687 research outputs found
On-sky observations with an achromatic hybrid phase knife coronagraph in the visible
CONTEXT: The four-quadrant phase mask stellar coronagraph, introduced by D.
Rouan et al., is capable of achieving very high dynamical range imaging and was
studied in the context of the direct detection of extra-solar planets.
Achromatic four-quadrant phase mask is currently being developed for broadband
IR applications. AIMS: We report on laboratory and on-sky tests of a prototype
coronagraph in the visible. This prototype, the achromatic hybrid phase knife
coronagraph, was derived from the four-quadrant phase mask principle. METHODS:
The instrumental setup implementing the coronagraph itself was designed to
record the pre- and post-coronagraphic images simultaneously so that an
efficient real-time image selection procedure can be performed. We describe the
coronagraph and the associated tools that enable robust and repeatable
observations. We present an algorithm of image selection that has been tested
against the real on-sky data of the binary star HD80081 (* 38 Lyn). RESULTS
Although the observing conditions were poor, the efficiency of the proposed
method is proven. From this experiment, we derive procedures that can apply to
future focal instruments associating adaptive optics and coronagraphy,
targeting high dynamic range imaging in astronomy, such as detecting
extra-solar planets
A framework for the prospective analysis of super-diversity coming from high levels of immigration
Background
Pressures to keep immigration rates at relatively high levels are likely to persist in most developed countries. At the same time, immigrant cohorts are becoming more and more diverse, leading host societies to become increasingly heterogeneous across multiple dimensions. For scholars who study demographic or socio-economic behaviours, the need to account for ethnocultural “super-diversity” brings new challenges and complications.
Objective
The main objective of this paper is to present a framework for the prospective analysis of super-diversity in several high immigration countries.
Methods
We developed microsimulation models that simultaneously project several population-dimensions for Canada, the United States and countries of the European Union, with the aim of studying the consequences of alternate future population and migration trends.
Results
The paper presents the projected progression of three indicators of diversity for Canada, the USA and the EU28: percentage of foreign-born population, percentage of the population using a non-official language at home and percentage of non-Christians under the reference scenario. Results from alternative scenarios show the potential impact of modifying the composition of migrant cohorts. The paper also examines the projected changes in the labour force for each region by education level and language. Finally, the paper proposes a new longitudinal indicator that counts the number of years lived as active and inactive over the life course for foreign- and native-born cohorts.
Contribution
The microsimulation models provide much more informative results than more traditional cohort-component or multi-state models to study the future effects of ethnocultural super-diversity on high immigration countries
The VAST Survey - III. The multiplicity of A-type stars within 75 pc
With a combination of adaptive optics imaging and a multi-epoch common proper
motion search, we have conducted a large volume-limited (D 75 pc)
multiplicity survey of A-type stars, sensitive to companions beyond 30 au. The
sample for the Volume-limited A-STar (VAST) survey consists of 435 A-type
stars: 363 stars were observed with adaptive optics, 228 stars were searched
for wide common proper motion companions and 156 stars were measured with both
techniques. The projected separation coverage of the VAST survey extends from
30 to 45,000 au. A total of 137 stellar companions were resolved, including 64
new detections from the VAST survey, and the companion star fraction, projected
separation distribution and mass ratio distribution were measured. The
separation distribution forms a log-normal distribution similar to the
solar-type binary distribution, but with a peak shifted to a significantly
wider value of 387 (+132,-98) au. Integrating the fit to the distribution over
the 30 to 10,000 au observed range, the companion star fraction for A-type
stars is estimated as 33.8%+-2.6%. The mass ratio distribution of closer (<125
au) binaries is distinct from that of wider systems, with a flat distribution
for close systems and a distribution that tends towards smaller mass ratios for
wider binaries. Combining this result with previous spectroscopic surveys of
A-type stars gives an estimate of the total companion star fraction of
68.9%+-7.0%. The most complete assessment of higher order multiples was
estimated from the 156-star subset of the VAST sample with both adaptive optics
and common proper motion measurements, combined with a literature search for
companions, yielding a lower limit on the frequency of single, binary, triple,
quadruple and quintuple A-type star systems of 56.4 (-4.0,+3.8), 32.1
(-3.5,+3.9), 9.0 (-1.8,+2.8), 1.9 (-0.6,+1.8) and 0.6 (-0.2,+1.4) per cent,
respectively.Comment: 46 pages, 24 figures. Accepted for publication in the Monthly Notices
of the Royal Astronomical Society, 7th October 201
The VAST Survey - IV. A wide brown dwarf companion to the A3V star Delphini
We report the discovery of a wide co-moving substellar companion to the
nearby ( pc) A3V star Delphini based on imaging and
follow-up spectroscopic observations obtained during the course of our
Volume-limited A-Star (VAST) multiplicity survey. Del was observed over
a five-year baseline with adaptive optics, revealing the presence of a
previously-unresolved companion with a proper motion consistent with that of
the A-type primary. The age of the Del system was estimated as
Myr based on the position of the primary on the colour-magnitude
and temperature-luminosity diagrams. Using intermediate-resolution
near-infrared spectroscopy, the spectrum of Del B is shown to be
consistent with a mid-L dwarf (L), at a temperature of K.
Combining the measured near-infrared magnitude of Del B with the
estimated temperature leads to a model-dependent mass estimate of
M, corresponding to a mass ratio of . At a
projected separation of au, Del B is among the most
widely-separated and extreme-mass ratio substellar companions to a
main-sequence star resolved to-date, providing a rare empirical constraint of
the formation of low-mass ratio companions at extremely wide separations.Comment: 12 pages, 11 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Society, 2014 September 25. Revised to incorporate
typographical errors noted during the proofing proces
Speckle noise and dynamic range in coronagraphic images
This paper is concerned with the theoretical properties of high contrast
coronagraphic images in the context of exoplanet searches. We derive and
analyze the statistical properties of the residual starlight in coronagraphic
images, and describe the effect of a coronagraph on the speckle and photon
noise. Current observations with coronagraphic instruments have shown that the
main limitations to high contrast imaging are due to residual quasi-static
speckles. We tackle this problem in this paper, and propose a generalization of
our statistical model to include the description of static, quasi-static and
fast residual atmospheric speckles. The results provide insight into the
effects on the dynamic range of wavefront control, coronagraphy, active speckle
reduction, and differential speckle calibration. The study is focused on
ground-based imaging with extreme adaptive optics, but the approach is general
enough to be applicable to space, with different parameters.Comment: 31 pages, 18 figure
MOST light-curve analysis of the gamma Dor pulsator HR 8799, showing resonances and amplitude variations
Context: The central star of the HR 8799 system is a gamma Doradus-type
pulsator. The system harbours four planetary-mass companions detected by direct
imaging, and is a good solar system analogue. The masses of the companions are
not known accurately, because the estimation depends strongly on the age of the
system, which is also not known with sufficient accuracy. Asteroseismic studies
of the star might help to better constrain the age of HR 8799. We organized an
extensive photometric and multi-site spectroscopic observing campaign for
studying the pulsations of the central star.
Aims: The aim of the present study is to investigate the pulsation properties
of HR 8799 in detail via the ultra-precise 47-d-long nearly continuous
photometry obtained with the MOST space telescope, and to find as many
independent pulsation modes as possible, which is the prerequisite of an
asteroseismic age determination.
Methods: We carried out Fourier analysis of the wide-band photometric time
series.
Results: We find that resonance and sudden amplitude changes characterize the
pulsation of HR 8799. The dominant frequency is always at f1 = 1.978 c/d. Many
multiples of one ninth of the dominant frequency appear in the Fourier spectrum
of the MOST data: n/9 f1, where n={1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 17,
18}. Our analysis also reveals that many of these peaks show strong amplitude
decrease and phase variations even on the 47-d time-scale. The dependencies
between the pulsation frequencies of HR 8799 make the planned subsequent
asteroseismic analysis rather difficult. We point out some resemblance between
the light curve of HR 8799 and the modulated pulsation light curves of Blazhko
RR Lyrae stars.Comment: Accepted for publication in Astronomy and Astrophysics. 8 pages, 4
figure
GQ Lup B Visible & Near-Infrared Photometric Analysis
We have re-analyzed archival HST and Subaru data of the recently discovered
planetary mass companion (PMC) GQ Lup B. With these we produce the first R and
I band photometry of the companion and fit a radius and effective temperature
using detailed model atmospheres. We find an effective temperature of 2335 +/-
100K, and a radius of 0.38 +/- 0.05 Rsol and luminosity of log(L/Lsol) = -2.42
+/- 0.07 (at 140pc). Since we fit wavelengths that span most of the emitted
radiation from GQ Lup this luminosity estimate is robust, with uncertainty
dominated by the distance uncertainty (+/- 50 pc). The radius obtained for
140pc (0.38Rsol) is significantly larger than the one originally derived and
larger than model predictions. The mass of the object is much more
model-dependent than the radiative properties, but for the GAIA dusty models we
find a mass between 10-20 MJup, in the range of the brown dwarf and PMC
deuterium burning boundary. Assuming a distance of 140pc, observations fit to
1sigma the Baraffe evolution model for a ~15 MJup brown dwarf. Additionally,
the F606W photometric band is significantly overluminous compared to model
predictions and other brown dwarfs. Such overluminosity could be explained by a
bright Halpha emission from chromospheric activity, interaction with another
undetected companion, or accretion. Assuming that GQ Lup B has a bright Halpha
emission line, its Halpha emission strength is 10^(-1.71 +/- 0.10) Lbol,
significantly larger than field late-type dwarfs. GQ Lup B might be strongly
accreting and still be in its formation phase.Comment: 14 pages, 3 figures, ApJL final accepted versio
Dissecting the Moth: Discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging
The debris disk known as "The Moth" is named after its unusually asymmetric
surface brightness distribution. It is located around the ~90 Myr old G8V star
HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6
microns. Polarimetric observations suggested that the circumstellar material
consists of two distinct components, a nearly edge-on disk or ring, and a
swept-back feature, the result of interaction with the interstellar medium. We
resolve both components at unprecedented resolution with VLT/NACO H-band
imaging. Using optimized angular differential imaging techniques to remove the
light of the star, we reveal the disk component as a distinct narrow ring at
inclination i=84.3 \pm 1.0{\deg}. We determine a semi-major axis of a=61.25 \pm
0.85 AU and an eccentricity of e=0.045 \pm 0.015, assuming that periastron is
located along the apparent disk major axis. Therefore, the ring center is
offset from the star by at least 2.75 \pm 0.85 AU. The offset, together with a
relatively steep inner rim, could indicate a planetary companion that perturbs
the remnant planetesimal belt. From our imaging data we set upper mass limits
for companions that exclude any object above the deuterium-burning limit for
separations down to 0.3". The ring shows a strong brightness asymmetry along
both the major and minor axis. A brighter front side could indicate
forward-scattering grains, while the brightness difference between the NE and
SW components can be only partly explained by the ring center offset,
suggesting additional density enhancements on one side of the ring. The
swept-back component appears as two streamers originating near the NE and SW
edges of the debris ring.Comment: 6 pages, 6 figures. Accepted to Astronomy and Astrophysics letter
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