`Mass without mass' from thin shells in Gauss-Bonnet gravity

Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity. There is the well known junction condition for the singular part of the stress tensor intrinsic to the shell, which we also prove to be well defined. There are also equations relating the geometry of the shell (jump and average of the extrinsic curvature as well as the intrinsic curvature) to the non-singular components of the bulk stress tensor on the sides of the thin shell. The equations are applied to spherically symmetric thin shells in vacuum. The shells are part of the vacuum, they carry no energy tensor. We classify these solutions of `thin shells of nothingness' in the pure Gauss-Bonnet theory. There are three types of solutions, with one, zero or two asymptotic regions respectively. The third kind of solution are wormholes. Although vacuum solutions, they have the appearance of mass in the asymptotic regions. It is striking that in this theory, exotic matter is not needed in order for wormholes to exist- they can exist even with no matter.Comment: 13 pages, RevTex, 8 figures. Version 2: includes discussion on the well-defined thin shell limit. Version 3: typos fixed, a reference added, accepted for publication in Phys. Rev.

Low energy effective gravitational equations on a Gauss-Bonnet brane

We present effective gravitational equations at low energies in a $Z_2$-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based on the geometrical projection approach, and we solve iteratively the bulk geometry using the gradient expansion scheme. Although the original field equations are quite complicated due to the presence of the Gauss-Bonnet term, our final result clearly has the form of the Einstein equations plus correction terms, which is simple enough to handle. As an application, we consider homogeneous and isotropic cosmology on the brane. We also comment on the holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.Comment: 10 pages, v2: minor clarification

Dust and molecules in the Local Group galaxy NGC 6822. III. The first-ranked HII region complex Hubble V

We present maps of the first-ranked HII region complex Hubble V in the metal-poor Local Group dwarf galaxy NGC 6822 in the first four transitions of CO, the 158 micron transition of C+, the 21-cm line of HI, the Pa-beta line of HII, and the continuum at 21 cm and 2.2 micron wavelengths. We have also determined various integrated intensities, notably of HCO+ and near-IR H2 emission. Although Hubble X is located in a region of relatively strong HI emission, our mapping failed to reveal any significant CO emission from it. The relatively small CO cloud complex associated with Hubble V is comparable in size to the ionized HII region. The CO clouds are hot (Tkin) = 150 K) and have high molecular gas densities (n(H2) = 10**4 cm**-3) Molecular hydrogen probably extends well beyond the CO boundaries. C+ column densities are more than an order of magnitude higher than those of CO. The total mass of the complex is about 10**6 M(sun) and molecular gas account for more than half of this. The complex is excited by luminous stars reddened or obscured at visual, but apparent at near-infrared wavelengths. The total embedded stellar mass may account for about 10% of the total mass, and the mass of ionized gas for half of that. Hubble V illustrates that modest star formation efficiencies may be associated with high CO destruction efficiencies in low-metallicity objects. The analysis of the Hubble V photon-dominated region (PDR) confirms in an independent manner the high value of the CO-to-H2 conversion factor X found earlier, characteristic of starforming low-metallicity regions.Comment: Accepted for publication in A&

On a Classical, Geometric Origin of Magnetic Moments, Spin-Angular Momentum and the Dirac Gyromagnetic Ratio

By treating the real Maxwell Field and real linearized Einstein equations as being imbedded in complex Minkowski space, one can interpret magnetic moments and spin-angular momentum as arising from a charge and mass monopole source moving along a complex world line in the complex Minkowski space. In the circumstances where the complex center of mass world-line coincides with the complex center of charge world-line, the gyromagnetic ratio is that of the Dirac electron.Comment: 17 page

Two-dimensional Quantum Black Holes, Branes in BTZ and Holography

We solve semiclassical Einstein equations in two dimensions with a massive source and we find a static, thermodynamically stable, quantum black hole solution in the Hartle-Hawking vacuum state. We then study the black hole geometry generated by a boundary mass sitting on a non-zero tension 1-brane embedded in a three-dimensional BTZ black hole. We show that the two geometries coincide and we extract, using holographic relations, information about the CFT living on the 1-brane. Finally, we show that the quantum black hole has the same temperature of the bulk BTZ, as expected from the holographic principle.Comment: 10 pages, 2 figures, RevTex, ``point particle of mass \mu '' changed with ``massive boundary source'' for better clarity. Action in (50) written in Z_2 symmetric form. Appendix clarified. Minor corrections and references added. Version accepted for pubblication in PRD15 (2006

Stability of Transparent Spherically Symmetric Thin Shells and Wormholes

The stability of transparent spherically symmetric thin shells (and wormholes) to linearized spherically symmetric perturbations about static equilibrium is examined. This work generalizes and systematizes previous studies and explores the consequences of including the cosmological constant. The approach shows how the existence (or not) of a domain wall dominates the landscape of possible equilibrium configurations.Comment: 12 pages, 7 figures, revtex. Final form to appear in Phys. Rev.

IDN1 and IDN2 are required for de novo DNA methylation in Arabidopsis thaliana.

DNA methylation is an epigenetic mark affecting genes and transposons. Screening for mutants that fail to establish DNA methylation yielded two we termed "involved in de novo" (idn) 1 and 2. IDN1 encodes DMS3, an SMC-related protein, and IDN2 encodes a previously unknown double-stranded RNA-binding protein with homology to SGS3. IDN1 and IDN2 control de novo methylation and small interfering RNA (siRNA)-mediated maintenance methylation and are components of the RNA-directed DNA methylation pathway

Neutral hydrogen in the starburst galaxy NGC3690/IC694

Researchers made observations of the neutral hydrogen (HI) emission structure surrounding the very deep absorption peak (observed earlier by Dickey (1986)) in the galaxy pair NGC3690/IC694. This galaxy pair is highly luminous in the far infrared, and known to exhibit extensive star formation as well as nuclear activity. Knowledge of the spatial distribution and velocity structure of the HI emission is of great importance to the understanding of the dynamics of the interaction and the resulting environmental effects on the galaxies