Post-T Tauri stars: a false problem

We consider the problem of the apparent lack of old T Tauri stars in low-mass star forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetime and ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. We conclude that the probability of finding a large population of old stars in a star forming region is intrinsically very small and that the post-T Tauri problem is by and large not existent.Comment: 6 pages (LaTeX), no Figures to be published in The Astrophysical Journal Letter

A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1

Recent optical and IR studies have revealed that the heavily-reddened starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (WR) stars, comprising the richest WR population of any galactic cluster. We present results of a senstive Chandra X-ray observation of Wd 1 which detected 12 of the 22 known WR stars and the mysterious emission line star W9. The fraction of detected WN stars is nearly identical to that of WC stars. The WN stars WR-A and WR-B as well as W9 are exceptionally luminous in X-rays and have similar hard heavily-absorbed spectra with strong Si XIII and S XV emission lines. The luminous high-temperature X-ray emission of these three stars is characteristic of colliding wind binary systems but their binary status remains to be determined. Spectral fits of the X-ray bright sources WR-A and W9 with isothermal plane-parallel shock models require high absorption column densities log N$_{H}$ = 22.56 (cm$^{-2}$) and yield characteristic shock temperatures kT_shock ~ 3 keV (T ~ 35 MK).Comment: ApJL, 2006, in press (3 figures, 1 table

Statistical Agent Based Modelization of the Phenomenon of Drug Abuse

We introduce a statistical agent based model to describe the phenomenon of drug abuse and its dynamical evolution at the individual and global level. The agents are heterogeneous with respect to their intrinsic inclination to drugs, to their budget attitude and social environment. The various levels of drug use were inspired by the professional description of the phenomenon and this permits a direct comparison with all available data. We show that certain elements have a great importance to start the use of drugs, for example the rare events in the personal experiences which permit to overcame the barrier of drug use occasionally. The analysis of how the system reacts to perturbations is very important to understand its key elements and it provides strategies for effective policy making. The present model represents the first step of a realistic description of this phenomenon and can be easily generalized in various directions.Comment: 12 pages, 5 figure

Triangle percolation in mean field random graphs -- with PDE

We apply a PDE-based method to deduce the critical time and the size of the giant component of the ``triangle percolation'' on the Erd\H{o}s-R\'enyi random graph process investigated by Palla, Der\'enyi and VicsekComment: Summary of the changes made: We have changed a remark about k-clique percolation in the first paragraph. Two new paragraphs are inserted after equation (4.4) with two applications of the equation. We have changed the names of some variables in our formula

Trace formula for noise corrections to trace formulas

We consider an evolution operator for a discrete Langevin equation with a strongly hyperbolic classical dynamics and Gaussian noise. Using an integral representation of the evolution operator we investigate the high order corrections to the trace of arbitary power of the operator. The asymptotic behaviour is found to be controlled by sub-dominant saddle points previously neglected in the perturbative expansion. We show that a trace formula can be derived to describe the high order noise corrections.Comment: 4 pages, 2 figure

Systemic lupus erythematosus and the economic perspective: a systematic literature review and points to consider

Systemic lupus erythematosus (SLE) is a chronic, disabling, progressive disease, with many associated comorbidities, affecting patients during prime working years resulting in a high economic burden on society, producing high direct, indirect and intangible costs. In this article, our goals are two-fold. First, we review and discuss studies published in the period 2002–2012 concerning costs of SLE and point out gaps in the published literature. Second, we propose further research studies to advance our understanding of the economic perspective in SLE in the current area of new and emerging therapies. The literature evaluating disease costs in SLE remains limited and to date has only included a small number of countries. Despite these limitations, available studies indicate that SLE has significant socio-economic ramifications. Future studies are needed, especially to assess novel biologic therapies which have been made available or currently under investigation for SLE. An interesting approach in these new economic evaluations in SLE may be represented by the selection of the targets of the treatment to include in the cost-effectiveness and cost-utility analyses. Future treat-to-target strategies will likely include evaluation of their pharmacoeconomic implications

DNA analysis as tool for identification of bacteria in archaeological waterlogged wood

Abstract In this work molecular techniques were applied in order to integrete the results obtained by Optical (OM) and Scanning Electron (SEM) Microscopy, to understanding and assessing the changes in the anatomical structure of archaeological waterlogged wood (Pinus sp.) induced by bacteria colonization. Observation of wooden thin sections revealed by OM showed the presence of black and dark -brown areas (must probably due to sulphur compounds) and mineral concretions. The SEM micrographs revealed a specific cell wall alteration attributable to bacterial activity and abundant pyrite framboids (as single structure or clustered). The presence of sulfur compounds in archaeological waterlogged wood, indicate both long-term burial in anoxic environment and colonization by sulfate-reducing bacteria. Molecular biology investigation was performed through ad hoc protocols by direct DNA extraction from wood samples and in vitro amplification of bacteria DNA target sequence (16S, ITS regions-r RNA). The results reveal and identify bacterial genus as Pseudomonas, Cellulomonas, Xanthomonas and Bacillus that, as reported in the related scientific literature, are the most common cellulosolytic and ligninolytic bacteria. Moreover were also revealed the presence of Marinobacter sp. and Desulforudis audaxviator, respectively iron- oxidizing and sulfate- reducing bacteria. The investigation protocol set up in this work can be applied to a range of wooden artifacts of archaeological findings for both identification of bacteria colonization shed some light on the degradation phenomena, indispensable for correct conservation and restoration strategies

12^{12}C/13^{13}C ratio in planetary nebulae from the IUE archives

We investigated the abundance ratio of $^{12}$C/$^{13}$C in planetary nebulae by examining emission lines arising from \ion{C}{3} 2s2p ^3P_{2,1,0} \to 2s^2 ^1S_0. Spectra were retrieved from the International Ultraviolet Explorer archives, and multiple spectra of the same object were coadded to achieve improved signal-to-noise. The $^{13}$C hyperfine structure line at 1909.6 \AA was detected in NGC 2440. The $^{12}$C/$^{13}$C ratio was found to be $\sim4.4\pm$1.2. In all other objects, we provide an upper limit for the flux of the 1910 \AA line. For 23 of these sources, a lower limit for the $^{12}$C/$^{13}$C ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high signal-to-noise \ion{C}{3} spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 flux ratio outside the low-electron density theoretical limit for $^{12}$C. A mixture of $^{13}$C with $^{12}$C helps to close the gap somewhat. Nevertheless, some observed 1907/1909 flux ratios still appear too high to conform to the presently predicted limits. It is shown that this limit, as well as the 1910/1909 flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet $^1S_0$--$^3P_J$ according to the statistical weights. A detailed calculation for the fine structure collision strengths between these individual levels would be valuable.Comment: ApJ accepted: 19 pages, 3 Figures, 2 Table

Persistence and Uncertainty in the Academic Career

Understanding how institutional changes within academia may affect the overall potential of science requires a better quantitative representation of how careers evolve over time. Since knowledge spillovers, cumulative advantage, competition, and collaboration are distinctive features of the academic profession, both the employment relationship and the procedures for assigning recognition and allocating funding should be designed to account for these factors. We study the annual production n_{i}(t) of a given scientist i by analyzing longitudinal career data for 200 leading scientists and 100 assistant professors from the physics community. We compare our results with 21,156 sports careers. Our empirical analysis of individual productivity dynamics shows that (i) there are increasing returns for the top individuals within the competitive cohort, and that (ii) the distribution of production growth is a leptokurtic "tent-shaped" distribution that is remarkably symmetric. Our methodology is general, and we speculate that similar features appear in other disciplines where academic publication is essential and collaboration is a key feature. We introduce a model of proportional growth which reproduces these two observations, and additionally accounts for the significantly right-skewed distributions of career longevity and achievement in science. Using this theoretical model, we show that short-term contracts can amplify the effects of competition and uncertainty making careers more vulnerable to early termination, not necessarily due to lack of individual talent and persistence, but because of random negative production shocks. We show that fluctuations in scientific production are quantitatively related to a scientist's collaboration radius and team efficiency.Comment: 29 pages total: 8 main manuscript + 4 figs, 21 SI text + fig

Shocks and PDRs in an intermediate mass star forming globule: the case of IC1396N

The dark globule IC1396N is a typical example of a star formation process induced by radiation driven implosion due to the strong UV field from a nearby O6 star. The IRAS source embedded in the globule and its associated molecular outflow have been observed with the Long Wavelength Spectrometer (LWS) on ISO revealing an extremely rich spectrum including: CO rotational lines from J=14-13 up to J=28-27, rotational lines from ortho-H2O, OH lines involving the first four rotational levels of both ladders, atomic (OI 63μm, OI 145μm) and ionic (CII 157μm, OIII 52μm, OIII 88μm) lines. A complex picture arises, where an externally illuminated PDR coexists with strong C-shocks within IC1396N and whose origin is not clear