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

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 $dv$ 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 $dv$ 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

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

Stability of Terrestrial Planets in the Habitable Zone of Gl 777 A, HD 72659, Gl 614, 47 Uma and HD 4208

We have undertaken a thorough dynamical investigation of five extrasolar planetary systems using extensive numerical experiments. The systems Gl 777 A, HD 72659, Gl 614, 47 Uma and HD 4208 were examined concerning the question of whether they could host terrestrial like planets in their habitable zones (=HZ). First we investigated the mean motion resonances between fictitious terrestrial planets and the existing gas giants in these five extrasolar systems. Then a fine grid of initial conditions for a potential terrestrial planet within the HZ was chosen for each system, from which the stability of orbits was then assessed by direct integrations over a time interval of 1 million years. The computations were carried out using a Lie-series integration method with an adaptive step size control. This integration method achieves machine precision accuracy in a highly efficient and robust way, requiring no special adjustments when the orbits have large eccentricities. The stability of orbits was examined with a determination of the Renyi entropy, estimated from recurrence plots, and with a more straight forward method based on the maximum eccentricity achieved by the planet over the 1 million year integration. Additionally, the eccentricity is an indication of the habitability of a terrestrial planet in the HZ; any value of e>0.2 produces a significant temperature difference on a planet's surface between apoapse and periapse. The results for possible stable orbits for terrestrial planets in habitable zones for the five systems are summarized as follows: for Gl 777 A nearly the entire HZ is stable, for 47 Uma, HD 72659 and HD 4208 terrestrial planets can survive for a sufficiently long time, while for Gl 614 our results exclude terrestrial planets moving in stable orbits within the HZ.Comment: 14 pages, 18 figures submitted 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