The statistical Analysis of Star Clusters

We review a range of stastistical methods for analyzing the structures of star clusters, and derive a new measure ${\cal Q}$ which both quantifies, and distinguishes between, a (relatively smooth) large-scale radial density gradient and multi-scale (fractal) sub-clustering. Q is derived from the normalised correlation length and the normalised edge length of the minimal spanning tree for each cluster

Synoptic physics

A new course has been introduced in the second year of undergraduate Physics studies at Cardiff University. Aimed at addressing problem solving, team working and communication, the course has been very well received by students, with exceptional levels of attendance and engagement. Students also displayed an increased level of enthusiasm for the degree and enhanced self-confidence. The structure and content of the course are outlined

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

SPH simulation of the formation and evolution of protoplanetary disks

The formation and evolution of protoplanetary disks is simulated by computer modelling, using the Smoothed Particle Hydrodynamics (SPH) method. The suitability of SPH for modelling disks is investigated, and problems are identified with the SPH implementation of Artificial Viscosity in disks with Keplerian velocity profiles. Analytical and experimental results reveal that the resultant viscous force for a simulated Keplerian disk is in the opposite direction to that produced by linear shear. Applying Artificial Viscosity only to approaching particles results in a radial force four times larger than the force in the direction of the orbit. The viscous force can change direction if temperature, and therefore sound speed, decreases. Techniques for activating Artificial Viscosity only when convergence is detected are found to fail in differentially rotating disks. Both the Balsara Switch and Time Dependent Artificial Viscosity use the SPH estimate of V v sph, which has a low freqency time varying component which is independent of h, and so cannot be removed by increasing the number of SPH particles. An alternative method, based on pattern recognition, is shown to reduce the viscous spread of a differentially rotating ring by an order of magnitude. We also identify problems associated with the gravitational field of disks. The use of an annulus to represent a portion of a much larger, continuous disk, may yield unrepresentative results. The edge effects can cause preferential accretion zones, where the Toomre Q parameter is not the same as it would be for the same region of an extended disk. SPH simulations of Protoplanetary disks produce condensations which do not persist long enough to collapse. The high tidal shearing forces in a Keplerian accretion disk disrupt the condensations before they accumulate enough mass to collapse. Including a more realistic treatment of the thermal physics, and reducing the effective shear viscosity, makes the situation wors

Video assignments: A powerful way to introduce creativity and fun into your program – A physics education story

The School of Physics and Astronomy has been setting video assessments for several years and they have been very popular with students. Now that video equipment and editing software is so easily available to students, and videos can now be uploaded to Learning Central, there are no technical barriers. This can enable students, particularly in technical subjects, to practice communication skills as well as show creativity and flair

Phosphine gas in the cloud decks of Venus

Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas

Measuring clustering in 2dv space

The statistical descriptor is a robust and useful tool for distinguishing and quantifying the degree of radial or multiscale clustering in objects such as open clusters. is calculated as m/s, where is the mean edge length of the minimum spanning tree and is the mean distance between cluster members, or correlation length. is obtained using only two-dimensional position data. Here, we investigate the performance of in three dimensions, both when true three-dimensional data are available and when the radial velocity of cluster components is used as a proxy for position: this is known as 2dv space. True three-dimensional data offer an improvement in the resolution of and as diagnostic indicators of clustering, a scatter plot of versus proving to be a particularly clear method of interpreting the information. Results are not satisfactory when 2dv information is used, as the data from cluster types which are clearly distinguishable using 2d information alone become overlapping and confused when 2dv information is used. We therefore recommend that the 2d method is used, unless true 3d positions of cluster members are available. The use of the versus plot is particularly recommended, as adding extra discrimination between cluster types, compared with that achieved using alon

New statistical methods for investigating submarine pockmarks

We investigate the applicability of some novel spatial analysis techniques, developed for studies of astrophysical datasets, to the analysis of spatial point data in sedimentary basins. The techniques are evaluated and compared with standard methods using two test areas that contain large numbers of submarine pockmarks developed in distributed arrays. The familiar Ripley K and Voronoi tesselation techniques are used, and the results are then compared with those obtained using more novel techniques, the correlation length and minimal spanning tree. The correlation length technique is found to identify the precise distances at which clustering occurs more accurately, making a physical interpretation more clear than is possible using the Ripley K. The minimal spanning tree is found to be powerful at identifying the space-filling nature of the pockmark distribution, and has the advantage of being immune to edge effects. The use of these two novel techniques permits more information to be extracted from the datasets, and demonstrates clear statistically significant differences between them, which are not detectable using standard techniques