Is prophylaxis of early-onset group B streptococcal disease appropriate for South Africa?

Background. Early-onset group B streptococcal (GBS) disease in neonates can be prevented by the use of intrapartum chemoprophylaxis. There are two prevention strategies, one based on risk factors and the other on culture screening for GBS. This study sought to establish whether GBS chemoprophylaxis is appropriate in a developing country such as South Africa.Methods. All neonates with early-onset GBS disease born at Johannesburg Hospital between 1 January 1995 and 21 December 1997 were reviewed. Data were collected prospectively between 1 January and 31 October 1998. Data included demographic information, obstetric information, disease characteristics, admission details and mortality. The approximate cost of implementing both strategies was determined.Results. The overal incidence of early-onset GBS was 1.16 per 1 000 live births. The rate was significantly greater in 1998 compared with the previous years. Most of the babies were born preterm (70%), and 60% required admission to the neonatal intensive care unit (lCU) (a total of 81 ICU days). Twelve of the babies died. Assuming that chemoprophylaxis would reduce the number of ICU days by half, this would save an amount of R52 000. Culture based chemoprophylaxis would cost R1O million, whereas an approach based on risk factors would cost R31140.Conclusion. In conclusion, we feel that early-onset GBS disease is sufficiently prevalent in our unit to justify the implementation of a chemoprophylaxis strategy based on risk factors. Whether other units should adopt a similar approach would depend on the local incidence of earlyonset GBS

of surfactant replacement therapy at Johannesburg Hospital, November 1991 December 1992

Objective. To assess the impact of surfactant replacement therapy (SRl) on the outcome of hyaline membrane disease (HMD) and to assess the cost implications of a policy of selective administration of artificial surfactant. Design. The short-term outcome of 103 newborns ventilated for HMD (61 selected for SRT according to initial and/or ongoing oxygen requirements) was compared with that of a historical control group of 173 infants ventilated for HMD before the introduction of SRT. Main outcome measures. Mortality and morbidity of HMD including death, bronchopulmonary dysplasia, pneumothorax, pulmonary haemorrhage, patent ductus arteriosus and intraventricular haemorrhage. Results. There were significant demographic differences between the treatment and control groups (black patients 74% v. 28%, P < 0,0001; unbooked mothers 72% v. 15%, P < 0,0001) as well as evidence of more severe lung disease in the treatment group (pressor support 44% v. 27%, P < 0,005; and paralysis during ventilation 38% v. 25%, P < 0,005). Pneumothorax was reduced in the SRT group (7% v. 17%, P < 0,01). There were no significant differences between the two groups in the incidence of BPO or mortality. The use of SRT added to the total cost of treating a patient ventilated for HMD. Conclusion. The selective use of SRT had the effect of converting severe -disease into moderate disease rather than achieVing maximal benefit in all cases of HMD through routine use of the product. A policy of restricting use may result in cost savings where resources are limited.S Afr Med J 1995; 85; 646-649

NN Scattering: Chiral Predictions for Asymptotic Observables

We assume that the nuclear potential for distances larger than 2.5 fm is given just by the exchanges of one and two pions and, for the latter, we adopt a model based on chiral symmetry and subthreshold pion-nucleon amplitudes, which contains no free parameters. The predictions produced by this model for nucleon-nucleon observables are calculated and shown to agree well with both experiment and those due to phenomenological potentials.Comment: 16 pages, 12 PS figures included, to appear in Physical Review

Quark Condensate in the Deuteron

We study the changes produced by the deuteron on the QCD quark condensate by means the Feynman-Hellmann theorem and find that the pion mass dependence of the pion-nucleon coupling could play an important role. We also discuss the relation between the many body effect of the condensate and the meson exchange currents, as seen by photons and pions. For pion probes, the many-body term in the physical amplitude differs significantly from that of soft pions, the one linked to the condensate. Thus no information about the many-body term of the condensate can be extracted from the pion-deuteron scattering length. On the other hand, in the Compton amplitude, the relationship with the condensate is a more direct one.Comment: to appear in Physics Review C (19 pages, 3 figures

The connection between stellar granulation and oscillation as seen by the Kepler mission

The long and almost continuous observations by Kepler show clear evidence of a granulation background signal in a large sample of stars, which is interpreted as the surface manifestation of convection. It has been shown that its characteristic timescale and rms intensity fluctuation scale with the peak frequency (\nu_{max}) of the solar-like oscillations. Various attempts have been made to quantify the observed signal, to determine scaling relations, and to compare them to theoretical predictions. We use a probabilistic method to compare different approaches to extracting the granulation signal. We fit the power density spectra of a large set of Kepler targets, determine the granulation and global oscillation parameter, and quantify scaling relations between them. We establish that a depression in power at about \nu_{max}/2, known from the Sun and a few other main-sequence stars, is also statistically significant in red giants and that a super-Lorentzian function with two components is best suited to reproducing the granulation signal in the broader vicinity of the pulsation power excess. We also establish that the specific choice of the background model can affect the determination of \nu_{max}, introducing systematic uncertainties that can significantly exceed the random uncertainties. We find the characteristic background frequency and amplitude to tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass dependency of the latter. To enable comparison with theoretical predictions, we computed effective timescales and intensity fluctuations and found them to approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran} \propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&

Determining global parameters of the oscillations of solar-like stars

Helioseismology has enabled us to better understand the solar interior, while also allowing us to better constrain solar models. But now is a tremendous epoch for asteroseismology as space missions dedicated to studying stellar oscillations have been launched within the last years (MOST and CoRoT). CoRoT has already proved valuable results for many types of stars, while Kepler, which was launched in March 2009, will provide us with a huge number of seismic data very soon. This is an opportunity to better constrain stellar models and to finally understand stellar structure and evolution. The goal of this research work is to estimate the global parameters of any solar-like oscillating target in an automatic manner. We want to determine the global parameters of the acoustic modes (large separation, range of excited pressure modes, maximum amplitude, and its corresponding frequency), retrieve the surface rotation period of the star and use these results to estimate the global parameters of the star (radius and mass).To prepare the analysis of hundreds of solar-like oscillating stars, we have developed a robust and automatic pipeline. The pipeline consists of data analysis techniques, such as Fast Fourier Transform, wavelets, autocorrelation, as well as the application of minimisation algorithms for stellar-modelling. We apply our pipeline to some simulated lightcurves from the asteroFLAG team and the Aarhus-asteroFLAG simulator, and obtain results that are consistent with the input data to the simulations. Our strategy gives correct results for stars with magnitudes below 11 with only a few 10% of bad determinations among the reliable results. We then apply the pipeline to the Sun and three CoRoT targets.In particular we determine the parameters of the Sun, HD49933, HD181906, and HD181420.Comment: 15 pages, 17 figures, accepted for publication in A&

Core properties of alpha Cen A using asteroseismology

A set of long and nearly continuous observations of alpha Centauri A should allow us to derive an accurate set of asteroseismic constraints to compare to models, and make inferences on the internal structure of our closest stellar neighbour. We intend to improve the knowledge of the interior of alpha Centauri A by determining the nature of its core. We combined the radial velocity time series obtained in May 2001 with three spectrographs in Chile and Australia: CORALIE, UVES, and UCLES. The resulting combined time series has a length of 12.45 days and contains over 10,000 data points and allows to greatly reduce the daily alias peaks in the power spectral window. We detected 44 frequencies that are in good overall agreement with previous studies, and found that 14 of these show possible rotational splittings. New values for the large and small separations have been derived. A comparison with stellar models indicates that the asteroseismic constraints determined in this study allows us to set an upper limit to the amount of convective-core overshooting needed to model stars of mass and metallicity similar to those of alpha Cen A.Comment: 8 pages, 11 figures, A&A accepte

Bayesian peak-bagging of solar-like oscillators using MCMC: A comprehensive guide

Context: Asteroseismology has entered a new era with the advent of the NASA Kepler mission. Long and continuous photometric observations of unprecedented quality are now available which have stimulated the development of a number of suites of innovative analysis tools. Aims: The power spectra of solar-like oscillations are an inexhaustible source of information on stellar structure and evolution. Robust methods are hence needed in order to infer both individual oscillation mode parameters and parameters describing non-resonant features, thus making a seismic interpretation possible. Methods: We present a comprehensive guide to the implementation of a Bayesian peak-bagging tool that employs a Markov chain Monte Carlo (MCMC). Besides making it possible to incorporate relevant prior information through Bayes' theorem, this tool also allows one to obtain the marginal probability density function for each of the fitted parameters. We apply this tool to a couple of recent asteroseismic data sets, namely, to CoRoT observations of HD 49933 and to ground-based observations made during a campaign devoted to Procyon. Results: The developed method performs remarkably well at constraining not only in the traditional case of extracting oscillation frequencies, but also when pushing the limit where traditional methods have difficulties. Moreover it provides an rigorous way of comparing competing models, such as the ridge identifications, against the asteroseismic data.Comment: Accepted for publication in A&

Stability of g-modes in rotating B-type stars

We have studied the stability of low degree $g$-modes in uniformly rotating B-type stars, taking into account the effects of the Coriolis force and the rotational deformation. From an analysis treating rotation frequency as a small parameter it is found that slow rotation tends to $destabilize$ high radial-order $retrograde$ $g$-modes, although the effect is very small or absent for relatively low order modes. Calculating eigenfrequencies at selected rotation rates, we find, on the other hand, that rapid rotation tends to $stabilize$ $retrograde$ $g$-modes. The stabilizing effect appears stronger for less massive B-type stars having low effective temperatures. If we change rotation rate continuously, the frequency of a $g$-mode belonging to ($l', m$) crosses frequencies of other $g$-modes belonging to ($l', m$). If the parity of the two encountering modes are the same, they interact each other and the stability (i.e., imaginary part of eigenfrequency) of each mode is modified. Using an asymptotic method we discuss the property of such mode crossings and couplings. For rapidly rotating stars mode couplings are important for the stability of low degree $g$-modes. In particular, we find that the stabilization of retrograde $g$-modes in rapidly rotating stars is due to many strong mode couplings, while %prograde sectoral% modes are exceptionally immune to the damping effects from the mode couplings.Comment: 13 pages, 15 figure

Electromagnetic transition form factors of negative parity nucleon resonances

We have calculated the transition form factors for the electromagnetic excitation of the negative parity resonances of the nucleon using different models previously proposed and we discuss their results and limits by comparison with experimental data.Comment: 13 pages, 6 figures, to be published on Journal of Physics