The Strauss conjecture on asymptotically flat space-times

By assuming a certain localized energy estimate, we prove the existence portion of the Strauss conjecture on asymptotically flat manifolds, possibly exterior to a compact domain, when the spatial dimension is 3 or 4. In particular, this result applies to the 3 and 4-dimensional Schwarzschild and Kerr (with small angular momentum) black hole backgrounds, long range asymptotically Euclidean spaces, and small time-dependent asymptotically flat perturbations of Minkowski space-time. We also permit lower order perturbations of the wave operator. The key estimates are a class of weighted Strichartz estimates, which are used near infinity where the metrics can be viewed as small perturbations of the Minkowski metric, and the assumed localized energy estimate, which is used in the remaining compact set.Comment: Final version, to appear in SIAM Journal on Mathematical Analysis. 17 page

A web-based resource for radiation safety courses

CQU runs regular courses for intending Radiation Safety Officers from a variety of industry sectors. Participants have varying levels of prior knowledge to bring to the intensive three day program. To cater for those students whose prior knowledge of radiation physics is weak the print-based media for the course have been converted into a CD-ROM incorporating animations, worked examples, progress checks and self-assessment items. This allows the students who need extra study time on the basic physics concepts to cover as much as possible of the material before commencement of the course. The first course sessions can then concentrate on students’ individual problems and the required program content. This approach also allows the student with strong prior learning to submit an assessment test and be awarded a partial course exemption. Although the material selection is specifically for Radiation Safety training the CD-ROM has also been used in bridging courses, as reinforcement for first year students, and within our Engineering Technology Instrumentation program. The number of students who have used the CD-ROM is not statistically significant but student feedback on the material is very positive

Testing White Dwarf Crystallization Theory with Asteroseismology of the Massive Pulsating DA Star BPM 37093

It was predicted more than 40 years ago that the cores of the coolest white dwarf stars should eventually crystallize. This effect is one of the largest sources of uncertainty in white dwarf cooling models, which are now routinely used to estimate the ages of stellar populations in both the Galactic disk and the halo. We are attempting to minimize this source of uncertainty by calibrating the models, using observations of pulsating white dwarfs. In a typical mass white dwarf model, crystallization does not begin until the surface temperature reaches 6000-8000 K. In more massive white dwarf models the effect begins at higher surface temperatures, where pulsations are observed in the ZZ Ceti (DAV) stars. We use the observed pulsation periods of BPM 37093, the most massive DAV white dwarf presently known, to probe the interior and determine the size of the crystallized core empirically. Our initial exploration of the models strongly suggests the presence of a solid core containing about 90% of the stellar mass, which is consistent with our theoretical expectations.Comment: minor changes for length, accepted for ApJ Letter

A maximum-likelihood method for improving faint source flux and color estimates

Flux estimates for faint sources or transients are systematically biased high because there are far more truly faint sources than bright. Corrections which account for this effect are presented as a function of signal-to-noise ratio and the (true) slope of the faint-source number-flux relation. The corrections depend on the source being originally identified in the image in which it is being photometered. If a source has been identified in other data, the corrections are different; a prescription for calculating the corrections is presented. Implications of these corrections for analyses of surveys are discussed; the most important is that sources identified at signal-to-noise ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS