a Novel Technique to Derive Distance and Mass of Mira Stars
We present infrared interferometric imaging of the S-type Mira star Chi
Cygni. The object was observed at four different epochs in 2005-2006 with the
IOTA optical interferometer (H band). Images show up to 40% variation in the
stellar diameter, as well as significant changes in the limb darkening and
stellar inhomogeneities. Model fitting gave precise time-dependent values of
the stellar diameter, and reveals presence and displacement of a warm molecular
layer. The star radius, corrected for limb darkening, has a mean value of 12.1
mas and shows a 5.1mas amplitude pulsation. Minimum diameter was observed at
phase 0.94+/-0.01. Maximum temperature was observed several days later at phase
1.02+/-0.02. We also show that combining the angular acceleration of the
molecular layer with CO (Delta v = 3) radial velocity measurements yields a
5.9+/-1.5 mas parallax. The constant acceleration of the CO molecules -- during
80% of the pulsation cycle -- lead us to argument for a free-falling layer. The
acceleration is compatible with a gravitational field produced by a
2.1(+1.5/-0.7) solar mass star. This last value is in agreement with
fundamental mode pulsator models. We foresee increased development of
techniques consisting in combining radial velocity with interferometric angular
measurements, ultimately allowing total mapping of the speed, density, and
position of the diverse species in pulsation driven atmospheres.Comment: 36 pages, accepted in Ap
Speckle Interferometry of Metal-Poor Stars in the Solar Neighborhood.II
The results of speckle interferometric observations of 115 metal-poor stars
[m/H]<-1 within 250 pc from the Sun and with proper motions mu <= 0.2"/yr, made
with the 6-m telescope of the Special Astrophysical Observatory of the Russian
Academy of Sciences, are reported. Close companions with separations ranging
from 0.034" to 1" were observed for 12 objects - G76-21, G59-1, G63-46,
G135-16, G168-42, G141-47, G142-44, G190-10, G28-43, G217-8, G130-7, and G89-14
- eight of them are astrometrically resolved for the first time. The newly
resolved systems include one triple star - G190-10. If combined with
spectroscopic and visual data, our results imply a
single:binary:triple:quadruple star ratio of 147:64:9:1 for a sample of 221
primary components of halo and thick-disk stars
Evolved star water maser cloud size determined by star size
Cool, evolved stars undergo copious mass loss but the details of how the
matter is returned to the ISM are still under debate. We investigated the
structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic
Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved
stars were imaged using MERLIN, at sub-AU resolution. Each source was observed
at between 2 and 7 epochs (several stellar periods). We compared our results
with long-term Pushchino single dish monitoring. The 22-GHz emission is located
in ~spherical, thick, unevenly filled shells. The outflow velocity doubles
between the inner and outer shell limits. Water maser clumps could be matched
at successive epochs separated by <2 years for AGB stars, or at least 5 years
for RSG. This is much shorter than the decades taken for the wind to cross the
maser shell, and comparison with spectral monitoring shows that some features
fade and reappear. In 5 sources, most of the matched features brighten or dim
in concert from one epoch to the next. One cloud in W Hya was caught in the act
of passing in front of a background cloud leading to 50-fold, transient
amplification. The masing clouds are 1-2 orders of magnitude denser than the
wind average and contain a substantial fraction of the mass loss in this
region, with a filling factor <1%. The RSG clouds are ~10x bigger than those
round the AGB stars. Proper motions are dominated by expansion, with no
systematic rotation. The maser clouds survive for decades (the shell crossing
time) but the masers are not always beamed in our direction. Radiative effects
cause changes in flux density throughout the maser shells on short timescales.
Cloud size is proportional to parent star size; clouds have a similar radius to
the star in the 22-GHz maser shell. Stellar properties such as convection cells
must determine the clumping scale.Comment: Accepted by A&A 2012 July 10 Main text 29 pages, 62 figures Appendix
44 pages, 23 figure
Differential photometry of speckle-interferometric binary and multiple stars
A method for differential photometry of
speckle-interferometric binary and multiple stars is presented.
Both the accuracy and sources of systematic errors
of the method are analysed.
The photometric accuracy ranges between 0\fm02 and 0\fm20,
depending on the atmospheric seeing, the brightness and the
separation of the system components.
A comparison between our magnitude differences and those
of other authors is presented
Mise au point d'une technique d'imprégnation d'un support comprimable par une dispersion solide
DIJON-BU Médecine Pharmacie (212312103) / SudocSudocFranceF