22,842 research outputs found
Photodissociative Regulation of Star Formation in Metal-Free Pregalactic Clouds
We study the H2 photodissociation regions around OB stars in primordial gas
clouds whose virial temperatures are between a few hundred and a few thousand
Kelvin. In such small objects, a single O star can photodissociate a mass equal
to that of the cloud itself. As a result, the clouds deplete their molecular
coolant and cannot cool in a free-fall time, and subsequent star formation is
totally quenched. This indicates that stars do not form efficiently in small
objects and that these objects contribute little to the reionization of the
universe.Comment: 9 pages. ApJ, 518, in pres
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Destruction of Molecular Hydrogen During Cosmological Reionization
We investigate the ability of primordial gas clouds to retain molecular
hydrogen (H_2) during the initial phase of the reionization epoch. We find that
before the Stromgren spheres of the individual ionizing sources overlap, the UV
background below the ionization threshold is able to penetrate large clouds and
suppress their H_2 abundance. The consequent lack of H_2 cooling could prevent
the collapse and fragmentation of clouds with virial temperatures T_vir < 10^4
K (or masses 10^8 Msun [(1+z_vir)/10]^{-3/2}). This negative feedback on
structure-formation arises from the very first ionizing sources, and precedes
the feedback due to the photoionization heating.Comment: 14 pages, uuencoded compressed Postscript, 4 figures included. To
appear in Ap
Models of Disk Evolution: Confrontation with Observations
We present simple models for disk evolution based on two different
approaches: a forward approach based on predictions generic to hierarchical
models for structure formation (e.g., Mo, Mao, & White 1998) and a backwards
approach based on detailed modeling of the Milky Way galaxy (e.g., Bouwens,
Cayon, & Silk 1997). We normalize these models to local observations and
predict high-redshift luminosities, sizes, circular velocities, and surface
brightnesses. Both approaches yield somewhat similar predictions for size,
surface brightness, and luminosity evolution though they clearly differ in the
amount of number evolution. These predictions seem to be broadly consistent
with the high-redshift observations of Simard et al. (1999), suggesting that
the B-band surface brightness of disks has indeed evolved by ~1.5 mag from z~0
to z~1 similar to the models and is not an artifact of selection effects as
previously claimed. We also find a lack of low surface brightness galaxies in
several high redshift samples relative to model predictions based on local
samples (de Jong & van der Kruit 1994; Mathewson, Ford, & Buchhorn 1992).Comment: 34 pages, 9 figures, accepted to Ap
Split-gate quantum point contacts with tunable channel length
We report on developing split-gate quantum point contacts (QPCs) that have a
tunable length for the transport channel. The QPCs were realized in a
GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below
its surface. The conventional design uses 2 gate fingers on the wafer surface
which deplete the 2DEG underneath when a negative gate voltage is applied, and
this allows for tuning the width of the QPC channel. Our design has 6 gate
fingers and this provides additional control over the form of the electrostatic
potential that defines the channel. Our study is based on electrostatic
simulations and experiments and the results show that we developed QPCs where
the effective channel length can be tuned from about 200 nm to 600 nm.
Length-tunable QPCs are important for studies of electron many-body effects
because these phenomena show a nanoscale dependence on the dimensions of the
QPC channel
Preliminary monetary values for the reliability of travel times in freight transport
www.ejtir.tbm.tudelft.n
Table of Contents
Table of contents for Volume 10, Issue 3 of the Linfield Magazin
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
PRODUS 1 d: Rendement MZI zaad op percelen: effect van wegvissen van krabben - perceelproef 2009
Verbeteren van het rendement van mosselzaad op percelen is een belangrijke factor die de duurzaamheid van de mosselkweek kan vergroten. Om meer inzicht te verkrijgen in het effect van behandelingen van kwekers op het rendement van MZI zaad zijn in 2009 meting en gedaan op percelen van verschillende ondernemers in de Waddenzee, waarbij de aandacht met name was gericht op rendement van MZI zaad in relatie tot de dichtheid van de krabben en het wegvangen ervan of de grootte van de MZI mosselen in die periode. De gepresenteerde resultaten blijken niet ondersteunend voor de hypothese dat het wegvissen van krabben het rendement van MZI zaad vergroot
- …