Relativistic Effects in Extrasolar Planetary Systems

This paper considers general relativistic (GR) effects in currently observed extrasolar planetary systems. Although GR corrections are small, they can compete with secular interactions in these systems and thereby play an important role. Specifically, some of the observed multiple planet systems are close to secular resonance, where the dynamics is extremely sensitive to GR corrections, and these systems can be used as laboratories to test general relativity. For the three-planet solar system Upsilon Andromedae, secular interaction theory implies an 80% probability of finding the system with its observed orbital elements if GR is correct, compared with only a 2% probability in the absence of GR. In the future, tighter constraints can be obtained with increased temporal coverage.Comment: Accepted for publication in International Journal of Modern Physics D; this paper received ``Honorable Mention'' in the 2006 Essay Competition of the Gravity Research Foundation; 9 pages including 1 figur

Possible Stellar Metallicity Enhancements from the Accretion of Planets

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central star. We study the ramifications of planet accretion for the final main sequence metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr, stars with masses near 1 solar mass are predicted to have virtually no metallicity enhancement. On the other hand, early F and late A type stars with masses of 1.5--2.0 solar masses can experience significant metallicity enhancements due to their considerably smaller convection zones during the first 10 Myr of pre-main-sequence evolution. We show that the metallicities of an aggregate of unevolved F stars are consistent with an average star accreting about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr dispersal time.Comment: 14 pages, AAS LaTeX, 3 figures, accepted to ApJ Letter

Photoevaporation of Circumstellar Disks due to External FUV Radiation in Stellar Aggregates

When stars form in small groups (N = 100 - 500 members), their circumstellar disks are exposed to little EUV radiation but a great deal of FUV radiation from massive stars in the group. This paper calculates mass loss rates for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius exceeds the critical radius (supercritical disks) where the sound speed in the FUV heated layer exceeds the escape speed. This paper shows that significant mass loss still takes place for subcritical systems. Some of the gas extends beyond the disk edge (above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The evaporation rate is a sensitive function of the stellar mass and disk radius, which determine the escape speed, and the external FUV flux, which determines the temperature structure of the flow. Disks around red dwarfs are readily evaporated and shrink to disk radii of 15 AU on short time scales (10 Myr) when exposed to moderate FUV fields with $G_0$ = 3000. Although disks around solar type stars are more durable, these disks shrink to 15 AU in 10 Myr for intense FUV radiation fields with $G_0$ = 30,000; such fields exist in the central 0.7 pc of a cluster with N = 4000 stars. If our solar system formed in the presence of such strong FUV radiation fields, this mechanism could explain why Neptune and Uranus in our solar system are gas poor, whereas Jupiter and Saturn are gas rich. This mechanism for photoevaporation can also limit the production of Kuiper belt objects and can suppress giant planet formation in sufficiently large clusters, such as the Hyades, especially for disks associated with low mass stars.Comment: 49 pages including 12 figures; accepted to Ap

On the CSFT approach to localized closed string tachyons

We compute the potential for localized closed string tachyons in bosonic string theory on the orbifold C/Z_4 using level-truncated closed string field theory. The critical points of the potential exhibit features which agree with their conjectured identification as lower-order orbifolds. However this case also raises some questions regarding the quantitative predictions associated with these conjectures.Comment: 20 pages, 3 figures, v2: The relation between the flat space and orbifold gravitational constants has been corrected. This resolves the puzzle of multiple predictions, but worsens the agreement between the depth of the potential and the change in the deficit angl

Localized Tachyons and the Quantum McKay Correspondence

The condensation of closed string tachyons localized at the fixed point of a C^d/\Gamma orbifold can be studied in the framework of renormalization group flow in a gauged linear sigma model. The evolution of the Higgs branch along the flow describes a resolution of singularities via the process of tachyon condensation. The study of the fate of D-branes in this process has lead to a notion of a ``quantum McKay correspondence.'' This is a hypothetical correspondence between fractional branes in an orbifold singularity in the ultraviolet with the Coulomb and Higgs branch branes in the infrared. In this paper we present some nontrivial evidence for this correspondence in the case C^2/Z_n by relating the intersection form of fractional branes to that of ``Higgs branch branes,'' the latter being branes which wrap nontrivial cycles in the resolved space.Comment: 25 pages; harvma

The Gravitational Demise of Cold Degenerate Stars

We consider the long term fate and evolution of cold degenerate stars under the action of gravity alone. Although such stars cannot emit radiation through the Hawking mechanism, the wave function of the star will contain a small admixture of black hole states. These black hole states will emit radiation and hence the star can lose its mass energy in the long term. We discuss the allowed range of possible degenerate stellar evolution within this framework.Comment: LaTeX, 18 pages, one figure, accepted to Physical Review

The rooting issue for a lattice fermion formulation similar to staggered fermions but without taste mixing

To investigate the viability of the 4th root trick for the staggered fermion determinant in a simpler setting, we consider a two taste (flavor) lattice fermion formulation with no taste mixing but with exact taste-nonsinglet chiral symmetries analogous to the taste-nonsinglet $U(1)_A$ symmetry of staggered fermions. M. Creutz's objections to the rooting trick apply just as much in this setting. To counter them we show that the formulation has robust would-be zero-modes in topologically nontrivial gauge backgrounds, and that these manifest themselves in a viable way in the rooted fermion determinant and also in the disconnected piece of the pseudoscalar meson propagator as required to solve the U(1) problem. Also, our rooted theory is heuristically seen to be in the right universality class for QCD if the same is true for an unrooted mixed fermion action theory.Comment: 22 revtex pages, to appear in PRD. v4: correction in the relation of the 2-flavor theory to twisted mass fermion

The Landlab v1.0 OverlandFlow component: a Python tool for computing shallow-water flow across watersheds

Representation of flowing water in landscape evolution models (LEMs) is often simplified compared to hydrodynamic models, as LEMs make assumptions reducing physical complexity in favor of computational efficiency. The Landlab modeling framework can be used to bridge the divide between complex runoff models and more traditional LEMs, creating a new type of framework not commonly used in the geomorphology or hydrology communities. Landlab is a Python-language library that includes tools and process components that can be used to create models of Earth-surface dynamics over a range of temporal and spatial scales. The Landlab OverlandFlow component is based on a simplified inertial approximation of the shallow water equations, following the solution of de Almeida et al. (2012). This explicit two-dimensional hydrodynamic algorithm simulates a flood wave across a model domain, where water discharge and flow depth are calculated at all locations within a structured (raster) grid. Here we illustrate how the OverlandFlow component contained within Landlab can be applied as a simplified event-based runoff model and how to couple the runoff model with an incision model operating on decadal timescales. Examples of flow routing on both real and synthetic landscapes are shown. Hydrographs from a single storm at multiple locations in the Spring Creek watershed, Colorado, USA, are illustrated, along with a map of shear stress applied on the land surface by flowing water. The OverlandFlow component can also be coupled with the Landlab DetachmentLtdErosion component to illustrate how the nonsteady flow routing regime impacts incision across a watershed. The hydrograph and incision results are compared to simulations driven by steady-state runoff. Results from the coupled runoff and incision model indicate that runoff dynamics can impact landscape relief and channel concavity, suggesting that on landscape evolution timescales, the OverlandFlow model may lead to differences in simulated topography in comparison with traditional methods. The exploratory test cases described within demonstrate how the OverlandFlow component can be used in both hydrologic and geomorphic applications

Jet Reconstruction in Heavy Ion Collisions

We examine the problem of jet reconstruction at heavy-ion colliders using jet-area-based background subtraction tools as provided by FastJet. We use Monte Carlo simulations with and without quenching to study the performance of several jet algorithms, including the option of filtering, under conditions corresponding to RHIC and LHC collisions. We find that most standard algorithms perform well, though the anti-kt and filtered Cambridge/Aachen algorithms have clear advantages in terms of the reconstructed transverse-momentum offset and dispersion.Comment: 31 pages, 17 figure

Employing culturally responsive pedagogy to foster literacy learning in schools

 In recent years it has become increasingly obvious that, to enable students in schools from an increasingly diverse range of cultural backgrounds to acquire literacy to a standard that will support them to achieve academically, it is important to adopt pedagogy that is responsive to, and respectful of, them as culturally situated. What largely has been omitted from the literature, however, is discussion of a relevant model of learning to underpin this approach. For this reason this paper adopts a socio-cultural lens (Vygotsky, 1978) through which to view such pedagogy and refers to a number of seminal texts to justify of its relevance. Use of this lens is seen as having a particular rationale. It forces a focus on the agency of the teacher as a mediator of learning who needs to acknowledge the learner’s cultural situatedness (Kozulin, 2003) if school literacy learning for all students is to be as successful as it might be. It also focuses attention on the predominant value systems and social practices that characterize the school settings in which students’ literacy learning is acquired. The paper discusses implications for policy and practice at whole-school, classroom and individual student levels of culturally-responsive pedagogy that is based on a socio-cultural model of learning. In doing so it draws on illustrations from the work of a number of researchers, including that of the author