Evolution of near UV Halley's spectrum in the inner coma

A direct way to observe the photodissociation of water vapor in a cometary coma is to detect the OH prompt emission. This emission is shifted of delta lambda = 4 nm with respect to the OH 309 nm fluorescence band. The extended data set obtained with the three-channel spectrometer on-board Vega 2 reveals at short distance of the nucleus (i.e., less than 600 km) an excess of emission on the right wing of the OH band which may be interpreted as being mainly due to prompt emission

Dust and gas jets: Evidence for a diffuse source in Halley's coma

The distribution of dust-scattered intensity in Halley's inner coma is measured with the Vega three-channel spectrometer at three selected wavelengths: 377, 482, and 607 nm. The variation along a cometo-centric radius may be described by a p(sup -s) law where p is the distance between nucleus and optical axis and s is an exponent which is equal to 1 except in an intermediate 3000 less than p less than 7000 km region where s = 1.5. The shape of the radial distribution may be explained with a model including solar radiation pressure effect and quantum scattering efficiencies calculated from Mie theory. Monochromatic images inside an angular sector having its apex at the nucleus show evidence of two dust jets which extend to 40,000 Km. The pixel-to-pixel ratio of two images of dust intensity at 377 and 482 nm shows that the scattered intensity presents an excess of blue coloration in a zone located around the jets between 10,000 and 25,000 km. This coloration is interpreted as being due to a population of sub-micronic grains which result of the fragmentation of dust particles transported in the jets. It is suggested that the diffuse source where an additional quantity of CO was detected might be connected with the presence of a dust jet. In the present scheme, grain particles with a size of several micron or 10 micron would be transported inside a dust jet to distances of several 10,000 km where they would suffer fragmentation and produce sub-micronic particles and a release of gas which would be at the origin of the diffuse source

Semi-automated stereoradiographic upper limb 3D reconstructions using a combined parametric and statistical model: a preliminary study

PURPOSE: Quantitative assessment of 3D clinical indices may be crucial for elbow surgery planning. 3D parametric modeling from bi-planar radiographs was successfully proposed for spine and lower limb clinical investigation as an alternative for CT-scan. The aim of this study was to adapt this method to the upper limb with a preliminary validation. METHODS: CT-scan 3D models of humerus, radius and ulna were obtained from 20 cadaveric upper limbs and yielded parametric models made of geometric primitives. Primitives were defined by descriptor parameters (diameters, angles...) and correlations between these descriptors were found. Using these correlations, a semi-automated reconstruction method of humerus using bi-planar radiographs was achieved: a 3D personalized parametric model was built, from which clinical parameters were computed [orientation and projections on bone surface of trochlea sulcus to capitulum (CTS) axis, trochlea sulcus anterior offset and width of distal humeral epiphysis]. This method was evaluated by accuracy compared to CT-scan and reproducibility. RESULTS: Points-to-surface mean distance was 0.9 mm (2 RMS = 2.5 mm). For clinical parameters, mean differences were 0.4-1.9 mm and from 1.7° to 2.3°. All parameters except from angle formed by CTS axis and bi-epicondylar axis in transverse plane were reproducible. Reconstruction time was about 5 min. CONCLUSIONS: The presented method provides access to morphological upper limb parameters with very low level of radiation. Preliminary in vitro validation for humerus showed that it is fast and accurate enough to be used in clinical daily practice as an alternative to CT-scan for total elbow arthroplasty pre operative evaluation

Astronomical observation through the NIR atmospheric emissive layer

International audienceThe emission of the upper atmosphere introduces an additional component into photometric observations of astronomical objects. In the I band for instance, the intensity of the atmospheric emission is of the order of 1 to 2 Imag20 per square arcsecond. The subtraction of this component is not easy because it varies during the night by as much as 50% and it is not homogeneous over the sky. A program aimed at understanding the main characteristics of the atmospheric emission was undertaken. A set of CCD images of the OH emission in the I band covering the sky was assembled in a panorama, it shows wide converging arches. An algorithm was developed in order to invert the perspective projection of the photographs. The result is a 2200 km wide view over Europe and Mediterranean Sea of the emission as seen from a virtual satellite. This image shows the presence of an extended wave field. A Fourier analysis allows to infer mean horizontal wavelength, mean temporal period and horizontal phase velocity. The atmospheric emission varies under the influence of atmospheric waves. A stereoscopic imaging program is currently under development to measure the amplitude and the energy of the atmospheric waves

Spectroscopic study of comet Halley by the Vega 2 three-channel spectrometer

International audienceThe three-channel spectrometer (TKS) aboard the Vega 2 spacecraft recorded infrared and visible spectra near the nucleus of comet Halley. Spectra in the range 0.95–1.9 µm at a distance of 300 km from the nucleus reveal the H2O 1.38-µm band and OH bands, the latter excited by dissociation of a parent molecule

Near-ultraviolet and visible spectrophotometry of comet Halley from Vega 2

International audienceWe report results obtained by the near-ultraviolet and visible channel (V) of the TKS instrument1 carried by Vega 2 during the hour preceding its closest approach to comet Halley. The brightness of the OH (0, 0) band is found to be 1.2, 1.4 and 1.8 MR (megarayleigh; 1 R = 106 photons cm−2 s−1 (4π sr)−1) at distances of 4,900, 2,450 and <1,000 km from the nucleus. The OH gas production rate is estimated to have been ∼9×1029 molecules s−1 on 9 March 1986. Modifications in the structure of the OH and CN bands observed as the distance from the line of sight to the nucleus decreases probably reflect the change in the vibration–rotation line intensity distribution of the radicals, due to the presence of radicals created in an excited state by the photolysis of their parent molecules