Studying the small scale ISM structure with supernovae

AIMS. In this work we explore the possibility of using the fast expansion of a Type Ia supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months. METHODS. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of Na I D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations. RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na I column densities exceeding a few 10^11 cm^-2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on a scale of 1000 AU. CONCLUSIONS. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.Comment: 6 pages, 3 figures. Accepted for publication in Astronomy & Astrophysic

Estimating the Effect of Leaders on Public Sector Productivity: The Case of School Principals

Although much has been written about the importance of leadership in the determination of organizational success, there is little quantitative evidence due to the difficulty of separating the impact of leaders from other organizational components – particularly in the public sector. Schools provide an especially rich environment for studying the impact of public sector management, not only because of the hypothesized importance of leadership but also because of the plentiful achievement data that provide information on institutional outcomes. Outcome-based estimates of principal value-added to student achievement reveal significant variation in principal quality that appears to be larger for high-poverty schools. Alternate lower-bound estimates based on direct estimation of the variance yield smaller estimates of the variation in principal productivity but ones that are still important, particularly for high poverty schools. Patterns of teacher exits by principal quality validate the notion that a primary channel for principal influence is the management of the teacher force. Finally, looking at principal transitions by quality reveals little systematic evidence that more effective leaders have a higher probability of exiting high poverty schools.

Charter School Quality and Parental Decision Making With School Choice

Charter schools have become a very popular instrument for reforming public schools, because they expand choices, facilitate local innovation, and provide incentives for the regular public schools while remaining under public control. Despite their conceptual appeal, evaluating their performance has been hindered by the selective nature of their student populations. This paper investigates the quality of charter schools in Texas in terms of mathematics and reading achievement and finds that, after an initial start-up period, average school quality in the charter sector is not significantly different from that in regular public schools. Perhaps most important, the parental decision to exit a charter school is much more sensitive to education quality than the decision to exit a regular public school, consistent with the notion that the introduction of charter schools substantially reduces the transactions costs of switching schools. Low income charter school families are, however, less sensitive to school quality than higher income families.

Spectroscopically Peculiar Type Ia Supernovae and Implications for Progenitors

In a recent paper Li et al. (2000) reported that 36 percent of 45 Type Ia supernovae (SNe Ia) discovered since 1997 in two volume-limited supernova searches were spectroscopically peculiar, and they suggested that because this peculiarity rate is higher than that reported for an earlier observational sample by Branch et al. (1993), it is now more likely that SNe Ia are produced by more than one kind of progenitor. In this paper I discuss and clarify the differences between the results of Li et al. and Branch et al. and I suggest that multiple progenitor systems are now less likely than they were before.Comment: 11 pages; accepted by PASP; several minor changes, 2 references added, main conclusions unchange

Spectral Consequences of Deviation from Spherical Composition Symmetry in Type Ia Supernovae

We investigate the prospects for constraining the maximum scale of clumping in composition that is consistent with observed Type Ia supernova flux spectra. Synthetic spectra generated without purely spherical composition symmetry indicate that gross asymmetries make prominent changes to absorption features. Motivated by this, we consider the case of a single unblended line forming in an atmosphere with perturbations of different scales and spatial distributions. Perturbations of about 1% of the area of the photodisk simply weaken the absorption feature by the same amount independent of the line of sight. Conversely, perturbations of about 10% of the area of the photodisk introduce variation in the absorption depth which does depend on the line of sight. Thus, 1% photodisk area perturbations may be consistent with observed profile homogeneity but 10% photodisk area perturbations can not. Based on this, we suggest that the absence of significant variation in the depths of Si II 6355 absorption features in normal Type Ia spectra near maximum light indicates that any composition perturbations in these events are quite small. This also constrains future three-dimensional explosion models to produce ejecta profiles with only small scale inhomogeneities.Comment: 11 pages, 6 figure

A Program to Help Prepare pre-Schoolers for Reading

Parents of young children are very often concerned with their child\u27s pre-reading and beginning reading development. They want to know what they can do to help their child have a good start in this critically important skill. All too frequently parents do not have the information available to them to help their children. this article describes a local project designed to provide parents with ideas and information that should contribute to their children\u27s maximum opportunity for success when formal school instruction begins. Although each parent-child relationship should be treated individually, there are some pre-reading experiences whicch are applicable to most family situations

The Probable Detection of SN 1923A: The Oldest Radio Supernova?

Based upon the results of VLA observations, we report the detection of two unresolved radio sources that are coincident with the reported optical position of SN 1923A in M83. For the source closest to the SN position, the flux density was determined to be 0.30 +/- 0.05 mJy at 20 cm and 0.093 +/- 0.028 mJy at 6 cm. The flux density of the second nearby source was determined to be 0.29 +/- 0.05 at 20 cm and 0.13 +/- 0.028 at 6 cm. Both sources are non-thermal with spectral indices of alpha = -1.0 +/- 0.30 and -0.69 +/- 0.24, respectively. SN 1923A has been designated as a Type II-P. No Type II-P (other than SN 1987A) has been detected previously in the radio. The radio emission from both sources appears to be fading with time. At an age of approximately 68 years when we observed it, this would be the oldest radio supernova (of known age) yet detected

Predation on the angulate tortoise Chersina angulata by the kelp gull Larus dominicanus on Dassen Island, Western Cape

The kelp gull (Larus dominicanus) is an important predator on the angulate tortoise (Chersina angulata) on Dassen Island, Western Cape, South Africa. Tortoise shell fragments (n= 142) were collected and measured. Using regression curves based on 184 live tortoises measured on the island, the size class of tortoises eaten by kelp gulls was determined to include tortoises from 38-124 mm (mean 84 mm) total length (mass 25-350 g; mean 130 g). It is probable that very small tortoises were also eaten, but totally consumed. It is suggested that predation on tortoises occurs during the autumn-winter by the small population of resident gulls (approximately 50 individuals)