63 research outputs found
Dome C site testing: surface layer, free atmosphere seeing and isoplanatic angle statistics
This paper analyses 3.5 years of site testing data obtained at Dome C,
Antarctica, based on measurements obtained with three DIMMs located at three
different elevations. Basic statistics of the seeing and the isoplanatic angle
are given, as well as the characteristic time of temporal fluctuations of these
two parameters, which we found to around 30 minutes at 8 m. The 3 DIMMs are
exploited as a profiler of the surface layer, and provide a robust estimation
of its statistical properties. It appears to have a very sharp upper limit
(less than 1 m). The fraction of time spent by each telescope above the top of
the surface layer permits us to deduce a median height of between 23 m and 27
m. The comparison of the different data sets led us to infer the statistical
properties of the free atmosphere seeing, with a median value of 0.36 arcsec.
The C_n^2 profile inside the surface layer is also deduced from the seeing data
obtained during the fraction of time spent by the 3 telescopes inside this
turbulence. Statistically, the surface layer, except during the 3-month summer
season, contributes to 95 percent of the total turbulence from the surface
level, thus confirming the exceptional quality of the site above it
First detection of dust clouds around R CrB variable stars
From VLT/NACO diffraction-limited images of RY~Sgr, we report the first
direct detection of heterogeneities in the circumstellar envelope of a R
Coronae Borealis variable star. Several bright and very large dust clouds are
seen in various directions at several hundred stellar radii from RY Sgr,
revealing high activity for the ejection of stellar material by R CrB
variables. These observations do support the current interpretation that
optically thick dust clouds are formed around the surface of this type of
variable stars and, when passing between the star and the observer, produce the
huge and sudden declines characterizing these objects in visible light. This is
the first direct confirmation of a scenario proposed about 70 years ago.Comment: Published in Astronomy and Astrophysics (A&A 428,L13-L16 (2004)
Photometric quality of Dome C for the winter 2008 from ASTEP South
ASTEP South is an Antarctic Search for Transiting Exo- Planets in the South
pole field, from the Concordia station, Dome C, Antarctica. The instrument
consists of a thermalized 10 cm refractor observing a fixed 3.88\degree x
3.88\degree field of view to perform photometry of several thousand stars at
visible wavelengths (700-900 nm). The first winter campaign in 2008 led to the
retrieval of nearly 1600 hours of data. We derive the fraction of photometric
nights by measuring the number of detectable stars in the field. The method is
sensitive to the presence of small cirrus clouds which are invisible to the
naked eye. The fraction of night-time for which at least 50% of the stars are
detected is 74% from June to September 2008. Most of the lost time (18.5% out
of 26%) is due to periods of bad weather conditions lasting for a few days
("white outs"). Extended periods of clear weather exist. For example, between
July 10 and August 10, 2008, the total fraction of time (day+night) for which
photometric observations were possible was 60%. This confirms the very high
quality of Dome C for nearly continuous photometric observations during the
Antarctic winter
Dust temperature and density profiles of AGB and post-AGB stars from mid-infrared observations
First mid-infrared images of a sample of AGB and post-AGB carbon stars (V
Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO,
Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the
9.7m image, Roberts 22 shows an envelope slightly elongated in the
north-east direction while images of V Hya and IRC+10216 are roughly
spherically symmetric. Using inversion technique, the dust emissivity was
derived from the observed intensity profiles, allowing a determination of the
grain temperature and density distributions inside the envelope for these
stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216.
Our results are comparable to those obtained in previous studies if dust grains
have dimensions in the range 0.01 - 0.2 m. Color maps suggest the
presence of temperature inhomogeneities in the central regions of the dust
envelopes. In the case of V Hya, an eccentric hot point, which direction
coincides with the jet previously seen in [SII] emission, suggest that we are
observing a material ejected in a previous mass-loss event. Bipolar lobes are
clearly seen in the color maps of Roberts 22 and IRC +10216.Comment: accepted for publication in Astronomy and Astrophysic
The secondary eclipses of WASP-19b as seen by the ASTEP 400 telescope from Antarctica
The ASTEP (Antarctica Search for Transiting ExoPlanets) program was
originally aimed at probing the quality of the Dome C, Antarctica for the
discovery and characterization of exoplanets by photometry. In the first year
of operation of the 40 cm ASTEP 400 telescope (austral winter 2010), we
targeted the known transiting planet WASP-19b in order to try to detect its
secondary transits in the visible. This is made possible by the excellent
sub-millimagnitude precision of the binned data. The WASP-19 system was
observed during 24 nights in May 2010. The photometric variability level due to
starspots is about 1.8% (peak-to-peak), in line with the SuperWASP data from
2007 (1.4%) and larger than in 2008 (0.07%). We find a rotation period of
WASP-19 of 10.7 +/- 0.5 days, in agreement with the SuperWASP determination of
10.5 +/- 0.2 days. Theoretical models show that this can only be explained if
tidal dissipation in the star is weak, i.e. the tidal dissipation factor Q'star
> 3.10^7. Separately, we find evidence for a secondary eclipse of depth 390 +/-
190 ppm with a 2.0 sigma significance, a phase consistent with a circular orbit
and a 3% false positive probability. Given the wavelength range of the
observations (420 to 950 nm), the secondary transit depth translates into a day
side brightness temperature of 2690(-220/+150) K, in line with measurements in
the z' and K bands. The day side emission observed in the visible could be due
either to thermal emission of an extremely hot day side with very little
redistribution of heat to the night side, or to direct reflection of stellar
light with a maximum geometrical albedo Ag=0.27 +/- 0.13. We also report a
low-frequency oscillation well in phase at the planet orbital period, but with
a lower-limit amplitude that could not be attributed to the planet phase alone,
and possibly contaminated with residual lightcurve trends.Comment: Accepted for publication in Astronomy and Astrophysics, 13 pages, 13
figure
ASTEP South: An Antarctic Search for Transiting ExoPlanets around the celestial South pole
ASTEP South is the first phase of the ASTEP project (Antarctic Search for
Transiting ExoPlanets). The instrument is a fixed 10 cm refractor with a 4kx4k
CCD camera in a thermalized box, pointing continuously a 3.88 degree x 3.88
degree field of view centered on the celestial South pole. ASTEP South became
fully functional in June 2008 and obtained 1592 hours of data during the 2008
Antarctic winter. The data are of good quality but the analysis has to account
for changes in the point spread function due to rapid ground seeing variations
and instrumental effects. The pointing direction is stable within 10 arcseconds
on a daily timescale and drifts by only 34 arcseconds in 50 days. A truly
continuous photometry of bright stars is possible in June (the noon sky
background peaks at a magnitude R=15 arcsec-2 on June 22), but becomes
challenging in July (the noon sky background magnitude is R=12.5 arcsec?2 on
July 20). The weather conditions are estimated from the number of stars
detected in the field. For the 2008 winter, the statistics are between 56.3 %
and 68.4 % of excellent weather, 17.9 % to 30 % of veiled weather and 13.7 % of
bad weather. Using these results in a probabilistic analysis of transit
detection, we show that the detection efficiency of transiting exoplanets in
one given field is improved at Dome C compared to a temperate site such as La
Silla. For example we estimate that a year-long campaign of 10 cm refractor
could reach an efficiency of 69 % at Dome C versus 45 % at La Silla for
detecting 2-day period giant planets around target stars from magnitude 10 to
15. This shows the high potential of Dome C for photometry and future planet
discoveries. [Short abstract
A mid-infrared imaging catalogue of post-asymptotic giant branch stars
The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright Royal Astronomical SocietyPost-asymptotic giant branch (post-AGB) stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VLT spectrometer and imager for the mid-infrared (VISIR)/VLT, T-Recs/Gemini-South and Michelle/Gemini-North. We found that all the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non-resolved. The resolved targets can be divided into two categories. (i) The nebulae with a dense central core, that are either bipolar and multipolar and (ii) the nebulae with no central core, with an elliptical morphology. The dense central torus observed likely hosts binary systems which triggered fast outflows that shaped the nebulae.Peer reviewe
The circumstellar envelope of IRC+10216 from milli-arcsecond to arcmin scales
Aims.Analysis of the innermost regions of the carbon-rich star IRC+10216 and
of the outer layers of its circumstellar envelope have been performed in order
to constrain its mass-loss history. Methods: .We analyzed the high dynamic
range of near-infrared adaptive optics and the deep V-band images of the
circumstellar envelope of IRC+10216 using high angular resolution, collected
with the VLT/NACO and FORS1 instruments. Results: .From the near-infrared
observations, we present maps of the sub-arcsecond structures, or clumps, in
the innermost regions. The morphology of these clumps is found to strongly vary
from J- to L-band. Their relative motion appears to be more complex than
proposed in earlier works: they can be weakly accelerated, have a constant
velocity, or even be motionless with respect to one another. From V-band
imaging, we present a high spatial resolution map of the shell distribution in
the outer layers of IRC+10216. Shells are resolved well up to a distance of
about 90'' to the core of the nebula and most of them appear to be composed of
thinner elongated shells. Finally, by combining the NACO and FORS1 images, a
global view is present to show both the extended layers and the bipolar core of
the nebula together with the real size of the inner clumps. Conclusions: .This
study confirms the rather complex nature of the IRC+10216 circumstellar
environment. In particular, the coexistence at different spatial scales of
structures with very different morphologies (clumps, bipolarity, and almost
spherical external layers) is very puzzling. This confirms that the formation
of AGB winds is far more complex than usually assumed in current models.Comment: Published in Astronomy & Astrophysics, 2006, 455, 18
MOA-2010-BLG-477Lb: constraining the mass of a microlensing planet from microlensing parallax, orbital motion and detection of blended light
Microlensing detections of cool planets are important for the construction of
an unbiased sample to estimate the frequency of planets beyond the snow line,
which is where giant planets are thought to form according to the core
accretion theory of planet formation. In this paper, we report the discovery of
a giant planet detected from the analysis of the light curve of a
high-magnification microlensing event MOA-2010-BLG-477. The measured
planet-star mass ratio is and the projected
separation is in units of the Einstein radius. The angular
Einstein radius is unusually large mas. Combining
this measurement with constraints on the "microlens parallax" and the lens
flux, we can only limit the host mass to the range . In
this particular case, the strong degeneracy between microlensing parallax and
planet orbital motion prevents us from measuring more accurate host and planet
masses. However, we find that adding Bayesian priors from two effects (Galactic
model and Keplerian orbit) each independently favors the upper end of this mass
range, yielding star and planet masses of
and at a distance of kpc,
and with a semi-major axis of AU. Finally, we show that the
lens mass can be determined from future high-resolution near-IR adaptive optics
observations independently from two effects, photometric and astrometric.Comment: 3 Tables, 12 Figures, accepted in Ap
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