Fluctuations and stability in front propagation

Propagating fronts arising from bistable reaction-diffusion equations are a purely deterministic effect. Stochastic reaction-diffusion processes also show front propagation which coincides with the deterministic effect in the limit of small fluctuations (usually, large populations). However, for larger fluctuations propagation can be affected. We give an example, based on the classic spruce-budworm model, where the direction of wave propagation, i.e., the relative stability of two phases, can be reversed by fluctuations.Comment: 5 pages, 5 figure

Measurement of the electron electric dipole moment using YbF molecules

The most sensitive measurements of the electron electric dipole moment d_e have previously been made using heavy atoms. Heavy polar molecules offer a greater sensitivity to d_e because the interaction energy to be measured is typically 10^3 times larger than in a heavy atom. We report the first measurement of this kind, for which we have used the molecule YbF. Together, the large interaction energy and the strong tensor polarizability of the molecule make our experiment essentially free of the systematic errors that currently limit d_e measurements in atoms. Our first result d_e = (- 0.2 \pm 3.2) x 10^-26 e.cm is less sensitive than the best atom measurement, but is limited only by counting statistics and demonstrates the power of the method.Comment: 4 pages, 4 figures. v2. Minor corrections and clarifications made in response to referee comment

The ideal gas as an urn model: derivation of the entropy formula

The approach of an ideal gas to equilibrium is simulated through a generalization of the Ehrenfest ball-and-box model. In the present model, the interior of each box is discretized, {\it i.e.}, balls/particles live in cells whose occupation can be either multiple or single. Moreover, particles occasionally undergo random, but elastic, collisions between each other and against the container walls. I show, both analitically and numerically, that the number and energy of particles in a given box eventually evolve to an equilibrium distribution $W$ which, depending on cell occupations, is binomial or hypergeometric in the particle number and beta-like in the energy. Furthermore, the long-run probability density of particle velocities is Maxwellian, whereas the Boltzmann entropy $\ln W$ exactly reproduces the ideal-gas entropy. Besides its own interest, this exercise is also relevant for pedagogical purposes since it provides, although in a simple case, an explicit probabilistic foundation for the ergodic hypothesis and for the maximum-entropy principle of thermodynamics. For this reason, its discussion can profitably be included in a graduate course on statistical mechanics.Comment: 17 pages, 3 figure

Online occupational education in community colleges: Prevalence and contextual factors

This study examined the current state of online occupational programs in community colleges and explored issues related to institutional, economic, and social indicators that influence (a) the offering of online programs and (b) the programmatic connection to workforce development needs. This project is the first national study that categorizes and inventories specific types of online occupational programs in community colleges. The study included a national random sample of 321 institutions in the United States. Data were collected through institutional websites, statewide websites, follow-up emails, and phone inquiries to institutions. The following sections summarize key findings

Temporal and Spatial Aspects of Gas Release During the 2010 Apparition of Comet 103P/Hartley-2

We report measurements of eight primary volatiles (H2O, HCN, CH4, C2H6, CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) in comet 103P/Hartley-2 using high dispersion infrared spectroscopy. We quantified the long- and short-term behavior of volatile release over a three-month interval that encompassed the comet's close approach to Earth, its perihelion passage, and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission. We present production rates for individual species, their mixing ratios relative to water, and their spatial distributions in the coma on multiple dates. The production rates for water, ethane, HCN, and methanol vary in a manner consistent with independent measures of nucleus rotation, but mixing ratios for HCN, C2H6, & CH3OH are independent of rotational phase. Our results demonstrate that the ensemble average composition of gas released from the nucleus is well defined, and relatively constant over the three-month interval (September 18 through December 17). If individual vents vary in composition, enough diverse vents must be active simultaneously to approximate (in sum) the bulk composition of the nucleus. The released primary volatiles exhibit diverse spatial properties which favor the presence of separate polar and apolar ice phases in the nucleus, establish dust and gas release from icy clumps (and also, directly from the nucleus), and provide insights into the driver for the cyanogen (CN) polar jet. The spatial distributions of C2H6 & HCN along the near-polar jet (UT 19.5 October) and nearly orthogonal to it (UT 22.5 October) are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water was 2.85 \pm 0.20; the lower bound (2.65) defines Tspin > 32 K. These values are consistent with results returned from ISO in 1997.Comment: 18 pages, 3 figures, to be published in: Astrophysical Journal Letter

Slowing heavy, ground-state molecules using an alternating gradient decelerator

Cold supersonic beams of molecules can be slowed down using a switched sequence of electrostatic field gradients. The energy to be removed is proportional to the mass of the molecules. Here we report deceleration of YbF, which is 7 times heavier than any molecule previously decelerated. We use an alternating gradient structure to decelerate and focus the molecules in their ground state. We show that the decelerator exhibits the axial and transverse stability required to bring these molecules to rest. Our work significantly extends the range of molecules amenable to this powerful method of cooling and trapping.Comment: 4 pages, 5 figure