1,678 research outputs found
Poly(thioether) vitrimers via transalkylation of trialkylsulfonium salts
Vitrimers are permanently cross-linked organic polymers that can be reshaped, molded, and recycled without loss of network integrity. Herein, we report poly(thioether) networks, prepared through a straightforward thiol-ene photopolymerization, that can be turned into catalyst-free vitrimer materials by partial alkylation of the thioethers (1-10%) to the corresponding trialkylsulfonium salts. Based on a classical S(N)2-type substitution, the resulting polyionic networks can be reshaped upon heating via swift transalkylation reactions. This novel exchange reaction for the design of vitrimers was studied both on low MW model compounds as well as on a material level. In addition, we demonstrated the recycling of these networks without significant loss of mechanical properties
Studying magnetic turbulence with radio polarimetry
Magnetised plasma permeates the Universe. It is present in stars, accretion discs, galaxies and clusters of galaxies. This dissertation investigates the characteristics of diffuse magnetised plasma in the inter-stellar-medium (ISM) and the intra-cluster-medium (ICM).
To probe turbulence in the ISM and ICM, we develop an estimator for the magnetic tension-force spectrum. It is based on polarised synchrotron emission data in the Faraday-rotation-free frequency regime. The tension force is the dynamically most important magnetic force in subsonic magnetic turbulence.
We consider two magnetic field scenarios: a statistically isotropic field distribution, and a statistically isotropic field upon a small mean field. In the latter case the magnetic power spectrum can also be obtained in addition to the tension-spectrum. The method is exact in the ideal case of a homogeneous cosmic ray electron distribution with a power law energy spectrum with a spectral index p=3, and isotropic magnetic fields. The method is applied to mock observations based on magneto-hydrodynamic simulations as a
prelude to an application to real observations. We confirm the robustness of the estimator by comparing its output to the corresponding quantities directly computed from the simulation.
Furthermore, to investigate the global Galactic field we developed hammurabi. This software tool takes three-dimensional theoretical models for the ISM components and generates mock observations from them. It focuses on the main tracers of our Galactic magnetised plasma such as synchrotron flux, Faraday rotation, dispersion measure and ultra-high-energy-cosmic-ray delflections. Hammurabi therefore permits us to confront models of the Galactic magnetised plasma with a broad range of real observations. We report on a number of works based on this code. In particular, due to simultaneous constraints of rotation measure, polarised and total synchrotron intensity, as well as theoretical predictions on the magnetic field and cosmic-ray electron distribution, the Galactic electron density scale height was suggested to be about twice as large as previously thought. This result obtained with hammurabi was recently confirmed independently by other authors using pulsar dispersion measure data.
Finally, we consider the role of the Galactic kinetic Sunyaev Zeldovich (kSZ) effect as a CMB foreground. We analyse the detectability of the Galactic kSZ effect by means of an optimally matched filter technique applied to a simulation of an ideal observation. We obtain a S/N ratio of 0.1, and demonstrate thereby that the Galactic kSZ effect can safely be ignored as a CMB foreground.
Furthermore, we rule out any significant contamination of the polarised CMB signal by second scattering of galactic kSZ photons
Infrared High-Resolution Spectroscopy of Post-AGB Circumstellar Disks. I. HR 4049 - The Winnowing Flow Observed?
High-resolution infrared spectroscopy in the 2.3-4.6 micron region is
reported for the peculiar A supergiant, single-lined spectroscopic binary HR
4049. Lines from the CO fundamental and first overtone, OH fundamental, and
several H2O vibration-rotation transitions have been observed in the
near-infrared spectrum. The spectrum of HR 4049 appears principally in emission
through the 3 and 4.6 micron region and in absorption in the 2 micron region.
The 4.6 micron spectrum shows a rich 'forest' of emission lines. All the
spectral lines observed in the 2.3-4.6 micron spectrum are shown to be
circumbinary in origin. The presence of OH and H2O lines confirm the
oxygen-rich nature of the circumbinary gas which is in contrast to the
previously detected carbon-rich material. The emission and absorption line
profiles show that the circumbinary gas is located in a thin, rotating layer
near the dust disk. The properties of the dust and gas circumbinary disk and
the spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolar
and M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocity
of 1 km/s. The severe depletion of refractory elements but near-solar
abundances of volatile elements observed in HR 4049 results from abundance
winnowing. The separation of the volatiles from the grains in the disk and the
subsequent accretion by the star are discussed. Contrary to prior reports, the
HR 4049 carbon and oxygen isotopic abundances are typical AGB values:
12C/13C=6^{+9}_{-4} and 16O/17O>200.Comment: 42 pages, 14 figures, Accepted by Ap
CMB Isotropy Anomalies and the Local Kinetic Sunyaev-Zel'dovich Effect
Several anomalies have been identified which may imply a breakdown of the
statistical isotropy of the cosmic microwave background (CMB). In particular,
an anomalous alignment of the quadrupole and octopole and a hemispherical power
asymmetry have increased in significance as the data have improved. There have
been several attempts to explain these observations which explore isotropy
breaking mechanisms within the early universe, but little attention has been
given to the possibility that these anomalies have their origin within the
local universe. We explore such a mechanism by considering the kinetic
Sunyaev-Zel'dovich effect due to a gaseous halo associated with the Milky Way.
Considering several physical models of an anisotropic free electron optical
depth contributed by such a halo, we find that the associated screening maps of
the primordial anisotropies have the necessary orientations to affect the
anomaly statistics very significantly, but only if the column density of free
electrons in the halo is at least an order of magnitude higher than indicated
by current observations.Comment: 10 pages, 4 figure
The dust disk of HR4049
We present the Spectral Energy Distribution of HR 4049 based on literature
data and new continuum measurements at 850 micron. The SED shows variable
absorption in the UV, and a large IR excess, both caused by circumstellar dust.
The shape of the IR excess from 1 micron all the way down to 850 micron can be
nearly perfectly fitted with a single blackbody function at T ~ 1150 K or
alternatively with a sum of blackbodies in a narrow temperature range. The
energy emitted in this IR continuum radiation is about one-third of the stellar
luminosity. We show that this blackbody radiation must be due to the presence
of a circumbinary disk with a large height. This disk must also be gas-rich, in
agreement with the observations of molecular bands in the ISO-SWS spectrum. We
present two possible scenario's for explaining the shape and the intensity of
the IR excess. The first scenario involves large grains (a >1 mm) that each
radiate like a blackbody. The second scenario argues that the blackbody
radiation is due to a very optically thick circumbinary disk. We investigate if
such a disk would indeed produce blackbody radiation by presenting results from
radiative transfer calculations. We further quantify the properties of such a
disk and its stability in the framework of (hydro)dynamics, grain settling,
radiation pressure and grain drift. The virtues and shortcomings of both models
for the origin of the IR blackbody are discussed by contrasting them with other
observations and assessing them in the framework of (binary) (post-)AGB
evolution.Comment: 16 pages, 12 figures, accepted for publication in A&
Variability and nature of the binary in the Red Rectangle Nebula
We present new observations of the central binary inside the Red Rectangle
nebula. The detection of zinc in the optical spectrum confirms that the
peculiar photospheric abundances are due to accretion of circumstellar gas.
Grey brightness variations with the orbital period are observed. They are
interpreted as being due to the variation of the scattering angle with orbital
phase. The small orbital separation of the system is not compatible with
previous normal evolution of the primary on the AGB. We point out the
similarity of the orbital history of this and other similar systems with those
of some close Barium stars and suggest that the nonzero eccentricity of the
orbit is the result of tidal interaction with the circumbinary disk.Comment: 4 pages, 3 figures, A&A Letters accepte
The Hypervelocity Star SDSS J090745.0+024507 is a Short-Period Variable
We present high-precision photometry of the hypervelocity star SDSS
J090745.0+024507 (HVS), which has a Galactic rest-frame radial velocity of
v=709 km/s, and so has likely been ejected from the supermassive black hole in
the Galactic center. Our data were obtained on two nights using the MMT 6.5m
telescope, and is supplemented by lower precision photometry obtained on four
nights using the FLWO 1.2m telescope. The high-precision photometry indicates
that the HVS is a short-period, low-amplitude variable, with period P=0.2-2
days and amplitude A = 2-10%. Together with the known effective temperature of
T_eff ~ 10,500 K (spectral type B9), this variability implies that the HVS is a
member of the class of slowly pulsating B-type main sequence stars, thus
resolving the previously-reported two-fold degeneracy in the luminosity and
distance of the star. The HVS has a heliocentric distance of 71 kpc, and an age
of ~0.35 Gyr. The time of ejection from the center of the Galaxy is < 100 Myr,
and thus the existence of the OS constitutes observational evidence of a
population of young stars in the proximity of the central supermassive black
hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that
was tidally disrupted by the central black hole; we discuss constraints on the
properties of the companion's orbit.Comment: ApJL, submitted, 4 pages, 4 figure
Can pulsational instabilities impact a massive star's rotational evolution?
We investigate whether angular momentum transport due to unstable pulsation
modes can play a significant role in the rotational evolution of massive stars.
We find that these modes can redistribute appreciable angular momentum, and
moreover trigger shear-instability mixing in the molecular weight gradient zone
adjacent to stellar cores, with significant evolutionary impact.Comment: 6 pages, to appear in Proc. IAU Symposium 250: Massive Stars as
Cosmic Engine
Complex asteroseismology of the Slowly Pulsating B-type star HD74560
We present the results of complex seismic modelling of the Slowly Pulsating
B-type star HD74560. The star pulsates in five frequencies detected in
photometric observations. For all these frequencies, we identify the mode
degree, . For two of them, found also in spectroscopic data, we are able
to derived the empirical values of the complex nonadiabatic parameter . We
test effects of chemical composition and opacity data. Our results show that
the properties of seismic models of SPB stars differ significantly from those
of the more massive Cephei stars.Comment: 4 pages, 2 figures, conference proceedings, to appear in ASS
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