Physical Constraints On Fast Radio Burst

Fast Radio Bursts (FRBs) are isolated, \ms radio pulses with dispersion measure (DM) of order 10^3\DMunit. Galactic candidates for the DM of high latitude bursts detected at \GHz frequencies are easily dismissed. DM from bursts emitted in stellar coronas are limited by free-free absorption and those from HII regions are bounded by the nondetection of associated free-free emission at radio wavelengths. Thus, if astronomical, FRBs are probably extra-galactic. FRB 110220 has a scattering tail of \sim 5.6\pm 0.1 \ms. If the electron density fluctuations arise from a turbulent cascade, the scattering is unlikely to be due to propagation through the diffuse intergalactic plasma. A more plausible explanation is that this burst sits in the central region of its host galaxy. Pulse durations of order \ms constrain the sizes of FRB sources implying high brightness temperatures that indicates coherent emission. Electric fields near FRBs at cosmological distances would be so strong that they could accelerate free electrons from rest to relativistic energies in a single wave period.Comment: 5 pages, accepted by ApJ

Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems

The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the solar system. Resonance locking predicts a similar migration timescale but a different effective tidal quality factor $Q$ governing the migration of each moon. It also predicts nearly constant migration timescales a function of semi-major axis, such that effective $Q$ values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective $Q$ values that are smaller than expected (and are different between moons), and which correspond to migration timescales of $\sim$10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small $Q$ governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates.Comment: Published in MNRA

Thermal Conductivity Of Rubble Piles

Rubble piles are a common feature of solar system bodies. They are composed of monolithic elements of ice or rock bound by gravity. Voids occupy a significant fraction of the volume of a rubble pile. They can exist up to pressure P ≈ є_yµ, where є_y is the monolithic material's yield strain and μ its rigidity. At low P, contacts between neighboring elements are confined to a small fraction of their surface areas. As a result, the effective thermal conductivity of a rubble pile, k_(con)≈ k(є_yµ)^(1/2), can be orders of magnitude smaller than the thermal conductivity of its monolithic elements, k. In a fluid-free environment, only radiation can transfer energy across voids. It contributes an additional component, k_(rad)=16ℓσT^3/3, to the total effective conductivity, k_(eff) = k_(con)+ k_(rad). Here ℓ, the inverse of the opacity per unit volume, is of the order of the size of the elements, and voids. An important distinction between k_(con) and k_(rad) is that the former is independent of the size of the elements, whereas the latter is proportional to it. Our expression for k_(eff) provides a good fit to the depth dependence of thermal conductivity in the top 140 cm of the lunar regolith. It also offers a good starting point for detailed modeling of thermal inertias for asteroids and satellites. Measurement of the response of surface temperature to variable insolation is a valuable diagnostic of a regolith. There is an opportunity for careful experiments under controlled laboratory conditions to test models of thermal conductivity such as the one we outline

DAVs: Red Edge and Outbursts

As established by photometric surveys, white dwarfs with hydrogen atmospheres and surface gravity, log(g) ≈ 8.0 pulsate as they cool across the temperature range of 12,500 K ≳ T_(eff) ≳ 10,800 K. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to gravity modes excited by convective driving. Overstability requires convective driving to exceed radiative damping. Previous works have demonstrated that ω ≳ max(τ_c^( −1), L_(ℓ,b)) is a necessary and sufficient condition for overstability. Here τ_c and L ℓ,b are the effective thermal timescale and Lamb frequency at the base of the surface convection zone. Below the observational red edge, L(ℓ,b) » τ_c^( −1), so overstable modes all have ωτ_c » 1. Consequently, their photometric amplitudes are reduced by that large factor rendering them difficult to detect. Although proposed previously, the condition ω ≳ L_(ℓ,b) has not been clearly interpreted. We show that modes with ω < L_(ℓ,b) suffer enhanced radiative damping that exceeds convective driving rendering them damped. A quasi-adiabatic analysis is adequate to account for this enhancement. Although this approximation is only marginally valid at the red edge, it becomes increasingly accurate toward both higher and lower T_(eff). Recently, Kepler discovered a number of cool DAVs that exhibit sporadic flux outbursts. Typical outbursts last several hours, are separated by days, and release ~10^(33) – 10^(34) erg. We attribute outbursts to limit cycles arising from sufficiently resonant 3-mode couplings between overstable parent modes and pairs of radiatively damped daughter modes. Limit cycles account for the durations and energies of outbursts and their prevalence near the red edge of the DAV instability strip

Classification of Satellite Resonances in the Solar System

Several pairs of solar system satellites occupy mean motion resonances (MMRs). We divide these into two groups according to their proximity to exact resonance. Proximity is measured by the existence of a separatrix in phase space. MMRs between Io–Europa, Europa–Ganymede, and Enceladus–Dione are too distant from exact resonance for a separatrix to appear. A separatrix is present only in the phase spaces of the Mimas–Tethys and Titan–Hyperion MMRs, and their resonant arguments are the only ones to exhibit substantial librations. Could there be a causal connection between the libration amplitude and the presence of a separatrix? Our suspicions were aroused by Goldreich & Schlichting, who demonstrate that sufficiently deep in a MMR, eccentricity damping could destabilize librations. However, our investigation reveals that libration amplitudes in both the Mimas–Tethys and Titan–Hyperion MMRs are fossils. Although the Mimas–Tethys MMR is overstable, its libration amplitude grows on the tidal damping timescale of Mimas's inclination, which is considerably longer than a Hubble time. On the other hand, the Titan–Hyperion MMR is stable, but tidal damping of Hyperion's eccentricity is too weak to have affected the amplitude of its libration

Financial Aid, Persistence, and Degree Completion in Masters Degree Programs

This article provides an overview of the research findings from a longitudinal study conducted at a large urban university on student financial aid, persistence, and degree completion of masters degree students. The purpose of the study was to determine how the types and amounts of student financial aid, along with students\u27 demographic and academic characteristics, are related to masters degree completion. Data were analyzed for a large cohort of masters degree students over a four-year period beginning in Fall, 1985 and ending in Summer, 1989