40 research outputs found
The distance and neutral environment of the massive stellar cluster Westerlund 1
The goal of this study is to determine a distance to Westerlund 1 independent
of the characteristics of the stellar population and to study its neutral
environment, using observations of atomic hydrogen. The HI observations are
taken from the Southern Galactic Plane Survey to study HI absorption in the
direction of the HII region created by the members of Westerlund 1 and to
investigate its environment as observed in the HI line emission. A Galactic
rotation curve was derived using the recently revised values for the Galactic
centre distance of kpc, and the velocity of the Sun around the
Galactic centre of km s. The newly determined
rotation model leads us to derive a distance of kpc to Westerlund
1, consistent with a location in the Scutum-Crux Arm. Included in this estimate
is a very careful investigation of possible sources of error for the Galactic
rotation curve. We also report on small expanding HI features around the
cluster with a maximum dynamic age of 600,000 years and a larger bubble which
has a minimum dynamic age of 2.5 million years. Additionally we re-calculated
the kinematic distances to nearby HII regions and supernova remnants based on
our new Galaxic rotation curve. We propose that in the early stages of the
development of Wd 1 a large interstellar bubble of diameter about 50 pc was
created by the cluster members. This bubble has a dynamic age similar to the
age of the cluster. Small expanding bubbles, with dynamical ages Myr
are found around Wd 1, which we suggest consist of recombined material lost by
cluster members through their winds.Comment: 8 pages, accepted for publication in A&
Waves in Schwarzschild spacetimes: How strong can imprints of the spacetime curvature be
Misprints corrected, two references added. To appear in the Phys. Rev. D
General Relativistic versus Newtonian: a universality in radiation hydrodynamics
We compare Newtonian and general relativistic descriptions of the stationary
accretion of self-gravitating fluids onto compact bodies. Spherical symmetry
and thin gas approximation are assumed. Luminosity depends, amongst other
factors, on the temperature and the contribution of gas to the total mass, in
both -- general relativistic () and Newtonian () -- models. We
discover a remarkable universal behaviour for transonic flows: the ratio of
respective luminosities is independent of the fractional mass of
the gas and depends on asymptotic temperature. It is close to 1 in the regime
of low asymptotic temperatures and can grow by one order of magnitude for high
temperatures. These conclusions are valid for a wide range of polytropic
equations of state.Comment: 8 pages, 4 figure
A counter-example to a recent version of the Penrose conjecture
By considering suitable axially symmetric slices on the Kruskal spacetime, we
construct counterexamples to a recent version of the Penrose inequality in
terms of so-called generalized apparent horizons.Comment: 12 pages. Appendix added with technical details. To appear in
Classical and Quantum Gravit
Axially Symmetric Bianchi I Yang-Mills Cosmology as a Dynamical System
We construct the most general form of axially symmetric SU(2)-Yang-Mills
fields in Bianchi cosmologies. The dynamical evolution of axially symmetric YM
fields in Bianchi I model is compared with the dynamical evolution of the
electromagnetic field in Bianchi I and the fully isotropic YM field in
Friedmann-Robertson-Walker cosmologies. The stochastic properties of axially
symmetric Bianchi I-Einstein-Yang-Mills systems are compared with those of
axially symmetric YM fields in flat space. After numerical computation of
Liapunov exponents in synchronous (cosmological) time, it is shown that the
Bianchi I-EYM system has milder stochastic properties than the corresponding
flat YM system. The Liapunov exponent is non-vanishing in conformal time.Comment: 18 pages, 6 Postscript figures, uses amsmath,amssymb,epsfig,verbatim,
to appear in CQ
Hysteresis effects and diagnostics of the shock formation in low angular momentum axisymmetric accretion in the Kerr metric
The secular evolution of the purely general relativistic low angular momentum
accretion flow around a spinning black hole is shown to exhibit hysteresis
effects. This confirms that a stationary shock is an integral part of such an
accretion disc in the Kerr metric. The equations describing the space gradient
of the dynamical flow velocity of the accreting matter have been shown to be
equivalent to a first order autonomous dynamical systems. Fixed point analysis
ensures that such flow must be multi-transonic for certain astrophysically
relevant initial boundary conditions. Contrary to the existing consensus in the
literature, the critical points and the sonic points are proved not to be
isomorphic in general. Homoclinic orbits for the flow flow possessing multiple
critical points select the critical point with the higher entropy accretion
rate, confirming that the entropy accretion rate is the degeneracy removing
agent in the system. However, heteroclinic orbits are also observed for some
special situation, where both the saddle type critical points of the flow
configuration possesses identical entropy accretion rate. Topologies with
heteroclinic orbits are thus the only allowed non removable degenerate
solutions for accretion flow with multiple critical points, and are shown to be
structurally unstable. Depending on suitable initial boundary conditions, a
homoclinic trajectory can be combined with a standard non homoclinic orbit
through an energy preserving Rankine-Hugoniot type of stationary shock. An
effective Lyapunov index has been proposed to analytically confirm why certain
class of transonic flow can not accommodate shock solutions even if it produces
multiple critical points. (Abridged)Comment: mn2e.cls format. 24 pages. 4 figure
From Geometry to Numerics: interdisciplinary aspects in mathematical and numerical relativity
This article reviews some aspects in the current relationship between
mathematical and numerical General Relativity. Focus is placed on the
description of isolated systems, with a particular emphasis on recent
developments in the study of black holes. Ideas concerning asymptotic flatness,
the initial value problem, the constraint equations, evolution formalisms,
geometric inequalities and quasi-local black hole horizons are discussed on the
light of the interaction between numerical and mathematical relativists.Comment: Topical review commissioned by Classical and Quantum Gravity.
Discussion inspired by the workshop "From Geometry to Numerics" (Paris, 20-24
November, 2006), part of the "General Relativity Trimester" at the Institut
Henri Poincare (Fall 2006). Comments and references added. Typos corrected.
Submitted to Classical and Quantum Gravit