2,328 research outputs found
Reduced classes and curve counting on surfaces II: calculations
We calculate the stable pair theory of a projective surface . For fixed
curve class the results are entirely topological, depending
on , , , , \emph{and}
invariants of the ring structure on such as the Pfaffian of
considered as an element of . Amongst other things, this
proves an extension of the G\"ottsche conjecture to non-ample linear systems.
We also give conditions under which this calculates the full 3-fold reduced
residue theory of . This is related to the reduced residue Gromov-Witten
theory of via the MNOP conjecture. When the surface has no holomorphic
2-forms this can be expressed as saying that certain Gromov-Witten invariants
of are topological.
Our method uses the results of \cite{KT1} to express the reduced virtual
cycle in terms of Euler classes of bundles over a natural smooth ambient space.Comment: 19 pages. Minor correction
Forecasting the money multiplier: implications for money stock control and economic activity
Forecasting ; Money supply
Accretion disk reversal and the spin-up/spin-down of accreting pulsars
We numerically investigate the hydrodynamics of accretion disk reversal and
relate our findings to the observed spin-rate changes in the accreting X-ray
pulsar GX~1+4. In this system, which accretes from a slow wind, the accretion
disk contains two dynamically distinct regions. In the inner part viscous
forces are dominant and disk evolution occurs on a viscous timescale. In the
outer part dynamical mixing of material with opposite angular momentum is more
important, and the externally imposed angular momentum reversal timescale
governs the flow. In this outer region the disk is split into concentric rings
of material with opposite senses of rotation that do not mix completely but
instead remain distinct, with a clear gap between them. We thus predict that
torque reversals resulting from accretion disk reversals will be accompanied by
minima in accretion luminosity.Comment: 13 pages, 7 figures, accepted for publication in Ap
Using New Submillimetre Surveys to Identify the Evolutionary Status of High-z Galaxies
This paper describes a key submillimetre survey which we are currently
conducting to address some of the outstanding questions in cosmology - how, at
what epoch and over what period of time did massive galaxies form at high
redshift? A summary of the technical feasibility of future submillimetre
observations with new ground-based, airborne and satellite telescopes is also
presented.Comment: 6 pages, 3 postscript figures, LaTex uses Kluwer book style file
crckapb10.sty, to appear in "Observational Cosmology with the New Radio
Surveys", 13-15 January 1997, Puerto de la Cruz, Tenerife, M.Bremer,
N.Jackson, I.Perez-Fournon (eds.), Kluwe
Verlinde formulae on complex surfaces: K-theoretic invariants
We conjecture a Verlinde type formula for the moduli space of Higgs sheaves
on a surface with a holomorphic 2-form. The conjecture specializes to a
Verlinde formula for the moduli space of sheaves. Our formula interpolates
between -theoretic Donaldson invariants studied by the first named author
and Nakajima-Yoshioka and -theoretic Vafa-Witten invariants introduced by
Thomas and also studied by the first and second named authors. We verify our
conjectures in many examples (e.g. on K3 surfaces).Comment: Published version. 37 page
Jet Acceleration by Tangled Magnetic Fields
We explore the possibility that extragalactic radio jets might be accelerated
by highly disorganized magnetic fields that are strong enough to dominate the
dynamics until the terminal Lorentz factor is reached. Following the
twin-exhaust model by Blandford & Rees (1974), the collimation under this
scenario is provided by the stratified thermal pressure from an external
medium. The acceleration efficiency then depends on the pressure gradient of
that medium. In order for this mechanism to work there must be continuous
tangling of the magnetic field, changing the magnetic equation of state away
from pure flux freezing (otherwise conversion of Poynting flux to kinetic
energy flux is suppressed). This is a complementary approach to models in which
the plasma is accelerated by large scale ordered fields. We include a simple
prescription for magnetic dissipation, which leads to tradeoffs among
conversion of magnetic energy into bulk kinetic energy, random particle energy,
and radiation. We present analytic dynamical solutions of such jets, assess the
effects of radiation drag, and comment on observational issues, such as the
predicted polarization and synchrotron brightness. Finally, we try to make the
connection to observed radio galaxies and gamma-ray bursts.Comment: 15 pages, 10 figures, accepted for publication in Ap
Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries
We present FUSE observations of the hot white dwarfs in the post-common
envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The
spectra show numerous photospheric absorption lines which trace the white dwarf
orbital motion. We report the detection of C III, O VI, P V, and Si IV in the
spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C
III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance
measurements support the possibility that white dwarfs in post-common envelope
binaries accrete material from the secondary star wind. The FUSE observations
of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE
spectra to measure the radial velocities traced by the white dwarf in the four
binaries, where the zero-point velocity were fixed using the ISM velocities in
the line of sight of the stellar systems. For BPM 6502 we determined a white
dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity
semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate
the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass
determination for the white dwarf, we infer a low secondary mass of M_RD =
0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity
semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE
J0720-317 cover approximately 30% of the binary period and combined with the
HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary
properties. FUSE observations of Feige 24 cover approximately 60% of the orbit
and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423)
data to update the binary properties.Comment: Accepted for publication in Ap
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