6,029 research outputs found
The inner regions of protoplanetary disks
To understand how planetary systems form in the dusty disks around
pre-main-sequence stars a detailed knowledge of the structure and evolution of
these disks is required. While this is reasonably well understood for the
regions of the disk beyond about 1 AU, the structure of these disks inward of 1
AU remains a puzzle. This is partly because it is very difficult to spatially
resolve these regions with current telescopes. But it is also because the
physics of this region, where the disk becomes so hot that the dust starts to
evaporate, is poorly understood. With infrared interferometry it has become
possible in recent years to directly spatially resolve the inner AU of
protoplanetary disks, albeit in a somewhat limited way. These observations have
partly confirmed current models of these regions, but also posed new questions
and puzzles. Moreover, it has turned out that the numerical modeling of these
regions is extremely challenging. In this review we give a rough overview of
the history and recent developments in this exciting field of astrophysics.Comment: 45 pages with 14 Figures. to appear in Annual Review of Astronomy and
Astrophysics (2010, Vol. 48
The Inner Rim of YSO Disks: Effects of dust grain evolution
Dust-grain growth and settling are the first steps towards planet formation.
An understanding of dust physics is therefore integral to a complete theory of
the planet formation process. In this paper, we explore the possibility of
using the dust evaporation front in YSO disks (`the inner rim') as a probe of
the dust physics operating in circumstellar disks. The geometry of the rim
depends sensitively on the composition and spatial distribution of dust. Using
radiative transfer and hydrostatic equilibrium calculations we demonstrate that
dust growth and settling can curve the evaporation front dramatically (from a
cylindrical radius of about 0.5 AU in the disk mid-plane to 1.2 AU in the disk
upper layers for an A0 star). We compute synthetic images and interferometric
visibilities for our representative rim models and show that the current
generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly
constrain the dust properties of circumstellar disks, shedding light on the
relatively poorly understood processes of grain growth, settling and turbulent
mixing.Comment: 26 pages, 9 figures. Accepted for publication in Ap
Multiplicity of Galactic Cepheids from long-baseline interferometry. II. The Companion of AX Circini revealed with VLTI/PIONIER
Aims: We aim at detecting and characterizing the main-sequence companion of
the Cepheid AX Cir ( 18 yrs). The long-term objective is
to estimate the mass of both components and the distance to the system.
Methods: We used the PIONIER combiner at the VLT Interferometer to obtain the
first interferometric measurements of the short-period Cepheid AX Cir and its
orbiting component. Results: The companion is resolved by PIONIER at a
projected separation mas and projection angle . We measured -band flux ratios between the companion and
the Cepheid of % and %, respectively at a
pulsation phase for the Cepheid and 0.48. The lower contrast at
is due to increased brightness of the Cepheid compared to the
. This gives an average apparent magnitude mag. The limb-darkened angular diameter of the Cepheid at the
two pulsation phases was measured to be
mas and mas, respectively at and 0.48. A lower limit on the total mass of the system was also derived
based on our measured separation, we found .Comment: Accepted for publication in Astronomy and Astrophysic
Endoscopic Findings of 100 Early-Stage Esophageal Cancers
The morphologic analysis of 100 early squamous cell carcinomas of the esophagus has shown that
the barely visible or invisible forms (erythroplakias and occult forms) are predominant. This explains
the poor yield of upper gastrointestinal (GI) endoscopies in detecting early cancers, at least in Western
countries. Leucoerythroplakias correspond to the most advanced form of early cancers (submucosal
invasion in approximately 38% of cases). Pure erythroplakias and occult forms correspond to in situ
or intramucosal cancers in over 90% of the cases
Fundamental properties and atmospheric structure of the red supergiant VY CMa based on VLTI/AMBER spectro-interferometry
We investigate the atmospheric structure and fundamental properties of the
red supergiant VY CMa. We obtained near-infrared spectro-interferometric
observations of VY CMa with spectral resolutions of 35 and 1500 using the AMBER
instrument at the VLTI. The visibility data indicate the presence of molecular
layers of water vapor and CO in the extended atmosphere with an asymmetric
morphology. The uniform disk diameter in the water band around 2.0 mu is
increased by \sim20% compared to the near-continuum bandpass at 2.20-2.25 mu
and in the CO band at 2.3-2.5 mu it is increased by up to \sim50%. The closure
phases indicate relatively small deviations from point symmetry close to the
photospheric layer, and stronger deviations in the extended H2O and CO layers.
Making use of the high spatial and spectral resolution, a near-continuum
bandpass can be isolated from contamination by molecular and dusty layers, and
the Rosseland-mean photospheric angular diameter is estimated to 11.3 +/- 0.3
mas based on a PHOENIX atmosphere model. Together with recent high-precision
estimates of the distance and spectro-photometry, this estimate corresponds to
a radius of 1420 +/- 120 Rsun and an effective temperature of 3490 +/- 90 K. VY
CMa exhibits asymmetric, possibly clumpy, atmospheric layers of H2O and CO,
which are not co-spatial, within a larger elongated dusty envelope. Our revised
fundamental parameters put VY CMa close to the Hayashi limit of recent
evolutionary tracks of initial mass 25 Msun with rotation or 32 Msun without
rotation, shortly before evolving blueward in the HR-diagram.Comment: 5 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics (A&A) as a Lette
High-pressure transport properties of CeRu_2Ge_2
The pressure-induced changes in the temperature-dependent thermopower S(T)
and electrical resistivity \rho(T) of CeRu_2Ge_2 are described within the
single-site Anderson model. The Ce-ions are treated as impurities and the
coherent scattering on different Ce-sites is neglected. Changing the
hybridisation \Gamma between the 4f-states and the conduction band accounts for
the pressure effect. The transport coefficients are calculated in the
non-crossing approximation above the phase boundary line. The theoretical S(T)
and \rho(T) curves show many features of the experimental data. The seemingly
complicated temperature dependence of S(T) and \rho(T), and their evolution as
a function of pressure, is related to the crossovers between various fixed
points of the model.Comment: 9 pages, 10 figure
Multisystem Langerhans' cell histiocytosis (Hand-SchĂŒller-Christian disease) in an adult: a case report and review of the literature
Langerhans' cell histiocytosis (LCH) is a rare and enigmatic clonal disorder that affects mainly children. It is characterized by single or multiple granulomatous mass lesions composed of cells with the Langerhans' cell phenotype. Clinical presentation and behavior are heterogeneous and can range from a solitary lytic bone lesion (i.e., eosinophilic granuloma) with a favorable course to a fatal disseminated leukaemia-like form, with a wide spectrum of intermediate clinical presentations between these two extremes. Although LCH typically involves the bone, lesions can be found in almost all organs. We are reporting the case of a multisystem LCH in a 47-year-old patient who presented with a panhypopituitarism and diabetes insipidus, and who, 5years later, developed mandibular, mastoid and femoral lesions. The final diagnosis of LCH was made on mandibular biops
Can we use shelterwoods in Mediterranean pine forests to promote oak seedling development?
International audienceThe use of shelterwoods to favour the development of natural or underplanted seedlings is common in temperate forests but rare in the pine forests of the Mediterranean area. Our aim was to assess the use of shelterwoods in Aleppo pine (Pinus halepensis) woodlands in southern France to promote the survival and growth of two co-occurring oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex. Twelve Aleppo pine stands were selected and differentially thinned to create a light shelterwood (G = 32 m2/ha, irradiance 13%), a medium shelterwood (G = 19 m2/ha, irradiance 33%) and a dense shelterwood (G = 10 m2/ha, irradiance 52%). A total of 1248 sowing points, half composed of Q. pubescens and half of Q. ilex, were then set up in these three conditions. Seedling survival and growth were monitored for three years and plant stress was assessed by measuring predawn leaf potential and photosynthetic performance through the Fv âFm ratio. Soil moisture was also recorded at two depths during two growing seasons. Survival was high for both species in all three conditions due to three consecutive wet years. The lowest survival was recorded for Q. pubescens in the dense shelterwoods. Growth in diameter and height increased from the dense to the light shelterwoods. Shrubs developed more strongly in the light shelterwood, and increasing shrub cover enhanced height growth. Photosynthetic performance was lowest for Q. pubescens in dense shelterwoods and highest in light shelterwoods, whereas the reverse was true for Q. ilex. The lowest predawn potentials were recorded in the dense shelterwoods even though higher soil water content values were measured in this treatment during the summer drought. We show that light shelterwoods were more beneficial to growth than denser ones, indicating control by light availability during the three years of the study. However, as lower soil moisture and faster understorey development were also recorded in this condition, more extended observation is needed to determine whether this benefit persists in subsequent years
Asymmetric Beam Combination for Optical Interferometry
Optical interferometers increasingly use single-mode fibers as spatial
filters to convert varying wavefront distortion into intensity fluctuations
which can be monitored for accurate calibration of fringe amplitudes. Here I
propose using an asymmetric coupler to allow the photometric intensities of
each telescope beam to be measured at the same time as the fringe visibility,
but without the need for dedicated photometric outputs, which reduce the light
throughput in the interferometric channels. In the read-noise limited case
often encountered in the infrared, I show that a 53% improvement in
signal-to-noise ratio for the visibility amplitude measurement is achievable,
when compared to a balanced coupler setup with 50% photometric taps (e.g., the
FLUOR experiment). In the Poisson-noise limit appropriate for visible light,
the improvement is reduced to only ~8%. This scheme also reduces the cost and
complexity of the beam combination since fewer components and detectors are
required, and can be extended to more than two telescopes for "all-in-one" or
pair-wise beam combination. Asymmetric beam combination can also be employed
for monitoring scintillation and throughput variations in systems without
spatial filtering.Comment: 13 pages, accepted by Publications of the Astronomical Society of the
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