1,653 research outputs found
Chemical tracers of episodic accretion in low-mass protostars
Aims: Accretion rates in low-mass protostars can be highly variable in time.
Each accretion burst is accompanied by a temporary increase in luminosity,
heating up the circumstellar envelope and altering the chemical composition of
the gas and dust. This paper aims to study such chemical effects and discusses
the feasibility of using molecular spectroscopy as a tracer of episodic
accretion rates and timescales.
Methods: We simulate a strong accretion burst in a diverse sample of 25
spherical envelope models by increasing the luminosity to 100 times the
observed value. Using a comprehensive gas-grain network, we follow the chemical
evolution during the burst and for up to 10^5 yr after the system returns to
quiescence. The resulting abundance profiles are fed into a line radiative
transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at
a series of time steps. We compare these spectra to observations taken from the
literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the
Herschel Space Observatory.
Results: The bursts are strong enough to evaporate CO throughout the
envelope, which in turn enhances the abundance of HCO+ and reduces that of
N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+
and N2H+ to return to normal. The chemical effects of the burst remain visible
in the rotational spectra for as long as 10^5 yr after the burst has ended,
highlighting the importance of considering luminosity variations when analyzing
molecular line observations in protostars. The spherical models are currently
not accurate enough to derive robust timescales from single-dish observations.
As follow-up work, we suggest that the models be calibrated against spatially
resolved observations in order to identify the best tracers to be used for
statistically significant source samples.Comment: Accepted by A&A; 12 pages, 7 figure
Hybrid Lateral Transshipments in a Multi-Location Inventory System
In managing networks of stock holding locations, two approaches to the pooling of inventory have been proposed. Reactive transshipm nts respond to stockouts at a location by moving inventory from elsewhere within the network, while proactive redistribution of stock seeks to minimise the chance of future shocks. This paper is the first to propose a hybrid approach in which transshipments are viewed as an opportunity for stock redistribution. We adopt a quasi-myopic approach to the development of a strongly performing hybrid transshipment policy. Numerical studies which utilise dynamic programming and simulation testify to the benefits of using transshipments proactively. In comparison to a purely reactive approach to transshipment, service levels are improved while a reduction in safety stock levels is achieved. The aggregate costs incurred in managing the system are significantly reduced, especially so for large networks facing high levels of demand.
Wage Mobility in Europe. A Comparative Analysis Using restricted Multinomial Logit Regression
In this paper, we investigate cross-country differences in wage mobility in Europe using the European Community Household Panel. The paper is particularly focused on examining the impact of economic conditions, welfare state regimes and employment regulation on wage mobility. We apply a log-linear approach that is very much similar to a restricted multinomial logit model and much more flexible than the standard probit approach. It appears that regime, economic conditions and employment regulation explain a substantial part of the cross-country variation. The findings also confirm the existence of an inverse U-shape pattern of wage mobility, showing a great deal of low and high-wage persistence in all countries.wages; wage mobility; wage dynamics; multinomial logit regression; loglinear models; welfare states
Does the fluid elasticity influence the dispersion in packed beds?
Reasons are given why the axial dispersion in a gas flowing through a packed bed may be influenced by the elasticity - or compressibility - of the fluid. To support this hypothesis, experiments have been done in a packed column at pressures from 0.13 to 2.0 MPa. The elasticity E of a gas is proportional to the pressure P and the compressibility to 1/P. The axial dispersion coefficients as determined were found to be a function of the pressure in the packed bed in the turbulent flow region of 3 < Rep < 150 if the Bodenstein number is plotted as a function of the particle Reynolds number. This is shown to be an artifact. The pressure influence is eliminated, if Bom, ax is plotted versus the ratio of the kinetic forces over the elastic forces u2/E. Regrettably, Bom, ax seems to be independent of u2/E. For the moment we only can conclude that Bom, ax in the turbulent region is a unique function of the velocity of the gas which flows through the packed bed. Although the fact that a constant Bo value is obtained when plotted against u2/E, the experimental results are so intriguing we wanted to make them public already now. The experimental work proceeds
In search of relevance: The changing contract between science and society
This paper presents a framework to study the historical development of the relationship between science and society. We elaborate this relationship as a contract that specifies the mission of scientific research, the rationales for public support for science, and the conditions under which scientists work. These three structural elements will always be part of the contract, but their specific content can vary. The credibility cycle, as a model for scientific practice, helps to describe and understand the consequences of a changing contract for the work of individual scientists. A brief case study of chemistry in the Netherlands demonstrates the usefulness of the framework. We show how concepts of relevance have changed since 1975 and how this affects the practice of academic chemistry.relevance, contract, credibility cycle, chemistry
Who benefits from a job change: The dwarfs or the giants?
Empirical studies have shown that voluntary job-to-job changes have a positive effect on wage mobility. In this paper, we suggest that the impact of a job change on wage growth depends on the position in the wage distribution. Using panel data from the UK and Germany, we investigate the effect of employer changes and within-firm job changes on year-to-year wage mobility. We show that a change of employer results into a wage increase for the low-paid workers but not for the high-paid workers. Within-firm job changes produce, on average, moderate wage gains for the low-paid workers in the UK, but have no effect in Germany. Results on 3-year wage mobility are shown to be very similar to the results on year-to-year wage growth.low pay ; high pay ; job mobility ; wage mobility
Molecule sublimation as a tracer of protostellar accretion: Evidence for accretion bursts from high angular resolution C18O images
The accretion histories of embedded protostars are an integral part of
descriptions of their physical and chemical evolution. In particular, are the
accretion rates smoothly declining from the earlier toward later stages or in
fact characterized by variations such as intermittent bursts? We aim to
characterize the impact of possible accretion variations in a sample of
embedded protostars by measuring the size of the inner regions of their
envelopes where CO is sublimated and relate those to their temperature profiles
dictated by their current luminosities. Using observations from the
Submillimeter Array we measure the extents of the emission from the C18O
isotopologue toward 16 deeply embedded protostars. We compare these
measurements to the predicted extent of the emission given the current
luminosities of the sources through dust and line radiative transfer
calculations. Eight out of sixteen sources show more extended C18O emission
than predicted by the models. The modeling shows that the likely culprit for
these signatures is sublimation due to increases in luminosities of the sources
by about a factor five or more during the recent 10,000 years - the time it
takes for CO to freeze-out again on dust grains. For four of those sources the
increase would have had to have been a factor 10 or more. The compact emission
seen toward the other half of the sample suggests that C18O only sublimates
when the temperature exceeds 30 K - as one would expect if CO is mixed with H2O
in the grain ice-mantles. The small-number statistics from this survey suggest
that protostars undergo significant bursts about once every 20,000 years. This
also illustrates the importance of taking the physical evolutionary histories
into account for descriptions of the chemical structures of embedded
protostars.Comment: Accepted by A&A; 11 pages, 5 figure
Hot water in the inner 100 AU of the Class 0 protostar NGC1333 IRAS2A
Evaporation of water ice above 100 K in the inner few 100 AU of low-mass
embedded protostars (the so-called hot core) should produce quiescent water
vapor abundances of ~10^-4 relative to H2. Observational evidence so far points
at abundances of only a few 10^-6. However, these values are based on spherical
models, which are known from interferometric studies to be inaccurate on the
relevant spatial scales. Are hot cores really that much drier than expected, or
are the low abundances an artifact of the inaccurate physical models? We
present deep velocity-resolved Herschel-HIFI spectra of the 3(12)-3(03) lines
of H2-16O and H2-18O (1097 GHz, Eup/k = 249 K) in the low-mass Class 0
protostar NGC1333 IRAS2A. A spherical radiative transfer model with a power-law
density profile is unable to reproduce both the HIFI data and existing
interferometric data on the H2-18O 3(13)-2(20) line (203 GHz, Eup/k = 204 K).
Instead, the HIFI spectra likely show optically thick emission from a hot core
with a radius of about 100 AU. The mass of the hot core is estimated from the
C18O J=9-8 and 10-9 lines. We derive a lower limit to the hot water abundance
of 2x10^-5, consistent with the theoretical predictions of ~10^-4. The revised
HDO/H2O abundance ratio is 1x10^-3, an order of magnitude lower than previously
estimated.Comment: Accepted by ApJ; 12 pages in emulateapj format; 7 figure
Production planning and control of closed-loop supply chains
More and more supply chains emerge that include a return flow of materials. Many original equipment manufacturers are nowadays engaged in the
remanufacturing business. In many process industries, production defectives and by-products are reworked. These closed-loop supply chains deserve special attention. Production planning and control in such hybrid systems is a real challenge, especially due to increased uncertainties. Even companies that are engaged in remanufacturing operations only, face more complicated planning situations than traditional manufacturing companies.
We point out the main complicating characteristics in closed-loop systems with
both remanufacturing and rework, and indicated the need for new or modified/extended
production planning and control approaches. An overview of the existing scientific
contributions is given. It appears that we only stand at the beginning of this line of research, and that many more contributions are needed and expected in the future
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