Context. The surface energy constraint puts a limit on the smallest fragment ssurf that can be produced after a collision. Based on analytical considerations, this mechanism has been recently ...identified as being having the potential to prevent the production of small dust grains in debris discs and to cut off their size distribution at sizes larger than the blow-out size. Aims. We numerically investigate the importance of this effect to find out under which conditions it can leave a signature in the small-size end of a disc’s particle size distribution (PSD). An important part of this work is to map out, in a disc at steady-state, what is the most likely collisional origin for μm-sized dust grains, in terms of the sizes of their collisional progenitors. Methods. For the first time, we implement the surface energy constraint into a collisional evolution code. We consider a typical debris disc extending from 50 to 100 au and two different stellar types: sun-like and A star. We also consider two levels of stirring in the disc: dynamically “hot” (⟨e⟩ = 0.075) and “cold” (⟨e⟩ = 0.01). In all cases, we derive ssurf maps as a function of target and projectile sizes, st and sp, and compare them to equivalent maps for the dust-production rate. We then compute disc-integrated profiles of the PSD and estimate the imprint of the surface energy constraint. Results. We find that the (sp,st) regions of high ssurf values do not coincide with those of high dust production rates. As a consequence, the surface energy constraint generally has a weak effect on the system’s PSD. The maximum ssurf-induced depletion of μm-sized grains is ~30% and is obtained for a sun-like star and a dynamically “hot” case. For the e = 0.01 cases, the surface energy effect is negligible compared to the massive small grain depletion that is induced by another mechanism: the “natural” imbalance between dust production and destruction rates in low-stirring discs.
We investigate classical planetesimal accretion in a binary star system of separation
a
b
⩽
50
AU
by numerical simulations, with particular focus on the region at a distance of 1 AU from the ...primary. The planetesimals orbit the primary, are perturbed by the companion and are in addition subjected to a gas drag force. We concentrate on the problem of relative velocities Δ
v among planetesimals of different sizes. For various stellar mass ratios and binary orbital parameters we determine regions where Δ
v exceed planetesimal escape velocities
v
esc
(thus preventing runaway accretion) or even the threshold velocity
v
ero
for which erosion dominates accretion. Gaseous friction has two crucial effects on the velocity distribution: it damps secular perturbations by forcing periastron alignment of orbits, but at the same time the size-dependence of this orbital alignment induces a significant Δ
v increase between bodies of different sizes. This differential phasing effect proves very efficient and almost always increases Δ
v to values preventing runaway accretion, except in a narrow
e
b
≃
0
domain. The erosion threshold
Δ
v
>
v
ero
is reached in a wide (
a
b
,
e
b
) space for small <10-km planetesimals, but in a much more limited region for bigger ≃50-km objects. In the intermediate
v
esc
<
Δ
v
<
v
ero
domain, a possible growth mode would be the type II runaway growth identified by Kortenkamp et al. Kortenkamp, S., Wetherill, G., Inaba, S., 2001. Science 293, 1127–1129.
Models on the move: Migration and imperialism Bradley, Seamus; Thébault, Karim P.Y.
Studies in history and philosophy of science. Part A,
October 2019, 2019-10-00, 20191001, Volume:
77
Journal Article
Peer reviewed
Open access
We introduce ‘model migration’ as a species of cross-disciplinary knowledge transfer whereby the representational function of a model is radically changed to allow application to a new disciplinary ...context. Controversies and confusions that often derive from this phenomenon will be illustrated in the context of econophysics and phylogeographic linguistics. Migration can be usefully contrasted with the concept of ‘imperialism’, which has been influentially discussed in the context of geographical economics. In particular, imperialism, unlike migration, relies upon extension of the original model via an expansion of the domain of phenomena it is taken to adequately describe. The success of imperialism thus requires expansion of the justificatory sanctioning of the original idealising assumptions to a new disciplinary context. Contrastingly, successful migration involves the radical representational re-interpretation of the original model, rather than its extension. Migration thus requires ‘re-sanctioning’ of new ‘counterpart idealisations’ to allow application to an entirely different class of phenomena. Whereas legitimate scientific imperialism should be based on the pursuit of some form of ontological unification, no such requirement is needed to legitimate the practice of model migration. The distinction between migration and imperialism will thus be shown to have significant normative as well as descriptive value.
•We introduce the concept of ´model migration' via two examples of cross-disciplinary knowledge transfer.•Our notion of migration is contrasted with the importantly different concept of ´imperialism'.•This contrast draws our focus upon ´re-sanctioning', which is the defining and crucial feature of successful model migration.
Aims. We explore planet formation in binary systems around the central star where the protoplanetary disk plane is highly inclined with respect to the companion star orbit. This might be the most ...frequent scenario for binary separations larger than 40 AU. We focus on planetesimal accretion and compute average impact velocities in the habitable region and up to 6 AU from the primary. Methods. Planetesimal trajectories are computed within the frame of the restricted 3-body problem determined by the central star, the companion star and massless planetesimals. Relative velocities are computed and interpreted in terms of accreting or eroding impacts. Results. We first show that, for binary inclinations higher than 10 degrees, planetesimals evolve, to a first approximation, in a gas-free environment. Planetesimal accretion is confined around the central star in a region determined by two main parameters, firstly by the mutual inclination between the binary plane and the disk, and, secondly, by the binary eccentricity. Conclusions. The onset of large mutual inclinations between planetesimals due to the nodal randomization causes an increase in the relative velocity. The chances for a successful planet accumulation process depend on the balance between the timescale for node randomization and that of planetesimal accretion. When the binary semimajor axis is larger than 70 AU, planet formation appears possible even for eccentric binaries (up to 0.4). For lower binary separations the region where planetesimals accumulate into protoplanets shrinks consistently. When the mutual inclination between the binary plane and that of the planetesimal disk is larger than $40^\circ$, the Kozai mechanism strongly inhibits planetesimal accumulation.
The canonical formalism of general relativity affords a particularly interesting characterization of the infamous hole argument. It also provides a natural formalism in which to relate the hole ...argument to the problem of time in classical and quantum gravity. Conceptual and formal inadequacies within the representative language of canonical gravity will be shown to be at the heart of both the canonical hole argument and the problem of time. Interesting and fruitful work at the interface of physics and philosophy relates to the challenge of resolving such inadequacies.
Outer edges of debris discs Thébault, P.; Wu, Y.
Astronomy & astrophysics,
04/2008, Volume:
481, Issue:
3
Journal Article
Peer reviewed
Open access
Context. Rings or annulus-like features have been observed in most imaged debris discs. Outside the main ring, while some systems (e.g., β Pictoris and AU Mic) exhibit smooth surface brightness ...profiles (SB) that fall off roughly as ~r-3.5, others (e.g. HR 4796A and HD 139664) display large drops in luminosity at the ring's outer edge and steeper radial luminosity profiles. Aims. We seek to understand this diversity of outer edge profiles under the “natural” collisional evolution of the system, without invoking external agents such as planets or gas. Methods. We use a multi-annulus statistical code to follow the evolution of a collisional population, ranging in size from dust grains to planetesimals and initially confined within a belt (the “birth ring”). The crucial effect of radiation pressure on the dynamics and spatial distribution of the smallest grains is taken into account. We explore the dependence of the resulting disc surface brightness profile on various parameters. Results. The disc typically evolves toward a “standard” steady state, where the radial surface brightness profile smoothly decreases with radius as r-3.5 outside the birth ring. This confirms and extends the semi-analytical study of Strubbe & Chiang (2006, ApJ, 648, 652) and provides a firm basis for interpreting observed discs. Deviations from this typical profile, in the form of a sharp outer edge and a steeper fall-off, occur for two “extreme” cases: 1) when the birth ring is so massive that it becomes radially optically thick for the smallest grains. However, the required disc mass is probably too high here to be realistic; 2) when the dynamical excitation of the dust-producing planetesimals is so low ($\langle e\rangle $ and $\langle i\rangle \leq 0.01$) that the smallest grains, which otherwise dominate the optical depth of the system, are preferentially depleted. This low-excitation case, although possibly not generic, cannot be ruled out by observations for most systems, . Conclusions. Our “standard” profile provides a satisfactory explanation for a large group of debris discs that show smooth outer edges and $SB \propto r^{-3.5}$. Systems with sharper outer edges, barring other confining agents, could still be explained by “natural” collisional evolution if their dynamical excitation is very low. We show that such a dynamically-cold case provides a satisfactory fit to the specific HR4796A ring.
Context. 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 also respond to the companion star’s perturbations. Aims. 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 in a stationary state. Methods. We numerically followed the evolution of 105 massless small grains, initially produced from a circumprimary disc of parent bodies following a size distribution in dN ∝ s-3.5ds . Grains were submitted to both stars’ gravity and radiation pressure. In addition, particles were assigned an empirically derived collisional lifetime. Results. For all the binary configurations, the disc extends far beyond the critical semi-major axis acrit for orbital stability. This is due to the steady production of small grains, placed by radiation pressure on eccentric orbits reaching beyond acrit. 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. Conclusions. We have shown that a companion star can never fully truncate a collisionally active disc. For eccentric companions, grains in the unstable regions can contribute significantly 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.
Prevention of bacterial adhesion and biofilm formation on the surfaces of materials is a topic of major medical and societal importance. Various synthetic approaches based on immobilization or ...release of bactericidal substances such as metal derivatives, polyammonium salts and antibiotics were extensively explored to produce antibacterial coatings. Although providing encouraging results, these approaches suffer from the use of active agents which may be associated with side-effects such as cytotoxicity, hypersensibility, inflammatory responses or the progressive alarming phenomenon of antibiotic resistance. In addition to these synthetic approaches, living organisms, e.g. animals and plants, have developed fascinating strategies over millions of years to prevent efficiently the colonization of their surfaces by pathogens. These strategies have been recently mimicked to create a new generation of bio-inspired biofilm-resistant surfaces. In this review, we discuss some of these bio-inspired methods devoted to the development of antibiofilm surfaces. We describe the elaboration of antibacterial coatings based on natural bactericidal substances produced by living organisms such as antimicrobial peptides, bacteriolytic enzymes and essential oils. We discuss also the development of layers mimicking algae surfaces and based on anti-quorum-sensing molecules which affect cell-to-cell communication. Finally, we report on very recent strategies directly inspired from marine animal life and based on surface microstructuring.
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