350 research outputs found
Impact of reindeer grazing on ground-dwelling Carabidae and Curculionidae assemblages in Lapland
Reindeer Rangifer tarandus L. grazing shapes forest vegetation, microclimate, and soil respiration in Lapland, especially due to grazing on lichens (Cladina). We studied how these changes and their magnitude affect ground-dwelling species of beetle families Carabidae (predators) and Curculionidae (herbivores), by using pitfall traps to collect invertebrates from pairs of grazed and ungrazed study plots over a wide range of site types. Changes in abundance, composition, richness and diversity of beetle assemblage were tested in relation to magnitude of the impacts on vegetation. The species compositions of Carabidae and Curculionidae differed between grazed and ungrazed plots in all sites. The relative difference between grazed and ungrazed plots in the number of individuals increased linearly with the impact of reindeer on vegetation cover. Carabid beetles, as a family, were more common in grazed plots in all sites. Curculionid beetles were more common in ungrazed plots in the birch dominated sites. This difference was mainly due to the species that feeds on deciduous leaves. In the pine dominated sites with high Cladina cover and more changes in ground vegetation, the number of curculionids feeding on conifers was higher in grazed plots. Species richness and diversity (H') of both families were higher in grazed plots. Of the total 27 species, 11 were found only in grazed plots, while not a single species was found only in ungrazed plots. The relative difference between plots in diversity and evennes (H'/H' max) had humped response to the difference in Cladina cover. The diversity values were greater in grazed plots at the intermediate levels of grazing impact, and only in sites with very low or extremely high Cladina cover difference was the diversity higher in ungrazed plots. The response of beetle diversity resembled the hypotheses suggested for the relationship between grazing and vegetation diversity: greatest positive effect at intermediate grazing intensity and negative effects at unproductive sites
Parameter constraints for high-energy models of colliding winds of massive stars: the case WR 147
We explore the ability of high energy observations to constrain orbital
parameters of long period massive binary systems by means of an inverse Compton
model acting in colliding wind environments. This is particular relevant for
(very) long period binaries where orbital parameters are often poorly known
from conventional methods, as is the case e.g. for the Wolf-Rayet (WR) star
binary system WR 147 where INTEGRAL and MAGIC upper limits on the high-energy
emission have recently been presented. We conduct a parameter study of the set
of free quantities describing the yet vaguely constrained geometry and
respective effects on the non-thermal high-energy radiation from WR 147. The
results are confronted with the recently obtained high-energy observations and
with sensitivities of contemporaneous high-energy instruments like Fermi-LAT.
For binaries with sufficient long periods, like WR 147, gamma-ray attenuation
is unlikely to cause any distinctive features in the high-energy spectrum. This
leaves the anisotropic inverse Compton scattering as the only process that
reacts sensitively on the line-of-sight angle with respect to the orbital
plane, and therefore allows the deduction of system parameters even from
observations not covering a substantial part of the orbit.
Provided that particle acceleration acts sufficiently effectively to allow
the production of GeV photons through inverse Compton scattering, our analysis
indicates a preference for WR 147 to possess a large inclination angle.
Otherwise, for low inclination angles, electron acceleration is constrained to
be less efficient as anticipated here.Comment: 33 pages, 9 figures; accepted by Ap
Coronary revascularisation in stable patients after an acute coronary syndrome: a propensity analysis of early invasive versus conservative management in a register-based cohort study
OBJECTIVES: To compare the effectiveness of in-hospital medical therapy versus coronary revascularisation added to medical therapy in patients who stabilised after an acute coronary syndrome (ACS).
DESIGN: Propensity score-matched cohort study from the database of the Tampere ACS registry.
SETTING: A single academic hospital in Finland.
PARTICIPANTS: 1149 patients with a recent ACS, but no serious coexisting conditions: recurrent ischaemic episodes despite adequate medical therapy, haemodynamic instability, overt congestive heart failure and serious ventricular arrhythmias. PRIMARY AND SECONDARY OUTCOME MEASURES: The composite endpoint of major acute cardiovascular events (MACEs): unstable angina requiring rehospitalisation, stroke, myocardial infarction and all-cause mortality, at 6-month follow-up.
RESULTS: Compared with standard medical treatment, revascularisation was associated with a lower rate of MACEs at 6 months in patients of the first quintile (HR 0.81; 95% CI 0.66 to 0.99), but a higher rate of MACEs in the fifth quintile (HR 4.74, CI 1.36 to 16.49; p=0.014). There were no significant differences in the rates of MACEs in the remaining three quintiles. Patients of the first quintile were the oldest (79.7\ub18.3 years) and had a more significant (p<0.001) history of prior myocardial infarction (37%) and poor renal function (creatine, \ub5mol/l: 114.9\ub170.7). They also showed the highest C reactive protein (7.3\ub19.5 mg/l) levels.
CONCLUSIONS: Our findings suggest that in-hospital coronary revascularisation did not lead to any advantage with signal of possible harm in the great majority of patients who stabilised after an ACS. An early invasive management strategy may be best reserved for elderly patients having high-risk clinical features and biochemical evidence of a strong inflammatory activity
Attractive Interaction Between Pulses in a Model for Binary-Mixture Convection
Recent experiments on convection in binary mixtures have shown that the
interaction between localized waves (pulses) can be repulsive as well as {\it
attractive} and depends strongly on the relative {\it orientation} of the
pulses. It is demonstrated that the concentration mode, which is characteristic
of the extended Ginzburg-Landau equations introduced recently, allows a natural
understanding of that result. Within the standard complex Ginzburg-Landau
equation this would not be possible.Comment: 7 pages revtex with 3 postscript figures (uuencoded
Coexisting Pulses in a Model for Binary-Mixture Convection
We address the striking coexistence of localized waves (`pulses') of
different lengths which was observed in recent experiments and full numerical
simulations of binary-mixture convection. Using a set of extended
Ginzburg-Landau equations, we show that this multiplicity finds a natural
explanation in terms of the competition of two distinct, physical localization
mechanisms; one arises from dispersion and the other from a concentration mode.
This competition is absent in the standard Ginzburg-Landau equation. It may
also be relevant in other waves coupled to a large-scale field.Comment: 5 pages revtex with 4 postscript figures (everything uuencoded
Large scale dynamics in turbulent Rayleigh-Benard convection
The progress in our understanding of several aspects of turbulent
Rayleigh-Benard convection is reviewed. The focus is on the question of how the
Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the
Prandtl number Pr, and on how the thicknesses of the thermal and the kinetic
boundary layers scale with Ra and Pr. Non-Oberbeck-Boussinesq effects and the
dynamics of the large-scale convection-roll are addressed as well. The review
ends with a list of challenges for future research on the turbulent
Rayleigh-Benard system.Comment: Review article, 34 pages, 13 figures, Rev. Mod. Phys. 81, in press
(2009
The Keck Aperture Masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars
We report the results of a high angular resolution near-infrared survey of
dusty Wolf-Rayet stars using the Keck-1 Telescope, including new
multi-wavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112.
Angular sizes were measured for an additional 8 dusty WR stars using aperture
masking interferometry, allowing us to probe characteristics sizes down to ~20
milliarcseconds (~40 AU for typical sources). With angular sizes and specific
fluxes, we can directly measure the wavelength-dependent surface brightness and
size relations for our sample. We discovered tight correlations of these
properties within our sample which could not be explained by simple
spherically-symmetric dust shells or even the more realistic ``pinwheel
nebula'' (3-D) radiative transfer model, when using optical constants of Zubko.
While the tightly-correlated surface brightness relations we uncovered offer
compelling indirect evidence of a shared and distinctive dust shell geometry
amongst our sample, long-baseline interferometers should target the
marginally-resolved objects in our sample in order to conclusively establish
the presence or absence of the putative underyling colliding wind binaries
thought to produce the dust shells around WC Wolf-Rayets.Comment: Accepted by Astrophysical Journa
A Spectroscopic Survey of WNL Stars in the LMC: General Properties and Binary Status
We report the results of an intense, spectroscopic survey of all 41
late-type, nitrogen-rich Wolf-Rayet (WR) stars in the Large Magellanic Cloud
(LMC) observable with ground-based telescopes. This survey concludes the
decade-long effort of the Montr\'eal Massive Star Group to monitor every known
WR star in the Magellanic Clouds except for the 6 crowded WNL stars in R136,
which will be discussed elsewhere. The focus of our survey was to monitor the
so-called WNL stars for radial-velocity (RV) variability in order to identify
the short- to intermediate-period (P \la 200 days) binaries among them. Our
results are in line with results of previous studies of other WR subtypes, and
show that the binary frequency among LMC WNL stars is statistically consistent
with that of WNL stars in the Milky Way. We have identified four previously
unknown binaries, bringing the total number of known WNL binaries in the LMC to
nine. Since it is very likely that none but one of the binaries are classical,
helium-burning WNL stars, but rather superluminous, hence extremely massive,
hydrogen-burning objects, our study has dramatically increased the number of
known binaries harbouring such objects, and thus paved the way to determine
their masses through model-independent, Keplerian orbits. It is expected that
some of the stars in our binaries will be among the most massive known. With
the binary status of each WR star now known, we also studied the photometric
and X-ray properties of our program stars using archival MACHO photometry as
well as Chandra and ROSAT data. We find that one of our presumably single WNL
stars is among the X-ray brightest WR sources known. We also identify a binary
candidate from its RV variability and X-ray luminosity which harbours the most
luminous WR star known in the Local Group.Comment: 25 pages, 11 figures; accepted for MNRA
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
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