Tilman Birnstiel and Sean M. Andrews

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

The expectation that aerodynamic drag will force the solids in a gas-rich protoplanetary disk to spiral in toward the host star on short timescales is one of the fundamental problems in planet formation theory. The nominal efficiency of this radial drift process is in conflict with observations, suggesting that an empirical calibration of solid transport mechanisms in a disk is highly desirable. However, the fact that both radial drift and grain growth produce a similar particle size segregation in a disk (such that larger particles are preferentially concentrated closer to the star) makes it difficult to disentangle a clear signature of drift alone. We highlight a new approach, by showing that radial drift leaves a distinctive “fingerprint” in the dust surface density profile that is directly accessible to current observational facilities. Using an analytical framework for dust evolution, we demonstrate that the combined effects of drift and (viscous) gas drag naturally produce a sharp outer edge in the dust distribution (or, equivalently, a sharp decrease in the dust-to-gas mass ratio). This edge feature forms during the earliest phase in the evolution of disk solids, before grain growth in the outer disk has made much progress, and is preserved over longer timescales when both growth and transport effects are more substantial. The key features of these analytical models are reproduced in detailed numerical simulations, and are qualitatively consistent with recent millimeter-wave observations that find gas/dust size discrepancies and steep declines in dust continuum emission in the outer regions of protoplanetary disks.

Reference
T Birnstiel and Andrews SM (2014) On the outer edges of protoplanetary dust disks. The Astrophysical Journal 780:153.
[doi:10.1088/0004-637X/780/2/153]

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P. R. Brook^1,5, A. Karastergiou¹, S. Buchner^2,6, S. J. Roberts³, M. J. Keith^4,7, S. Johnston⁴ and R. M. Shannon⁴

¹Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²Hartebeesthoek Radio Astronomy Observatory, P.O. Box 443, Krugersdorp 1740, South Africa
³Information Engineering, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
⁴CSIRO Astronomy and Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia
⁵CSIRO Astronomy and Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia
⁶School of Physics, University of Witwatersrand, Johannesburg, South Africa
⁷Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Debris disks and asteroid belts are expected to form around young pulsars due to fallback material from their original supernova explosions. Disk material may migrate inward and interact with a pulsar’s magnetosphere, causing changes in torque and emission. Long-term monitoring of PSR J0738–4042 reveals both effects. The pulse shape changes multiple times between 1988 and 2012. The torque, inferred via the derivative of the rotational period, changes abruptly from 2005 September. This change is accompanied by an emergent radio component that drifts with respect to the rest of the pulse. No known intrinsic pulsar processes can explain these timing and radio emission signatures. The data lead us to postulate that we are witnessing an encounter with an asteroid or in-falling debris from a disk.

Reference
Brook PR, Karastergiou A, Buchner S, Roberts SJ, Keith MJ, Johnston S and Shannon RM (2014) Evidence of an Asteroid Encountering a Pulsar. The Astrophysical Journal – Letters 780:L31.
[doi:10.1088/2041-8205/780/2/L31]

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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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Natalia A. Konovalova^1,*, Jose M. Madiedo², Josep M. Trigo-Rodríguez³

¹Institute of Astrophysics of the Academy of Sciences of the Republic of Tajikistan, Dushanbe, Tajikistan
²Facultad de Ciencias Experimentales, Universidad de Huelva, Huelva, Spain
³Facultat de Ciencies, Institute of Space Sciences (CSIC-IEEC), Campus UAB, Belaaterra, Spain

The results of the atmospheric trajectory, radiant, heliocentric orbit, and preliminary strewn field calculations for an extremely bright slow-moving fireball are presented. In the evening hours of July 23, 2008, a bright object entered Earth’s atmosphere over Tajikistan. The fireball had a −20.3 maximum absolute magnitude and a spectacularly long persistent dust trail remained visible over a widespread region of Tajikistan for about 28 minutes after sunset. The fireball was also recorded by a visible-light satellite system at 14 h 45 min 25 s UT, and the dust trail was imaged by video and photocameras. A unique aspect of this event is that it was detected by two infrasound and five seismic stations too. The bolide was first recorded at a height of 38.2 km, reached its maximum brightness at a height of 35.0 km, and finished at a height of 19.6 km. The first breakup occurred under an aerodynamic pressure of approximately 1.6 MPa, similar to the values derived for breakups of the scarcely reported meteorite-dropping bolides. The fireball’s trajectory and dynamic results suggest that meteorite survival is likely. The meteoroid followed an Apollo-like asteroid orbit comparable to those derived for previously recovered meteorites with accurately known orbits.

Reference
Konovalova NA, Madiedo JM and Trigo-Rodríguez JM (in press) The Tajikistan superbolide of July 23, 2008. I. Trajectory, orbit, and preliminary fall data. Meteoritics & Planetary Science
[doi:10.1111/maps.12217]
Published by arrangement with John Wiley & Sons

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Philippe Rousselot¹, Olivier Pirali², Emmanuël Jehin³, Michel Vervloet², Damien Hutsemékers³, Jean Manfroid³, Daniel Cordier¹, Marie-Aline Martin-Drumel², Sébastien Gruet², Claude Arpigny³, Alice Decock³, and Olivier Mousis¹

¹Institut UTINAM-UMR CNRS 6213, Observatoire des Sciences de l’Univers THETA, University of Franche-Comté, BP 1615, F-25010 Besançon Cedex, France
²Synchrotron SOLEIL, ligne AILES, UMR 8214 CNRS, L’orme des Merisiers, Saint-Aubin, F-91192 Gif-Sur-Yvette, France
³Département d’Astrophysique, de Géophysique et d’Océanographie, Université de Liège, Allée du Six Aohat ut, B-4000 Liège, Belgium

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar system’s origin. We unambiguously identified emission lines in comets due to the ¹⁵NH₂ radical produced by the photodissociation of ¹⁵NH₃. Analysis of our data has permitted us to measure the ¹⁴N/¹⁵N isotopic ratio in comets for a molecule carrying the amine (–NH) functional group. This ratio, within the error, appears similar to that measured in comets in the HCN molecule and the CN radical, and lower than the protosolar value, suggesting that N₂ and NH₃ result from the separation of nitrogen into two distinct reservoirs in the solar nebula. This ratio also appears similar to that measured in Titan’s atmospheric N₂, supporting the hypothesis that, if the latter is representative of its primordial value in NH₃, these bodies were assembled from building blocks sharing a common formation location.

Reference
Rousselot P, Pirali O, Jehin E, Vervloet M, Hutsemékers D, Manfroid J, Cordier D, Martin-Drumel M-A, Gruet S, Arpigny C, Decock A and Mousis O (2014) Toward a Unique Nitrogen Isotopic Ratio in Cometary Ices. The Astrophysical Journal – Letters 780:L17.
[doi:10.1088/2041-8205/780/2/L17]

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L. O’Rourke¹, D. Bockelée-Morvan², N. Biver², B. Altieri¹, D. Teyssier¹, L. Jorda³, V. Debout², C. Snodgrass⁴, M. Küppers¹, M. A’Hearn⁵, T. G. Müller⁶ and T. Farnham⁵

¹European Space Astronomy Centre, ESAC, ESA, 28691 Villanueva de la Cañada, Spain
²LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
³Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
⁴Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁵Dept. of Astronomy, Univ. of Maryland, College Park, MD 20742-2421, USA
⁶Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany

Context. The sungrazer comet C/2012 S1 (ISON) (perihelion at r_h = 0.0125 AU from the Sun) was bright and active when discovered in September 2012 at 6.3 AU from the Sun.
Aims. Our goal was to characterize the distant gaseous and dust activity of this comet, inbound, from observations of H₂O, CO and the dust coma in the far-infrared and submillimeter domains.
Methods. We report observations undertaken with the Herschel space observatory on 8 and 13 March 2013 (r_h = 4.54–4.47AU) and with the 30 m telescope of Institut de Radioastronomie Millimétrique (IRAM) in March and April 2013 (r_h = 4.45–4.18 AU). The HIFI instrument aboard Herschel was used to observe the H₂O 1₁₀ − 1₀₁ line at 557 GHz, whereas images of the dust coma at 70 μm and 160 μm were acquired with the PACS instrument. Spectra acquired at the IRAM 30 m telescope cover the CO J(2–1) line at 230.5 GHz. The spectral observations were analysed with excitation and radiative transfer models. A model of dust thermal emission taking into account a range of dust sizes is used to analyse the PACS maps.
Results. While H₂O was not detected in our 8 March 2013 observation, we derive a sensitive 3σ upper limit of Q_H2O < 3.5 × 10²⁶ molecules s^-1 for this date. A marginal 3.2σdetection of CO is found, corresponding to a CO production rate of Q_CO = 3.5 × 10²⁷ molecules s^-1. The Herschel PACS measurements show a clear detection of the coma and tail in both the 70 μm and 160 μm maps. Under the assumption of a 2-km radius nucleus, we infer dust production rates in the range 10–13 kg s^-1 or 40–70 kg s^-1, depending on whether a low or high gaseous activity from the nucleus surface is assumed. We constrain the size distribution of the emitted dust by comparing PACS 70 and 160 μm data, and considering optical data. Size indices between –4 and –3.6 are suggested. The morphology of the tail observed on 70 μm images can be explained by the presence of grains with ages older than 60 days.

Reference
O’Rourke L, Bockelée-Morvan D, Biver N, Altieri B, Teyssier D, Jorda L, Debout V, Snodgrass C, Küppers M, A’Hearn M, Müller TG and Farnham T (2013) Herschel and IRAM-30 m observations of comet C/2012 S1 (ISON) at 4.5 AU from the Sun. Astronomy & Astrophysics 560:A101.
[doi:10.1051/0004-6361/201322756]
Reproduced with permission © ESO

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Patrick Lajeunesse^1,*, Guillaume St-Onge², Jacques Locat³, Mathieu J. Duchesne⁴, Michael D. Higgins⁵, Richard Sanfaçon⁶, Joseph Ortiz⁷

¹Centre d’études nordiques and Département de géographie, Université Laval, Québec City, Québec, Canada
²Canada Research Chair in Marine Geology, Institut des sciences de la mer de Rimouski and GEOTOP, Université du Québec à Rimouski, Rimouski, Québec, Canada
³Département de géologie et de génie géologique, Université Laval, Québec City, Québec, Canada
⁴Natural Resources Canada, Geological Survey of Canada, Québec City, Québec, Canada
⁵Sciences de la Terre, Université du Québec à Chicoutimi, Saguenay, Québec, Canada
⁶Canadian Hydrographic Service, Institut Maurice-Lamontagne, Mont-Joli, Québec, Canada
⁷Department of Geology, Kent State University, Kent, Ohio, USA

We report on a 4.1 (±0.2) km diameter and 185 m deep circular submarine structure exposed on the seabed in >40 m water depths in the northwestern Gulf of St. Lawrence (Eastern Canada) from the analysis of high-resolution multibeam bathymetric and seismic data. The presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures. Also, other origins, such as kimberlites, intrusions, karsts, or diapirs, can be eliminated on geological criteria. A single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition, assumed to be impact melts, and a single quartz grain with planar intersection features thought to be shock-induced planar deformation features (PDFs). The absolute age of this possible impact structure is unknown, but its geological setting indicates that it was formed long after the Mid-Ordovician and before regional pre-Quaternary sea-level lowstands. Present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure.

Reference
Lajeunesse P, St-Onge G, Locat J, Duchesne MJ, Higgins MD, Sanfaçon R and Ortiz J (in press) The Corossol structure: A possible impact crater on the seafloor of the northwestern Gulf of St. Lawrence, Eastern Canada. Meteoritics & Planetary Science
[doi:10.1111/maps.12224]
Published by arrangement with John Wiley & Sons

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O. Pravdivtseva^1,*, A. Meshik¹, C. M. Hohenberg¹, G. Kurat^2,†

¹McDonnell Center for the Space Sciences and Physics Department of Washington University, Saint Louis, Missouri, USA
²Department of Lithospheric Sciences, University of Vienna, Vienna, Austria
^†Deceased

Using in situ laser analyses of a polished thin section from the IAB iron meteorite Campo del Cielo, we identified two silicate grains rich in radiogenic ^129*Xe, Cr-diopside, and oligoclase, excavated them from the metal, and irradiated them with thermal neutrons for I-Xe dating. The release profiles of ^129*Xe and ^128*Xe are consistent with these silicates being diopside and oligoclase, with activation energies, estimated using Arrhenius plots, of ∼201 and ∼171 kcal mole⁻¹, respectively. The 4556.4 ± 0.4 Ma absolute I-Xe age of the more refractory diopside isyounger than the 4558.0 ± 0.7 Ma I-Xe age of the less refractory oligoclase. We suggest that separate impact events at different locations and depths on a porous initial chondritic IAB parent body led to the removal of the melt and recrystallization of diopside and oligoclase at the times reflected by their respective I-Xe ages. The diopside and oligoclase grains were later brought into the studied inclusion by a larger scale catastrophic collision that caused breakup and reassembly of the debris, but did not reset the I-Xe ages dating the first events. The metal melt most probably was <1250 °C when it surrounded studied silicate grains. This reassembly could not have occurred earlier than the I-Xe closure in diopside at 4556.4 ± 0.4 Ma.

Reference
Pravdivtseva O, Meshik A, Hohenberg CM and Kurat G (in press) I-Xe ages of Campo del Cielo silicates as a record of the complex early history of the IAB parent body. Meteoritics & Planetary Science
[doi:10.1111/maps.12233]
Published by arrangement with John Wiley & Sons

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