Planetary embryos embedded in a gas disc suffer a decay in semimajor axis -- type I migration -- due to the asymmetric torques produced by the interior and exterior wakes raised by the body (Goldreich & Tremaine 1980; Ward 1986). This presents a challenge for standard oligarchic approaches to forming the terrestrial planets (Kokubo & Ida 1998) as the timescale to grow the progenitor objects near 1 AU is longer than that for them to decay into the Sun. In this paper we investigate the middle and late stages of oligarchic growth using both semi-analytic methods (based upon Thommes et al. 2003) and N-body integrations, and vary gas properties such as dissipation timescale in different models of the protoplanetary disc. We conclude that even for near-nominal migration efficiencies and gas dissipation timescales of ~1 Myr it is possible to maintain sufficient mass in the terrestrial region to form Earth and Venus if the disc mass is enhanced by factors of ~2-4 over the minimum mass model. The resulting configurations differ in several ways from the initial conditions used in previous simulations of the final stages of terrestrial accretion (e.g. Chambers 2001), chiefly in (1) larger inter-embryo spacings, (2) larger embryo masses, and (3) up to ~0.4 Earth masses of material left in the form of planetesimals when the gas vanishes. The systems we produce are reasonably stable for ~100 Myr and therefore require an external source to stir up the embryos sufficiently to produce final systems resembling the terrestrial planets.Comment: 49 pages, 22 figures; accepted in AJ, expected Dec '0

The discovery of short-period Neptune-mass objects, now including the remarkable system HD69830 (Lovis et al. 2006) with three Neptune analogues, raises difficult questions about current formation models which may require a global treatment of the protoplanetary disc. Several formation scenarios have been proposed, where most combine the canonical oligarchic picture of core accretion with type I migration (e.g. Terquem & Papaloizou 2007) and planetary atmosphere physics (e.g. Alibert et al. 2006). To date, published studies have considered only a small number of progenitors at late times. This leaves unaddressed important questions about the global viability of the models. We seek to determine whether the most natural model -- namely, taking the canonical oligarchic picture of core accretion and introducing type I migration -- can succeed in forming objects of 10 Earth masses and more in the innermost parts of the disc. This problem is investigated using both traditional semianalytic methods for modelling oligarchic growth as well as a new parallel multi-zone N-body code designed specifically for treating planetary formation problems with large dynamic range (McNeil & Nelson 2009). We find that it is extremely difficult for oligarchic tidal migration models to reproduce the observed distribution. Even under many variations of the typical parameters, we form no objects of mass greater than 8 Earth masses. By comparison, it is relatively straightforward to form icy super-Earths. We conclude that either the initial conditions of the protoplanetary discs in short-period Neptune systems were substantially different from the standard disc models we used, or there is important physics yet to be understood.Comment: 19 pages, 18 figures. Final version accepted to MNRAS 30 September 200

Modern N‐body techniques for planetary dynamics are generally based on symplectic algorithms specially adapted to the Kepler problem. These methods have proven very useful in studying planet formation, but typically require the time‐step for all objects to be set to a small fraction of the orbital period of the innermost body. This computational expense can be prohibitive for even moderate particle number for many physically interesting scenarios, such as recent models of the formation of hot exoplanets, in which the semimajor axis of possible progenitors can vary by orders of magnitude. We present new methods which retain most of the benefits of the standard symplectic integrators but allow for radial zones with distinct time‐steps. These approaches should make simulations of planetary accretion with large dynamic range tractable. As proof‐of‐concept, we present preliminary science results from an implementation of the algorithm as applied to an oligarchic migration scenario for forming hot Neptunes.

We developed early warning algorithms for influenza using data from the Alberta Real-Time Syndromic Surveillance Net (ARTSSN). In addition to looking for signatures of potential pandemics, the model was operationalized by using the algorithms to provide regular weekly forecasts on the influenza trends in Alberta during 2012-2014. We describe the development of the early warning model and the predicted influenza peak time and attack rate results. We report on the usefulness of this model using real-time ARTSSN data, discuss how it was used by decision makers and suggest future enhancements for this promising tool in influenza planning and preparedness.

Symplectic integrators have been the backbone of much theoretical solar system research over the past decade. As implemented, they involve the direct computation of the distances between each pair of N particles, a process whose effort grows as O(N 2 ). A variety of fast [that is, with effort growing more slowly than O(N 2 )] but approximate force calculation methods have been developed in other areas of research. Several of these algorithms are examined here, and their speed and accuracy are compared with traditional methods, with an eye toward their suitability for solar system research in particular. We find that approximate force algorithms can provide, in some situations, a suitable alternative to traditional ones, with break-even in terms of computation time at particle numbers as low as a few hundred, and often with only modest increases in the short-term error. Though empirically stable on the systems tested here, the effect of approximate methods on the phase-space manifold of the problem remains a concern.