8,970 research outputs found
OH-selected AGB and post-AGB stellar objects II.Blue versus red evolution off the AGB
Using objects found in a systematic survey of the galactic Plane in the
1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution,
a red and a blue. We argue that the red and the blue groups separate by initial
mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy
distributions, outflow velocities and scaleheight. The higher-mass (blue)
objects may have earlier AGB termination. The lower-mass (red) objects undergo
very sudden reddening for IRAS colour R21\sim1.2; these sources must all
undergo a very similar process at AGB termination. The transition colour
corresponds to average initial masses of 1.7Msun. A combined IRAS-MSX colour
proves a very sensitive tool to distinguish lower-mass, early post-AGB objects
from sources still on the AGB and also to distinguish more evolved post-AGB
objects from star-forming regions. The high-mass blue objects are the likely
precursors of bipolar planetary nebulae, whereas the low-mass red objects will
evolve into elliptical planetary nebulae.Comment: 12 pages, LaTex, 7 figures (1 colour), AJ (accepted
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 (<sup>12</sup>C<sup>16</sup>O<sup>17</sup>O,
<sup>12</sup>C<sup>16</sup>O<sup>18</sup>O, <sup>13</sup>C<sup>16</sup>O<sup>16</sup>O) in nanomolar quantities
of CO<sub>2</sub> extracted from stratospheric air samples. CO<sub>2</sub> is
separated from bulk air by gas chromatography and CO<sub>2</sub> isotope ratio
measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio
mass spectrometry (IRMS). The <sup>17</sup>O excess (Δ<sup>17</sup>O) is
derived from isotope measurements on two different CO<sub>2</sub> aliquots:
unmodified CO<sub>2</sub> and CO<sub>2</sub> after complete oxygen isotope exchange with
cerium oxide (CeO<sub>2</sub>) at 700 °C. Thus, a single measurement of
Δ<sup>17</sup>O requires two injections of 1 mL of air with a CO<sub>2</sub>
mole fraction of 390 μmol mol<sup>−1</sup> at 293 K and 1 bar pressure
(corresponding to 16 nmol CO<sub>2</sub> 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
<sup>17</sup>O excess analysis is 1.7‰. 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‰ for Δ
<sup>17</sup>O in 2.5 h of analysis. 100 repeat analyses of one air sample
decrease the standard error to 0.20‰. The instrument
performance was demonstrated by measuring CO<sub>2</sub> on stratospheric air
samples obtained during the EU project RECONCILE with the high-altitude
aircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for δ<sup>13</sup>C, 0.07‰ (1σ) for δ<sup>18</sup>O and 0.55‰ (1σ) for δ<sup>17</sup>O for a sample of 10
measurements. This is sufficient to examine stratospheric enrichments, which
at altitude 33 km go up to 12‰ for δ<sup>17</sup>O
and up to 8‰ for δ<sup>18</sup>O with respect to
tropospheric CO<sub>2</sub> : δ<sup>17</sup>O ~
21‰ Vienna Standard Mean Ocean Water (VSMOW), δ<sup>18</sup>O ~
41‰ VSMOW (Lämmerzahl et al., 2002). The samples
measured with our analytical technique agree with available data for
stratospheric CO<sub>2</sub>
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
Surface defreezing of glasses
A glass surface may still flow below the bulk glass transition temperature,
where the underlying bulk is frozen. Assuming the existence at T=T* of a bulk
thermodynamical glass transition, we show that the glass-vapor interface is
generally wetted by a liquid layer of thickness ~ -ln(T*-T) when T--> T*.
Contrary to standard surface melting of crystals however, the integrated value
of the diffusivity across the interface remains finite for T-->T*. Difference
in shape induced by bulk and by surface flow is discussed as a possible means
of experimental detection of surface defreezing.Comment: five pages, three figure
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 1 up to
10% with increasing monomer diameter from 0.3 m to 1.5 m (i.e.
tensile strength decreasing from 12 to 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.
- …