Detecting the Dusty Debris of Terrestrial Planet Formation

We use a multiannulus accretion code to investigate debris disks in the terrestrial zone, at 0.7-1.3 AU around a 1 solar mass star. Terrestrial planet formation produces a bright dusty ring of debris with a lifetime of at least 1 Myr. The early phases of terrestrial planet formation are observable with current facilities; the late stages require more advanced instruments with adaptive optics.Comment: 11 pages of text, 3 figures, accepted for ApJ Letters, additional info at http://cfa-www.harvard.edu/~kenyon/pf/terra/td

Duration judgements in patients with schizophrenia

Background. The ability to encode time cues underlies many cognitive processes. In the light of schizophrenic patients' compromised cognitive abilities in a variety of domains, it is noteworthy that there are numerous reports of these patients displaying impaired timing abilities. However, the timing intervals that patients have been evaluated on in prior studies vary considerably in magnitude (e.g. 1 s, 1 min, 1 h etc.). Method. In order to obviate differences in abilities in chronometric counting and place minimal demands on cognitive processing, we chose tasks that involve making judgements about brief durations of time (<1 s). Results. On a temporal generalization task, patients were less accurate than controls at recognizing a standard duration. The performance of patients was also significantly different from controls on a temporal bisection task, in which participants categorized durations as short or long. Although time estimation may be closely intertwined with working memory, patients' working memory as measured by the digit span task did not correlate significantly with their performance on the duration judgement tasks. Moreover, lowered intelligence scores could not completely account for the findings. Conclusions. We take these results to suggest that patients with schizophrenia are less accurate at estimating brief time periods. These deficits may reflect dysfunction of biopsychological timing processes

Analytic frameworks for assessing dialogic argumentation in online learning environments

Over the last decade, researchers have developed sophisticated online learning environments to support students engaging in argumentation. This review first considers the range of functionalities incorporated within these online environments. The review then presents five categories of analytic frameworks focusing on (1) formal argumentation structure, (2) normative quality, (3) nature and function of contributions within the dialog, (4) epistemic nature of reasoning, and (5) patterns and trajectories of participant interaction. Example analytic frameworks from each category are presented in detail rich enough to illustrate their nature and structure. This rich detail is intended to facilitate researchers’ identification of possible frameworks to draw upon in developing or adopting analytic methods for their own work. Each framework is applied to a shared segment of student dialog to facilitate this illustration and comparison process. Synthetic discussions of each category consider the frameworks in light of the underlying theoretical perspectives on argumentation, pedagogical goals, and online environmental structures. Ultimately the review underscores the diversity of perspectives represented in this research, the importance of clearly specifying theoretical and environmental commitments throughout the process of developing or adopting an analytic framework, and the role of analytic frameworks in the future development of online learning environments for argumentation

Surveyor batteries Final engineering report

Design and performance of Surveyor spacecraft silver-zinc main batter

Cosmic-ray induced background intercomparison with actively shielded HPGe detectors at underground locations

The main background above 3\,MeV for in-beam nuclear astrophysics studies with $\gamma$-ray detectors is caused by cosmic-ray induced secondaries. The two commonly used suppression methods, active and passive shielding, against this kind of background were formerly considered only as alternatives in nuclear astrophysics experiments. In this work the study of the effects of active shielding against cosmic-ray induced events at a medium deep location is performed. Background spectra were recorded with two actively shielded HPGe detectors. The experiment was located at 148\,m below the surface of the Earth in the Reiche Zeche mine in Freiberg, Germany. The results are compared to data with the same detectors at the Earth's surface, and at depths of 45\,m and 1400\,m, respectively.Comment: Minor errors corrected; final versio

Solvent extraction study of the thorium nitrate, nitric acid, and tributyl phosphate-dodecane system: density and acidity relationships

A solvent extraction study to determine equilibrium conditions of thorium nitrate-nitric acid with 30% tributyl phosphate in normal dodecane has been completed. Experimental conditions studied were 30 to 60{sup 0}C, 0.05 to 1.5 M Th(NO{sub 3}){sub 4}, and 0.0 to 3.0 M HNO{sub 3}. The extractant concentration was constant at 30% tributyl phosphate. The equilibrium experiments have produced data which demonstrate that thorium nitrate concentration, free acid, and density are related in equilibrium behavior between the aqueous and organic phases from 30 to 60{sup 0}C in the 30% tributyl phosphate-dodecane solvent extraction system. The concentration interactions apply to both the two- and three-phase regions. A linear correlation was observed for the density (D) of the aqueous or organic phase and the concentration of thorium and free acid. The general form of the equation is D = a(C/sub Th/ + bC/sub H/) + c, where a is the slope, b is the constant, c is the intercept, and C/sub Th/ and C/sub H/ are the molar concentrations of thorium and free acid respectively. The relationship of temperature, thorium nitrate, and free acid makes possible the definitions of the boundaries between the two- and three-phase regions. This dependence, in turn, permits operational control or simulation studies of the system within the two-phase region. The data demonstrate the interactions of the components of the Thorex system and can be used to improve the mathematical description of equilibrium in the SEPHIS-Thorex computer program

Numerically improved computational scheme for the optical conductivity tensor in layered systems

The contour integration technique applied to calculate the optical conductivity tensor at finite temperatures in the case of layered systems within the framework of the spin-polarized relativistic screened Korringa-Kohn-Rostoker band structure method is improved from the computational point of view by applying the Gauss-Konrod quadrature for the integrals along the different parts of the contour and by designing a cumulative special points scheme for two-dimensional Brillouin zone integrals corresponding to cubic systems.Comment: 17 pages, LaTeX + 4 figures (Encapsulated PostScript), submitted to J. Phys.: Condensed Matter (19 Sept. 2000

Non-collinear magnetic structures: a possible cause for current induced switching

Current induced switching in Co/Cu/Co trilayers is described in terms of ab-initio determined magnetic twisting energies and corresponding sheet resistances. In viewing the twisting energy as an energy flux the characteristic time thereof is evaluated by means of the Landau-Lifshitz-Gilbert equation using ab-initio parameters. The obtained switching times are in very good agreement with available experimental data. In terms of the calculated currents, scalar quantities since a classical Ohm's law is applied, critical currents needed to switch magnetic configurations from parallel to antiparallel and vice versa can unambiguously be defined. It is found that the magnetoresistance viewed as a function of the current is essentially determined by the twisting energy as a function of the relative angle between the orientations of the magnetization in the magnetic slabs, which in turn can also explain in particular cases the fact that after having switched off the current the system remains in the switched magnetic configuration. For all ab-initio type calculations the fully relativistic Screened Korringa-Kohn-Rostoker method and the corresponding Kubo-Greenwood equation in the context of density functional theory are applied.Comment: 20 pages, 4 tables and 15 figures, submitted to PR

Speckle Imaging Measurements of the Relative Tangential Velocities of the Components of T Tauri Binary Stars

Over a five year period, we have used speckle imaging to monitor 20 T Tauri binary stars with separations ranging from 0".09 to 1" (13-140 AU). This project is aimed at detecting the relative motion of the component stars to ascertain whether or not the observed companions (1) are stellar in nature, as opposed to being HH objects, and (2) are gravitationally bound to the primary stars. These observations demonstrate that speckle imaging measurements of close binary stars' separations can be made with an accuracy of a few milliarcseconds. The majority of the observed systems show significant relative velocities which (1) are not consistent with the motion expected for HH objects, (2) are greater than the velocity dispersion of these star-forming regions and thus are not the result of differential proper motion, and (3) are consistent with orbital motion. This is the first demonstration that these systems are physically bound. Furthermore, these relative velocity measurements provide dynamical evidence that the average total mass of these T Tauri binary star systems is ~l.7M_⊙