The Location of the Snow Line in Protostellar Disks

The snow line in a gas disk is defined as the distance from the star beyond which the water ice is stable against evaporation. Since oxygen is the most abundant element after hydrogen and helium, the presence of ice grains can have important consequences for disk evolution. However, determining the position of the snow line is not simple. I discuss some of the important processes that affect the position of the snow line.Comment: 8 pages, 5 figures. Invited talk at IAU Symposium 263 - Icy Bodies in the Solar System. Rio de Janeiro, Aug. 200

The evolution of the stellar populations in low surface brightness galaxies

We investigate the star formation history and chemical evolution of low surface brightness (LSB) disk galaxies by modelling their observed spectro-photometric and chemical properties using a galactic chemical and photometric evolution model incorporating a detailed metallicity depen dent set of stellar input data. For a large fraction of the LSB galaxies in our sample, observed properties are best explained by models incorporating an exponentially decreasing global star formation rate (SFR) ending at a present-day gas fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For some galaxies small amplitude star formation bursts are required to explain the contribution of the young (5-50 Myr old) stellar population to the galaxy integrated luminosity. This suggests that star formation has proceeded in a stochastic manner. The presence of an old stellar population in many late-type LSB galaxies suggests that LSB galaxies roughly follow the same evolutionary history as HSB galaxies, except at a much lower rate. In particular, our results imply that LSB galaxies do not form late, nor have a delayed onset of star formation, but simply evolve slowly.Comment: To be published in A&

Benchmarking Burgerzaken : een empirisch onderzoek naar de kostendoelmatigheid van burgerzaken

De noodzaak van productiviteitsgroei in de publieke sector is nu groter dan ooit. Aan deze noodzaak liggen twee ontwikkelingen ten grondslag. In de eerste plaats staan de financiën van de publieke sector onder druk als gevolg van bezuinigingen. In de tweede plaats worden er op de langere termijn knelpunten op de arbeidsmarkt verwacht als gevolg van vergrijzing en ontgroening van de bevolking. In de marksector dwingen concurrentieoverwegingen organisaties ertoe om voortdurend aandacht te hebben voor productiviteitsverbetering en deze waar mogelijk te realiseren. In de publieke sector ontbreken de prikkels van de markt en lijken productiviteitsverbeteringen moeizaam tot stand te komen

Many-body wave scattering by small bodies

Scattering problem by several bodies, small in comparison with the wavelength, is reduced to linear algebraic systems of equations, in contrast to the usual reduction to some integral equations

KK246, a dwarf galaxy with extended H I disk in the Local Void

We have found that KK 246, the only confirmed galaxy located within the nearby Tully Void, is a dwarf galaxy with an extremely extended H I disk and signs of an H I cloud with anomalous velocity. It also exhibits clear misalignment between the kinematical major and minor axes, indicative of an oval distortion, and a general misalignment between the H I and optical major axes. We measure a H I mass of 1.05 +- 0.08 x 10^8 M_sun, and a H I extent 5 times that of the stellar disk, one of the most extended H I disks known. We estimate a dynamical mass of 4.1 x 10^9 M_sun, making this also one of the darkest galaxies known, with a mass-to-light ratio of 89. The relative isolation and extreme underdense environment make this an interesting case for examining the role of gas accretion in galaxy evolution.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in A

Star formation and the interstellar medium in low surface brightness galaxies. II. Deep CO observations of low surface brightness disk galaxies

We present deep, pointed $^{12}$CO($J=2-1$) observations of three late-type LSB galaxies. The beam-size was small enough that we could probe different environments (\HI maximum, \HI mininum, star forming region) in these galaxies. No CO was found at any of the positions observed. We argue that the implied lack of molecular gas is real and not caused by conversion factor effects. The virtual absence of a molecular phase may explain the very low star formation rates in these galaxies.Comment: 9 pages, 3 figure, uses aa.cls. Typos in Tables and text correcte

Optical Albedo Theory of Strongly-Irradiated Giant Planets: The Case of HD 209458b

We calculate a new suite of albedo models for close-in extrasolar giant planets and compare with the recent stringent upper limit for HD 209458b of Rowe et al. using MOST. We find that all models without scattering clouds are consistent with this optical limit. We explore the dependence on wavelength and waveband, metallicity, the degree of heat redistribution, and the possible presence of thermal inversions and find a rich diversity of behaviors. Measurements of transiting extrasolar giant planets (EGPs) at short wavelengths by MOST, Kepler, and CoRoT, as well as by proposed dedicated multi-band missions, can complement measurements in the near- and mid-IR using {\it Spitzer} and JWST. Collectively, such measurements can help determine metallicity, compositions, atmospheric temperatures, and the cause of thermal inversions (when they arise) for EGPs with a broad range of radii, masses, degrees of stellar insolation, and ages. With this paper, we reappraise and highlight the diagnostic potential of albedo measurements of hot EGPs shortward of $\sim$1.3 $\mu$m.Comment: 6 pages, 1 table, 1 color figure; accepted to the Astrophysical Journa

Radiative Heat Transfer between Neighboring Particles

The near-field interaction between two neighboring particles is known to produce enhanced radiative heat transfer. We advance in the understanding of this phenomenon by including the full electromagnetic particle response, heat exchange with the environment, and important radiative corrections both in the distance dependence of the fields and in the particle absorption coefficients. We find that crossed terms of electric and magnetic interactions dominate the transfer rate between gold and SiC particles, whereas radiative corrections reduce it by several orders of magnitude even at small separations. Radiation away from the dimer can be strongly suppressed or enhanced at low and high temperatures, respectively. These effects must be taken into account for an accurate description of radiative heat transfer in nanostructured environments.Comment: 22 pages, 9 figures, fully self-contained derivation

Identification of Super- and Sub-critical Regions in Shocks driven by Coronal Mass Ejections

In this work, we focus on the analysis of a CME-driven shock observed by SOHO/LASCO. We show that white-light coronagraphic images can be employed to estimate the compression ratio X = rho_d / rho_u all along the front of CME-driven shocks. X increases from the shock flanks (where X ~ 1.2) to the shock center (where X ~ 3.0 is maximum). From the estimated X values, we infer the Alfv\'en Mach number for the general case of an oblique shock. It turns out that only a small region around the shock center is supercritical at earlier times, while higher up in the corona the whole shock becomes subcritical. This suggests that CME-driven shocks could be efficient particle accelerators at the initiation phases of the event, while at later times they progressively loose energy, also losing their capability to accelerate high energy particles. This result has important implications on the localization of particle acceleration sites and in the context of predictive space weather studies