Extraction and analysis of microcrater residues using focused ion beam microscopy

We describe results from a new technique using dual beam FIB/SEM with which impact residues can be extracted from microcraters and analysed by EDS. This will allow the determination of residue compositions from Stardust craters

Flavour Democracy in Strong Unification

We show that the fermion mass spectrum may naturally be understood in terms of flavour democratic fixed points in supersymmetric theories which have a large domain of attraction in the presence of "strong unification". Our approach provides an alternative to the approximate Yukawa texture zeroes of the Froggatt-Nielsen mechanism. We discuss a particular model based on a broken gauged $SU(3)_L\times SU(3)_R$ family symmetry which illustrates our approach.Comment: 21 Pages plain latex; includes 5 eps figure

Dilaton Stabilisation in DD-term Inflation

Dilaton stabilisation is usually considered to pose a serious obstacle to successful $D$-term inflation in superstring theories. We argue that the physics of gaugino condensation is likely to be modified during the inflationary phase in such a way as to enhance the gaugino condensation scale. This enables dilaton stabilisation during inflation with the $D$-term still dominating the vacuum energy at the stable minimum.Comment: 13 pages, Latex, 1 figur

The Refractory-to-Ice Mass Ratio in Comets

We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc

T-Duality, Space-time Spinors and R-R Fields in Curved Backgrounds

We obtain the T-duality transformations of space-time spinors (the supersymmetry transformation parameters, gravitinos and dilatinos) of type-II theories in curved backgrounds with an isometry. The transformation of the spinor index is shown to be a consequence of the twist that T-duality introduces between the left and right-moving local Lorentz frames. The result is then used to derive the T-duality action on Ramond-Ramond field strengths and potentials in a simple way. We also discuss the massive IIA theory and, using duality, give a short derivation of ``mass''-dependent terms in the Wess-Zumino actions on the D-brane worldvolumes.Comment: 19 pages, LaTeX; (V3): typos corrected and minor comments added (to appear in Nucl. Phys. B

An approach to F-theory

We consider BPS configurations in theories with two timelike directions from the perspective of the supersymmetry algebra. We show that whereas a BPS state in a theory with one timelike variable must have positive energy, in a theory with two times any BPS state must have positive angular momentum in the timelike plane, in that $Z_{0\tilde{0}}>0$, where $0$ and $\tilde{0}$ are the two timelike directions. We consider some generic BPS solutions of theories with two timelike directions, and then specialise to the study of the (10,2) dimensional superalgebra for which the spinor operators generate 2-forms and 6-forms. We argue that the BPS configurations of this algebra relate to F-theory in the same way that the BPS configurations of the eleven dimensional supersymmetry algebra relate to M-theory. We show that the twelve dimensional theory is one of fundamental 3-branes and 7-branes, along with their dual partners. We then formulate the new intersection rules for these objects. Upon reduction of this system we find the algebraic description of the IIB-branes and the M-branes. Given these correspondences we may begin an algebraic study of F-theory.Comment: 20 pages, no figures, late

General Non-minimal Kinetic coupling to gravity

We study a new model of scalar field with a general non-minimal kinetic coupling to itself and to the curvature, as a source of dark energy, and analyze the cosmological dynamics of this model and the issue of accelerated expansion. A wide variety of scalar fields and potentials giving rise to power-law expansion have been found. The dynamical equation of state is studied for the two cases, without and with free kinetic term . In the first case, a behavior very close to that of the cosmological constant was found. In the second case, a solution was found, which match the current phenomenology of the dark energy. The model shows a rich variety of dynamical scenarios.Comment: 25 pages, 3 figures; figure added, references adde