5,604 research outputs found
Gauge Symmetry and Gravito-Electromagnetism
A tensor description of perturbative Einsteinian gravity about an arbitrary
background spacetime is developed. By analogy with the covariant laws of
electromagnetism in spacetime, gravito-electromagnetic potentials and fields
are defined to emulate electromagnetic gauge transformations under
substitutions belonging to the gauge symmetry group of perturbative
gravitation. These definitions have the advantage that on a flat background,
with the aid of a covariantly constant timelike vector field, a subset of the
linearised gravitational field equations can be written in a form that is fully
analogous to Maxwell's equations (without awkward factors of 4 and extraneous
tensor fields). It is shown how the remaining equations in the perturbed
gravitational system restrict the time dependence of solutions to these
equations and thereby prohibit the existence of propagating vector fields. The
induced gravito-electromagnetic Lorentz force on a test particle is evaluated
in terms of these fields together with the torque on a small gyroscope. It is
concluded that the analogy of perturbative gravity to Maxwell's description of
electromagnetism can be valuable for (quasi-)stationary gravitational phenomena
but that the analogy has its limitations.Comment: 29 pages no-fig
Radio observations of the massive stellar cluster Westerlund 1
High-dynamic range radio observations of Westerlund 1 are presented that
detect a total of 21 stars in the young massive stellar cluster, the richest
population of radio emitting stars known for any young massive galactic cluster
in the Galaxy. We will discuss some of the more remarkable objects, including
the highly radio luminous supergiant B[e] star W9, with an estimated mass-loss
rate ~10^{-3} solarmass/yr, comparable to that of eta Carina, along with the
somewhat unusual detection of thermal emission from almost all the cool red
supergiants and yellow hypergiants. There is strong supporting evidence from
X-ray observations that each of the WR stars with radio emission are likely to
be colliding-wind binariesComment: To appear in the proceedings of "Massive Stars: Fundamental
Parameters and Circumstellar Interactions". 2 pages, 1 figur
Westerlund 1 as a Template for Massive Star Evolution
With a dynamical mass M_dyn ~ 1.3x10e5 M_sun and a lower limit M_cl>5x10e4
M_sun from star counts, Westerlund 1 is the most massive young open cluster
known in the Galaxy and thus the perfect laboratory to study massive star
evolution. We have developed a comprehensive spectral classification scheme for
supergiants based on features in the 6000-9000A range, which allows us to
identify >30 very luminous supergiants in Westerlund 1 and ~100 other less
evolved massive stars, which join the large population of Wolf-Rayet stars
already known. Though detailed studies of these stars are still pending,
preliminary rough estimates suggest that the stars we see are evolving to the
red part of the HR diagram at approximately constant luminosity.Comment: To be published in Proceedings of IAU Symposium 250: Massive Stars as
Cosmic Engines, held in Kaua'i (Hawaii, USA), Dec 2007, edited by F.
Bresolin, P.A. Crowther & J. Puls (Cambridge University Press
Optimizing optical Bragg scattering for single-photon frequency conversion
We develop a systematic theory for optimising single-photon frequency
conversion using optical Bragg scattering. The efficiency and phase-matching
conditions for the desired Bragg scattering conversion as well as spurious
scattering and modulation instability are identified. We find that third-order
dispersion can suppress unwanted processes, while dispersion above the fourth
order limits the maximum conversion efficiency. We apply the optimisation
conditions to frequency conversion in highly nonlinear fiber, silicon nitride
waveguides and silicon nanowires. Efficient conversion is confirmed using full
numerical simulations. These design rules will assist the development of
efficient quantum frequency conversion between multicolour single photon
sources for integration in complex quantum networks.Comment: 9 pages, 14 figure
The population of M-type supergiants in the starburst cluster Stephenson 2
The open cluster Stephenson 2 contains the largest collection of red
supergiants known in the Galaxy, and at present is the second most massive
young cluster known in the Milky Way. We have obtained multi-epoch,
intermediate-resolution spectra around the CaII triplet for more than 30 red
supergiants in Stephenson~2 and its surroundings. We find a clear separation
between a majority of RSGs having spectral types M0-M2 and the brightest
members in the NIR, which have very late spectral types and show strong
evidence for heavy mass loss. The distribution of spectral types is similar to
that of RSGs in other clusters, such as NGC 7419, or associations, like Per
OB1. The cluster data strongly support the idea that heavy mass loss and maser
emission is preferentially associated with late-M spectral types, suggesting
that they represent an evolutionary phase.Comment: Contribution to the Betelgeuse workshop, November 2012, Paris. To be
published in the European Astronomical Society Publications Series, editors:
Pierre Kervella, Thibaut Le Bertre & Guy Perri
CO-dark gas and molecular filaments in Milky Way type galaxies
We use the moving mesh code AREPO coupled to a time-dependent chemical
network to investigate the formation and destruction of molecular gas in
simulated spiral galaxies. This allows us to determine the characteristics of
the gas that is not traced by CO emission. Our extremely high resolution AREPO
simulations allow us to capture the chemical evolution of the disc, without
recourse to a parameterised `clumping factor'. We calculate H2 and CO column
densities through our simulated disc galaxies, and estimate the CO emission and
CO-H2 conversion factor. We find that in conditions akin to those in the local
interstellar medium, around 42% of the total molecular mass should be in
CO-dark regions, in reasonable agreement with observational estimates. This
fraction is almost insensitive to the CO integrated intensity threshold used to
discriminate between CO-bright and CO-dark gas, as long as this threshold is
less than 10 K km/s. The CO-dark molecular gas primarily resides in extremely
long (>100 pc) filaments that are stretched between spiral arms by galactic
shear. Only the centres of these filaments are bright in CO, suggesting that
filamentary molecular clouds observed in the Milky Way may only be small parts
of much larger structures. The CO-dark molecular gas mainly exists in a
partially molecular phase which accounts for a significant fraction of the
total disc mass budget. The dark gas fraction is higher in simulations with
higher ambient UV fields or lower surface densities, implying that external
galaxies with these conditions might have a greater proportion of dark gas.Comment: Accepted by MNRA
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