Cavity Reactor Engineering Mockup Critical Experiment

Critical mass of uranium 235 for stainless steel lined cavities in nuclear research and test reactors with heavy water reflecto

Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus

We present Hubble Space Telescope Planetary Camera UVI data for the little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing cluster R136. The average reddening of Hodge 301 is found to be = (0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two different sets of evolutionary models, we derive an age of ~ 20-25 Myr for Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the most prominent representative of the oldest population in the 30 Dor starburst region; a region that has undergone multiple star formation events. This range of ages is an important consideration for the modelling of starburst regions. Hodge 301 shows a widened upper main sequence largely caused by Be stars. We present a list of Be star candidates. The slope of the initial mass function for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no indication for a truncation or change of slope of the IMF within this mass range. In accordance with the age of Hodge 301 no obvious pre-main-sequence stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with more than 12 solar masses may have turned into supernovae since the formation of the cluster. Multiple supernova explosions are the most likely origin of the extremely violent gas motions and the diffuse X-ray emission observed in the cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in two-column style. 9 separate figures, in part in significantly reduced resolution for space reasons (bitmapped postscript or jpg

Tracking the Irish adult population during the first year of the COVID-19 pandemic : a methodological report of the COVID-19 psychological research consortium (C19PRC) study in Ireland

The COVID-19 Psychological Research Consortium (C19PRC) study was established to determine the impact of the COVID-19 pandemic on the population of multiple countries. Here, we provide a methodological overview, cohort profile, data access, and summary of key findings from the Republic of Ireland arm of the C19PRC study. A longitudinal internet panel survey was designed to collect data from a nationally representative sample of Irish adults (N = 1041) who were tracked from March/April 2020 to March/April 2021. Quota sampling methods were used to produce a sample that was representative of the population with respect to sex, age, and regional distribution. Data were collected in five waves, and new participants were recruited at follow-up waves to cover sample attrition and produce nationally representative samples at various points during the first year of the pandemic. A comprehensive battery of measures was used throughout the project to assess an array of sociodemographic, political, social, psychological, physical health, COVID-19, and mental health variables. Analyses were conducted to compare sample characteristic to known population parameters from available census data. These analyses showed that the sample was representative of the general adult population of Ireland on the three quota variables and was reasonable representative of the population across a diverse range of sociodemographic variables. These data representative the first and only nationally representative, longitudinal survey of the mental health of the Irish population. These data are made freely available to interested users (https://osf.io/2huzd/files/) and the findings of this study provide a methodological basis for the future use of these data

Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004

A Census of the Chamaeleon I Star-Forming Region

Optical spectroscopy has been obtained for 179 objects that have been previously identified as possible members of the cluster, that lack either accurate spectral types or clear evidence of membership, and that are optically visible (I<18). I have used these spectroscopic data and all other available constraints to evaluate the spectral classifications and membership status of a total sample of 288 candidate members of Chamaeleon I that have appeared in published studies of the cluster. The latest census of Chamaeleon I now contains 158 members, 8 of which are later than M6 and thus are likely to be brown dwarfs. I find that many of the objects identified as members of Chamaeleon I in recent surveys are actually field stars. Meanwhile, 7 of 9 candidates discovered by Carpenter and coworkers are confirmed as members, one of which is the coolest known member of Chamaeleon I at a spectral type of M8 (~0.03 M_sun). I have estimated extinctions, luminosities, and effective temperatures for the members and used these data to construct an H-R diagram for the cluster. Chamaeleon I has a median age of ~2 Myr according to evolutionary models, and hence is similar in age to IC 348 and is slightly older than Taurus (~1 Myr). The measurement of an IMF for Chamaeleon I from this census is not possible because of the disparate methods with which the known members were originally selected, and must await an unbiased, magnitude-limited survey of the cluster.Comment: 59 pages, 22 figure

Supersonic water masers in 30 Doradus

We report on extremely high velocity molecular gas, up to -80 km/s relative to the ambient medium, in the giant star-formation complex 30 Doradus in the Large Magellanic Cloud (LMC), as observed in new 22 GHz H2O maser emission spectra obtained with the Mopra radio telescope. The masers may trace the velocities of protostars, and the observed morphology and kinematics indicate that current star formation occurs near the interfaces of colliding stellar-wind blown bubbles. The large space velocities of the protostars and associated gas could result in efficient mixing of the LMC. A similar mechanism in the Milky Way could seed the galactic halo with relatively young stars and gas.Comment: 11 pages plus 1 PS and 1 EPS figure, uses AASTeX preprint style; accepted for publication in Astrophysical Journal Letter

Immediate Elaborated Feedback Personalization in Online Assessment

Providing a student with feedback that is timely, most suitable and useful for her personality and the performed task is a challenging problem of online assessment within Web-based Learning Systems (WBLSs). In our recent work we suggested a general approach of feedback adaptation in WBLS and through a series of experiments we demonstrated the possibilities of tailoring the feedback that is presented to a student as a result of her response to questions of an online test, taking into account the individual learning styles (LS), certitude in a response and correctness of this response. In this paper we present the result of the most recent experimental field study where we tested two feedback adaptation strategies in real student assessment settings (73 students had to answer 15 multiple-choice questions for passing the midterm exam). The first strategy is based on the correctness and certitude of the response, while the second strategy takes student LS into account as well. The analysis of assessment results and students’ behaviour demonstrate that both strategies perform reasonably well, yet the analysis also provide some evidence that the second strategy does a better job

On the Identification of High Mass Star Forming Regions using IRAS: Contamination by Low-Mass Protostars

We present the results of a survey of a small sample (14) of low-mass protostars (L_IR < 10^3 Lsun) for 6.7 GHz methanol maser emission performed using the ATNF Parkes radio telescope. No new masers were discovered. We find that the lower luminosity limit for maser emission is near 10^3 Lsun, by comparison of the sources in our sample with previously detected methanol maser sources. We examine the IRAS properties of our sample and compare them with sources previously observed for methanol maser emission, almost all of which satisfy the Wood & Churchwell criterion for selecting candidate UCHII regions. We find that about half of our sample satisfy this criterion, and in addition almost all of this subgroup have integrated fluxes between 25 and 60 microns that are similar to sources with detectable methanol maser emission. By identifying a number of low-mass protostars in this work and from the literature that satisfy the Wood & Churchwell criterion for candidate UCHII regions, we show conclusively for the first time that the fainter flux end of their sample is contaminated by lower-mass non-ionizing sources, confirming the suggestion by van der Walt and Ramesh & Sridharan.Comment: 8 pages with 2 figures. Accepted by Ap

Near-infrared Spectra of Chamaeleon I Stars

We present low resolution (R = 500) near-infrared spectra of 46 candidate young stellar objects in the Chamaeleon I star-forming region recently detected in several deep photometric surveys of the cloud. Most of these stars have K < 12. In addition, we present spectra of 63 previously known southern hemisphere young stars mainly belonging to the Chamaeleon I and Lupus dark clouds. We describe near-infrared spectroscopic characteristics of these stars and use the water vapor indexes to derive spectral types for the new objects. Photometric data from the literature are used to estimate the bolometric luminosities of all sources. We apply D'Antona & Mazzitelli (1998) pre-main sequence evolutionary tracks and isocrones to derive masses and ages. We detect two objects with mass below the H burning limit among the 46 new candidates. One of this object (PMK99 IR Cha INa1) is the likely driving source of a bipolar outflow in the northern region of the cloud.Comment: 48 pages, 9 figures, 9 tables, manuscript format - Accepted by Astron. J. (April 2003 issue)