Search

AT2016dah and AT2017fyp are fairly typical Andromeda Galaxy (M31) classical novae. AT2016dah is an almost text book example of a 'very fast' declining, yet uncommon, Fe II'b' (broad-lined) nova, discovered during the rise to peak optical luminosity, and decaying with a smooth broken power-law light curve. AT2017fyp is classed as a 'fast' nova, unusually for M31, its early decline spectrum simultaneously shows properties of both Fe II and He/N spectral types - a 'hybrid'. Similarly, the light curve of AT2017fyp has a broken power-law decline but exhibits an extended flat-topped maximum. Both novae were followed in the UV and X-ray by the Neil Gehrels Swift Observatory, but no X-ray source was detected for either nova. The pair were followed photometrically and spectroscopically into their nebular phases. The progenitor systems were not visible in archival optical data, implying that the mass donors are main sequence stars. What makes AT2016dah and AT2017fyp particularly interesting is their position with respect to M31. The pair are close on the sky but are located far from the centre of M31, lying almost along the semi-minor axis of their host. Radial velocity measurements and simulations of the M31 nova population leads to the conclusion that both novae are members of the Andromeda Giant Stellar Stream (GSS). We find the probability of at least two M31 novae appearing coincident with the GSS by chance is ~1%. Therefore, we claim that these novae arose from the GSS progenitor, not M31 - the first confirmed novae discovered in a tidal steam., Comment: 22 pages, 14 figures, 4 tables. Accepted for publication in MNRAS

We attempt to take a strategic view of the development and application of HTE techniques across a broad spectrum of chemical, material and earth sciences, looking for unifying assumptions and approaches. We consider why much of the development of HTE technologies and techniques, as well as the majority of their application, have taken place in industry or in institutes or centers working closely with industry. And we look for commonalities and synergies across diverse HTE application areas, taking examples from the energy, catalysis, formulations and biotechnology fields.

Over the last two decades the Andromeda Galaxy (M31) has been something of a test-bed for methods aimed at obtaining accurate time-domain relative photometry within highly crowded fields. Difference imaging methods, originally pioneered towards M31, have evolved into sophisticated methods, such as the Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are most widely used to survey variable stars, transients and microlensing events in our own Galaxy. We show that modern difference image (DIA) algorithms such as OIS, whilst spectacularly successful towards the Milky Way bulge, may perform badly towards high surface brightness targets such as the M31 bulge. Poor results can occur in the presence of common systematics which add spurious flux contributions to images, such as internal reflections, scattered light or fringing. Using data from the Angstrom Project microlensing survey of the M31 bulge, we show that very good results are usually obtainable by first performing careful photometric alignment prior to using OIS to perform point-spread function (PSF) matching. This separation of background matching and PSF matching, a common feature of earlier M31 photometry techniques, allows us to take full advantage of the powerful PSF matching flexibility offered by OIS towards high surface brightness targets. We find that difference images produced this way have noise distributions close to Gaussian, showing significant improvement upon results achieved using OIS alone. We show that with this correction light-curves of variable stars and transients can be recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to implement and is quick enough to be incorporated within real-time DIA pipelines. (Abridged)

We report Hubble Space Telescope imaging, obtained 155 and 449 days after the 2006 outburst of the recurrent nova RS Ophiuchi, together with ground-based spectroscopic observations, obtained from the Observatorio Astron\'omico Nacional en San Pedro M\'artir, Baja California, M\'exico and at the Observatorio Astrof\'isico Guillermo Haro, at Cananea, Sonora, M\'exico. The observations at the first epoch were used as inputs to model the geometry and kinematic structure of the evolving RS Oph nebular remnant. We find that the modeled remnant comprises two distinct co-aligned bipolar components; a low-velocity, high-density innermost (hour glass) region and a more extended, high-velocity (dumbbell) structure. This overall structure is in agreement with that deduced from radio observations and optical interferometry at earlier epochs. We find that the asymmetry observed in the west lobe is an instrumental effect caused by the profile of the HST filter and hence demonstrate that this lobe is approaching the observer. We then conclude that the system has an inclination to the line of sight of 39$^{+1}_{-10}$ degrees. This is in agreement with the inclination of the binary orbit and lends support to the proposal that this morphology is due to the interaction of the outburst ejecta with either an accretion disk around the central white dwarf and/or a pre-existing red giant wind that is significantly denser in the equatorial regions of the binary than at the poles. The second epoch HST observation was also modeled. However, as no spectra were taken at this epoch, it is more difficult to constrain any model. Nevertheless, we demonstrate that between the two HST epochs the outer dumbbell structure seems to have expanded linearly., Comment: 33 pages, 9 figures, 1 table, accepted for publication in ApJ

The Angstrom Project is using a distributed network of two-meter class telescopes to conduct a high cadence pixel-lensing survey of the bulge of the Andromeda Galaxy (M31). With the expansion of global telescope network, the detection efficiency of pixel-lensing surveys is rapidly improving. In this paper, we estimate the detection rate of binary lens events expected from high-cadence pixel-lensing surveys toward M31 such as the Angstrom Project based on detailed simulation of events and application of realistic observational conditions. Under the conservative detection criteria that only high signal-to-noise caustic-crossing events with long enough durations between caustic crossings can be firmly identified as binary lens events, we estimate that the rate would be $\Gamma_{\rm b}\sim (7-15)f_{\rm b}(N/50)$ per season, where $f_{\rm b}$ is the fraction of binaries with projected separations of $10^{-3} {\rm AU}, Comment: 6 pages, 4 figures

The X-ray emission from quasars such as 3C273 is generally agreed to arise from Compton scattering of low energy seed photons by relativistic electrons in a relativistic jet oriented close to the line of sight. However there are a number of possible models for the origin of the seed photons. In Paper I (McHardy et al 1999) we showed that the X-ray and IR variability from 3C273 was highly correlated in 1997, with the IR flux leading the X-rays by ~0.75 +/- 0.25 days. The strong correlation, and lag, supports the Synchrotron Self-Compton (SSC) model, where the seed photons are synchroton photons from the jet itself. The previous correlation was based on one moderately well sampled flare and another poorly sampled flare, so the possibility of chance correlated variability exists. Here we report on further X-ray and IR observations of 3C273 which confirm the behaviour seen in Paper I. During a 2 week period of observations we see a flare of amplitude ~25%, lasting for ~5 days, showing a high correlation between IR and X-ray variations, with the X-rays lagging by ~1.45+/- 0.15 days. These observations were not scheduled at any special time, implying that the same mechanism - almost certainly SSC - dominates the X-ray emission on most occasions and that the structure of the emission region is similar in most small flares.

The Andromeda Galaxy Stellar Robotic Microlensing Project (The Angstrom Project) aims to use stellar microlensing events to trace the structure and composition of the inner regions of the Andromeda Galaxy (M31). We present microlensing rate and timescale predictions and spatial distributions for stellar and sub-stellar lens populations in combined disk and barred bulge models of M31. We show that at least half of the stellar microlenses in and around the bulge are expected to have characteristic durations between 1 and 10 days, rising to as much as 80% for brown-dwarf dominated mass functions. These short-duration events are mostly missed by current microlensing surveys that are looking for Macho candidates in the M31 dark matter halo. Our models predict that an intensive monitoring survey programme such as Angstrom, which will be able to detect events of durations upwards of a day, could detect around 30 events per season within ~5 arcminutes of the M31 centre, due to ordinary low-mass stars and remnants. This yield increases to more than 60 events for brown-dwarf dominated mass functions. The overall number of events and their average duration are sensitive diagnostics of the bulge mass, in particular the contribution of low-mass stars and brown dwarfs. The combination of an inclined disk, an offset bar-like bulge, and differences in the bulge and disk luminosity functions results in a four-way asymmetry in the number of events expected in each quadrant defined by the M31 disk axes. The asymmetry is sensitive to the bar prolongation, orientation and mass.

We present the results of a deep X-ray survey conducted with XMM-Newton, centred on the UK ROSAT 13H deep field area. This region covers 0.18 deg^2 and is the first of two areas covered with XMM-Newton as part of an extensive multi-wavelength survey designed to study the nature and evolution of the faint X-ray source population. We have produced detailed Monte-Carlo simulations to obtain a quantitative characterisation of the source detection procedure and to assess the reliability of the resultant sourcelist. We use the simulations to establish a likelihood threshold above which we expect less than 7 (3%) of our sources to be spurious. We present the final catalogue of 225 sources. Within the central 9 arcmin, 68 per cent of source positions are accurate to 2 arcsec, making optical follow-up relatively straightforward. We construct the N(>S) relation in four energy bands: 0.2-0.5 keV, 0.5-2 keV, 2-5 keV and 5-10 keV. In all but our highest energy band we find that the source counts can be represented by a double powerlaw with a bright end slope consistent with the Euclidean case and a break around 10^-14 cgs. Below this flux the counts exhibit a flattening. Our source counts reach densities of 700, 1300, 900 and 300 deg^-2 at fluxes of 4.1x10^-16, 4.5x10^-16, 1.1x10^-15 and 5.3x10^-15 cgs in the 0.2-0.5, 0.5-2, 2-5 and 5-10 keV energy bands respectively. We have compared our source counts with those in the two Chandra deep fields and Lockman hole and find our source counts to be amongst the highest of these fields in all energy bands. We resolve >51% (>50%) of the X-ray background emission in the 1-2 keV (2-5 keV) energy bands.

The POINT-AGAPE survey is an optical search for gravitational microlensing events towards the Andromeda Galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients. Here we describe the automated detection and selection pipeline used to identify M31 classical novae (CNe) and we present the resulting catalogue of 20 CN candidates observed over three seasons. CNe are observed both in the bulge region as well as over a wide area of the M31 disk. Nine of the CNe are caught during the final rise phase and all are well sampled in at least two colours. The excellent light-curve coverage has allowed us to detect and classify CNe over a wide range of speed class, from very fast to very slow. Among the light-curves is a moderately fast CN exhibiting entry into a deep transition minimum, followed by its final decline. We have also observed in detail a very slow CN which faded by only 0.01 mag day$^{-1}$ over a 150 day period. We detect other interesting variable objects, including one of the longest period and most luminous Mira variables. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of classical novae in M31, such as the global nova rate, the reliability of novae as standard-candle distance indicators and the dependence of the nova population on stellar environment. The findings of this statistical study will be reported in a follow-up paper., 21 pages, 13 figures, re-submitted for publication in MNRAS, typos corrected, references updated, figures 5-9 made clearer

High throughput methodologies were applied for the synthesis and testing of late transition metal complexes of Fe with bidentate diazabutadiene or tridentate diiminopyridine ligand in the polymerization of ethylene. Fe DIP complexes with ligands derived from amines with two or three annealed aromatic rings were found to be highly active catalysts when activated with methylaluminoxane. This finding can be rationalized based on models previously developed.

In the field of high throughput experimentation (HTE) related to materials sciences and catalysis a large number of test methods and analytical tools have been developed in the last years. As a consequence of the increasing parallelization and integration of the reactor and analysis systems, the requirements for the synthesis methodologies are rising extremely: smallest amounts of material samples of, i.e. different multi component mixed oxides in the μg- or gram-range have to be synthesized reproducibly and fully automated. In order to meet these challenges, the combination of different expert skills in the fields of inorganic synthesis, robotics, automation and related fields are of major importance. Real combinatorial chemistry, meaning the combinatorial permutation of element combinations or synthesis parameters involving statistical synthetic approaches leading to true combinatorial libraries of new functional materials supported by computer aided designs, was not presented yet in scientific community of inorganic solid state chemistry. We present here for the first time a tool for synthetic chemistry, originally developed with the application focus on molecular libraries, to produce inorganic solid state materials. With the so-called Split&Pool (S&P)-principle, it is possible to produce highly diverse materials libraries in the range of 10 3 –10 8 samples with a very simple workflow. Characterization with μ-XRF, optical microscopy and other analysis tools prove the potential of the presented synthesis technique. A powerful enhancement of this synthesis tool is our recently presented “single bead reactor” (SBR), allowing efficient testing of the libraries created by the Split&Pool methodology.

Our VLA observations of the XMM-Newton/Chandra 13hr deep survey field (see Page et al., this proceedings) result in one of the two deepest 1.4GHz radio maps ever made. Within the 15' radius field covered by the deep X-ray data (0.19 sq deg), a total of 556 radio sources are detected, down to a 4 sigma flux limit of 28uJy. Of the 214 Chandra sources, 55 have radio counterparts. The sub-arcsecond accuracy of the VLA and Chandra positions enable us to determine with high confidence the sources common to both surveys. Here we present the relationship between the X-ray and radio source populations at the faintest radio flux limits yet probed by such a study. We discuss how the X-ray/radio relationship differs as a function of optical morphology, ie between unresolved `stellar' objects and well resolved galaxies. We then discuss the origin of the X-ray and radio emission, ie AGN, starburst or a mixture of both, in these two classes of object., Comment: Astronomische Nachrichten, in press (Feb 2003). Proceedings of "X-ray Surveys, in the Light of New Observatories", 4-6 September, Santander, Spain

We describe some of the first X-ray detections of groups of galaxies at high redshifts (z~0.4), based on the UK deep X-ray survey of McHardy et al (1998). Combined with other deep ROSAT X-ray surveys with nearly complete optical identifications, we investigate the X-ray evolution of these systems. We find no evidence for evolution of the X-ray luminosity function up to z=0.5 at the low luminosities of groups of galaxies and poor clusters (Lx>3e42 erg/s), although the small sample size precludes very accurate measurements. This result confirms and extends to lower luminosities current results based on surveys at brighter X-ray fluxes. The evolution of the X-ray luminosity function of these low luminosity systems is more sensitive to the thermal history of the intra-group medium (IGM) than to cosmological parameters. Energy injection into the IGM (from for example supernovae or AGN winds) is required to explain the X-ray properties of nearby groups. The observed lack of evolution suggests that the energy injection occured at redshifts z>0.5., 13 pages, MNRAS accepted

The Liverpool Telescope is one of the world's premier facilities for time domain astronomy. The time domain landscape is set to radically change in the coming decade, with surveys such as LSST providing huge numbers of transient detections on a nightly basis; transient detections across the electromagnetic spectrum from other facilities such as SVOM, SKA and CTA; and the era of `multi-messenger astronomy', wherein events are detected via non-electromagnetic means, such as gravitational wave emission. We describe here our plans for Liverpool Telescope 2: a new robotic telescope designed to capitalise on this new era of time domain astronomy. LT2 will be a 4-metre class facility co-located with the LT at the Observatorio del Roque de Los Muchachos on the Canary island of La Palma. The telescope will be designed for extremely rapid response: the aim is that the telescope will take data within 30 seconds of the receipt of a trigger from another facility. The motivation for this is twofold: firstly it will make it a world-leading facility for the study of fast fading transients and explosive phenomena discovered at early times. Secondly, it will enable large-scale programmes of low-to-intermediate resolution spectral classification of transients to be performed with great efficiency. In the target-rich environment of the LSST era, minimising acquisition overheads will be key to maximising the science gains from any follow-up programme. The telescope will have a diverse instrument suite which is simultaneously mounted for automatic changes, but it is envisaged that the primary instrument will be an intermediate resolution, optical/infrared spectrograph for scientific exploitation of transients discovered with the next generation of synoptic survey facilities. In this paper we outline the core science drivers for the telescope, and the requirements for the optical and mechanical design., Comment: 53 pages including 7 figures and 1 table. Accepted by Experimental Astronomy. First revision modified in light of comments from the anonymous referee. This revision fixed some minor typographical errors introduced in the first revision

Based on a small number of early experimental results, it can be anticipated that high throughput experimentation (HTE) will play a significant role in the discovery of new zeolites and related materials. The four prime stages in the HTE approach are introduced, in the specific context of work on zeolite synthesis. Key experimental and computational issues are detailed and illustrated by recent literature results. Bottlenecks to achieving the throughput estimated as necessary for a planned level of productivity in new materials discovery are discussed.

We report the discovery of a narrow-line QSO (NLQSO) RX J1334.2+3759 with a steep soft X-ray spectrum. Soft X-ray emission from the NLQSO is highly variable. Changes in the intensity by a factor of about 2 have been detected in the ROSAT PSPC observations of RX J1334.2+3759 on time scales of 20000-40000s. Rapid variability events have also been observed from RX J1334.2+3759. The most extreme variable event has Delta L/Delta t = (1.95+/-1.02)E42 erg/s/s corresponding to a change in intensity by a factor of about 4 within 400s. The PSPC spectra of the NLQSO can be well represented by a power-law of photon index 3.8, modified by an absorbing column local to the source over and above that due to our own Galaxy. The intrinsic soft X-ray luminosity of RX J1334.2+3759 is estimated to be 2.2E44 erg/s in the energy band of 0.1-2.0 keV. The optical spectrum of RX J1334.2+3759 is typical of the NLS1 galaxies showing strong Balmer Hbeta, Halpha, and forbidden line [O III]5007. Fe II multiplets, usually present in the optical spectra of NLS1 galaxies, are also detected from RX J1334.2+3759. Decomposition of the Hbeta and Halpha line profiles shows the presence of narrow (FWHM = 880 km/s) and broad (FWHM = 2850 km/s) components in the spectrum of RX J1334.2+3759. The narrow-line region of RX J1334.2+3759 appears to be significantly different from those of normal Seyfert galaxies. A possible explanation for the observed properties of the narrow line region and the broad line region is suggested in terms of density enhancements.

First principles calculations based on the nonlocal density approximation to the density functional theory were performed to study structures, stabilities, and vibrational properties of small (monomer, triatomic, and dimer) neutral and ionized clusters of AlN, GaN, and InN. As a general trend, triatomic isomers prefer doublet spin states, whereas triplets are predicted for the monomer and the linear dimer clusters. Both nitrogenexcess and metal-excess triatomic clusters show minimum energy configurations to be approximately linear. The most stable isomer of Al2N2 and Ga2N2 is a rhombus with a singlet spin state, though In 2N2 is predicted not to be stable against dissociation into In 2 and N2. A strong dominance of the N-N bond over the metalnitrogen and metal-metal bonds appears to control the structural skeletons and the chemistry of these clusters. This is manifested in the dissociation of neutral and singly-ionized clusters, where the loss of metal atoms is shown to be the most likely fragmentation channel, except in the case of the dimer, in which the formation of two homonuclear diatomics is favored. The vibrational modes and frequencies are also explained in terms of the different bond strengths found in the diatomic clusters.

The magnetic moments of neutral Sc, V, and Ti atoms supported on a benzene molecule are found to be substantially enhanced over their free-atom value while those of neutral Mn, Fe, and Co atoms experience a similar reduction. The magnetic moment of the neutral Cr atom, the highest (6 μ B ) in the 3d-transition metal series, remains unchanged and that of neutral Ni is completely quenched. These results, based on density functional theory and the generalized gradient approximation, are shown to be a simple consequence of the Pauli exclusion principle. Similar calculations on positively and negatively charged 3d-metal–benzene complexes are carried out.

We report the results of first principles calculations on the Cr2O2, Cr2O3, and Cr2O4 clusters for which some of the considered configurations were proposed in the recent infrared spectroscopy experiments (Chertihin, G. V.; Bare, W. D.; Andrews, L. J. Chem. Phys. 1997, 107, 2798). Both linear and ring-like isomers are predicted to be equally probable for Cr2O2, while the (Cr2O2)O isomer is preferred over the chainlike (OCrOCrO) isomer for Cr2O3. For Cr2O4, a clear preference for the O(Cr2O2)O isomer over the (CrO2)2 isomer is predicted. Calculations of the vibrational frequencies for the lowest energy isomers of these clusters yield the stretching mode involving the stronger Cr−O bond to be around 900−1000 cm-1 and the bending mode involving the puckered-ring to be around 550−700 cm-1. Overall, the calculated normal modes of the cluster vibrations provide a very satisfactory description of the observed IR spectrum of the chromia clusters.

From a combination of high quality X-ray observations from the NASA Rossi X-ray Timing Explorer (RXTE), and infrared observations from the UK Infrared Telescope (UKIRT) we show that the medium energy X-ray (3-20 keV) and near infrared fluxes in the quasar 3C273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C273 arises from Compton scattering of low energy seed photons and our observations provide the first reliable detection of correlated variations in 3C273 between the X-ray band and any lower energy band. For a realistic electron distribution we demonstrate that it is probable that each decade of the seed photon distribution from the mm to IR waveband contributes roughly equally to the medium energy X-ray flux. However the expected mm variations are too small to be detected above the noise, probably explaining the lack of success of previous searches for a correlation between X-ray and mm variations. In addition we show that the infrared leads the X-rays by 0.75+/-0.25 days. These observations rule out the `External Compton' emission process for the production of the X-rays., 7 pages, 5 figures. Uses mn style file and epsf. Accepted for publication in the MNRAS

Crystallographic symmetry plays an important role in structure determination from diffraction or scattering data, in spectroscopy and in simulations. It is convenient and insightful to integrate the display and use of such symmetry data with data analysis and modeling methods. We outline the integration of a suite of crystallographic algorithms, closely coupled with interactive graphical displays. These include techniques for identifying the unit cell of a solid, for automatically determining space and point group symmetries, for generalized displays of symmetry elements overlaid on structural models, and for construction, editing, and transformation of models subject to symmetry constraints. In addition, electron densities derived from periodic density functional calculations can be symmetrized and displayed with the corresponding symmetry elements. Applications of these various capabilities in crystallographic research are illustrated by topical examples.

Some of the areas in which we anticipate, over the next five years, notable advances in the application of molecular simulation to problems in heterogeneous catalysis are considered, in the context of recent progress to date. The areas specifically addressed are: • expanding access to methods, • quantitative structure-property relationships, • building structural models to focus or pre-screen experiments, • confidence in predicting local and extended structure • reaction mechanisms, barriers and kinetics, and • data for chemical process simulations. In each of these areas, we indicate why we consider the topic significant, provide reference to topical work and suggest opportunities for future developments.

A new grid-based algorithm developed at Molecular Simulations, and molecular dynamics method have been applied to modeling the locations of non-framework species in zeolites. This new method locates the energy minima for various frameworks and populates these sites with the non-framework species. The cation locations were predicted in dehydrated zeolite adsorbents and catalysts, such as Na88X, Ca48X, mixed cation zeolite 3A (K60Na36A), and Cu-mordenite (Si/Al=5.0) using only well-known framework structure models. Furthermore, the locations of benzene molecules in a supercage of zeolite Ca48X were correctly predicted via the application of the Monte Carlo docking and molecular dynamics methods. These examples demonstrate that when the framework type is known, the new technique can provide a realistic initial structure input for the challenging task of solving crystal structures of zeolites containing non-framework species.

We consider the application of high-throughput experimentation (HTE), including combinatorial methods, to catalyst discovery and early-phase optimization. While combinatorial- and parallel-testing methods have an already substantial history in catalysis, recent work by several groups promises significant efficiency gains. In molecular catalysis, progress is noted in library design, library synthesis by pooled, parallel, and discrete formats, catalyst testing and reaction optimization; the prime constraint for organometallic catalysts is the limited scope of synthesis procedures for non-peptide-based ligand libraries. Routes described for the synthesis of heterogeneous catalysts include hydrothermal synthesis, arraying of solution precursors, automated impregnation and precipitation, and arraying of solid precursors. The key challenge in applying HTE to heterogeneous catalysis is testing; we distinguish here between Stage 1, or discovery testing and Stage 2, optimization testing, and describe techniques with potential in each case. Recent examples from the literature and our own work are used to illustrate these principles and the prospects for HTE applied to catalysis.

In blazar research the key to answering such questions as ‘What is the origin the emission mechanism of the high energy photons?’ are the time lags between different wavebands. Progress towards measuring these lags has been hampered by the difficulty of obtaining longterm high energy monitoring, but this situation changed with the launch of RXTE. We report on and discuss the results we have obtained from 18 months of monitoring of 3C 279 and 3C 273 by RXTE in conjunction with other wavebands. Highlights include simultaneous flares in the X-ray/γ-ray and X-ray/IR, both new ground breaking results.

The synthesis and crystal structure of potassium tantalate (KTa(2)(O,OH)(6).1.8H(2)O) with a defect pyrochlore structure are reported. The compound was crystallized directly from tantalum pentoxide in basic solution under hydrothermal conditions at temperatures as low as 200 degrees C. Ion exchange of this substance in acid yielded an H-type defect pyrochlore. Both highly crystalline K-type and H-type materials absorb argon and have BET surface areas of about 14 m(2)/g. The crystal structures were refined by the Rietveld least-squares method from powder X-ray diffraction data. The final R(wp) and chi(2) were 7.70% and 2.79, respectively. The high thermal stability and control of acidity of these materials, along with the simple and cheap synthesis, may find many applications in sorption, catalysis, and microelectronics.

We investigate the feasibility of reconstructing the radial intensity profile of extended stellar sources by inverting their microlensed light curves. Using a simple, linear, limb darkening law as an illustration, we show that the intensity profile can be accurately determined, at least over the outer part of the stellar disc, with realistic light curve sampling and photometric errors. The principal requirement is that the impact parameter of the lens be less than or equal to the stellar radius. Thus, the analysis of microlensing events provides a powerful method for testing stellar atmosphere models., 4 pages LaTeX, to appear in New Astronomy Reviews - proceedings of the Oxford Workshop `Gravitational Lensing: Nature's Own Weighing Scales'. Uses elsart.cls. Paper also available at ftp://info.astro.gla.ac.uk/pub/martin/extended.ps

It has already been shown that microlensing can give rise to a non-zero variable polarisation signal. Here we use realistic simulations to demonstrate the additional information that can be gained from polarimetric observations of lensing events., Comment: 4 pages, LaTeX, 2 figures from 6 postscript files. Uses elsart.sty, graphicx.sty and harvard.sty. To appear in New Astronomy Reviews - proceedings of the Oxford Workshop `Gravitational Lensing: Nature's Own Weighing Scales'

We report the results of density functional theory calculations on chromium oxide clusters responding to the formula CrmOn (m = 1−2, n = 1−3). Double numeric basis sets supplemented by polarization functions have been used in both local and nonlocal spin density approximations. Geometry optimizations of different spin states have been performed at the unrestricted spin level for the selected initial configurations. We have found that the covalent polarized Cr−O bonds dominate the chemical description of the CrOn series, while for the Cr2On series the presence of the Cr−Cr bond reduces the metal to oxygen charge transfer, yielding much softer clusters, as reflected by the smaller HOMO−LUMO gaps. The stability of all the isomers has been checked by computing their harmonic vibrational frequencies and the energetics of different fragmentation paths. The calculations reveal that the linear isomers are not true minima and that the oxygen-rich clusters are preferred over the metal-rich clusters. Overall, our re...

We present deep K-band imaging at the positions of four very faint X-ray sources found in the UK ROSAT Deep Survey to have no optical counterpart brighter than R~23. Likely identifications are found within the ROSAT error circle in all four fields with R-K colours of between 3.2 +/- 0.4 and 6.4 +/- 0.6. From a consideration of the R-K colours and X-ray to optical luminosity ratios of the candidate identifications, we tentatively classify two of the X-ray sources as very distant (z ~ 1) clusters of galaxies, one as a narrow emission line galaxy and one as an obscured QSO., 7 pages, LaTeX, 7 figures (1 postscript file each). Uses mn.sty and epsf.sty. Accepted by MNRAS. For more information see http://www.astro.soton.ac.uk/~amn/UKdeep

A molecular simulation technique for the efficient energy-based placement of cations within a matrix is described. The method is based on the use of non-bonded energy grids and rapid grid operations and is shown to reproduce known cation distributions efficiently. Results are reported for a range of inorganic and microporous materials.

We present the anharmonicity and roughening of Cu surfaces derived from the derivatives of the layer spacings and the surface energies with respect to temperature using the embedded-atom molecular dynamics method. The anharmonicity and roughening of the Cu(110) surface started to increase rapidly around 600 K and 1000 K respectively. In addition, we present the dependence of the surface energy on the temperature for the heteroepitaxial Au/Ag(110) system. The slope of the surface energy was found to be less pronounced compared with that of the free Ag(110) surface system.

Molecular dynamics and Gibbs ensemble Monte Carlo simulations have been used to investigate solvation structures in a dilute supercritical solution of two particle types interacting via Lennard-Jones potentials. Spatial correlations evidenced in the solute-solvent radial distribution functions near the critical point are found to depend strongly on the simulation cell size. There are significant differences between the solvent-solvent and solute-solvent radial distribution functions in the liquid and supercritical states. The mechanism of the development of a dynamic solvation structure around the solute is revealed in the density fluctuations in the critical region. In the liquid state thermal motion prevents strong pair correlations from appearing, masking underlying differences in the solvent-solvent and solute-solvent interaction potentials.

Intergrowths of cubic (FAU) and hexagonal (EMT) structural phases in a number of faujasite-related zeolite materials have been studied using transmission electron microscopy, and high-resolution powder X-ray diffraction. In Y, CSZ-1, CSZ-3, ZSM-2, ZSM-3, ZSM-20, ECR-35, ECR-30 and EMC-2 materials, the FAU and EMT components segregate non-randomly into contiguous blocks. This segregation is most pronounced in ZSM-20 and ECR-35. ECR-30 and EMC-2 are both predominantly EMT framework materials, within which FAU intergrowths occur in clumps of 1, 2 or 3 layers. No evidence is found for extended superlattice ordering of intergrowths. Intergrowth segregation acts to reduce both the configurational entropy associated with the stacking statistics and the elastic energy associated with coherency strain.

The embedded atom method (EAM) was used to simulate the dynamics of deposition of supported metal clusters on fcc transition metals and the dynamic behavior of surfaces. The formation of metal clusters was observed before deposition and the geometries, stabilities and dynamics of a series of metal clusters were discussed as obtained by EAM and compared with first-principle calculations. The calculations indicate that the structure of a cluster in the vapor phase is governed by its zero kelvin stability. We find that if the cluster forms in the vapor phase, it is difficult to get epitaxial growth since the metal cohesive energy is large and the chester can not be broken by thermal motions. The effect of impurities on the deposition is also discussed. The anharmonicity, roughening and reconstruction of surface were also investigated. Three-body interactions were found to he important to understand these phenomena

The dissocn. of NO and CO has been studied on cluster models representing the copper (100) and -(111) single-crystal faces using d. functional quantum calcns. For each surface, several possible reaction paths are proposed, for which we fully optimized the reactant, product, and transition states at the local d. level (LDA). Nonlocal d. functional calcns. (NLDA) were then performed on these optimized geometries. The clusters we used, varying in size between 13 and 31 atoms, produced dissocn. barriers and energies that were reasonably well converged with cluster size. Classical transition-state theory was used to calc. the rates of dissocn. and recombination on the basis of computed frequencies of the predicted transition state and the reactant and product states. The transition states for NO and CO dissocn. on all surfaces can be described as \"tight\" transition states corresponding to preexponentials for dissocn. in the range 1010-1013 s-1. The dissocn. barrier for NO is significantly lower than that for CO. In addn., the more open Cu(100) surface is more reactive toward dissocn. than the close-packed Cu(111) surface. Nonlocal corrections to the LDA functional were found to have a small effect on dissocn. barrier height, but the effect was found to be more profound on the recombination barrier and overall dissocn. energies

First principles, density functional theory embodied in the DMol program has been applied to agostic ethyl-Ti-complexes, including the dmpe complex, [Ti(-CH 2 CH 3 )C1 3 (dmpe)], where dmpe=(Me 2 PCH 2 ) 2 and its model complex, [Ti(−CH 2 CH 3 )Cl 3 (PH 3 ) 2 ]. The ethyl moiety of the complexes can adopt two limiting conformations, staggered and eclipsed. In the model complex, [Ti(−CH 2 CH 3 )C1 3 (PH 3 ) 2 ], both conformers are found to form agostic structures upon geometry optimization subject to Cs symmetry constraint, with the agostic eclipsed structure being the lower in energy. Full geometry optimization of the dmpe complex, [Ti(−CH 2 CH 3 )C1 3 (dmpe)], yields an agostic structure with geometrical features similar to those measured by single crystal X-ray analysis. It is shown that the HOMO orbital contributes substantially to the agostic bonding.

The structural and electronic properties of neutral cube-like MgO clusters comprising of 8, 18, 32, 48 and 64 atoms have been studied using density functional theory. Geometry optimization of all these stoichiometric clusters was done with C 2v symmetry. The binding energy and the nearest-neighbor separation in these clusters show a progressive approach towards their corresponding bulk values. The densities of states indicate an increase in the width of valence bands with increasing cluster size. Total charge density plots show a relatively stronger dependence of magnesium charge density on the coordination number as compared to that of oxygen in the 64-atom cluster.

We describe the implementation of the mesh-based first-principles density functional code DMol on nCUBE 2 parallel computers. The numerical mesh nature of DMol makes it naturally suited for a massively parallel computational environment. Our parallelization strategy consists of a domain decomposition of mesh points. This evenly distributes mesh points to all available processors and leads to a substantial computational speedup with limited communication overhead and good node balancing. To achieve better performance and circumvent memory storage limitations, the torus wrap method is used to distribute both the Hamiltonian and overlap matrices, and a parallel matrix diagonalization routine is employed to calculate eigenvalues and eigenvectors. Benchmark calculations on a 128-node nCUBE 2 are presented. © 1995 by John Wiley & Sons, Inc.

Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular siev (OMS) having the 2x2 hollandite structure with a one-dimensional pore diameter of 4.6 {Angstrom} Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnO{sub 4} and Mn{sup 2+}. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600 {degrees}C. OMS-2 formed by oxidation of Mn{sup 2+} under basic conditions and calcined at higher temperature (200-800 {degrees}C) is thermally stable up to 800 {degrees}C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu{sup 2+} and Fe{sup 3+}. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu{sup 2+} and Fe{sup 3+} contain nonexchangeable Mn{sup 2+}. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and A value of 85 G indicative of octahedral Mn{sup 2+} coordination. The Mn{sup 2+} EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra,more » ion-exchange data, X-ray diffraction patterns, and the theoretical simulations of diffraction data suggest that Cu{sup 2+} and Fe{sup 3+} are located in the tunnels of OMS-2. 29 refs., 8 figs., 3 tabs.« less

Pervasive streaking observed in electron diffraction patterns indicates that natural ferrierite from Lovelock, Nevada, frequently contains planar defects. Modeling and diffraction simulations of seven candidate fault structures indicate that the most commonly observed fault involves c 2 a shear on the (040) planes and can occur with a frequency of up to 30% on the (040) planes in an individual crystallite. Also observed are faults involving a c 2 shear on the (400) planes, which occur with a lower frequency of ∼ 5%.

This article gives a brief overview of density functional theory and discusses two specific implementations: a numerical localized basis approach (DMol) and the pseudopotential plane-wave method. Characteristic examples include Cu, clusters, CO and NO dissociation on copper surfaces, Li-, K-, and O-endohedral fullerenes, tris-quaternary ammonium cations as zeolite template, and oxygen defects in bulk SiO2. The calculations reveal the energetically favorable structures (estimated to be within ± 0.02 A of experiment), the energetics of geometric changes, and the electronic structures underlying the bonding mechanisms. A characteristic DMo1 calculation on a 128-node nCUBE 2 parallel computer shows a speedup of about 107 over a single processor. A plane-wave calculation on a unit cell with 64 silicon atoms using 1024 nCUBE 2 processors runs about five times faster than on a single-processor CRAY YMP.

The 105 zero-wavevector vibrational modes of an infinite sodalite framework with atomic positions corresponding to the sodalites M 8 [Al 6 Si 6 O 24 ]Cl 2 (M=Li, Na, K) and silica sodalite are calculated by the Wilson GF matrix method. Not all of the modes which are formally infrared or Raman active have significant spectral intensity, and point-charge or bond-polarizability models are used to generate synthetic infrared or Raman spectra in order to identify the modes which give rise to the observed bands. The synthetic spectra reproduce most of the features of the experimental spectra very satisfactorily, and for the aluminosilicate sodalites the wavenumbers of the bands are simulated to within 30 cm -1

By extending the theoretical treatments of Cowley and Hendricks and Teller, we have shown that the diffracted wave functions from successive slabs in crystals containing planar defects are related by simple recurrence equations. Recurrence equations greatly simplify the simulation of diffraction from coherently faulted crystals. Selected-area electron diffraction patterns for zeolite beta are simulated and compared with experimental data. Results are consistent with previous observations, made by structure imaging and powder X-ray diffraction, that zeolite beta is a highly disordered intergrowth of two unfaulted end-member structures, with a probability of a stacking fault being close to 50%. The quantitative accuracy of the recursion model of electron diffraction is limited by dynamical scattering effects.