Planetesimal collisions in binary systems

We study the collisional evolution of km-sized planetesimals in tight binary star systems to investigate whether accretion towards protoplanets can proceed despite the strong gravitational perturbations from the secondary star. The orbits of planetesimals are numerically integrated in two dimensions under the influence of the two stars and gas drag. The masses and orbits of the planetesimals are allowed to evolve due to collisions with other planetesimals and accretion of collisional debris. In addition, the mass in debris can evolve due to planetesimal-planetesimal collisions and the creation of new planetesimals. We show that it is possible in principle for km-sized planetesimals to grow by two orders of magnitude in size if the efficiency of planetesimal formation is relatively low. We discuss the limitations of our two-dimensional approach.Comment: 5 pages, 5 figures, accepted for publication in MNRA

Debris discs in binaries: a numerical study

Debris disc analysis and modelling provide crucial information about the structure and the processes at play in extrasolar planetary systems. In binary systems, this issue is more complex because the disc should in addition respond to the companion star's perturbations. We explore the dynamical evolution of a collisionally active debris disc for different initial parent body populations, diverse binary configurations and optical depths. We focus on the radial extent and size distribution of the disc at a stationary state. We numerically follow the evolution of $10^{5}$ massless small grains, initially produced from a circumprimary disc of parent bodies following a size distribution in $dN \propto s^{-3.5}$ds . Grains are submitted to both stars' gravity as well as radiation pressure. In addition, particles are assigned an empirically derived collisional lifetime. For all the binary configurations the disc extends far beyond the critical semimajor axis $a_crit$ for orbital stability. This is due to the steady production of small grains, placed on eccentric orbits reaching beyond $a_crit$ by radiation pressure. The amount of matter beyond acrit depends on the balance between collisional production and dynamical removal rates: it increases for more massive discs as well as for eccentric binaries. Another important effect is that, in the dynamically stable region, the disc is depleted from its smallest grains. Both results could lead to observable signatures. We have shown that a companion star can never fully truncate a collisionally active disc. For eccentric companions, grains in the unstable regions can significantly contribute to the thermal emission in the mid-IR. Discs with sharp outer edges, especially bright ones such as HR4796A, are probably shaped by other mechanisms.Comment: accepted for publication in A&

Dynamics of Planetesimals due to Gas Drag from an Eccentric Precessing Disk

We analyze the dynamics of individual kilometer-size planetesimals in circumstellar orbits of a tight binary system. We include both the gravitational perturbations of the secondary star and a non-linear gas drag stemming from an eccentric gas disk with a finite precession rate. We consider several precession rates and eccentricities for the gas, and compare the results with a static disk in circular orbit. The disk precession introduces three main differences with respect to the classical static case: (i) The equilibrium secular solutions generated by the gas drag are no longer fixed points in the averaged system, but limit cycles with frequency equal to the precession rate of the gas. The amplitude of the cycle is inversely dependent on the body size, reaching negligible values for $\sim 50$ km size planetesimals. (ii) The maximum final eccentricity attainable by small bodies is restricted to the interval between the gas eccentricity and the forced eccentricity, and apsidal alignment is no longer guaranteed for planetesimals strongly coupled with the gas. (iii) The characteristic timescales of orbital decay and secular evolution decrease significantly with increasing precession rates, with values up to two orders of magnitude smaller than for static disks. Finally, we apply this analysis to the $\gamma$-Cephei system and estimate impact velocities for different size bodies and values of the gas eccentricity. For high disk eccentricities, we find that the disk precession decreases the velocity dispersion between different size planetesimals, thus contributing to accretional collisions in the outer parts of the disk. The opposite occurs for almost circular gas disks, where precession generates an increase in the relative velocities.Comment: 11 pages, 9 figures. Accepted in MNRA

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

Secular dynamics of planetesimals in tight binary systems: Application to Gamma-Cephei

The secular dynamics of small planetesimals in tight binary systems play a fundamental role in establishing the possibility of accretional collisions in such extreme cases. The most important secular parameters are the forced eccentricity and secular frequency, which depend on the initial conditions of the particles, as well as on the mass and orbital parameters of the secondary star. We construct a second-order theory (with respect to the masses) for the planar secular motion of small planetasimals and deduce new expressions for the forced eccentricity and secular frequency. We also reanalyze the radial velocity data available for Gamma-Cephei and present a series of orbital solutions leading to residuals compatible with the best fits. Finally, we discuss how different orbital configurations for Gamma-Cephei may affect the dynamics of small bodies in circunmstellar motion. For Gamma-Cephei, we find that the classical first-order expressions for the secular frequency and forced eccentricity lead to large inaccuracies around 50 % for semimajor axes larger than one tenth the orbital separation between the stellar components. Low eccentricities and/or masses reduce the importance of the second-order terms. The dynamics of small planetesimals only show a weak dependence with the orbital fits of the stellar components, and the same result is found including the effects of a nonlinear gas drag. Thus, the possibility of planetary formation in this binary system largely appears insensitive to the orbital fits adopted for the stellar components, and any future alterations in the system parameters (due to new observations) should not change this picture. Finally, we show that planetesimals migrating because of gas drag may be trapped in mean-motion resonances with the binary, even though the migration is divergent.Comment: 11 pages, 9 figure

Collisional processes and size distribution in spatially extended debris discs

We present a new multi-annulus code for the study of collisionally evolving extended debris discs. We first aim to confirm results obtained for a single-annulus system, namely that the size distribution in "real" debris discs always departs from the theoretical collisional equilibrium dN\proptoR^{-3.5}dR power law, especially in the crucial size range of observable particles (<1cm), where it displays a characteristic wavy pattern. We also aim at studying how debris discs density distributions, scattered light luminosity profiles, and SEDs are affected by the coupled effect of collisions and radial mixing due to radiation pressure affected small grains. The size distribution evolution is modeled from micron-sized grains to 50km-sized bodies. The model takes into account the crucial influence of radiation pressure-affected small grains. We consider the collisional evolution of a fiducial a=120AU radius disc with an initial surface density in $\Sigma(a)\propto a^{\alpha}$. We show that the system's radial extension plays a crucial role: in most regions the collisional and size evolution of the dust is imposed by small particles on eccentric or unbound orbits produced further inside the disc. The spatial distribution of small grains strongly departs from the initial profile, while the bigger objects, containing most of the system's mass, still follow the initial distribution. This has consequences on the scattered--light radial profiles which get significantly flatter, and we propose an empirical law to trace back the distribution of large unseen parent bodies from the observed profiles. We finally provide empirical formula for the collisional size distribution and collision timescale that can be used for future debris disc modeling.Comment: Accepted for publication in Astronomy and Astrophysics (with better figures) (note: full abstract in the *.pdf file

Collisional Velocities and Rates in Resonant Planetesimal Belts

We consider a belt of small bodies around a star, captured in one of the external or 1:1 mean-motion resonances with a massive perturber. The objects in the belt collide with each other. Combining methods of celestial mechanics and statistical physics, we calculate mean collisional velocities and collisional rates, averaged over the belt. The results are compared to collisional velocities and rates in a similar, but non-resonant belt, as predicted by the particle-in-a-box method. It is found that the effect of the resonant lock on the velocities is rather small, while on the rates more substantial. The collisional rates between objects in an external resonance are by about a factor of two higher than those in a similar belt of objects not locked in a resonance. For Trojans under the same conditions, the collisional rates may be enhanced by up to an order of magnitude. Our results imply, in particular, shorter collisional lifetimes of resonant Kuiper belt objects in the solar system and higher efficiency of dust production by resonant planetesimals in debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and Dynamical Astronomy

The Color Distribution in the Edgeworth-Kuiper Belt

We have started since 1997 the Meudon Multicolor Survey of Outer Solar System Objects with the aim of collecting a large and homogeneous set of color data for Trans-Neptunian and Centaurs objects [...] We have a combined sample of 52 B-R color measurements for 8 Centaurs, 22 Classicals, 13 Plutinos, 8 Scattered objects and 1 object with unidentified dynamical class. This dataset is the largest single and homogeneous published dataset to date [...]. A strong (color) correlation with mean excitation velocity points toward a space weathering/impact origin for the color diversity. However, thorough modeling of the collisional/dynamical environment in the Edgeworth-Kuiper belt needs to be done in order to confirm this scenario. We found also that the Classical TNOs consist in the superposition of two distinct populations: the dynamically Cold Classical TNOs (red colors, low i, small sizes) and the dynamically Hot Classical TNOs (diverse colors, moderate and high i, larger sizes). [...] Our specific observation strategy [...] permitted us to highlight a few objects suspected to have true compositional and/or texture variation on their surfaces. These are 1998 HK151, 1999 DF9, 1999 OY3, 2000 GP183, 2000 OK67, and 2001 KA77 and should be prime targets for further observations [...]. Our survey has also highlighted 1998 SN165 whose colors and dynamical properties puts it in a new dynamical class distinct from the Classicals, its previously assigned dynamical class.Comment: Accepted for publication in Astronomical Journal (38 pages, inc. 11 figures

Relations between cognition and motricity in children with neonatal arterial ischemic stroke

Introduction and goalPerinatal arterial ischemic stroke (PAIS) affects one child for 4000 births. The few studies about cognitive development specific to PAIS showed that cognitive performances in this population do not follow up a normal development (Westmacott et al., 2010; Ricci et al., 2008). Based on new data about relation between motricity and cognition (Smits-Engelsman et Hill, 2012), and on the theory of the embodied cognition, led us to hypothesize that cognitive performances would be correlated to the motor performances in children with PAIS.Patients and methodologyWe tested 77 7 years old children meeting the criteria of neonatal AIS, with a diagnosis before the 28th day of life relying on cerebral imagery. After excluding children with seizure and bi-hemispheric lesion, 56 children participated to our study. The cognitive evaluation was performed with the Wechsler Intelligence Scale for Children (WISC-4), the motor evaluation relied on testing of gross motor of the upper arm (Box and Block Test) and fine prehension test (“Nine Hole Peg Test”). The localisation of the lesion, the economic level of parents, the gender, sensory impairments and the presence of hemiplegia were collected. We analyzed these results with simple linear regression.ResultsThe main result of our study is the significative correlation (P<0.03) between scores of the WISC4 (except for working memory index) and motor results. In contrast we did not find any correlation between the scores of the WISC4 and the presence of hemiplegia or with lesion localization.DiscussionMany brain networks develop during the first year through sensorimotor experiences, which contribute to the emergence of knowledge. This concept of development, supported by the approach embodied cognition, can explain the correlations between cognition and motor found in our work and in several studies with children with other early neurological damage

Spring molybdenum enrichment in scallop shells: a potential tracer of diatom productivity in temperate coastal environments (Brittany, NW France)

Skeletal molybdenum/calcium ([Mo]/[Ca])_shell ratios were examined in shells of the Great Scallop <i>Pecten maximus</i> collected in temperate coastal environments of Western Europe (42 to 49&deg; N). These ratios were determined by quantitative LA-ICP-MS analyses of daily striae taken every third day (i.e. a temporal resolution of 3 days) in 36 flat valves (2-years old; 3 shells/year). Variations of ([Mo]/[Ca])_shell ratios were significant and reproducible for scallops from the same population, from different years (1998–2004) and temperate coastal locations (NW France). The [Mo]/[Ca])_shell ratios exhibit typical profiles characterized by a background content, below the detection limit for this method (&lt;0.003 &mu;mol/mol) for most of the shell growth period, which is punctuated by a significant transient enrichment (0.031–2.1 &mu;mol/mol) mainly occurring from May to June. The Bay of Brest (France) was investigated in particular because of its long term observations on scallop communities, environmental variables, and high resolution analyses of dissolved Mo in bottom seawater in 2000. In 2000, dissolved Mo exhibited a significant increase in concentration just preceding the maximum ([Mo]/[Ca])_shell ratio. Both the intense monitoring survey in 2000 and over the 7-year period indicates that the ([Mo]/[Ca])_shell maximum is directly influenced by spring changes of environmental conditions at the sediment water interface (SWI), occurring subsequent to the intense and periodic spring bloom. Spring maxima of ([Mo]/[Ca])_shell ratios are closely correlated to the extent of silicic acid and nitrate depletion in seawater between winter and late spring (<i>r</i>²=0.878 and 0.780, <i>p</i><0.05, <i>n</i>=6) that reflects diatom uptake and productivity in the Bay of Brest. The Mo inputs in bottom waters and subsequent shell enrichment are thus suggested to be directly or indirectly influenced by such biogenic material input at the SWI. The [Mo]/[Ca])_shell records thus reveal unexpected biogeochemical cycles of Mo influenced by coastal spring productivity, faithfully recorded in scallop shells