304 research outputs found
Asteroseismic test of rotational mixing in low-mass white dwarfs
We exploit the recent discovery of pulsations in mixed-atmosphere (He/H),
extremely low-mass white dwarf precursors (ELM proto-WDs) to test the
proposition that rotational mixing is a fundamental process in the formation
and evolution of low-mass helium core white dwarfs. Rotational mixing has been
shown to be a mechanism able to compete efficiently against gravitational
settling, thus accounting naturally for the presence of He, as well as traces
of metals such as Mg and Ca, typically found in the atmospheres of ELM
proto-WDs. Here we investigate whether rotational mixing can maintain a
sufficient amount of He in the deeper driving region of the star, such that it
can fuel, through HeII-HeIII ionization, the observed pulsations in this type
of stars. Using state-of-the-art evolutionary models computed with MESA, we
show that rotational mixing can indeed explain qualitatively the very existence
and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs.
Moreover, such objects are very likely to pulsate again during their final WD
cooling phase.Comment: accepted for publication in A&A Letter
It has to be cool: on supergiant progenitors of binary black hole mergers from common-envelope evolution
Common-envelope (CE) evolution in massive binary systems is thought to be one
of the most promising channels for the formation of compact binary mergers. In
the case of merging binary black holes (BBHs), the essential CE phase takes
place at a stage when the first BH is already formed and the companion star
expands as a supergiant. We study which BH binaries with supergiant companions
will evolve through and potentially survive a CE phase. To this end, we compute
envelope binding energies from detailed massive stellar models at different
evolutionary stages and metallicities. We make multiple physically extreme
choices of assumptions that favor easier CE ejection as well as account for
recent advancements in mass transfer stability criteria. We find that even with
the most optimistic assumptions, a successful CE ejection in BH (and also NS)
binaries is only possible if the donor is a massive convective-envelope giant,
a red supergiant (RSG). In other words, pre-CE progenitors of BBH mergers are
BH binaries with RSG companions. We find that due to its influence on the
radial expansion of massive giants, metallicity has an indirect but a very
strong effect on the envelope structure and binding energies of RSGs. Our
results suggest that merger rates from population synthesis models could be
severely overestimated, especially at low metallicity. Additionally, the lack
of observed RSGs with luminosities above log() = 5.6-5.8,
corresponding to stars with , puts into question the
viability of the CE channel for the formation of the most massive BBH mergers.
Either such RSGs elude detection due to very short lifetimes, or they do not
exist and the CE channel can only produce BBH systems with total mass . We discuss an alternative CE scenario, in which a partial envelope
ejection is followed by a phase of possibly long and stable mass transfer.Comment: 20 pages + App., accepted for publication in A&A. For fits, see: https://ftp.science.ru.nl/astro/jklencki
Poultry manure gasification and its energy yield
The disposal of poultry manure is one of the main problems of the poultry sector. The results of some tests developed in a gasification pilot plant are presented in this paper. The aim was to preliminarily analyze the energy generation from poultry manure treated by gasification. The level of energy yields and the quantification of the produced syngas are also reported. The good results suggests an possible application at full scale
Estudo dos agentes causais de diarreia em crianças menores de 5 anos no Hospital Geral do Bengo
A diarreia pode ser causada por vÃrus, parasitas e bactérias e constitui uma das principais causas de doença e morte em crianças menores de cinco anos em Angola. O presente estude assume como principal objectivo identificar os agentes patogénicos causadores de diarreia em crianças admitidas no Hospital Geral do Bengo
Estudo dos agentes causais de diarreia em crianças menores de 5 anos no Hospital Geral do Bengo
A diarreia pode ser causada por vÃrus, parasitas e bactérias e constitui uma das principais causas de doença e morte em crianças menores de cinco anos em Angola. O presente estude assume como principal objectivo identificar os agentes patogénicos causadores de diarreia em crianças admitidas no Hospital Geral do Bengo
A Dense Companion to the Short-Period Millisecond Pulsar Binary PSR J0636+5128
PSR J0636+5128 is a millisecond pulsar in one of the most compact pulsar
binaries known, with a 96\,min orbital period. The pulsar mass function
suggests a very low-mass companion, similar to that seen in so-called "black
widow" binaries. Unlike in most of those, however, no radio eclipses by
material driven off from the companion were seen leading to the possibility
that the companion was a degenerate remnant of a carbon-oxygen white dwarf. We
report the discovery of the optical counterpart of its companion in images
taken with the Gemini North and Keck~I telescopes. The companion varies between
and on the 96\,min orbital period of the binary, caused by
irradiation from the pulsar's energetic wind. We modeled the multi-color
lightcurve using parallax constraints from pulsar timing and determine a
companion mass of , a radius of
, and a mean density of , all for an assumed neutron star mass of . This
makes the companion to PSR J0636+5128 one of the densest of the "black widow"
systems. Modeling suggests that the composition is not predominantly hydrogen,
perhaps due to an origin in an ultra-compact X-ray binary.Comment: 4 figures, 1 table. Submitted to ApJ on June 29, 2018. Accepted on
July 20, 201
Causes of industrial protein a column degradation, explored using Raman spectroscopy.
Monoclonal antibodies (mAbs) are used extensively as biotherapeutics for chronic and acute conditions. Production of mAbs is lengthy and expensive, with protein A affinity capture the most costly step, due both to the nature of the resin and its marked reduction in binding capacity with repeated use. Our previous studies using in situ ATR-FTIR spectroscopy indicated that loss in protein A binding capacity is not the result of leaching or degradation of protein A ligand, suggesting fouling is the principal cause. Here we explore binding behavior and resin capacity loss using Raman spectroscopy. Our data reveal a distinct Raman spectral fingerprint for mAb bound to the protein A ligand of MabSelect SuRe. The results show that the drop in static binding capacity (SBC) previously observed for used protein A resin is discernible by Raman spectroscopy in combination with partial least-squares regression. The SBC is lowest (35.76 mg mL-1) for used inlet resin compared to used outlet (40.17 mg mL-1) and unused resin samples (70.35 mg mL-1). Depth profiling by Raman spectroscopy indicates that at below saturating concentrations (∼18 mg mL-1), binding of mAb is not homogeneous through used resin beads with protein binding preferentially to the outer regions of the bead, in contrast to fully homogeneous distribution through unused control MabSelect SuRe resin beads. Analysis of the Raman spectra indicates that one foulant is irreversibly bound mAb. The presence of irreversibly bound mAb and host cell proteins was confirmed by mass spectrometric analysis of used resin beads
The Peculiar Phase Structure of Random Graph Bisection
The mincut graph bisection problem involves partitioning the n vertices of a
graph into disjoint subsets, each containing exactly n/2 vertices, while
minimizing the number of "cut" edges with an endpoint in each subset. When
considered over sparse random graphs, the phase structure of the graph
bisection problem displays certain familiar properties, but also some
surprises. It is known that when the mean degree is below the critical value of
2 log 2, the cutsize is zero with high probability. We study how the minimum
cutsize increases with mean degree above this critical threshold, finding a new
analytical upper bound that improves considerably upon previous bounds.
Combined with recent results on expander graphs, our bound suggests the unusual
scenario that random graph bisection is replica symmetric up to and beyond the
critical threshold, with a replica symmetry breaking transition possibly taking
place above the threshold. An intriguing algorithmic consequence is that
although the problem is NP-hard, we can find near-optimal cutsizes (whose ratio
to the optimal value approaches 1 asymptotically) in polynomial time for
typical instances near the phase transition.Comment: substantially revised section 2, changed figures 3, 4 and 6, made
minor stylistic changes and added reference
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