J. Leliwa-Kopystynski^a and M. Arakawa^b

^aUniversity of Warsaw, Institute of Geophysics, 02-093 Warsaw, Pasteura 7, Poland
^bGraduate School of Science, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

A series of impact experiments onto solid decimeter-sized cylinders made of porous gypsum admixed with approximately one centimeter-sized pebbles have been performed. The target densities and their heterogeneous structures could be representative of those of the asteroids Ida, Eros and many others, because asteroid sub-surface could be the consolidated boulders made by self-compaction and/or by impact compaction. Impact velocities in the experiments ranged from 2.0 km/s to 6.7 km/s (collision velocity in the asteroid main belt is approximately 5 km/s). It was found that the slope of the cumulative number distribution of post-impact fragments strongly depends on the specific energy of the impact. The presence of pebbles strongly influences the impact strength of the target as well as the size distribution of the post-impact fragments. Results of the experiments presented here are aimed at identifying the analogy between the laboratory results and the damage of small asteroids or their catastrophic disruption after impacts.

Reference
Leliwa-Kopystynski J and Arakawa M (in press) Impacts experiments onto heterogeneous targets simulating impact breccia: Implications for impact strength of asteroids and formation of the asteroid families. Icarus
[doi:10.1016/j.icarus.2014.03.012]
Copyright Elsevier

Link to Article

Takeru K. Suzuki and Shu-ichiro Inutsuka

Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

We report results of three-dimensional magnetohydrodynamical (MHD) simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence excited by magnetorotational instability are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ~0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallower slopes, ~m⁰ – m^–1, than those of velocity and density. The Poynting flux associated with the MHD turbulence drives intermittent and structured disk winds as well as sound-like waves toward the midplane. The mass accretion mainly occurs near the surfaces, and the gas near the midplane slowly moves outward in the time domain of the present simulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks.

Reference
Suzuki TK and Inutsuka S-I (2014) Magnetohydrodynamic Simulations of Global Accretion Disks with Vertical Magnetic Fields. The Astrophysical Journal 784:121.
[doi:10.1088/0004-637X/784/2/121]

Link to Article

Anthony L. Piro¹ and Ehud Nakar²

¹Theoretical Astrophysics, California Institute of Technology, 1200 E California Boulevard, M/C 350-17, Pasadena, CA 91125, USA
²Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

Ongoing transient surveys are presenting an unprecedented account of the rising light curves of Type Ia supernovae (SNe Ia). This early emission probes the shallowest layers of the exploding white dwarf (WD), which can provide constraints on the progenitor star and the properties of the explosive burning. We use semianalytic models of radioactively powered rising light curves to analyze these observations. As we have summarized in previous work, the main limiting factor in determining the surface distribution of ⁵⁶Ni is the lack of an unambiguously identified time of explosion, as would be provided by detection of shock breakout or shock-heated cooling. Without this the SN may in principle exhibit a “dark phase” for a few hours to days, where the only emission is from shock-heated cooling that is too dim to be detected. We show that by assuming a theoretically motivated time-dependent velocity evolution, the explosion time can be better constrained, albeit with potential systematic uncertainties. This technique is used to infer the surface ⁵⁶Ni distributions of three recent SNe Ia that were caught especially early in their rise. In all three we find fairly similar ⁵⁶Ni distributions. Observations of SN 2011fe and SN 2012cg probe shallower depths than SN 2009ig, and in these two cases ⁵⁶Ni is present merely ~10^-2 M_☉ from the WDs’ surfaces. The uncertainty in this result is up to an order of magnitude given the difficulty of precisely constraining the explosion time. We also use our conclusions about the explosion times to reassess radius constraints for the progenitor of SN 2011fe, as well as discuss the roughly t² power law that is inferred for many observed rising light curves.

Reference
Piro AL and Nakar E (2014) Constraints on Shallow 56Ni from the Early Light Curves of Type Ia Supernovae. The Astrophysical Journal 784:85.
[doi:10.1088/0004-637X/784/1/85]

Link to Article

Bin Yang^1,2, Jacqueline Keane¹, Karen Meech^1,3, Tobias Owen³ and Richard Wainscoat³

¹NASA Astrobiology Institute, University of Hawaii, Honolulu, HI 96822, USA
²European Southern Observatory, Santiago, Chile
³Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA

The dynamically new comet C/2011 L4 (Pan-STARRS) is one of the brightest comets observed since the great comet C/1995 O1 (Hale-Bopp). Here, we present our multi-wavelength observations of C/2011 L4 during its in-bound passage to the inner solar system. A strong absorption band of water ice at 2.0 μm was detected in the near-infrared spectra, obtained with the 8 m Gemini-North and 3 m Infrared Telescope Facility Telescopes. The companion 1.5 μm band of water ice, however, was not observed. Spectral modeling shows that the absence of the 1.5 μm feature can be explained by the presence of sub-micron-sized fine ice grains. No gas lines (i.e., CN, HCN, or CO) were observed pre-perihelion in either the optical or the submillimeter. We derived 3σ upper limits for the CN and CO production rates. The comet exhibited a very strong continuum in the optical and its slope seemed to become redder as the comet approached the Sun. Our observations suggest that C/2011 L4 is an unusually dust-rich comet with a dust-to-gas mass ratio >4.

Reference
Yang B, Keane J, Meech K, Owen T and Wainscoat R (2014) TMulti-wavelength Observations of Comet C/2011 L4 (Pan-STARRS). The Astrophysical Journal – Letters 784:L23.
[doi:10.1088/2041-8205/784/2/L23]

Link to Article

Shoshana Z. Weider^a,d, Larry R. Nittler^a,d, Richard D. Starr^b,d, Timothy J. McCoy^c,d and Sean C. Solomon^a,c,d

^aDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
^bPhysics Department, The Catholic University of America, Washington, DC 20064, USA
^cDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
^dLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

We present measurements of Mercury’s surface composition from the analysis of MESSENGER X-Ray Spectrometer data acquired during 55 large solar flares, which each provide a statistically significant detection of Fe X-ray fluorescence. The Fe/Si data display a clear dependence on phase angle, for which the results are empirically corrected. Mercury’s surface has a low total abundance of Fe, with a mean Fe/Si ratio of ∼0.06 (equivalent to ∼1.5 wt % Fe). The absolute Fe/Si values are subject to a number of systematic uncertainties, including phase-angle correction and possible mineral mixing effects. Individual Fe/Si measurements have an intrinsic error of ∼10%. Observed Fe/Si values display small variations (significant at two standard deviations) from the planetary average value across large regions in Mercury’s southern hemisphere. Larger differences are observed between measured Fe/Si values from more spatially resolved footprints on volcanic smooth plains deposits in the northern hemisphere and from those in surrounding terrains. Fe is most likely contained as a minor component in sulfide phases (e.g., troilite, niningerite, daubréelite) and as Fe metal, rather than within mafic silicates. Variations in surface reflectance (i.e., differences in albedo and spectral slope) across Mercury are unlikely to be caused by variations in the abundance of Fe.

Reference
Weider SZ, Nittler LR, Starr RD, McCoy TJ and Solomon SC (in press) Variations in the abundance of iron on Mercury’s surface from MESSENGER X-Ray Spectrometer observations. Icarus
[doi:10.1016/j.icarus.2014.03.002]
Copyright Elsevier

Link to Article

Mainzer¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Only a very small fraction of the asteroid population at size scales comparable to the object that exploded over Chelyabinsk, Russia has been discovered to date, and physical properties are poorly characterized. We present previously unreported detections of 105 close approaching near-Earth objects (NEOs) by the Wide-field Infrared Survey Explorer (WISE) mission’s NEOWISE project. These infrared observations constrain physical properties such as diameter and albedo for these objects, many of which are found to be smaller than 100 m. Because these objects are intrinsically faint, they were detected by WISE during very close approaches to the Earth, often at large apparent on-sky velocities. We observe a trend of increasing albedo with decreasing size, but as this sample of NEOs was discovered by visible light surveys, it is likely that selection biases against finding small, dark NEOs influence this finding.

Reference
Mainzer et al. (2014) The Population of Tiny Near-Earth Objects Observed by NEOWISE. The Astrophysical Journal 784:110.
[doi:10.1088/0004-637X/784/2/110]

Link to Article

D. Turrini^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aa Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via del Fosso del Cavaliere 100, 00133, Rome, Italy

The Dawn spacecraft recently observed the presence of dark material, which in turn proved to be associated with the presence of OH and H-rich material, on the surface of Vesta. The source of this dark material has been almost unanimously identified with the low albedo asteroids, likely analogous to the carbonaceous chondrites found on Earth, that impacted on Vesta over its lifetime. However, it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events, to micrometeorites or to the continuous, secular flux of impactors on Vesta. The “continuous flux” scenario, in particular, predicts that a significant fraction of the exogenous material accreted by Vesta should be due to non-dark impactors likely analogous to ordinary chondrites, which instead represent only a minor contaminant in the Howardite-Eucrite-Diogenite meteorites. In this work, we explored the “continuous flux” scenario and its implications for the composition of the vestan regolith, taking advantage of the data from the Dawn mission and the Howardite-Eucrite-Diogenite meteorites to constrain the contamination history of Vesta. We developed a model for the delivery of the exogenous material to Vesta and verified how the results it supplies are sensitive to the different parameters we consider. We calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the Rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of Cornelia crater and of its dark deposits with a hydrocode simulation. We used our calibrated model to show that the “stochastic events” scenario and the “micrometeoritic flux” scenario are just natural consequences of the “continuous flux” scenario. We then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on Vesta since the Late Heavy Bombardment and we showed how our results match well with the values estimated by the Dawn mission. We finally used our model to assess the amount of Fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith: concerning the siderophile elements, we focused our attention on the role of Ni. The results we obtained are in agreement with the data available on the Fe and Ni content of the Howardite-Eucrite-Diogenite meteorites and can be used as a reference frame in future studies of the data from the Dawn mission and of the Howardite-Eucrite-Diogenite meteorites. Our model cannot yet provide an answer to the conundrum of the fate of the missing non-carbonaceous contaminants, but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results.

Reference
Turrini et al. (in press) The contamination of the surface of Vesta by impacts and the delivery of the dark material. Icarus
[doi:10.1016/j.icarus.2014.02.021]
Copyright Elsevier

Link to Article

Kun Wang^a, James M.D. Day^b, Randy L. Korotev^a, Ryan A. Zeigler^c and Frédéric Moynier^a,d

^aDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
^bGeosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA
^cAstromaterials Research and Explorations Science Directorate, Acquisition and Curation, NASA Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058, USA
^dInstitut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238, Paris Cedex 05, France

New Fe isotope data of feldspar-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9) reveal that they are the only known examples of crustal materials with isotopically light Fe isotope compositions (; δ  ⁵⁶Fe is defined as the per mille   deviation of a sample’s ⁵⁶Fe/⁵⁴Fe ratio from the IRMM-014 standard) in the Solar System. In contrast, associated brachinites, as well as brachinite-like achondrites, have Fe isotope compositions () that are isotopically similar to carbonaceous chondrites and the bulk terrestrial mantle. In order to understand the cause of Fe isotope variations in the GRA 06128/9 and brachinite parent body, we also report the Fe isotope compositions of metal, silicate and sulfide fractions from three ordinary chondrites (Semarkona, Kernouve, Saint-Séverin). Metals from ordinary chondrites are enriched in the heavier isotopes of Fe (average ), sulfide fractions are enriched in the lighter isotopes of Fe (average ), and the δ  ⁵⁶Fe values of the silicates are coincident with that of the bulk rock (average ).
The enrichment of light isotopes of Fe isotopes in GRA 06128/9 is consistent with preferential melting of sulfides in precursor chondritic source materials leading to the formation of Fe–S-rich felsic melts. Conceptual models show that melt generation to form a GRA 06128/9 parental melt occurred prior to the onset of higher-temperature basaltic melting (<1200 °C) in a volatile-rich precursor and led to the generation of buoyant felsic melt with a strong Fe–S signature. These models not only reveal the origin of enrichment in light isotopes of Fe for GRA 06128/9, but are also consistent with petrological and geochemical observations, experimental studies for the origin of Fe–S-rich felsic melts, and for the cessation of early melting on some asteroidal parent bodies because of the effective removal of the major radioactive heat-source, ²⁶Al. The mode of origin for GRA 06128/9 contrasts strongly with crust formation on Earth, the Moon, Mars and other asteroids, where mantle differentiation and/or oxygen activity are the major controls on crustal Fe isotope compositions.

Reference
Wang K, Day JMD, Korotev RL, Zeigler RA and Moynier F (2014) Iron isotope fractionation during sulfide-rich felsic partial melting in early planetesimals. Earth and Planetary Science Letters 392:124–132.
[doi:10.1016/j.epsl.2014.02.022]
Copyright Elsevier

Link to Article

Gonzalo Tancredi

Departamento de Astronomí a, Facultad de Ciencias, Iguá 4225, 11400. Montevideo, URUGUAY

The classification criterion between asteroids and comets has evolved in recent decades, but the main distinction remains unchanged. Comets present gas and dust ejection from the surface at some point of their orbits, therefore, these objects are considered to be active. On the other hand, asteroids do not show any kind of large scale gas and dust ejection, they are inert. Nevertheless, this classification scheme is impractical when we have more than 500,000 asteroids already discovered. In addition, comets are not active all along their orbits. In order for a comet to display activity at present or in the recent past in the inner region of the Solar System (heliocentric distance <2AU), the cometary orbit must be unstable in the time scale on the order of ten thousands of years; otherwise, the object should have completely consumed its volatile component. Close encounters with the most massive planets is the only mechanism that could produce ”macroscopic” instabilities on a short time scale. The macroscopic changes in the orbital elements can be detected in a numerical integration of the dynamical evolution of the object over a time scale of several thousand years. This procedure to identify asteroids in cometary-like orbits is also impractical because it would require months of computing time. Therefore, a classification scheme based on the orbital elements to identify the border cases between the asteroid and comet populations is urgently required.
We present a criterion to classify asteroids and comets and to find the border case based on the Tisserand’s parameter, the Minimum Orbital Intersection Distance (MOID), and considering some information regarding the aphelion and perihelion distances. Objects in mean-motion are disregarded. After applying a filter to the sample of over half a million asteroids already discovered to select the precise orbits and to the sample of 487 short-period comets, we apply the proposed classification criterion. The resulting sample consists of ~331 Asteroids in Cometary Orbits (ACOs). The ACOs are further classified in subclasses similar to the cometary classification. There are 436 Jupiter Family Comets and 203 ACOs of the Jupiter Family type. This new criterion is more strict that the criteria used by other authors to identify ACOs; nonetheless, with the new criterion we ensure that the ACOs have a chaotic dynamical evolution similar to the periodic comets. The discovered dormant or extinct comets seems, if they exist at all, to be a small fraction of the active comets.
We also analyse the available photometric data of ACOs to identify possible large brightness variations. Among the sample of ACOs, there is only one object with brightness variations typical of an active comet: 174P/(60558) Echeclus. But this object has already been double classified as asteroid and comet.

Reference
Tancredi G (in press) A criterion to classify asteroids and comets based on the orbital parameters. Icarus
[doi:10.1016/j.icarus.2014.02.013]
Copyright Elsevier

Link to Article

Miloslav Druckmüller¹, Shadia Rifai Habbal², Peter Aniol^3,4, Adalbert Ding⁵ and Huw Morgan⁶

¹Faculty of Mechanical Engineering, Brno University of Technology, 616 69 Brno, Czech Republic
²Institute for Astronomy, University of Hawaii, Honolulu 96822, Hawaii, USA
³ASTELCO Systems GmbH, D-82152 Martinsried, Germany
⁴KACCOLR, King Abdulaziz University, Jeddah 22254, Saudi Arabia
⁵Institute of Optics and Atomic Physics, Technische Universitaet Berlin, and Institute of Technical Physics, Berlin, Germany
⁶Institute of Mathematics, Physics and Computer Science, Aberystwyth University, Ceredigion, Cymru SY23 3BZ, UK

Much anticipation and speculation were building around comet ISON, or C/2012 S1, discovered on 2012 September 21 by the International Scientific Optical Network telescope in Russia, and bound for the Sun on 2013 November 28, with a closest heliocentric approach distance of 2.7 R_☉. Here we present the first white light image of the comet’s trail through the inner corona. The image was taken with a wide field Lyot-type coronagraph from the Mees Observatory on Haleakala at 19:12 UT, past its perihelion passage at 18:45 UT. The perfect match between the comet’s trail captured in the inner corona and the trail that had persisted across the field of view of 2-6 R_☉ of the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph Experiment/C2 coronagraph at 19:12 UT demonstrates that the comet survived its perihelion passage.

Reference
Druckmüller M, Habbal SR, Aniol P, Ding A and Morgan H (2014) Imaging Comet ISON C/2012 S1 in the Inner Corona at Perihelion. The Astrophysical Journal – Letters 784:L22.
[doi:10.1088/2041-8205/784/2/L22]

Link to Article