2,262 research outputs found
On the stability of bow shocks generated by red supergiants: the case of IRC-10414
In this Letter, we explore the hypothesis that the smooth appearance of bow
shocks around some red supergiants (RSGs) might be caused by the ionization of
their winds by external sources of radiation. Our numerical simulations of the
bow shock generated by IRC-10414 (the first-ever RSG with an optically detected
bow shock) show that the ionization of the wind results in its acceleration by
a factor of two, which reduces the difference between the wind and space
velocities of the star and makes the contact discontinuity of the bow shock
stable for a range of stellar space velocities and mass-loss rates. Our best
fit model reproduces the overall shape and surface brightness of the observed
bow shock and suggests that the space velocity and mass-loss rate of IRC-10414
are 50 and , respectively, and that the number density of the local ISM is
3 . It also shows that the bow shock emission comes
mainly from the shocked stellar wind. This naturally explains the enhanced
nitrogen abundance in the line-emitting material, derived from the spectroscopy
of the bow shock. We found that photoionized bow shocks are 1550
times brighter in optical line emission than their neutral counterparts, from
which we conclude that the bow shock of IRC-10414 must be photoionized.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
Exponential Renormalization II: Bogoliubov's R-operation and momentum subtraction schemes
This article aims at advancing the recently introduced exponential method for
renormalisation in perturbative quantum field theory. It is shown that this new
procedure provides a meaningful recursive scheme in the context of the
algebraic and group theoretical approach to renormalisation. In particular, we
describe in detail a Hopf algebraic formulation of Bogoliubov's classical
R-operation and counterterm recursion in the context of momentum subtraction
schemes. This approach allows us to propose an algebraic classification of
different subtraction schemes. Our results shed light on the peculiar algebraic
role played by the degrees of Taylor jet expansions, especially the notion of
minimal subtraction and oversubtractions.Comment: revised versio
Radial Velocity Observations and Light Curve Noise Modeling Confirm That Kepler-91b is a Giant Planet Orbiting a Giant Star
Kepler-91b is a rare example of a transiting hot Jupiter around a red giant
star, providing the possibility to study the formation and composition of hot
Jupiters under different conditions compared to main-sequence stars. However,
the planetary nature of Kepler-91b, which was confirmed using phase-curve
variations by Lillo-Box et al., was recently called into question based on a
re-analysis of Kepler data. We have obtained ground-based radial velocity
observations from the Hobby-Eberly Telescope and unambiguously confirm the
planetary nature of Kepler-91b by simultaneously modeling the Kepler and radial
velocity data. The star exhibits temporally correlated noise due to stellar
granulation which we model as a Gaussian Process. We hypothesize that it is
this noise component that led previous studies to suspect Kepler-91b to be a
false positive. Our work confirms the conclusions presented by Lillo-Box et al.
that Kepler-91b is a 0.73+/-0.13 Mjup planet orbiting a red giant star.Comment: Published in Ap
Projective Fourier Duality and Weyl Quantization
The Weyl-Wigner correspondence prescription, which makes large use of Fourier
duality, is reexamined from the point of view of Kac algebras, the most general
background for noncommutative Fourier analysis allowing for that property. It
is shown how the standard Kac structure has to be extended in order to
accommodate the physical requirements. An Abelian and a symmetric projective
Kac algebras are shown to provide, in close parallel to the standard case, a
new dual framework and a well-defined notion of projective Fourier duality for
the group of translations on the plane. The Weyl formula arises naturally as an
irreducible component of the duality mapping between these projective algebras.Comment: LaTeX 2.09 with NFSS or AMSLaTeX 1.1. 102Kb, 44 pages, no figures.
requires subeqnarray.sty, amssymb.sty, amsfonts.sty. Final version with text
improvements and crucial typos correction
Fourier Duality as a Quantization Principle
The Weyl-Wigner prescription for quantization on Euclidean phase spaces makes
essential use of Fourier duality. The extension of this property to more
general phase spaces requires the use of Kac algebras, which provide the
necessary background for the implementation of Fourier duality on general
locally compact groups. Kac algebras -- and the duality they incorporate -- are
consequently examined as candidates for a general quantization framework
extending the usual formalism. Using as a test case the simplest non-trivial
phase space, the half-plane, it is shown how the structures present in the
complete-plane case must be modified. Traces, for example, must be replaced by
their noncommutative generalizations - weights - and the correspondence
embodied in the Weyl-Wigner formalism is no more complete. Provided the
underlying algebraic structure is suitably adapted to each case, Fourier
duality is shown to be indeed a very powerful guide to the quantization of
general physical systems.Comment: LaTeX 2.09 with NFSS or AMSLaTeX 1.1. 97Kb, 43 pages, no figures.
requires subeqnarray.sty, amssymb.sty, amsfonts.sty. Final version with (few)
text and (crucial) typos correction
Low-Mass Relics of Early Star Formation
The earliest stars to form in the Universe were the first sources of light,
heat and metals after the Big Bang. The products of their evolution will have
had a profound impact on subsequent generations of stars. Recent studies of
primordial star formation have shown that, in the absence of metals (elements
heavier than helium), the formation of stars with masses 100 times that of the
Sun would have been strongly favoured, and that low-mass stars could not have
formed before a minimum level of metal enrichment had been reached. The value
of this minimum level is very uncertain, but is likely to be between 10^{-6}
and 10^{-4} that of the Sun. Here we show that the recent discovery of the most
iron-poor star known indicates the presence of dust in extremely
low-metallicity gas, and that this dust is crucial for the formation of
lower-mass second-generation stars that could survive until today. The dust
provides a pathway for cooling the gas that leads to fragmentation of the
precursor molecular cloud into smaller clumps, which become the lower-mass
stars.Comment: Offprint of Nature 422 (2003), 869-871 (issue 24 April 2003
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