2,134 research outputs found
Spectroscopy of PTCDA attached to rare gas samples: clusters vs. bulk matrices. I. Absorption spectroscopy
The interaction between PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride)
and rare gas or para-hydrogen samples is studied by means of laser-induced
fluorescence excitation spectroscopy. The comparison between spectra of PTCDA
embedded in a neon matrix and spectra attached to large neon clusters shows
that these large organic molecules reside on the surface of the clusters when
doped by the pick-up technique. PTCDA molecules can adopt different
conformations when attached to argon, neon and para-hydrogen clusters which
implies that the surface of such clusters has a well-defined structure and has
not liquid or fluxional properties. Moreover, a precise analysis of the doping
process of these clusters reveals that the mobility of large molecules on the
cluster surface is quenched, preventing agglomeration and complex formation
Planet formation in Binaries
Spurred by the discovery of numerous exoplanets in multiple systems, binaries
have become in recent years one of the main topics in planet formation
research. Numerous studies have investigated to what extent the presence of a
stellar companion can affect the planet formation process. Such studies have
implications that can reach beyond the sole context of binaries, as they allow
to test certain aspects of the planet formation scenario by submitting them to
extreme environments. We review here the current understanding on this complex
problem. We show in particular how each of the different stages of the
planet-formation process is affected differently by binary perturbations. We
focus especially on the intermediate stage of kilometre-sized planetesimal
accretion, which has proven to be the most sensitive to binarity and for which
the presence of some exoplanets observed in tight binaries is difficult to
explain by in-situ formation following the "standard" planet-formation
scenario. Some tentative solutions to this apparent paradox are presented. The
last part of our review presents a thorough description of the problem of
planet habitability, for which the binary environment creates a complex
situation because of the presence of two irradation sources of varying
distance.Comment: Review chapter to appear in "Planetary Exploration and Science:
Recent Advances and Applications", eds. S. Jin, N. Haghighipour, W.-H. Ip,
Springer (v2, numerous typos corrected
Origin and Dynamical Evolution of Neptune Trojans - II: Long Term Evolution
We present results examining the fate of the Trojan clouds produced in our
previous work. We find that the stability of Neptunian Trojans seems to be
strongly correlated to their initial post-migration orbital elements, with
those objects that survive as Trojans for billions of years displaying
negligible orbital evolution. The great majority of these survivors began the
integrations with small eccentricities (e < 0.2) and small libration amplitudes
(A < 30 - 40{\deg}). The survival rate of "pre-formed" Neptunian Trojans (which
in general survived on dynamically cold orbits (e < 0.1, i < 5 - 10{\deg}))
varied between ~5 and 70%. By contrast, the survival rate of "captured" Trojans
(on final orbits spread across a larger region of e-i element space) were
markedly lower, ranging between 1 and 10% after 4 Gyr. Taken in concert with
our earlier work, we note that planetary formation scenarios which involve the
slow migration (a few tens of millions of years) of Neptune from an initial
planetary architecture that is both resonant and compact (aN < 18 AU) provide
the most promising fit of those we considered to the observed Trojan
population. In such scenarios, we find that the current day Trojan population
would number ~1% of that which was present at the end of the planet's
migration, with the bulk being sourced from captured, rather than pre-formed
objects. We note, however, that even those scenarios still fail to reproduce
the currently observed portion of the Neptune Trojan population moving on
orbits with e 20{\deg}. Dynamical integrations of the currently
observed Trojans show that five out of the seven are dynamically stable on 4
Gyr timescales, while 2001 QR322, exhibits significant dynamical instability.
The seventh Trojan object, 2008 LC18, has such large orbital uncertainties that
only future studies will be able to determine its stability.Comment: 24 pages, 6 figures, accepted for publication in MNRAS (The abstract
was shortened. Original version can be found in the pdf file
Against all odds? Forming the planet of the HD196885 binary
HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose
orbit places it at the limit for orbital stability. The presence of a planet in
such a highly perturbed region poses a clear challenge to planet-formation
scenarios. We investigate this issue by focusing on the planet-formation stage
that is arguably the most sensitive to binary perturbations: the mutual
accretion of kilometre-sized planetesimals. To this effect we numerically
estimate the impact velocities amongst a population of circumprimary
planetesimals. We find that most of the circumprimary disc is strongly hostile
to planetesimal accretion, especially the region around 2.6AU (the planet's
location) where binary perturbations induce planetesimal-shattering of
more than 1km/s. Possible solutions to the paradox of having a planet in such
accretion-hostile regions are 1) that initial planetesimals were very big, at
least 250km, 2) that the binary had an initial orbit at least twice the present
one, and was later compacted due to early stellar encounters, 3) that
planetesimals did not grow by mutual impacts but by sweeping of dust (the
"snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab
was formed not by core-accretion but by the concurent disc instability
mechanism. All of these 4 scenarios remain however highly conjectural.Comment: accepted for publication by Celestial Mechanics and Dynamical
Astronomy (Special issue on EXOPLANETS
Quantitative analysis by renormalized entropy of invasive electroencephalograph recordings in focal epilepsy
Invasive electroencephalograph (EEG) recordings of ten patients suffering
from focal epilepsy were analyzed using the method of renormalized entropy.
Introduced as a complexity measure for the different regimes of a dynamical
system, the feature was tested here for its spatio-temporal behavior in
epileptic seizures. In all patients a decrease of renormalized entropy within
the ictal phase of seizure was found. Furthermore, the strength of this
decrease is monotonically related to the distance of the recording location to
the focus. The results suggest that the method of renormalized entropy is a
useful procedure for clinical applications like seizure detection and
localization of epileptic foci.Comment: 10 pages, 5 figure
Origin and Dynamical Evolution of Neptune Trojans - I: Formation and Planetary Migration
We present the results of detailed dynamical simulations of the effect of the
migration of the four giant planets on both the transport of pre-formed Neptune
Trojans, and the capture of new Trojans from a trans-Neptunian disk. We find
that scenarios involving the slow migration of Neptune over a large distance
(50Myr to migrate from 18.1AU to its current location) provide the best match
to the properties of the known Trojans. Scenarios with faster migration (5Myr),
and those in which Neptune migrates from 23.1AU to its current location, fail
to adequately reproduce the current day Trojan population. Scenarios which
avoid disruptive perturbation events between Uranus and Neptune fail to yield
any significant excitation of pre-formed Trojans (transported with efficiencies
between 30 and 98% whilst maintaining the dynamically cold nature of these
objects). Conversely, scenarios with periods of strong Uranus-Neptune
perturbation lead to the almost complete loss of such pre-formed objects. In
these cases, a small fraction (~0.15%) of these escaped objects are later
recaptured as Trojans prior to the end of migration, with a wide range of
eccentricities (<0.35) and inclinations (<40 deg). In all scenarios (including
those with such disruptive interaction between Uranus and Neptune) the capture
of objects from the trans-Neptunian disk (through which Neptune migrates) is
achieved with efficiencies between ~0.1 and ~1%. The captured Trojans display a
wide range of inclinations (<40 deg for slow migration, and <20 deg for rapid
migration) and eccentricities (<0.35), and we conclude that, given the vast
amount of material which undoubtedly formed beyond the orbit of Neptune, such
captured objects may be sufficient to explain the entire Neptune Trojan
population. (Shortened version)Comment: 25 pages, 6 figure
The PHASES Differential Astrometry Data Archive. III. Limits to Tertiary Companions
The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES)
monitored 51 subarcsecond binary systems to evaluate whether tertiary
companions as small as Jovian planets orbited either the primary or secondary
stars, perturbing their otherwise smooth Keplerian motions. Twenty-one of those
systems were observed 10 or more times and show no evidence of additional
companions. A new algorithm is presented for identifying astrometric companions
and establishing the (companion mass)-(orbital period) combinations that can be
excluded from existence with high confidence based on the PHASES observations,
and the regions of mass-period phase space being excluded are presented for 21
PHASES binaries.Comment: 16 pages, Accepted to A
Vascular permeability, vascular hyperpermeability and angiogenesis
The vascular system has the critical function of supplying tissues with nutrients and clearing waste products. To accomplish these goals, the vasculature must be sufficiently permeable to allow the free, bidirectional passage of small molecules and gases and, to a lesser extent, of plasma proteins. Physiologists and many vascular biologists differ as to the definition of vascular permeability and the proper methodology for its measurement. We review these conflicting views, finding that both provide useful but complementary information. Vascular permeability by any measure is dramatically increased in acute and chronic inflammation, cancer, and wound healing. This hyperpermeability is mediated by acute or chronic exposure to vascular permeabilizing agents, particularly vascular permeability factor/vascular endothelial growth factor (VPF/VEGF, VEGF-A). We demonstrate that three distinctly different types of vascular permeability can be distinguished, based on the different types of microvessels involved, the composition of the extravasate, and the anatomic pathways by which molecules of different size cross-vascular endothelium. These are the basal vascular permeability (BVP) of normal tissues, the acute vascular hyperpermeability (AVH) that occurs in response to a single, brief exposure to VEGF-A or other vascular permeabilizing agents, and the chronic vascular hyperpermeability (CVH) that characterizes pathological angiogenesis. Finally, we list the numerous (at least 25) gene products that different authors have found to affect vascular permeability in variously engineered mice and classify them with respect to their participation, as far as possible, in BVP, AVH and CVH. Further work will be required to elucidate the signaling pathways by which each of these molecules, and others likely to be discovered, mediate the different types of vascular permeability
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