Mass-luminosity relation and pulsational properties of Wolf-Rayet stars

Evolution of Population I stars with initial masses from 70M_\odot to 130M_\odot is considered under various assumptions on the mass loss rate \dot M. The mass-luminosity relation of W-R stars is shown to be most sensitive to the mass loss rate during the helium burning phase \dot M_{3\alpha}. Together with the mass-luminosity relation obtained for all evolutionary sequences several more exact relations are determined for the constant ratio f_{3\alpha}=\dot M/\dot M_{3\alpha} with 0.5 \le f_{3\alpha} \le 3. Evolutionary models of W-R stars were used as initial conditions in hydrodynamic computations of radial nonlinear stellar oscillations. The oscillation amplitude is larger in W-R stars with smaller initial mass or with lower mass loss rate due to higher surface abundances of carbon and oxygen. In the evolving W-R star the oscillation amplitude decreases with decreasing stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects allow us to calculate the integral of the mechanical work W done over the pulsation cycle in each mass zone of the hydrodynamical model. The only positive maximum on the radial dependence of W is in the layers with temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot are shown to be also excited by the kappa-mechanism but the instability driving zone is at the bottom of the envelope and pulsation motions exist in the form of nonlinear running waves propagating outward from the inner layers of the envelope.Comment: 15 pages, 10 figures, submitted to Astronomy Letter

Continuous-flow IRMS technique for determining the 17O excess of CO2 using complete oxygen isotope exchange with cerium oxide

This paper presents an analytical system for analysis of all single substituted isotopologues (¹²C¹⁶O¹⁷O, ¹²C¹⁶O¹⁸O, ¹³C¹⁶O¹⁶O) in nanomolar quantities of CO₂ extracted from stratospheric air samples. CO₂ is separated from bulk air by gas chromatography and CO₂ isotope ratio measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio mass spectrometry (IRMS). The ¹⁷O excess (Δ¹⁷O) is derived from isotope measurements on two different CO₂ aliquots: unmodified CO₂ and CO₂ after complete oxygen isotope exchange with cerium oxide (CeO₂) at 700 °C. Thus, a single measurement of Δ¹⁷O requires two injections of 1 mL of air with a CO₂ mole fraction of 390 μmol mol⁻¹ at 293 K and 1 bar pressure (corresponding to 16 nmol CO₂ each). The required sample size (including flushing) is 2.7 mL of air. A single analysis (one pair of injections) takes 15 minutes. The analytical system is fully automated for unattended measurements over several days. The standard deviation of the ¹⁷O excess analysis is 1.7&permil;. Multiple measurements on an air sample reduce the measurement uncertainty, as expected for the statistical standard error. Thus, the uncertainty for a group of 10 measurements is 0.58&permil; for &Delta; ¹⁷O in 2.5 h of analysis. 100 repeat analyses of one air sample decrease the standard error to 0.20&permil;. The instrument performance was demonstrated by measuring CO₂ on stratospheric air samples obtained during the EU project RECONCILE with the high-altitude aircraft Geophysica. The precision for RECONCILE data is 0.03&permil; (1&sigma;) for δ¹³C, 0.07&permil; (1&sigma;) for δ¹⁸O and 0.55&permil; (1&sigma;) for &delta;¹⁷O for a sample of 10 measurements. This is sufficient to examine stratospheric enrichments, which at altitude 33 km go up to 12&permil; for &delta;¹⁷O and up to 8&permil; for δ¹⁸O with respect to tropospheric CO₂ : &delta;¹⁷O ~ 21&permil; Vienna Standard Mean Ocean Water (VSMOW), δ¹⁸O ~ 41&permil; VSMOW (Lämmerzahl et al., 2002). The samples measured with our analytical technique agree with available data for stratospheric CO₂

OH-selected AGB and post-AGB objects I.Infrared and maser properties

Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz OH line, new light is cast on the infrared properties of evolved stars on the TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS colours, largely fail to distinguish early post-AGB stages. A two-colour diagram from narrower-band MSX flux densities, with bimodal distributions, provides a better tool to do the latter. Four mutually consistent selection criteria for OH-masing red PPNe are given, as well as two for early post-AGB masers and one for all post--AGB masers, including the earliest ones. All these criteria miss a group of blue, high-outflow post-AGB sources with 60-mum excess; these will be discussed in detail in Paper II. The majority of post-AGB sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly low outflow velocities, although the fractions of single peaks and irregular spectra may vary with age and mass. The OH flux density shows a fairly regular relation with the stellar flux and the envelope optical depth, with the maser efficiency increasing with IRAS colour R21. The OH flux density is linearly correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted

The footprint of cometary dust analogues: II. Morphology as a tracer of tensile strength and application to dust collection by the Rosetta spacecraft

The structure of cometary dust is a tracer of growth processes in the formation of planetesimals. Instrumentation on board the Rosetta mission to comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in situ. However, these deposits are a product of a collision within the instrument. We conducted laboratory experiments with cometary dust analogues, simulating the collection process by Rosetta instruments (specifically COSIMA, MIDAS). In Paper I we reported that velocity is a key driver in determining the appearance of deposits. Here in Paper II we use materials with different monomer sizes, and study the effect of tensile strength on the appearance of deposits. We find that mass transfer efficiency increases from $\sim$1 up to $\sim$10% with increasing monomer diameter from 0.3 $\mu$m to 1.5 $\mu$m (i.e. tensile strength decreasing from $\sim$12 to $\sim$3 kPa), and velocities increasing from 0.5 to 6 m/s. Also, the relative abundance of small fragments after impact is higher for material with higher tensile strength. The degeneracy between the effects of velocity and material strength may be lifted by performing a closer study of the deposits. This experimental method makes it possible to estimate the mass transfer efficiency in the COSIMA instrument. Extrapolating these results implies that more than half of the dust collected during the Rosetta mission has not been imaged. We analysed two COSIMA targets containing deposits from single collisions. The collision that occurred closest to perihelion passage led to more small fragments on the target.Comment: 13 pages, 11 figures, accepted for publication in MNRA

The footprint of cometary dust analogs: I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data

Cometary dust provides a unique window on dust growth mechanisms during the onset of planet formation. Measurements by the Rosetta spacecraft show that the dust in the coma of comet 67P/Churyumov-Gerasimenko has a granular structure at size scales from sub-um up to several hundreds of um, indicating hierarchical growth took place across these size scales. However, these dust particles may have been modified during their collection by the spacecraft instruments. Here we present the results of laboratory experiments that simulate the impact of dust on the collection surfaces of COSIMA and MIDAS, instruments onboard the Rosetta spacecraft. We map the size and structure of the footprints left by the dust particles as a function of their initial size (up to several hundred um) and velocity (up to 6 m/s). We find that in most collisions, only part of the dust particle is left on the target; velocity is the main driver of the appearance of these deposits. A boundary between sticking/bouncing and fragmentation as an outcome of the particle-target collision is found at v ~ 2 m/s. For velocities below this value, particles either stick and leave a single deposit on the target plate, or bounce, leaving a shallow footprint of monomers. At velocities > 2 m/s and sizes > 80 um, particles fragment upon collision, transferring up to 50 per cent of their mass in a rubble-pile-like deposit on the target plate. The amount of mass transferred increases with the impact velocity. The morphologies of the deposits are qualitatively similar to those found by the COSIMA instrument.Comment: 14 pages, 12 figures, accepted for publication in MNRA

A Flood disaster in the Netherlands:a trade-off between paying for protection and undertaking action?

Effectively, one of the principal challenges for the Netherlands in the face of climate change is: How should we manage flood risk so that regional needs are adequately captured? Recent studies however point at the existing gap between risk perceptions of the public and the experts, which seems to become an apple of discord in the Dutch flood management. While public authorities are not willing to emphasize the growing threats that flooding are posing to the country, the issue is pressing hard to be addressed. In this paper we offer a summary of a recent study on flood risk perceptions in the Netherlands that precludes the findings regarding the willingness to pay for (public) measures contrasted to taking (individual) protective action. Clearly, arising debates around the issue of public-private (shared) responsibility in addressing flood risk add more fuel to the fire, where views seem to be polarized. We attempt thus to link this variation in behavioural intention to the willingness to pay for improvements in flood safety and explore the nature of this relationship. We observe in addition that regional differences do matter, which has direct implications for policy and practice. In particular, we suggest that measures tailored to serve local needs and attuned to local perceptions should prove to provide most feasible solutions

Proton Motive Force-Dependent Hoechst 33342 Transport by the ABC Transporter LmrA of Lactococcus lactis

The fluorescent compound Hoechst 33342 is a substrate for many multidrug resistance (MDR) transporters and is widely used to characterize their transport activity. We have constructed mutants of the adenosine triphosphate (ATP) binding cassette (ABC)-type MDR transporter LmrA of Lactococcus lactis that are defective in ATP hydrolysis. These mutants and wild-type LmrA exhibited an atypical behavior in the Hoechst 33342 transport assay. In membrane vesicles, Hoechst 33342 transport was shown to be independent of the ATPase activity of LmrA, and it was not inhibited by orthovanadate but sensitive to uncouplers that collapse the proton gradient and to N,N'-dicyclohexylcarbodiimide, an inhibitor of the F0F1-ATPase. In contrast, transport of Hoechst 33342 by the homologous, heterodimeric MDR transporter LmrCD showed a normal ATP dependence and was insensitive to uncouplers of the proton gradient. With intact cells, expression of LmrA resulted in an increased rate of Hoechst 33342 influx while LmrCD caused a decrease in the rate of Hoechst 33342 influx. Cellular toxicity assays using a triple knockout strain, i.e., L. lactis ΔlmrA ΔlmrCD, demonstrate that expression of LmrCD protects cells against the growth inhibitory effects of Hoechst 33342, while in the presence of LmrA, cells are more susceptible to Hoechst 33342. Our data demonstrate that the LmrA-mediated Hoechst 33342 transport in membrane vesicles is influenced by the transmembrane pH gradient due to a pH-dependent partitioning of Hoechst 33342 into the membrane.

An air shower array for LOFAR: LORA

LOFAR is a new form of radio telescope which can detect radio emission from air showers induced by very high-energy cosmic rays. It can also look for radio emission from particle cascades on the Moon induced by ultra high-energy cosmic rays or neutrinos. To complement the radio detection, we are setting up a small particle detector array LORA (LOfar Radboud Air shower array) within an area of $\sim 300$ m diameter in the LOFAR core. It will help in triggering and confirming the radio detection of air showers with the LOFAR antennas. In this paper, we present a short overview about LORA and discuss its current status.Comment: 10 pages (using article.cls), 6 figures, accepted for the proceedings of 22nd European Cosmic Ray Symposium, 3-6 August 2010, Finlan