3,211 research outputs found
The population of white dwarf binaries with hot subdwarf companions
Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their
entire hydrogen envelope in the red-giant phase. Since a high fraction of those
stars are in close binary systems, common envelope ejection is an important
formation channel. We identified a total population of 51 close sdB+WD binaries
based on time-resolved spectroscopy and multi-band photometry, derive the WD
mass distribution and constrain the future evolution of these systems. Most WDs
in those binaries have masses significantly below the average mass of single
WDs and a high fraction of them might therefore have helium cores. We found 12
systems that will merge in less than a Hubble time and evolve to become either
massive C/O WDs, AM\,CVn systems, RCrB stars or even explode as supernovae type
Ia.Comment: 5 pages, 2 figures, to appear in the proceedings of the 19th European
White Dwarf Workshop, ASP Conf. Se
Study of Viscoelastic Effect on the Frequency Shift of Microcantilever Chemical Sensors (proceedings)
Microcantilevers coated with a chemically sensitive layer are increasingly being used in chemical detection systems. The sensitive coating, often a polymer, absorbs specific molecules, which can be detected by monitoring the shift in the mechanical resonant frequency. Usually, the frequency shift resulting from molecular absorption is interpreted as a mass loading effect. However, mass loading is not the only effect that has an impact on the frequency shift; the viscoelastic properties of the sensitive coating are also affected by the sorption process. Sorption-induced modulus changes are typically difficult to characterize. However, it is known that the sorption of analyte molecules in a polymer coating results in the plasticization of the coating. In most cases, the polymer becomes more rubbery with increasing concentration of analyte molecules, i.e., the coating becomes softer with increasing loss modulus while the storage modulus decreases. Using a new analytical model developed for the resonant frequency expression of a hybrid microcantilever (elastic base and viscoelastic layer), the effects of the modification of the storage and loss moduli of the sensitive layer on the resonant frequency are examined. The main conclusion of this analytical study is that, even if the sensitive coating moduli are small compared to the base cantilever\u27s Young\u27s modulus, the effect of the change in the viscoelastic coating properties could contribute significantly to the overall frequency shift (8-23% in the simulations depending on the coating thickness, with even higher contributions for other sets of problem parameters)
Effect of Viscoelasticity on Quality Factor of Microcantilever Chemical Sensors: Optimal Coating Thickness for Minimum Limit of Detection
Microcantilevers with polymer coatings hold great promise as resonant chemical sensors. It is known that the coated cantilever sensitivity increases with coating thickness; however, the drawback of increasing the coating thickness is the increase of the frequency noise and thus the deterioration of the sensor\u27s limit of detection. In this paper, an analytical expression for the viscoelastic losses in the coating, hence the quality factor is established and is used to explain the observed increase of the frequency noise with the polymer thickness. This result is then used to demonstrate that an optimum coating thickness exists that minimise the limit of detectio
Distribution of graph-distances in Boltzmann ensembles of RNA secondary structures
Large RNA molecules often carry multiple functional domains whose spatial
arrangement is an important determinant of their function. Pre-mRNA splicing,
furthermore, relies on the spatial proximity of the splice junctions that can
be separated by very long introns. Similar effects appear in the processing of
RNA virus genomes. Albeit a crude measure, the distribution of spatial
distances in thermodynamic equilibrium therefore provides useful information on
the overall shape of the molecule can provide insights into the interplay of
its functional domains. Spatial distance can be approximated by the
graph-distance in RNA secondary structure. We show here that the equilibrium
distribution of graph-distances between arbitrary nucleotides can be computed
in polynomial time by means of dynamic programming. A naive implementation
would yield recursions with a very high time complexity of O(n^11). Although we
were able to reduce this to O(n^6) for many practical applications a further
reduction seems difficult. We conclude, therefore, that sampling approaches,
which are much easier to implement, are also theoretically favorable for most
real-life applications, in particular since these primarily concern long-range
interactions in very large RNA molecules.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
Dual Magnetic Separator for TRIP
The TRIP facility, under construction at KVI, requires the production
and separation of short-lived and rare isotopes. Direct reactions,
fragmentation and fusion-evaporation reactions in normal and inverse kinematics
are foreseen to produce nuclides of interest with a variety of heavy-ion beams
from the superconducting cyclotron AGOR. For this purpose, we have designed,
constructed and commissioned a versatile magnetic separator that allows
efficient injection into an ion catcher, i.e., gas-filled stopper/cooler or
thermal ionizer, from which a low energy radioactive beam will be extracted.
The separator performance was tested with the production and clean separation
of Na ions, where a beam purity of 99.5% could be achieved. For
fusion-evaporation products, some of the features of its operation as a
gas-filled recoil separator were tested.Comment: accepted by Nucl.Instr. Meth., final versio
Searching for O in the SMC:Constraints on Oxygen Chemistry at Low Metallicities
We present a 39 h integration with the Odin satellite on the ground-state
118.75 GHz line of O2 towards the region of strongest molecular emission in the
Small Magellanic Cloud. Our 3sigma upper limit to the O2 integrated intensity
of <0.049 K km/s in a 9'(160 pc) diameter beam corresponds to an upper limit on
the O2/H2 abundance ratio of <1.3E-6. Although a factor of 20 above the best
limit on the O2 abundance obtained for a Galactic source, our result has
interesting implications for understanding oxygen chemistry at sub-solar metal
abundances. We compare our abundance limit to a variety of astrochemical models
and find that, at low metallicities, the low O2 abundance is most likely
produced by the effects of photo-dissociation on molecular cloud structure.
Freeze-out of molecules onto dust grains may also be consistent with the
observed abundance limit, although such models have not yet been run at
sub-solar initial metallicities.Comment: 4 pages, accepted to A&A Letter
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
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