382 research outputs found
Linear polarization sensitivity of SeGA detectors
Parity is a key observable in nuclear spectroscopy. Linear polarization
measurements of -rays are a probe to access the parities of energy
levels. Utilizing the segmentation of detectors in the Segmented Germanium
Array (SeGA) at the NSCL and analyzing the positions of interaction therein
allows the detectors to be used as Compton polarimeters. Unlike other segmented
detectors, SeGA detectors are irradiated from the side to utilize the
transversal segmentation for better Doppler corrections. Sensitivity in such an
orientation has previously been untested. A linear polarization sensitivity has been measured in the 350-keV energy range for SeGA detectors
using - correlations from a \nuc{249}{Cf} source.Comment: 7 pages, 9 figure
Asymptotic models for the generation of internal waves by a moving ship, and the dead-water phenomenon
This paper deals with the dead-water phenomenon, which occurs when a ship
sails in a stratified fluid, and experiences an important drag due to waves
below the surface. More generally, we study the generation of internal waves by
a disturbance moving at constant speed on top of two layers of fluids of
different densities. Starting from the full Euler equations, we present several
nonlinear asymptotic models, in the long wave regime. These models are
rigorously justified by consistency or convergence results. A careful
theoretical and numerical analysis is then provided, in order to predict the
behavior of the flow and in which situations the dead-water effect appears.Comment: To appear in Nonlinearit
Gas and dust in the Beta Pictoris Moving Group as seen by the Herschel Space Observatory
Context. Debris discs are thought to be formed through the collisional
grinding of planetesimals, and can be considered as the outcome of planet
formation. Understanding the properties of gas and dust in debris discs can
help us to comprehend the architecture of extrasolar planetary systems.
Herschel Space Observatory far-infrared (IR) photometry and spectroscopy have
provided a valuable dataset for the study of debris discs gas and dust
composition. This paper is part of a series of papers devoted to the study of
Herschel PACS observations of young stellar associations.
Aims. This work aims at studying the properties of discs in the Beta Pictoris
Moving Group (BPMG) through far-IR PACS observations of dust and gas.
Methods. We obtained Herschel-PACS far-IR photometric observations at 70, 100
and 160 microns of 19 BPMG members, together with spectroscopic observations of
four of them. Spectroscopic observations were centred at 63.18 microns and 157
microns, aiming to detect [OI] and [CII] emission. We incorporated the new
far-IR observations in the SED of BPMG members and fitted modified blackbody
models to better characterise the dust content.
Results. We have detected far-IR excess emission toward nine BPMG members,
including the first detection of an IR excess toward HD 29391.The star HD
172555, shows [OI] emission, while HD 181296, shows [CII] emission, expanding
the short list of debris discs with a gas detection. No debris disc in BPMG is
detected in both [OI] and [CII]. The discs show dust temperatures in the range
55 to 264 K, with low dust masses (6.6*10^{-5} MEarth to 0.2 MEarth) and radii
from blackbody models in the range 3 to 82 AU. All the objects with a gas
detection are early spectral type stars with a hot dust component.Comment: 12 pages, 7 figures, 6 table
HST/NICMOS observations of a proto-brown dwarf candidate
We present deep HST/NICMOS observations peering through the outflow cavity of
the protostellar candidate IRAS 04381+2540 in the Taurus Molecular Cloud-1. A
young stellar object as central source, a jet and a very faint and close (0.6")
companion are identified. The primary and the companion have similar colours,
consistent with strong reddening. We argue that the companion is neither a
shock-excited knot nor a background star. The colour/magnitude information
predicts a substellar upper mass limit for the companion, but the final
confirmation will require spectroscopic information. Because of its geometry,
young age and its rare low-mass companion, this system is likely to provide a
unique insight into the formation of brown dwarfs.Comment: Astronomy & Astrophysics Letters, in press; 4 pages, 2 figure
Influence of grape juice extraction methods on basic analytical parameters
Currently, for monitoring the ripening of grape berries, different devices are used to produce the juices to be analysed. Crushing the berries is a key step that determines the quantity of juice extracted and may impact it composition. The effect of different devices on analytical parameters of the musts produced were compared in this study. Samples from four grape varieties ('Cabernet-Sauvignon', 'EkigaĂŻna', 'Marselan' and 'Vermentino'), showing a variability of berry size and precocity, were crushed using six different devices (ASieves, Bag mixerÂź, Crusher, Manual, TPress and Blender). Whatever the pressing equipment, sugar concentrations of the must were not modified by the extraction method, unlike other parameters. pH and titratable acidity were slightly impacted by the crushing method without changing the ranking of the varieties. However, potassium concentrations were more impacted by the pressing method. Differences in mechanical forces applied to skins and seeds according to the pressing equipment used may release more or less potassium. This study clearly discarded a complete grinding of the samples for grape ripening monitoring: this method strongly modified the potassium content and, consequently, the pH and the titratable acidity of the musts
Stellar Multiplicity
Stellar multiplicity is an ubiquitous outcome of the star formation process.
Characterizing the frequency and main characteristics of multiple systems and
their dependencies on primary mass and environment is therefore a powerful tool
to probe this process. While early attempts were fraught with selection biases
and limited completeness, instrumentation breakthroughs in the last two decades
now enable robust analyses. In this review, we summarize our current empirical
knowledge of stellar multiplicity for Main Sequence stars and brown dwarfs, as
well as among populations of Pre-Main Sequence stars and embedded protostars.
Clear trends as a function of both primary mass and stellar evolutionary stage
are identified that will serve as a comparison basis for numerical and
analytical models of star formation.Comment: original version submitted to ARA&A, final version to appear in vol.
51, comments welcome, 38 pages, 5 figure
FIRST, a pupil-remapping fiber interferometer at the Subaru Telescope: on-sky results
FIRST, the Fibered Imager foR a Single Telescope, is a spectro-imager using single-mode fibers for pupil remap- ping, allowing measurements beyond the telescope diffraction limit. Integrated on the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru Telescope, it benefits from a very stable visible light wave- front allowing to acquire long exposure and operate on significantly fainter sources than previously possible. On-sky results demonstrated the ability of the instrument to detect stellar companions separated 43mas in the case of the Capella binary system. A similar approach on an extremely large telescope would offer unique scientific opportunities for companion detection and characterization at very high angular resolution
FIRST, a pupil-remapping fiber interferometer at the Subaru Telescope: on-sky results
FIRST, the Fibered Imager foR a Single Telescope, is a spectro-imager using single-mode fibers for pupil remap- ping, allowing measurements beyond the telescope diffraction limit. Integrated on the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru Telescope, it benefits from a very stable visible light wave- front allowing to acquire long exposure and operate on significantly fainter sources than previously possible. On-sky results demonstrated the ability of the instrument to detect stellar companions separated 43mas in the case of the Capella binary system. A similar approach on an extremely large telescope would offer unique scientific opportunities for companion detection and characterization at very high angular resolution
Laboratory characterization of FIRSTv2 photonic chip for the study of substellar companions
FIRST (Fibered Imager foR a Single Telescope instrument) is a post-AO instrument that enables high contrast imaging and spectroscopy at spatial scales below the diffraction limit. FIRST achieves sensitivity and accuracy by a unique combination of sparse aperture masking, spatial filtering by single-mode fibers and cross-dispersion in the visible. The telescope pupil is divided into sub-pupils by an array of microlenses, coupling the light into single-mode fibers. The output of the fibers are rearranged in a non redundant configuration, allowing the measurement of the complex visibility for every baseline over the 600-900 nm spectral range. A first version of this instrument is currently integrated to the Subaru Extreme AO bench (SCExAO). This paper focuses on the on-going instrument upgrades and testings, which aim at increasing the instrumentâs stability and sensitivity, thus improving the dynamic range. FIRSTv2âs interferometric scheme is based on a photonic chip beam combiner. We report on the laboratory characterization of two different types of 5-input beam combiner with enhanced throughput. The interferometric recombination of each pair of sub-pupils is encoded on a single output. Thus, to sample the fringes we implemented a temporal phase modulation by pistoning the segmented mirrors of a Micro-ElectroMechanical System (MEMS). By coupling high angular resolution and spectral resolution in the visible, FIRST offers unique capabilities in the context of the detection and spectral characterization of close companions, especially on 30m-class telescopes
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