Place matters: challenges and opportunities in four rural Americas

A survey of 7,800 rural Americans in 19 counties across the country has led to the Carsey Institute\u27s first major publication that outlines four distinctly different rural Americas—amenity, decline, chronic poverty, and those communities in decline that are also amenity-rich—each has unique challenges in this modern era that will require different policies than their rural neighbors

Knowledge-based system V and V in the Space Station Freedom program

Knowledge Based Systems (KBS's) are expected to be heavily used in the Space Station Freedom Program (SSFP). Although SSFP Verification and Validation (V&V) requirements are based on the latest state-of-the-practice in software engineering technology, they may be insufficient for Knowledge Based Systems (KBS's); it is widely stated that there are differences in both approach and execution between KBS V&V and conventional software V&V. In order to better understand this issue, we have surveyed and/or interviewed developers from sixty expert system projects in order to understand the differences and difficulties in KBS V&V. We have used this survey results to analyze the SSFP V&V requirements for conventional software in order to determine which specific requirements are inappropriate for KBS V&V and why they are inappropriate. Further work will result in a set of recommendations that can be used either as guidelines for applying conventional software V&V requirements to KBS's or as modifications to extend the existing SSFP conventional software V&V requirements to include KBS requirements. The results of this work are significant to many projects, in addition to SSFP, which will involve KBS's

Secular Dynamics of Binaries in Stellar Clusters

The orbital evolution of two bound point masses (a 'binary') perturbed by external tidal forces represents one of the oldest problems in celestial mechanics. Most obviously, tidal perturbations may arise due to an external point mass bound to the binary, as in the Lidov-Kozai (LK) theory of hierarchical triples, but they can also stem from the gravitational field of an extended stellar system (e.g. galaxy or globular cluster) in which the binary resides. Due to the weakness of the external perturbation, the resulting orbital evolution is usually secular in nature, i.e. it occurs on timescales much longer than any characteristic orbital period. This thesis is concerned with the secular dynamical evolution of tidally perturbed binary systems. If problems of this sort are centuries old, what motivation is there to further study them now? In fact, interest in the problem of tidally perturbed binaries has surged recently due to the discovery of various exotic astrophysical phenomena, not least the mergers of compact object (black hole and/or neutron star) binaries by the LIGO/Virgo collaboration. The question of how these binaries shrink rapidly enough to merge within a Hubble time is still an open one, but tidal perturbations may provide the answer. For instance, LK oscillations driven by a tertiary companion can naturally drive a binary orbit to become highly eccentric, boosting gravitational wave emission and substantially speeding up binary coalescence. Similar ideas (with different sources of dissipation at pericentre) have been previously considered for explaining the origin of other exotic objects, such as hot Jupiters, blue stragglers, and Type 1a supernovae. Thus, understanding the tidally-forced eccentricity evolution and possible mergers of binary systems has become a central focus of modern research in astrophysical dynamics. In this thesis we consider the secular evolution of binaries driven by the tidal gravitational field of an arbitrary axisymmetric host system ('cluster') in which the binary moves. We formulate the most general possible theory of tide-driven secular evolution of two bound point masses, applicable to a wide variety of astrophysical systems. Our secular Hamiltonian theory (averaged over both the inner Keplerian orbit of the binary and its outer orbit within the cluster) reproduces classical results — such as LK evolution and the effect of the Galactic tide on Oort Cloud comets — in appropriate limits, but is more general. We then investigate the secular dynamics in detail, uncovering new dynamical characteristics that are far removed from the canonical LK behaviour. We also extend the secular theory by accounting for the important non-Newtonian effects of general relativistic (GR) perihelion precession and gravitational wave (GW) emission, and the non-secular effect of short-timescale fluctuations in the perturbing torque. These three effects, unavoidably important in many practical applications, add further levels of complexity and richness to the binary dynamics. The central result of the theory is that the mean-field gravitational tidal potential of a star cluster is often sufficient to torque a binary so that it performs large-amplitude eccentricity oscillations. This result has significant consequences for the dynamical evolution of compact object binaries, many of which reside in stellar clusters. We show that it leads to mergers of compact object binaries which could not have merged if they were isolated, and calculate the resulting observable merger rate. In summary, then, the purpose of this thesis is three-fold: to formulate a general unified theory of binary dynamical evolution; to propose a possible origin for LIGO/Virgo compact object merger events; and to uncover and explain a range of new, important and beautiful dynamical phenomena.STFC Studentshi

Saturation of spiral instabilities in disk galaxies

Spiral density waves can arise in galactic disks as linear instabilities of the underlying stellar distribution function. Such an instability grows exponentially in amplitude at some fixed growth rate $\beta$ before saturating nonlinearly. However, the mechanisms behind saturation, and the resulting saturated spiral amplitude, have received little attention. Here we argue that one important saturation mechanism is the nonlinear trapping of stars near the spiral's corotation resonance. Under this mechanism, we show analytically that an $m$-armed spiral instability will saturate when the libration frequency of resonantly trapped orbits reaches $\omega_\mathrm{lib} \sim \mathrm{a\,\, few}\times m^{1/2} \beta$. For a galaxy with a flat rotation curve this implies a maximum relative spiral surface density $\vert \delta\Sigma/\Sigma_0\vert \sim \mathrm{a\,\,few} \times (\beta/\Omega_\mathrm{p})^2 \cot \alpha$, where $\Omega_\mathrm{p}$ is the spiral pattern speed and $\alpha$ is its pitch angle. This result is in reasonable agreement with recent $N$-body simulations, and suggests that spirals driven by internally-generated instabilities reach relative amplitudes of at most a few tens of percent; higher amplitude spirals, like in M51 and NGC 1300, are likely caused by very strong bars and/or tidal perturbations.Comment: Revised version, accepted for publication in MNRA

Place effects on environmental views

How people respond to questions involving the environment depends partly on individual characteristics. Characteristics such as age, gender, education, and ideology constitute the well-studied social bases of environmental concern, which have been explained in terms of cohort effects or of cognitive and cultural factors related to social position. It seems likely that people\u27s environmental views depend not only on personal characteristics but also on their social and physical environments. This hypothesis has been more difficult to test, however. Using data from surveys in 19 rural U.S. counties, we apply mixed-effects modeling to investigate simple place effects with respect to locally focused environmental views. We find evidence for two kinds of place effects. Net of individual characteristics, specific place characteristics have the expected effect on related environmental views. Local changes are related to attitudes about regulation and growth. For example, respondents more often perceive rapid development as a problem, and favor environmental rules that restrict development, in rural counties with growing populations. Moreover, they favor conserving resources for the future rather than using them now to create jobs in counties that have low unemployment. After we controlled for county growth, unemployment and jobs in resource based industries, and individual social-position and ideological factors, there remains significant place-to-place variation in mean levels of environmental concern. Even with both kinds of place effects in the models, the individual level predictors of environmental concern follow patterns expected from previous research. Concern increases with education among Democrats, whereas among Republicans, the relationship is attenuated or reversed. The interaction marks reframing of environmental questions as political wedge issues, through nominally scientific counterarguments aimed at educated, ideologically receptive audiences. © 2010, by the Rural Sociological Society

Eccentricity dynamics of wide binaries -- I. The effect of Galactic tides

A major puzzle concerning the wide stellar binaries (semimajor axes $a\gtrsim 10^3$ AU) in the Solar neighborhood is the origin of their observed superthermal eccentricity distribution function (DF), which is well-approximated by $P(e) \propto e^\alpha$ with $\alpha \approx 1.3$. This DF evolves under the combined influence of (i) tidal torques from the Galactic disk and (ii) scattering by passing stars, molecular clouds, and substructure. Recently, Hamilton (2022) (H22) demonstrated that Galactic tides alone cannot produce a superthermal eccentricity DF from an initially isotropic, non-superthermal one, under the restrictive assumptions that the eccentricity DF was initially of power law form and then was rapidly phase-mixed toward a steady state by the tidal perturbation. In this paper we first prove analytically that H22's conclusions are in fact valid at all times, regardless of these assumptions. We then adopt H22's Galactic disk model and numerically integrate the equations of motion for several ensembles of isotropically oriented wide binaries to study the time evolution in detail. We find that even non-power law DFs can be described by an effective power law index $\alpha_\mathrm{eff}$ which accurately characterizes both their initial and final states. Any DF with initial (effective or exact) power law index $\alpha_\mathrm{i}$ is transformed by Galactic tides into another power law with index $\alpha_\mathrm{f} \approx (1+\alpha_\mathrm{i})/2$ on a timescale $\sim 2$ Gyr $(a/10^4\mathrm{AU})^{-3/2}$. In a companion paper, we investigate separately the effect of stellar scattering. As the GAIA data continues to improve, these results will place strong constraints on wide binary formation channels.Comment: 13 pages, 9 figures. Main result in Figure 7. Submitted to MNRAS, comments are welcome