A Study of the Near-Ultraviolet Spectrum of Vega

UV, optical, and near-IR spectra of Vega have been combined to test our understanding of stellar atmospheric opacities, and to examine the possibility of constraining chemical abundances from low-resolution UV fluxes. We have carried out a detailed analysis assuming Local Thermodynamic Equilibrium (LTE) to identify the most important contributors to the UV continuous opacity: H, H$^{-}$, C I, and Si II. Our analysis also assumes that Vega is spherically symmetric and its atmosphere is well described with the plane parallel approximation. Comparing observations and computed fluxes we have been able to discriminate between two different flux scales that have been proposed, the IUE-INES and the HST scales, favoring the latter. The effective temperature and angular diameter derived from the analysis of observed optical and near-UV spectra are in very good agreement with previous determinations based on different techniques. The silicon abundance is poorly constrained by the UV observations of the continuum and strong lines, but the situation is more favorable for carbon and the abundances inferred from the UV continuum and optical absorption lines are in good agreement. Some spectral intervals in the UV spectrum of Vega that the calculations do not reproduce well are likely affected by deviations from LTE, but we conclude that our understanding of UV atmospheric opacities is fairly complete for early A-type stars.Comment: 13 pages, 9 figures, to be published in Ap

An Atlas of K-line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)

We present a short atlas illustrating the unusual Ca {\sc ii} K-line profiles in upper main sequence stars with anomalous abundances. Slopes of the profiles for 10 cool, magnetic chemically peculiar (CP) stars change abruptly at the very core, forming a deep "nib." The nibs show the same or nearly the same radial velocity as the other atomic lines. The near wings are generally more shallow than in normal stars. In three magnetic CP stars, the K-lines are too weak to show this shape, though the nibs themselves are arguably present. The Ca {\sc ii} H-lines also show deep nibs, but the profiles are complicated by the nearby, strong H$\epsilon$ absorption. The K-line structure is nearly unchanged with phase in $\beta$ CrB and $\alpha$ Cir. Calculations, including NLTE, show that other possibilities in addition to chemical stratification may yield nib-like cores.Comment: 6 pages, 2 figures, and 8 figures; accepted for publication in ApJ

Abundance analysis of two late A-type stars HD 32115 and HD 37594

We have performed abundance analysis of two slowly rotating, late A-type stars, HD 32115 (HR 1613) and HD 37594 (HR 1940), based on obtained echelle spectra covering the spectral range 4000-9850 AAngstrom. These spectra allowed us to identify an extensive line list for 31 chemical elements, the most complete to date for A-type stars. Two approaches to abundance analysis were used, namely a ``manual'' (interactive) and a semi-automatic procedure for comparison of synthetic and observed spectra and equivalent widths. For some elements non-LTE (NLTE) calculations were carried out and the corresponding corrections have been applied. The abundance pattern of HD 32115 was found to be very close to the solar abundance pattern, and thus may be used as an abundance standard for chemical composition studies in middle and late A stars. Further, its H-alpha line profile shows no core-to-wing anomaly like that found for cool Ap stars and therefore also may be used as a standard in comparative studies of the atmospheric structures of cool, slowly rotating Ap stars. HD 37594 shows a metal deficiency at the level of -0.3 dex for most elements and triangle-like cores of spectral lines. This star most probably belongs to the Delta Scuti group.Comment: 10 pages, 4 figure

A-type Supergiant Abundances in the SMC: Probes of Evolution

New abundances of N, O, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Sr, Zr, and Ba are presented for 10 A-type supergiants in the SMC, plus upper limits for C. In interpreting the CNO results for constraints on stellar evolution theories, careful attention has been paid to the comparison abundances, i.e., the present day abundances of SMC nebulae and B-dwarf stars. These new results are also compared to published results from F-K supergiant analyses, and found to be in good agreement when both sets of data are carefully examined as differential (SMC minus Galactic standard) abundances. With the exception of nitrogen, very small star-to-star abundance variations are found for all elements in this analysis. The N variations are not predicted by standard stellar evolution models. Instead, the results support the new predictions reported from rotating stellar models, where the range in nitrogen is the result of partial mixing of CN-cycled gas from the stellar interior due to main-sequence rotation at different rates (c.f., Langer & Heger 1998). The overall overabundance of nitrogen in the sampled stars also implies these stars have undergone the first dredge-up in addition to having been mixed while on the main-sequence. The alpha-elements (O, Mg, Si, Ca, Ti) have similar underabundances to Fe, which is not the same as seen in metal-poor stars in the solar neighborhood of the Galaxy. In addition, certain light s-process elements (Zr, Ba) are slightly more underabundant than Fe, which is predicted by the bursting chemical evolution model presented by Pagel & Tautvaisiene (1998) for the SMC.Comment: Accepted for publication in the Astrophysical Journal, Manuscript #39295. Accepted January 4, 199

Cost comparison of MRSA screening and management – a decision tree analysis

<p>Abstract</p> <p>Background</p> <p>Methicillin-resistant <it>Staphylococcus aureus</it> (MRSA) infections represent a serious challenge for health-care institutions. Rapid and precise identification of MRSA carriers can help to reduce both nosocomial transmissions and unnecessary isolations and associated costs. The practical details of MRSA screenings (who, how, when and where to screen) remain a controversial issue.</p> <p>Methods</p> <p>Aim of this study was to determine which MRSA screening and management strategy causes the lowest expected cost for a hospital. For this cost analysis a decision analytic cost model was developed, primary based on data from peer-reviewed literature. Single and multiplex sensitivity analyses of the parameters “costs per MRSA case per day”, “costs for pre-emptive isolation per day”, “MRSA rate of transmission not in isolation per day” and “MRSA prevalence” were conducted.</p> <p>Results</p> <p>The omission of MRSA screening was identified as the alternative with the highest risk for the hospital. Universal MRSA screening strategies are by far more cost-intensive than targeted screening approaches. Culture confirmation of positive PCR results in combination with pre-emptive isolation generates the lowest costs for a hospital. This strategy minimizes the chance of false-positive results as well as the possibility of MRSA cross transmissions and therefore contains the costs for the hospital. These results were confirmed by multiplex and single sensitivity analyses. Single sensitivity analyses have shown that the parameters “MRSA prevalence” and the “rate of MRSA of transmission per day of non-isolated patients” exert the greatest influence on the choice of the favorite screening strategy.</p> <p>Conclusions</p> <p>It was shown that universal MRSA screening strategies are far more cost-intensive than the targeted screening approaches. In addition, it was demonstrated that all targeted screening strategies produce lower costs than not performing a screening at all.</p