Time-resolved luminescence of low sensitivity quartz from crystalline rocks

preprintTime-resolved luminescence spectra of low sensitivity natural quartz from crystalline rocks are presented. The luminescence was pulse-stimulated at width using 470 nm blue light from quartz separated from plutonic, metamorphic, volcanic and hydrothermal samples. Measurements were made at 20 °C. All samples show evidence of a short lifetime component less than long although in several cases too weak in intensity to be evaluated accurately. On the other hand, the value of the principal lifetime component varies considerably being about in metamorphic quartz, in plutonic quartz, and in one example of hydrothermal quartz. The results illustrate a new feature of luminescence from quartz for which lifetimes less than or greater than have never been reported at room temperature before. It is argued that the thermal provenance of the quartz and so the annealing it will have experienced influences the size of the observed lifetime. In particular, the results are explained in terms of a model consisting of three luminescence centers with the dominant lifetime linked to preferential recombination at one center depending on the thermal history of the sample and hence the hole concentration of the center

Cathodoluminescence of Recent biogenic carbonates: environmental and ontogenetic fingerprint

Cathodoluminescence (CL) examination of Recent biogenic carbonates shows that they are often luminescent regardless of their mineralogical composition (calcite v. aragonite), habitat (marine v. fresh water), way of life (sessile v. vagile) or environment (hyper- v. hyposaline water). Thus, the presence of luminescence in biogenic particles is not a reliable indicator of diagenetic alteration as some authors have suggested. In addition, CL can reveal variations in the mineralogy of shell material (e.g. regenerated calcitic v. primary aragonitic) and can highlight growth-related structures. Manganese (Mn2+) is the most likely activator of this luminescence, and its content in the shells of benthic organisms seems to be linked to growth rate, ontogeny, open sea conditions, bathymetry and salinity. In neritic environments the Mn2+ content and the CL of molluscs and foraminifera appear to increase with decreasing salinity. This study indicates that CL may be an important tool for the determination of environmental and ontogenetic parameters in biogenic carbonates in addition to its current use indiagenetic studie

The secondary star and distance of the polar V1309 Ori

The first phase-resolved JHK light curves of the eclipsing polar (AM Herculis binary) V1309 Ori are presented and interpreted. We separate the contributions from the secondary star and from other sources with the aim of determining a photometric distance. Simple model calculations show that the accretion stream and the cyclotron source on the accreting white dwarf are minor contributors to the infrared light, allowing an accurate determination of spectral type and absolute flux of the secondary star. The unilluminated backside of the secondary star as seen in eclipse has spectral type dM0 to dM0+. Its dereddened magnitude is K = 13.58 at orbital phase phi = 0 (eclipse). Using the calibrated surface brightness of M-stars and the published mass of the secondary, M2 = 0.46 Msun, we obtain a distance d = 600 +/- 25 pc which scales as M2^(1/2). The radius of the Roche-lobe filling secondary exceeds the main-sequence radius of an M0 star by 21 +11/-6 %. The debated origin of the infrared light of V1309 Ori has been settled in favor of the secondary star as the main contributor and an accurate distance has been derived that will place estimates of the luminosity and synchronization time scale on a more secure basis.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and Astrophysic

Discrete-row growth of xenon adsorbed on the vicinal Pt(997) surface: Comparison between theory and experiment

Xe exhibits a discrete-row growth mode on the vicinal Pt(997) surface by sequential attachment to the substrate steps. In order to interpret experimental results obtained by grazing incidence helium scattering, potential calculations are performed. A mean-field Hamiltonian within the two-dimensional Ising model is shown to explain the sequential-row growth observed in helium-atom diffraction studies. More specifically, the calculated temperatures for the occurrence of each row depend mainly on the shape of the potential increment due to the steps and countersteps. They are in good agreement with the experimental values associated with maxima in the scattered He intensity versus coverage curves

Landing together: how flocks arrive at a coherent action in time and space in the presence of perturbations

Collective motion is abundant in nature, producing a vast amount of phenomena which have been studied in recent years, including the landing of flocks of birds. We investigate the collective decision making scenario where a flock of birds decides the optimal time of landing in the absence of a global leader. We introduce a simple phenomenological model in the spirit of the statistical mechanics-based self-propelled particles (SPP-s) approach to interpret this process. We expect that our model is applicable to a larger class of spatiotemporal decision making situations than just the landing of flocks (which process is used as a paradigmatic case). In the model birds are only influenced by observable variables, like position and velocity. Heterogeneity is introduced in the flock in terms of a depletion time after which a bird feels increasing bias to move towards the ground. Our model demonstrates a possible mechanism by which animals in a large group can arrive at an egalitarian decision about the time of switching from one activity to another in the absence of a leader. In particular, we show the existence of a paradoxical effect where noise enhances the coherence of the landing process.Comment: 15 pages, 7 figure

New Rotation Periods in the Pleiades: Interpreting Activity Indicators

We present results of photometric monitoring campaigns of G, K and M dwarfs in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation periods for 18 stars in this cluster. In this paper, we examine the validity of using observables such as X-ray activity and amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds

Grating formation in step flow heterogeneous growth and wavelength selection induced by confinement

Based on kinetic Monte Carlo simulations, we show that modulated wires and island gratings can be formed at vicinal surfaces. The modulation (grating) wavelength along the steps can be tailored by external conditions (coverage and temperature) and intrinsic surface properties (diffusion, terrace width) via a scaling law. Above 220 K a thermodynamic saturation value for the wavelength occurs, which depends only on the terrace width. This morphological behavior can be understood in terms of nucleation arguments applied to heteroepitaxial growth of Ag on stepped Pt(111) surfaces. (C) 2004 Elsevier B.V. All rights reserved