2,222 research outputs found
Binary planetary nebulae nuclei towards the Galactic bulge. II. A penchant for bipolarity and low-ionisation structures
Considerable effort has been applied towards understanding the precise
shaping mechanisms responsible for the diverse range of morphologies exhibited
by planetary nebulae (PNe). A binary companion is increasingly gaining support
as a dominant shaping mechanism, however morphological studies of the few PNe
that we know for certain were shaped by binary evolution are scarce or biased.
Newly discovered binary central stars (CSPN) from the OGLE-III photometric
variability survey have significantly increased the sample of post
common-envelope (CE) nebulae available for morphological analysis. We present
Gemini South narrow-band images for most of the new sample to complement
existing data in a qualitative morphological study of 30 post-CE nebulae.
Nearly 30% of nebulae have canonical bipolar morphologies, however this rises
to 60% once inclination effects are incorporated with the aid of geometric
models. This is the strongest observational evidence yet linking CE evolution
to bipolar morphologies. A higher than average proportion of the sample shows
low-ionisation knots, filaments or jets suggestive of a binary origin. These
features are also common around emission-line nuclei which may be explained by
speculative binary formation scenarios for H-deficient CSPN.Comment: Accepted for publication in A&
Turbulent transport of heat and momentum in a boundary layer subject to deceleration, suction and variable wall temperature
The relationship between the turbulent transport of heat and momentum in an adverse pressure gradient boundary layer was studied. An experimental study was conducted of turbulent boundary layers subject to strong adverse pressure gradients with suction. Near-equilibrium flows were attained, evidenced by outer-region similarity in terms of defect temperature and defect velocity profiles. The relationship between Stanton number and enthalpy thickness was shown to be the same as for a flat plate flow both for constant wall temperature boundary conditions and for steps in wall temperature. The superposition principle used with the step-wall-temperature experimental result was shown to accurately predict the Stanton number variation for two cases of arbitrarily varying wall temperature. The Reynolds stress tensor components were measured for strong adverse pressure gradient conditions and different suction rates. Two peaks of turbulence intensity were found: one in the inner and one in the outer regions. The outer peak is shown to be displaced outward by an adverse pressure gradient and suppressed by suction
Linearisation Instabilities of the Massive Nonsymmetric Gravitational Theory
The massive nonsymmetric gravitational theory is shown to posses a
linearisation instability at purely GR field configurations, disallowing the
use of the linear approximation in these situations. It is also shown that
arbitrarily small antisymmetric sector Cauchy data leads to singular evolution
unless an ad hoc condition is imposed on the initial data hypersurface.Comment: 14 pages, IOP style for submission to CQG. Minor changes and
additional background material adde
NGC 3603 - a Local Template for Massive Young Clusters
We present a study of the star cluster associated with the massive Galactic
HII region NGC3603 based on near-IR broad-- and narrowband observations taken
with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss
color-color diagrams and address the impact of the high UV flux on the disk
evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt
The spin rates and spin evolution of the O components in WR+O binaries
Despite 50 years of extensive binary research we have to conclude that the
Roche lobe overflow/mass transfer process that governs close binary evolution
is still poorly understood. It is the scope of the present paper to lift a tip
of the veil by studying the spin-up and spin-down processes of the O-type
components of WR+O binaries. We critically analyze the available observational
data of rotation speeds of the O-type components in WR+O binaries. By combining
a binary evolutionary code and a formalism that describes the effects of tides
in massive stars with an envelope in radiative equilibrium, we compute the
corresponding rotational velocities during the Roche lobe overflow of the
progenitor binaries. In all the studied WR+O binaries, we find that the O-type
stars were affected by accretion of matter during the RLOF of the progenitor.
This means that common envelope evolution which excludes any accretion onto the
secondary O-star, has not played an important role to explain the WR+O
binaries. Moreover, although it is very likely that the O-type star progenitors
were spun-up by the mass transfer, many ended the RLOF/mass transfer phase with
a rotational velocity that is significantly smaller than the critical rotation
speed. This may indicate that during the mass transfer phase there is a
spin-down process which is of the same order as, although significantly less
than that of the spin-up process. We propose a Spruit-Tayler type dynamo
spin-down suggested in the past to explain the rotation speeds of the mass
gainers in long-period Algols.Comment: 6 pages, accepted for publication in A&A; accepted versio
Understanding the X-ray Flaring from Eta Car
We quantify the rapid variations in X-ray brightness ("flares") from the
extremely massive colliding wind binary Eta Carinae seen during the past three
orbital cycles by RXTE. The observed flares tend to be shorter in duration and
more frequent as periastron is approached, although the largest ones tend to be
roughly constant in strength at all phases. Plausible scenarios include (1) the
largest of multi-scale stochastic wind clumps from the LBV component entering
and compressing the hard X-ray emitting wind-wind collision (WWC) zone, (2)
large-scale corotating interacting regions in the LBV wind sweeping across the
WWC zone, or (3) instabilities intrinsic to the WWC zone. The first one appears
to be most consistent with the observations, requiring homologously expanding
clumps as they propagate outward in the LBV wind and a turbulence-like
power-law distribution of clumps, decreasing in number towards larger sizes, as
seen in Wolf-Rayet winds.Comment: accepted by Ap
VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136
Using the Very Large Telescope's Spectrograph for INtegral Field Observation
in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR
spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of
the very young (~1 Myr), massive and dense cluster R136, in the Large
Magellanic Cloud (LMC). We also de-archived available images that were obtained
with the Hubble Space Telescope's Space Telescope Imaging Spectrograph
(HST/STIS), and extracted high-quality, differential photometry of our target
stars to check for any variability related to binary motion.
Previous studies, relying on spatially unresolved, integrated, optical
spectroscopy, had reported that one of these stars was likely to be a 4.377-day
binary. Our study set out to identify the culprit and any other short-period
system among our targets. However, none displays significant photometric
variability, and only one star, BAT99-112 (R136c), located on the outer fringe
of R136, displays a marginal variability in its radial velocities; we
tentatively report an 8.2-day period. The binary status of BAT99-112 is
supported by the fact that it is one of the brightest X-ray sources among all
known WR stars in the LMC, consistent with it being a colliding-wind system.
Follow-up observations have been proposed to confirm the orbital period of this
potentially very massive system.Comment: 9 pages, 6 figures; accepted for publication in MNRA
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