The Production Rate and Employment of Ph.D. Astronomers

In an effort to encourage self-regulation of the astronomy job market, I examine the supply of, and demand for, astronomers over time. On the supply side, I document the production rate of Ph.D. astronomers from 1970 to 2006 using the UMI Dissertation Abstracts database, along with data from other independent sources. I compare the long-term trends in Ph.D. production with federal astronomy research funding over the same time period, and I demonstrate that additional funding is correlated with higher subsequent Ph.D. production. On the demand side, I monitor the changing patterns of employment using statistics about the number and types of jobs advertised in the AAS Job Register from 1984 to 2006. Finally, I assess the sustainability of the job market by normalizing this demand by the annual Ph.D. production. The most recent data suggest that there are now annual advertisements for about one postdoctoral job, half a faculty job, and half a research/support position for every new domestic Ph.D. recipient in astronomy and astrophysics. The average new astronomer might expect to hold up to 3 jobs before finding a steady position.Comment: 5 pages, 4 figures, 2 tables, PASP accepte

Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology

During helium burning in the core of a red giant, the relative rates of the 3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in the 3&alpha reaction at stellar energies due to the extrapolation from high-energy laboratory measurements is relatively small, but this is not the case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the analysis of asteroseismological data on pulsating white dwarf stars now make it possible to obtain precise measurements of the central ratio of carbon to oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O reaction rate at stellar energies. We assess the systematic uncertainties of this approach and quantify small shifts in the measured central oxygen abundance originating from the observations and from model settings that are kept fixed during the optimization. Using new calculations of white dwarf internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O reaction that is significantly higher than most published values. The accuracy of this method may improve as we modify some of the details of our description of white dwarf interiors that were not accessible through previous model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for publication in the Astrophysical Journa

White Dwarf Asteroseismology and the ^12C(alpha,gamma)^16O Rate

Due to a new global analysis method, it is now possible to measure the internal composition of pulsating white dwarf stars, even with relatively simple theoretical models. The precise internal mixture of carbon and oxygen is the largest single source of uncertainty in ages derived from white dwarf cosmochronometry, and contains information about the rate of the astrophysically important, but experimentally uncertain, ^12C(alpha,gamma)^16O nuclear reaction. Recent determinations of the internal composition and structure of two helium-atmosphere variable (DBV) white dwarf stars, GD 358 and CBS 114, initially led to conflicting implied rates for the ^12C(alpha,gamma)^16O reaction. If both stars were formed through single-star evolution, then the initial analyses of their pulsation frequencies must have differed in some systematic way. I present improved fits to the two sets of pulsation data, resolving the tension between the initial results and leading to a value for the ^12C(alpha,gamma)^16O reaction rate that is consistent with recent laboratory measurements.Comment: 4 pages, 2 figures, 1 table, uses emulateapj5.sty; Accepted for publication in ApJ Letter

The core/envelope symmetry in pulsating stars

We demonstrate that there is an inherent symmetry in the way high-overtone stellar pulsations sample the core and the envelope, which can potentially lead to an ambiguity in the asteroseismologically derived locations of internal structures. We provide an intuitive example of the source of this symmetry by analogy with a vibrating string. For the stellar case, we focus on the white dwarf stars, establishing the practical consequences for high-order white dwarf pulsations both analytically and numerically. In addition, we verify the effect empirically by cross-fitting two different structural models, and we discuss the consequences that this approximate symmetry may have for past and present asteroseismological fits of the pulsating DBV, GD 358. Finally, we show how the signatures of composition transition zones that are brought about by physically distinct processes may be used to help alleviate this potential ambiguity in our asteroseismological interpretation of the pulsation frequencies observed in white dwarf stars.Comment: Accepted for publication in MNRAS; 8 pages, 8 figure

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

Producing Birdsfoot Trefoil Seed

Supplies of birdsfoot trefoil seed haven\u27t met the demand in Iowa. Seed is scarce and relatively high priced. Seed production can be profitable for those willing to take the precautions necessary for good seed yields