Simulating star formation in molecular cloud cores I. The influence of low levels of turbulence on fragmentation and multiplicity

We present the results of an ensemble of simulations of the collapse and fragmentation of dense star-forming cores. We show that even with very low levels of turbulence the outcome is usually a binary, or higher-order multiple, system. We take as the initial conditions for these simulations a typical low-mass core, based on the average properties of a large sample of observed cores. All the simulated cores start with a mass of $M = 5.4 M_{\odot}$, a flattened central density profile, a ratio of thermal to gravitational energy $\alpha_{\rm therm} = 0.45$ and a ratio of turbulent to gravitational energy $\alpha_{\rm turb} = 0.05$. Even this low level of turbulence is sufficient to produce multiple star formation in 80% of the cores; the mean number of stars and brown dwarfs formed from a single core is 4.55, and the maximum is 10. At the outset, the cores have no large-scale rotation. The only difference between each individual simulation is the detailed structure of the turbulent velocity field. The multiple systems formed in the simulations have properties consistent with observed multiple systems. Dynamical evolution tends preferentially to eject lower mass stars and brown dwarves whilst hardening the remaining binaries so that the median semi-major axis of binaries formed is $\sim 30$ au. Ejected objects are usually single low-mass stars and brown dwarfs, yielding a strong correlation between mass and multiplicity. Our simulations suggest a natural mechanism for forming binary stars that does not require large-scale rotation, capture, or large amounts of turbulence.Comment: 20 pages, 12 figures submitted to A&

Discs in misaligned binary systems

We perform SPH simulations to study precession and changes in alignment between the circumprimary disc and the binary orbit in misaligned binary systems. We find that the precession process can be described by the rigid-disc approximation, where the disc is considered as a rigid body interacting with the binary companion only gravitationally. Precession also causes change in alignment between the rotational axis of the disc and the spin axis of the primary star. This type of alignment is of great important for explaining the origin of spin-orbit misaligned planetary systems. However, we find that the rigid-disc approximation fails to describe changes in alignment between the disc and the binary orbit. This is because the alignment process is a consequence of interactions that involve the fluidity of the disc, such as the tidal interaction and the encounter interaction. Furthermore, simulation results show that there are not only alignment processes, which bring the components towards alignment, but also anti-alignment processes, which tend to misalign the components. The alignment process dominates in systems with misalignment angle near 90 degrees, while the anti-alignment process dominates in systems with the misalignment angle near 0 or 180 degrees. This means that highly misaligned systems will become more aligned but slightly misaligned systems will become more misaligned.Comment: 15 pages, 16 figures, 1 table, accepted for publication in MNRA

How to identify the youngest protostars

We study the transition from a prestellar core to a Class 0 protostar, using SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer code to generate the SED and isophotal maps. For a prestellar core illuminated by the standard interstellar radiation field, the luminosity is low and the SED peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to a growing contribution from accretion onto the protostar) and the peak of the SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the Class 0 phase, the accretion rate is falling, the luminosity has decreased, and the peak of the SED shifts back towards longer wavelengths (90-150 micron). In our simulations, the density of material around the protostar remains sufficiently high well into the Class 0 phase that the protostar only becomes visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm maps of Class 0 protostars tend to be much more centrally condensed than those of prestellar cores. However, when convolved with a typical telescope beam, the difference in central concentration is less marked, although the Class 0 protostars appear more circular. Our results suggest that, if a core is deemed to be prestellar on the basis of having no associated IRAS source, no cm radio emission, and no outflow, but it has a circular appearance and an SED which peaks at wavelengths below ~170 micron, it may well contain a very young Class 0 protostar.Comment: Accepted by A&A (avaliable with high-res images at http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications

Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics

We perform SPH simulations of the collapse and fragmentation of low-mass cores having different initial levels of turbulence (alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation which captures the transport of cooling radiation against opacity due to both dust and gas (including the effects of dust sublimation, molecules, and H^- ions). We also perform comparison simulations using a standard barotropic equation of state. We find that -- when compared with the barotropic equation of state -- our more realistic treatment of the energy equation results in more protostellar objects being formed, and a higher proportion of brown dwarfs; the multiplicity frequency is essentially unchanged, but the multiple systems tend to have shorter periods (by a factor ~3), higher eccentricities, and higher mass ratios. The reason for this is that small fragments are able to cool more effectively with the new treatment, as compared with the barotropic equation of state. We find that the process of fragmentation is often bimodal. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&

Effective forecasting for supply-chain planning: an empirical evaluation and strategies for improvement

Demand forecasting is a crucial aspect of the planning process in supply-chain companies. The most common approach to forecasting demand in these companies involves the use of a simple univariate statistical method to produce a forecast and the subsequent judgmental adjustment of this by the company's demand planners to take into account market intelligence relating to any exceptional circumstances expected over the planning horizon. Based on four company case studies, which included collecting more than 12,000 forecasts and outcomes, this paper examines: i) the extent to which the judgmental adjustments led to improvements in accuracy, ii) the extent to which the adjustments were biased and inefficient, iii) the circumstances where adjustments were detrimental or beneficial, and iv) methods that could lead to greater levels of accuracy. It was found that the judgmentally adjusted forecasts were both biased and inefficient. In particular, market intelligence that was expected to have a positive impact on demand was used far less effectively than intelligence suggesting a negative impact. The paper goes on to propose a set of improvements that could be applied to the forecasting processes in the companies and to the forecasting software that is used in these processes

Deep space network

Background, current status, and sites of Deep Space Network stations are briefly discussed

A robust method for measuring the Hubble parameter

We obtain a robust, non-parametric, estimate of the Hubble constant from galaxy linear diameters calibrated using HST Cepheid distances. Our method is independent of the parametric form of the diameter function and the spatial distribution of galaxies and is insensitive to Malmquist bias. We include information on the galaxy rotation velocities; unlike Tully-Fisher, however, we retain a fully non-parametric treatment. We find $H_0=66\pm6$ km/s/Mpc, somewhat larger than previous results using galaxy diameters.Comment: 4 pages, 1 figure, Cosmic Flows Workshop, Victoria B.C. Canada, July 1999, ed. S. Courteau, M. Strauss & J. Willick, ASP conf. serie