H. Jay Melosh^1,*, David J. Stevenson², Robin Canup³

¹EAPS, Purdue University, West Lafayette, IN 47907, USA.
²Department of Planetary Science, California Institute of Technology, Pasadena, CA 91125-2100, USA.
³Planetary Science Directorate, Southwest Research Institute Boulder, CO 80302, USA.

This is a short letter without abstract.

Reference
Melosh HJ, Stevenson DJ and Robin Canup R (2013) Credit for Impact Theory. Science 342:1445-1446.
[doi:10.1126/science.342.6165.1445-b]
Reprinted with permission from AAAS

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E. Mazzotta Epifani¹, D. Perna², L. Di Fabrizio³, M. Dall’Ora¹, P. Palumbo⁴, C. Snodgrass⁵, J. Licandro^6,7, V. Della Corte⁴ and G. P. Tozzi⁸

¹INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
²LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France 
³Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, TF, Spain
⁴Universitá Parthenope, Dip. Scienze Applicate, Centro Direzionale Isola C4, 80143 Napoli, Italy
⁵Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁶Instituto de Astrofísica de Canarias, c/vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁷Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁸INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Context. With this work we start a systematic analysis of the distant activity of several long-period comets in order to investigate the evolution of activity throughout the solar system and explore differences between comets that pass their perihelion at far or very close distances from the Sun.
Aims. We present observational data for eight long-period comets, observed for the first time beyond r = 5 AU. Three targets have been characterised on their inward orbital branch. The others have passed their perihelion at quite large heliocentric distances (r_q from 4.5 to 7.5 AU).
Methods. We analyse multicolour broadband images (V,R, and I filters) taken at the Telescopio Nazionale Galileo to characterise the dust coma of the comets and investigate their morphology, photometry, colours, and dust production.
Results. The morphological analysis shows many differences among the sample, from the large twisted structure present in the coma of comet C/2005 L3 to the regular coma envelope of C/2010 R1. The colour of the dust coma of all the comets is redder than the Sun. The Afρ value (measured in a reference aperture of radius ρ = 10⁴ km) ranges from 114 ± 2 (C/2005 S4) to 5091 ± 47 (C/2005 L3) cm, depicting a scenario of bodies from moderately to very active. This is confirmed by the first-order quantitative estimate of the dust mass-loss rate for the comets that was obtained from the photometric data: assuming a grain velocity of v = 20 m/s, the dust production rate is comparable with, or even significantly larger than, that measured for many short-period (“old”) comets at much smaller heliocentric distances.

Reference
Epifani, EM, Perna D, Di Fabrizio L, Dall’Ora M, Palumbo P, Snodgrass C, Licandro J, Corte VD and Tozzi GP (2014) Observational results for eight long-period comets observed far from the Sun. Astronomy & Astrophysics 561:A6.
[doi:10.1051/0004-6361/201321290]
Reproduced with permission © ESO

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P. Thebault¹, Q. Kral¹ and J.-C. Augereau²

¹LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 6, Univ. Paris-Diderot, France
²Université Joseph Fourier/CNRS, LAOG, UMR5571, Grenoble, France

Context. In most debris discs, dust grain dynamics is strongly affected by stellar radiation pressure. Because this mechanism is size-dependent, we expect dust grains to be spatially segregated according to their sizes. However, because of the complex interplay between radiation pressure, grain processing by collisions, and dynamical perturbations, this spatial segregation of the particle size distribution (PSD) has proven difficult to investigate and quantify with numerical models.
Aims. We propose to thoroughly investigate this problem by using a new-generation code that can handle some of the complex coupling between dynamical and collisional effects. We intend to explore how PSDs behave in both unperturbed discs at rest and in discs pertubed by planetary objects.
Methods. We used the DyCoSS code to investigate the coupled effect of collisions, radiation pressure, and dynamical perturbations in systems that have reached a steady-state. We considered two setups: a narrow ring perturbed by an exterior planet, and an extended disc into which a planet is embedded. For both setups we considered an additional unperturbed case without a planet. We also investigated the effect of possible spatial size segregation on disc images at different wavelengths.
Results. We find that PSDs are always spatially segregated. The only case for which the PSD follows a standard dn ∝ s^-3.5ds law is for an unperturbed narrow ring, but only within the parent-body ring itself. For all other configurations, the size distributions can strongly depart from such power laws and have steep spatial gradients. As an example, the geometrical cross-section of the disc is very rarely dominated by the smallest grains on bound orbits, as it is expected to be in standard PSDs in s^q with q ≤ −3. Although the exact profiles and spatial variations of PSDs are a complex function of the set-up that is considered, we are still able to derive some reliable results that will be useful for image or SED-fitting models of observed discs.

Reference
Thebault P, Kral Q and Augereau J-C (2014) Grain size segregation in debris discs. Astronomy & Astrophysics 561:A16.
[doi:10.1051/0004-6361/201322052]
Reproduced with permission © ESO

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