45,213 research outputs found
Wolf-Rayet Stars in Starburst Galaxies
Wolf-Rayet stars have been detected in a large number of galaxies
experiencing intense bursts of star formation. All stars initially more massive
than a certain, metallicity-dependent, value are believed to experience the
Wolf-Rayet phase at the end of their evolution, just before collapsing in
supernova explosion. The detection of Wolf-Rayet stars puts therefore important
constraints on the evolutionary status of starbursts, the properties of their
Initial Mass Functions and their star formation regime. In this contribution we
review the properties of galaxies hosting Wolf-Rayet stars, with special
emphasis on the factors that determine their presence and evolution, as well as
their impact on the surrounding medium.Comment: Contribution to the Proceedings of the JENAM 99 conference "The
interplay between massive stars and the ISM", held in Toulouse in September
7-11, 1999. 10 pages, 5 figures. Requires elsart.cls latex macr
Information transmission around block trades on the Spanish stock market
Current fmancial research is placing increasing attention on the effects of large transactions, or Block Trades (BT), on the fmancial markets. In order to analyze whether BT transmit information, we assume that information can be better reflected by changes in asset true value, proxied by the midpoint of bid-ask best quotes, instead of
transactions prices or returns. Moreover, following market microstructure literature, we also look at changes in relative spread and in their adverse selection component. The Madrid Stock Exchange offers us a particularly appropriate testing ground for examining these issues, since this topic has not been facilitated as in other markets
till 1998. We analyze 195 BT, classified according with trading volume, the side of the market initiating the BT (buyer,
seller or indeterminate initiated), its type (inside the spread, sweeping or not classified) and if they change or not
the asset true value. The main result of the paper is that it seems that there is BT information transmission when we look at adverse selection spread component in the different subsample classification, but there is no significant permanent effect in
returns. We also observe changes in liquidity around BTs but the effect is related with temporary spread component
q-deformations of two-dimensional Yang-Mills theory: Classification, categorification and refinement
We characterise the quantum group gauge symmetries underlying q-deformations
of two-dimensional Yang-Mills theory by studying their relationships with the
matrix models that appear in Chern-Simons theory and six-dimensional N=2 gauge
theories, together with their refinements and supersymmetric extensions. We
develop uniqueness results for quantum deformations and refinements of gauge
theories in two dimensions, and describe several potential analytic and
geometric realisations of them. We reconstruct standard q-deformed Yang-Mills
amplitudes via gluing rules in the representation category of the quantum group
associated to the gauge group, whose numerical invariants are the usual
characters in the Grothendieck group of the category. We apply this formalism
to compute refinements of q-deformed amplitudes in terms of generalised
characters, and relate them to refined Chern-Simons matrix models and
generalized unitary matrix integrals in the quantum beta-ensemble which compute
refined topological string amplitudes. We also describe applications of our
results to gauge theories in five and seven dimensions, and to the dual
superconformal field theories in four dimensions which descend from the N=(2,0)
six-dimensional superconformal theory.Comment: 71 pages; v2: references added; final version to be published in
Nuclear Physics
Matrix models and stochastic growth in Donaldson-Thomas theory
We show that the partition functions which enumerate Donaldson-Thomas
invariants of local toric Calabi-Yau threefolds without compact divisors can be
expressed in terms of specializations of the Schur measure. We also discuss the
relevance of the Hall-Littlewood and Jack measures in the context of BPS state
counting and study the partition functions at arbitrary points of the Kaehler
moduli space. This rewriting in terms of symmetric functions leads to a unitary
one-matrix model representation for Donaldson-Thomas theory. We describe
explicitly how this result is related to the unitary matrix model description
of Chern-Simons gauge theory. This representation is used to show that the
generating functions for Donaldson-Thomas invariants are related to
tau-functions of the integrable Toda and Toeplitz lattice hierarchies. The
matrix model also leads to an interpretation of Donaldson-Thomas theory in
terms of non-intersecting paths in the lock-step model of vicious walkers. We
further show that these generating functions can be interpreted as
normalization constants of a corner growth/last-passage stochastic model.Comment: 31 pages; v2: comments and references added; v3: presentation
improved, comments added; final version to appear in Journal of Mathematical
Physic
The bubble wall velocity in the minimal supersymmetric light stop scenario
We build on existing calculations of the wall velocity of the expanding
bubbles of the broken symmetry phase in a first-order electroweak phase
transition within the light stop scenario (LSS) of the MSSM. We carry out the
analysis using the 2-loop thermal potential for values of the Higgs mass
consistent with present experimental bounds. Our approach relies on describing
the interaction between the bubble and the hot plasma by a single friction
parameter, which we fix by matching to an existing 1-loop computation and
extrapolate it to our regime of interest. For a sufficiently strong phase
transition (in which washout of the newly-created baryon asymmetry is
prevented) we obtain values of the wall velocity, v_w~0.05, far below the speed
of sound in the medium, and not very much deviating from the previous 1-loop
calculation. We also find that the phase transition is about 10% stronger than
suggested by simply evaluating the thermal potential at the critical
temperature.Comment: 17pages, 3 figure
Some new results on an old controversy: is perturbation theory the correct asymptotic expansion in nonabelian models?
Several years ago it was found that perturbation theory for two-dimensional
O(N) models depends on boundary conditions even after the infinite volume limit
has been taken termwise, provided . There ensued a discussion whether the
boundary conditions introduced to show this phenomenon were somehow anomalous
and there was a class of `reasonable' boundary conditions not suffering from
this ambiguity. Here we present the results of some computations that may be
interpreted as giving some support for the correctness of perturbation theory
with conventional boundary conditions; however the fundamental underlying
question of the correctness of perturbation theory in these models and in
particular the perturbative function remain challenging problems of
mathematical physics.Comment: 4 pages, 3 figure
- …