10,241 research outputs found
A bioenergetic model for zebrafish \u3ci\u3eDanio rerio\u3c/i\u3e (Hamilton)
A bioenergetics model was developed from observed consumption, respiration and growth rates for zebrafish Danio rerio across a range (18–32° C) of water temperatures, and evaluated with a 50 day laboratory trial at 28° C. No significant bias in variable estimates was found during the validation trial; namely, predicted zebrafish mass generally agreed with observed mass
A Minimalist Turbulent Boundary Layer Model
We introduce an elementary model of a turbulent boundary layer over a flat
surface, given as a vertical random distribution of spanwise Lamb-Oseen vortex
configurations placed over a non-slip boundary condition line. We are able to
reproduce several important features of realistic flows, such as the viscous
and logarithmic boundary sublayers, and the general behavior of the first
statistical moments (turbulent intensity, skewness and flatness) of the
streamwise velocity fluctuations. As an application, we advance some heuristic
considerations on the boundary layer underlying kinematics that could be
associated with the phenomenon of drag reduction by polymers, finding a
suggestive support from its experimental signatures.Comment: 5 pages, 10 figure
Higher-spin strings and W minimal models
We study the spectrum of physical states for higher-spin generalisations of
string theory, based on two-dimensional theories with local spin-2 and spin-
symmetries. We explore the relation of the resulting effective Virasoro string
theories to certain minimal models. In particular, we show how the
highest-weight states of the minimal models decompose into Virasoro
primaries.Comment: 13 pages, CTP TAMU-43/93, KUL-TF-93/9
Canonical BRST Quantisation of Worldsheet Gravities
We reformulate the BRST quantisation of chiral Virasoro and worldsheet
gravities. Our approach follows directly the classic BRST formulation of
Yang-Mills theory in employing a derivative gauge condition instead of the
conventional conformal gauge condition, supplemented by an introduction of
momenta in order to put the ghost action back into first-order form. The
consequence of these simple changes is a considerable simplification of the
BRST formulation, the evaluation of anomalies and the expression of Wess-Zumino
consistency conditions. In particular, the transformation rules of all fields
now constitute a canonical transformation generated by the BRST operator ,
and we obtain in this reformulation a new result that the anomaly in the BRST
Ward identity is obtained by application of the anomalous operator ,
calculated using operator products, to the gauge fermion.Comment: 27 pages. (Some factors of \hbar corrected
X-ray observations of highly obscured τ_(9.7 μm) > 1 sources: an efficient method for selecting Compton-thick AGN?
Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ_(9.7 μm) > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine)
in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si
absorption, with the most notable example being NGC1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey.
All these have been selected to have no PAH features (EW_(6.2 μm) 10^(42) erg s^(−1)) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L_(X)/L_(6 μm) luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN
Unifying W-Algebras
We show that quantum Casimir W-algebras truncate at degenerate values of the
central charge c to a smaller algebra if the rank is high enough: Choosing a
suitable parametrization of the central charge in terms of the rank of the
underlying simple Lie algebra, the field content does not change with the rank
of the Casimir algebra any more. This leads to identifications between the
Casimir algebras themselves but also gives rise to new, `unifying' W-algebras.
For example, the kth unitary minimal model of WA_n has a unifying W-algebra of
type W(2,3,...,k^2 + 3 k + 1). These unifying W-algebras are non-freely
generated on the quantum level and belong to a recently discovered class of
W-algebras with infinitely, non-freely generated classical counterparts. Some
of the identifications are indicated by level-rank-duality leading to a coset
realization of these unifying W-algebras. Other unifying W-algebras are new,
including e.g. algebras of type WD_{-n}. We point out that all unifying quantum
W-algebras are finitely, but non-freely generated.Comment: 13 pages (plain TeX); BONN-TH-94-01, DFTT-15/9
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