The intrinsic shapes of starless cores in Ophiuchus

Using observations of cores to infer their intrinsic properties requires the solution of several poorly constrained inverse problems. Here we address one of these problems, namely to deduce from the projected aspect ratios of the cores in Ophiuchus their intrinsic three-dimensional shapes. Four models are proposed, all based on the standard assumption that cores are randomly orientated ellipsoids, and on the further assumption that a core's shape is not correlated with its absolute size. The first and simplest model, M1, has a single free parameter, and assumes that the relative axes of a core are drawn randomly from a log-normal distribution with zero mean and standard deviation \sigma o. The second model, M2a, has two free parameters, and assumes that the log-normal distribution (with standard deviation \sigma o) has a finite mean, \mu o, defined so that \mu o<0 means elongated (prolate) cores are favoured, whereas \mu o>0 means flattened (oblate) cores are favoured. Details of the third model (M2b, two free parameters) and the fourth model (M4, four free parameters) are given in the text. Markov chain Monte Carlo sampling and Bayesian analysis are used to map out the posterior probability density functions of the model parameters, and the relative merits of the models are compared using Bayes factors. We show that M1 provides an acceptable fit to the Ophiuchus data with \sigma o ~ 0.57+/-0.06; and that, although the other models sometimes provide an improved fit, there is no strong justification for the introduction of their additional parameters.Comment: 10 pages, 8 figures. Accepted by MNRA

Synthetic observations of protostellar multiple systems

Observations of protostars are often compared with synthetic observations of models in order to infer the underlying physical properties of the protostars. The majority of these models have a single protostar, attended by a disc and an envelope. However, observational and numerical evidence suggests that a large fraction of protostars form as multiple systems. This means that fitting models of single protostars to observations may be inappropriate. We produce synthetic observations of protostellar multiple systems undergoing realistic, non-continuous accretion. These systems consist of multiple protostars with episodic luminosities, embedded self-consistently in discs and envelopes. We model the gas dynamics of these systems using smoothed particle hydrodynamics and we generate synthetic observations by post-processing the snapshots using the SPAMCART Monte Carlo radiative transfer code. We present simulation results of three model protostellar multiple systems. For each of these, we generate 4 × 104 synthetic spectra at different points in time and from different viewing angles. We propose a Bayesian method, using similar calculations to those presented here, but in greater numbers, to infer the physical properties of protostellar multiple systems from observations

Simulations of star formation in Ophiuchus

The way in which stars form from the interstellar medium is poorly understood. In this thesis we investiage the process star formation in molecular clouds via core fragmentation using Smoothed Particle Hydrodynamics (SPH). The initial conditions of the simulations are informed as closely as possible by observations of Ophiuchus. We run large ensebles of individual core simulations and compare the collective results with observations of stars and brown dwarfs. We use observations of Ophiuchus by Motte et al. (1998) and Andre et al. (2007) to calibrate a lognormal distribution from which we draw correlated masses, sizes and velocity dispersions. We assume that the cores are intrinsically triaxial. The distribution of core shapes is then inferred by fitting a single parameter family of ellipsoidal shapes to the observed core aspect ratios. Each core is given the density profile of a critical Bonnor-Ebert sphere and a turbulent velocity field which is modified to include ordered rotation and radial excursions. We evolve one hundred of the model prestellar cores using the Seren SPH code (Hubber et al., 2011). The simulations are repeated with continuous accretion heating, no accretion heating and episodic accretion heating (Stamatellos et al., 2012). We find that simulations with episodic accretion heating can reproduce the general features of the Chabrier (2005) initial mass function. This includs the ratio of stars to brown dwarfs and the turn-over at 0:2M�. We demonstrate that the mass of a star is not related to the mass of the prestellar core in which it formed. Low mass cores with Mcore � 0:1M� tend to collapse into single objects whereas higher mass cores with Mcore & 1M� can fragment into tens of objects. We finally show that the multiplicy statistics of the protostars formed in these simulations are well matched by observations. Multiplicity frequencies are higher than those of field stars and we note the presence of long-lived quadruple, quintuple and sextuple systems

SPAMCART: a code for smoothed particle Monte Carlo radiative transfer

We present a code for generating synthetic SEDs and intensity maps from Smoothed Particle Hydrodynamics simulation snapshots. The code is based on the Lucy (1999) Monte Carlo Radiative Transfer method, i.e. it follows discrete luminosity packets as they propagate through a density field, and then uses their trajectories to compute the radiative equilibrium temperature of the ambient dust. The sources can be extended and/or embedded, and discrete and/or diffuse. The density is not mapped onto a grid, and therefore the calculation is performed at exactly the same resolution as the hydrodynamics. We present two example calculations using this method. First, we demonstrate that the code strictly adheres to Kirchhoff's law of radiation. Second, we present synthetic intensity maps and spectra of an embedded protostellar multiple system. The algorithm uses data structures that are already constructed for other purposes in modern particle codes. It is therefore relatively simple to implement

Inter-fractional Respiratory Motion Modelling from Abdominal Ultrasound: A Feasibility Study

Motion management strategies are crucial for radiotherapy of mobile tumours in order to ensure proper target coverage, save organs at risk and prevent interplay effects. We present a feasibility study for an inter-fractional, patient-specific motion model targeted at active beam scanning proton therapy. The model is designed to predict dense lung motion information from 2D abdominal ultrasound images. In a pretreatment phase, simultaneous ultrasound and magnetic resonance imaging are used to build a regression model. During dose delivery, abdominal ultrasound imaging serves as a surrogate for lung motion prediction. We investigated the performance of the motion model on five volunteer datasets. In two cases, the ultrasound probe was replaced after the volunteer has stood up between two imaging sessions. The overall mean prediction error is 2.9 mm and 3.4 mm after repositioning and therefore within a clinically acceptable range. These results suggest that the ultrasound-based regression model is a promising approach for inter-fractional motion management in radiotherapy

The State of the Region: Hampton Roads 2001

This is Old Dominion University\u27s second annual State of the Region Report. While it represents the work of many individuals connected in various ways to the university, the Report does not constitute an official viewpoint of the University, or it\u27s president, Dr. Roseann Runte. This year\u27s Report was produced with the goal of making Hampton Roads an even better place to live. Those who know and love the region of Hampton Roads frequently boast of its numerous strengths and successes, but we do so with the understanding that it would be possible for us to improve the region\u27s performance in certain areas. However, in order to improve our situation, we must have accurate knowledge about where we are, and what the policy implications are of the various choices in front of us. This year\u27s Report should be quite helpful in supplying such information. At the end of the day, we hope to have stimulated thought and discussion about things that matter.https://digitalcommons.odu.edu/economics_books/1017/thumbnail.jp

Collisions in young triple systems

We perform N-body simulations of young triple systems consisting of two low-mass companions orbiting around a significantly more massive primary. We find that, when the orbits of the companions are coplanar and not too widely separated, the chance of a collision between the two companions can be as high as 20 per cent. Collisions between one of the companions (always the less massive) and the primary can also occur in systems with unequal-mass companions. The chance of collisions is a few per cent in systems with more realistic initial conditions, such as with slightly non-coplanar orbits and unequal-mass companions. If the companions start widely separated then collision are very rare except in some cases when the total mass of the companions is large. We suggest that collisions between members of young multiple systems may explain some unusual young multiple systems such as apparently non-coeval companions

A consistent, scalable model for Eulerian spray modeling

Despite great practical interest in how sprays emanate from fuel injectors, the near-nozzle region has remained a challenge for spray modelers. Recently, Eulerian models have shown promise in capturing the fast gas-liquid interactions in the near field. However, with the inclusion of compressibility, it can be difficult to maintain consistency between the hydrodynamic and thermodynamic variables. In order to resolve numerical inconsistencies that occur in segregated solutions of Eulerian spray model equations as well as to provide good scalability and stability, a new construction of a -Y model is introduced. This construction is built around an IMEX-RK3 algorithm which offers accuracy and efficiency. The new algorithm is compared to an existing implementation for speed and is validated against experimental measurements of spray evolution in order to test the accuracy. The predictions of the new construction are slightly more accurate and, when tested on 256 processors, are 34 times faster.Also this research used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. The authors gratefully acknowledge the computing resources provided on the Texas Advanced Computing Center (TACC) at The University of Texas at Austin that have contributed to the research results reported within this paper URL: http://www.tacc.utexas.edu.Pandal-Blanco, A.; Pastor Enguídanos, JM.; García Oliver, JM.; Baldwin, E.; Schmidt, D. (2016). A consistent, scalable model for Eulerian spray modeling. International Journal of Multiphase Flow. 83:162-171. doi:10.1016/j.ijmultiphaseflow.2016.04.003S1621718