Discovery of Blue Luminescence in the Red Rectangle: Possible Fluorescence from Neutral Polycyclic Aromatic Hydrocarbon Molecules?

Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infra-red (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our galaxy as well as in other galaxies, and are considered likely carriers of the UIR-band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system, as the set of mid-infra-red emission features attributed to these molecules between the wavelengths of 3.3 micron and 16.4 micron is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene (C14H10) and pyrene (C16H10). These small PAHs would then be the largest molecules specifically identified in the interstellar medium.Comment: 4 pages, 4 figures, Accepted for publication in ApJL (LaTeX, uses emulateapj.sty

The Flux Ratio Method for Determining the Dust Attenuation of Starburst Galaxies

The presence of dust in starburst galaxies complicates the study of their stellar populations as the dust's effects are similar to those associated with changes in the galaxies' stellar age and metallicity. This degeneracy can be overcome for starburst galaxies if UV/optical/near-infrared observations are combined with far-infrared observations. We present the calibration of the flux ratio method for calculating the dust attenuation at a particular wavelength, Att(\lambda), based on the measurement of F(IR)/F(\lambda) flux ratio. Our calibration is based on spectral energy distributions (SEDs) from the PEGASE stellar evolutionary synthesis model and the effects of dust (absorption and scattering) as calculated from our Monte Carlo radiative transfer model. We tested the attenuations predicted from this method for the Balmer emission lines of a sample starburst galaxies against those calculated using radio observations and found good agreement. The UV attenuation curves for a handful of starburst galaxies were calculated using the flux ratio method, and they compare favorably with past work. The relationship between Att(\lambda) and F(IR)/F(\lambda) is almost completely independent of the assumed dust properties (grain type, distribution, and clumpiness). For the UV, the relationship is also independent of the assumed stellar properties (age, metallicity, etc) accept for the case of very old burst populations. However at longer wavelengths, the relationship is dependent on the assumed stellar properties.Comment: accepted by the ApJ, 18 pages, color figures, b/w version at http://mips.as.arizona.edu/~kgordon/papers/fr_method.htm

Interakcija H2 (D2) i kalija sa Ag(111) i Ag(110)

Adsorption and coadsorption of H2 (D2) and potassium on the single-crystal silver surfaces (111) and (110) have been studied using thermal desorption spectroscopy, work function measurements and LEED. Atomic hydrogen was used to cover the silver surfaces, since a very high activation barrier makes molecular adsorption impossible. From the Ag(111) surface hydrogen desorbs in a single peak with a close to second order reaction and a desorption energy of 29.4 kJ/mol. Hydrogen desorption from Ag(110) is characterized by two overlapping desorption peaks which are very sensitive to small amounts of water coadsorption. Upon hydrogen saturation (0.65 ML) on Ag(111), the work function increases by 240 meV. Coadsorbed potassium shifts the desorption temperature for H2 dramatically by 250 K to higher temperature on both surfaces. Simultaneous desorption of hydrogen and potassium on both silver surfaces with a ratio of 1 K-atom to 4H2 molecules indicates the formation of a potassium-hydrogen complex in the coadsorbate.Adsorpcija i koadsorpcija H2(D2) i kalija na monokristalnim (111) i (110) površinama srebra proučavane su termalnom desorpcijskom spektroskopijom, mjerenjima izlaznog rada i LEED. Sa Ag(111) površine vodik desorbira s jednim vrhom s desorpcijskom energijom 29.4 kJ/mol. Desorpcija vodika sa Ag(110) ima dva preklopljena desorpcijska vrha koji su vrlo osjetljivi na male količine adsorbirane vode. Koadsorbiran kalij pomiče desorpcijsku temperaturu naviše za 250 K na obje površine. Istovremena desorpcija vodika i kalija s obje površine srebra ukazuje na stvaranje kompleksa kalij–vodik u koadsorbatu

Measurement techniques for evaluating encapsulant thermophysical properties during cure

Sandia now has the capability to evaluate stresses during cure of epoxies with finite element codes. Numerous material parameters are needed as input to these codes. I present a relatively quick set of tests which enable evaluation of the required thermophysical properties. Ease and accuracy of the tests improve as the reaction rate of the thermoset slows. Material parameters for common encapsulants at Sandia are presented in tables

A model for gelation with explicit solvent effects: Structure and dynamics

We study a two-component model for gelation consisting of $f$-functional monomers (the gel) and inert particles (the solvent). After equilibration as a simple liquid, the gel particles are gradually crosslinked to each other until the desired number of crosslinks has been attained. At a critical crosslink density the largest gel cluster percolates and an amorphous solid forms. This percolation process is different from ordinary lattice or continuum percolation of a single species in the sense that the critical exponents are new. As the crosslink density $p$ approaches its critical value $p_c$, the shear viscosity diverges: $\eta(p)\sim (p_c-p)^{-s}$ with $s$ a nonuniversal concentration-dependent exponent.Comment: 6 pages, 9 figure

A Modelling Study of Developmental Stage and Environmental Variability Effects on Copepod Foraging

We used a stochastic Lagrangian model to study how behaviour contributes to copepod grazing success. The model simulates distinct foraging behaviours of Clausocalanus furcatus, Paracalanus aculeatus, and Oithona plumifera. Three sets of simulations were performed to investigate the effects of (a) prey-size preference; (b) variation in prey-size spectra; and (c) turbulence intensity on these species’ grazing rates. The size preference simulations demonstrate that, compared with copepodites, mature females have cell ingestion rates that are an order of magnitude lower, while carbon uptake is reduced by 35%. A prey spectrum that is skewed towards cells ,\u3c6 μm promotes copepodite success because the basal metabolic needs of the adult females require a prey concentration of 850–1000 cells ml-1. Variations in turbulence intensity reveal distinct ecological niches, with stronger mixing favouring O. plumifera and stable conditions favouring C. furcatus. Differences in theoretically derived and simulated prey-encounter rates demonstrate that the hopping behaviour of O. plumifera provides an order of magnitude increase in prey encounter, whereas the feeding behaviour of C. furcatus can result in localized depletion of prey. These simulations highlight the importance of species-specific feeding behaviour in defining oceanic copepod distributions