COBE ground segment attitude determination

The Cosmic Background Explorer (COBE) spacecraft was launched in November 1989 by NASA to survey the sky for primordial radiation left from the Big Bang explosion. The success of the mission requires an accurate determination of the spacecraft attitude. While the accuracy of the attitude obtained from the attitude sensors is adequate for two of the experiments, the higher accuracy required by the Diffuse Infrared Background Experiment (DIRBE) is obtained by using the DIRBE instrument as a special type of star sensor. Presented here is an overview of the attitude processing algorithms used at the Cosmology Data Analysis Center (CDAC) and the results obtained from the flight data

γ∗NΔ\gamma^{*}N\Delta Form Factors from a Relativistic Dynamical Model of Pion Electroproduction

We obtain the electromagnetic form factors of the $\gamma N\Delta$ transition by analyzing recent pion-electroproduction data using a fully relativistic dynamical model. Special care is taken to satisfy Ward-Takahashi identities for the Born term in the presence of form factors thereby allowing the use of realistic electromagnetic form factors of the nucleon and pion. We parametrize the $Q^2$ dependence of the {\it bare} $\gamma N \Delta$ form factors by a three-parameter form which is consistent with the asymptotic behavior inferred from QCD. The parameters of the bare $\gamma N \Delta$ form factors are the only free parameters of the model and are fitted to the differential cross-section and multipole-analysis data up to $Q^2=4$ (GeV/c)$^2$ in the $\Delta(1232)$-resonance region. This analysis emphasizes the significance of the pion-cloud effects in the extraction of the resonance parameters.Comment: 9 pages, 5 figures, several small corrections, to be published in Phys. Rev.

Separation of Q from carbon in CR meteorites during stepped combustion

Introduction: The nature of the planetary noble gas carrier (Q) in meteorites remains uncertain. It is known that it is likely to be carbonaceous, but represents only a small fraction of the total macromolecular material. Q is oxidisible with nitric and other oxidizing acids. It seems to be partly destroyed with pyridine and may have an organic structure. Previously, we have shown that during parent body thermal metamorphism Q is less affected than the majority of other carbonaceous materials. If organic matter is graphitized, as has happened in the enstatite chondrite parent bodies, Q remains unaffected. In the present study we have found that Q is also separable from the majority of carbon in type 2 and 3 CR chondrites during stepped combustion. It is possible that this is because Q has become encased within the matrix, in contrast to other carbon phases, during parent body metamorphism

On the variations of the elemental composition of the P3 component in presolar diamonds

Analysis of noble gas element variations in the P3 component of presolar diamonds indicates that a special mechanism of element fractionation is required to explain the variations

Presolar diamonds in Krymka: C, N and Xe isotope data from grain-size separates and comparison with other meteorites

Comparison of carbon isotope variations in grain size fractions of presolar diamonds separated from three meteorites (Efremovka, Boriskino and Krymka) suggest that diamond from various parts of solar nebular have had diffrent C isotope signature

Is Xe-HL a real component?

Analysis of noble gases in the grain-size fractions of Boriskino meteorite indicate that Xe-HL is a real component implanted into diamonds after mixing of isotopically anomalous and normal components

Infrared Emission from the Radio Supernebula in NGC 5253: A Proto-Globular Cluster?

Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253 lies an intense radio source with an unusual spectrum which could be interpreted variously as nebular gas ionized by a young stellar cluster or nonthermal emission from a radio supernova or an AGN. We have obtained 11.7 and 18.7 micron images of this region at the Keck Telescope and find that it is an extremely strong mid-infrared emitter. The infrared to radio flux ratio rules out a supernova and is consistent with an HII region excited by a dense cluster of young stars. This "super nebula" provides at least 15% of the total bolometric luminosity of the galaxy. Its excitation requires 10^5-10^6 stars, giving it the total mass and size (1-2 pc diameter) of a globular cluster. However, its high obscuration, small size, and high gas density all argue that it is very young, no more than a few hundred thousand years old. This may be the youngest globular cluster yet observed.Comment: 6 pages, 2 color figures, Submitted to the ApJL, Revised 4/6/01 based on referee's comment

COBE ground segment gyro calibration

Discussed here is the calibration of the scale factors and rate biases for the Cosmic Background Explorer (COBE) spacecraft gyroscopes, with the emphasis on the adaptation for COBE of an algorithm previously developed for the Solar Maximum Mission. Detailed choice of parameters, convergence, verification, and use of the algorithm in an environment where the reference attitudes are determined form the Sun, Earth, and star observations (via the Diffuse Infrared Background Experiment (DIRBE) are considered. Results of some recent experiments are given. These include tests where the gyro rate data are corrected for the effect of the gyro baseplate temperature on the spacecraft electronics