1,021 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
X-ray emission from the remarkable A-type star HR 8799
We present a Chandra observation of the exceptional planet bearing A5V star
HR 8799, more precisely classified as a kA5hF0mA5 star and search for intrinsic
X-ray emission. We clearly detect HR 8799 at soft X-ray energies with the
ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is
present during the observation. The coronal plasma is described well by a model
with a temperature of around 3 MK and an X-ray luminosity of about Lx = 1.3 x
10^28 erg/s in the 0.2-2.0 keV band, corresponding to an activity level of log
Lx/Lbol ~ -6.2. Altogether, these findings point to a rather weakly active and
given a RASS detection, long-term stable X-ray emitting star. The X-ray
emission from HR 8799 resembles those of a late A/early F-type stars, in
agreement with its classification from hydrogen lines and effective temperature
determination and thus resolving the apparent discrepancy with the standard
picture of magnetic activity that predicts mid A-type stars to be virtually
X-ray dark.Comment: 4 pages, 3 figures, accepted by A&
Direct Imaging of Multiple Planets Orbiting the Star HR 8799
Direct imaging of exoplanetary systems is a powerful technique that can
reveal Jupiter-like planets in wide orbits, can enable detailed
characterization of planetary atmospheres, and is a key step towards imaging
Earth-like planets. Imaging detections are challenging due to the combined
effect of small angular separation and large luminosity contrast between a
planet and its host star. High-contrast observations with the Keck and Gemini
telescopes have revealed three planets orbiting the star HR 8799, with
projected separations of 24, 38, and 68 astronomical units. Multi-epoch data
show counter-clockwise orbital motion for all three imaged planets. The low
luminosity of the companions and the estimated age of the system imply
planetary masses between 5 and 13 times that of Jupiter. This system resembles
a scaled-up version of the outer portion of our Solar System.Comment: 30 pages, 5 figures, Research Article published online in Science
Express Nov 13th, 200
Morphology of the very inclined debris disk around HD 32297
Direct imaging of circumstellar disks at high angular resolution is mandatory
to provide morphological information that bring constraints on their
properties, in particular the spatial distribution of dust. New techniques
combining observing strategy and data processing now allow very high contrast
imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and
complement space telescopes while improving angular resolution at near infrared
wavelengths. We carried out a program at the VLT with NACO to image known
debris disks with higher angular resolution in the near IR than ever before in
order to study morphological properties and ultimately to detect signpost of
planets. The observing method makes use of advanced techniques: Adaptive
Optics, Coronagraphy and Differential Imaging, a combination designed to
directly image exoplanets with the upcoming generation of "planet finders" like
GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast
Exoplanet REsearch). Applied to extended objects like circumstellar disks, the
method is still successful but produces significant biases in terms of
photometry and morphology. We developed a new model-matching procedure to
correct for these biases and hence to bring constraints on the morphology of
debris disks. From our program, we present new images of the disk around the
star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an
unprecedented angular resolution (~65 mas). The images show an inclined thin
disk detected at separations larger than 0.5-0.6". The modeling stage confirms
a very high inclination (i=88{\deg}) and the presence of an inner cavity inside
r_0~110AU. We also found that the spine (line of maximum intensity along the
midplane) of the disk is curved and we attributed this feature to a large
anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged
...Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Discovery of a 66 mas Ultracool Binary with Laser Guide Star Adaptive Optics
We present the discovery of 2MASS J21321145+1341584AB as a closely separated
(0.066") very low-mass field dwarf binary resolved in the near-infrared by the
Keck II Telescope using laser guide star adaptive optics. Physical association
is deduced from the angular proximity of the components and constraints on
their common proper motion. We have obtained a near-infrared spectrum of the
binary and find that it is best described by an L5+/-0.5 primary and an
L7.5+/-0.5 secondary. Model-dependent masses predict that the two components
straddle the hydrogen burning limit threshold with the primary likely stellar
and the secondary likely substellar. The properties of this sytem - close
projected separation (1.8+/-0.3 AU) and near unity mass ratio - are consistent
with previous results for very low-mass field binaries. The relatively short
estimated orbital period of this system (~7-12 yr) makes it a good target for
dynamical mass measurements. Interestingly, the system's angular separation is
the tightest yet for any very low-mass binary published from a ground-based
telescope and is the tightest binary discovered with laser guide star adaptive
optics to date.Comment: 10 pages, 3 figures; accepted for publication to A
High Contrast Imaging of the Close Environment of HD 142527 -
Context. It has long been suggested that circumstellar disks surrounding
young stars may be the signposts of planets, and still more since the recent
discoveries of embedded substellar companions. The planet-disk interaction may
create, according to models, large structures, gaps, rings or spirals, in the
disk. In that sense, the Herbig star HD 142527 is particularly compelling as,
its massive disk displays intriguing asymmetries that suggest the existence of
a dynamical peturber of unknown nature. Aims. Our goal was to obtain deep
thermal images of the close circumstellar environment of HD 142527 to re-image
the reported close-in structures (cavity, spiral arms) of the disk and to
search for stellar and substellar companions that could be connected to their
presence. Results. The circumstellar environment of HD 142527 is revealed at an
unprecedented spatial resolution down to the sub arcsecond level for the first
time at 3.8 microns. Our images reveal important radial and azimuthal
asymmetries which invalidate an elliptical shape for the disk as previously
proposed. It rather suggests a bright inhomogeneous spiral arm plus various
fainter spiral arms. We also confirm an inner cavity down to 30 AU and two
important dips at position angles of 0 and 135 deg. The detection performance
in angular differential imaging enables the exploration of the planetary mass
regime for projected physical separations as close as 40 AU. The use of our
detection map together with Monte Carlo simulations sets stringent constraints
on the presence of planetary mass, brown dwarf or stellar companions as a
function of the semi-major axis. They severely constrain the presence of
massive giant planets with semi-major axis beyond 50AU, i.e. probably within
the large disk's cavity that radially extends up to 145 AU or even further
outside.Comment: 8 pages, 7 figures, accepted in A&
Education Policies and Intergenerational Educational Mobility in China: New Evidence for the 1986–95 Birth Cohort
Research on educational mobility for Chinese born in or before 1976–85 abounds. Although the Compulsory Education Law implemented in 1986 and the expansion of higher education introduced in 1999 changed Chinese millennials’ educational achievements, little is known about the educational mobility for the 1986–95 birth cohort and where it stands in the long-term trends. In this study, we calculated population-level educational percentile ranks by birth cohort and gender using data from the 1982 to 2020 China Censuses before linking these ranks to respondents in Chinese General Social Survey (CGSS) or China Family Panel Studies (CFPS) to document 1986–95 birth cohort's educational mobility and its historical position. We also explored the role played by offspring's hukou origin (urban or rural) and ethnicity (Han or ethnic minorities). In the 1986–95 birth cohort, women's educational percentile ranks for secondary and tertiary levels fell below men's for the first time in China, suggesting that the proportion of women in higher education overtook men's. From 1976–85 to 1986–95 birth cohorts, while educational rank-rank correlations remained stable in all parent–child dyads and were constantly higher for offspring with urban hukou origin, there is suggestive evidence on increased educational mobility for women with rural hukou origin. Ethnicity differences were not found. Our findings imply that China's Compulsory Education Law and higher education expansion may have contributed to greater educational mobility for women with rural hukou origin in the 1986–95 birth cohort and their diminished disadvantage in education
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