A CATALOG OF NEAR-IR SOURCES FOUND TO BE UNRESOLVED WITH MILLIARCSECOND RESOLUTION

\n \nIn the framework of an ongoing series of high angular resolution\nobservations by lunar occultations in the near infrared, we present\naccurate angular measurements\nfor ten late-type giant stars, seven of which were measured\nfor the first time.\nThe sample includes one K and seven\nM stars. It also includes two S stars, representing to\nour knowledge the first direct determination of the angular\ndiameter for this class of stars.\nThe measured angular\ndiameters range from 2.50 to 4.82 milliarcsec, with\nan average accuracy <5% . An exception is represented\nby the case of a star with a faint companion, where we have been\nable to investigate in detail the bias on the diameter determination,\nand hence on the effective temperature, which would be caused\nif the companion had not been included in the analysis. We\nsuggest that faint undetected companions could explain at\nleast some of the hitherto known cases of stars with\neffective temperatures remarkably lower than the standard\ncalibration.\nFor seven of the stars in the sample, we have used\nour own photometry in conjunction with values from the\nliterature, to compute the bolometric fluxes and the\neffective temperatures, which we discuss in the context of\nexisting calibrations.\nWith the achieved accuracy level in the angular\ndiameter, such measurements are also\nvaluable as calibrators or science verification targets for\nmodern large ground-based interferometers.\n\n

We present the design study of an electrostatic analyzer that permits combined high temporal, energy, and angular resolution measurements of solar wind ions. The requirements for high temporal, energy, and angular resolutions, combined with the need for sufficient counting statistics, lead to an electrostatic analyzer with large radius (140 mm) and large geometric factor. The resulting high count rates require the use of channel electron multipliers (CEMs), instead of microchannel plates, to avoid saturation. The large radius further permits the placement of 32 CEM detectors at the analyzer focal plane, thereby providing very high angular resolution in azimuth (1.5°). Electrostatic simulations were performed to define the analyzer geometric factor, energy resolution, analyzer constant (K), elevation response, etc. Simulations were also performed to define the geometry of the deflectors and collimator that are used to provide the proper energy resolution, field of view, and angular resolution (1.5°) in elevation as well (the total field of view of the design is ±24° × ±24°). We show how this design permits unprecedented measurements of the fine structure of the solar wind proton beam and other important features such as temperature anisotropy. This design is used for the Cold Solar Wind instrument of the medium‐class Turbulent Heating ObserveR mission, currently in phase A at the European Space Agency. These unprecedented measurement capabilities are in accordance with and even beyond the requirements of the mission.

Q-ball imaging (QBI) is one of the typical data models for quantifying white matter (WM) anisotropy in diffusion-weighted MRI (DwMRI) studies. Brain and spinal investigation by high angular resolution DwMRI (high angular resolution imaging (HARDI)) protocols exhibits higher angular resolution in diffusion imaging compared to low angular resolution models, although with longer acquisition times. We aimed to assess the difference between QBI-derived anisotropy values from high and low angular resolution DwMRI protocols and their potential advantages or shortcomings in neuroradiology. Brain DwMRI data sets were acquired in seven healthy volunteers using both HARDI (b = 3000 s/mm(2), 54 gradient directions) and low angular resolution (b = 1000 s/mm(2), 32 gradient directions) acquisition schemes. For both sequences, tract of interest tractography and generalized fractional anisotropy (GFA) measures were extracted by using QBI model and were compared between the two data sets. QBI tractography and voxel-wise analyses showed that some WM tracts, such as corpus callosum, inferior longitudinal, and uncinate fasciculi, were reconstructed as one-dominant-direction fiber bundles with both acquisition schemes. In these WM tracts, mean percent different difference in GFA between the two data sets was less than 5%. Contrariwise, multidirectional fiber bundles, such as corticospinal tract and superior longitudinal fasciculus, were more accurately depicted by HARDI acquisition scheme. Our results suggest that the design of optimal DwMRI acquisition protocols for clinical investigation of WM anisotropy by QBI models should consider the specific brain target regions to be explored, inducing researchers to a trade-off choice between angular resolution and acquisition time.

\n Context. Lunar occultations (LO) are a very efficient and powerful\n technique that achieves the best combination of high angular resolution and sensitivity\n possible today at near-infrared wavelengths. Given that the events are fixed in time, that\n the sources are occulted randomly, and that the telescope use is minimal, the technique is\n very well suited for service mode observations. \n Aims. We have established a program of routine LO observations at the\n VLT observatory, especially designed to take advantage of short breaks available\n in-between other programs. We have used the ISAAC instrument in burst mode, capable of\n producing continuous read-outs at millisecond rates on a suitable subwindow. Given the\n random nature of the source selection, our aim has been primarily the investigation of a\n large number of stellar sources at the highest angular resolution in order to detect new\n binaries. Serendipitous results such as resolved sources and detection of circumstellar\n components were also anticipated. \n Methods. We have recorded the signal from background stars for a few\n seconds, around the predicted time of occultation by the Moon’s dark limb. At millisecond\n time resolution, a characteristic diffraction pattern can be observed. Patterns for two or\n more sources superimpose linearly, and this property is used for the detection of binary\n stars. The detailed analysis of the diffraction fringes can be used to measure specific\n properties such as the stellar angular size and the presence of extended light sources\n such as a circumstellar shell. \n Results. We present a list of 191 stars for which LO data could be\n recorded and analyzed. Results include the detection of 16 binary and 2 triple stars, all\n but one of which were previously unknown. The projected angular separations are as small\n as 4 milliarcsec and magnitude differences as high as ΔK = 5.8 mag.\n Additionally we derive accurate angular diameters for 2 stars and resolve circumstellar\n emission around another one, also all for the first time. We have established upper limits\n on the angular size of 177 stars, mostly in the 1 to 5 mas range, and we plan to include\n them in a future list of sources well suited for the calibration of interferometers.\n Conclusions. We confirm the performance of the technique already\n established in our previous work. LO at an 8 m-class telescope can achieve an angular\n resolution close to \n $0\\farcs001$ with a sensitivity\n K ≈12 mag. \n

This PhD thesis presents a study of active galaxies carried out from cm- to mm-wavelengths with high angular resolution. The mechanism of the activity in these objects is supposed to be strongly correlated with the accretion of matter onto a super-massive black hole in their centres. One approach to increase the understanding of these highly interesting sources is to observe and analyse the molecular gas. The characteristics of its distribution and dynamics are indispensible diagnostic tools to investigate the accretion processes at different angular scales. A second approach is the study of emission originating in the direct vicinity of the black hole, such as non-thermal radio emission. To account for different activity levels, five objects were chosen ranging from nearby, Low Luminosity Active Galaxies (LLAGs; NGC3718 at z=0.003 and NGC1068 at z=0.004) to higher redshifted, High LAGs (HLAGs; HE1029-1831 at z=0.039, 3C48 at z=0.367, and Q0957+561 at z=1.414). The first two sources are part of the NU(clei of)-GA(laxy) project, that aims at analysing the distribution and kinematics of the molecular gas at high angular resolution/sensitivity in a sample of 30 nearby LLAGs. HE1029-1831 is part of a complementary sample of nearby HLAGs with similar aims. All sources were observed in carbon-monoxide (CO), known to be a good molecular gas tracer, with the IRAM Plateau de Bure Interferometer (PdBI) providing high sensitivity and high angular resolution. CO emission was detected in all five galaxies. In NGC3718, the molecular gas disk, having roughly two hundred million solar masses, is highly warped. The distribution of the gas reveals large scale asymmetries witnessing a possible tidal tail interaction with a close companion, and as well small scale asymmetries most likely tracing accretion onto the nucleus ...

view Abstract Citations (9) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Composite Spectra in Young Stars: Accretion or Close Companions? Koresko, Chris D. Abstract Three young low-mass stars which exhibit composite optical spectra were imaged at high angular resolution in the near-infrared to determine whether their spectral peculiarities are due to the presence of close binary companions. Companions to two of the three stars were found to be capable of producing the observed spectra, but only if they are much younger than their primaries. If the inferred age differences are real, they suggest that a mechanism must exist which can perturb the evolution of a young star in a binary system from its Hayashi track. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175312 Bibcode: 1995ApJ...440..764K Keywords: ACCRETION; ACCRETION DISKS; STARS: BINARIES: CLOSE; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (10)

Trade-off between angular and spatial resolutions in in vivo fiber tractography

The current state of knowledge of angular diameters of stars is reviewed and, based on this review and the requirements for the determination of surface fluxes, effective temperatures, radii and masses, targets of sensitivity, angular resolution and accuracy for future programs of stellar angular diameter measurements are established. Long baseline interferometry is the only technique with the potential to meet all the targets. The necessary improvements in sensitivity, angular resolution and accuracy are promised by the approach adopted in the modern Michelson stellar interferometer under development at the University of Sydney and the prototype instrument, which is currently nearing completion, is briefly described to illustrate how the atmospheric and mechanical problems which have inhibited the development of amplitude interferometry may be overcome using modern technology. This program together with the developments taking place at CERGA lead to the conclusion that the prospects for contributions by high angular resolution measurements to the determination of fundamental stellar quantities during the next decade are excellent.

The current state of knowledge of angular diameters of stars is reviewed and, based on this review and the requirements for the determination of surface fluxes, effective temperatures, radii and masses, targets of sensitivity, angular resolution and accuracy for future programs of stellar angular diameter measurements are established. Long baseline interferometry is the only technique with the potential to meet all the targets. The necessary improvements in sensitivity, angular resolution and accuracy are promised by the approach adopted in the modern Michelson stellar interferometer under development at the University of Sydney and the prototype instrument, which is currently nearing completion, is briefly described to illustrate how the atmospheric and mechanical problems which have inhibited the development of amplitude interferometry may be overcome using modern technology. This program together with the developments taking place at CERGA lead to the conclusion that the prospects for contributions by high angular resolution measurements to the determination of fundamental stellar quantities during the next decade are excellent.

Angle‐enabled wavefront sensing based on metasurface can realize high‐spatial‐resolution wavefront reconstruction without requiring reference‐beam interference, which has great potential in applications in various fields ranging from biological cell characterization to surface metrology. However, the current metasurface wavefront encoding scenario exhibits weak modulation capabilities with an oversimplified angular modulation function, resulting in a limited angular resolution. Here, a wavefront sensor based on random wavefront coding with a diverse angle response function, achieved through a diffractive optical element (DOE), is proposed, which can simultaneously provide a large dynamic range, high spatial resolution, and high angular resolution. By experimentally calibrating the mapping between DOE diffraction patterns and incident angles, one can precisely decode the local incident angles. Combined with zonal wavefront reconstruction algorithms, the unknown wavefront can be reconstructed accurately. Owing to the drastically improved resolution, the wavefront sensor can be used for quantitative phase imaging, which shows a good capability in surface topography measurement for both static and dynamically evolving samples. The method provides valuable insights for high‐resolution wavefront sensing and quantitative phase imaging.

The particle size of submicron particles significantly affects their properties; thus, the accurate measurement of submicron particle size is essential to ensure its excellent properties. Polarized light scattering is an important tool for measuring the particle size of the ensemble of particles in suspension. However, in the existing measurement systems, the polarized scattered light is detected using a CCD detector or an array of single-point detectors. The CCD detector misses a large part of the polarized scattered light due to its narrow detection range of scattering angles, and the array of single-point detectors has the problem of low angular resolution due to the limited number of detectors. According to the above problems, this paper designs a submicron particle size measurement method based on the polarization difference in polarized scattered light with high angular resolution. The vertically and horizontally polarized scattered light was acquired with high angular resolution (angular separation = 2°) over a scattering angle range of 50°–110° using a photomultiplier coupled with a turntable. The scattering angle of the acquired vertically and horizontally polarized scattered light were corrected to eliminate the scattering angle deviations caused by obliquely incident light, and then the polarization difference in the vertically and horizontally polarized scattered light was computed, from which the submicron particle size distribution was inverted subsequently. Experiments were performed using polystyrene microsphere standard particles with particle sizes of 350 nm, 200 nm, and 100 nm. The experimental results show that (1) the Pearson correlation coefficient of the linearly fitted curve of the corrected polarization difference to the theoretical polarization difference is larger than 0.997, and the slope and intercept of the linearly fitted curve are, respectively, close to 1 and 0, indicating that the corrected polarization difference is highly consistent with the theoretical polarization difference; (2) the mean relative error and coefficient of variation of the particle size distribution parameter D50 obtained from the polarization difference with high angular resolution (angular separation = 2°) are better than those of the parameter D50 obtained from the polarization difference with low angular resolution (angular separation = 12°), indicating better accuracy and repeatability of the particle size distribution inverted from the polarization difference with high angular resolution; and (3) for the particle size distribution parameters D10, D50, and D90 obtained from the scattering angle-corrected polarization difference with high angular resolution, the deviation of the measured values from the average value are all smaller than the thresholds given in the international standard, indicating a good repeatability of the proposed method.

The Fresnel interferometric imager is a new kind of high angular resolution space instrument for the UV domain, and the related astrophysical targets. This optical concept is meant to allow larger and lighter apertures in space than solid state optics. It yields high dynamic range images and same resolution as that of a solid aperture of the same size. The long focal lengths of the Fresnel imager (a few kilometers) require operation by two-vessel formation flying in space. The first vessel holds a large and thin opaque foil punched with thousands of holes: the interferometric array, the second vessel holds the focal instrumentation. This Fresnel imager has been designed for mapping high contrast stellar environments: dust disks, close companions and (we hope) exoplanets. Compact objects such as large stellar photospheres may be imaged with array sizes of a few meters in the UV. Larger and more complex fields can also be imaged, although with a lesser dynamic range, such as small fields on galactic clouds or extragalactic fields, or in an other domain: small solar system bodies. We present the first images obtained on artificial sources with an 8 cm laboratory testbed array having 26680 apertures, the measured dynamic range of these images and their diffraction limited angular resolution. A 3 m class probatory space mission will be studied and follow a validation path, It has been submitted as a proposal to the ESA Cosmic Vision program.

The GAMMA-400 gamma-ray telescope is designed to measure the gamma-ray fluxes in the energy range from ∼20 MeV to ∼1 TeV, performing a sensitive search for high-energy gamma-ray emission when annihilating or decaying dark matter particles. Such measurements will be also associated with the following scientific goals: searching for new and studying known Galactic and extragalactic discrete high-energy gamma-ray sources (supernova remnants, pulsars, accreting objects, microquasars, active galactic nuclei, blazars, quasars). It will be possible to study their structure with high angular resolution and measuring their energy spectra and luminosity with high-energy resolution; identify discrete gamma-ray sources with known sources in other energy ranges. The major advantage of the GAMMA-400 instrument is excellent angular and energy resolutions for gamma rays above 10 GeV. The gamma-ray telescope angular and energy resolutions for the main aperture at 100-GeV gamma rays are ∼0.01% and ∼1%, respectively.The motivation of presented results is to improve physical characteristics of the GAMMA-400 gamma-ray telescope in the energy range of ∼20-100 MeV, most unexplored range today. Such observations are crucial today for a number of high-priority problems faced by modern astrophysics and fundamental physics, including the origin of chemical elements and cosmic rays, the nature of dark matter, and the applicability range of the fundamental laws of physics. To improve the reconstruction accuracy of incident angle for low-energy gamma rays the special analysis of topology of pair-conversion events in thin layers of converter performed. Choosing the pair-conversion events with more precise vertical localization allows us to obtain significantly better angular resolution in comparison with previous and current space and ground-based experiments. For 50-MeV gamma rays the GAMMA-400 gamma-ray telescope angular resolution is better than 50.

High angular resolution images of IRC +10216 taken at various bandpasses within the near-infrared H, K, and L bands are presented. The maps have the highest angular resolution yet recovered and were reconstructed from interferometric measurements obtained at the Keck I telescope in 1997 December and 1998 April, forming a subset of a seven-epoch monitoring program presented earlier by Tuthill and coworkers in Paper I. Systematic changes with observing wavelength are found and discussed in the context of present geometrical models for the circumstellar envelope. With these new high-resolution, multiwavelength data and contemporaneous photometry, we also revisit the hypothesis that the bright compact core of the nebula (component A) marks the location of the central carbon star. We find that directly measured properties of the core (angular size, flux density, color temperature) are consistent with a reddened carbon star photosphere (line-of-sight τ2.2 = 5.3).

The diameter of a star is a major observable that serves to test the validity of stellar structure theories. It is also a difficult observable that is mostly obtained with indirect methods since the stars are so remote. Today only ~600 apparent star diameters have been measured by direct methods: optical interferometry and lunar occultations. Accurate star diameters are now required in the new field of exoplanet studies, since they condition the planets' sizes in transit observations, and recent publications illustrate a visible renewal of interest in this topic. Our analysis is based on the modeling of the relationship between measured angular diameters and photometries. It makes use of two new reddening-free concepts: a distance indicator called pseudomagnitude, and a quasi-experimental observable that is independent of distance and specific to each star, called the differential surface brightness (DSB). The use of all the published measurements of apparent diameters that have been collected so far, and a careful modeling of the DSB allow us to estimate star diameters with a median statistical error of 1%, knowing their spectral type and, in the present case, the VJHKs photometries. We introduce two catalogs, the JMMC Measured Diameters Catalog (JMDC), containing measured star diameters, and the second version of the JMMC Stellar Diameter Catalog (JSDC), augmented to about 453000 star diameters. Finally, we provide simple formulas and a table of coefficients to quickly estimate stellar angular diameters and associated errors from (V, Ks) magnitudes and spectral types.