464 research outputs found
Power Spectra for Galaxy Shape Correlations
It has recently been argued that the observed ellipticities of galaxies may
be determined at least in part by the primordial tidal gravitational field in
which the galaxy formed. Long-range correlations in the tidal field could thus
lead to an ellipticity-ellipticity correlation for widely separated galaxies. I
present results of a calculation of the angular power spectrum of intrinsic
galaxy shape correlations using a new model relating ellipticity to angular
momentum. I show that for low redshift galaxy surveys, the model predicts that
intrinsic correlations will dominate correlations induced by weak lensing, in
good agreement with previous theoretical work and observations. The model also
produces `E-mode' correlations enhanced by a factor of 3.5 over `B-modes' on
small scales, making it harder to disentangle intrinsic correlations from weak
lensing.Comment: 4 pages, 1 figure. in ``The Shapes of Galaxies and Their Dark
Halos,'' Yale Cosmology Workshop, Ed. P. Natarajan (New Haven CT, May 2001).
Revised web version corrects lensing curve normalisation in fig.1; text as
publishe
Dynamical Models for the Formation of Elephant Trunks in H II Regions
The formation of pillars of dense gas at the boundaries of H II Regions is
investigated with hydrodynamical numerical simulations including ionising
radiation from a point source. We show that shadowing of ionising radiation by
an inhomogeneous density field is capable of forming so-called elephant trunks
(pillars of dense gas as in e.g. M16) without the assistance of self-gravity,
or of ionisation front and cooling instabilities. A large simulation of a
density field containing randomly generated clumps of gas is shown to naturally
generate elephant trunks with certain clump configurations. These
configurations are simulated in isolation and analysed in detail to show the
formation mechanism and determine possible observational signatures. Pillars
formed by the shadowing mechanism are shown to have rather different velocity
profiles depending on the initial gas configuration, but asymmetries mean that
the profiles also vary significantly with perspective, limiting their ability
to discriminate between formation scenarios. Neutral and molecular gas cooling
are shown to have a strong effect on these results.Comment: 17 pages, 11 figures, MNRAS. Minor revisions: typos corrected,
figures re-ordered to match published versio
3D Smoothed Particle Hydrodynamics Models of Betelgeuse's Bow Shock
Betelgeuse, the bright red supergiant (RSG) in Orion, is a runaway star. Its
supersonic motion through the interstellar medium has resulted in the formation
of a bow shock, a cometary structure pointing in the direction of motion. We
present the first 3D hydrodynamic simulations of the formation and evolution of
Betelgeuse's bow shock. We show that the bow shock morphology depends
substantially on the growth timescale for Rayleigh-Taylor versus
Kelvin-Helmholtz instabilities. We discuss our models in light of the recent
Herschel, GALEX and VLA observations. If the mass in the bow shock shell is low
(~few x 0.001 Msun), as seems to be implied by the AKARI and Herschel
observations, then Betelgeuse's bow shock is very young and is unlikely to have
reached a steady state. The circular, smooth bow shock shell is consistent with
this conclusion. We further discuss the implications of our results, in
particular, the possibility that Betelgeuse may have only recently entered the
RSG phase.Comment: 9 pages, 4 figures, Betelgeuse workshop, November 2012, Paris. To be
published in the European Astronomical Society Publications Series, editors:
Pierre Kervella, Thibaut Le Bertre & Guy Perri
Theoretical Estimates of Intrinsic Galaxy Alignment
It has recently been argued that the observed ellipticities of galaxies may
be determined at least in part by the primordial tidal gravitational field in
which the galaxy formed. Long-range correlations in the tidal field could thus
lead to an ellipticity-ellipticity correlation for widely separated galaxies.
We present a new model relating ellipticity to angular momentum, which can be
calculated in linear theory. We use this model to calculate the angular power
spectrum of intrinsic galaxy shape correlations. We show that for low redshift
galaxy surveys, our model predicts that intrinsic correlations will dominate
correlations induced by weak lensing, in good agreement with previous
theoretical work and observations. We find that our model produces `E-mode'
correlations enhanced by a factor of 3.5 over B-modes on small scales, making
it harder to disentangle intrinsic correlations from those induced by weak
gravitational lensing.Comment: 14 pages, 2 figures, MNRAS in press. Error corrected in lensing
calculation; revised versio
Cold gas in hot star clusters: the wind from the red supergiant W26 in Westerlund 1
The massive red supergiant (RSG) W26 in Westerlund 1 is one of a growing
number of RSGs shown to have winds that are ionized from the outside in. The
fate of this dense wind material is important for models of second generation
star formation in massive star clusters. Mackey et al. (2014) showed that
external photoionization can stall the wind of RSGs and accumulate mass in a
dense static shell. We use 1D R-HD simulations of an externally photoionized
wind to predict the Halpha and [NII] emission arising from photoionized winds
both with and without a dense shell. We analyse spectra of the Halpha and [NII]
emission in the environment around W26 and compare them with predicted
synthetic emission. Simulations of slow winds that are decelerated into a dense
shell show strongly limb-brightened line emission, with line radial velocities
that are independent of the wind speed. Faster winds (>22 km/s) do not form a
dense shell, have less limb-brightening, and the line radial velocity is a good
tracer of the wind speed. The brightness of the [NII] and Halpha lines as a
function of distance from W26 agrees reasonably well with observations when
only the line flux is considered. The radial velocity disagrees, however: the
brightest observed emission is blueshifted by ~25 km/s relative to the radial
velocity of the star, whereas a spherically symmetric wind has the brightest
emission at zero radial velocity. Our results show that the bright nebula
surrounding W26 must be asymmetric; we suggest it is confined by external ram
pressure from the wind of the nearby supergiant W9. We obtain a lower limit on
the nitrogen abundance within the nebula of 2.35 times solar. The line ratio
strongly favours photoionization over shock ionization, and so even if the
observed nebula is pressure confined there should still be an ionization front
and a photoionization-confined shell closer to the star.Comment: 12 pages plus appendices, accepted for publication in Astronomy &
Astrophysics; abstract shortened to fit arXiv limit
Wind bubbles within H II regions around slowly moving stars
Interstellar bubbles around O stars are driven by a combination of the star's
wind and ionizing radiation output. The wind contribution is uncertain because
the boundary between the wind and interstellar medium is difficult to observe.
Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust
emission around O stars, contained well within the H II region bubble. These
arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by
density gradients and/or stellar motion. We present two-dimensional,
radiation-hydrodynamics simulations investigating the evolution of wind bubbles
and H II regions around massive stars moving through a dense (n=3000 cm^{-3}),
uniform medium with velocities ranging from 4 to 16 km/s. The H II region
morphology is strongly affected by stellar motion, as expected, but the wind
bubble is also very aspherical from birth, even for the lowest space velocity
considered. Wind bubbles do not fill their H II regions (we find filling
factors of 10-20%), at least for a main sequence star with mass M~30 Msun.
Furthermore, even for supersonic velocities the wind bow shock does not
significantly trap the ionization front. X-ray emission from the wind bubble is
soft, faint, and comes mainly from the turbulent mixing layer between the wind
bubble and the H II region. The wind bubble radiates <1 per cent of its energy
in X-rays; it loses most of its energy by turbulent mixing with cooler
photoionized gas. Comparison of the simulations with the H II region RCW 120
shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is
allowed, implying that the ionizing source is unlikely to be a runaway star but
more likely formed in situ. The region's youth, and apparent isolation from
other O or B stars, makes it very interesting for studies of massive star
formation and of initial mass functions.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics (new version corrects an error in the simulation postprocessing,
figs 6,7,11 are modified slightly, conclusions unchanged
Revealing the binary origin of Type Ic superluminous supernovae through nebular hydrogen emission
We propose that nebular H-alpha emission as detected in the Type Ic
superluminous supernova iPTF13ehe stems from matter which is stripped from a
companion star when the supernova ejecta collide with it. The temporal
evolution, the line broadening, and the overall blueshift of the emission are
consistent with this interpretation. We scale the nebular H-alpha luminosity
predicted for Type Ia supernovae in single-degenerate systems to derive the
stripped mass required to explain the H-alpha luminosity of iPTF13ehe. We find
a stripped mass of 0.1 - 0.9 solar masses, assuming that the supernova
luminosity is powered by radioactivity or magnetar spin down. Because a central
heating source is required to excite the H-alpha emission, an
interaction-powered model is not favored for iPTF13ehe if the H-alpha emission
is from stripped matter. We derive a companion mass of more than 20 solar
masses and a binary separation of less than about 20 companion radii based on
the stripping efficiency during the collision, indicating that the supernova
progenitor and the companion formed a massive close binary system. If Type Ic
superluminous supernovae generally occur in massive close binary systems, the
early brightening observed previously in several Type Ic superluminous
supernovae may also be due to the collision with a close companion.
Observations of nebular hydrogen emission in future Type Ic superluminous
supernovae will enable us to test this interpretation.Comment: 4 pages, 1 figure, 1 table, accepted by Astronomy & Astrophysics
Letter
- …