Solar Models with Non-Standard Chemical Composition

The OPAL monochromatic opacity tables are used to evaluate the impact of a non-standard chemical composition on solar models. A calibrated solar model with consistent diffusion including the effect of radiative forces and ionization on drift velocities is presented. It is shown that surface abundances are predicted to change slightly more than in traditional solar models where these refinements are not included. All elements included in the model settle at similar rates which is reflected in the relative variation in surface abundances ranging from 7.5% for calcium to 8.8% for argon. The structural difference between the consistent model and the traditional model is small, with a maximum effect of 0.3% for the isothermal sound speed at the base of the convection zone. The settling of CNO is only marginally affected. Opacity profiles have also been calculated with varying abundances for volatile elements, for which the abundances are poorly known, and other selected elements. It is shown that if one allows a 10% variation of these elements individually one can expect a peak Rosseland mean opacity variation of 3% for oxygen, a little less 2% for Si and Ne, and around 1% for Mg and S in the radiative zone. Other light metals and volatile elements have no significant impact on the opacity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sedimenting particles and swimming micro-organisms in a rotating fluid.

Experiments and calculations on the trajectories of micron-sized spheres, suspended in a fluid that fills a dosed container which rotates about an axis perpendicular to g, relate to the planning and interpretation of clinostat experiments. For low Reynolds number motion, the orbits are nearly circular, the radius being inversely proportional to the rotation rate. The swimming direction of micro-organisms can be affected by light, gravity, vorticity etc. The trajectories of algae swimming in steadily rotating environments have been observed and compared with theoretical predictions for ideal gyrotactic micro-organisms, thus providing some insights into the mechanisms of gravitaxis, gyrotaxis and the behaviour of the cells.     

Maps of hydrogen isotopes at low altitudes in the inner zone from sampex observations

The PET instrument aboard the SAMPEX satellite has provided us with long-term intra-calibrated observations of geomagnetically trapped protons and deuterons in the inner zone, suitable for use in constraining the low-altitude portions of radiation belt models being developed as successors to AP-8. These observations have been summarized elsewhere (Looper et al., 1996). Here we report a detection of geomagnetically-trapped tritium at energies from 14 to 35 MeV/nuc below L = 1.2, at about 1/8 the flux of deuterium previously reported at that location and at similar energy per nucleon. We also demonstrate the utility of the SAMPEX/PET observations for measuring the east-west anisotropy in the trapped particle flux at low altitudes, which is due to displacement of particle gyrocenters from the position of observation in a region of strong flux gradients. This anisotropy is implicitly ignored in omnidirectional radiation-flux models, but it can be important to mission planners considering how to distribute shielding over the surface of oriented spacecraft in low Earth orbit.     

Effect of track structure and radioprotectors on the induction of oncogenic transformation in murine fibroblasts by heavy ions

The oncogenic potential of high-energy ⁵⁶Fe particles (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at the Brookhaven National Laboratory was examined utilizing the mouse C3H 10T1/2 cell model. The dose-averaged LET for high-energy ⁵⁶Fe is estimated to be 143 keV/μm with the exposure conditions used in this study. For ⁵⁶Fe ions, the maximum relative biological effectiveness (RBE_max) values for cell survival and oncogenic transformation were 7.71 and 16.5 respectively. Compared to 150 keV/μm ⁴He nuclei, high-energy ⁵⁶Fe nuclei were significantly less effective in cell killing and oncogenic induction. The prostaglandin E₁ analog misoprostol, an effective oncoprotector of C3H 10T1/2 cells exposed to X rays, was evaluated for its potential as a radioprotector of oncogenic transformation with high-energy ⁵⁶Fe. Exposure of cells to misoprostol did not alter ⁵⁶Fe cytotoxicity or the rate of ⁵⁶Fe-induced oncogenic transformation.     

Clinorotation reduces number, but not size, of cartilaginous nodules formed in micromass cultures of mouse limbbud cells.

In previous studies we used a ground based model to investigate the cellular responses to microgravity by exposing micromass cultures of embryonic limb cells to simulated weightlessness on a clinostat. Cultures set up in T-flasks and rotated at 30 rpm showed that clinostatted cultures had less chondrocyte differentiation than stationary or rotation controls, as assessed by number of nodules/culture stained with cartilage specific Alcian blue. In the current study, nodule size and shape of these nodules was assessed by interactive measurement of area, perimeter, circularity, and equivalent diameters, using the Optimas imaging software. Results show no significant difference in any of the measurements, indicating that clinorotation has no effect on expansion of the nodules either by differentiation of cells within the nodule, or by recruitment of cells into the nodule. The reduction in number of nodules without an alteration in size and shape indicates that the effect of simulated microgravity is to reduce the cell interactions required for the initial condensation of cells into a nodule, probably by interference with cell adhesion molecules.     

EXOSAT LE-CMA observations of the Vela supernova remnant

Presented here are Exosat LE1-CMA images of fields in the Vela SNR. Soft X-ray emission is observed in the north of the remnant indicating a filamentry structure. No X-ray emission was seen in two fields to the west and south-west which have optical filaments but were not studied by Einstein. The Vela pulsar is observed and is significantly broader than a point source indicating the presence of a small nebula as seen by the Einstein HRI. The flux seen from the point source in the CMA is consistent with blackbody radiation from a 10⁶ degree neutron star of approx. 10 km radius. A bright ridge of emission is seen north of the pulsar which may be part of the extended synchroton nebula seen in the 2–10 keV range.     

Tissue responses to low protracted doses of high LET radiations or photons: early and late damage relevant to radio-protective countermeasures.

Early and late murine tissue responses to single or fractionated low doses of heavy charged particles, fission-spectrum neutrons or gamma rays are considered. Damage to the hematopoietic system is emphasized, but results on acute lethality, host response to challenge with transplanted leukemia cells and life-shortening are presented. Low dose rates per fraction were used in some neutron experiments. Split-dose lethality studies (LD 50/30) with fission neutrons indicated greater accumulation of injury during a 9 fraction course (over 17 days) than was the case for gamma-radiation. When total doses of 96 or 247 cGy of neutrons or gamma rays were given as a single dose or in 9 fractions, a significant sparing effect on femur CFU-S depression was observed for both radiation qualities during the first 11 days, but there was not an earlier return to normal with dose fractionation. During the 9 fraction sequence, a significant sparing effect of low dose rate on CFU-S depression was observed in both neutron and gamma-irradiated mice. CFU-S content at the end of the fractionation sequence did not correlate with measured LD 50/30. Sustained depression of femur and spleen CFU-S and a significant thrombocytopenia were observed when a total neutron dose of 240 cGy was given in 72 fractions over 24 weeks at low dose rates. The temporal aspects of CFU-S repopulation were different after a single versus fractionated neutron doses. The sustained reduction in the size of the CFU-S population was accompanied by an increase in the fraction in DNA synthesis. The proliferation characteristics and effects of age were different for radial CFU-S population closely associated with bone, compared with the axial population that can be readily aspirated from the femur. In aged irradiated animals, the CFU-S proliferation/redistribution response to typhoid vaccine showed both an age and radiation effect. After high single doses of neutrons or gamma rays, a significant age- and radiation-related deficiency in host defense mechanisms was detected by a shorter mean survival time following challenge with transplantable leukemia cells. Comparison of dose-response curves for life shortening after irradiation with fission-spectrum neutrons or high energy silicon particles indicated high initial slopes for both radiation qualities at low doses, but for higher doses of silicon, the effect per Gy decreased to a value similar to that for gamma rays. The two component life-shortening curve for silicon particles has implications for the potential efficacy of radioprotectants. Recent studies on protection against early and late effects by aminothiols, prostaglandins, and other compounds are discussed.     

Dynamics of a Solenoidal-Torqued Gyro-Stabilized Seeker Assembly for Guidance and Tracking

Several low-cost gyro-stabilized, bang-bang seeker assemblies for missile guidance and tracking applications have used similar physical structures. A magnetic dipole is an integral part of the gyro rotor assembly. The nominal spin axis of the gyro is colinear with the axis of a cylinder about which is wound a helix of wire. By properly modulating the current that is passed through that helix, the precession torque acting upon the gyroscope is controlled. One rather generalized model of such a seeker assembly is modeled, the equations of motion are derived, and the dominant error sources are identified and parametrically evaluated. Because this is such a commonly encountered structure, yet no such analysis has been published, to the author's knowledge, it is hoped that this is a useful contribution to the literature.