Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration

The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with 'hotter' kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations.Comment: 28 pages, 15 figure, accepted for publication in MNRA

Uniqueness of Current Cosmic Acceleration

One of the strongest arguments against the cosmological constant as an explanation of the current epoch of accelerated cosmic expansion is the existence of an earlier, dynamical acceleration, i.e. inflation. We examine the likelihood that acceleration is an occasional phenomenon, putting stringent limits on the length of any accelerating epoch between recombination and the recent acceleration; such an epoch must last less than 0.05 e-fold (at z>2) or the matter power spectrum is modified by more than 20%.Comment: 6 pages, 5 figures; v2 corrected typo in Eq.

A simple theory of bimodal star formation

A model of bimodal star formation is presented, wherein massive stars form in giant molecular clouds (GNC), at a rate regulated by supernovae energy feedback through the interstellar medium, the heat input also ensuring that the initial mass function (IMF) remains skewed towards massive stars. The low mass stars form at a constant rate. The formation of the GMC is governed by the dynamics of the host galaxy through the rotation curve and potential perturbations such as a spiral density wave. The characteristic masses, relative normalizations, and rates of formation of the massive and low mass modes of star formation may be tightly constrained by the requirements of the chemical evolution in the Solar Neighborhood. Good fits were obtained for the age metallicity relation and the metallicity structure of thin disk and spheroid stars only for a narrow range of these parameters

Validation of the Air Force Family Needs Screener

The Air Force Family Needs Screener (FNS) has been utilized as a screening measure for risk of both child maltreatment and intimate partner violence (IPV) within the context of a military primary prevention program called the New Parent Support Program (NPSP) since 1988. However, this measure has only been validated with the initial pilot samples used in its development. The current study sought to assess the reliability, dependability, factor structure and predictive capacity of the FNS using a new sample of mothers. In addition, this study sought to develop shortened versions of the FNS in order to determine if they performed as well as or better than the original version of the measure. Results indicated the original FNS has adequate reliability and dependability, but the cut-score currently used in practice resulted in a fairly low sensitivity rate and corresponding high false negative rate. Since there was no evidence of measurement invariance of a common items model tested across four different population groups, three shortened versions of the FNS made up only of common items to all four groups were also tested. All shortened models appeared to perform as well as the original FNS, suggesting the FNS could be shortened, while retaining its predictive capacity. A shortened model made up of only thirty-six common items was recommended as a revised version of the FNS for implementation with the Air Force. Although not the most parsimonious model, this model retained the highest rate of sensitivity while still improving the acceptability of the measure for staff and respondents alike