346 research outputs found
Density Variations over Subparsec Scales in Diffuse Molecular Gas
We present high-resolution observations of interstellar CN, CH, CH^{+},
\ion{Ca}{1}, and \ion{Ca}{2} absorption lines toward the multiple star systems
HD206267 and HD217035. Substantial variations in CN absorption are observed
among three sight lines of HD206267, which are separated by distances of order
10,000 AU; smaller differences are seen for CH, CH^{+}, and \ion{Ca}{1}. Gas
densities for individual velocity components are inferred from a chemical
model, independent of assumptions about cloud shape. While the component
densities can differ by factors of 5.0 between adjacent sightlines, the
densities are always less than 5000 cm^{-3}. Calculations show that the derived
density contrasts are not sensitive to the temperature or reaction rates used
in the chemical model. A large difference in the CH^{+} profiles (a factor of 2
in column density) is seen in the lower density gas toward HD217035.Comment: 9 pages, 2 figures. Accepted for publication in ApJ
A stochastic individual based model for the growth of a stand of Japanese knotweed including mowing as a management technique
Invasive alien species are a growing threat for environment and health. They
also have a major economic impact, as they can damage many infrastructures. The
Japanese knotweed (Fallopia japonica), present in North America, Northern and
Central Europe as well as in Australia and New Zealand, is listed by the World
Conservation Union as one of the world's worst invasive species. So far, most
models have dealt with how the invasion spreads without management. This paper
aims at providing a model able to study and predict the dynamics of a stand of
Japanese knotweed taking into account mowing as a management technique. The
model we propose is stochastic and individual-based, which allows us taking
into account the behaviour of individuals depending on their size and location,
as well as individual stochasticity. We set plant dynamics parameters thanks to
a calibration with field data, and study the influence of the initial
population size, the mean number of mowing events a year and the management
project duration on mean area and mean number of crowns of stands. In
particular, our results provide the sets of parameters for which it is possible
to obtain the stand eradication, and the minimal duration of the management
project necessary to achieve this latter
Plant chemical defence: a partner control mechanism stabilising plant - seed-eating pollinator mutualisms
<p>Abstract</p> <p>Background</p> <p>Mutualisms are inherently conflictual as one partner always benefits from reducing the costs imposed by the other. Despite the widespread recognition that mutualisms are essentially reciprocal exploitation, there are few documented examples of traits that limit the costs of mutualism. In plant/seed-eating pollinator interactions the only mechanisms reported so far are those specific to one particular system, such as the selective abortion of over-exploited fruits.</p> <p>Results</p> <p>This study shows that plant chemical defence against developing larvae constitutes another partner sanction mechanism in nursery mutualisms. It documents the chemical defence used by globeflower <it>Trollius europaeus </it>L. (Ranunculaceae) against the seed-eating larvae of six pollinating species of the genus <it>Chiastocheta </it>Pokorny (Anthomyiidae). The correlative field study carried out shows that the severity of damage caused by <it>Chiastocheta </it>larvae to globeflower fruits is linked to the accumulation in the carpel walls of a C-glycosyl-flavone (adonivernith), which reduces the larval seed predation ability per damaged carpel. The different <it>Chiastocheta </it>species do not exploit the fruit in the same way and their interaction with the plant chemical defence is variable, both in terms of induction intensity and larval sensitivity to adonivernith.</p> <p>Conclusion</p> <p>Adonivernith accumulation and larval predation intensity appear to be both the reciprocal cause and effect. Adonivernith not only constitutes an effective chemical means of partner control, but may also play a key role in the sympatric diversification of the <it>Chiastocheta </it>genus.</p
Polymérisation radicalaire contrôlée : le défi de l'éthylène
The work presented in this thesis displays the controlled radical polymerization of ethylene at low temperature (70 °C) and low pressure (200 bar) and the synthesis of block copolymers featuring polyethylene segments. Four polymerization techniques, commonly used in macromolecular engineering, were studied: NMP, CMRP, RAFT/MADIX and ESCP. Our investigation of the use of SG1 nitroxide (NMP) and cobalt (II) acetylacetonate (CMRP) as controlling agents demonstrated their inability to control the polymerization of ethylene. Nonetheless, an unexpected reaction with cobalt (II) acetylacetonate was observed. The coupling reaction between propagating radicals appeared to be favored by the presence of this compound. On the other hand, the first controlled polymerization of ethylene was successfully achieved by using xanthate (RAFT/MADIX). A linear increase of molecular weight with conversion and low polydispersities were observed for the produced polyethylenes. The reaction was demonstrated to be a pseudo-living polymerization by the synthesis of block copolymers poly(vinyl acetate)-b-polyethylene. In addition, midchain-functionalized polyethylenes and ABA type block copolymers, with polystyrene or polyacrylate as the A block and polyethylene as the B block, were also prepared using nitrone based polymerization technique (ESCP)Les travaux présentés dans cette thèse portent sur le contrôle de la polymérisation radicalaire de l'éthylène à basse pression (200 bar) et basse température (70 °C) et sur la synthèse de copolymères à blocs contenant au moins un segment de polyéthylène. Quatre techniques de polymérisation, couramment utilisées en ingénierie macromoléculaire, ont été étudiées : NMP, CMRP, RAFT/MADIX et ESCP. Nos études sur le nitroxyde SG1 (NMP) et le bis(acétylacétonate) de cobalt (CMRP) ont montré que ces composés sont inefficaces pour contrôler la polymérisation de l'éthylène. Un comportement inattendu du bis(acétylacétonate) de cobalt a cependant été mis en évidence. Il semblerait que ces complexes de cobalt favorisent les réactions de couplage entre les radicaux propagateurs. En revanche, la première polymérisation radicalaire contrôlée de l'éthylène a été atteinte grâce à l'utilisation de xanthates (RAFT/MADIX). Les polyéthylènes synthétisés possèdent des masses molaires qui augmentent linéairement avec la conversion et des dispersités faibles. Le caractère pseudo-vivant de la réaction a été démontré par la synthèse de copolymères à blocs poly(acétate de vinyle)-b-polyéthylène. L'utilisation de nitrones (ESCP) a également permis l'introduction d'une fonction réactive au centre des chaînes de polyéthylène et la synthèse de copolymères triblocs de type ABA, où les blocs latéraux A sont en polystyrène ou polyacrylate et le bloc central B est en Polyéthylèn
Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalogue
We present refined coordinates and proper motion data for the high proper
motion (HPM) stars in the Luyten Half-Second (LHS) catalogue. The positional
uncertainty in the original Luyten catalogue is typically >10" and is often
>30". We have used the digital scans of the Palomar Observatory Sky Survey
(POSS) I and POSS II plates to derive more accurate positions and proper
motions of the objects. Out of the 4470 candidates in the LHS catalogue, 4323
objects were manually re-identified in the POSS I and POSS II scans. A small
fraction of the stars were not found due to the lack of finder charts and
digitized POSS II scans. The uncertainties in the revised positions are
typically ~2", but can be as high as ~8" in a few cases; this is a large
improvement over the original data. Cross-correlation with the Tycho-2 and
Hipparcos catalogues yielded 819 candidates (with m_R < 12). For these brighter
sources, the position and proper motion data have been replaced with the more
accurate Tycho/Hipparcos data. In total, we have revised proper motion
measurements and coordinates for 4040 stars and revised coordinates for 4330
stars, which are presented here.Comment: 108 pages. Accepted for Publication in ApJ Suppl. Some errors caused
by the transcription errors in the original LHS catalogue have been corrected
in this resubmission. The most current version of the catalogue is also
available online at http://www.stsci.edu/~ksahu/lh
Asymmetric transit curves as indication of orbital obliquity: clues from the late-type dwarf companion in KOI-13
KOI-13.01, a planet-sized companion in an optical double star was announced
as one of the 1235 Kepler planet candidates in February 2011. The transit
curves show significant distortion that was stable over the ~130 days time-span
of the data. Here we investigate the phenomenon via detailed analyses of the
two components of the double star and a re-reduction of the Kepler data with
pixel-level photometry. Our results indicate that KOI-13 is a common proper
motion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We
identify the host star of KOI-13.01 and conclude that the transit curve
asymmetry is consistent with a companion orbiting a rapidly rotating, possibly
elongated star on an oblique orbit. After correcting the Kepler light curve to
the second light of the optical companion star, we derive a radius of 2.2 R_J
for the transiter, implying an irradiated late-type dwarf, probably a hot brown
dwarf rather than a planet. KOI-13 is the first example for detecting orbital
obliquity for a substellar companion without measuring the Rossiter-McLaughlin
effect from spectroscopy.Comment: Accepted by ApJ Letters. A new part added about KOI-13.01 (Sect. 3.4
pars 2-4
Photometric study of the OB star clusters NGC 1502 and NGC 2169 and mass estimation of their members at the University Observatory Jena
In this work we present detailed photometric results of the trapezium like
galactic nearby OB clusters NGC 1502 and NGC 2169 carried out at the University
Observatory Jena. We determined absolute magnitudes of the mostly
resolved components using Landolt standard stars. This multi colour photometry
enables us to estimate spectral type and absorption as well as the masses of
the components, which were not available for most of the cluster members in the
literature so far, using models of stellar evolution. Furthermore, we
investigated the optical spectrum of the components ADS 2984A and SZ Cam of the
sextuple system in NGC 1502. Our spectra clearly confirm the multiplicity of
these components, which is the first investigation of this kind at the
University Observatory Jena.Comment: seven pages, eight pictures. Submitted to AN in Feb. 21st, accepted
April 30t
Lucky Imaging survey for southern M dwarf binaries
While M dwarfs are the most abundant stars in the Milky Way, there is still
large uncertainty about their basic physical properties (mass, luminosity,
radius, etc.) as well as their formation environment. Precise knowledge of
multiplicity characteristics and how they change in this transitional mass
region, between Sun-like stars on the one side and very low mass stars and
brown dwarfs on the other, provide constraints on low mass star and brown dwarf
formation. In the largest M dwarf binary survey to date, we search for
companions to active, and thus preferentially young, M dwarfs in the solar
neighbourhood. We study their binary/multiple properties, such as the
multiplicity frequency and distributions of mass ratio and separation, and
identify short period visual binaries, for which orbital parameters and hence
dynamical mass estimates can be derived in the near future. The observations
are carried out in the SDSS i' and z' band using the Lucky Imaging camera
AstraLux Sur at the ESO 3.5 m New Technology Telescope. In the first part of
the survey, we observed 124 M dwarfs of integrated spectral types M0-M6 and
identified 34 new and 17 previously known companions to 44 stars. We derived
relative astrometry and component photometry for these systems. More than half
of the binaries have separations smaller than 1 arcsec and would have been
missed in a simply seeing-limited survey. Correcting our sample for selection
effects yields a multiplicity fraction of 32+/-6% for 108 M dwarfs within 52 pc
and with angular separations of 0.1-6.0 arcsec, corresponding to projected
separation 3-180 AU at median distance 30 pc. Compared to early-type M dwarfs
(M>0.3M_Sun), later type (and hence lower mass) M dwarf binaries appear to have
closer separations, and more similar masses.Comment: 18 pages, 9 figures. Minor corrections and changes. Revised to match
accepted A&A versio
Masses and Luminosities of O and B - type stars and red super giants
Massive stars are of interest as progenitors of super novae, i.e. neutron
stars and black holes, which can be sources of gravitational waves. Recent
population synthesis models can predict neutron star and gravitational wave
observations but deal with a fixed super nova rate or an assumed initial mass
function for the population of massive stars. Here we investigate those massive
stars, which are supernova progenitors, i.e. with O and early B type stars, and
also all super giants within 3kpc. We restrict our sample to those massive
stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars
with parallax and precise photometry. To determine the luminosities we
calculated the extinctions from published multi-colour photometry, spectral
types, luminosity class, all corrected for multiplicity and recently revised
Hipparcos distances. We use luminosities and temperatures to estimate the
masses and ages of these stars using different models from different authors.
Having estimated the luminosities of all our stars within 3kpc, in particular
for all O- and early B-type stars, we have determined the median and mean
luminosities for all spectral types for luminosity classes I, III, and V. Our
luminosity values for super giants deviate from earlier results: Previous work
generally overestimates distances and luminosities compared to our data, this
is likely due to Hipparcos parallaxes (generally more accurate and larger than
previous ground-based data) and the fact that many massive stars have recently
been resolved into multiples of lower masses and luminosities. From
luminosities and effective temperatures we derived masses and ages using mass
tracks and isochrones from different authors. From masses and ages we estimated
lifetimes and derived a lower limit for the supernova rate of ~20 events/Myr
averaged over the next 10 Myrs within 600 pc from the sun. These data are then
used to search for areas in the sky with higher likelihood for a supernova or
gravitational wave event (like OB associations).Comment: 13 pages, 13 figures, submitted to AN Dec 12 200
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