Outbursts of Young Stellar Objects

We argue that the outbursts of the FU Orionis stars occur on timescales which are much longer than expected from the standard disc instability model with \alpha_{c} \gtrsim 10^{-3}. The outburst, recurrence, and rise times are consistent with the idea that the accretion disc in these objects is truncated at a radius R_{i} \sim 40 \rsun. In agreement with a number of previous authors we suggest that the inner regions of the accretion discs in FU Ori objects are evacuated by the action of a magnetic propeller anchored on the central star. We develop an analytic solution for the steady state structure of an accretion disc in the presence of a central magnetic torque, and present numerical calculations to follow its time evolution. These calculations confirm that a recurrence time that is consistent with observations can be obtained by selecting appropriate values for viscosity and magnetic field strength.Comment: 13 pages, 7 figures, accepted by MNRA

Temporal fuzzy association rule mining with 2-tuple linguistic representation

This paper reports on an approach that contributes towards the problem of discovering fuzzy association rules that exhibit a temporal pattern. The novel application of the 2-tuple linguistic representation identifies fuzzy association rules in a temporal context, whilst maintaining the interpretability of linguistic terms. Iterative Rule Learning (IRL) with a Genetic Algorithm (GA) simultaneously induces rules and tunes the membership functions. The discovered rules were compared with those from a traditional method of discovering fuzzy association rules and results demonstrate how the traditional method can loose information because rules occur at the intersection of membership function boundaries. New information can be mined from the proposed approach by improving upon rules discovered with the traditional method and by discovering new rules

Infrared properties of serendipitous X-ray quasars

Near infrared measurements were obtained of 30 quasars originally found serendipitously as X-ray sources in fields of other objects. The observations show that the infrared characteristics of these quasars do not differ significantly from those of quasars selected by other criteria. Because this X-ray selected sample is subject to different selection biases than previous radio and optical surveys, this conclusion is useful in validating previous inferences regarding the infrared colors of 'typical' quasars

The steady-state structure of accretion discs in central magnetic fields

We develop a new analytic solution for the steady-state structure of a thin accretion disc under the influence of a magnetic field that is anchored to the central star. The solution takes a form similar to that of Shakura and Sunyaev and tends to their solution as the magnetic moment of the star tends to zero. As well as the Kramer's law case, we obtain a solution for a general opacity. The effects of varying the mass transfer rate, spin period and magnetic field of the star as well as the opacity model applied to the disc are explored for a range of objects. The solution depends on the position of the magnetic truncation radius. We propose a new approach for the identification of the truncation radius and present an analytic expression for its position.Comment: 11 pages, 7 figures, accepted by MNRA

Evolving temporal fuzzy itemsets from quantitative data with a multi-objective evolutionary algorithm

We present a novel method for mining itemsets that are both quantitative and temporal, for association rule mining, using multi-objective evolutionary search and optimisation. This method successfully identifies temporal itemsets that occur more frequently in areas of a dataset with specific quantitative values represented with fuzzy sets. Current approaches preprocess data which can often lead to a loss of information. The novelty of this research lies in exploring the composition of quantitative and temporal fuzzy itemsets and the approach of using a multi-objective evolutionary algorithm. This preliminary work presents the problem, a novel approach and promising results that will lead to future work. Results show the ability of NSGA-II to evolve target itemsets that have been augmented into synthetic datasets. Itemsets with different levels of support have been augmented to demonstrate this approach with varying difficulties

Coherent Time Evolution and Boundary Conditions of Two-Photon Quantum Walks

Multi-photon quantum walks in integrated optics are an attractive controlled quantum system, that can mimic less readily accessible quantum systems and exhibit behavior that cannot in general be accurately replicated by classical light without an exponential overhead in resources. The ability to observe time evolution of such systems is important for characterising multi-particle quantum dynamics---notably this includes the effects of boundary conditions for walks in spaces of finite size. Here we demonstrate the coherent evolution of quantum walks of two indistinguishable photons using planar arrays of 21 evanescently coupled waveguides fabricated in silicon oxynitride technology. We compare three time evolutions, that follow closely a model assuming unitary evolution, corresponding to three different lengths of the array---in each case we observe quantum interference features that violate classical predictions. The longest array includes reflecting boundary conditions.Comment: 7 pages,7 figure

Diffraction-Limited Imaging and Photometry of NGC 1068

The nearby Seyfert 2 Galaxy NGC 1068 was observed with speckle imaging techniques in the near-infrared H-band (1.6 microns) at the Hale 200-inch Telescope and K-band (2.2 microns) at the 10 m Keck I Telescope. Images with diffraction limited or near-diffraction limited resolutions of 0.''05 - 0.''1 were obtained and used to search for structure in the nuclear region. Images of the nucleus of NGC 1068 reveal an extended region of emission which accounts for nearly 50% of the nuclear flux at K-band. This region extends 10 pc on either side of an unresolved point source nucleus which is at most, 0.''02 or 1.4 pc in size. Both the point source and the newly imaged extended emission are very red, with identical H-K colors corresponding to a color temperature of 800 K. While the point source is of a size to be consistent with grains in thermal equilibrium with the nuclear source, the extended emission is not. It must consist either of nuclear emission which has been reflected off an extended dusty disk or of small grains raised to transiently high temperatures by reflected UV photons.Comment: accepted to AJ, AAS LaTeX and epsfig, 22 pages incl. 5 ps figure

A Two-Dimensional Near-Infrared Speckle Imaging Survey of T Tauri Stars in Taurus and Ophiuchus

We present the results of a magnitude limited (K≤8.5 mag) multiplicity survey of T Tauri stars in the two nearest star forming regions, Taurus-Auriga and Ophiuchus-Scorpius (D = 150 pc), observable from the northern hemisphere. Each of the 70 stars in the sample was imaged at 2.2 μm using two-dimensional speckle interferometry resulting in a survey sensitive to binary stars with separations ranging from 0.″09 to about 2″.5. The frequency of double stars with separation in this range is 46±8%. A division between the classical T Tauri stars and the weak-lined T Tauri stars shows no distinction. Furthermore, no difference is observed between the binary frequencies in the two star forming regions although the clouds have very different properties. Given the limited angular separation range that this survey is sensitive to, both the spectroscopic and wide binaries will be missed. The rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions

Near Infrared Imaging of the Hubble Deep Field with The Keck Telescope

Two deep K-band ($2.2 \mu m$) images, with point-source detection limits of $K=25.2$ mag (one sigma), taken with the Keck Telescope in subfields of the Hubble Deep Field, are presented and analyzed. A sample of objects to K=24 mag is constructed and $V_{606}-I_{814}$ and $I_{814}-K$ colors are measured. By stacking visually selected objects, mean $I_{814}-K$ colors can be measured to very faint levels; the mean $I_{814}-K$ color is constant with apparent magnitude down to $V_{606}=28$ mag.Comment: Replaced with slightly revised source positions and corrected V-I magnitudes (which were incorrect in the Tables and Figure 5). 18 pages. The data are publicly available at http://www.cco.caltech.edu/~btsoifer/hdf.html along with a high-resolution version of Fig.