106,028 research outputs found
On the Survival of Short-Period Terrestrial Planets
The currently feasible method of detection of Earth-mass planets is transit
photometry, with detection probability decreasing with a planet's distance from
the star. The existence or otherwise of short-period terrestrial planets will
tell us much about the planet formation process, and such planets are likely to
be detected first if they exist. Tidal forces are intense for short-period
planets, and result in decay of the orbit on a timescale which depends on
properties of the star as long as the orbit is circular. However, if an
eccentric companion planet exists, orbital eccentricity () is induced and
the decay timescale depends on properties of the short-period planet, reducing
by a factor of order if it is terrestrial. Here we examine the
influence companion planets have on the tidal and dynamical evolution of
short-period planets with terrestrial structure, and show that the relativistic
potential of the star is fundamental to their survival.Comment: 13 pages, 2 figures, accepted for publication in Ap
Dissociation Transition of a Composite Lattice of Magnetic Vortices in the Flux-Flow Regime of Two-Band Superconductors
In multiband superconductors, each superconducting condensate supports
vortices with fractional quantum flux. In the ground state, vortices in
different bands are spatially bounded together to form a composite vortex,
carrying one quantum flux \Phi_0. Here we predict dissociation of the composite
vortices lattice in the flux flow state due to the disparity of the vortex
viscosity and flux of the vortex in different bands. For a small driving
current, composite vortices start to deform, but the constituting vortices in
different bands move with the same velocity. For a large current, composite
vortices dissociate and vortices in different bands move with different
velocities. The dissociation transition shows up as an increase of flux flow
resistivity. In the dissociated phase, Shapiro steps are developed when an ac
current is superimposed with a dc current.Comment: 4.5 pages, 3 figure
Polar plasmas as observed by Dynamics Explorers 1 and 2
Plasma measurements from the Dynamics Explorer 1 and 2 satellites were used to characterize the polar cap environment. Analysis of numerous polar-cap passes indicate that, in general, three major regimes of plasma exist: (1) polar rain--electrons with magnetosheath-like energy spectra but much lower densities, most intense near the cusp and weakening toward the central polar cap; (2) polar wind--low energy upward flowing ions with both field-aligned and conical distributions; and (3) acceleration events--sporadic events consistent with the acceleration of electrons and positive ions by parallel electric fields. (1) to (3) were observed at high altitudes by Dynamics Explorer 1, while (1) and (3) were also observed at low altitudes by Dynamics Explorer 2. The plasma parameters associated with these plasma regimes are presented and discussed in terms of source and acceleration mechanisms
Evidence for Differential Rotation on a T Tauri Star
Five years of photometric monitoring of the T Tauri star HBC 338 in NGC 1333
has revealed that it is a periodic variable, but the period has changed
significantly with time. From 2000-2003, a period near 5.6 days was observed,
while in the last two seasons, the dominant period is near 4.6 days. No other T
Tauri star has been seen to change its period by such a large percentage. We
propose a model in which a differentially rotating star is seen nearly
equator-on and a high latitude spot has gradually been replaced by a low
latitude spot. We show that this model provides an excellent fit to the
observed shapes of the light curves at each epoch. The amplitude and sense of
the inferred differential rotation is similar to what is seen on the Sun. This
may be surprising given the likely high degree of magnetic surface activity on
the star relative to the Sun but we note that HBC 338 is clearly an exceptional
T Tauri star.Comment: Acepted for publication in PAS
Aspects of Integrability in N =4 SYM
Various recently developed connections between supersymmetric Yang-Mills
theories in four dimensions and two dimensional integrable systems serve as
crucial ingredients in improving our understanding of the AdS/CFT
correspondence. In this review, we highlight some connections between
superconformal four dimensional Yang-Mills theory and various integrable
systems. In particular, we focus on the role of Yangian symmetries in studying
the gauge theory dual of closed string excitations. We also briefly review how
the gauge theory connects to Calogero models and open quantum spin chains
through the study of the gauge theory duals of D3 branes and open strings
ending on them. This invited review, written for Modern Physics Letters-A, is
based on a seminar given at the Institute of Advanced Study, Princeton.Comment: Invited brief review for Mod. Phys. Lett. A based on a talk at I.A.S,
Princeto
Experimental studies on the impact properties of water ice
Experimental studies on the impact of ice particles at very low velocity were continued. These measurements have applications in the dynamics of Saturn's rings. Initially data were obtained on the coefficient of restitution for ice spheres of one radius of curvature. The type of measurements were expanded to include restitution data for balls with a variety of surfaces as well as sticking forces between ice particles. Significant improvements were made to this experiment, the most important being the construction of a new apparatus. The new apparatus consists of a smaller version of the disk pendulum and a stainless steel, double-walled cryostat. The apparatus has proved to be a significant improvement over the old one. Measurements can now be made at temperatures near 90 K, comparable to the temperature of the environment of Saturn's rings, and with much greater temperature stability. It was found that a roughened contact surface or the presence of frost can cause a much larger change in the restitution measure than the geometrical effect of the radius of curvature
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