Mass Loss Due to Sputtering and Thermal Processes in Meteoroid Ablation

Conventional meteoroid theory assumes that the dominant mode of ablation is by evaporation following intense heating during atmospheric flight. In this paper we consider the question of whether sputtering may provide an alternative disintegration process of some importance.For meteoroids in the mass range from 10^-3 to 10^-13 kg and covering a meteor velocity range from 11 to 71 km/s, we numerically modeled both thermal ablation and sputtering ablation during atmospheric flight. We considered three meteoroid models believed to be representative of asteroidal (3300 kg m^-3 mass density), cometary (1000 kg m^-3) and porous cometary (300 kg m^-3) meteoroid structures. Atmospheric profiles which considered the molecular compositions at different heights were used in the sputtering calculations. We find that while in many cases (particularly at low velocities and for relatively large meteoroid masses) sputtering contributes only a small amount of mass loss during atmospheric flight, in some cases sputtering is very important. For example, a 10^-10 kg porous meteoroid at 40 km/s will lose nearly 51% of its mass by sputtering, while a 10^-13 kg asteroidal meteoroid at 60 km/s will lose nearly 83% of its mass by sputtering. We argue that sputtering may explain the light production observed at very great heights in some Leonid meteors. The impact of this work will be most dramatic for very small meteoroids such as those observed with large aperture radars.Comment: in pdf form, 48 pgs incl figures and table

The Uncertainty in Newton's Constant and Precision Predictions of the Primordial Helium Abundance

The current uncertainty in Newton's constant, G_N, is of the order of 0.15%. For values of the baryon to photon ratio consistent with both cosmic microwave background observations and the primordial deuterium abundance, this uncertainty in G_N corresponds to an uncertainty in the primordial 4He mass fraction, Y_P, of +-1.3 x 10^{-4}. This uncertainty in Y_P is comparable to the effect from the current uncertainty in the neutron lifetime, which is often treated as the dominant uncertainty in calculations of Y_P. Recent measurements of G_N seem to be converging within a smaller range; a reduction in the estimated error on G_N by a factor of 10 would essentially eliminate it as a source of uncertainty in the calculation of the primordial 4He abundance.Comment: 3 pages, no figures, fixed typos, to appear in Phys. Rev.

Associations between family relationships and symptoms/strengths at kindergarten age : what is the role of children's parental representations?

Background: The quality of the family relationships plays an important role in the development of children’s psychopathology and in their socio-emotional development. This longitudinal study aims to investigate whether family relationships are related to children’s symptoms/strengths at kindergarten, and whether family relationships are predictors of changes in children’s symptoms/strengths between 5 and 6. Our main interest is to examine the role of children’s parental representations. Methods: One hundred and fifty-three kindergarten children (67 girls, 86 boys) participated in this study. Children’s parental representations were assessed using a story stem task (age 5). The family environment was assessed using parents’ ratings (age 5). A multi-informant approach (parent, teacher, child) was employed to assess children’s symptoms/strengths at 5 and 6. Children were interviewed using a standardized puppet interview. Parents and teachers completed questionnaires. Results: Children’s parental representations as well as the family environment were associated with children’s symptoms/strengths at 5 and 6. When controlled for gender, children’s representations were the only predictor of changes in symptoms/strengths between 5 and 6. A large number of negative parental representations at 5 predicted an increase in conduct problems. A large number of positive parental representations at 5 predicted an increase in pro-social behaviour. In terms of emotional symptoms and hyperactivity/ inattention, symptoms at 5 were the only predictor for symptoms one year later. Conclusions: Our results suggest that kindergarten children’s narrative representations may elaborate the processes underlying the expression of child disturbance and strengths, and underline the relevance of the play and narration of young children for diagnostics and therapy