989 research outputs found
Cocycle twisting of E(n)-module algebras and applications to the Brauer group
We classify the orbits of coquasi-triangular structures for the Hopf algebra
E(n) under the action of lazy cocycles and the Hopf automorphism group. This is
applied to detect subgroups of the Brauer group of E(n) that are
isomorphic. For a triangular structure on E(n) we prove that the subgroup
of arising from is isomorphic to a direct
product of , the Brauer-Wall group of the ground field , and
, the group of symmetric matrices under addition. For a
general quasi-triangular structure on E(n) we construct a split short
exact sequence having as a middle term and as a left term a
central extension of the group of symmetric matrices of order (
depending on ). We finally describe how the image of the Hopf automorphism
group inside acts on .Comment: Accidentally an old version of the paper was posted. Main corrections
are in Section 2 and in Section 4.
Feedback-limited Accretion: Luminous Signatures from Growing Planets
Planets form in discs of gas and dust around stars, and keep growing by
accretion of disc material while available. Massive planets clear a gap in that
protoplanetary disc, but still accrete through spiral wakes. On its way to the
planet, the gas will settle on a \emph{circumplanetary} disc around the planet
and slowly accrete on to it. The energy of the accreted gas will be released,
heating the planet surroundings in a feedback process. For high enough
accretion rates the planet should be detectable at infrared wavelengths. We aim
to find whether detectable planet luminosities, , can occur when considering that the planet luminosity is
coupled to the accretion, and also to study which other effects has the
feedback on the dynamics of the circumplanetary and the gap regions. We model a
planet with mass ratio , orbiting at 10 AU from a solar mass star,
using a modified version of the 2D code FARGO-AD, which includes a prescription
for the accretion and feedback luminosity of the planet. We find that the
planetary feedback is able to partially deplete the circumplanetary disc, and
to reduce the accretion rate onto the planet. However, detectable luminosities
of are still produced. The
feedback also contributes to partially refilling the gap, to heat up the
coorbital region, and to perturb the orbital velocity of the gas.Comment: Submitted to MNRA
Galactic Centre stellar winds and Sgr A* accretion
(ABRIDGED) We present in detail our new 3D numerical models for the accretion
of stellar winds on to Sgr A*. In our most sophisticated models, we put stars
on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account
for radiative cooling. We first model only one phase `fast' stellar winds (1000
km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5
Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models.
In contrast, Mdot decreases by an order of magnitude for stars following
circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to
produce slow winds. Much of these winds cool radiatively, forming cold clumps
immersed into the X-ray emitting gas. We test two orbital configurations for
the stars in this scenario, an isotropic distribution and two rotating discs
with perpendicular orientation. The morphology of cold gas is quite sensitive
to the orbits. In both cases, however, most of the accreted gas is hot, with an
almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The
cold gas accretes in intermittent, short but powerful episodes which may give
rise to large amplitude variability in the luminosity of Sgr A* on time scales
of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and
1e4 Rsch, for the one and two-phase wind simulations, respectively, never
forming the quasi-spherical accretion flows suggested in some previous work.
Our work suggests that, averaged over time scales of 100s to 1000s of years,
the radiative and mechanical luminosity of Sgr A* may be substantially higher
than it is in its current state. Further improvements of the wind accretion
modelling of Sgr A* will rely on improved observational constraints for the
wind properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper
and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2:
minor changes
Santiago Ibarra: Historia de un inmigrante vasco = euskal etorkin baten historia
Reseña bibliográfica de la obra "Santiago Ibarra: Historia de un inmigrante vasco = euskal etorkin baten historia" de Ángeles de Dios de Martina. El libro es el resultado del análisis detallado de una extensa carta escrita por Santiago Ibarra desde Argentina a su hermana Leoncia de Bilbao en 1954. Este estudio incluye ilustraciones de la época y análisis del contexto histórico de los eventos que narra en la carta
The X-ray Ridge Surrounding Sgr A* at the Galactic Center
We present the first detailed simulation of the interaction between the
supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc
region at the Galactic center. The passage of the supernova ejecta through this
medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive
black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray
intensity of this feature match very well with recently obtained Chandra
images, and we infer a supernova remnant age of less than 2,000 years. This
young age--a factor 3--4 lower than previous estimates--arises from our
inclusion of stellar wind effects in the initial (pre-explosion) conditions in
the medium. The supernova does not clear out the central ~ 0.2-pc region around
Sgr~A* and does not significantly alter the accretion rate onto the central
black hole upon passage through the Galactic center.Comment: 10 pages, 3 figures, submitted to ApJ
General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*
Near-IR and X-ray flares have been detected from the supermassive black hole
Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes,
suggesting an emission region only a few Schwarzschild radii above the event
horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its
detailed lightcurve, yielding not only the highest quality period thus far, but
also important structure reflecting the geometry of the emitting region. Recent
MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave
instability, that enhances the accretion rate for several hours, possibly
accounting for the observed flares. In this Letter, we carry out ray-tracing
calculations in a Schwarzschild metric to determine as accurately as possible
the lightcurve produced by general relativistic effects during such a
disruption. We find that the Rossby wave induced spiral pattern in the disk is
an excellent fit to the data, implying a disk inclination angle of ~77 deg.
Note, however, that if this association is correct, the observed period is not
due to the underlying Keplerian motion but, rather, to the pattern speed. The
favorable comparison between the observed and simulated lightcurves provides
important additional evidence that the flares are produced in Sgr A*'s inner
disk.Comment: 5 Pages, 3 Figures, accepted for publication in ApJ Lette
Binary formation and mass function variations in fragmenting discs with short cooling times
Accretion discs at sub-pc distances around supermassive black holes are
likely to cool rapidly enough that self-gravity results in fragmentation. Here,
we use high-resolution hydrodynamic simulations of a simplified disc model to
study how the outcome of fragmentation depends upon numerical resolution and
cooling time, and to investigate the incidence of binary formation within
fragmenting discs. We investigate a range of cooling times, from the relatively
long cooling time-scales that are marginally unstable to fragmentation down to
highly unstable cooling on a time-scale that is shorter than the local
dynamical time. The characteristic mass of fragments decreases with reduced
cooling time, though the effect is modest and dependent upon details of how
rapidly bound clumps radiate. We observe a high incidence of capture binaries,
though we are unable to determine their final orbits or probability of
survival. The results suggest that faster cooling in the parent disc results in
an increased binary fraction, and that a high primordial binary fraction may
result from disc fragmentation. We discuss our results in terms of the young
massive stars close to the Galactic Centre, and suggest that observations of
some stellar binaries close to the Galactic Centre remain consistent with
formation in a fragmenting accretion disc.Comment: 10 pages, 5 figures. Accepted for publication in MNRAS. Figures 1 and
3 degraded to meet arXiv size limits - version with high resolution figures
available at http://www.strw.leidenuniv.nl/~rda/publications.htm
Picard groups and strongly graded coalgebras
AbstractIn this paper we study strongly graded coalgebras and its relation to the Picard group. A classification theorem for this kind of coalgebras is given via the second Doi's cohomology group. The strong Picard group of a coalgebra is introduced in order to characterize those graded coalgebras with strongly graded dual ring. Finally, for a Hopf algebra H we also characterize the H∗-Galois coextensions with dual H-Galois extension solving the question proposed in Dăscălescu et al., J. Algebra 178 (1995) 400–413
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